Langmuir 2000, 16, 5831-5836
5831
Solubilization of Oil in Silicate-Surfactant Mesostructures Mika Linde´n,* Patrik A° gren, and Stefan Karlsson Department of Physical Chemistry, A° bo Akademi University, Porthansgatan 3-5, FIN-20500, Finland
Patrick Bussian Max-Planck Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm Platz 1, D-45470, Mu¨ lheim an der Ruhr, Germany
Heinz Amenitsch Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17/VI, A-8010 Graz, Austria Received December 22, 1999. In Final Form: March 24, 2000 The solubilization of n-hexane and toluene by silicate-surfactant hexagonal mesophases has been studied by time-resolved in situ synchrotron small-angle X-ray scattering. It is suggested that the extent of solubilization is mainly governed by the oil solubility in the continuous phase. The availability of the oil will largely influence the phase behavior and the long-range order of the silicatropic mesophase. While the addition of n-hexane to the synthesis results in a loss of order, a substantial amount of toluene can be solubilized without affecting the long-range order. Many of the observed effects can be rationalized based on time-dependent changes in the properties of the system due to silicate hydrolysis and condensation reactions, in addition to the water solubility of the solubilizate. The results are compared with those obtained for corresponding systems in the absence of silica.
Introduction Silicate-surfactant composite structures that possess mesoscopic order have attracted a great amount of interest since the pioneering work of the Mobil Oil group published in 1992.1,2 The similarity between silicatropic mesostructures and liquid crystalline phases observed in aqueous surfactant systems suggests that the silicate organizes around arrays of surfactant molecules. This is opposite to zeolite structures, where single molecules serve as structure-directing agents. MCM-41, a member of the M41S family of materials, contains regular, two-dimensional, hexagonal arrangement of pores. It is synthesized in the presence of positively charged surfactant, typically CnTAB (n ) 8-18), at elevated pH, where the silicate species are negatively charged. As noted by several authors, the synthesis can be carried out at surfactant concentrations well below those needed for surfactant liquid crystals to form. Normally, the surfactant concentration is even below those needed for rodlike micelles to be present in the surfactant solution. Stucky and coworkers3,4 concluded that the mesophase formation is a cooperative self-assembly process where multidentate binding of silicate oligomers, preferred polymerization of silicate at the surfactant-silicate interface, and charge * To whom correspondence should be addressed (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (4) Huo, Q.; Margolese, D.; Ciesla, U.; Demuth, D.; Feng, P.; Gier, T.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176.
density matching are key factors in the formation of a surfactant-silicate mesophase. Recently, Zana and coworkers5 confirmed that some degree of prepolymerization of silica seems to be a prerequisite for preferential adsorption to the positively charged micellar interface to occur. Association of oligomeric/polymeric silica and surfactant induces a sphere-to-rod transition of the micelles, a necessary step in the formation of these materials at low surfactant concentrations. It has been suggested that the final structure of the mesophase be determined by the interplay of forces acting in the inorganic-inorganic, inorganic-organic, and organic-organic interfacial regions. While early stages of mesophase formation are determined mainly by the surfactant properties, further structural changes to lower energy conformations may be induced as changes in the charge density of the silicate occurs upon silicate condensation. More striking evidence for the lyotropic nature of the silicate-surfactant mesophase is the ability to solubilize auxiliary organics within the hydrophobic regions of the surfactant aggregates.1,2,4 Usually aromatic oils, like trimethylbenzene, have been used for this purpose, although saturated hydrocarbons have been used as well. It has even been proposed that there is a correlation between the degree of swelling of the silicatropic mesophase and the hydrocarbon chain length when n-alkanes are used as the swelling agent.6 However, dilute, aqueous solutions of CTAB alone cannot solubilize large amounts of oil, due to the preferred high curvature of the charged interface of the spherical micelles. Thus, the equilibrium solubilities of toluene and n-hexane in aqueous C16TAB solutions are only 0.20 g and 0.05 g per gram of surfactant, respectively.7 The pronounced (5) Zana, R.; Frasch, J.; Soulard, M.; Lebeau, B.; Patarin, J. Langmuir 1999, 15, 2603. (6) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759.
