Journal of Colloid and Interface Science 415 (2014) 18–25
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Surfactant micelles containing solubilized oil decrease foam film thickness stability Jongju Lee, Alex Nikolov, Darsh Wasan ⇑ Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA
a r t i c l e
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Article history: Received 25 March 2013 Accepted 9 October 2013 Available online 22 October 2013 Keywords: Foam stability Single foam film stability Swollen micelles Aqueous foams
a b s t r a c t Many practical applications involving three-phase foams (aqueous foams containing oil) commonly employ surfactants at several times their critical micelle concentration (CMC); in these applications, the oil can exist in two forms: (1) oil drops or macroemulsions and (2) oil solubilized within the micelles. We have recently observed that in the case of aqueous foams stabilized with sodium dodecyl sulfate (SDS) and n-dodecane as an oil, the oil drops did not alter the foam stability but the solubilized oil (swollen micelles) greatly influenced the foam’s stability. In order to explain the effect of oil solubilized in the surfactant micelles on foam stability, we studied the stability of a single foam film containing swollen micelles of SDS using reflected light microinterferometry. The film thinning occurs in stepwise manner (stratification). In addition, we obtained data for the film-meniscus contact angle versus film thickness (corresponding to the different number of micellar layers) and used it to calculate the film structural energy isotherm. The results of this study showed that the structural energy stabilization barrier decreased in the presence of swollen micelles in the film, thereby decreasing the foam stability. These results provide a better understanding of the role of oil solubilized by the micelles in affecting foam stability. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Liquid films are the basic structural elements of any foaming system containing surfactant micelles. Consequently, the stability of the foam is highly dependent on stability of the intervening film between the bubbles. There have been numerous studies [1,2] on the stability of a single liquid film separating the gas bubbles in a two-phase (gas and liquid) foam system. However, studies examining the stability of the intervening micellar film in a three-phase aqueous foam containing oil are rare. This is especially true when solubilized oil is present within the micelles (swollen micelles). To the best of our knowledge, the only reported study is that of Nikolov et al. [3], Wasan et al. [4], and Lobo et al. [5] who had examined the stability of a foam film and/or foam containing swollen micelles of a nonionic surfactant. More recently, Lee et al. [6] reported the results of a study on the importance of dispersed versus solubilized oil on the foam stability in a three-phase aqueous foaming system containing n-dodecane and micelles of an anionic surfactant of sodium dodecyl sulfate (SDS). Their results revealed that the oil drop did not alter the foam stability but the oil solubilized in the surfactant micelles greatly influenced the foam’s stability.
⇑ Corresponding author. Fax: +1 6309854451. E-mail address: [email protected] (D. Wasan). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.014
In order to elucidate the effect of oil solubilized in the surfactant micelles on the foam stability, we studied the stability of a single foam film containing swollen micelles of SDS using the reflected light microinterferometric technique. We observed the microstructuring phenomenon in a thinning foam film. In addition, we measured the film-meniscus contact angle and calculated the film interaction energy as a function of the film thickness. We then estimated the structural energy stabilization/destabilization barrier from the calculated interaction energy isotherm. We also conducted a three-phase foam stability experiment using the Bartsch method, and rationalized the results of these tests in terms of the changes in the structural stabilization barrier and the effective micellar volume for foaming systems with and without the swollen micelles.
2. Experimental 2.1. Materials and preparation of solutions We used n-dodecane (the density at 20 °C is 0.75 g/cm3) from Aldrich Chemical, and pre-equilibrated the surfactant solution to produce swollen micelles. An anionic surfactant, sodium dodecyl sulfate (SDS), was supplied by Fisher; the surfactant molecule has an average molecular weight of 288.38. We used 0.03 M (mol/L) and 0.06 M surfactant
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solutions. These concentrations are higher than the critical micelle concentration (CMC = 8.2 103 M). The surfactant solutions were prepared by dissolving SDS in deionized water using a ‘‘Milli-Q’’ water system deionizer from Millipore Corporation. The surfactant system was contacted with the 1000 ppm n-dodecane and preequilibrated for more than 10 days. During pre-equilibration, the oil phase was dispersed into the aqueous surfactant solution as drops, and left overnight for phase separation in a glass container. The pre-equilibrated surfactant solutions were centrifuged for 2 h, and subsequently filtrated through a 0.1 lm filter. In order to minimize the surfactant adsorption on the filter, the filter was saturated with 0.06 M SDS surfactant solution before filtration. We used suction repeatedly to remove the pre-equilibrated oil from the top of the container.
2.2. Surface and interfacial tension measurements The surface tension was measured by a Kruss tensiometer using a platinum Wilhelmy plate for the oil-free surfactant solution as well as the pre-equilibrated surfactant solution.
2.3. Refractive index measurement The refractive index of the micellar solutions was measured using a Fisher refractometer at 25 °C ± 1 °C.
2.4. Foam stability measurements The foams were produced from micellar surfactant solutions of sodium dodecyl sulfate (SDS). The stability of the foam was quantified by the Bartsch method [6]. The three-phase foams were generated in a graduated glass cylinder (250 mL) by shaking 50 mL of the surfactant solution 20 times. The foam height was monitored as a function of time. We compared the stability of the foams formed by shaking the aqueous solutions containing the SDS swollen micelles (the micelles with solubilized oil) with the stability of the foams without swollen micelles.
2.5. Film thinning We used the reflected light microinterferometric method to observe the film stratification phenomenon (stepwise thinning) [7– 10]. Microscopic films were formed in a cylindrical capillary with a hydrophilic inner wall of radius R (1.35 mm). The films were formed by sucking out the liquid from a biconcave drop inside the capillary through an orifice in the wall. The resulting horizontal flat film was encircled by a biconcave meniscus. The glass cell assembly was enclosed in an environment with a thermostat and the temperature was maintained within ±0.2 °C in the range of 20–50 °C. The entire assembly was placed on the stage of the Max Zhender differential interference microscope, which was mounted on a vibration-free table to keep any external disturbances from affecting the film thinning process. In the reflected light mode of the microscope, monochromatic light (with a wavelength of 546 nm) through the top of the glass cell was incident on the film surface. The thickness of the foam film changes produced the interference patterns. The film lamella thickness as a function of time was evaluated from the color of the interference patterns. The video camera, in conjunction with the monitor and digital video recorder, recorded the process of film thinning. The effective micellar diameter and volume fraction were calculated from the stepwise film thinning phenomena.
