DOI: 10.1002/cphc.201500126
Articles
Sol-to-Gel Transition in Fast Evaporating Systems Observed by in Situ Time-Resolved Infrared Spectroscopy Plinio Innocenzi,*[a] Luca Malfatti,[a] Davide Carboni,[a] and Masahide Takahashi[b] The in situ observation of a sol-to-gel transition in fast evaporating systems is a challenging task and the lack of a suitable experimental design, which includes the chemistry and the analytical method, has limited the observations. We synthesise an acidic sol, employing only tetraethylorthosilicate, SiCl4 as catalyst and deuterated water; the absence of water added to the sol allows us to follow the absorption from the external environment and the evaporation of deuterated water. The timeresolved data, obtained by attenuated total reflection infrared
spectroscopy on an evaporating droplet, enables us to identify four different stages during evaporation. They are linked to specific hydrolysis and condensation rates that affect the uptake of water from external environment. The second stage is characterized by a decrease in hydroxyl content, a fast rise of condensation rate and an almost stationary absorption of water. This stage has been associated with the sol-to-gel transition.
1. Introduction The sol-to-gel transition is a chemical–physical transformation of a colloidal solution (sol) into a three-dimensional continuous network that encloses a liquid phase (gel).[1] The formation of the continuous network marks the gel point while the gelation time spans from hours to days as a function of the pH of the sol. The sol-to-gel transition is not a thermodynamic event such as the glass transition and its exact determination is difficult to achieve. As underlined by Hench and West[2] “the gelation point of any system, including sol–gel silica, is easy to observe qualitatively and easy to define in abstract terms but extremely difficult to measure analytically”. This is particularly true in sol–gel films because the kinetics of the different chemical–physical phenomena is very fast.[3] The environment plays also a crucial role because the relative humidity in the deposition chamber influences the evaporation process in quite a complex way. The in situ analysis of evaporating droplets of alcohol[4] and water[5] has shown, in fact, that the absorption of water from the external environment has a key role in the process. This also occurs in the deposition of a film from a liquid phase which undergoes through the sol-to-gel transition during processing. The lack of suitable analytical techniques has not allowed up to now a direct investigation of the overall phenomenon. Several different methods have been applied so
far, mostly using fluorescent probes, but only a partial understanding of the process has been reached.[6–8] An attempt to correlate the water content with the formation of a mesostructured gel has been made using Karl Fisher titration.[9] An alternative technique is in situ time-resolved infrared spectroscopy, which can be applied to different types of evaporation processes.[10] The technique is relatively simple even if data analysis has to be carried out with attention. The experiment can be performed by attenuated total reflection (ATR) infrared analysis or with an infrared microscope. This analytical method has been applied to ethanol[4] and water droplets,[5, 11] mesoporous films,[12–16] sol–gel films,[17] hybrid materials[18] and tri-block copolymers.[19, 20] The technique has been also combined with small angle X-ray scattering (SAXS) for simultaneous time-resolved analysis of self-assembly and evaporation phenomena in mesoporous ordered films.[21] These experiments have shown that evaporative phenomena at the time scale of seconds can be followed in detail by time-resolved infrared spectroscopy. In situ analysis has allowed understanding the details of evaporation of water and ethanol as a function of relative humidity in the evaporation chamber. These data have been used for a better comprehension of self-assembly, especially by combining in situ time-resolved FTIR with other techniques (SAXS), which allow getting a direct observation of micelles organization. In the present work we have applied time-resolved infrared analysis to a silica sol droplet with the purpose of getting a better insight into the sol-to-gel transition in fast evaporating systems. In general, the chemistry of the precursor sol, which means the pH, the concentration of the silicon alkoxide and the relative amount of water, control the properties of the final material. If the silica sol is used to deposit thin films, the environment in the deposition chamber, the temperature and relative humidity are the parameters to be controlled. On the
[a] Prof. Dr. P. Innocenzi, Dr. L. Malfatti, Dr. D. Carboni Laboratorio di Scienza dei Materiali e Nanotecnologie DADU, Universit di Sassari, Palazzo Pou Salid Piazza Duomo 6, 07041 Alghero (Italy) E-mail: [email protected] [b] Prof. Dr. M. Takahashi Department of Materials Science Graduate School of Engineering and International Laboratory of Materials Science and Nanotechnology (iLMNT) Osaka Prefecture University, Sakai, Osaka 599-8531 (Japan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500126.
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Articles other hand the kinetics of the process, which depends on the withdrawal speed in the case of dip-coating or rotation rate for spin-coating, are also of paramount importance. This means that even in the simplest case, such as a silica sol obtained by tetraethylorthosilicate (TEOS) and water, the sol-togel transition reaches a high level of complexity, which hampers a real understanding of the overall process. In particular, as we have previously underlined, the atmospheric moisture plays an important role and in situ studies are quite difficult because of the limited techniques available that would allow separating the different chemical–physical phenomena during sol evaporation. For these reasons, we have designed an experiment employing deuterated water and TEOS to acquire a better understanding of the processes behind the evaporation of a silica sol and to discriminate between reacting and absorbing water. This has allowed for the first time, to our knowledge, obtaining a direct observation of a sol-to-gel transition in a fast evaporating system.
sorption bands at 1637 cm1 (bending, n2); 2123 cm1 (combination of n2 + libration mode, L2) and 3300 cm1 (n1, symmetric stretching with an overlapped mode around 3490 cm1 due to antisymmetric stretching, v3, and the overtone of n2). The spectrum of deuterated water in the same range is characterized by three bands due to the same vibrations of water, but shifted to lower energies and detected at 1209 (n2), 1547 (n2 + L2) and 2472 cm1 (n1, n3, n2 overtone). Despite showing the same symmetry, D2O has a distinct mass with respect to water, because of the different atomic weight of the two deuterium atoms. This leads to a modified value of reduced mass (m) in the Hamiltonian which describes the vibration of the molecule, that is, a different energy value for the characteristic infrared vibration modes. During evaporation of a D2O droplet, water is also absorbed and an exchange of protons to form HOD molecules will occur [Eq. (1)]:
2. Results and Discussion
The amount of HOD species that are formed increases with the time of evaporation, as we have observed following the process in situ (Figure S1, Supporting Information). The reference infrared spectrum of HOD[22] is also shown in Figure 1, with the three main absorption bands at 1450, 2472 and 3397 cm1; the band at 2472 cm1 overlaps with that of D2O while the band at 3397 cm1 overlaps with that of water.[23] As a first step to understand the details of sol-to-gel transition in a silica sol, we have followed the evaporation process of a pure TEOS droplet in an open chamber, measuring the actual relative humidity (RH) value, and the results are shown in Figure S2. The TEOS droplet evaporates without absorbing water from the external environment and no signs of hydrolysis and condensation were detected in the FTIR spectra. In absence of any catalyst, in fact, the atmospheric water is unable to trigger the hydrolysis of the alkoxide and therefore TEOS cannot undergo any polycondensation reaction. At the end of the process TEOS has been completely evaporated and no residues have been observed on the ATR diamond substrate.
