Phase Behavior of Binary Water− Trimethylsilane Surfactant Systems

902

Langmuir 1999, 15, 902-905

Letters Phase Behavior of Binary Water-Trimethylsilane Surfactant Systems: Origin of the Dilute Lamellar Phase R. Wagner† Max-Planck-Institut fu¨ r Kolloid und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12489 Berlin, Germany

R. Strey* Institut fu¨ r Physikalische Chemie, Universita¨ t zu Ko¨ ln, Luxemburger Strasse 116, D-50939 Ko¨ ln, Germany Received October 20, 1998. In Final Form: December 2, 1998 We have used the highly purified, homogeneous trimethylsilane surfactant CH3Si(CH2)6(OCH2CH2)5OCH3 to determine the full binary phase diagram with water. The trimethylsilane surfactants display similar properties to those of the well-known superspreading trisiloxane surfactants but have higher hydrolytic stability. Although the richness and sequence of phases found for the present system resemble that known from water-nonionic surfactant systems of the alkylpolyglycolether type, a peculiarity is observed: there are two disconnected lamellar phases, a concentrated one and a dilute one. Furthermore, an L3 phase is found located at lower concentrations than the dilute lamellar phase. The position, the extent, and the phase sequence around the L3 and dilute lamellar phases allows a clear assignment of seven two-phase regions, which require the existence of four triple lines. Each two-phase region has been identified, and all triple lines have been located to within a few hundredths of a degree.

I. Introduction The phase behavior of water-surfactant systems has been under study for more than 100 years. A useful historic account can be found in Laughlin’s monograph.1 In that book a number of open questions are addressed. One such question is the origin of the dilute lamellar and L3 phases.2-12 Coincidentally, the present investigation sheds light onto this hitherto unclear situation. The original purpose, which is still served, was to use a pure, stable, homogeneous trisilane surfactant to clarify the principal phase behavior of so-called superspreading surfactants.13 Binary systems of water-nonionic surfactant had been * To whom correspondence should be sent. † Permanent address: Auwa-Chemie GmbH, Ulrich-HofmeisterStr. 45, D-86159 Augsburg, Germany. (1) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: New York, 1994. (2) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (3) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233. (4) Benton, W. J.; Miller, C. A. J. Phys. Chem. 1983, 87, 4981. (5) Larche, F. C.; Appell, J.; Porte, G.; Bassereau, P.; Margnan, J. Phys. Rev. Lett. 1986, 56, 1700. (6) Safinya, C. R.; Roux, D.; Smith, G. S.; Sinha, S. K.; Dimon, P.; Clark, N. A.; Belocq, A. M. Phys. Rev. Lett. 1986, 57, 2718. (7) Satoh, N.; Tsujii, K. J. Phys. Chem. 1987, 91, 6629. (8) Porte, G.; Marignan, J.; Bassereau, P.; May, R. Europhys. Lett. 1988, 7, 713. (9) Olsson, U.; Stro¨m, P.; So¨derman, O.; Wennerstro¨m, H. J. Phys. Chem. 1989, 93, 4572. (10) Gazeau, D.; Bellocq, A. M.; Roux, D.; Zemb, T. Europhys. Lett. 1989, 9, 447. (11) Miller, C. A.; Gradzielski, M.; Hoffmann, H.; Thunig, C. Colloid Polym. Sci. 1990, 268, 1066. (12) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (13) For a useful review, see: Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247.

