Langmuir 1993,9, 2789-2798
2789
Articles Lyotropic Liquid Crystal Phase Behavior of Polymeric Siloxane Surfactants Randal M. Hill* Central Research and Development, Dow Corning Corporation, 2200 West Salzburg, Midland, Michigan 48686-0994
Mengtao He, Zuchen Lin,H. Ted Davis, and L. E. Scriven Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455 Received April 18,1992. In Final Form: May 17,199P
Optical microscopy, cryo-TEM, and small angle X-ray scattering were used to characterize the liquid crystal phase behavior of two ABA and two comb-type polymeric siloxane surfactants in water. These polymeric surfactants follow the usual sequence of LC phases, progressing from lamellar phase to inverse hexagonal phase upon increasing temperature or decreasing the hydrophilic head group size. Bilayer thicknesses are consistently shorter than extended molecular lengths, indicating that the molecules are packed into the bilayer in a relatively coiled and interdigitated conformation. Polydispersity appears to stabilize vesicles, allowing the curvature requirements of all species to be met while maintaining zero net curvature. Three of the four surfactants studied form vesicles spontaneously on contact with water. At low concentrations we found vesicle dispersions, or vesicle/micelle mixed dispersions.' A diverse array of vesicle morphologies were observed,includinground and tubular,unilamellarand multilamellarstructures, as well as possible instances of vesicle fission. The bilayer bending constant is quite small due to the properties of the siloxane hydrophobe and polydispersity.
Introduction energy, are responsible for the unusual properties of siloxane ~urfactants.'~J~ Most commonly used siloxane Siloxane surfactants are widely used in textile manusurfactants are low to medium MW copolymerswith combfacture, cosmeticsformulations, as agricultural adjuvants type or ABA-typemolecular structures. A technologically and as paint additives.24 Although many of these important variation of the comb-type structure is liquid applications are aqueous systems, and some involve the crystal side chain polymers based on the polysiloxane use of siloxane surfactants in combination with hydrobackbone14which form thermotropic liquid crystals.1616 carbon or silicone oils, the liquid crystal (LC) phase A few such polymers have been made which are water behavior of this class of surfactants is essentiallyunknown. soluble and for which the lyotropic LC phase behavior has Since aggregation into LC phases strongly influences been reported.17-19 surfactant function in many applications, we undertook Gradzielskiet aL20foundglobularmicellesand hexagonal to determine the LC phase behavior of a series of siloxane and lamellar LC phases in aqueous solutions of several Surfactants. This paper reports the results for four anionic, cationic,and nonionic siloxane surfactants. From nonionic polymeric siloxane surfactants of the comb-type micellar radii resulta they concludedthat the siloxanechain and ABA structures. Results for cationic and nonionic trisiloxane surfactants are being reported e l s e ~ h e r e . ~ ~ ~must be coiled in the aggregates. Schmaucks et al.21also Siloxane surfactants are amphiphilic materials containing a methylated siloxane hydrophobe coupled to one (10) Bailey,D. L.;Peterson,I. H.;Reid, W. G . Chem.Phys. Appl. Surf. Act. Subst. 1967, 1, 173. or more polar groups.s11 They are surface active in both (11) Kanner, B.; Reid, W. G.; Petersen, I. H. Ind. Eng. Chem., Prod. aqueous and nonaqueous media.12 T w o properties of the Res. Deu. 1967,6, 88. dimethylsiloxane chain, its flexibility and low cohesive (12) Owen, M.J. Znd. Eng. Chem., Prod. Res. Dev. 1980,19, 97. @Abstractpublished in Advance ACS Abstracts, September 1, 1993. (l)Lin, Z.;Hill, R. M.; Scriven, L. E.; Davis, H. T.; Talmon, Y., Langmuir, in press. (2) Schmidt, G. Tenside, Surfactants, Deterg. 1990,27, 324. (3) Gould, C. Spec. Chem. 1991, 354. (4) Gruning, B.; Koemer, G. Tenside, Surfactants, Deterg. 1989,26,
".
a ln l.
(5) Schaefer, D. Tenside, Surfactants, Deterg. 1990,27, 154. (6) Smid-Korbar,J.; Kristl, J.; Stare, M. Int. J. Cosmet. Sci. 1990,12, 135. (7) Lin, Z.;He, M.; Scriven, L. E.; Davis, H. T.; Snow, S. A. J . Phys. Chem. 1993,97,3571. (8) He, M.; Hill, R. M.; Scriven, L. E.; Davis, H. T. J . Phys. Chem. 1993, 97, 8820. (9) Schwarz, E. G.; Reid, W. G. Znd. Eng. Chem. 1963,56, 26.