H2 O þ D2 O ! 2 HOD
Deuterium oxide and TEOS form a biphasic mixture which needs a catalyst to initiate the hydrolysis reaction. To avoid the addition of an acidic source containing external protons, a chemical reagent capable of generating in situ the soughtafter acidity has been used. We have selected SiCl4 that reacts with D2O to form deuterated hydrochloric acid, which in turn creates a slightly acidic environment capable of catalysing the hydrolysis and condensation reactions with an in situ generated source of D + . The advantage of this approach is that no external H2O is added to the precursor sol and this allows following by FTIR the absorption of H2O from the external environment and D2O evaporation from the sol. 2.1. Reference Spectra In Figure 1 the reference infrared absorption spectra, in the 3800–1000 cm1 range, of liquid water, deuterated water and HOD are shown. The spectrum of water shows three main ab-
ð1Þ
2.2. Water Absorption during Evaporation We have designed the experiments with the specific purpose of understanding in more detail the chemical–physical processes occurring during the evaporation of a silica sol. As said above, we have used deuterated water instead of water in the precursor sol; this has enabled following the interaction of the sol with the external environment during evaporation. In particular, it has been possible to identify the contribution of water absorbed from the external environment since the infrared vibration of deuterated water is shifted at much lower wavenumbers with respect to water. The water absorption from the atmosphere can be revealed by the infrared band peaking at 1637 cm1. This band does not overlap with other vibrational modes and gives a direct indication of the water exchange during the absorption–evaporation process. The experiments have been carried out with silica sols prepared by using the three different sols with D2O/
Figure 1. ATR-FTIR reference spectra of water (c), deuterated water (a) and partially deuterated water (g) in the range 3800–1000 cm1.
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Articles is then followed by a third phase where the absorption increases at a slower rate and a fourth one where absorption finally decreases with the completion of the sol evaporation. A similar trend is observed during the evaporation of the sol with a supra-stoichiometric amount of D2O (R = 8), (Figure 2 e). The change in the absorption maxima shows also four steps, with similar stages with respect to the sol with stoichiometric D2O (Figure 2 f).
2.3. Formation of Hydroxyls With the progress of evaporation TEOS goes through a progressive hydrolysis and condensation. To avoid the addition of water (vide supra) we have used SiCl4 to catalyse the acidic hydrolysis of TEOS [Eq. (2)]: Figure 2. 3D ATR-FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1725–1575 cm1 range, of the three of evaporation experiments corresponding to R = 1 (a), 4 (c) and 8 (e); the absorbance intensity at 1637 cm1 is represented by a false grey scale. The corresponding changes of absorption maxima as a function of time are also shown (b, d, and f, respectively).
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where the deuterated DCl acts as the catalyst of the hydrolysis and condensation of TEOS with deuterated water. Although formally written as DCl, we have to remember that, in aqueous solution, this molecule mostly exists in the hydrated form, that is, the deuterated hydroxonium ion and its counter ion according to the equilibrium given by Equation (3):
TEOS ratios (R) equal to 1, 4 and 8, while the RH was constant at 37 % in all the evaporation runs. The evaporation process has been followed until no further changes could be observed in the FTIR spectra and a stable silica gel formed as final product. Figures 2 a,c,e shows the 3D FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1725– 1575 cm1 range, of the three different series of evaporation experiments. The absorbance intensity is represented by a false grey scale. In Figures 2 b,d,f the corresponding changes of absorption maxima at 1637 cm1 as a function of time are shown. During the evaporation of the sol with a sub-stoichiometric amount of deuterated water (R = 1), a small absorption of water (Figure 2 a), which follows an asymptotic trend (Figure 2 b), is observed. The gel, which is formed at the end of the evaporation process, retains the absorbed water and no further changes are observed in the spectra. The trend, in the silica sol containing a stoichiometric (R = 4) amount of D2O (Figure 2 c), changes. In this case water is absorbed in a larger amount and reaches a maximum which then decreases as the evaporation proceeds. This sol presents another peculiarity in the absorption of water, as the curve obtained from the absorption maxima shows the presence of different evaporation stages (Figure 2 d). There is a first fast absorption of water; after this stage the absorption stops for few seconds. This step ChemPhysChem 0000, 00, 0 – 0
SiCl4 þ 4 D2 O ! SiðODÞ4 þ 4 DCl
D2 O þ DCl Ð D3 Oþ þ Cl
ð3Þ
As expected from an acid-catalysed hydrolysis, the first step is the protonation of the ethoxy groups of TEOS by the deuterated hydroxonium ion (D3O + ), which is the true catalytic species in solution. This weakens the bond between the alkoxy group and the silicon, leaving the deuterated water free to attack the silicon centre. The nucleophilic attack thus results in a substitution of a deuterated ethanol with a molecule of deuterated water, giving rise to a protonated silanol, Si+ OD2 (Scheme 1). This silanol further reacts with a molecule of D2O to restore the deuterated hydroxonium ion, which is able to start a new catalytic cycle. The sequence of reactions involving a complete acidic hydrolysis of a TEOS molecule can be, for the sake of clarity, summarised as it follows: Si-ðOC2 H5 Þ4 þ 4 D2 O ! Si-ðODÞ4 þ 4 C2 H5 OD 3
ð4Þ
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Articles a second short phase when the rise of OH stops, a third stage with a further increase of OH and a last phase when a decrease is finally observed. In particular, the second stage is observed after 146 (R = 4) and 240 sec (R = 4) (Figures 3 d,f). Scheme 1. Catalytic cycle of TEOS hydrolysis operated by deuterated hydroxonium ions. The wide band peaking at 3450 cm1 results by different overlapped contributions, which have to be discussed in detail. In that region, the OH stretchHowever, it has to be kept in mind that the presence in soluing in water has two components, one of high intensity at tion of deuterated hydroxonium ions and other protic species 3300 cm1 (n1) and a second one at 3490 cm1 (n3, and overbrings a number of side reactions due to proton exchange occurring between deuterated species and water absorbed on tone of n2). The first one can be directly correlated with the abthe droplet surface once exposed to air moisture. The deutersorption of water from the external environment; the second ated silanols will then be involved in the condensation reacone of lower intensity overlaps completely with the OH band tion, giving rise to the silica network and releasing more deudue to silanols. The 3300 cm1 gives rise to a shoulder (see terated water [Eq. (5)]: Supporting Information, Figure S3, where the details of the evaporating spectra are reported) and produces an enlargeSiOD þ DOSi ! SiOSi þ D2 O ð5Þ ment of the 3450 cm1 main band as the evaporation proceeds. Another contribution is given by the HOD species at At the same time, as we have seen in the previous para3397 cm1, which progressively form with evaporation graphs, with the progress of evaporation, water is gradually (Figure 1). This is completely overlapped with the main OH viabsorbed and several intermediate species will be formed, inbration at 3450 cm1 but its contribution is negligible when cluding HOD through the exchange of protons with D2O. The rise of the wide infrared band around 3450 cm1, due to vibrations of hydroxyl species, allows following the progress of the hydrolysis during evaporation. Figures 3 a,c,e shows the 3D FTIR time-resolved absorption spectra (absorbance–wavenumber–time) of the three different series of evaporation experiments in the 3710–3015 cm1 range. The absorbance intensity is represented by a false grey scale. The evaporation of the sol with a sub-stoichiometric amount of deuterated water (R = 1) shows a progressive increase of OH species (Figure 3 a) up to a maximum at 540 s (Figure 3 b), after which the OH content decreases. The other two sols, which have stoichiometric and supra-stoichiometric amounts of D2O, show instead a different trend, in accordance with that observed for water absorption. The OH content of Figure 3. 3D ATR-FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 3710–3015 cm1 these sols, in fact, evolves in difrange, of the three of evaporation experiments corresponding to R = 1 (a), 4 (c) and 8 (e); the absorbance intensity ferent well-defined stages (Figur- is represented by a false grey scale. The corresponding changes of absorption maxima as a function of time are es 3 c,e): a first fast increase, also shown (b, d, and f, respectively).