studied by Goodman and co-workers.14 in the 60s. The first observation of the L3 phase in these systems was reported in 1979 by Harusawa et al.15 A thorough investigation of the L3 phase, termed there “anomalous”, in relation to the detailed phase diagram of water-C10E4 was performed in 1980 by Lang and Morgan.2 A systematic study in 1983 by Tiddy and co-workers16 clarified the variations of the phase diagrams of nonionic surfactants of the CiEj type with both hydrophilic (j) and hydrophobic chain lengths (i). From this work it can be seen that the occurrence of the L3 phase is related to the intersection of the lamellar phase with the cloud point curve. However, due to their experimental procedure, these authors missed the dilute lamellar phase, LR. In a more detailed study of the phase behavior of water-C12E5 by Strey et al.12 in 1990, the existence of the dilute lamellar phase and the L3 phase was demonstrated. The bilayer properties of these phases were clarified by scattering and electrical conductivity experiments. Siloxane-based surfactants in water have been under investigation for about 30 years because of their interesting spreading behavior on hydrophobic surfaces.17 The relation of the striking spreading behavior, the surfactant nature, the phase behavior, and the surface tension dynamics has been intensely investigated.18-22 Although similarities in (14) Clunie, J. S.; Corkill, J. M.; Goodman, J. F.; Symons, P. C.; Tate, J. R. J. Chem. Soc., Faraday Trans. 1967, 63, 2839. (15) Harusawa, F.; Nakamura, S.; Mitsui, T. Colloid Polym. Sci. 1979, 252, 613. (16) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T. A.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1983, 79, 975. (17) Bailey, D. L. Siloxane wetting agents. U.S. Patent 3299112, 1967. (18) Zhu, X.; Miller, W. G.; Scriven, L. E.; Davis, H. T. Colloid Surf., A 1994, 90, 63. (19) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1996, 12, 337.

10.1021/la9814687 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/22/1999

Letters

Langmuir, Vol. 15, No. 4, 1999 903

the phase diagrams of the trisiloxane surfactants with those of the above-mentioned nonionic surfactants have been noted,18,23,24 the phase diagram determinations have remained semiquantitative. One reason for this unsatisfactory situation is the thermal instability of the trisiloxane surfactants.2,25 Another reason is the use of technical grade surfactants, which limits the reproducibility. In a recent series of papers the wetting properties of pure, homogeneous trisiloxane surfactants have been analyzed and compared to those of the technical grade mixtures.26-28 Even more recent investigations show that similiar properties are exhibited by trimethylsilane surfactants.29 This class of surfactants is thermally stable and may be highly purified. Therefore, other workers should be able to reproduce the results. Here we present the first detailed determination of the phase diagram of water and a trimethylsilane surfactant.29 II. Experimental Section Materials. The trimethylsilane surfactant CH3Si(CH2)6(OCH2CH2)5OCH3 was synthesized according to the procedure of Klein et al.25 From gas chromatography it is judged that the surfactant has a purity > 99%. This finding is supported by the phase diagram analysis below. Doubly quartz-distilled water was used. Sample Preparation. The typical procedure for sample preparation proceeded as follows. An aliquot of surfactant was weighed on a microbalance into a 20 mL test tube equipped with a polyethylene stopper. Then water was added. The balance reading was recorded with a precision of (1 mg. After determination of the phase sequence for the given composition, the sample was further diluted. The test tube was always brought to room temperature before opening the stopper. Also, care was taken to avoid contact of the solution with the stopper or the inlet in order to avoid loss of material or contamination. Mixing of the components was achieved with a small Teflon-coated magnetic stirring bar. Phase Behavior. The phase behavior was studied in fish tanks containing 4 L of water. The temperature was regulated with immersion thermostats to a precision of (0.01 K. The optical properties of the sample in the test tube were studied in transmitted light, under various angles including 90° and under crossed polarizers. In this fashion any occurrence of turbidity (a sign of two phases), opalescence (typical for the critical region and the L3 phase), anisotropy (an indication of the lamellar phase), or streaming birefringence (sign of anisotropic structures under shear, L3 phase) could be detected. While the magnetic stirrer was used for rapid thermal equilibration, the final judgment on the state of the phases was performed after the stirrer was turned off. In some cases phase separation was awaited and the nature, the position, and the relative magnitude of the phases in the test tube were recorded. In this fashion a unique assignment of onephase, two-phase, and three-phase regions was possible. In all references to two- and three-phase regions, the order of the phases as they appear in the test tube from bottom to top is maintained. This feature, a consequence of the respective densities of the (20) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7270. (21) Stoebe, T.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7276. (22) Svitova, T.; Hoffmann, H.; Hill, R. M. Langmuir 1996, 12, 1712. (23) Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Langmuir 1994, 10, 1724. (24) He, M.; Hill, R. M.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1996, 97, 8820. (25) Klein, K. D.; Knott, W.; Koerner, G. Silicone Surfactantss Developments of Hydrolytically Stable Wetting Agents. In Organosilicon Chemistry II; Auner, N., Weis, J., Eds.; VCH: New York, 1996; p 613. See also German Patent DE 43 20 920 C1. (26) Wagner, R.; Wu, Y.; Czichocki, G.; Berlepsch, H. v.; Weiland, B.; Rexin, F.; Perepelittchenko, L. Appl. Organomet. Chem., submitted. (27) Wagner, R.; Wu, Y.; Czichocki, G.; Berlepsch, H. v.; Rexin, F.; Perepelittchenko, L. Appl. Organomet. Chem., submitted. (28) Wagner, R.; Czichocki, G.; Wu, Y.; Berlepsch, H. v.; Rexin, F.; Rexin, T.; Perepelittchenko, L. Appl. Organomet. Chem., submitted. (29) Wagner, R.; Wu, Y.; Berlepsch, H. v.; Perepelittchenko, L. Appl. Organomet. Chem., to be submitted.