0143-1463/93/2409-2189$04.00/0
(13) Sonnek.G .Abh. Akad. Wiss. DDR. Abt. Math..Naturwiss..Tech. 1987. i53. ' (14) Hsieh, C.-J.; Hsu, C.-S.;Hsiue, G.-H. Polym. Mater. Sci. Eng. 1988,59, 1014. (15) Gray, G. W. In Side Chain Liquid Crystal Polymers; McArdle, C. B., Ed.; Blackie: Glasgow, U K 1989, p 106. (16) Gray,G. W.;Hill,J. S.;Lacey,D.Mol.Cryst.Liq.Cryst. 1991,197, 43. See also Gray, G. W.; Hawthome, W. D.; Hill, J. 5.;Lacey, D.; Lee, M. S. K.; Nestor, G.; White, M. S. Polymer 1989,30, 964. (17) Finkelmann, H.; L h a n n , B.; Rehage, G. Colloid Polym. Sci. 1982,260, 56. (18) Lijhmann, B.; Finkelmann, H. Colloid Polym. Sci. 1987,265,506. (19) Lijhmann, B.; Finkelmann, H. Makromol. Chem. 1985,186,1059. Hoffmann, H.;Robisch, P.;Ulbricht, W. Tenside, (20) Gradzielski,M.; Surfactants, Deterg. 1990, 27, 366. (21) Schmaucks, G.;Sonnek,G.;Wustneck, R.; Herbst, M.; Ramm, M. Langmuir 1992,8, 1724.
0 1993 American Chemical Society
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2790 Langmuir, Vol. 9,No.11, 1993
present evidence that the siloxane hydrophobe is highly flexible. L h a n n and Finkelmannl7-lginvestigated the lyotropic LC phase behavior of several amphiphilic liquid crystal side chain polymers in which an amphiphilic molecule was attached to each monomer unit of a siloxane backbone. They found that these polymers formed the usual progression of cubic-hexagonal-lamellarphases. Although one might expect the linkage of the amphiphiles to the polysiloxane backbone to hinder their ability to pack into liquid crystalline structures, these authors found no decrease in mesophase stability with increasingpolymer chain length, indicating that the polymer backbone conforms freely to the packing requirements imposed by the LC phases. Yang and Wegner22*23 investigated several ABA-type siloxane surfactants and found a solubility boundary at high temperature, a broad mesophase region below this boundary, and the usual progression of cubichexagonal-lamellar phases. Thus, a few studies of the aggregation of siloxane Surfactantsin aqueoussolution have been carried out, but many questions remain. The objective of this work was to determine the microstructure of LC phases formed by siloxane surfactants, especially how such irregularly shaped, branched, and polydisperse molecules pack into highly ordered LC phases. We also wanted to determine if any patterns or trends in phase behavior as a function of molecular structure were discernible that could be used to guide further work. Small angle X-ray scattering (SAXS)by LC phases generates a series of scattering peaks in I@), where l i s the scattered intensity and 8 is the scattering angle. What is usually plotted is I(@, where q is the magnitude of the scattering q = (4a/X) sin 8. Each type of LC phase gives rise to a unique sequence of relative peak positions.2s27 If at least two peaks can be detected, then the type of LC phase can be determined from the relative peak positions. If the scattering arises from a regular array of scatterers, then a so-called “d-spacing”,or characteristic length associated with the array, is related to q by d = 2 d q . The physical meaning of this length depends on the structure of the LC phase. Lamellar phase interplanar spacings were taken directly from the first peak position while hexagonal phase center-to-center inter-rod spacings were calculated from the first peak position using hexagonal lattice geometry. The interplanar spacing of the lamellar phase, db, can be divided into a surfactant part and a water partB (1) db = db,s + db,w where dba is the thickness of the surfactant portion of the bilayer and db,wis the thickness of the water portion. A simple geometrical analysis shows that db,a = db‘h (2) where & is the volume fraction of surfactant. A similar analysis applied to H2, the inverse hexagonal phase, yields (3) where dawis the diameter of the water-filled rods, dh is (22) Yang, J.; Wegner, G. Macromolecules 1992,!?5, 1786. (23) Yang,J.; Wegner, G.Macromolecules 1992,25, 1791. (24) Glatter, 0. InNeutron, X-ray andlight Scattering: Introduction
to an Investigate Tool for Colloidal and Polymeric Systems; Lmdner, P., Zemb, Th.,Ed.;Elaevier Science Publishing Co.: New York, 1991; p 33. (25) Bleasdale,T. A.;Tiddy, G. J. T. NATO ASZSer., Ser. C 1990,324, 397. (26) Mariani, P.; Luzzati, V.; Delcroix, H. J.Mo1. Biol. 1988,204,165. (27) Tiddy, G. J. T. Mod. Trends Colloid Sci. Chem. Biol.,Znt. Symp. Colloid Surf. Sci. 1985, 148.
(28)Carvell,M.;Hall, D. G.;Lyle, I. G.;Tiddy,G.J. T. Faraday Discuss. Chem. SOC.1986,81,223.