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3. Results and discussion 3.1. Foam stability in a three-phase foam system The main criterion for the foam stability was the decrease in the foam height versus time. Fig. 1 shows the effect of swollen micelles on the foam height versus time. Data are shown for the two different micellar concentrations of 0.03 M and 0.06 M. In the case of the SDS solution without swollen micelles, the foam formed from a higher micellar concentration solution was more stable than that formed from a lower micellar concentration. The foam height started to decrease in a stepwise manner after 4 h and 7.5 h, respectively. The decrease in the foam height as a function of time was more pronounced in the samples containing swollen micelles than those without them. About 80% of the foam in the presence of the swollen micelles containing n-dodecane collapsed in about 2 h. It has been demonstrated [10–13] that the foam films or lamellae formed from higher SDS micellar concentrations exhibit a more ordered micelle microstructure during thinning (stratification) due to the repulsive intermicellar interactions and the micelles with a narrow size distribution (these are forced into the restricted volume of the film). The drainage time of the liquid film increases with the increase in the micellar volume phase. Consequently, foams with a slower drainage time exhibit increased foam stability with an increase in the micellar concentration. In the case of n-dodecane pre-equilibrated micellar system containing swollen micelles, the solubilized oil (swollen micelle) reduces the foam stability. As n-dodecane solubilization limit is approached, the stability of foam decreases gradually versus amount of solubilized oil. In order to verify the role of solubilized oil on the foam stability, we study the solubilization limit, the solubilization time and concentration of n-dodecane into SDS micelles. 3.2. Solubilization limit versus concentration of n-dodecane and pre-equilibration time We investigated the solubilization limit of n-dodecane into 0.06 M SDS surfactant solution in two aspects: (1) time to reach the solubilization limit at high and low concentrations of n-dodecane and (2) the solubilization limit of n-dodecane concentration into 0.06 M SDS surfactant solution. The time for reaching solubilization limit versus the concentraion of n-dodecane (200 ppm and 1000 ppm) was stuied first. The 0.06 M SDS surfactant solutions were contacted with n-dodecane
Fig. 1. Normalized foam height versus time.
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and pre-equilibrated for 1–360 h respectively. The stability of the foam was quantified by the decrease the foam height as a function of time (Bartsch method). We observed that the foam stability decreases as increasing the pre-equilibration time. The data presented in Fig. 2 depict the normalized foam stability versus time and versus the pre-equilibration time of 1000 ppm n-dodecane with 0.06 M SDS solution. The dashed line marks the solubilization time limit. The foam formed by 0.06 M SDS solution pre-equilibration for 48 h with 1000 ppm of n-dodecane collapsed 80% within 2 h (see Fig. 2). As expected in the case of low concentration of n-dodecane (200 ppm), it took longer time to reach the solubilization limit of n-dodecane into 0.06 M SDS surfactant solution. From Fig. 3, about 80% of foam collapsed within 2 h when 0.06 M SDS surfactant solution was pre-equilibrated with 200 ppm of n-dodecane for 144 h. It represents that the system reaches the solubilization limit. The solubilization limit of n-dodecane concentration into 0.06 M SDS surfactant solution was also investigated (see Fig. 4). The 0.06 M SDS surfactant solutions were pre-equilibrated with different concentrations of n-dodecane (from 100 to 1000 ppm) for 10 days – enough time to reach the solubilization limit from the results of the time for low (200 ppm) and high (1000 ppm) concentration of n-dodecane solubilization into 0.06 M SDS micelles. Then, the foam stability was measured by the decrease in the foam height with time using the Bartsch method. The results are presented in Fig. 4. About 80% of the foam collapsed after 2 h when the 0.06 M SDS pre-equilibrated with more than 200 ppm of ndodecane for 10 days. The data indicate that the concentration range between 200 and 300 ppm of n-dodecane is the solubilization limit of n-dodecane into 0.06 M SDS surfactant solution. When the system reaches the solubilization limit, about 80% of the foam collapse within 2 h. Based on the study of solubilization limit of n-dodecane into 0.06 M SDS surfactnat solution, 0.06 M SDS surfactnat solution pre-equilibrated with 1000 ppm n-dodecane for 10 days reaches the solubilization limit. And, results of these foam stability tests clearly indicate that the foam stability significantly decreased when the system contained swollen micelles at the solubilization limit. To gain understanding into the role of the interaction between swollen micelles on the contribution of structural disjoining pressure of foam lamella stability we investigated the stability of a
Fig. 3. 3-D bar plot depicts the normalized foam stability to collapse by 80% versus time and versus the pre-equilibration time of 200 ppm n-dodecane with 0.06 M SDS. The dashed line marks the solubilization time limit.
Fig. 4. 3-D bar plot depicts the normalized foam stability to collapse by 80% versus time and versus the pre-equilibration time of n-dodecane with 0.06 M SDS during 10 days. The dashed line marks the solubilization time limit.
single foam film by examining the drainage characteristics of the film with and without swollen micelles.
3.3. Film stratification phenomena
Fig. 2. 3-D bar plot depicts the normalized foam stability to collapse by 80% versus time and versus the pre-equilibration time of 1000 ppm n-dodecane with 0.06 M SDS. The dashed line marks the solubilization time limit.
Using the reflected light microinterferometric method [7–10,14], we observed the drainage characteristics of the microscopic horizontal foam films formed from SDS 0.06 M (7.3 times higher than CMC) solutions (with and without swollen micelles). The thinning evolution of one of the films is depicted in the movie clip available in the ‘‘Supplementary Information’’ section accompanying this paper.
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After the film is formed, it immediately starts to decrease its thickness. At first, the film rests for a few seconds in a metastable state of uniform thickness (a homogeneous color) at h = 102 nm. Then the darker spots appear locally inside of the uniform color, indicating that this area is thinner than the remaining part of the film. The area of the darker spot increases gradually until it covers the whole film area. Fig. 5 shows the stepwise thinning process for the 0.06 M SDS micellar solution. The first thickness transition occurs at above 17 s and the film reduces its thickness from 70 nm to 59 nm. This is followed by the second, third, fourth, and fifth transitions. During the final stage of the film thinning process, the film remains stable (at h = 26 nm) with one layer of micelles. Fig. 6 shows the photomicrographs of the stepwise thinning phenomena for the film formed for the 0.06 M SDS solution pre-equilibrated with n-dodecane. This system displayed only four thickness transitions and its final film contained no micelles. It is worth noting that the film containing micelles (without the solubilized oil) exhibited rather regular stepwise thickness transitions, whereas the transitions were less regular and occurred with greater ease for the film containing the swollen micelles, as seen in Figs. 5 and 6, and the video clip (in the Supplementary Information). The values for the film thicknesses and their lifetimes are tabulated in Table 1. Fig. 7 depicts the evolution, manner and features of film thinning of foam film from 0.06 M SDS solution with and without swollen micelles regarding: the number of film thickness transitions, the amplitude of transitions, the time it takes for the thickness transitions to occur, the final (equilibrium) film thickness, and the manner of transition (e.g., how many dark spot/s formed during the transitions). It is important to note that the effect of the micelles (or the role of the structural disjoining pressure) on the manner of film thinning becomes apparent in film thicknesses less than three micellar layers, where the film surfaces confinement effect on the structural disjoining pressure has to be considered. The foam film in the presence of the swollen micelles exhibits four thickness transitions before reaching its equilibrium thickness without micelles; the film with the SDS micelles exhibits five thickness transitions and its equilibrium thickness remains at one micellar layer (see Table 1). The amplitude of thickness transitions for film with the swollen micelles was 15 ± 2 nm while the amplitude of the foam film with the SDS micelles was 11 ± 2 nm, and it was more regular than that of the film containing swollen micelles. As a result of the swollen micelles’ polydispersity, the thickness transitions of the film with the swollen micelles were less regular than that of the film with the SDS micelles. The polydispersity affects the time it takes for the thickness transitions to occur and in what manner they occur. The foam film in the
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presence of swollen micelles reduces its film layer thickness (stratify) quickly by simultaneous appearance of several dark spots in film area, corresponding to a film without micelles. The foam film with the SDS micelles stratifies slowly with the appearance of one spot. The film changed in thickness corresponding to the effective micellar diameter. The number of thickness transitions indicates the micellar effective volume fraction, and the amplitude of stepwise thinning indicates the effective micellar diameter. The effective micellar volume concentration was estimated to be 30 vol.% based on the number of film thickness transitions [15] and the effective micellar diameter is about 11 nm (corresponding to the micellar stepwise thickness) for the solution containing the 0.06 M SDS micelles. However, there are only four thickness transitions for the 0.06 M SDS solution containing the swollen micelles which corresponded to an 18 vol.% concentration with an effective micellar diameter of about 15 nm. The micelles swell with the solubilization of the oil and become polydisperse in size. The effect of oil solubilization upon film thinning is relevant because of the relationship between solubilization and micellar microstructuring in the film. The decrease in the micellar volume phase from 30 vol.% to 18 vol.% results in a decrease in the intermicellar interaction forces. Recently, Lee et al. [6] determined the second virial coefficient (which is indicative of intermicellar interactions) for the 0.06 M SDS solution containing swollen micelles and found it to be negative, signifying attractive interactions between the micelles. But the second virial coefficient was found to be positive for the SDS micelles without the solubilized oil, signifying repulsive interactions. The uniformity in the micelle size and the repulsive intermicellar interactions are known to result in better ordered microstructuring (layered structure) inside the confined surfaces of the film. In contrast, polydispersity in size and less repulsive (or attractive) interactions yield films with less ordered microstructuring, as seen in Fig. 6. 3.4. Film-meniscus profile and contact angle Using the reflected light interferometric technique, we monitored the film-meniscus profile and the contact angle between the film and the meniscus [8,9,16–20]. Fig. 8A shows a photograph of a foam film surrounded by the meniscus, indicated by the meniscus, indicated by the consecutive dark and bright Newton interference rings around the periphery of the film. Fig. 8B depicts the schematic of the film-meniscus region and Fig. 8C shows the interferogram depicting the successive maxima and minima in the intensity of the reflective light, representing the change in
Fig. 5. Evolution of film thinning for the 0.06 M SDS solution (the film is undergoing stratification thickness transitions).