Figure 1. Phase diagram of water-CH3Si(CH2)6(OCH2CH2)5OCH3. Note the existence of two separate lamellar phases. phases, may already be taken as the first hint of the composition of the phases.

III. Results Cloud Point Curve and Lamellar Phase. In Figure 1 the phase diagram of the binary system H2O-CH3Si(CH2)6(OCH2CH2)5OCH3 is shown. We will discuss the occurring phases in the order of increasing complexity. The gross features are the cloud point curve at high temperatures and the lamellar phase at a concentration around 65 wt %. Starting at high concentrations, the pure surfactant is liquid. When water is added, an isotropic solution is formed, which we denote by L2, as customary in the literature. At about 95%, an upper miscibility gap appears, which is the high concentration end of the cloud point curve. The latter term is frequently used in surfactant science for an upper closed-loop coexistence curve.1,2 The relatively large extension of the cloud point in the present system seems to be related to the methyl end cap of the surfactant.1 As more water is added, at low temperatures, a lamellar phase, denoted LR, is observed. It exhibits itself by a moderately high viscosity. The magnetic stirring bar in the test tube still moves, but small gas bubbles are trapped and move along with the liquid. The lamellar phase is strongly anisotropic. In the phase diagram in Figure 1 the first appearance of anisotropy is indicated by the full line. The width of the two-phase regions L1 + LR and LR + L2, which are expected from the phase rule,1 has not explicitly been determined and is therefore shown as a dashed line. It is interesting to note that the lamellar phase has an inclination toward the water-rich side of the phase diagram with increasing temperature. The uppermost temperature of the lamellar phase is 28.78 ( 0.02 K. The concentration of the maximum is located at 57.9 wt %. Above the lamellar phase there is a gap of 7 K, where the mixtures are isotropic before the cloud point curve is met. It is noteworthy that the cloud point curve shows an indentation in the concentration range above the lamellar phase. The gap constitutes an optically isotopic, homogeneous channel permitting a continuous transition from the L2 phase region to the L1 phase region.

904 Langmuir, Vol. 15, No. 4, 1999

Figure 2. Enlarged portion of the phase diagram in Figure 1. Note the relative position of the L3 and LR phases.