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Articles taking into account the low intensity and the trend. In fact, if we observe the increase in intensity of the HOD band at lower wavenumbers, around 1450 cm1, this appears negligible in comparison to the increase of the OH stretching band (see Supporting Information, Figure S4). The main band at 3450 cm1 follows a similar general trend in all the samples: an increase in intensity up to a maximum and a following decrease. This process shows, however, a discontinuity at higher R ratios. This band is assigned to OD and OH stretching in SiOH and SiOD species that form upon reaction of TEOS with D2O in the sol or H2O, which is absorbed during evaporation. As condensation proceeds, the band decreases in intensity and the silica network is formed through the reaction shown by Equation (5), which also includes intermediate species.
Figure 4. 3D ATR-FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1195–963 cm1 range, of the three of evaporation experiments corresponding to R = 1 (a), 4 (c) and 8 (e); the absorbance intensity is represented by a false grey scale. The corresponding changes of absorption maxima as a function of time are also shown (b, d, and f, respectively).
2.4. Silica Condensation The time–wavenumber spectra Figures 4 b,d,f show that, in the sample with sub-stoichiometric water, there is a wavenumber shift from 1025 to 1010 cm1, while, in the other samples, this is much larger, from 1045 to 1010 cm1. The curves show a similar trend and again it is possible to identify different stages of silica condensation during the evaporation.
To get a full overview of the silica sol evaporation process is important to follow also the formation of the silica network through condensation. The silica species have three main vibrational modes in the mid-infrared range. The most intense band is detected around 1100 cm1 and is assigned to the Si OSi symmetric stretching mode. This band has a shoulder at a higher wavenumber (1135 cm1), which is due to a broadened signature of the LO component of the TO3 antisymmetric stretching mode.[24] The progress of silica condensation produces an increase in intensity and a wavenumber shift. The evolution of this band gives therefore a direct clue of the relative degree of condensation in the silica network. Figures 4 a,c,e shows the 3D FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1195–963 cm1 interval, of the three different series of evaporation experiments. The absorbance intensity is represented by a false grey scale. The spectra clearly show the increase in intensity and the shift to lower wavenumbers of the silica stretching band, which is the signature of the formation of small oligomeric silica species. Even if this is counterintuitive because, in general, a strengthening of the network should be reflected in a wavenumber increase, this is a peculiar response of silica sol–gel materials (see Ref. [24] for a detailed discussion) that in the first stage of condensation are characterised by signals at lower wavenumbers. ChemPhysChem 0000, 00, 0 – 0
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2.5. From Sol to Gel during Solvent Evaporation The analysis of the time-resolved infrared spectra has shown that the chemical–physical conditions during evaporation undergo well-defined changes, which allow the identification of four different stages in the process. Under the present experimental conditions, however, the timescale cannot be quantitatively considered, as the evaporation process is mainly controlled by the surface-to-volume ratio of the droplets and the water exchange and evaporation processes occur at the interface between the droplet and the atmosphere. Nevertheless we can comparatively discuss the kinetics of the four stages of evaporation during sol-to-gel transition. In Figure 5 the overlapping time-dependent curves (time– absorbance and time–wavenumber) for the stoichiometric sample R = 4 (Figures 2 d, 3 d and 4 d) are shown. We have identified four different stages during the silica sol evaporation, which are shown using the grey-scale areas in the figure. 5
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Articles tion of an extended network. At this stage the sol reaches the xerogel phase although the hydrolysis and condensation will continue at different rates in the next stages.
3. Conclusions The combination of chemical and analytical techniques, designed to study in situ evaporating systems, has been shown to be very effective to investigate the different chemical–physical phenomena involved in a fast-evaporating sol. Using deuterated water has allowed following the absorption of water from external environment in the different stages of evaporation. Four different phases in the process have been identified. These are characterized by a change in hydroxyl and condensation rates which in turn affect the water absorption. After a first fast hydrolysis, a critical amount of silanol is reached, which induces the condensation of silica; the sol-to-gel transition occurs during the second evaporation stage when a decrease in hydroxyl content, a fast rise of condensation rate and an almost stationary absorption of water are observed.
Figure 5. Time-dependent curves (time–absorbance) of water (black dots) and OH (light grey circles) superimposed with the time dependent curve (time–wavenumber) of the SiOSi antisymmetric stretching (grey triangles). The three curves are referred to the stoichiometric sol R = 4. The lines are guides for the eyes.
The first stage, which is quite fast, is due to the hydrolysis of TEOS.[25] In this phase the condensation is very little, while the hydroxyl content quickly rises together with absorbed water. The formation of hydroxyls changes the nature of the liquid, which loses the hydrophobicity given by unreacted TEOS. The hydrolysis step is followed by a second short stage characterised by a decrease in hydroxyl content, a fast rise of condensation rate, while water absorption is stationary. These features clearly indicate that silica condensation starts quite abruptly only when a critical amount of silanol is formed. The sudden condensation of the silanols releases water (in this case deuterated water) which is available in excess at this stage. The depletion of silanols due to condensation also reduces the absorption of water from the external environment. After this critical stage, which marks the evaporation process, there is a third phase where an increase in silanols and absorption of water are observed once more, but at a lower rate with respect to phase 1 because silica condensation is also proceeding. Water absorption and silanol formation reach a maximum at the end of this stage. The fourth stage is characterized by the final evaporation phase; the amount of absorbed water decreases, the amount of hydroxyls and condensation rate is small and a gel is finally formed, as experimentally observed. A similar response, with different kinetics, is observed in the sample containing a supra-stoichiometric amount of water, but not when water is present in sub-stoichiometric conditions. In general, a complete hydrolysis requires an excess of water. If the water amount is low, the critical amount of silanols capable of triggering a fast polycondensation is difficult to reach. In this case the sol evaporation leads to a smaller extent of silica condensation. In the different evaporation stages the hydrolysis and condensation reactions are not separated in time and take place simultaneously. However, they do have different reaction rates that mark the evaporation stages. These data show that the critical evaporation stage occurs after the initial hydrolysis of TEOS and the sol-to-gel transition takes place within this short time range. The formation of a critical amount of hydroxyls triggers the silica condensation which can lead to the forma-
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Experimental Section Chemicals Deuterium oxide (D2O) (Aldrich, 99.9 % atom D), tetraethylorthosilicate (TEOS) (ABCR, 99 %) and silicon tetrachloride (SiCl4) (Aldrich, 99.998 %) were used as received without further purification.