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Fig. 6. Evolution of film thinning for 0.06 M SDS solution pre-equilibrated by n-dodecane (the film is undergoing stratification thickness transitions).
Table 1 Estimation of the film thickness (h) at the metastable state based on the mean height of a stepwise film thickness transition (~d), lifetime (s) corresponding to the number of layers (n) and film-meniscus contact angle (heq). n
0.06 M SDS ~ ¼ 11 nmÞ ðd h (nm) b
5 4a 3a 2b 1b
70 ± 2 59 ± 2 48 ± 2 37 ± 2 26 ± 3
Film contact angle heq (deg)
n
0.06 M SDS pre-equilibrated by n-dodecane ~ ¼ 15 nmÞ ðd
s (s) 11 – – 35 51
– – – 0.7 ± 0.1 1.1 ± 0.1
b
4 3a 2a 1b Film without micellesb
h (nm)
s (s)
81 ± 2 66 ± 2 51 ± 2 36 ± 2 21 ± 3 (final film thickness)
11 – – 35 42
Film contact angle heq (deg)
– – – 0.8 ± 0.1 1.3 ± 0.1
a The transitions happen simultaneously for a short period of time. The film thickness was estimated by the mean height of a stepwise film thickness transition (~ d) corresponding to the number of layers. b All the measured values have an accuracy of ±10%.
noting that thinner films have larger contact angles. The contact angle increases when the film becomes thinner due to the larger interaction between the film surfaces. The data for the film-meniscus contact angle versus film thickness were used to calculate the film structural energy isotherm as shown in the following section. 3.5. Structural interaction energy between film surfaces
Fig. 7. Time for accruing the stepwise thickness transitions of systems with and without the swollen micelles.
the thickness in the meniscus region. The film-meniscus profile shown in the figure was obtained using the procedure described in Ref. [9]. The contact angle, heq, is the angle subtended between the film and the meniscus, which is determined from the slope of the point of intersection of the meniscus profile with the film contacting one layer of SDS micelles. Table 1 lists the values of the film-meniscus microscopic contact angle for the 0.06 M SDS and the 0.06 M SDS pre-equilibrated with the n-dodecane solution versus the film thickness corresponding to the number of micellar layers. The accuracy of the contact angle determination is ±0.1°. It is worth
We used the procedure described in Ref. [9] to calculate the film structural energy isotherm. As discussed in Ref. [9], the structural energy is oscillatory due to the micellar layering within the film (as seen in Figs. 5 and 6), and it dominates the van der Waals contribution to the film energy given by the conventional DLVO theory. The amplitude of oscillation (A), and the experimentally measured parameters—namely, the contact angle (heq), and film thickness (h)—can be linked by the relation,
A cos
2p h h exp ¼ ra=l ðcos heq 1Þ d d
ð1Þ
where ‘d’ is the effective diameter of the micelle given by the average distance between the film thickness transitions, ‘ra/l’ is the surface tension, and ‘h’ is the film thickness. It should be noted that this equation justifiably neglects the contribution of the surface interaction force, which is given by the sum of the capillary and hydrostatic pressures multiplied by the equilibrium film thickness as the micellar structuring overwhelms the effect of the surface interaction force [18]. As noted in Ref. [9], parameter ‘A’ is determined by choosing the film thickness corresponding to one micellar layer.
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Fig. 8. (A) Photomicrograph depicting the film and the surrounding meniscus. (B) Sketch of the film-meniscus region. (C) Inset of light interferograph and the film-meniscus profile.
Fig. 9 displays the oscillatory structural interaction energy (W) [9,21] for the two systems; this figure shows a plot of the nondimensionalized structural energy (Wd2/kT) versus the film thickness (h). The structural energy barriers for the two systems are also shown in this figure. From the first maximum to the second minimum, the structural energy barrier for the pre-equilibrated system is lower (0.53 kT) than that (1.53 kT) for the system without pre-equilibration. Therefore, the film with a larger structural barrier will take longer to reach its final thickness and this is what was observed experimentally (see Figs. 5 and 6, and Table 1). Fig. 9 shows that the structural stabilization barrier increases as the film becomes thinner. If the film is thick (more than 2 micellar layers), the stepwise thinning transitions occur within a shorter period of time because the contribution of the film interaction energy is minor. In order to understand the role of swollen micelle on the foam stability, we investigated the single foam film by using the refracted light microinterferometric technique. And, we analyzed the experimental observations to estimate the structural stabilization barrier on the film to reduce the thickness to explain decreas-
Fig. 9. Estimated oscillatory structural energy isotherm as a function of the film thickness.
ing the film stability due to swollen micelles. Based on the results, we explain the stability of single foam film in the presence of micelles and swollen micelles characterized by the structural stabilization barrier. 3.6. The role of micellar interactions in foam stability The classical model, DLVO (Derjaguin, Landau, Verwey, and Overbeek), based on the disjoining pressure isotherm provides a phenomenological understanding for the thickness stability of foam film. The classical understanding for film stability is sketched in Fig. 10. It assumes that the disjoining pressure isotherm has ‘S’ shape and predicts the film may have two equilibrium thickness states: a thicker film (h1) in equilibrium within the meniscus with a small contact angle and a thinner film (h2) with a meniscus in equilibrium with a large contact angle. The DLVO model postulates that a film could change its thickness in a single stepwise manner if the capillary pressure (Pc) exceeds the disjoining pressure P(h) barrier. Based on the DLVO model, Scheludko [22] proposed for the film instability the criteria Pc > P and dP/dh > 0. A film at its critical thickness (hcr) becomes kinetically unstable with respect to small,
Fig. 10. Disjoining pressure isotherm and the critical film thickness (hcr) based on DLVO model.