10.1021/la991671h CCC: $19.00 © 2000 American Chemical Society Published on Web 05/24/2000
5832
Langmuir, Vol. 16, No. 13, 2000
swelling of the silicatropic liquid crystalline mesophase synthesized at low surfactant/water ratios can therefore not be rationalized based on equilibrium solubilization properties of isotropic surfactant solutions. For this reason, we wanted to examine the oil solubilization in silicatropic liquid crystalline mesophases by utilizing in situ smallangle X-ray scattering. This technique has proven to be a powerful tool for high-resolution investigations of timedependent phase behavior of similar systems.8 Here it was shown that, for the synthesis composition used in this study, a hexagonal mesophase with an initial s value of 0.205 nm-1 (s ) 1/d) forms within 80 s without going through any intermediate phases. The hexagonal structure contracts during the first minutes but reaches a steady value of 0.227 nm-1 after approximately 7 min. Since this synthesis is a rapid room-temperature synthesis, it was found to be most suitable for the experimental setup used in the present study. Experimental Section Materials. The following chemicals were used in the synthesis: Ammonia (25%) was supplied by Merck. Hexadecyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), n-hexane (99%), and toluene (>99.5%) were obtained from Aldrich. All the above-mentioned chemicals were used as received. The water was purified by distillation and deionization. Synthesis. CTAB (m ) 2.4 g) was dissolved in water (m ) 120 g) by stirring. After complete dissolution of the CTAB, hexane or toluene and ammonia were added with stirring at 400 rpm for 5 min. The system was allowed to phase separate for 5 min after which the TEOS (m ) 9.4 g) was cautiously added to the separated oil phase in the reaction vessel in order to avoid initial mixing with the aqueous solution. The final molar composition was 157/ 3/0.15/1/x H2O/NH3/C16TAB/TEOS/auxiliary organics. The reaction was initiated by turning on the magnetic stirrer (400 rpm). Batch Reactor. The reactor system used has been described in detail in ref 8. The setup consisted of a beaker containing the synthesis mixture, a peristaltic pump, tubing, and a quartz capillary. Homogenizing the two-phase system with stirring started the reaction, and at the same time the pumping was started. The emulsified synthesis solution was pumped through a quartz capillary (1 mm diameter) at a pumping rate of 25 mL/ min. The synthesis mixture flowing through the capillary was recycled back to the beaker. The experiments were carried out at room temperature. X-ray Analysis. The quartz capillary was focused in the X-ray beam. The small-angle X-ray scattering (SAXS) measurements were performed in an s range from 0.035 to 0.85 nm-1, with an exposure time down to 0.3 s, at the Austrian high flux SAXS beam-line of the 2 GeV electron storage ring ELETTRA, Trieste, Italy.9 The radiation wavelength was 1.542 Å. A linear positionsensitive Gabriel detector was used, which enabled simultaneous detection of the whole angular range studied.
Linde´ n et al.
Figure 1. Time-resolved in situ synchrotron X-ray diffraction (XRD) pattern of the development of the mesoscopic order in the presence of n-hexane. n-Hexane/CTAB mass ratio mhexane/ mCTAB ) 1.68,. The intensities between s ) 0.3 nm-1 and s ) 0.6 nm-1 have been multiplied by 3 for clarity.
Results Hexane Added. The time-resolved evolution of the diffractogram for mhexane/mCTAB ) 1.68 is shown in Figure 1. At short reaction times, only diffuse scattering from micelles was observed. However, after about 120 s a reflection at s ) 0.157 nm-1 (H′) was observed followed by a second reflection with a s value of 0.193 nm-1 (H′′) at t ) 140 s. Soon after the appearance of this second low-angle reflection, H′′, high-order reflections appeared, which could be indexed assuming a two-dimensional hexagonal symmetry, characteristic for MCM-41. The inten(7) Dam, Th.; Engberts, J. B. N. F.; Ka¨rtha¨user, J.; Karaborni, S.; van Os, N. M. Colloids Surf., A 1996, 118, 41. (8) A° gren P.; Linde´n, M.; Rosenholm, J. B.; Schwarzenbacher, R.; Kriechbaum ,M.; Amenitsch, H.; Laggner, P.; Blanchard, J.; Schu¨th, F. J. Phys. Chem. 1999, 103, 5943. (9) Amenitsch, H.; Bernstorff, S.; Laggner, P. Rev. Sci. Instrum. 1995, 66, 1624.
