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Synergistic behaviour of ZnO nanoparticles and gemini surfactants on the dynamic and equilibrium oil/water interfacial tension† Tahereh Fereidooni Moghadam,a Saeid Azizian*a and Shawn Wettigb In this work the effect of ZnO nanoparticles on the interfacial behaviour of gemini surfactants (12-3-12 and 14-3-14) at the oil/water interface was investigated. Equilibrium and dynamic interfacial tension in the absence and presence of ZnO was measured and compared. The results show that the synergistic
Received 27th January 2015, Accepted 6th February 2015 DOI: 10.1039/c5cp00510h
interactions between the surfactants and nanoparticles decrease the interfacial tension beyond that observed for each component, alone. Modelling of dynamic data with two different models indicates that the mechanism of surfactant migration (with and without ZnO) is mixed diffusion-kinetic-control. The Gibbs free energy of micellization and the Gibbs free energy of adsorption in the absence and presence of ZnO were calculated and compared. Finally the effect of addition of ZnO nanoparticles on emulsion
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stability was also examined.
1. Introduction Gemini surfactants are made up of two amphiphilic surfactant moieties linked at or near the head group by a spacer group. In comparison with the corresponding monomeric surfactants, gemini surfactants have lower critical micelle concentration (cmc) values, and are more efficient in reducing the surface or interfacial tension.1–6 Thus, gemini surfactants are used in a number of industrial applications such as foaming, cosmetics, detergents, coatings, biological and biomedical fields.7 The study of the combination of surfactants and nanoparticles and their interaction at the air–liquid and liquid– liquid interfaces is a topic of increasing interest, due to their application especially to improve the stability of emulsions and foams.8–10 The study of interfacial tension as a macroscopic parameter for understanding the interactions of nanoparticles and surfactants at the interface is very important. Despite this importance, the interfacial tension of a mixture of nanoparticles and surfactants has not been extensively studied.11 Ravera et al.12 showed that silica nanoparticles increase the surface and interfacial tension of a solution of a
Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, 65147, Hamedan, Iran. E-mail: [email protected], [email protected]; Fax: +98-8138380709; Tel: +98-8138282807 b School of Pharmacy, University of Waterloo, Waterloo, ON N2L 3G1, Canada † Electronic supplementary information (ESI) available: The plots of equilibrium and dynamic interfacial tension for the 14-3-14 surfactant, dynamic surface concentrations, the SRT kinetic model, apparent and effective diffusion coefficients for 12-3-12 and 14-3-14 surfactants, the photograph of the vessel and the optical microscopy image of 14-3-14. See DOI: 10.1039/c5cp00510h
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cetyltrimethylammonium bromide (CTAB). The effect of negatively charged silica nanoparticles on the interfacial tension of CTAB at oil–water interfaces was studied by Lan et al.13 According to their study the interfacial tension was reduced due to a synergistic effect between nanoparticles and CTAB. Ma et al.11 investigated the effect of silica nanoparticles on surface and interfacial tension in combination with anionic and nonionic surfactants. Their results demonstrated that the addition of silica nanoparticles increased the efficiency of SDS in reducing interfacial tension. Also, the effect of the surfactant type on the interfacial tension in the presence of hydrophilic silica nanoparticles at the oil/water interface was reported by Pichot et al.14 According to their results, silica nanoparticles, in the presence of w/o surfactants, have no effect on interfacial tension, while they affect interfacial tension in the presence of o/w surfactants. Recently, the influence of ZnO nanoparticles on the oil/water interfacial tension of single chain surfactants CTAB (cationic),15 SDS (anionic)16 and F-127 (non-ionic)17 solution has been studied by our group. The results showed that ZnO nanoparticles increase the surfactant efficiency in decreasing the interfacial tension due to the synergistic effect between surfactants and ZnO nanoparticles, similar to that described in the previous paragraph for silica nanoparticles. The potential applications of gemini surfactants are in foaming, cosmetics, detergents, coatings, chemical industry, biomedical and biological fields.7 Also ZnO is one of the compounds used in cosmetic and skincare products extensively, and therefore, the study of its synergistic effect with gemini surfactants and also its effect on the emulsion stability is important for this industry. According to
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our knowledge, the influence of nanoparticles on the interfacial tension of gemini surfactant solution has not been examined. In this work the effect of ZnO nanoparticles on the oil/water interfacial tension of gemini surfactants N,N0 -bis(dimethylalkyl)-a,o-alkanediammonium dibromide (m-s-m type, m = the length of the n-alkyl tail and s = the length of the polymethylene spacer), 12-3-12 and 14-3-14 has been studied using both equilibrium and dynamic surface tension methods. The effect of ZnO nanoparticles on the ability of these surfactants to stabilize emulsions was also investigated.
2. Experimental Materials and methods The N,N0 -bis(dimethyldodecyl)-1,3-propanediammonium dibromide (12-3-12) and N,N0 -bis(dimethyltetradecyl)-1,3-propanediammonium dibromide (14-3-14) gemini surfactants used in this study were synthesized according to procedures previously reported.1 The oil used in this study was n-decane (Merck Co., 499%) that was purified via column chromatography over basic alumina for removal of polar impurities before use.18 The ZnO nanoparticles were prepared according to a procedure previously reported.19 The synthesized ZnO nanoparticles were characterized using scanning electron microscopy (SEM) (MIRA3TESCAN) and X-ray diffraction (XRD) (ADP2000ITALSTRUCTURE Italia) methods. Aqueous ZnO nanoparticle dispersions were prepared using an ultrasonic (UNIVERSAL ULTRASONIC) bath. For the study of dispersion of ZnO in aqueous solution ZnO was dispersed in surfactant solution and turbidity of the solution was measured after magnetic stirring or sonication for 10, 20, 30, 40 and 60 minutes. The plot is shown in Fig. S1 (ESI†); this figure shows that the turbidity of magnetically stirred solution is low and this means that the particles are mainly in aggregate form; but the turbidity increases with increased sonication time and after 40 minutes reached a constant value. This high turbidity by 40 min sonication in comparison to magnetic stirring shows that the particles are mainly dispersed and 40 min sonication provides the maximum possible dispersion of ZnO nanoparticles. This result was in agreement with previously reported results that show ultrasonication of nanoparticles as an effective tool for the elimination of agglomerates.20–22 A drop volume tensiometer (DVT30 Kruss, Germany) was used to measure the equilibrium and dynamic interfacial tension between the oil phase and the surfactant aqueous phase. Emulsions of n-decane and aqueous surfactant solution or dispersions of ZnO nanoparticles in aqueous surfactant solution were prepared at equal volumes using an ultrasonic probe (SONOPULS) for 60 s and their stability after 24 h were monitored. All experiments were carried out at 25 1C and at a pH of approximately 7 (except for the investigation of pH effects). Also measurements were executed at 0.01 wt% ZnO nanoparticles concentration. The n-decane/water interfacial tension value at 25 1C was obtained as 48 mN m1, in agreement with the previously reported value.23
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3. Results and discussion 3.1.