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spontaneous film surfaces perturbations or fluctuations k(hcr) so that, in spite of increasing the film’s free energy due to van der Waals attraction forces, the film’s total energy decreases. In addition, Vrij [23] suggested a model for the film’s instability based on Scheludko’s criteria of film instability. After thinning film reaches its critical thickness (hcr), the local wave fluctuation growth spontaneously promoted by the van der Waals attractive forces and overcame the local capillary repulsive force, and triggered the local film thickness to decrease or the film to rupture. Therefore, the film ruptures when the film becomes thin that the rate of the local fluctuations at the critical thickness (hcr) is faster than the film thinning process. The DLVO model provides phenomenological explanation for the reason film becomes unstable and but does not provide understanding of the mechanism of how a film thickness changes or how a film ruptures. The classical model does not account for the role of micelles in film stability. In the case of the foam film containing micelles, the effect of structural disjoining pressure on the film stability is important. In order to elucidate the role of the structural disjoining pressure on instability of the film, we analyze the relationship between the structural disjoining pressure (Pst) and the capillary pressure (Pc) in the case of 0.06 M SDS foam. We estimated the structural disjoining pressure (Pst) of 0.06 M SDS micellar solution based on the simple calculation proposed by Trokhymchuk et al. [24]. According to the proposed model in the case of monodispersed nanofluid, the structural disjoining pressure (Pst) is proportional to micellar osmotic pressure (Posm). The osmotic pressure of 0.06 M SDS was estimated by light scattering measurement (12 kPa) and Carnahan–Starling equation (8 kPa) [24]. The structural pressure (Pst) of the 0.06 M SDS micellar solution was estimated of 8 kP when the capillary pressure of foam lamellae meniscuses at the top of foam height was estimated of 3 kP (see the data for structural, capillary and osmotic pressures presented in Fig. 11). Based on DLVO model, the foam lamellae are not able to rupture with the criteria Pc < P. Therefore, the role of lamella structural disjoining pressure has to be considered as a major foam stability controlling factor in order to understand the instability of the foam containing micelles. There is no direct correlation between the single film stability and the bulk foam stability. However, we have observed that the foam lamella containing micelles inside of the 3-D foam becomes thinner in the same stepwise manner and contains the same number of transitions (stratification) as the thinning of the single foam film (see Fig. 12). It represents that the same intermicellar interaction operates in the lamella inside of the bulk foam and the single foam film. A simple way to probe the role of intermicellar interactions and the role of the structural disjoining pressure’s contribution to the stability of the foam lamella is to study the thinning
Fig. 11. Structural disjoining pressure (Pst) isotherm, the osmotic pressure (Posm), and the capillary pressure (Pc) in the case of 0.06 M SDS micellar solution (h: film thickness, d: effective diameter of micelle).
of a single microscopic foam film. The single microscopic foam film thinning was studied to elucidate the role of solubilized oil on the intermicellar interaction contribution to the structural disjoining pressure and the film interaction energy. To get comprehensive understanding of foam stability, the effect of lamella size on the foam stability should be considered. We observed that during the foam drainage, the foam lamellae at the top on the bulk foam did not reduce its thickness significantly, but slowly increased in size before rupture. This indicates that a critical lamella size exists, and the critical size of lamella inside of the foam represents the single film. We expect that the study of the role of film size on the film and/or foam stability will provide the understanding of the correlation between the foam stability and the single film stability. This study is underway and results will be presented in a forthcoming paper.
4. Concluding remarks The effect of oil solubilized in the surfactant micelles of SDS on foam stability was probed in this study. The stability of an aqueous foam decreased significantly when the SDS micellar system contained solubilized n-dodecane. In order to understand the role of the interaction between swollen micelles on the contribution of structural disjoining pressure of foam lamella stability, we investigated the single foam film thickness stability and the film-meniscus contact angle using microinterferometry. We found that in the presence of n-dodecane swollen micelles, there are decreases
Fig. 12. Foam film thinning of lamella inside of the foam cell.
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in the number of film thickness transitions, the film drainage time, and the intermicellar interaction forces. Also, there is an increase in the polydispersity in the micellar size. All these figures lead to a lower film thickness stability in the foam film containing swollen micelles. The microscopic contact angle in the film-meniscus region increases with the decreasing film thickness due to the larger interaction between the two film surfaces. The contact angle measurements were used to estimate the structural energy barriers for the film with and without the swollen micelles. It was found that the structural energy barrier was smaller for the film containing the swollen micelles, indicating that the film is less stable. This was also confirmed by the negative second virial coefficient for this system signifying attractive micellar interactions. The role of polydispersity in micelle size due to the solubilized oil as well as the effects of the depletion interaction forces and the film size on the film stability, and consequently on the foam stability, warrant further study. Also, different oils and surfactant systems need to be investigated in order to develop a more comprehensive understanding of the role of solubilized oil in surfactant micelles on foam stability. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.014. References [1] R.V. Craster, O.K. Matar, Rev Mod Phys 81 (2009) 1131. [2] D.T. Wasan, A.D. Nikolov, Curr. Opin. Colloid Interface Sci. 13 (3) (2008) 128–133.