As the system is further diluted, the cloud point curve extends to lower temperature and runs through a minimum at 3.9 ( 0.2 wt % with the lowest (critical) temperature being Tc ) 19.78 ( 0.03 °C. No attempt was made to determine the critical composition and temperature more precisely. The features described until here are similar to those found in conventional systems of the water-CiEj type.16 An inclined lamellar phase may for instance be seen in the water-C12E6 system. A cloud point curve with similar coordinates of the critical point is exhibited by the waterC10E4 system.2 Dilute Lamellar Phase and L3 Phase. More striking features concern the surprising occurrence of the dilute lamellar phase above and within the cloud point curve. The position of the dilute lamellar phase is located temperature-wise above the high concentration lamellar phase discussed in the preceding section. The lowest temperature at which the dilute lamellar phase is observed is 30.61 ( 0.02 °C, while the high-concentration lamellar phase disappears at 28.78 °C, as mentioned above. The dilute lamellar phase disappears at 33.31 ( 0.03 °C. Concentration-wise the dilute lamellar phase ranges between 15.8 and 34.8 wt %. It has an almond shape with the sharp edge being located at the lowest concentration and temperature. This can be seen more clearly in Figure 2, where we show an enlarged portion of Figure 1. At low concentrations the lamellar phase is bounded by the L3 phase which extends from 6.8 to 25.8 wt %. Temperature-wise the L3 phase is found to begin existing earlier than the dilute lamellar phase at 30.38 ( 0.02 °C (vs 30.61 °C) and also lasts to higher temperature 33.97 ( 0.03 °C (vs 33.31 °C). The extent of the dilute lamellar phase and the L3 phase and their location inside the cloud point region introduce a wealth of seven two-phase regions, which in turn require the existence of four triple lines, all of which have been located to within a few hundredths of a degree. In the following, we describe the observed phase sequence in the order of increasing temperature (cf. Figure 2). At low temperatures, one finds first the phase separation of the L1 phase into two phases, L1′ + L1′′. Both phase are optically isotropic and clear. Only in the vicinity of Tc is strong opalescence observed in both phases. At 29.87 °C, phase separation of a 22.0 wt % sample proceeds quite

Letters

rapidly into two clear phases of which the lower L1′ phase appears to be very dilute in surfactant. Neglecting the surfactant concentration in the L1′ phase, the upper L1′′ phase is found to have a composition of 36.2 wt % from the lever rule, consistent with the cloud point curve determined optically. The L1′′ phase is faintly bluish. The same 22.0 wt % sample phase separates at 30.59 °C into the two phases L3 + L1′′ with a volume ratio of 0.625/ 0.375. The lower L3 phase has an intense bluish tint, much stronger than that of the upper, otherwise clear L1′′ phase. Taking 12.0 wt % for the L3 phase concentration at this temperature, one finds 36.4 wt % for the L1′′ phase, again consistent with the phase boundary measured directly. Between the two phase regions there is a triple line L1′ + L3 + L1′′ at 30.38 ( 0.02 °C. The crossing of the triple line has been observed directly with samples at 0.445, 2.00, and 7.10 wt %. While this phase behavior is an interesting subject in itself, the existence of the two-phase region L3 + L1′′ clearly precludes the dilute lamellar phase LR being in direct contact with a dilute aqueous phase, like L1′. At 30.61 ( 0.02 °C another triple line is encountered, L3 + LR + L1′′. This line announces itself already in the stirred 17.1 wt % sample by anisotropic striations in the middle of the test tube. Phase separation, however, takes prohibitively long to be awaited. At slightly higher temperature, one finds the two-phase region LR + L2. The phase separation here is faster with the lower phase displaying anisotropy while the upper phase is clear. We have tacitly switched from the notation L1′′ to L2. We will argue below in favor of this change in notation at the triple line L3 + LR + L1′′()L2) at 30.61 °C. The LR + L2 region can be traced until 33.31 ( 0.06 °C, where a change in anisotropy indicates another triple line, L3 + LR + L2, clearly observed for the 41.4 wt % sample. Above this temperature, the L3 + L2 region is found, which terminates with a sudden change in stirring appearance from creamy to whitish indicative of the highest triple line at 33.97 ( 0.02 K. The phase sequence here is L1′ + L3 + L2. The slight fluctuations ((0.01 K) in the thermostat temperature (set to a temperature of 33.97 °C) are sufficient to let a sample at 8.99 wt % separate into the three phases. In this fashion one can actually “see” the triple line in a two-component system. The reason is that at this concentration the temperature fluctuations sample both the L1′ + L2 and the L1′ + L3 coexistence when crossing the triple line. The commonly observed L1′ + L3 two-phase region2,3,12 extends from the lowest triple line L1′ + L3 + L1′′ at 30.38 °C to the highest L1′ + L3 + L2 triple line at 33.97 °C. It in a way acts like a ferry transporting the surfactant from the normal curvature of the L1′′ phase to the inverted curvature of the L2 phase. The discussion in terms of curvature will be continued below. IV. Discussion The results described have a number of similarities with those of other surfactant systems but also one new feature. The new feature is that there are two lamellar phases. Compared to the related water-C10E4 system, where the lamellar phase bends over to low surfactant concentrations and continues to low concentrations (see for example refs 1 and 2), in the present system the lamellar phase is disrupted by an isotropic corridor connecting the L1 and L2 phases. Here an interesting continuous transition from the L1 phase to the L2 phase may be studied. We have, by coincidence, found a system that permits us to demonstrate experimentally the evolution of the