Characterization of the prepared ZnO
The SEM image of the synthesized ZnO nanoparticles is shown in Fig. 1a. From this figure, we can deduce that nanoparticles are of spherical shape with a diameter of approximately 40 nm. The XRD pattern is shown in Fig. 1b, and indicates that the synthesised nanoparticles are pure ZnO, as described in our previous study. The characteristic peaks of ZnO at 2y = 31.6, 34.2, 36.1, 48.1, 56.4, 63.0, 67.9 and 68.91 correspond to the 100, 002, 101, 102, 110, 103, 112 and 201 planes respectively.15,19 For further characterization of prepared ZnO nanoparticles, the contact angle experiments were performed too. As shown in Fig. S2 (ESI†), the water droplet on the surface of thin film of the prepared ZnO is spread (wetting behaviour) and the contact angle is lower than 901, which indicates the hydrophilic nature of ZnO nanoparticles. The images of the water droplet covered by n-decane on a quartz substrate in the absence and presence of ZnO (Fig. S3, ESI†) show that ZnO has no effect on the contact angle of the oil/water system; i.e. these particles are hydrophilic and do not change the interfacial tension of this system. 3.2. Effect of ZnO nanoparticles in the presence of gemini surfactants In our previous study it has been shown that ZnO nanoparticles themselves have no effect on the interfacial tension of the oil/water system.15 The dynamic and equilibrium interfacial tension of the oil/water system was measured at different concentrations of 12-3-12 and 14-3-14 without nanoparticles or mixed with 0.01 wt% of ZnO nanoparticles.
Fig. 1 (a) The SEM image and (b) the XRD pattern of the prepared ZnO nanoparticles.
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3.2.1. Equilibrium studies. The resulting equilibrium interfacial tension changes at the n-decane/water interface in the absence and presence of ZnO nanoparticles (0.01 wt%), with increasing 12-3-12 concentration, are shown in Fig. 2. This figure illustrates that for both systems the interfacial tension decreases as the surfactant concentration increases, and then becomes approximately constant. Comparing the two plots shows that addition of ZnO nanoparticles to the surfactant system influences its efficiency in decreasing the interfacial tension; in the presence of ZnO nanoparticles, the interfacial tension reduction is greater than that of the sole surfactant system. It is suggested that a synergistic interaction between surfactants and nanoparticles is responsible for the observed increase in the efficiency of lowering the interfacial tension. Fig. 2 also shows that the presence of nanoparticles do not change the cmc of the surfactant, which is in agreement with the report of Lan et al. for the CTAB–silica nanoparticle system.13 Fig. S4 (ESI†) presents the same results for the 14-3-14 surfactant. Comparing Fig. 2 and Fig. S4 (ESI†) shows that 14-3-14, with longer alkyl tails, gives a lower interfacial tension for the water/ n-decane system; however, it can be seen that the effect of nanoparticles on interfacial tension is higher for the 12-3-12 surfactant. It is also noted that for a given concentration of bulk surfactant, the efficiency of gemini surfactants in reducing the interfacial tension is greater than for a corresponding single chain surfactant. For example at 0.0001 M surfactant the n-decane/water interfacial tension in the presence of the 12-3-12 surfactant is approximately 13 mN m1 while for CTAB it is approximately 18 mN m1.15 The Gibbs equation and the equilibrium interfacial tension data were used to obtain the equilibrium surface concentration: dge ¼ 3Ge RT d ln C0
(1)
where ge is the equilibrium interfacial tension, Ge is the equilibrium surface concentration, C0 is the bulk concentration, T is the absolute temperature and R is the gas constant. The surface concentration changes vs. 12-3-12 and 14-3-14 bulk concentration in the absence and presence of ZnO nanoparticles are
Table 1 Saturation surface concentration (Gsat) and the Langmuir constant (K) from the Langmuir isotherm for adsorption of 12-3-12 and 14-3-14 at the water–n-decane system at 25 1C
Gsat/mol m2
System (12-3-12 (12-3-12 (14-3-14 (14-3-14
solution) –n-decane solution + ZnO) –n-decane solution) –n-decane solution + ZnO) –n-decane
3.4 2.9 2.7 2.4
107 107 107 107
K/L mol1 5.8 6.2 6.1 6.8
105 106 105 105
R2 0.995 0.994 0.994 0.99
shown in Fig. S5 and S6 (ESI†), respectively. For both systems the Ge values increase with increasing surfactant concentration and reach saturation at the cmc. The Langmuir isotherm was used for modelling of equilibrium surface concentration data: Ge KC0 ¼ Gsat 1 þ KC0
(2)
where Gsat is the saturated surface concentration and K is the Langmuir constant. Table 1 shows the results of this fitting for the gemini surfactants. These results demonstrate that in the presence of ZnO the surface concentrations of the surfactants are lower than that for the particle free system. The larger Langmuir constant in the presence of nanoparticles gives rise to the observed greater decrease in the interfacial tension. For all systems the values of the Gibbs free energy of micellization DG0mic were calculated according to eqn (3):11,24 DG0mic = 2RT (1.5 a) ln(cmc/o)
(3)
where cmc is the critical micelle concentration of the surfactant (M), a is the degree of ionization of the surfactant and o is the number of moles of pure water in 1 liter. Here, an a value of 0.23 for 12-3-121 and 0.74 for 14-3-14,25 and the o value of 55.5 M (ref. 26) at 25 1C were used. From this, the Gibbs free energy of micellization per alkyl tail, DG0mic,tail = DG0mic/2,24 was also calculated. The Gibbs free energy of adsorption DG0ads was obtained using the DG0mic values and the following equation:27 DG0ads = DG0mic (pcmc/Gsat)
(4)
where pcmc is the surface pressure (p = g0 g) at cmc. The obtained values of DG0mic, DG0mic,tail and DG0ads are normalized using the thermal energy (RT) and are listed in Table 2. These results illustrate that for both surfactants the addition of ZnO nanoparticles has no effect on DG0mic and DG0mic,tail because ZnO nanoparticles do not change the cmc. The DG0ads values are more negative in the presence of ZnO nanoparticles. This is explained by the observed decrease in Gsat in the presence of ZnO Table 2 Calculated DG0mic, DG0mic,tail and DG0ads for the gemini surfactant–ZnO nanoparticle systems
DG0mic/RT DG0mic,tail/RT DG0ads/RT
System Fig. 2 The equilibrium interfacial tension of aqueous 12-3-12 solution/ n-decane vs. 12-3-12 concentration in the absence (J), and presence (’) of 0.01 wt% ZnO nanoparticles at 25 1C.