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[3] A.D. Nikolov, D.T. Wasan, D.W. Huang, et al., The effect of oil on foam stability: mechanisms and implications for oil, displacement by foam in porous media, in: 61st SPE Annual Technical Conference and Exhibition, vol. Preprint SPE 15443, 1986. [4] D.T. Wasan, A.D. Nikolov, D.D. Huang, et al., ACS Symp. Ser. 373 (1988) 136– 162. [5] L.A. Lobo, A.D. Nikolov, D.T. Wasan, J. Dispersion Sci. Technol. 10 (2) (1989) 143–161. [6] J. Lee, A.D. Nikolov, D.T. Wasan, Ind. Eng. Chem. Res. 52 (1) (2012) 66–72. [7] A.D. Nikolov, A.S. Dimitrov, P.A. Kralchevsky, Optica Acta 33 (11) (1986) 1359– 1368. [8] A.S. Dimitrov, P.A. Kralchevsky, A.D. Nikolov, et al., Colloids Surf. 47 (July) (1990) 299–321. [9] A.D. Nikolov, K. Kondiparty, D.T. Wasan, Langmuir 26 (11) (2010) 7665–7670. [10] A.D. Nikolov, D.T. Wasan, J. Colloid Interface Sci. 133 (1) (1989) 1–12. [11] A.D. Nikolov, P.A. Kralchevsky, I.B. Ivanov, et al., J. Colloid Interface Sci. 133 (1) (1989) 13–22. [12] A.D. Nikolov, P.A. Kralchevsky, I.B. Ivanov, Foam film stability: role of micellar interaction on the formation and expansion of spots in stratifying film. An overview, in: M.G. Velarde, C.I. Christov (Eds.), Fluid Physics, Lecture Notes of Summer Schools, Would Scientific Series on Nonlinear Science, Series B, vol. 5, 1994, pp. 209–228. [13] D.T. Wasan, K. Koczo, A.D. Nikolov, Foams: Fundam. Appl. Pet. Ind. 242 (1994) 47–114. [14] A. Sheludko, Adv. Colloid Interface Sci. 1 (4) (1967) 391–464. [15] A.D. Nikolov, D.T. Wasan, Langmuir 8 (12) (1992) 2985–2994. [16] L. Lobo, D.T. Wasan, Langmuir 9 (7) (1993) 1668–1677. [17] T. Kolarov, A. Scheludko, D. Exerowa, Trans. Faraday Soc. 64 (1968) 2864– 2873. [18] J.A. De Feijter, J.B. Rijnbout, A.J. Vrij, J. Colloid Interface Sci. 64 (1978) 258–268. [19] L.A. Lobo, A.D. Nikolov, A.S. Dimitrov, et al., Langmuir 6 (5) (1990) 995–1001. [20] R. Krustev, H. Müller, J. Toca-Herrera, Colloid Polym. Sci. 276 (6) (1998) 518– 523. [21] E.S. Basheva, P.A. Kralchevsky, K.D. Danov, et al., Physical Chemistry Chemical Physics 9 (38) (2007) 5183–5198. [22] A. Scheludko, Proc. Konikl. Ned. Akad. Wet. B 65 (1962) 86–99. [23] A. Vrij, Discuss. Faraday Soc. 42 (1966) 23–33. [24] A. Trokhymchuk, D. Henderson, A.D. Nikolov, et al., Langmuir 17 (16) (2001) 4940–4947.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Surfactant micelles containing solubilized oil decrease foam film thickness stability Jongju Lee, Alex Nikolov, Darsh Wasan ⇑ Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA
a r t i c l e
i n f o
Article history: Received 25 March 2013 Accepted 9 October 2013 Available online 22 October 2013 Keywords: Foam stability Single foam film stability Swollen micelles Aqueous foams
a b s t r a c t Many practical applications involving three-phase foams (aqueous foams containing oil) commonly employ surfactants at several times their critical micelle concentration (CMC); in these applications, the oil can exist in two forms: (1) oil drops or macroemulsions and (2) oil solubilized within the micelles. We have recently observed that in the case of aqueous foams stabilized with sodium dodecyl sulfate (SDS) and n-dodecane as an oil, the oil drops did not alter the foam stability but the solubilized oil (swollen micelles) greatly influenced the foam’s stability. In order to explain the effect of oil solubilized in the surfactant micelles on foam stability, we studied the stability of a single foam film containing swollen micelles of SDS using reflected light microinterferometry. The film thinning occurs in stepwise manner (stratification). In addition, we obtained data for the film-meniscus contact angle versus film thickness (corresponding to the different number of micellar layers) and used it to calculate the film structural energy isotherm. The results of this study showed that the structural energy stabilization barrier decreased in the presence of swollen micelles in the film, thereby decreasing the foam stability. These results provide a better understanding of the role of oil solubilized by the micelles in affecting foam stability. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Liquid films are the basic structural elements of any foaming system containing surfactant micelles. Consequently, the stability of the foam is highly dependent on stability of the intervening film between the bubbles. There have been numerous studies [1,2] on the stability of a single liquid film separating the gas bubbles in a two-phase (gas and liquid) foam system. However, studies examining the stability of the intervening micellar film in a three-phase aqueous foam containing oil are rare. This is especially true when solubilized oil is present within the micelles (swollen micelles). To the best of our knowledge, the only reported study is that of Nikolov et al. [3], Wasan et al. [4], and Lobo et al. [5] who had examined the stability of a foam film and/or foam containing swollen micelles of a nonionic surfactant. More recently, Lee et al. [6] reported the results of a study on the importance of dispersed versus solubilized oil on the foam stability in a three-phase aqueous foaming system containing n-dodecane and micelles of an anionic surfactant of sodium dodecyl sulfate (SDS). Their results revealed that the oil drop did not alter the foam stability but the oil solubilized in the surfactant micelles greatly influenced the foam’s stability.
⇑ Corresponding author. Fax: +1 6309854451. E-mail address: [email protected] (D. Wasan). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.014
In order to elucidate the effect of oil solubilized in the surfactant micelles on the foam stability, we studied the stability of a single foam film containing swollen micelles of SDS using the reflected light microinterferometric technique. We observed the microstructuring phenomenon in a thinning foam film. In addition, we measured the film-meniscus contact angle and calculated the film interaction energy as a function of the film thickness. We then estimated the structural energy stabilization/destabilization barrier from the calculated interaction energy isotherm. We also conducted a three-phase foam stability experiment using the Bartsch method, and rationalized the results of these tests in terms of the changes in the structural stabilization barrier and the effective micellar volume for foaming systems with and without the swollen micelles.
2. Experimental 2.1. Materials and preparation of solutions We used n-dodecane (the density at 20 °C is 0.75 g/cm3) from Aldrich Chemical, and pre-equilibrated the surfactant solution to produce swollen micelles. An anionic surfactant, sodium dodecyl sulfate (SDS), was supplied by Fisher; the surfactant molecule has an average molecular weight of 288.38. We used 0.03 M (mol/L) and 0.06 M surfactant
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solutions. These concentrations are higher than the critical micelle concentration (CMC = 8.2 103 M). The surfactant solutions were prepared by dissolving SDS in deionized water using a ‘‘Milli-Q’’ water system deionizer from Millipore Corporation. The surfactant system was contacted with the 1000 ppm n-dodecane and preequilibrated for more than 10 days. During pre-equilibration, the oil phase was dispersed into the aqueous surfactant solution as drops, and left overnight for phase separation in a glass container. The pre-equilibrated surfactant solutions were centrifuged for 2 h, and subsequently filtrated through a 0.1 lm filter. In order to minimize the surfactant adsorption on the filter, the filter was saturated with 0.06 M SDS surfactant solution before filtration. We used suction repeatedly to remove the pre-equilibrated oil from the top of the container.
2.2. Surface and interfacial tension measurements The surface tension was measured by a Kruss tensiometer using a platinum Wilhelmy plate for the oil-free surfactant solution as well as the pre-equilibrated surfactant solution.
2.3. Refractive index measurement The refractive index of the micellar solutions was measured using a Fisher refractometer at 25 °C ± 1 °C.
2.4. Foam stability measurements The foams were produced from micellar surfactant solutions of sodium dodecyl sulfate (SDS). The stability of the foam was quantified by the Bartsch method [6]. The three-phase foams were generated in a graduated glass cylinder (250 mL) by shaking 50 mL of the surfactant solution 20 times. The foam height was monitored as a function of time. We compared the stability of the foams formed by shaking the aqueous solutions containing the SDS swollen micelles (the micelles with solubilized oil) with the stability of the foams without swollen micelles.