the center-to-center inter-rod spacing,and r$w is the volume fraction of water. The thickness of the surfactant layer separating the rods is dh,s = d h - dh,w. The bending constants of surfactant bilayers are important factors in theoretical models of surfactant aggregation structures and microemulsion stability.2s34 Large interplanar spacings of surfactant bilayers, especially in nonionic surfactants, are thought to be stabilized by thermal undulati0ns.3~When the mean curvature bending constant ( K ) of the bilayer is of the order of kT or smaller, the repulsive undulation Yforce”exceeds the van der Waals attraction and thus stabilizes the interplanar spacing.33 Larger spacings thus reflect smaller bending constants. For common binary surfactant/water systems, a cosurfactant must to be added to decrease the rigidity of the surfactant bilayer sufficiently to give rise to large interplanar spacing^.^^.^^ Di Meglio et al.36measured a value of 0.16kT in a surfactant/cosurfactant/water system, compared with 49kT for the lecithin/water system. Polydispersity should lead to similar effects as adding cosurfactant.36~37 For midrange microemulsions and lamellar phases stabilized by thermal undulations, the persistence length (,$) is related to the bending constant ( K ) by31p36 (4) To evaluate the persistence length tK,a variety of models and phenomenological theories have been proposed for midrange microemulsions and disordered bilayers and vesicles.39 We assumed that at small scattering angles, most of the surfactant bilayers selected by the incident beam are nearly parallel to the incident beama and used the disordered lamellar model suggested by Vonk, Billman, and Kalefll to relate the line shape of the first-order diffraction peak to the “distortion length” of an idealized bilayer. The value of the “distortion length” should be close to the persistence length, 5K.39$42p43 Finally this estimate of lKwas used to calculate the bilayer bending constant by eq 4.
Experimental Section Materials. Four siloxane surfactantswere investigated two comb-type Surfactants and two ABA-type surfactants. Their structures are given in Table I. The two comb-type surfactants were prepared by hydrmilylation of MDaD’zM (R = H) and H&=CHCH2EnOH (I) using chloroplatinic acid catalyst, where n was 12 and 8. MD22D’aM denotes an average structure containing a distributionof chain lengths and number of D’ units per chain. The D’ units are situated randomly along the chain. The given poly(oxyethy1ene)chain lengths of 12 and 8aleodenote (29) Ott, A.; Urbach, W.; Langevin, D.; Hoffmann, H. Langmuir 1992, 8, 345. (30) Roux, D.; Coulon, C.; Cates, M. E. J . Phys. Chem. 1992,96,4174. (31) Helfrich, W. J . Phys. (Paris) 1985, 46, 1263. (32) Helfrich, W. 2.Naturforsch. 1973,28,693. (33) Roux, D.; Safhaya, C. R. J . Phys. (Paris) 1988,49, 307. (34) Wenneratrom, H.; Ohon, U. Langmuir 1993,9,365. (35) Marignan, J.; Appell, J.; Bassereau, P.; Porte, G.h o g . Colloid Polym. Sci. 1988, 76, 101. (36) Di Meglio, J.-M.;Dvolaitzky, M.;Taupin, C. J. Phys. Chem. 1985, 89, 871. (37) Milner, S. T.; Witten, T. A. J. Phys. Fr. 1988,49, 1951. (38) De Gennes, P.; Taupin, C. J. Phys. Chem. 1982,86,2294. (39) Schubert, K.-V.;Strey, R. J . Chem. Phys. 1991,95,8532.
(40)Sheu, E. Y.; Chen, S.-H.; Carvalho, B. L.; Lin, J. S.; Capel, M. Langmuir 1991, 7, 1895. (41) Vonk, C. G.;Billman,J. F.; Kaler, E. W. J . Chem. Phys. 1988,88, 3970; see also Billman, J. F.; Kaler, E. W. Langmuir 1990,6, 611. (42) Barnes, I. S.; Hyde, S.T.; Ninham, B. W.; Derian, P. J.; Drifford, M.; Warr, G.G.; Zemb, T. N. h o g . Colloid Polym. Sei. 1988, 76, 90. (43) Abillon, 0.;Binks, B. P.; Langevin, D.; Meunier, J. h o g . Colloid Polym. Sci. 1988, 76, 84.
Langmuir, Vol. 9, No. 11, 1993 2791
Liquid Crystal Phase Behavior of Siloxane Surfactants Table I ~
type
nomenclaturea
~
~~
~~~~
molecular structure
comb-type MDn(D'E12)zM MesSiO(Si(Me)zO)n(Si(Me)(Rl)O)zSiMes R1 = (CH2)s(OCH&Hz)laOH comb-type MDn(D'E&M Me3SiO(Si(Me)nO)~&i(Me)(R2)O)zSiMea R2 = (CHz)s(OCH2CHz)sOH ABA-type E12D16E12 Rl(Si(Me)20)1Si(Me)~Rl R1 = (CH~)~(OCH~CH~)I~OH ABA-type ED&a R2(Si(Me)zO)lSi(Mez)R2 R2 = (CH~)~(OCH~CHZ)~OH
M stands for MesSiO-; D, -Si(Me)zO-; D', -Si(Me)(R)D-, E, -0CHzCHr. the average of a distribution. Thus, these comb-type surfactants are polydispersein length as well as in the isomericspeciespresent. The two ABA-type surfactanta were prepared similarly by hydrosilylation of M'DiaM' and I, where M' stands for HSi(Mez)O-. Although this structure represents a distribution of chain lengths, every chain has exactly two SiH groups, and they are situated only on the ends of the chains. Thus the ABA-type surfactants are polydisperse only in chain length. As studied, all four surfactants contained about 5 wt % of unreacted HzC=CHCH&,OH and 3-5 wt % cyclic siloxanes (D3-6). Methods. Surfactant solutions were prepared by weight using MilliQ deionized water and equilibrated in a water bath for at least 8 h before any observations were made. The temperature at which samples were observed was varied in two different ways. The temperature of the water bath was varied between 0 and 70 (h0.2) OC. The SAXS sample holder was thermostated and was varied between 0 and 70 (hO.1) OC. Qualitative changes in viscosity were characterized by comparing flow rates upon inverting sample vials or by observing the movementof air bubbles trapped in the solutions. An optical microscope equipped with differential interference contrast (DIC, also called Nemarsky) enhancement was used to observe the presence of vesicles in dilute solutions. Contrast was further enhanced using the gain and offset features of a Hamamatau C2400 CCD video camera. The video camera also allowed