Silica Sol Preparation All the reagents were sampled under inert atmosphere from newly opened bottles of chemical reagents using a bag full of nitrogen or nitrogen balloons. A stock solution of “acidic TEOS” was prepared by adding 100 mL of SiCl4 to 15 mL of TEOS under nitrogen. 3 mL of the stock solution (2.8 g, 0.01344 mmol) were sampled and transferred under nitrogen into a vial equipped with a magnetic stirrer. Deuterium oxide was sampled under nitrogen with a syringe and then added in different amounts (0.25, 1 and 2 mL) to the TEOS solution containing 20 mL of SiCl4 in order to have a molar ratio, R = D2O:TEOS, equal to 1, 4 and 8, respectively. The hydrolysis reaction took places exothermically soon after the addition of deuterium oxide to the acidic TEOS solution.
Evaporation Experiments The evaporation experiments were carried out using the attenuated total reflection (ATR) infrared analysis that was performed with an interferometer Bruker infrared Vertex 70v equipped with a Platinum ATR attachment. The experiments were performed at a temperature of 26 0.5 8C and a relative humidity (RH) of 37 %. Each analysis was performed by depositing 1 mL of sol directly onto the diamond of the ATR attachment and the acquisition was started 1 second after the deposition. The spectra were acquired with a resolution of 4 cm1, averaging 16 scans per spectrum and with a spectrum delay of 1 second between two consecutive acquisitions. The background was recorded in air and 100 spectra were acquired for each experiment to follow the full evaporation–condensation profile. The results were analysed with the Bruker Opus
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[10] P. Innocenzi, T. Kidchob, L. Malfatti, S. Costacurta, M. Takahashi, M. Piccinini, A. Marcelli, J. Sol – Gel Sci. Technol. 2008, 48, 253 – 259. [11] P. Innocenzi, L. Malfatti, M. Piccinini, D. Sali, U. Schade, A. Marcelli, J. Phys. Chem. A 2009, 113, 9418 – 9423. [12] P. Innocenzi, L. Malfatti, T. Kidchob, P. Falcaro, M. C. Guidi, M. Piccinini, A. Marcelli, Chem. Commun. 2005, 2384 – 2386. [13] P. Innocenzi, T. Kidchob, J. M. Bertolo, M. Piccinini, M. C. Guidi, A. Marcelli, J. Phys. Chem. B 2006, 110, 10837 – 10841. [14] P. Falcaro, S. Costacurta, G. Mattei, H. Amenitsch, A. Marcelli, M. C. Guidi, M. Piccinini, A. Nucara, L. Malfatti, T. Kidchob, P. Innocenzi, J. Am. Chem. Soc. 2005, 127, 3838 – 3846. [15] V. R. Koganti, S. Das, S. E. Rankin, J. Phys. Chem. C 2014, 118, 19450 – 19461. [16] D. A. Doshi, A. Gibaud, V. Goletto, M. Lu, H. Gerung, B. Ocko, S. M. Han, C. J. Brinker, J. Am. Chem. Soc. 2003, 125, 11646 – 11655. [17] H. De Paz-Simon, A. Chemtob, C. Croutx-Barghorn, D. Le Nouen, S. Rigolet, J. Phys. Chem. B 2012, 116, 5260 – 5268. [18] P. Innocenzi, C. Figus, M. Takahashi, M. Piccinini, L. Malfatti, J. Phys. Chem. A 2011, 115, 10438 – 10444. [19] P. Innocenzi, L. Malfatti, M. Piccinini, A. Marcelli, J. Phys. Chem. A 2010, 114, 304 – 308. [20] H. De Paz-Simon, A. Chemtob, C. Croutx-Barghorn, S. Rigolet, L. Michelin, L. Vidal, B. Lebeau, Langmuir 2013, 29, 1963 – 1969. [21] P. Innocenzi, L. Malfatti, T. Kidchob, S. Costacurta, P. Falcaro, M. Piccinini, A. Marcelli, P. Morini, D. Sali, H. Amenitsch, J. Phys. Chem. C 2007, 111, 5345 – 5350. [22] B. Auer, R. Kumar, J. R. Schmidt, J. L. Skinner, Proc. Natl. Acad. Sci. USA 2007, 104, 14215 – 14220. [23] J. C. Dek, S. T. Rhea, L. K. Iwaki, D. D. Dlott, J. Phys. Chem. A 2000, 104, 4866 – 4875. [24] P. Innocenzi, J. Non-Cryst. Solids 2003, 316, 309 – 319. [25] F. Rubio, J. Rubio, J. L. Oteo, Spectrosc. Lett. 1998, 31, 199 – 219.
Acknowledgements D.C. acknowledges the financial support by the Sardinian Regional Government (P.O.R. SARDEGNA F.S.E. 2007–2013 -Obiettivo competitivit regionale e occupazione, Asse IV Capitale umano, Linea di Attivit l.3.1.). S. Iola is gratefully acknowledged for technical support. M.T. acknowledges the financial support by JSPS “Brain Circulation” Project (R2507) and University of Sassari “visiting scientist program”. Keywords: absorption · infrared spectroscopy · silica · sol–gel transitions · time-resolved analysis [1] C. J. Brinker, G. W. Scherer, Sol – Gel Science: The Physics and Chemistry of Sol – Gel Processing, Academic Press, San Diego, 1990. [2] L. L. Hench, J. K. West, Chem. Rev. 1990, 90, 33 – 72. [3] M. Faustini, C. Boissire, L. Nicole, D. Grosso, Chem. Mater. 2014, 26, 709 – 723. [4] P. Innocenzi, L. Malfatti, S. Costacurta, T. Kidchob, M. Piccinini, A. Marcelli, J. Phys. Chem. A 2008, 112, 6512 – 6516. [5] P. Innocenzi, L. Malfatti, M. Piccinini, A. Marcelli, D. Grosso, J. Phys. Chem. A 2009, 113, 2745 – 2749. [6] M. H. Huang, H. M. Soyez, B. S. Dunn, J. I. Zink, Chem. Mater. 2000, 12, 231 – 235. [7] R. Gupta, N. K. Chaudhury, J. Sol – Gel Sci. Technol. 2009, 49, 78 – 87. [8] G. L. G. Goring, J. D. Brennan, J. Mater. Chem. 2002, 12, 3400 – 3406. [9] F. Cagnol, D. Grosso, G. Soler-Illia, E. L. Crepaldi, F. Babonneau, H. Amenitsch, C. Sanchez, J. Mater. Chem. 2003, 13, 61 – 66.
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ARTICLES P. Innocenzi,* L. Malfatti, D. Carboni, M. Takahashi
A new view: Attenuated total reflection infrared spectroscopy on an evaporating droplet allows a direct in situ observation of a sol-to-gel transition in a fastevaporating system.