2.5. Film thinning We used the reflected light microinterferometric method to observe the film stratification phenomenon (stepwise thinning) [7– 10]. Microscopic films were formed in a cylindrical capillary with a hydrophilic inner wall of radius R (1.35 mm). The films were formed by sucking out the liquid from a biconcave drop inside the capillary through an orifice in the wall. The resulting horizontal flat film was encircled by a biconcave meniscus. The glass cell assembly was enclosed in an environment with a thermostat and the temperature was maintained within ±0.2 °C in the range of 20–50 °C. The entire assembly was placed on the stage of the Max Zhender differential interference microscope, which was mounted on a vibration-free table to keep any external disturbances from affecting the film thinning process. In the reflected light mode of the microscope, monochromatic light (with a wavelength of 546 nm) through the top of the glass cell was incident on the film surface. The thickness of the foam film changes produced the interference patterns. The film lamella thickness as a function of time was evaluated from the color of the interference patterns. The video camera, in conjunction with the monitor and digital video recorder, recorded the process of film thinning. The effective micellar diameter and volume fraction were calculated from the stepwise film thinning phenomena.
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3. Results and discussion 3.1. Foam stability in a three-phase foam system The main criterion for the foam stability was the decrease in the foam height versus time. Fig. 1 shows the effect of swollen micelles on the foam height versus time. Data are shown for the two different micellar concentrations of 0.03 M and 0.06 M. In the case of the SDS solution without swollen micelles, the foam formed from a higher micellar concentration solution was more stable than that formed from a lower micellar concentration. The foam height started to decrease in a stepwise manner after 4 h and 7.5 h, respectively. The decrease in the foam height as a function of time was more pronounced in the samples containing swollen micelles than those without them. About 80% of the foam in the presence of the swollen micelles containing n-dodecane collapsed in about 2 h. It has been demonstrated [10–13] that the foam films or lamellae formed from higher SDS micellar concentrations exhibit a more ordered micelle microstructure during thinning (stratification) due to the repulsive intermicellar interactions and the micelles with a narrow size distribution (these are forced into the restricted volume of the film). The drainage time of the liquid film increases with the increase in the micellar volume phase. Consequently, foams with a slower drainage time exhibit increased foam stability with an increase in the micellar concentration. In the case of n-dodecane pre-equilibrated micellar system containing swollen micelles, the solubilized oil (swollen micelle) reduces the foam stability. As n-dodecane solubilization limit is approached, the stability of foam decreases gradually versus amount of solubilized oil. In order to verify the role of solubilized oil on the foam stability, we study the solubilization limit, the solubilization time and concentration of n-dodecane into SDS micelles. 3.2. Solubilization limit versus concentration of n-dodecane and pre-equilibration time We investigated the solubilization limit of n-dodecane into 0.06 M SDS surfactant solution in two aspects: (1) time to reach the solubilization limit at high and low concentrations of n-dodecane and (2) the solubilization limit of n-dodecane concentration into 0.06 M SDS surfactant solution. The time for reaching solubilization limit versus the concentraion of n-dodecane (200 ppm and 1000 ppm) was stuied first. The 0.06 M SDS surfactant solutions were contacted with n-dodecane
Fig. 1. Normalized foam height versus time.
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and pre-equilibrated for 1–360 h respectively. The stability of the foam was quantified by the decrease the foam height as a function of time (Bartsch method). We observed that the foam stability decreases as increasing the pre-equilibration time. The data presented in Fig. 2 depict the normalized foam stability versus time and versus the pre-equilibration time of 1000 ppm n-dodecane with 0.06 M SDS solution. The dashed line marks the solubilization time limit. The foam formed by 0.06 M SDS solution pre-equilibration for 48 h with 1000 ppm of n-dodecane collapsed 80% within 2 h (see Fig. 2). As expected in the case of low concentration of n-dodecane (200 ppm), it took longer time to reach the solubilization limit of n-dodecane into 0.06 M SDS surfactant solution. From Fig. 3, about 80% of foam collapsed within 2 h when 0.06 M SDS surfactant solution was pre-equilibrated with 200 ppm of n-dodecane for 144 h. It represents that the system reaches the solubilization limit. The solubilization limit of n-dodecane concentration into 0.06 M SDS surfactant solution was also investigated (see Fig. 4). The 0.06 M SDS surfactant solutions were pre-equilibrated with different concentrations of n-dodecane (from 100 to 1000 ppm) for 10 days – enough time to reach the solubilization limit from the results of the time for low (200 ppm) and high (1000 ppm) concentration of n-dodecane solubilization into 0.06 M SDS micelles. Then, the foam stability was measured by the decrease in the foam height with time using the Bartsch method. The results are presented in Fig. 4. About 80% of the foam collapsed after 2 h when the 0.06 M SDS pre-equilibrated with more than 200 ppm of ndodecane for 10 days. The data indicate that the concentration range between 200 and 300 ppm of n-dodecane is the solubilization limit of n-dodecane into 0.06 M SDS surfactant solution. When the system reaches the solubilization limit, about 80% of the foam collapse within 2 h. Based on the study of solubilization limit of n-dodecane into 0.06 M SDS surfactnat solution, 0.06 M SDS surfactnat solution pre-equilibrated with 1000 ppm n-dodecane for 10 days reaches the solubilization limit. And, results of these foam stability tests clearly indicate that the foam stability significantly decreased when the system contained swollen micelles at the solubilization limit. To gain understanding into the role of the interaction between swollen micelles on the contribution of structural disjoining pressure of foam lamella stability we investigated the stability of a
Fig. 3. 3-D bar plot depicts the normalized foam stability to collapse by 80% versus time and versus the pre-equilibration time of 200 ppm n-dodecane with 0.06 M SDS. The dashed line marks the solubilization time limit.
Fig. 4. 3-D bar plot depicts the normalized foam stability to collapse by 80% versus time and versus the pre-equilibration time of n-dodecane with 0.06 M SDS during 10 days. The dashed line marks the solubilization time limit.
single foam film by examining the drainage characteristics of the film with and without swollen micelles.
3.3. Film stratification phenomena
Fig. 2. 3-D bar plot depicts the normalized foam stability to collapse by 80% versus time and versus the pre-equilibration time of 1000 ppm n-dodecane with 0.06 M SDS. The dashed line marks the solubilization time limit.
Using the reflected light microinterferometric method [7–10,14], we observed the drainage characteristics of the microscopic horizontal foam films formed from SDS 0.06 M (7.3 times higher than CMC) solutions (with and without swollen micelles). The thinning evolution of one of the films is depicted in the movie clip available in the ‘‘Supplementary Information’’ section accompanying this paper.