thermal motion, such a rapidly undulating bilayer structures, to be stopped using freeze-frame (see description of Figure 2 below). Penetration or diffusion experimentsz were carried out by contacting neat surfactant with water under a microscopeslide. This experiment allows the entire concentration range to be scanned for the existence of vesicles or liquid crystal phases in a few minutes. The morphology of the aggregates present in dilute solutions was further determined by cryo-TEM methods previously described." Briefly, this procedure involvesthe preparation and extremely rapid freezingof a very thin film of surfactant solution. If the f i i is frozen rapidly enough to prevent crystallization of the water, then vesicles, micelles, rod-shaped, and wormlike micelles are readily observed.Surfactant solutions were sonicated for 5-10 min before being prepared for the cryo-TEM. Exact location of phase boundaries in polymeric surfactant systems is difficult because polydispersity smears out phase transitions, and high molecular weights cause slow equilibration. The change from cloudy to transluscent or clear was taken to indicate a two-phase-bone-phase transiti~n.'~ Because of problems with faint birefringence patterns on the polarized light microscope (presumably due to alignment of the surfactant bilayerswith the plane of the microscopeslide?, this study relied on SAXS for positive phase determination. Small angleX-ray scattering experiments were performed using a Kratky camera. The X-ray source was a Rigaku-Rotaflex (44) Bellare, J. R., Davis, H. T.;Scriven, L. E.;Talmon,Y. J. Electron Microsc. Tech. 1988,10,87. (46)Clausen,T.M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.;Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992,96,474. (46)Burns,J. L.;Cohen,Y.;Talmon, Y. J. Phys. Chem. 1990,94,5308. (47) Miller, D. D.; Bellare, 3. R.; Ev-E, D. F.; Talmon, Y.; Ninham, B. W . J. Phys. Chem. 1987, 91, 674. 148) Sieael. D. P.; Burns,J. L.; Chestnut, M. H.; Talmon,Y. Biophys. J. 1989,SS; 161. (49) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1960, 73,5849.
rotating anode with a copper target operating at 7.5-10 kW. The K a wavelength of 1.54 A was selected using Nichol filters. The energywindowon the 10 cm OED linear position sensitivedetector was set to accept only the scattering signal with energy close to Ka. The input collimators and rectangular holes produced a 2 X 0.13 mm X-ray spot on the sample, which was sealed in a specialglass capiUary (diameter = 1mm). The sampleto-detector distance was 68.5 cm. The accessiblerange of q was 0.014.4A-1. Numerical desmearing of the scattering data was not necessary for the purposes of this study. Fourier transform Sia NMR spectra of E&& dissolved in D2O were acquired on a Varian VXR 4005 run at 79.5 MHz. probe. Chemical shifts were Samples were placed in a 16" referenced to external TMS. The resulting spectra were used to detect and semiquantify (no relaxation reagent was used) the hydrolysis of the siloxane backbone in aqueous solution. Molecular areas for E&,Es and E12D1812 were derived from ?rA(surface pressure vs area) isotherms of these two surfactants measured with a KSV 5OOO Langmuir film balance. The surfactanta were spread on MilliQ water at 25 "C using chloroform as the spreading solvent. Although both surfactants have some solubility in water, we were able to obtain isotherms with no significanthysteresis at compressionrates of 10-50 cm/min. Both surfactanta compress from a gaseous film to a liquid expanded f i i . Molecular areas were obtained by extrapolating the linear portion of the liquid expanded region to zero surface pressure.
Results MD22(D'E&M/Water System. The phase diagram of MD22(D'EA2)2Mis shown in Figure la. At low concentrations this surfactant forms a milky lamellar (La) phase dispersion. The solutions remain cloudy down to 0 OC and up to at leaat 90 "C. At 20 "C this system forme lamellar phase liquid crystal, La, between 45 and 92 wt % Solutions in the one phase La region are translucent, viscous, and brightly birefringent when observed as macroscopic samples. When microscope slidea are first prepared, the birefringence textures in this region have the usual focal conic or "oily streaks" appearanceof La.60*s1 However, the pattern fades with time, and the problem becomes worse as the upper boundary of the La region is approached. A similar effect was reported by Gradzielski et aL2Owho attributed it to homeotropic alignment. The lower boundary of the La region appears to slope slightly toward higher concentration with increasing temperature, but the phase persists up to at least 95 "C. Above 95 wt % the isotropicsurfactant-rich phase, L2, is found. There were no significant changes in the locations of the phase boundaries for at least 4-6 weeks. Figure 2 shows a DIC micrograph of MD22(D'E12)2M contacting water. A variety of different bilayer structures are visible. Round unilamellar vesicles are present in area A. An irregular multilamellar structure is seen in area B. The lamellae visible in area B are "wavy" and undulate rapidly-the appearance is similar to the quivering and "dust storms" described by Miller et aL4' It should be emphasized that vesicle formation in this system is rapid and spontaneous, driven only by the agitation resulting from the concentration gradient on contacting water and surfactant. Figure 3 shows a cryo-TEM micrograph of a 5 wt % solution of MD22(D'E&M. Small unilamellar vesicles and a few multilamellar structures are present. Most of the vesicles are approximately round, but a few short tubular vesicles are also visible. A number of distorted structures can be seen. No objects clearly identifiable as micelles are seen.