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(12-3-12 (12-3-12 (14-3-14 (14-3-14
solution) –n-decane solution + ZnO) –n-decane solution) –n-decane solution + ZnO) –n-decane
27.82 27.82 19.70 19.70
13.91 13.91 9.85 9.85
77.48 89.46 93.42 100.2
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nanoparticles, which, according to eqn (4) results in DG0ads being more negative. 3.2.2. Dynamic studies. The obtained plots of dynamic interfacial tension of 12-3-12 aqueous solution/n-decane for systems containing 12-3-12 only and 12-3-12 with nanoparticles at different surfactant concentrations are shown in Fig. 3. It can be observed that the trend in dynamic interfacial tension is very similar for both systems (with and without nanoparticles). For both systems, at all surfactant concentrations, dynamic interfacial tension values decrease rapidly at time zero and reach an equilibrium value overtime. At higher concentrations equilibrium was approached much sooner, whereas lower concentrations required longer times to reach equilibrium. In the presence of nanoparticles the reduction rate of interfacial tension is greater than that of the particle free system. Fig. S7 (ESI†) presents dynamic interfacial tension vs. time for 14-3-14 with and without ZnO, it is observed that the evolution of dynamic interfacial tension is very similar to 12-3-12. By using dynamic interfacial tension data and eqn (5) the values of dynamic surface concentration (G) can be obtained: G p ¼ 3kB TGsat ln 1 Gsat
G G vs. t þ ln 1 Ge Ge should be linear if the system follows the SRT model. The plot G G vs. t for the close to equilibrium surfactant þ ln 1 of Ge Ge
where b and k are constants. The plot of (5)
The calculated dynamic surface concentrations of 12-3-12 and 14-3-14 in the absence and presence of ZnO nanoparticles are
Fig. 3 The dynamic interfacial tension of 12-3-12 aqueous solution/ n-decane vs. time at 25 1C and at different SDS concentrations: 0.00001 (’), 0.0001 (J), 0.0002 (E), and 0.001 (D) M. (a) In the absence and (b) in the presence of 0.01 wt% ZnO nanoparticles.
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plotted versus time in Fig. 4 and Fig. S8 (ESI†), respectively. For both surfactant systems, both in the absence and presence of ZnO, the surface concentration values increase at time zero until a constant equilibrium value was attained at longer times. By modelling dynamic surface tension data with two models the mechanism of surfactant migration from bulk to the interface was determined. The mechanism of surfactant migration includes two elementary processes: (i) the migration of surfactant molecules from bulk to the subsurface layer by diffusion; and (ii) adsorption from the subsurface to the interface. The overall rate of migration is determined by the rate limiting step process. The statistical rate theory (SRT) model28 and mixed diffusion-kinetic control model29,30 were used for modelling the dynamic data. For an SRT model close to equilibrium the relationship between surface concentration and time is:28 G G ¼ b kt (6) þ ln 1 Ge Ge
Fig. 4 Dynamic surface concentration vs. time at different 12-3-12 concentrations: 0.000004 (&) and 0.00002 (K) M. (a) In the absence and (b) in the presence of 0.01 wt% ZnO nanoparticles at 25 1C.
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systems in the absence and presence of ZnO is observed to be nonlinear, as shown in Fig. S9 and S10 (ESI†), respectively, for the 12-3-12 and 14-3-14 surfactants. These results indicate that neither 12-3-12 nor 14-3-14 surfactant systems, with or without nanoparticles, follow the SRT model and therefore the mechanism of surfactant migration is not pure adsorption. Given this, the dynamic data were modelled using the mixed diffusionkinetic-controlled model, which was proposed by Ward and Tordai29 and developed by Azizian et al.30 For close to equilibrium data this model is:30 1=2 RTGe 2 p gðtÞt!1 ge ¼ (7) f ðtÞ2 Da t 2C0 where f (t) is defined by:30 Ge Ge G G f ðtÞ ¼ 1 1 Gsat Gsat Gsat Gsat
(8)
and Da is: Da
DE 2 2Ea ¼ D exp D kT
(9)
absence and presence of ZnO the evolution of DE is very similar to Da, and that addition of nanoparticles decreases DE. Based on eqn (5) the activation energy of adsorption for 12-3-12 and 14-3-14 in the absence and presence of nanoparticles was calculated. Fig. 6 and Fig. S16 (ESI†) show that Ea increases as surfactant concentration increases, and then reached a constant value. Small values of Ea for low surfactant concentrations indicate that at low surfactant concentrations adsorption is negligible and the mechanism of surfactant migration is pure diffusion. At higher surfactant concentrations adsorption becomes important (as evidenced by a larger value for Ea) and the migration mechanism surfactant changes to a mixed diffusion and adsorption mechanism. The presence of nanoparticles decreases the Ea values, again likely due to the synergistic interaction between the surfactants and nanoparticles that decreases the interfacial tension. It should be noted that the effect of nanoparticles on the activation energy for 12-3-12 is greater than that for14-3-14, similar to the greater effect of the nanoparticles on the equilibrium interfacial tension for 12-3-12 systems, discussed in Section 3.2.1.
3.3.