&& – && Sol-to-Gel Transition in Fast Evaporating Systems Observed by in Situ Time-Resolved Infrared Spectroscopy
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Sol-to-Gel Transition in Fast Evaporating Systems Observed by in Situ Time-Resolved Infrared Spectroscopy Plinio Innocenzi,*[a] Luca Malfatti,[a] Davide Carboni,[a] and Masahide Takahashi[b] The in situ observation of a sol-to-gel transition in fast evaporating systems is a challenging task and the lack of a suitable experimental design, which includes the chemistry and the analytical method, has limited the observations. We synthesise an acidic sol, employing only tetraethylorthosilicate, SiCl4 as catalyst and deuterated water; the absence of water added to the sol allows us to follow the absorption from the external environment and the evaporation of deuterated water. The timeresolved data, obtained by attenuated total reflection infrared
spectroscopy on an evaporating droplet, enables us to identify four different stages during evaporation. They are linked to specific hydrolysis and condensation rates that affect the uptake of water from external environment. The second stage is characterized by a decrease in hydroxyl content, a fast rise of condensation rate and an almost stationary absorption of water. This stage has been associated with the sol-to-gel transition.
1. Introduction The sol-to-gel transition is a chemical–physical transformation of a colloidal solution (sol) into a three-dimensional continuous network that encloses a liquid phase (gel).[1] The formation of the continuous network marks the gel point while the gelation time spans from hours to days as a function of the pH of the sol. The sol-to-gel transition is not a thermodynamic event such as the glass transition and its exact determination is difficult to achieve. As underlined by Hench and West[2] “the gelation point of any system, including sol–gel silica, is easy to observe qualitatively and easy to define in abstract terms but extremely difficult to measure analytically”. This is particularly true in sol–gel films because the kinetics of the different chemical–physical phenomena is very fast.[3] The environment plays also a crucial role because the relative humidity in the deposition chamber influences the evaporation process in quite a complex way. The in situ analysis of evaporating droplets of alcohol[4] and water[5] has shown, in fact, that the absorption of water from the external environment has a key role in the process. This also occurs in the deposition of a film from a liquid phase which undergoes through the sol-to-gel transition during processing. The lack of suitable analytical techniques has not allowed up to now a direct investigation of the overall phenomenon. Several different methods have been applied so
far, mostly using fluorescent probes, but only a partial understanding of the process has been reached.[6–8] An attempt to correlate the water content with the formation of a mesostructured gel has been made using Karl Fisher titration.[9] An alternative technique is in situ time-resolved infrared spectroscopy, which can be applied to different types of evaporation processes.[10] The technique is relatively simple even if data analysis has to be carried out with attention. The experiment can be performed by attenuated total reflection (ATR) infrared analysis or with an infrared microscope. This analytical method has been applied to ethanol[4] and water droplets,[5, 11] mesoporous films,[12–16] sol–gel films,[17] hybrid materials[18] and tri-block copolymers.[19, 20] The technique has been also combined with small angle X-ray scattering (SAXS) for simultaneous time-resolved analysis of self-assembly and evaporation phenomena in mesoporous ordered films.[21] These experiments have shown that evaporative phenomena at the time scale of seconds can be followed in detail by time-resolved infrared spectroscopy. In situ analysis has allowed understanding the details of evaporation of water and ethanol as a function of relative humidity in the evaporation chamber. These data have been used for a better comprehension of self-assembly, especially by combining in situ time-resolved FTIR with other techniques (SAXS), which allow getting a direct observation of micelles organization. In the present work we have applied time-resolved infrared analysis to a silica sol droplet with the purpose of getting a better insight into the sol-to-gel transition in fast evaporating systems. In general, the chemistry of the precursor sol, which means the pH, the concentration of the silicon alkoxide and the relative amount of water, control the properties of the final material. If the silica sol is used to deposit thin films, the environment in the deposition chamber, the temperature and relative humidity are the parameters to be controlled. On the
[a] Prof. Dr. P. Innocenzi, Dr. L. Malfatti, Dr. D. Carboni Laboratorio di Scienza dei Materiali e Nanotecnologie DADU, Universit di Sassari, Palazzo Pou Salid Piazza Duomo 6, 07041 Alghero (Italy) E-mail: [email protected] [b] Prof. Dr. M. Takahashi Department of Materials Science Graduate School of Engineering and International Laboratory of Materials Science and Nanotechnology (iLMNT) Osaka Prefecture University, Sakai, Osaka 599-8531 (Japan) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500126.
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Articles other hand the kinetics of the process, which depends on the withdrawal speed in the case of dip-coating or rotation rate for spin-coating, are also of paramount importance. This means that even in the simplest case, such as a silica sol obtained by tetraethylorthosilicate (TEOS) and water, the sol-togel transition reaches a high level of complexity, which hampers a real understanding of the overall process. In particular, as we have previously underlined, the atmospheric moisture plays an important role and in situ studies are quite difficult because of the limited techniques available that would allow separating the different chemical–physical phenomena during sol evaporation. For these reasons, we have designed an experiment employing deuterated water and TEOS to acquire a better understanding of the processes behind the evaporation of a silica sol and to discriminate between reacting and absorbing water. This has allowed for the first time, to our knowledge, obtaining a direct observation of a sol-to-gel transition in a fast evaporating system.
sorption bands at 1637 cm1 (bending, n2); 2123 cm1 (combination of n2 + libration mode, L2) and 3300 cm1 (n1, symmetric stretching with an overlapped mode around 3490 cm1 due to antisymmetric stretching, v3, and the overtone of n2). The spectrum of deuterated water in the same range is characterized by three bands due to the same vibrations of water, but shifted to lower energies and detected at 1209 (n2), 1547 (n2 + L2) and 2472 cm1 (n1, n3, n2 overtone). Despite showing the same symmetry, D2O has a distinct mass with respect to water, because of the different atomic weight of the two deuterium atoms. This leads to a modified value of reduced mass (m) in the Hamiltonian which describes the vibration of the molecule, that is, a different energy value for the characteristic infrared vibration modes. During evaporation of a D2O droplet, water is also absorbed and an exchange of protons to form HOD molecules will occur [Eq. (1)]:
2. Results and Discussion
The amount of HOD species that are formed increases with the time of evaporation, as we have observed following the process in situ (Figure S1, Supporting Information). The reference infrared spectrum of HOD[22] is also shown in Figure 1, with the three main absorption bands at 1450, 2472 and 3397 cm1; the band at 2472 cm1 overlaps with that of D2O while the band at 3397 cm1 overlaps with that of water.[23] As a first step to understand the details of sol-to-gel transition in a silica sol, we have followed the evaporation process of a pure TEOS droplet in an open chamber, measuring the actual relative humidity (RH) value, and the results are shown in Figure S2. The TEOS droplet evaporates without absorbing water from the external environment and no signs of hydrolysis and condensation were detected in the FTIR spectra. In absence of any catalyst, in fact, the atmospheric water is unable to trigger the hydrolysis of the alkoxide and therefore TEOS cannot undergo any polycondensation reaction. At the end of the process TEOS has been completely evaporated and no residues have been observed on the ATR diamond substrate.