.
(50)Gray,G.W.,Winsor,P. A.,Eda.Liquid Crystals& P b t i c CrY8tak John Wdey & Sons: New York, 1974. (51) Rosevear, F. B. J. Am. Oil Chem. SOC.1954,31,628.
Hill et al.
2792 hngmuir, Vol. 9, No. 11, 1993
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Figure 1. (a) Phase behavior of MD=(D'E&M in water. (b) Phase behavior of MDn(D'IM2M in water. Observed compositions and temperatures are marked with the following symbols: A = macroscopic observation, 0 = SAXS,X = microscopic observation.
Figure2. DIC optical micrographof MDz(D'Ed2M contacting water at room temperature, 22.5 OC. This micrographis a freezeframe video image, taken during a penetration, or diffusion, experiment.
Figure 4 shows another cryo-"EM micrograph of 5 w t This micrograph shows an unusual example of a multilamellar structure in which several concentric vesicles are remarkably well centered inside each other but the interlayer spacingsare a t least an order of magnitude larger than any apparent undulations in the bilayers. Since large interlayer spacingsare thought to be stabilized by forces arising from the undulations, this micrograph raises the question of what other forces act between widely separated bilayers. Interplanar spacings derived from SAXS are shown in Figure 5a. In the one-phase La region, dba is nearly constant a t about 77A. Because of the comb-type structure of this surfactant, it is difficult to calculate a molecular % MD22(D'Et&M.
Figure 3. Cryo-TEM micrograph of a 5% solution of MDw (D'E12)2M in water at 25 OC.
length to compare with this length. Assuming an average molecular structure (see Discussion), we calculated an extended hydrophobe length of 24 A. The extended length of an EO12 group is about 32 A giving an total length of 56 A,or a bilayer thickness of 112 A. Since the measured spacingof 77 A is significantlysmallerthan this, the chains must be interdigitated, or partly coiled, in agreement with previous work. I t is surprising that such an irregular and polydisperse surfactant structure forms an ordered LC phase a t all, but we note that MD22(D'E&M actually forms a lamellar phase that is stable over a wide range of concentration and temperature. We suspect that this is due to the flexibility of the siloxane chain which allows it to adapt its conformation to the packing constraints of the lamellar phase. 7920921
Liquid Crystal Phase Behavior of Siloxane Surfactants \
\
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i j
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Figure 5. (a) Interplanar spacings of MDdl'Eh)2Mlwater system at 20 "c. db, db4, and db,r are defined in the Introduction. (b) Inter-rod spacings of MDn(D'E&Mlwater system at 20 "C. dh, dhr, and dh,r are defined in the Introduction.
MDB(D'E&M/Water System. The phase diagram of MD22(D'E&M in water is shown in Figure lb. A t 20 "C there is a broad region of white opaque gel between 42 and 85 wt %. The sequence of SAXS peaks (similar to Figure 9below) identifies this phase as a hexagonal phase. We identified it as H2 rather than HI by analogy with the E&& system discussed below. A t concentrations lower than 42 wt % this system forms a two-phase dispersion which can be separated by centrifugation into a clear isotropic layer and a white opaque layer. Above 93 wt % L2 is found. DIC optical microscopy showed no evidence of a lamellar phase region, or vesicles present in dilute solutions. This system also exhibited little sensitivity to temperature, the H2 phase was stable to at least 70 "C. Because of the opacity of the phase, macroscopic birefringence in our 13 mm diameter test tubes was difficult to observe. On the polarized light microscopethis H2 gave an ill-defined, grainy birefringence texture. There were no significant changes in the locations of the phase boundaries for at least 4 4 weeks. Comparing the phase behavior of the E012 and EO8 versions of the two comb-type surfactants shown in Figures 1demonstrates that decreasing the size of the hydrophilic group from 12 to 8shifts the phase behavior from lamellar to inverse hexagonal. This is consistent with the usual progression of LC phasesz7 and with previous observations. * 7*zz Inter-rod spacings derived from SAXS are shown in Figure 5b, along with calculated surfactant layer thicknesses. The inter-rod spacing decreases with increasing surfactant concentration. The thickness of the surfactant
Langmuir, Vol. 9, No. 11, 1993 2793