Effect of solution pH
where Da is the apparent diffusion coefficient, D and DE are the monomer diffusion coefficient and the effective diffusion coefficient, respectively, and Ea is the adsorption activation energy. The plots of g(t)t-N ge vs. t1/2 for all 12-3-12 and 14-3-14 concentrations in the absence and presence of ZnO nanoparticles were plotted (see for example Fig. 5 and Fig. S11, ESI†). It can be seen that the plots are linear, indicating that the adsorption rate of gemini surfactants is controlled by both diffusion and adsorption. By using eqn (7) and the slope of plot g(t)t-N ge vs. t1/2 apparent diffusion coefficients Da for all systems (with and without of ZnO) were calculated. Fig. S12 (12-3-12) and S13 (14-3-13) (ESI†) show that Da decreases with increasing surfactant concentration until a constant value was reached. It can also be seen that Da decreases upon addition of ZnO nanoparticles. The results for the determination of DE are shown in Fig. S14 and S15 (ESI†) for the 12-3-12 and 14-3-14 surfactants, respectively. It can be seen that for both surfactants in the
In order to examine the impact of pH on the reduction of interfacial tension by gemini surfactant/ZnO nanoparticle systems, we chose to measure interfacial tension for 3 pH conditions. The results are shown in Fig. 7 (12-3-12) and Fig. S17 (14-3-14) (ESI†). It is seen that in the absence of nanoparticles pH has a negligible effect on the interfacial tension (Fig. 7a and Fig. S17a, ESI†), but in the presence of ZnO nanoparticles a greater effect of the nanoparticles in reducing the interfacial tension is observed under basic and neutral pH values as compared to an acidic pH. The pH of the point of zero charge (pHpzc) for ZnO nanoparticles is 7,15 indicating that the surface charge of ZnO nanoparticles is positive at acidic pH, uncharged at neutral pH and negative at basic pH. Therefore at basic pH the interactions between cationic surfactants and nanoparticles are attractive while at acidic pH they are repulsive. The repulsive forces between
Fig. 5 Plot of g(t)t-N ge vs. t1/2 for close to equilibrium data at 0.0001 M 12-3-12, in the absence (J) and presence (’) of 0.01 wt% ZnO nanoparticles at 25 1C.
Fig. 6 Activation energy of adsorption, Ea, as a function of 12-3-12 bulk concentration in the absence (J) and presence (’) of ZnO nanoparticles at 25 1C.
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Fig. 7 Effect of solution pH on the dynamic interfacial tension for C12-3-12 = 0.0001 M, pH = 5.5 (D), pH = 7 (J) and pH = 9.5 (E) (a) in the absence and (b) in the presence of 0.01 wt% ZnO nanoparticles at 25 1C.
cationic surfactants and positively charged nanoparticles can reduce the interfacial tension. It should be related to the promotion of surfactant diffusion towards the interface by the repulsive forces between cationic surfactants and positively charged nanoparticles.11 At basic pH, nanoparticles with negative surface charge adsorb the cationic surfactants and the modified nanoparticles migrate to the interface leading to more reduction of interfacial tension.13,16 Therefore the more reduction of interfacial tension at basic and neutral pH values of gemini surfactants suggested that the effect of attractive force on decreasing the interfacial tension is more than that of the repulsive force.
3.4.
Effect of ZnO concentration
For the investigation of the effect of the ZnO amount, the interfacial tension of 12-3-12 surfactant aqueous solution/ndecane vs. time at two concentrations of ZnO was measured (Fig. S18, ESI†). This figure shows that by increasing the amount of ZnO nanoparticles twice, a negligible change in interfacial tension (about 1 mN m1) was observed. Also, at higher concentrations of ZnO the turbidity of solution increases and measurement of interfacial tension becomes difficult by the drop volume method (the sensor cannot detect the oil drop in very turbid solution).
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Fig. 8 Photograph of vessels containing emulsions of decane and water (1 : 1) stabilized by (a) 12-3-12 surfactant alone at concentrations of 0.001, 0.0001, 0.00004, 0.00001 and 0.000004 M (left to right). (b) A fixed concentration of ZnO nanoparticles (0.01 wt%) and 12-3-12 with the same concentrations of (a).
3.5.
Emulsion stability
In this section the stability of o/w emulsions as a function of the surfactant concentration either with or without ZnO nanoparticles (0.01 wt%) was compared. The emulsion system examined was oil in water (o/w) emulsion, which was confirmed by drop dilution and dye solubility tests. The photographic images of emulsions after 24 h are shown in Fig. 8 for the 12-3-12 and in Fig. S19 (ESI†) for the 14-3-14 surfactants. It is seen that 12-3-12 and 14-3-14 alone are good emulsifiers; however, the addition of ZnO nanoparticles is observed to increase the stability of the emulsions (because the amount of residual emulsion after 24 h is higher in the presence of nanoparticles). Fig. 9 shows optical microscopy images of emulsions including 12-3-12 alone (a) and mixtures of ZnO and 12-3-12 (b), (c), and (d). It can be seen that a decrease in the droplet size is observed with increasing surfactant concentration. The same results are observed for 14-3-14 (Fig. S20, ESI†). These results are
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Acknowledgements
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The authors acknowledge the financial support of Bu-Ali Sina University, Hamedan, Iran Grant No. 32-1879. Financial assistance from the Natural Sciences and Engineering Research Council (NSERC) of Canada, and the Ontario (Canada) Ministry of Research and Innovation for Dr Wettig is gratefully acknowledged.
References
Fig. 9 Optical microscopy image of n-decane-in-water emulsions (1 : 1) stabilized by (a) 0.00004 M 12-3-12 alone, and mixture of 0.01 wt% ZnO particles, and (b) 0.00004, (c) 0.00002, and (d) 0.0001 M 12-3-12 with 400 magnification.
similar to the literature studies for single chain surfactants.9,15,31 Overall, the synergism between the gemini surfactants and ZnO nanoparticles observed from the equilibrium and dynamic interfacial tension studies has a dramatic impact on emulsion stability, with the addition of ZnO nanoparticles enhancing emulsion stability.
4. Conclusion The efficiency of two gemini surfactants in reducing the oil/water interfacial tension in the absence and presence of ZnO nanoparticles was compared. The experimental results show that the addition of ZnO nanoparticles increases the efficiency of the surfactants in reducing oil/water interfacial tension. This effect was greater for the 12-3-12 surfactant as compared to the 14-3-14 surfactant. Calculations of the Gibbs energies of micellization and adsorption for the 12-3-12 and 14-3-14 surfactants show that DG0mic is unaffected by the addition of ZnO, while DG0ads becomes more negative with the addition of ZnO. The dynamic interfacial tension study determined that the migration mechanism of both gemini surfactants (both with and without ZnO) is a diffusion-kinetic control mechanism. The impact of the addition of ZnO nanoparticles is greater under basic or neutral pH conditions, and is reduced under acidic conditions, likely due to decreased interaction between cationic gemini surfactants and cationic ZnO nanoparticles that exist under acidic pH conditions. The results also show that in the presence of nanoparticles the emulsion stability for water– n-decane–gemini surfactant emulsions is improved and the droplet size is decreased.