H2 O þ D2 O ! 2 HOD
Deuterium oxide and TEOS form a biphasic mixture which needs a catalyst to initiate the hydrolysis reaction. To avoid the addition of an acidic source containing external protons, a chemical reagent capable of generating in situ the soughtafter acidity has been used. We have selected SiCl4 that reacts with D2O to form deuterated hydrochloric acid, which in turn creates a slightly acidic environment capable of catalysing the hydrolysis and condensation reactions with an in situ generated source of D + . The advantage of this approach is that no external H2O is added to the precursor sol and this allows following by FTIR the absorption of H2O from the external environment and D2O evaporation from the sol. 2.1. Reference Spectra In Figure 1 the reference infrared absorption spectra, in the 3800–1000 cm1 range, of liquid water, deuterated water and HOD are shown. The spectrum of water shows three main ab-
ð1Þ
2.2. Water Absorption during Evaporation We have designed the experiments with the specific purpose of understanding in more detail the chemical–physical processes occurring during the evaporation of a silica sol. As said above, we have used deuterated water instead of water in the precursor sol; this has enabled following the interaction of the sol with the external environment during evaporation. In particular, it has been possible to identify the contribution of water absorbed from the external environment since the infrared vibration of deuterated water is shifted at much lower wavenumbers with respect to water. The water absorption from the atmosphere can be revealed by the infrared band peaking at 1637 cm1. This band does not overlap with other vibrational modes and gives a direct indication of the water exchange during the absorption–evaporation process. The experiments have been carried out with silica sols prepared by using the three different sols with D2O/
Figure 1. ATR-FTIR reference spectra of water (c), deuterated water (a) and partially deuterated water (g) in the range 3800–1000 cm1.
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Articles is then followed by a third phase where the absorption increases at a slower rate and a fourth one where absorption finally decreases with the completion of the sol evaporation. A similar trend is observed during the evaporation of the sol with a supra-stoichiometric amount of D2O (R = 8), (Figure 2 e). The change in the absorption maxima shows also four steps, with similar stages with respect to the sol with stoichiometric D2O (Figure 2 f).
2.3. Formation of Hydroxyls With the progress of evaporation TEOS goes through a progressive hydrolysis and condensation. To avoid the addition of water (vide supra) we have used SiCl4 to catalyse the acidic hydrolysis of TEOS [Eq. (2)]: Figure 2. 3D ATR-FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1725–1575 cm1 range, of the three of evaporation experiments corresponding to R = 1 (a), 4 (c) and 8 (e); the absorbance intensity at 1637 cm1 is represented by a false grey scale. The corresponding changes of absorption maxima as a function of time are also shown (b, d, and f, respectively).
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where the deuterated DCl acts as the catalyst of the hydrolysis and condensation of TEOS with deuterated water. Although formally written as DCl, we have to remember that, in aqueous solution, this molecule mostly exists in the hydrated form, that is, the deuterated hydroxonium ion and its counter ion according to the equilibrium given by Equation (3):
TEOS ratios (R) equal to 1, 4 and 8, while the RH was constant at 37 % in all the evaporation runs. The evaporation process has been followed until no further changes could be observed in the FTIR spectra and a stable silica gel formed as final product. Figures 2 a,c,e shows the 3D FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1725– 1575 cm1 range, of the three different series of evaporation experiments. The absorbance intensity is represented by a false grey scale. In Figures 2 b,d,f the corresponding changes of absorption maxima at 1637 cm1 as a function of time are shown. During the evaporation of the sol with a sub-stoichiometric amount of deuterated water (R = 1), a small absorption of water (Figure 2 a), which follows an asymptotic trend (Figure 2 b), is observed. The gel, which is formed at the end of the evaporation process, retains the absorbed water and no further changes are observed in the spectra. The trend, in the silica sol containing a stoichiometric (R = 4) amount of D2O (Figure 2 c), changes. In this case water is absorbed in a larger amount and reaches a maximum which then decreases as the evaporation proceeds. This sol presents another peculiarity in the absorption of water, as the curve obtained from the absorption maxima shows the presence of different evaporation stages (Figure 2 d). There is a first fast absorption of water; after this stage the absorption stops for few seconds. This step ChemPhysChem 0000, 00, 0 – 0
SiCl4 þ 4 D2 O ! SiðODÞ4 þ 4 DCl
D2 O þ DCl Ð D3 Oþ þ Cl
ð3Þ
As expected from an acid-catalysed hydrolysis, the first step is the protonation of the ethoxy groups of TEOS by the deuterated hydroxonium ion (D3O + ), which is the true catalytic species in solution. This weakens the bond between the alkoxy group and the silicon, leaving the deuterated water free to attack the silicon centre. The nucleophilic attack thus results in a substitution of a deuterated ethanol with a molecule of deuterated water, giving rise to a protonated silanol, Si+ OD2 (Scheme 1). This silanol further reacts with a molecule of D2O to restore the deuterated hydroxonium ion, which is able to start a new catalytic cycle. The sequence of reactions involving a complete acidic hydrolysis of a TEOS molecule can be, for the sake of clarity, summarised as it follows: Si-ðOC2 H5 Þ4 þ 4 D2 O ! Si-ðODÞ4 þ 4 C2 H5 OD 3
ð4Þ
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Articles a second short phase when the rise of OH stops, a third stage with a further increase of OH and a last phase when a decrease is finally observed. In particular, the second stage is observed after 146 (R = 4) and 240 sec (R = 4) (Figures 3 d,f). Scheme 1. Catalytic cycle of TEOS hydrolysis operated by deuterated hydroxonium ions. The wide band peaking at 3450 cm1 results by different overlapped contributions, which have to be discussed in detail. In that region, the OH stretchHowever, it has to be kept in mind that the presence in soluing in water has two components, one of high intensity at tion of deuterated hydroxonium ions and other protic species 3300 cm1 (n1) and a second one at 3490 cm1 (n3, and overbrings a number of side reactions due to proton exchange occurring between deuterated species and water absorbed on tone of n2). The first one can be directly correlated with the abthe droplet surface once exposed to air moisture. The deutersorption of water from the external environment; the second ated silanols will then be involved in the condensation reacone of lower intensity overlaps completely with the OH band tion, giving rise to the silica network and releasing more deudue to silanols. The 3300 cm1 gives rise to a shoulder (see terated water [Eq. (5)]: Supporting Information, Figure S3, where the details of the evaporating spectra are reported) and produces an enlargeSiOD þ DOSi ! SiOSi þ D2 O ð5Þ ment of the 3450 cm1 main band as the evaporation proceeds. Another contribution is given by the HOD species at At the same time, as we have seen in the previous para3397 cm1, which progressively form with evaporation graphs, with the progress of evaporation, water is gradually (Figure 1). This is completely overlapped with the main OH viabsorbed and several intermediate species will be formed, inbration at 3450 cm1 but its contribution is negligible when cluding HOD through the exchange of protons with D2O. The rise of the wide infrared band around 3450 cm1, due to vibrations of hydroxyl species, allows following the progress of the hydrolysis during evaporation. Figures 3 a,c,e shows the 3D FTIR time-resolved absorption spectra (absorbance–wavenumber–time) of the three different series of evaporation experiments in the 3710–3015 cm1 range. The absorbance intensity is represented by a false grey scale. The evaporation of the sol with a sub-stoichiometric amount of deuterated water (R = 1) shows a progressive increase of OH species (Figure 3 a) up to a maximum at 540 s (Figure 3 b), after which the OH content decreases. The other two sols, which have stoichiometric and supra-stoichiometric amounts of D2O, show instead a different trend, in accordance with that observed for water absorption. The OH content of Figure 3. 3D ATR-FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 3710–3015 cm1 these sols, in fact, evolves in difrange, of the three of evaporation experiments corresponding to R = 1 (a), 4 (c) and 8 (e); the absorbance intensity ferent well-defined stages (Figur- is represented by a false grey scale. The corresponding changes of absorption maxima as a function of time are es 3 c,e): a first fast increase, also shown (b, d, and f, respectively).