Figure 2. In situ XRD patterns measured after a reaction time of 600 s as a function of added amount of n-hexane: (a) mhexane/mCTAB ) 1.6; (b) mhexane/mCTAB ) 2.48; (c) mhexane/mCTAB ) 3.35.
sities of all reflections increased with time. However, the structures contracted with time, and the corresponding s-values at t ) 600 s were 0.161 and 0.202 nm-1, respectively. If the amount of added hexane was increased, the time at which the reflections appeared in the diffractogram increased slightly. More swollen, but still bimodal, structures were observed with increasing hexane concentration, where the intensity of the larger structure increased continuously with oil increasing concentration while the intensity of the smaller structure decreased.
Solubilization of Oil
Langmuir, Vol. 16, No. 13, 2000 5833
Table 1. Observed d-Spacings for H′ and H′′ at the Time When the Respective Primary Reflection Appeared in the Diffractogram, tIni, and at t ) 600 s as a Function of the Amount of Oil Added and the Intensity Ratio IH′/(IH′ + IH′′) at t ) 600 s H′ d100 (nm)
H′′ d100 (nm)
oil
moil (g)
Voil (mL)
moil/mCTAB
tini (s)
at tini
at t ) 600 s
tini (s)
at tini
at t ) 600 s
IH′/(IH′ + IH′′)600s
toluene toluene toluene toluene hexane hexane hexane
3.17 4.05 10.01 14.04 4.02 5.99 8.02
3.66 4.67 11.55 16.20 6.09 9.07 12.14
1.32 1.67 4.15 5.85 1.68 2.48 3.35
150 120 150 240 120 120 130
7.97 7.77 7.97 9.46 6.37 6.50 6.90
8.04 8.41 (10.79) 10.92 6.20 6.50 6.90
120 120 150 180 140 140 390
5.27 5.56 6.28 6.60 5.18 5.36 5.46
4.88 5.35 6.84 7.91 4.96 5.12 5.21
0.04 0.101 (0.153) 0.215 0.206 0.367 0.739
Figure 3. Time-resolved in situ synchrotron XRD pattern of the development of the mesoscopic order in a MCM-41 synthesis in the presence of toluene. Toluene/CTAB mass ratio, mtoluene/ mCTAB ) 5.85. The intensities between s ) 0.2 nm-1 and s ) 0.4 nm-1 have been multiplied by 3 for clarity.
This is shown in Figure 2, where diffractograms measured after a reaction time of 600 s are given for different hexane concentrations. The long-range order of the larger structure is not as good as that for the less swollen structure, indicated by the broadening of the reflections and the poorly resolved higher order reflections. H′′ always contracted with time, while the contraction of H′ diminished with increasing oil concentration and eventually a slight swelling with time was observed at high oil concentrations. A summary of the data is given in Table 1. Toluene Added. The mesophase behavior was quite different in the presence of toluene as compared to that observed for hexane. The evolution of the diffractogram with time for mtoluene/mCTAB ) 5.85 is shown in Figure 3. A low angle reflection appears in the diffractogram after t ) 180 s, and soon after two higher order reflections appear, which could be indexed as the 110 and 200 reflections of a two-dimensional hexagonal symmetry. A marked swelling of the hexagonal mesophase with time was observed. While the initial position of the 100 reflection was at s ) 0.152 nm-1 (t ) 180 s), s equaled 0.126 nm-1 at t ) 450 s. The hexagonal symmetry was maintained throughout the swelling process, as indicated by the accompanying shifts in the positions of the 110 and 200 reflections. Even the 210 reflection could be resolved at longer reactions times. An additional broad low-angle reflection was also observed at higher concentrations of toluene. However, the intensity of this reflection was much lower than that observed in the presence of high amounts of hexane. Diffractograms measured at t ) 600 s at different toluene concentrations are shown in Figure 4. The s-values of the hexagonal mesophase were decreasing in a continuous sequence with increasing toluene con-
Figure 4. In situ XRD patterns measured after a reaction time of 600 s as a function of added amount toluene: (a) mtoluene/ mCTAB ) 1.32; (b) mtoluene/mCTAB ) 1.67; (c) mtoluene/mCTAB ) 4.15; (d) mtoluene/mCTAB ) 5.85.