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Synergistic behaviour of ZnO nanoparticles and gemini surfactants on the dynamic and equilibrium oil/water interfacial tension† Tahereh Fereidooni Moghadam,a Saeid Azizian*a and Shawn Wettigb In this work the effect of ZnO nanoparticles on the interfacial behaviour of gemini surfactants (12-3-12 and 14-3-14) at the oil/water interface was investigated. Equilibrium and dynamic interfacial tension in the absence and presence of ZnO was measured and compared. The results show that the synergistic
Received 27th January 2015, Accepted 6th February 2015 DOI: 10.1039/c5cp00510h
interactions between the surfactants and nanoparticles decrease the interfacial tension beyond that observed for each component, alone. Modelling of dynamic data with two different models indicates that the mechanism of surfactant migration (with and without ZnO) is mixed diffusion-kinetic-control. The Gibbs free energy of micellization and the Gibbs free energy of adsorption in the absence and presence of ZnO were calculated and compared. Finally the effect of addition of ZnO nanoparticles on emulsion
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stability was also examined.
1. Introduction Gemini surfactants are made up of two amphiphilic surfactant moieties linked at or near the head group by a spacer group. In comparison with the corresponding monomeric surfactants, gemini surfactants have lower critical micelle concentration (cmc) values, and are more efficient in reducing the surface or interfacial tension.1–6 Thus, gemini surfactants are used in a number of industrial applications such as foaming, cosmetics, detergents, coatings, biological and biomedical fields.7 The study of the combination of surfactants and nanoparticles and their interaction at the air–liquid and liquid– liquid interfaces is a topic of increasing interest, due to their application especially to improve the stability of emulsions and foams.8–10 The study of interfacial tension as a macroscopic parameter for understanding the interactions of nanoparticles and surfactants at the interface is very important. Despite this importance, the interfacial tension of a mixture of nanoparticles and surfactants has not been extensively studied.11 Ravera et al.12 showed that silica nanoparticles increase the surface and interfacial tension of a solution of a
Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, 65147, Hamedan, Iran. E-mail: [email protected], [email protected]; Fax: +98-8138380709; Tel: +98-8138282807 b School of Pharmacy, University of Waterloo, Waterloo, ON N2L 3G1, Canada † Electronic supplementary information (ESI) available: The plots of equilibrium and dynamic interfacial tension for the 14-3-14 surfactant, dynamic surface concentrations, the SRT kinetic model, apparent and effective diffusion coefficients for 12-3-12 and 14-3-14 surfactants, the photograph of the vessel and the optical microscopy image of 14-3-14. See DOI: 10.1039/c5cp00510h
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cetyltrimethylammonium bromide (CTAB). The effect of negatively charged silica nanoparticles on the interfacial tension of CTAB at oil–water interfaces was studied by Lan et al.13 According to their study the interfacial tension was reduced due to a synergistic effect between nanoparticles and CTAB. Ma et al.11 investigated the effect of silica nanoparticles on surface and interfacial tension in combination with anionic and nonionic surfactants. Their results demonstrated that the addition of silica nanoparticles increased the efficiency of SDS in reducing interfacial tension. Also, the effect of the surfactant type on the interfacial tension in the presence of hydrophilic silica nanoparticles at the oil/water interface was reported by Pichot et al.14 According to their results, silica nanoparticles, in the presence of w/o surfactants, have no effect on interfacial tension, while they affect interfacial tension in the presence of o/w surfactants. Recently, the influence of ZnO nanoparticles on the oil/water interfacial tension of single chain surfactants CTAB (cationic),15 SDS (anionic)16 and F-127 (non-ionic)17 solution has been studied by our group. The results showed that ZnO nanoparticles increase the surfactant efficiency in decreasing the interfacial tension due to the synergistic effect between surfactants and ZnO nanoparticles, similar to that described in the previous paragraph for silica nanoparticles. The potential applications of gemini surfactants are in foaming, cosmetics, detergents, coatings, chemical industry, biomedical and biological fields.7 Also ZnO is one of the compounds used in cosmetic and skincare products extensively, and therefore, the study of its synergistic effect with gemini surfactants and also its effect on the emulsion stability is important for this industry. According to
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our knowledge, the influence of nanoparticles on the interfacial tension of gemini surfactant solution has not been examined. In this work the effect of ZnO nanoparticles on the oil/water interfacial tension of gemini surfactants N,N0 -bis(dimethylalkyl)-a,o-alkanediammonium dibromide (m-s-m type, m = the length of the n-alkyl tail and s = the length of the polymethylene spacer), 12-3-12 and 14-3-14 has been studied using both equilibrium and dynamic surface tension methods. The effect of ZnO nanoparticles on the ability of these surfactants to stabilize emulsions was also investigated.
2. Experimental Materials and methods The N,N0 -bis(dimethyldodecyl)-1,3-propanediammonium dibromide (12-3-12) and N,N0 -bis(dimethyltetradecyl)-1,3-propanediammonium dibromide (14-3-14) gemini surfactants used in this study were synthesized according to procedures previously reported.1 The oil used in this study was n-decane (Merck Co., 499%) that was purified via column chromatography over basic alumina for removal of polar impurities before use.18 The ZnO nanoparticles were prepared according to a procedure previously reported.19 The synthesized ZnO nanoparticles were characterized using scanning electron microscopy (SEM) (MIRA3TESCAN) and X-ray diffraction (XRD) (ADP2000ITALSTRUCTURE Italia) methods. Aqueous ZnO nanoparticle dispersions were prepared using an ultrasonic (UNIVERSAL ULTRASONIC) bath. For the study of dispersion of ZnO in aqueous solution ZnO was dispersed in surfactant solution and turbidity of the solution was measured after magnetic stirring or sonication for 10, 20, 30, 40 and 60 minutes. The plot is shown in Fig. S1 (ESI†); this figure shows that the turbidity of magnetically stirred solution is low and this means that the particles are mainly in aggregate form; but the turbidity increases with increased sonication time and after 40 minutes reached a constant value. This high turbidity by 40 min sonication in comparison to magnetic stirring shows that the particles are mainly dispersed and 40 min sonication provides the maximum possible dispersion of ZnO nanoparticles. This result was in agreement with previously reported results that show ultrasonication of nanoparticles as an effective tool for the elimination of agglomerates.20–22 A drop volume tensiometer (DVT30 Kruss, Germany) was used to measure the equilibrium and dynamic interfacial tension between the oil phase and the surfactant aqueous phase. Emulsions of n-decane and aqueous surfactant solution or dispersions of ZnO nanoparticles in aqueous surfactant solution were prepared at equal volumes using an ultrasonic probe (SONOPULS) for 60 s and their stability after 24 h were monitored. All experiments were carried out at 25 1C and at a pH of approximately 7 (except for the investigation of pH effects). Also measurements were executed at 0.01 wt% ZnO nanoparticles concentration. The n-decane/water interfacial tension value at 25 1C was obtained as 48 mN m1, in agreement with the previously reported value.23
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3. Results and discussion 3.1.