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Articles taking into account the low intensity and the trend. In fact, if we observe the increase in intensity of the HOD band at lower wavenumbers, around 1450 cm1, this appears negligible in comparison to the increase of the OH stretching band (see Supporting Information, Figure S4). The main band at 3450 cm1 follows a similar general trend in all the samples: an increase in intensity up to a maximum and a following decrease. This process shows, however, a discontinuity at higher R ratios. This band is assigned to OD and OH stretching in SiOH and SiOD species that form upon reaction of TEOS with D2O in the sol or H2O, which is absorbed during evaporation. As condensation proceeds, the band decreases in intensity and the silica network is formed through the reaction shown by Equation (5), which also includes intermediate species.
Figure 4. 3D ATR-FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1195–963 cm1 range, of the three of evaporation experiments corresponding to R = 1 (a), 4 (c) and 8 (e); the absorbance intensity is represented by a false grey scale. The corresponding changes of absorption maxima as a function of time are also shown (b, d, and f, respectively).
2.4. Silica Condensation The time–wavenumber spectra Figures 4 b,d,f show that, in the sample with sub-stoichiometric water, there is a wavenumber shift from 1025 to 1010 cm1, while, in the other samples, this is much larger, from 1045 to 1010 cm1. The curves show a similar trend and again it is possible to identify different stages of silica condensation during the evaporation.
To get a full overview of the silica sol evaporation process is important to follow also the formation of the silica network through condensation. The silica species have three main vibrational modes in the mid-infrared range. The most intense band is detected around 1100 cm1 and is assigned to the Si OSi symmetric stretching mode. This band has a shoulder at a higher wavenumber (1135 cm1), which is due to a broadened signature of the LO component of the TO3 antisymmetric stretching mode.[24] The progress of silica condensation produces an increase in intensity and a wavenumber shift. The evolution of this band gives therefore a direct clue of the relative degree of condensation in the silica network. Figures 4 a,c,e shows the 3D FTIR time-resolved absorption spectra (absorbance–wavenumber–time), in the 1195–963 cm1 interval, of the three different series of evaporation experiments. The absorbance intensity is represented by a false grey scale. The spectra clearly show the increase in intensity and the shift to lower wavenumbers of the silica stretching band, which is the signature of the formation of small oligomeric silica species. Even if this is counterintuitive because, in general, a strengthening of the network should be reflected in a wavenumber increase, this is a peculiar response of silica sol–gel materials (see Ref. [24] for a detailed discussion) that in the first stage of condensation are characterised by signals at lower wavenumbers. ChemPhysChem 0000, 00, 0 – 0
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2.5. From Sol to Gel during Solvent Evaporation The analysis of the time-resolved infrared spectra has shown that the chemical–physical conditions during evaporation undergo well-defined changes, which allow the identification of four different stages in the process. Under the present experimental conditions, however, the timescale cannot be quantitatively considered, as the evaporation process is mainly controlled by the surface-to-volume ratio of the droplets and the water exchange and evaporation processes occur at the interface between the droplet and the atmosphere. Nevertheless we can comparatively discuss the kinetics of the four stages of evaporation during sol-to-gel transition. In Figure 5 the overlapping time-dependent curves (time– absorbance and time–wavenumber) for the stoichiometric sample R = 4 (Figures 2 d, 3 d and 4 d) are shown. We have identified four different stages during the silica sol evaporation, which are shown using the grey-scale areas in the figure. 5
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Articles tion of an extended network. At this stage the sol reaches the xerogel phase although the hydrolysis and condensation will continue at different rates in the next stages.
3. Conclusions The combination of chemical and analytical techniques, designed to study in situ evaporating systems, has been shown to be very effective to investigate the different chemical–physical phenomena involved in a fast-evaporating sol. Using deuterated water has allowed following the absorption of water from external environment in the different stages of evaporation. Four different phases in the process have been identified. These are characterized by a change in hydroxyl and condensation rates which in turn affect the water absorption. After a first fast hydrolysis, a critical amount of silanol is reached, which induces the condensation of silica; the sol-to-gel transition occurs during the second evaporation stage when a decrease in hydroxyl content, a fast rise of condensation rate and an almost stationary absorption of water are observed.
Figure 5. Time-dependent curves (time–absorbance) of water (black dots) and OH (light grey circles) superimposed with the time dependent curve (time–wavenumber) of the SiOSi antisymmetric stretching (grey triangles). The three curves are referred to the stoichiometric sol R = 4. The lines are guides for the eyes.
The first stage, which is quite fast, is due to the hydrolysis of TEOS.[25] In this phase the condensation is very little, while the hydroxyl content quickly rises together with absorbed water. The formation of hydroxyls changes the nature of the liquid, which loses the hydrophobicity given by unreacted TEOS. The hydrolysis step is followed by a second short stage characterised by a decrease in hydroxyl content, a fast rise of condensation rate, while water absorption is stationary. These features clearly indicate that silica condensation starts quite abruptly only when a critical amount of silanol is formed. The sudden condensation of the silanols releases water (in this case deuterated water) which is available in excess at this stage. The depletion of silanols due to condensation also reduces the absorption of water from the external environment. After this critical stage, which marks the evaporation process, there is a third phase where an increase in silanols and absorption of water are observed once more, but at a lower rate with respect to phase 1 because silica condensation is also proceeding. Water absorption and silanol formation reach a maximum at the end of this stage. The fourth stage is characterized by the final evaporation phase; the amount of absorbed water decreases, the amount of hydroxyls and condensation rate is small and a gel is finally formed, as experimentally observed. A similar response, with different kinetics, is observed in the sample containing a supra-stoichiometric amount of water, but not when water is present in sub-stoichiometric conditions. In general, a complete hydrolysis requires an excess of water. If the water amount is low, the critical amount of silanols capable of triggering a fast polycondensation is difficult to reach. In this case the sol evaporation leads to a smaller extent of silica condensation. In the different evaporation stages the hydrolysis and condensation reactions are not separated in time and take place simultaneously. However, they do have different reaction rates that mark the evaporation stages. These data show that the critical evaporation stage occurs after the initial hydrolysis of TEOS and the sol-to-gel transition takes place within this short time range. The formation of a critical amount of hydroxyls triggers the silica condensation which can lead to the forma-
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Experimental Section Chemicals Deuterium oxide (D2O) (Aldrich, 99.9 % atom D), tetraethylorthosilicate (TEOS) (ABCR, 99 %) and silicon tetrachloride (SiCl4) (Aldrich, 99.998 %) were used as received without further purification.