layer between the rods and the diameter of the rods varies monotonically-there is no plateau in the one-phase region. In the normal hexagonal phase the diameter of the rods is fixed by the surfactant moleculardimensions. However, the rods of the H2 phase contain water and the hydrated poly(oxyethy1ene) chains. The surfactant molecules fill the interstitialvolume between the rods. It is noteworthy that the thickness of both the water and surfactant portions of the structure varies monotonically across the concentration range and there is no detectable break or change in slope in the vicinity of the phase boundary. ElpD15EldWater System. The phase diagram of E12DlsEl2 is shown in Figure 6a. Like MD22(D'E12)2M, this surfactant also forms a cloudy La dispersion at low concentrations. Vesicles are readily visible in this region using DIC optical microscopy. La is found between about 50 and 90 w t % up to at least 90 "C. No hexagonal phase region was located. Above 95wt % a solid hydrated crystal phase exists below 18 "C, and L2 at temperatures above 18 OC. There were no significant changes in the locations of the phase boundaries for a t least 4-6 weeks. Interplanar spacings a t 20 O C determined by SAXS are shownin Figure 7a. The thickness of the surfactant portion of the bilayer is approximately constant in the one-phase region with an average d b g of 74 A. Since the length of E12D15E12 is about 113A, this indicates a relatively coiled and interdigitated chain conformation. Figure 8 shows a cryo-TEM micrograph of a 5 wt 5% solution of E12D15E12. Severalvesicles are visible,including round and long tubular structures, along with globular and wormlike micelles (which resemble strings of beads in this micrograph). Unlike the comb-type structures discussed above, cryo-TEM indicates that this ABA-type surfactant formsmicelles. The vesicle-to-wormlike micelle transition observed in this system has been discussed elsewhere.' The coexistence of vesicles and micelles in this system can be attributed to polydispersity. A t low concentrations the different molecular speciesapparently segregate: those molecules with long EO chains (or high E/D ratio) form micelles while those with smaller EO groups form bilayer aggregates. At higher concentrations, the curvature requirements of the larger EO groups are met by forming vesicles.52 Safran and ~o-workers"~ have shown that mixtures of surfactants can stabilize the opposite curvatures of the inner and outer halves of a vesicle bilayer by incorporating the larger polar groups into the outer layer and the smaller ones into the inner layer. Thus, while the average size polar group might prefer wormlike micelles, the distribution of small and large polar groups stabilizes vesicles. Et&&/Water System. The phase diagram of E&& is shown in Figure 6b. Below 30 wt % this surfactant forms a cloudy La dispersion a t 20 "C. Vesiclesare readily seen in this region usually DIC optical microscopy; structures similar to those shown in Figure 2 are also observed for this ABA-type surfactant. Between 30-70 wt 5% this surfactant forms La at 20 "C and H2 at higher temperatures. The La is transluscent, viscous, and birefringent. The H2 is an opaque white gel which gives an ill-definedgrainy birefringencetexture on the polarized light microscope. There is a sharp increase in viscosity associated with formation of the H2 phase. Above 70 wt % Lp is found. (52) Bagdaesarian, C. K.; Roux, D.; Ben-Shad,A.; Gelbart, W.M. J. Chem. Phys. 1991,94,3030. (53) Safran, S. A.; Pincus, P. A.; Andelman, D.; MacKintush, F. C. Phys. Rev. A 1991,43, 1071.
2794 Langmuir, Vol. 9, No. 11, 1993
a
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0 2 0 4 0 6 0 8 0 1 0 0 W% E12DlSE12
Figure 6. (a) Phase behaviorof E12D16E12 in water. (b) Phasebehavior of EeDlaES in water (asobserved for freshly prepared solutions). Observed compositions and temperatures are marked with the following symbols: A = macroscopic observation, 0 = SAXS, X = microscopic observation.
Figure 7. (a) Interplanarspacings of E12DlsEldwatersystem at 20 "c. db, dbr, and db," are defined in the Introduction. (b) Interplanarspacings of E&&$water system at 20 "C. db, dbr, and db,r are defined the Introduction.
Comparing the phase behavior of the E012 and EO8 versions of the two ABA-type surfactants shown in Figure 6 shows that decreasing the size of the hydrophilic group from 12 to 8 again shifts the phase behavior toward the inverse hexagonal phase. However, unlike the comb-type surfactants discussed above, the inverse hexagonal phase is only found at higher temperatures. Figure 9 shows SAXS results for this system. Curve a illustrates a typical SAXS spectrum for the La phase. Two peaks are seen, at q = 0.0405 and 0.0811. The ratio of these peak positions is 1:2, identifying the phase as lamellar.2sn The interplanar spacing,taken from the f i t peak is 155.2 A. Curve c illustrates a typical SAXS spectrum for the white opaque gel found in this system. (Similarspectra were observed for MD22(D'&)M.) Three peaks are detected, at 0.0390,0.0674,and 0.1032. The identifying ratios of these peak positions are 1:31/2:7*/2, the phase as hexagonal.2s27 The inter-rod distance, calculated from the firstpeak position assuming hexagonal geometry, is 186.1 A. Curve b shows the SAXS spectrum
Figure 8. Cryo-TEM micrograph of a 5%solution of ElzD1aE12 in water at 25 O C .