Letters

dilute L3 and LR phases as one-phase islands within the cloud point. These phases have repeatedly been observed in multicomponent mixtures inside the two-phase regions ranging from L1 to L2,4,5,8,9,10,30 often by tuning the surfactant to cosurfactant ratio.31 Here, however, they are observed with a single surfactant. It may be speculated that the origin for the stability of the dilute bilayer phases is the existence of an undulation force which is predicted on theoretical grounds.32 The idea is that thermal undulations provide an additional stabilizing mechanism due to the liberation of configurational entropy normally buried in the lamellar stacks. There is an interesting similarity with the so-called phase inversion typically seen in microemulsion systems. In those systems also an isotropic channel exists, permitting the continuous transition between an oil/water and (30) Porte, G.; Appell, J.; Bassereau, P.; Marignan, J. J. Phys. 1989, 50, 1335. (31) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 6091. (32) Helfrich, W. Z. Naturforsch. 1978, 33a, 305. (33) Lichterfeld, F.; Schmeling, T.; Strey, R. J. Phys. Chem. 1986, 90, 5762. (34) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90, 4083. (35) Kahlweit, M.; Strey, R. Angew. Chem. 1985, 79, 655. (36) Alibert, I.; Coulon, C.; Coulon, Am. M.; Gulick-Krzywicki, T. Europhys. Lett. 1997, 39, 563. (37) Anderson, D.; Wennerstro¨m, H.; Olsson, U. J. Phys. Chem. 1989, 93, 4243.

Langmuir, Vol. 15, No. 4, 1999 905

a water/oil microemulsion33,34 while at the same time providing the highest mutual solubility of water and hydrocarbon.35 The origin of the similarity may be seen in the change of the so-called spontaneous curvature with temperature. At low temperature, the curvature is around the hydrocarbon while, at high temperature, due to the lowering of the head-group hydration, the curvature is around the water. In bilayer phases this change of spontaneous curvature leads to frustration of the curving tendencies of the monolayers comprising the bilayer, which therefore leads to the relative position of the L3 and dilute lamellar phases on the temperature scale.30,36,37 The temperature where the spontaneous curvature changes sign is presumably close to the maximum extension temperature of the dilute lamellar phase, T ) 30.61 °C. This temperature therefore appears to be the natural point to change the nomenclature from L1′′ to L2. V. Conclusions We have identified various two- and three-phase coexistences in a model system for a superspreading surfactant including a dilute lamellar phase and a L3 phase. Future research will be directed toward variation of the number of oxyethelene groups and the number of methyl groups in the hydrophobic chain and the spreading behavior of these compounds. LA9814687