Characterization of the prepared ZnO
The SEM image of the synthesized ZnO nanoparticles is shown in Fig. 1a. From this figure, we can deduce that nanoparticles are of spherical shape with a diameter of approximately 40 nm. The XRD pattern is shown in Fig. 1b, and indicates that the synthesised nanoparticles are pure ZnO, as described in our previous study. The characteristic peaks of ZnO at 2y = 31.6, 34.2, 36.1, 48.1, 56.4, 63.0, 67.9 and 68.91 correspond to the 100, 002, 101, 102, 110, 103, 112 and 201 planes respectively.15,19 For further characterization of prepared ZnO nanoparticles, the contact angle experiments were performed too. As shown in Fig. S2 (ESI†), the water droplet on the surface of thin film of the prepared ZnO is spread (wetting behaviour) and the contact angle is lower than 901, which indicates the hydrophilic nature of ZnO nanoparticles. The images of the water droplet covered by n-decane on a quartz substrate in the absence and presence of ZnO (Fig. S3, ESI†) show that ZnO has no effect on the contact angle of the oil/water system; i.e. these particles are hydrophilic and do not change the interfacial tension of this system. 3.2. Effect of ZnO nanoparticles in the presence of gemini surfactants In our previous study it has been shown that ZnO nanoparticles themselves have no effect on the interfacial tension of the oil/water system.15 The dynamic and equilibrium interfacial tension of the oil/water system was measured at different concentrations of 12-3-12 and 14-3-14 without nanoparticles or mixed with 0.01 wt% of ZnO nanoparticles.
Fig. 1 (a) The SEM image and (b) the XRD pattern of the prepared ZnO nanoparticles.
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3.2.1. Equilibrium studies. The resulting equilibrium interfacial tension changes at the n-decane/water interface in the absence and presence of ZnO nanoparticles (0.01 wt%), with increasing 12-3-12 concentration, are shown in Fig. 2. This figure illustrates that for both systems the interfacial tension decreases as the surfactant concentration increases, and then becomes approximately constant. Comparing the two plots shows that addition of ZnO nanoparticles to the surfactant system influences its efficiency in decreasing the interfacial tension; in the presence of ZnO nanoparticles, the interfacial tension reduction is greater than that of the sole surfactant system. It is suggested that a synergistic interaction between surfactants and nanoparticles is responsible for the observed increase in the efficiency of lowering the interfacial tension. Fig. 2 also shows that the presence of nanoparticles do not change the cmc of the surfactant, which is in agreement with the report of Lan et al. for the CTAB–silica nanoparticle system.13 Fig. S4 (ESI†) presents the same results for the 14-3-14 surfactant. Comparing Fig. 2 and Fig. S4 (ESI†) shows that 14-3-14, with longer alkyl tails, gives a lower interfacial tension for the water/ n-decane system; however, it can be seen that the effect of nanoparticles on interfacial tension is higher for the 12-3-12 surfactant. It is also noted that for a given concentration of bulk surfactant, the efficiency of gemini surfactants in reducing the interfacial tension is greater than for a corresponding single chain surfactant. For example at 0.0001 M surfactant the n-decane/water interfacial tension in the presence of the 12-3-12 surfactant is approximately 13 mN m1 while for CTAB it is approximately 18 mN m1.15 The Gibbs equation and the equilibrium interfacial tension data were used to obtain the equilibrium surface concentration: dge ¼ 3Ge RT d ln C0
(1)
where ge is the equilibrium interfacial tension, Ge is the equilibrium surface concentration, C0 is the bulk concentration, T is the absolute temperature and R is the gas constant. The surface concentration changes vs. 12-3-12 and 14-3-14 bulk concentration in the absence and presence of ZnO nanoparticles are
Table 1 Saturation surface concentration (Gsat) and the Langmuir constant (K) from the Langmuir isotherm for adsorption of 12-3-12 and 14-3-14 at the water–n-decane system at 25 1C
Gsat/mol m2
System (12-3-12 (12-3-12 (14-3-14 (14-3-14
solution) –n-decane solution + ZnO) –n-decane solution) –n-decane solution + ZnO) –n-decane
3.4 2.9 2.7 2.4
107 107 107 107
K/L mol1 5.8 6.2 6.1 6.8
105 106 105 105
R2 0.995 0.994 0.994 0.99
shown in Fig. S5 and S6 (ESI†), respectively. For both systems the Ge values increase with increasing surfactant concentration and reach saturation at the cmc. The Langmuir isotherm was used for modelling of equilibrium surface concentration data: Ge KC0 ¼ Gsat 1 þ KC0
(2)
where Gsat is the saturated surface concentration and K is the Langmuir constant. Table 1 shows the results of this fitting for the gemini surfactants. These results demonstrate that in the presence of ZnO the surface concentrations of the surfactants are lower than that for the particle free system. The larger Langmuir constant in the presence of nanoparticles gives rise to the observed greater decrease in the interfacial tension. For all systems the values of the Gibbs free energy of micellization DG0mic were calculated according to eqn (3):11,24 DG0mic = 2RT (1.5 a) ln(cmc/o)
(3)
where cmc is the critical micelle concentration of the surfactant (M), a is the degree of ionization of the surfactant and o is the number of moles of pure water in 1 liter. Here, an a value of 0.23 for 12-3-121 and 0.74 for 14-3-14,25 and the o value of 55.5 M (ref. 26) at 25 1C were used. From this, the Gibbs free energy of micellization per alkyl tail, DG0mic,tail = DG0mic/2,24 was also calculated. The Gibbs free energy of adsorption DG0ads was obtained using the DG0mic values and the following equation:27 DG0ads = DG0mic (pcmc/Gsat)
(4)
where pcmc is the surface pressure (p = g0 g) at cmc. The obtained values of DG0mic, DG0mic,tail and DG0ads are normalized using the thermal energy (RT) and are listed in Table 2. These results illustrate that for both surfactants the addition of ZnO nanoparticles has no effect on DG0mic and DG0mic,tail because ZnO nanoparticles do not change the cmc. The DG0ads values are more negative in the presence of ZnO nanoparticles. This is explained by the observed decrease in Gsat in the presence of ZnO Table 2 Calculated DG0mic, DG0mic,tail and DG0ads for the gemini surfactant–ZnO nanoparticle systems
DG0mic/RT DG0mic,tail/RT DG0ads/RT
System Fig. 2 The equilibrium interfacial tension of aqueous 12-3-12 solution/ n-decane vs. 12-3-12 concentration in the absence (J), and presence (’) of 0.01 wt% ZnO nanoparticles at 25 1C.