obtained in the two-phase region between La and H2. The first and third peaks (q = 0.0292, 0.0507, ratio 1:2) correspond to the f i i tand second peaks of (a). The second and fourth peaks (q = 0.0375,0.0751,ratio 1:31/2) correspond to the first and second peaks of (c). The interplanar spacing is 167.6 A; the inter-rod spacing is 248.1 A. Note that curve c was obtained at 65 "C while (a) and (b) were measured at 20 "C. Thus, the evidence for identifying the white opaque phase as H2 rather than HI is as follows: (i) The relative peak positions in SAXS identify the phase as having hexagonal symmetry. (ii) It has neither the macroscopic appearance, nor the microscopic birefringence texture commonly associated with the normal hexagonal phase.51 (iii) A similar transition from an La dispersion or La to H2 has been observed in a number of other systems.aa
Liquid Crystal Phase Behavior of Siloxane Surfactants
Lartgmuir, Vol. 9, No. 11, 1993 2795
.A
w
m
E w
CI
E
t
i
q3
I
t-u
I 0
0.04
0.08
0.12
Wave Vector q
0.16
0.2
(A-')
Figure 9. SAXS spectra for 50 w t 95 solutions of WI&in water: curve a, freshly prepared sample aged less than 3 days at 20 "C; curve b, sample held at 65 OC for 5 h, SAXS spectrum obtained after cooling to 20 OC; curve c, sample quickly heated to 65 "C, SAXS spectrum obtained at 65 "C.
Figure 11. (a) Cryo-"EM micrographof a freshlyprepared 10% solutionof E&DI~J& in water at 25 "C. (b)Cryo-TEM micrograph of a freshly prepared 10%solution of E&& in water at 25 "C showing several examples of tubular vesicles appearing to neckdown prior to fissioning.
Figure 10. Cryo-TEM micrograph of 10%solution of E&I& in water in the Ht phase region.
Figure 10 shows a cryeTEM micrograph of a 10 wt % solution of E&jE8 in the H2phase. The structures visible in this micrograph are quite similar to those shown by Siege1et al.48as representing an inverse hexagonal phase. (iv) Because raising the temperature decreases the head group area of poly(oxyethy1ene)-containing surfactants, a hexagonal phase at higher temperatures than a lamellar phase must be an inverse hexagonal phase. at 20 O C Interplanar spacings for solutions of are shown in Figure 8b. In the one-phase, La region, dbs is nearly constant at 77 A. Since the extended length of the nominal molecule is about 94 A, the molecules must be packed into the bilayer in a partly coiled, interdigitated conformation. Figure 1l a shows a cryeTEM micrograph of a 10wt 95 E&,& solution at 20 OC. This micrograph shows the presence of a variety of structures including unilamellar and multilamellar vesiclesranging in shape from round to very long bent tubes. The interlayer spacing estimated from the region marked A is about 95 A, from B about 286 A. Thus, a wide range of layer spacings is present, which should give a broad SAXS peak corresponding to the average spacing. C and D mark tubular vesicles that appear to be in the process of necking-down or fissioning from tubular to smaller, round form. The variety of
morphologiesvisiblemost likely reflects the polydispersity of the surfactant. Figure l l b shows another cryo-"EM micrograph of 10 wt % E8D15E8 a t 20 "C. A number of long tubules are visible in this photograph, including three regions (A, B, and C) where vesicle fission appears to be taking place. Regions D and E appear to show holes opening in vesicle walls. Aging of EaD& Solutions. All of the observations above relate to solutions of E&& which were less than 3days old. At times longer than 10days we found a highly reproducible, time-dependent phase behavior for this surfactant which we attribute to hydrolysis. We will discuss our observations and interpretation of this timedependent behavior in some detail because there is substantial uncertainty and confusion regardingthe degree and significance of hydrolysis of siloxane surfactants. Observations to determine the effect of aging at 25 O C were made at 2 and 4 weeks. Temperature effects were investigated in two ways: (i) Samples were quickly (5min) heated in the SAXS sample holder to 45 or 65 O C and scattering data collectedfor 30min after which the samples were quickly cooled back to 20 "C and a further 30 min of scattering data collected. (ii) Samples were held a t 65 "C in an oven for 5 h after which they were slowly cooled to 20 "C over 8h. SAXS measurements were subsequently performed at 20 and 45 "C. Aging solutions of ED15E8for 2 weeks at 25 "cconverted them to H2. At concentrations greater than 50 w t % some La was still present after 2 weeks, coexisting with the H2.
2796 Langmuir, Vol. 9, No. 11, 1993
Hill et al.
60[i..
. . . .. .
.
e
.
.