centration, as shown in Table 1. The long-range order was preserved for all the investigated toluene/surfactant ratios. The position of the 100 reflection as a function of reaction time is shown in Figure 5. Two features are evident: (i) the time at which the first indication of mesoscopic order was observed is increasing with increasing toluene concentration and (ii) while a contraction of the hexagonal mesophase with time was observed at low toluene concentrations, a pronounced swelling was observed at higher concentrations. Toluene Added at Different Reaction Times. To investigate the swelling behavior observed in the presence of toluene in more detail, additional experiments were carried out in which a constant amount of 12.1 g of toluene (mtoluene/mCTAB ) 5.04) was added at different times after the formation of the hexagonal mesophase. An immediate swelling of the mesophase was observed when toluene was added at an early stage of the reaction as shown in Figure 6. Here, the s-values of the most intense (see below) 100 reflections observed are plotted as a function of reaction time. There was a delay of about 10 s between
5834
Langmuir, Vol. 16, No. 13, 2000
Figure 5. Position of the 100 reflection of the hexagonal phase as a function of reaction time: (a) mtoluene/mCTAB ) 0; (b) mtoluene/ mCTAB ) 1.32; (c) mtoluene/mCTAB ) 1.67; (d) mtoluene/mCTAB ) 4.15; (e) mtoluene/mCTAB ) 5.85.
the time at which the toluene was added and the time at which a swelling of the mesophase was observed, due to the residence time of about 10 s between the reactor and the measurement cell. The longer the time interval between the initiation of the reaction and the addition of the toluene, the lower was the degree of swelling of the mesophase. If the toluene was added to the reactant mixture at t ) 600 s, no swelling of the hexagonal mesophase was observed. Note that the degree of swelling in each case was lower than that observed when toluene was added at the beginning of the synthesis. While only one hexagonal phase was detected if the toluene was added at t ) 250 or 600 s, three coexisting hexagonal phases were observed when the toluene was added at t ) 135 s, as shown in Figure 7. H′ and H′′ form soon after the addition of toluene, while H′′′ forms at a later stage. The corresponding s values at t ) 600 s were 0.175, 0.20, and 0.22 nm-1 for H′, H′′, and H′′′, respectively. Discussion Oil Solubilization in Surfactant Solutions in the Absence and Presence of Silica. Solubilization of oils by aqueous surfactant solutions has been studied in much more detail than that of inorganic-surfactant aggregates. Therefore, we will first focus on some important properties of aqueous surfactant systems before discussing the solubilization behavior of silicate-surfactant systems. It has been shown that the transport of oil between emulsion droplets or from emulsion droplets to micelles through the aqueous continuous phase may proceed at a significant rate if the oil is slightly water soluble. Here the rate of oil transport is mainly determined by the water solubility of the oil, i.e., the oil availability.10 If the oil has very low
Linde´ n et al.
Figure 6. Position of the most intense 100 reflection (see text for details) of the hexagonal phase as a function of reaction time when a constant amount of toluene, mtoluene/mCTAB ) 5.04, was added at different stages of the reaction. Toluene was added at (a) t ) 135 s, (b) 250 s, and (c) 615 s, respectively.
water solubility, however, the observed oil transport rates are better described by a micellar transport mechanism.11 It has been suggested that oil transport by molecular diffusion is predominant if the water solubility of the oil exceeds 10-6% w/w,11 which is the case for both oils used in the present study. The water solubility calculated as a weight fraction of n-hexane12 and toluene12 is 6.6 × 10-4 and 6.0 × 10-5, respectively. Furthermore, it has been shown7,13 and also thermodynamically modeled14 that aromatic compounds are solubilized in larger amounts, both gram/gram and mole/mole, than the corresponding saturated hydrocarbon in aqueous solutions of ionic surfactants, especially CTA+. This effect originates from both the smaller molecular volume and the higher polarity of the former. The reduction of the micellar core-water interaction energy with increasing polarity of the solubilizate is also responsible for the increase in solubilization of aromatic compounds. While solubilization in the palisade layer of the micelle would have little influence on the micellar diameter, solubilization in the core of the aggregates would increase the micellar diameter.15 Furthermore, core solubilization would preferentially lead to the formation of more spherical structures.16 (10) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 3442. (11) Carroll, B. J. J. Colloid Interface Sci. 1981 79 126. (12) American Petroleum Institute Technical Data Book; Washington, DC, 1979. (13) Chaiko, M. A.; Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1984, 99, 168. (14) Nagarajan, R.; Ruckenstein, E. In Surfactants in Solutions;. Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 923. (15) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. 1998, 102, 831.