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After the film is formed, it immediately starts to decrease its thickness. At first, the film rests for a few seconds in a metastable state of uniform thickness (a homogeneous color) at h = 102 nm. Then the darker spots appear locally inside of the uniform color, indicating that this area is thinner than the remaining part of the film. The area of the darker spot increases gradually until it covers the whole film area. Fig. 5 shows the stepwise thinning process for the 0.06 M SDS micellar solution. The first thickness transition occurs at above 17 s and the film reduces its thickness from 70 nm to 59 nm. This is followed by the second, third, fourth, and fifth transitions. During the final stage of the film thinning process, the film remains stable (at h = 26 nm) with one layer of micelles. Fig. 6 shows the photomicrographs of the stepwise thinning phenomena for the film formed for the 0.06 M SDS solution pre-equilibrated with n-dodecane. This system displayed only four thickness transitions and its final film contained no micelles. It is worth noting that the film containing micelles (without the solubilized oil) exhibited rather regular stepwise thickness transitions, whereas the transitions were less regular and occurred with greater ease for the film containing the swollen micelles, as seen in Figs. 5 and 6, and the video clip (in the Supplementary Information). The values for the film thicknesses and their lifetimes are tabulated in Table 1. Fig. 7 depicts the evolution, manner and features of film thinning of foam film from 0.06 M SDS solution with and without swollen micelles regarding: the number of film thickness transitions, the amplitude of transitions, the time it takes for the thickness transitions to occur, the final (equilibrium) film thickness, and the manner of transition (e.g., how many dark spot/s formed during the transitions). It is important to note that the effect of the micelles (or the role of the structural disjoining pressure) on the manner of film thinning becomes apparent in film thicknesses less than three micellar layers, where the film surfaces confinement effect on the structural disjoining pressure has to be considered. The foam film in the presence of the swollen micelles exhibits four thickness transitions before reaching its equilibrium thickness without micelles; the film with the SDS micelles exhibits five thickness transitions and its equilibrium thickness remains at one micellar layer (see Table 1). The amplitude of thickness transitions for film with the swollen micelles was 15 ± 2 nm while the amplitude of the foam film with the SDS micelles was 11 ± 2 nm, and it was more regular than that of the film containing swollen micelles. As a result of the swollen micelles’ polydispersity, the thickness transitions of the film with the swollen micelles were less regular than that of the film with the SDS micelles. The polydispersity affects the time it takes for the thickness transitions to occur and in what manner they occur. The foam film in the
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presence of swollen micelles reduces its film layer thickness (stratify) quickly by simultaneous appearance of several dark spots in film area, corresponding to a film without micelles. The foam film with the SDS micelles stratifies slowly with the appearance of one spot. The film changed in thickness corresponding to the effective micellar diameter. The number of thickness transitions indicates the micellar effective volume fraction, and the amplitude of stepwise thinning indicates the effective micellar diameter. The effective micellar volume concentration was estimated to be 30 vol.% based on the number of film thickness transitions [15] and the effective micellar diameter is about 11 nm (corresponding to the micellar stepwise thickness) for the solution containing the 0.06 M SDS micelles. However, there are only four thickness transitions for the 0.06 M SDS solution containing the swollen micelles which corresponded to an 18 vol.% concentration with an effective micellar diameter of about 15 nm. The micelles swell with the solubilization of the oil and become polydisperse in size. The effect of oil solubilization upon film thinning is relevant because of the relationship between solubilization and micellar microstructuring in the film. The decrease in the micellar volume phase from 30 vol.% to 18 vol.% results in a decrease in the intermicellar interaction forces. Recently, Lee et al. [6] determined the second virial coefficient (which is indicative of intermicellar interactions) for the 0.06 M SDS solution containing swollen micelles and found it to be negative, signifying attractive interactions between the micelles. But the second virial coefficient was found to be positive for the SDS micelles without the solubilized oil, signifying repulsive interactions. The uniformity in the micelle size and the repulsive intermicellar interactions are known to result in better ordered microstructuring (layered structure) inside the confined surfaces of the film. In contrast, polydispersity in size and less repulsive (or attractive) interactions yield films with less ordered microstructuring, as seen in Fig. 6. 3.4. Film-meniscus profile and contact angle Using the reflected light interferometric technique, we monitored the film-meniscus profile and the contact angle between the film and the meniscus [8,9,16–20]. Fig. 8A shows a photograph of a foam film surrounded by the meniscus, indicated by the meniscus, indicated by the consecutive dark and bright Newton interference rings around the periphery of the film. Fig. 8B depicts the schematic of the film-meniscus region and Fig. 8C shows the interferogram depicting the successive maxima and minima in the intensity of the reflective light, representing the change in
Fig. 5. Evolution of film thinning for the 0.06 M SDS solution (the film is undergoing stratification thickness transitions).
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Fig. 6. Evolution of film thinning for 0.06 M SDS solution pre-equilibrated by n-dodecane (the film is undergoing stratification thickness transitions).
Table 1 Estimation of the film thickness (h) at the metastable state based on the mean height of a stepwise film thickness transition (~d), lifetime (s) corresponding to the number of layers (n) and film-meniscus contact angle (heq). n
0.06 M SDS ~ ¼ 11 nmÞ ðd h (nm) b
5 4a 3a 2b 1b
70 ± 2 59 ± 2 48 ± 2 37 ± 2 26 ± 3
Film contact angle heq (deg)
n
0.06 M SDS pre-equilibrated by n-dodecane ~ ¼ 15 nmÞ ðd
s (s) 11 – – 35 51
– – – 0.7 ± 0.1 1.1 ± 0.1
b
4 3a 2a 1b Film without micellesb
h (nm)
s (s)
81 ± 2 66 ± 2 51 ± 2 36 ± 2 21 ± 3 (final film thickness)
11 – – 35 42
Film contact angle heq (deg)
– – – 0.8 ± 0.1 1.3 ± 0.1
a The transitions happen simultaneously for a short period of time. The film thickness was estimated by the mean height of a stepwise film thickness transition (~ d) corresponding to the number of layers. b All the measured values have an accuracy of ±10%.
noting that thinner films have larger contact angles. The contact angle increases when the film becomes thinner due to the larger interaction between the film surfaces. The data for the film-meniscus contact angle versus film thickness were used to calculate the film structural energy isotherm as shown in the following section. 3.5. Structural interaction energy between film surfaces
Fig. 7. Time for accruing the stepwise thickness transitions of systems with and without the swollen micelles.
the thickness in the meniscus region. The film-meniscus profile shown in the figure was obtained using the procedure described in Ref. [9]. The contact angle, heq, is the angle subtended between the film and the meniscus, which is determined from the slope of the point of intersection of the meniscus profile with the film contacting one layer of SDS micelles. Table 1 lists the values of the film-meniscus microscopic contact angle for the 0.06 M SDS and the 0.06 M SDS pre-equilibrated with the n-dodecane solution versus the film thickness corresponding to the number of micellar layers. The accuracy of the contact angle determination is ±0.1°. It is worth
We used the procedure described in Ref. [9] to calculate the film structural energy isotherm. As discussed in Ref. [9], the structural energy is oscillatory due to the micellar layering within the film (as seen in Figs. 5 and 6), and it dominates the van der Waals contribution to the film energy given by the conventional DLVO theory. The amplitude of oscillation (A), and the experimentally measured parameters—namely, the contact angle (heq), and film thickness (h)—can be linked by the relation,
A cos
2p h h exp ¼ ra=l ðcos heq 1Þ d d
ð1Þ
where ‘d’ is the effective diameter of the micelle given by the average distance between the film thickness transitions, ‘ra/l’ is the surface tension, and ‘h’ is the film thickness. It should be noted that this equation justifiably neglects the contribution of the surface interaction force, which is given by the sum of the capillary and hydrostatic pressures multiplied by the equilibrium film thickness as the micellar structuring overwhelms the effect of the surface interaction force [18]. As noted in Ref. [9], parameter ‘A’ is determined by choosing the film thickness corresponding to one micellar layer.
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Fig. 8. (A) Photomicrograph depicting the film and the surrounding meniscus. (B) Sketch of the film-meniscus region. (C) Inset of light interferograph and the film-meniscus profile.