Silica Sol Preparation All the reagents were sampled under inert atmosphere from newly opened bottles of chemical reagents using a bag full of nitrogen or nitrogen balloons. A stock solution of “acidic TEOS” was prepared by adding 100 mL of SiCl4 to 15 mL of TEOS under nitrogen. 3 mL of the stock solution (2.8 g, 0.01344 mmol) were sampled and transferred under nitrogen into a vial equipped with a magnetic stirrer. Deuterium oxide was sampled under nitrogen with a syringe and then added in different amounts (0.25, 1 and 2 mL) to the TEOS solution containing 20 mL of SiCl4 in order to have a molar ratio, R = D2O:TEOS, equal to 1, 4 and 8, respectively. The hydrolysis reaction took places exothermically soon after the addition of deuterium oxide to the acidic TEOS solution.
Evaporation Experiments The evaporation experiments were carried out using the attenuated total reflection (ATR) infrared analysis that was performed with an interferometer Bruker infrared Vertex 70v equipped with a Platinum ATR attachment. The experiments were performed at a temperature of 26 0.5 8C and a relative humidity (RH) of 37 %. Each analysis was performed by depositing 1 mL of sol directly onto the diamond of the ATR attachment and the acquisition was started 1 second after the deposition. The spectra were acquired with a resolution of 4 cm1, averaging 16 scans per spectrum and with a spectrum delay of 1 second between two consecutive acquisitions. The background was recorded in air and 100 spectra were acquired for each experiment to follow the full evaporation–condensation profile. The results were analysed with the Bruker Opus
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[10] P. Innocenzi, T. Kidchob, L. Malfatti, S. Costacurta, M. Takahashi, M. Piccinini, A. Marcelli, J. Sol – Gel Sci. Technol. 2008, 48, 253 – 259. [11] P. Innocenzi, L. Malfatti, M. Piccinini, D. Sali, U. Schade, A. Marcelli, J. Phys. Chem. A 2009, 113, 9418 – 9423. [12] P. Innocenzi, L. Malfatti, T. Kidchob, P. Falcaro, M. C. Guidi, M. Piccinini, A. Marcelli, Chem. Commun. 2005, 2384 – 2386. [13] P. Innocenzi, T. Kidchob, J. M. Bertolo, M. Piccinini, M. C. Guidi, A. Marcelli, J. Phys. Chem. B 2006, 110, 10837 – 10841. [14] P. Falcaro, S. Costacurta, G. Mattei, H. Amenitsch, A. Marcelli, M. C. Guidi, M. Piccinini, A. Nucara, L. Malfatti, T. Kidchob, P. Innocenzi, J. Am. Chem. Soc. 2005, 127, 3838 – 3846. [15] V. R. Koganti, S. Das, S. E. Rankin, J. Phys. Chem. C 2014, 118, 19450 – 19461. [16] D. A. Doshi, A. Gibaud, V. Goletto, M. Lu, H. Gerung, B. Ocko, S. M. Han, C. J. Brinker, J. Am. Chem. Soc. 2003, 125, 11646 – 11655. [17] H. De Paz-Simon, A. Chemtob, C. Croutx-Barghorn, D. Le Nouen, S. Rigolet, J. Phys. Chem. B 2012, 116, 5260 – 5268. [18] P. Innocenzi, C. Figus, M. Takahashi, M. Piccinini, L. Malfatti, J. Phys. Chem. A 2011, 115, 10438 – 10444. [19] P. Innocenzi, L. Malfatti, M. Piccinini, A. Marcelli, J. Phys. Chem. A 2010, 114, 304 – 308. [20] H. De Paz-Simon, A. Chemtob, C. Croutx-Barghorn, S. Rigolet, L. Michelin, L. Vidal, B. Lebeau, Langmuir 2013, 29, 1963 – 1969. [21] P. Innocenzi, L. Malfatti, T. Kidchob, S. Costacurta, P. Falcaro, M. Piccinini, A. Marcelli, P. Morini, D. Sali, H. Amenitsch, J. Phys. Chem. C 2007, 111, 5345 – 5350. [22] B. Auer, R. Kumar, J. R. Schmidt, J. L. Skinner, Proc. Natl. Acad. Sci. USA 2007, 104, 14215 – 14220. [23] J. C. Dek, S. T. Rhea, L. K. Iwaki, D. D. Dlott, J. Phys. Chem. A 2000, 104, 4866 – 4875. [24] P. Innocenzi, J. Non-Cryst. Solids 2003, 316, 309 – 319. [25] F. Rubio, J. Rubio, J. L. Oteo, Spectrosc. Lett. 1998, 31, 199 – 219.
Acknowledgements D.C. acknowledges the financial support by the Sardinian Regional Government (P.O.R. SARDEGNA F.S.E. 2007–2013 -Obiettivo competitivit regionale e occupazione, Asse IV Capitale umano, Linea di Attivit l.3.1.). S. Iola is gratefully acknowledged for technical support. M.T. acknowledges the financial support by JSPS “Brain Circulation” Project (R2507) and University of Sassari “visiting scientist program”. Keywords: absorption · infrared spectroscopy · silica · sol–gel transitions · time-resolved analysis [1] C. J. Brinker, G. W. Scherer, Sol – Gel Science: The Physics and Chemistry of Sol – Gel Processing, Academic Press, San Diego, 1990. [2] L. L. Hench, J. K. West, Chem. Rev. 1990, 90, 33 – 72. [3] M. Faustini, C. Boissire, L. Nicole, D. Grosso, Chem. Mater. 2014, 26, 709 – 723. [4] P. Innocenzi, L. Malfatti, S. Costacurta, T. Kidchob, M. Piccinini, A. Marcelli, J. Phys. Chem. A 2008, 112, 6512 – 6516. [5] P. Innocenzi, L. Malfatti, M. Piccinini, A. Marcelli, D. Grosso, J. Phys. Chem. A 2009, 113, 2745 – 2749. [6] M. H. Huang, H. M. Soyez, B. S. Dunn, J. I. Zink, Chem. Mater. 2000, 12, 231 – 235. [7] R. Gupta, N. K. Chaudhury, J. Sol – Gel Sci. Technol. 2009, 49, 78 – 87. [8] G. L. G. Goring, J. D. Brennan, J. Mater. Chem. 2002, 12, 3400 – 3406. [9] F. Cagnol, D. Grosso, G. Soler-Illia, E. L. Crepaldi, F. Babonneau, H. Amenitsch, C. Sanchez, J. Mater. Chem. 2003, 13, 61 – 66.
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Received: February 13, 2015 Published online on && &&, 2015
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ARTICLES P. Innocenzi,* L. Malfatti, D. Carboni, M. Takahashi
A new view: Attenuated total reflection infrared spectroscopy on an evaporating droplet allows a direct in situ observation of a sol-to-gel transition in a fastevaporating system.
&& – && Sol-to-Gel Transition in Fast Evaporating Systems Observed by in Situ Time-Resolved Infrared Spectroscopy
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