Solubilization of Oil
Langmuir, Vol. 16, No. 13, 2000 5835
Several kinetic effects have to be taken into account when comparing the behavior of the present silicatesurfactant systems corresponding to thermodynamically stable systems in the absence of silicate. First of all, kinetic energy is continuously introduced to the system through stirring. Therefore, the oil will be present as molecularly dissolved, solubilized in micelles, as kinetically stabilized emulsion droplets and, especially at high oil contents, as macrodroplets. The continuous hydrolysis of TEOS will release substantial amounts of ethanol, up to a theoretical maximum of 0.062 mEtOH/(mEtOH + mH2O), to the aqueous phase, thus gradually increasing the oil solubility in the aqueous phase with time and thereby increase the transferability of oil in the system. Preliminary experiments showed that the solubility of both toluene and hexane in a solution of 0.06 mass fraction of ethanol in water is more than three times the solubility in pure water, in agreement with reports in the literature.17 On the other hand, the silicatropic structure will gradually become rigid as silicate polymerization proceeds, which puts a limit to the time available for diffusion of oil into the mesophase. The interplay between all these nonequilibrium effects will influence the solubilization capacity of silicatesurfactant systems. Solubilization in the Silicatropic, Hexagonal Phase. The time at which the first indication of the hexagonal mesophase is observed was gradually shifted toward longer reaction times upon addition of oil (see, e.g., Figure 5). TEOS is completely miscible with both hexane and toluene, which will slow down the rate of
hydrolysis by reducing TEOS-water contact time. The presence of oil in the synthesis will also lead to an increase in the initial d-spacing of the hexagonal phase. This increase and the time-dependent swelling of the hexagonal phase followed the order toluene . hexane, i.e., the order of water solubility of the oils. Similar experiments using octane as the solubilizate (results not shown) gave similar results as those observed for hexane. However, hexane was a more effective swelling agent than octane, again suggesting that the water solubility of the additive is important. At low oil contents, a contraction of the hexagonal mesophase with time is observed, as also observed in the absence of oil8,18-20 or in the presence of short-chain alcohols.8 This contraction has been suggested to originate from silicate condensation.19,20 While the d-spacing of the hexagonal phase remained, at the most, more or less constant with time in the presence of hexane (Figure 1), a pronounced expansion with time was observed with increasing concentration of toluene (Figures 3 and 5). However, it should be noticed that in all cases the structures effectively swell, since the observed d-spacing is a sum of the contraction originating from silicate condensation and the swelling originating from the solubilization of oil inside the core of the silicatesurfactant aggregates. The availability, i.e., the water solubility, of the oil is the key parameter, since there is only a limited time to incorporate oil into the silicatesurfactant aggregates due to the ongoing condensation reactions that will eventually solidify the structure. However, the pronounced solubilization capacity of the silicate-surfactant mesophase at short reaction times clearly shows that the oligomeric/polymeric silicatesurfactant molecular complexes are capable of solubilizing large amounts of oil. This is opposite to the hexagonal liquid crystalline phase observed in aqueous solutions of ionic surfactants, which is only capable of solubilizing a fairly small amount of oil in the absence of cosurfactant. Unfortunately, there are extreme experimental difficulties involved in determining the exact composition of silicate species at any given reaction time in the rapidly evolving system, which is why we can only qualitatively conclude that solubilization capacity is determined by the degree of silicate polymerization in the silicatropic mesophase. This conclusion is supported by the three coexisting hexagonal phases, observed here for the first time, when toluene was added 135 s into the reaction (see Figure 7). The strong dependence on the time at which the solubilizate is introduced to the synthesis (Figure 6 + refs 2 and 4) originates from the distribution of silicate-surfactant species at different degrees of hydrolysis/condensation at the time when toluene is added as well as time-dependent changes in the concentration of free surfactant. Formation of the Less Ordered, More Swollen Phase. As described in the results section, increasing the oil content generally led to an increase in the fraction of a less ordered, more swollen structure, this behavior being much more pronounced for hexane (Figure 2c) than for toluene (Figure 4). While the well-ordered hexagonal phase was predominant at all toluene concentrations studied, the more swollen, less ordered phase was the only phase observed at higher concentrations of hexane. However, the extent of swelling of the well-ordered hexagonal phase in the presence of toluene was even higher than that in the swollen, poorly ordered structures observed in the presence of hexane. Preliminary analyses of the diffuse
(16) Hoffman, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (17) Coupland, J. H.; Brathwaite, D.; Fairley, P.; McClements, D. J. J. Colloid Interface Sci. 1997, 190, 71.