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(12-3-12 (12-3-12 (14-3-14 (14-3-14
solution) –n-decane solution + ZnO) –n-decane solution) –n-decane solution + ZnO) –n-decane
27.82 27.82 19.70 19.70
13.91 13.91 9.85 9.85
77.48 89.46 93.42 100.2
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nanoparticles, which, according to eqn (4) results in DG0ads being more negative. 3.2.2. Dynamic studies. The obtained plots of dynamic interfacial tension of 12-3-12 aqueous solution/n-decane for systems containing 12-3-12 only and 12-3-12 with nanoparticles at different surfactant concentrations are shown in Fig. 3. It can be observed that the trend in dynamic interfacial tension is very similar for both systems (with and without nanoparticles). For both systems, at all surfactant concentrations, dynamic interfacial tension values decrease rapidly at time zero and reach an equilibrium value overtime. At higher concentrations equilibrium was approached much sooner, whereas lower concentrations required longer times to reach equilibrium. In the presence of nanoparticles the reduction rate of interfacial tension is greater than that of the particle free system. Fig. S7 (ESI†) presents dynamic interfacial tension vs. time for 14-3-14 with and without ZnO, it is observed that the evolution of dynamic interfacial tension is very similar to 12-3-12. By using dynamic interfacial tension data and eqn (5) the values of dynamic surface concentration (G) can be obtained: G p ¼ 3kB TGsat ln 1 Gsat
G G vs. t þ ln 1 Ge Ge should be linear if the system follows the SRT model. The plot G G vs. t for the close to equilibrium surfactant þ ln 1 of Ge Ge
where b and k are constants. The plot of (5)
The calculated dynamic surface concentrations of 12-3-12 and 14-3-14 in the absence and presence of ZnO nanoparticles are
Fig. 3 The dynamic interfacial tension of 12-3-12 aqueous solution/ n-decane vs. time at 25 1C and at different SDS concentrations: 0.00001 (’), 0.0001 (J), 0.0002 (E), and 0.001 (D) M. (a) In the absence and (b) in the presence of 0.01 wt% ZnO nanoparticles.
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plotted versus time in Fig. 4 and Fig. S8 (ESI†), respectively. For both surfactant systems, both in the absence and presence of ZnO, the surface concentration values increase at time zero until a constant equilibrium value was attained at longer times. By modelling dynamic surface tension data with two models the mechanism of surfactant migration from bulk to the interface was determined. The mechanism of surfactant migration includes two elementary processes: (i) the migration of surfactant molecules from bulk to the subsurface layer by diffusion; and (ii) adsorption from the subsurface to the interface. The overall rate of migration is determined by the rate limiting step process. The statistical rate theory (SRT) model28 and mixed diffusion-kinetic control model29,30 were used for modelling the dynamic data. For an SRT model close to equilibrium the relationship between surface concentration and time is:28 G G ¼ b kt (6) þ ln 1 Ge Ge
Fig. 4 Dynamic surface concentration vs. time at different 12-3-12 concentrations: 0.000004 (&) and 0.00002 (K) M. (a) In the absence and (b) in the presence of 0.01 wt% ZnO nanoparticles at 25 1C.
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systems in the absence and presence of ZnO is observed to be nonlinear, as shown in Fig. S9 and S10 (ESI†), respectively, for the 12-3-12 and 14-3-14 surfactants. These results indicate that neither 12-3-12 nor 14-3-14 surfactant systems, with or without nanoparticles, follow the SRT model and therefore the mechanism of surfactant migration is not pure adsorption. Given this, the dynamic data were modelled using the mixed diffusionkinetic-controlled model, which was proposed by Ward and Tordai29 and developed by Azizian et al.30 For close to equilibrium data this model is:30 1=2 RTGe 2 p gðtÞt!1 ge ¼ (7) f ðtÞ2 Da t 2C0 where f (t) is defined by:30 Ge Ge G G f ðtÞ ¼ 1 1 Gsat Gsat Gsat Gsat
(8)
and Da is: Da
DE 2 2Ea ¼ D exp D kT
(9)
absence and presence of ZnO the evolution of DE is very similar to Da, and that addition of nanoparticles decreases DE. Based on eqn (5) the activation energy of adsorption for 12-3-12 and 14-3-14 in the absence and presence of nanoparticles was calculated. Fig. 6 and Fig. S16 (ESI†) show that Ea increases as surfactant concentration increases, and then reached a constant value. Small values of Ea for low surfactant concentrations indicate that at low surfactant concentrations adsorption is negligible and the mechanism of surfactant migration is pure diffusion. At higher surfactant concentrations adsorption becomes important (as evidenced by a larger value for Ea) and the migration mechanism surfactant changes to a mixed diffusion and adsorption mechanism. The presence of nanoparticles decreases the Ea values, again likely due to the synergistic interaction between the surfactants and nanoparticles that decreases the interfacial tension. It should be noted that the effect of nanoparticles on the activation energy for 12-3-12 is greater than that for14-3-14, similar to the greater effect of the nanoparticles on the equilibrium interfacial tension for 12-3-12 systems, discussed in Section 3.2.1.
3.3.