Fig. 9 displays the oscillatory structural interaction energy (W) [9,21] for the two systems; this figure shows a plot of the nondimensionalized structural energy (Wd2/kT) versus the film thickness (h). The structural energy barriers for the two systems are also shown in this figure. From the first maximum to the second minimum, the structural energy barrier for the pre-equilibrated system is lower (0.53 kT) than that (1.53 kT) for the system without pre-equilibration. Therefore, the film with a larger structural barrier will take longer to reach its final thickness and this is what was observed experimentally (see Figs. 5 and 6, and Table 1). Fig. 9 shows that the structural stabilization barrier increases as the film becomes thinner. If the film is thick (more than 2 micellar layers), the stepwise thinning transitions occur within a shorter period of time because the contribution of the film interaction energy is minor. In order to understand the role of swollen micelle on the foam stability, we investigated the single foam film by using the refracted light microinterferometric technique. And, we analyzed the experimental observations to estimate the structural stabilization barrier on the film to reduce the thickness to explain decreas-
Fig. 9. Estimated oscillatory structural energy isotherm as a function of the film thickness.
ing the film stability due to swollen micelles. Based on the results, we explain the stability of single foam film in the presence of micelles and swollen micelles characterized by the structural stabilization barrier. 3.6. The role of micellar interactions in foam stability The classical model, DLVO (Derjaguin, Landau, Verwey, and Overbeek), based on the disjoining pressure isotherm provides a phenomenological understanding for the thickness stability of foam film. The classical understanding for film stability is sketched in Fig. 10. It assumes that the disjoining pressure isotherm has ‘S’ shape and predicts the film may have two equilibrium thickness states: a thicker film (h1) in equilibrium within the meniscus with a small contact angle and a thinner film (h2) with a meniscus in equilibrium with a large contact angle. The DLVO model postulates that a film could change its thickness in a single stepwise manner if the capillary pressure (Pc) exceeds the disjoining pressure P(h) barrier. Based on the DLVO model, Scheludko [22] proposed for the film instability the criteria Pc > P and dP/dh > 0. A film at its critical thickness (hcr) becomes kinetically unstable with respect to small,
Fig. 10. Disjoining pressure isotherm and the critical film thickness (hcr) based on DLVO model.
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spontaneous film surfaces perturbations or fluctuations k(hcr) so that, in spite of increasing the film’s free energy due to van der Waals attraction forces, the film’s total energy decreases. In addition, Vrij [23] suggested a model for the film’s instability based on Scheludko’s criteria of film instability. After thinning film reaches its critical thickness (hcr), the local wave fluctuation growth spontaneously promoted by the van der Waals attractive forces and overcame the local capillary repulsive force, and triggered the local film thickness to decrease or the film to rupture. Therefore, the film ruptures when the film becomes thin that the rate of the local fluctuations at the critical thickness (hcr) is faster than the film thinning process. The DLVO model provides phenomenological explanation for the reason film becomes unstable and but does not provide understanding of the mechanism of how a film thickness changes or how a film ruptures. The classical model does not account for the role of micelles in film stability. In the case of the foam film containing micelles, the effect of structural disjoining pressure on the film stability is important. In order to elucidate the role of the structural disjoining pressure on instability of the film, we analyze the relationship between the structural disjoining pressure (Pst) and the capillary pressure (Pc) in the case of 0.06 M SDS foam. We estimated the structural disjoining pressure (Pst) of 0.06 M SDS micellar solution based on the simple calculation proposed by Trokhymchuk et al. [24]. According to the proposed model in the case of monodispersed nanofluid, the structural disjoining pressure (Pst) is proportional to micellar osmotic pressure (Posm). The osmotic pressure of 0.06 M SDS was estimated by light scattering measurement (12 kPa) and Carnahan–Starling equation (8 kPa) [24]. The structural pressure (Pst) of the 0.06 M SDS micellar solution was estimated of 8 kP when the capillary pressure of foam lamellae meniscuses at the top of foam height was estimated of 3 kP (see the data for structural, capillary and osmotic pressures presented in Fig. 11). Based on DLVO model, the foam lamellae are not able to rupture with the criteria Pc < P. Therefore, the role of lamella structural disjoining pressure has to be considered as a major foam stability controlling factor in order to understand the instability of the foam containing micelles. There is no direct correlation between the single film stability and the bulk foam stability. However, we have observed that the foam lamella containing micelles inside of the 3-D foam becomes thinner in the same stepwise manner and contains the same number of transitions (stratification) as the thinning of the single foam film (see Fig. 12). It represents that the same intermicellar interaction operates in the lamella inside of the bulk foam and the single foam film. A simple way to probe the role of intermicellar interactions and the role of the structural disjoining pressure’s contribution to the stability of the foam lamella is to study the thinning
Fig. 11. Structural disjoining pressure (Pst) isotherm, the osmotic pressure (Posm), and the capillary pressure (Pc) in the case of 0.06 M SDS micellar solution (h: film thickness, d: effective diameter of micelle).
of a single microscopic foam film. The single microscopic foam film thinning was studied to elucidate the role of solubilized oil on the intermicellar interaction contribution to the structural disjoining pressure and the film interaction energy. To get comprehensive understanding of foam stability, the effect of lamella size on the foam stability should be considered. We observed that during the foam drainage, the foam lamellae at the top on the bulk foam did not reduce its thickness significantly, but slowly increased in size before rupture. This indicates that a critical lamella size exists, and the critical size of lamella inside of the foam represents the single film. We expect that the study of the role of film size on the film and/or foam stability will provide the understanding of the correlation between the foam stability and the single film stability. This study is underway and results will be presented in a forthcoming paper.
4. Concluding remarks The effect of oil solubilized in the surfactant micelles of SDS on foam stability was probed in this study. The stability of an aqueous foam decreased significantly when the SDS micellar system contained solubilized n-dodecane. In order to understand the role of the interaction between swollen micelles on the contribution of structural disjoining pressure of foam lamella stability, we investigated the single foam film thickness stability and the film-meniscus contact angle using microinterferometry. We found that in the presence of n-dodecane swollen micelles, there are decreases
Fig. 12. Foam film thinning of lamella inside of the foam cell.
J. Lee et al. / Journal of Colloid and Interface Science 415 (2014) 18–25
in the number of film thickness transitions, the film drainage time, and the intermicellar interaction forces. Also, there is an increase in the polydispersity in the micellar size. All these figures lead to a lower film thickness stability in the foam film containing swollen micelles. The microscopic contact angle in the film-meniscus region increases with the decreasing film thickness due to the larger interaction between the two film surfaces. The contact angle measurements were used to estimate the structural energy barriers for the film with and without the swollen micelles. It was found that the structural energy barrier was smaller for the film containing the swollen micelles, indicating that the film is less stable. This was also confirmed by the negative second virial coefficient for this system signifying attractive micellar interactions. The role of polydispersity in micelle size due to the solubilized oil as well as the effects of the depletion interaction forces and the film size on the film stability, and consequently on the foam stability, warrant further study. Also, different oils and surfactant systems need to be investigated in order to develop a more comprehensive understanding of the role of solubilized oil in surfactant micelles on foam stability. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.10.014. References [1] R.V. Craster, O.K. Matar, Rev Mod Phys 81 (2009) 1131. [2] D.T. Wasan, A.D. Nikolov, Curr. Opin. Colloid Interface Sci. 13 (3) (2008) 128–133.
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