(18) Zhang, J.; Luz, Z.; Goldfarb, D. J. Phys. Chem. 1997, 101, 7087. (19) Regev, O.; Langmuir. 1996, 12, 4940. (20) Rathousky´, J.; Schulz-Ekloff, G.; Had, J.; Zukal, A. Phys. Chem. Chem. Phys. 1999, 1, 3053.
Figure 7. In situ XRD pattern measured after a reaction time of 600 s when mtoluene/mCTAB ) 5.04 was added at t ) 135 s.
5836
Langmuir, Vol. 16, No. 13, 2000
scattering region suggest the coexistence of two droplet sizes as a result of the stirring. However, it should be noted that the size of these droplets is much smaller than the size of emulsion droplets in normal emulsions, which usually lie in the micrometer range. The bimodal size distribution is a consequence of the kinetic nature of the system and does not correspond to an equilibrium distribution. This suggestion is supported by the observations of White et al.21 that the observed d-spacing of MCM41 type materials will depend on the stirring speed when saturated hydrocarbons were added to the synthesis mixture in order to swell the pores. A high stirring speed led to the exclusive formation of the larger structure while a bimodal distribution was obtained with lower stirring speeds in the presence of a constant amount of dodecane. We have observed the same phenomena with increasing hexane contents at a constant stirring speed or with increasing stirring rate at a constant hexane content (results not shown). The kinetic stabilization results in a wider size distribution of the droplets leading to a decrease in the long-range order of the silicatropic structure. However, it is not possible to determine the exact nature of the pre-existing aggregates based on our results, since diffraction patterns only give information about the mesoscopic order of the aggregates formed. Although solubilization of oil in the core of individual silicatesurfactant aggregates would promote a spherical aggregate shape, the silica-silica attractive forces will (21) Branton, P. J.; Dougherty, J.; Lockhart, G.; White, J. W. In Characterization of Porous Solids IV; McEnaney, B., Mays, T. J., Rouquerol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; The Royal Society of Chemistry: Cambridge, 1997; p 668.
Linde´ n et al.
promote the transition from spherical to rod-shaped structures in order to maximize silica-silica interactions in the silicatropic mesophase. Intra- and interaggregate silicate condensation reactions will stabilize these aggregates further. However, the nonequilibrium nature of the system is clearly demonstrated by the fact that the surfactant-silicate mesophase behavior was affected by addition of oil far above that what could possibly be solubilized by either the aqueous surfactant solution or the surfactant-silicate aggregates. Summary The solubilization of hydrophobic compounds by the silicatropic, hexagonal phase is mainly governed by the availability, i.e., water solubility, of the additive and the degree of silicate polymerization. However, silicate hydrolysis and condensation reactions as well as the adsorption of silicate oligomers and polymers to the surfactant will introduce important nonequilibrium effects that make solubilization in silicatropic systems different from that observed in corresponding systems in the absence of silicate. Much more oil can be solubilized in silicatropic systems than in aqueous solutions of ionic surfactant. Acknowledgment. The authors wish to thank Dr. Sune Backlund for fruitful discussions. The Ministry of Education (Finland) (GSMR), MATRA project, and EU project ERB-FMRX CT96-0084 are acknowledged for financial support. LA991671H