Effect of solution pH
where Da is the apparent diffusion coefficient, D and DE are the monomer diffusion coefficient and the effective diffusion coefficient, respectively, and Ea is the adsorption activation energy. The plots of g(t)t-N ge vs. t1/2 for all 12-3-12 and 14-3-14 concentrations in the absence and presence of ZnO nanoparticles were plotted (see for example Fig. 5 and Fig. S11, ESI†). It can be seen that the plots are linear, indicating that the adsorption rate of gemini surfactants is controlled by both diffusion and adsorption. By using eqn (7) and the slope of plot g(t)t-N ge vs. t1/2 apparent diffusion coefficients Da for all systems (with and without of ZnO) were calculated. Fig. S12 (12-3-12) and S13 (14-3-13) (ESI†) show that Da decreases with increasing surfactant concentration until a constant value was reached. It can also be seen that Da decreases upon addition of ZnO nanoparticles. The results for the determination of DE are shown in Fig. S14 and S15 (ESI†) for the 12-3-12 and 14-3-14 surfactants, respectively. It can be seen that for both surfactants in the
In order to examine the impact of pH on the reduction of interfacial tension by gemini surfactant/ZnO nanoparticle systems, we chose to measure interfacial tension for 3 pH conditions. The results are shown in Fig. 7 (12-3-12) and Fig. S17 (14-3-14) (ESI†). It is seen that in the absence of nanoparticles pH has a negligible effect on the interfacial tension (Fig. 7a and Fig. S17a, ESI†), but in the presence of ZnO nanoparticles a greater effect of the nanoparticles in reducing the interfacial tension is observed under basic and neutral pH values as compared to an acidic pH. The pH of the point of zero charge (pHpzc) for ZnO nanoparticles is 7,15 indicating that the surface charge of ZnO nanoparticles is positive at acidic pH, uncharged at neutral pH and negative at basic pH. Therefore at basic pH the interactions between cationic surfactants and nanoparticles are attractive while at acidic pH they are repulsive. The repulsive forces between
Fig. 5 Plot of g(t)t-N ge vs. t1/2 for close to equilibrium data at 0.0001 M 12-3-12, in the absence (J) and presence (’) of 0.01 wt% ZnO nanoparticles at 25 1C.
Fig. 6 Activation energy of adsorption, Ea, as a function of 12-3-12 bulk concentration in the absence (J) and presence (’) of ZnO nanoparticles at 25 1C.
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Fig. 7 Effect of solution pH on the dynamic interfacial tension for C12-3-12 = 0.0001 M, pH = 5.5 (D), pH = 7 (J) and pH = 9.5 (E) (a) in the absence and (b) in the presence of 0.01 wt% ZnO nanoparticles at 25 1C.
cationic surfactants and positively charged nanoparticles can reduce the interfacial tension. It should be related to the promotion of surfactant diffusion towards the interface by the repulsive forces between cationic surfactants and positively charged nanoparticles.11 At basic pH, nanoparticles with negative surface charge adsorb the cationic surfactants and the modified nanoparticles migrate to the interface leading to more reduction of interfacial tension.13,16 Therefore the more reduction of interfacial tension at basic and neutral pH values of gemini surfactants suggested that the effect of attractive force on decreasing the interfacial tension is more than that of the repulsive force.
3.4.
Effect of ZnO concentration
For the investigation of the effect of the ZnO amount, the interfacial tension of 12-3-12 surfactant aqueous solution/ndecane vs. time at two concentrations of ZnO was measured (Fig. S18, ESI†). This figure shows that by increasing the amount of ZnO nanoparticles twice, a negligible change in interfacial tension (about 1 mN m1) was observed. Also, at higher concentrations of ZnO the turbidity of solution increases and measurement of interfacial tension becomes difficult by the drop volume method (the sensor cannot detect the oil drop in very turbid solution).
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Fig. 8 Photograph of vessels containing emulsions of decane and water (1 : 1) stabilized by (a) 12-3-12 surfactant alone at concentrations of 0.001, 0.0001, 0.00004, 0.00001 and 0.000004 M (left to right). (b) A fixed concentration of ZnO nanoparticles (0.01 wt%) and 12-3-12 with the same concentrations of (a).
3.5.
Emulsion stability
In this section the stability of o/w emulsions as a function of the surfactant concentration either with or without ZnO nanoparticles (0.01 wt%) was compared. The emulsion system examined was oil in water (o/w) emulsion, which was confirmed by drop dilution and dye solubility tests. The photographic images of emulsions after 24 h are shown in Fig. 8 for the 12-3-12 and in Fig. S19 (ESI†) for the 14-3-14 surfactants. It is seen that 12-3-12 and 14-3-14 alone are good emulsifiers; however, the addition of ZnO nanoparticles is observed to increase the stability of the emulsions (because the amount of residual emulsion after 24 h is higher in the presence of nanoparticles). Fig. 9 shows optical microscopy images of emulsions including 12-3-12 alone (a) and mixtures of ZnO and 12-3-12 (b), (c), and (d). It can be seen that a decrease in the droplet size is observed with increasing surfactant concentration. The same results are observed for 14-3-14 (Fig. S20, ESI†). These results are
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Acknowledgements
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The authors acknowledge the financial support of Bu-Ali Sina University, Hamedan, Iran Grant No. 32-1879. Financial assistance from the Natural Sciences and Engineering Research Council (NSERC) of Canada, and the Ontario (Canada) Ministry of Research and Innovation for Dr Wettig is gratefully acknowledged.
References
Fig. 9 Optical microscopy image of n-decane-in-water emulsions (1 : 1) stabilized by (a) 0.00004 M 12-3-12 alone, and mixture of 0.01 wt% ZnO particles, and (b) 0.00004, (c) 0.00002, and (d) 0.0001 M 12-3-12 with 400 magnification.
similar to the literature studies for single chain surfactants.9,15,31 Overall, the synergism between the gemini surfactants and ZnO nanoparticles observed from the equilibrium and dynamic interfacial tension studies has a dramatic impact on emulsion stability, with the addition of ZnO nanoparticles enhancing emulsion stability.
4. Conclusion The efficiency of two gemini surfactants in reducing the oil/water interfacial tension in the absence and presence of ZnO nanoparticles was compared. The experimental results show that the addition of ZnO nanoparticles increases the efficiency of the surfactants in reducing oil/water interfacial tension. This effect was greater for the 12-3-12 surfactant as compared to the 14-3-14 surfactant. Calculations of the Gibbs energies of micellization and adsorption for the 12-3-12 and 14-3-14 surfactants show that DG0mic is unaffected by the addition of ZnO, while DG0ads becomes more negative with the addition of ZnO. The dynamic interfacial tension study determined that the migration mechanism of both gemini surfactants (both with and without ZnO) is a diffusion-kinetic control mechanism. The impact of the addition of ZnO nanoparticles is greater under basic or neutral pH conditions, and is reduced under acidic conditions, likely due to decreased interaction between cationic gemini surfactants and cationic ZnO nanoparticles that exist under acidic pH conditions. The results also show that in the presence of nanoparticles the emulsion stability for water– n-decane–gemini surfactant emulsions is improved and the droplet size is decreased.
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