50
.I.\* j
li
Xdblwk LOdb4wkl
inverse hexagonal liquid crystal
. . . . .. . . x .
lamellar liquid crystal phase
0
20
40
60
80
100
W% E8D15E8 Figure 13. Phase behavior of "aged" E&&
in water.
at which equilibrium is approached is catalyzed by either acid or base and occurs very slowly at neutral pH.2*u*55 Knoche et al." showed that hydrolysis of siloxane surfactanta involves primarily unassociated surfactant molecules. Siloxane groups in the interior of micelles react much slower than those outside the micelle. If the siloxanechain of EsD188 cleaves randomly, then hydrolysis should generate both higher and lower E/D ratio fragments. If fragments with higher E/D ratio preferentially partition into the water-rich phase while the lower E/D ratio fragmentspartition into the surfactantrich phase, hydrolysis would shift the composition of the surfactant-rich phase toward more hydrophobic. It is wellknown that ethoxylated surfactants become less soluble with increasing temperature, shifting the amphipldic nature of the surfactant toward more hydrophobic. Thus hydrolysis of EEDI~E~ could lead to similar effects as an increase in temperature. 29SiNMR was used to follow the hydrolysis reaction and to analyze the hydrolysis products. The 29SiNMR spectrum for freshly prepared solutionsof E8D15E8 in DzO contained two peaks, corresponding to internal and terminal units of the chain. After 2 weeks three additional lines appeared corresponding to various fragments containing -SiOH groups. Semiquantitative analysis indicated about 5 wt % of the surfactant had undergone hydrolysis after 2 weeks. Thus, the evidence is that EsDlsE8 undergoes a small amount of hydrolysis in a time frame consistent with the changes in phase behavior, and we have argued how such hydrolysis could shift the phase behavior in the manner observed. However, all four surfactants should have hydrolyzed to a similar degree. Why, then, were significant changes in the phase behavior only found for EDIsEB? We suggest the explanation involves two factors: (i) the (54) Knoche,M.; Tamura, H.; Bukovac, 1991, 39, 202.
M. J. J.Agric. Food Chem.
(55) Noll, W. The Chemistry and Technology of Silicones; Academic Press, New York, 1968.
Langmuir, Vol. 9, No. 11, 1993 2797
Liquid Crystal Phase Behavior of Siloxane Surfactants
phase partitioning may be greater for this surfactant than for the others, and (ii) the relative size of the EO group in this molecule compared with its hydrophobe places its aggregationbehavior near the boundary between a lamellar phase and an inverse phase, causing a small compositional change to induce a large change in the phase behavior.
Discussion Phase Behavior. Gradzielski et aL20 found globular micelles and hexagonal and lamellar LC phases in aqueous solutions of several (mostlylow molecular weight) siloxane surfactants. They did not report any time-dependent effects similar to what we observed for E&sEa and they did not report the presence of vesicles in any the systems they investigated. For both pairs of surfactants in this study the hydrophobe was held constant while the hydrophilic groups were changed. In both pairs decreasing the size of the hydrophilic group caused the phase behavior to change in a consistent manner. For the ABA pair we saw a progression from wormlike micelles for the E012 version at low concentrations to lamellar phase to inverse hexagonal phase for the EO8 version. This progression in aggregate geometry is from positive to neutral to negative curvature. A similar progression shifted toward negative curvature was also found for the rake pair. This progression is consistent with the usual sequence of LC phases27and with previous observations for polymeric siloxane surfactant~.'~?~~ The LC phase behavior of low molecular weight organic Surfactantshas been rationalized in terms of a model based on molecular packing consideration^.^^@-^^ This model quantifies surfactant molecular shape in terms of the parameter
s = VIAL where V is the hydrophobe molecular volume, A is the hydrophile cross-sectional area, and L is the extended length of the hydrophobe. Ratios of 1,1/2, and 1/3 indicate a preference for lamellar, hexagonal, and cubic LC phases, respectively. Ratios larger than 1 indicate inverse phases. Although this simple heuristic model explains the sequence of phases observed for a series of monodisperse CiEj alkyl surfactants, discrepancies appear when polydisperse, branched materials are considered.M Nevertheless, if such a calculation is carried out for the four surfactants in this study, the results are consistent with the observed phase behavior. The structures of the surfactants used in this study are given above. The molecular volume of each hydrophobe was calculated from the densities and molecular weights of the Si-H functional siloxane polymers from which the surfactants were prepared.59 3.27 A3 was added for the -CH2CH2CH2- group between the EO chain and the siloxane backbone (this group was considered to be part of the hydrophobe). Using published bond lengths and
Table I1 preferred surfactant
S
phase behavior
MDdD'Eh)zM
0.86 1.26 0.49
lamellar inverse hexagonal/lamellar lamellar
MDn(D'E&M
EizDibiz EeDisEe
0.72
observed phase behavior lamellar inverse hexagonal
lamellar lamellar
angles,6o the extended length of -CHzCH2CH&i0)14SiCH2CH2CHrwas calculated to be 50 A.61 Since there are two hydrophilic groups in this molecule, the length per hydrophilic group is 25 A. Because the hydrophilic groups of the comb-type surfactants are situated along the backbone rather than on the ends, the average hydrophobelength for the combtype surfactants was taken to be the overall length of the siloxane backbone divided by twice the number of hydrophilic groups62plus the length of the C-C-C group. This equals 24 A for the MD22D'M hydrophobe. In dilute solutions, the area of an E07 polar head group is about 51 A2, whereas the area of an E012 is about 85 A2.58 Interpolating, the area of an EO8 should be about 58 A2. Thus the molecular packing model predicts the preferred phase behavior shown in Table 11. Except for E12D15E12,the model predicts the observed phase behavior. Although we saw wormlike micelles in dilute solutions of E12D15E12, no hexagonal phase region was found. SAXS results gave dbg = 77 and 74 A for E8D15E8 and E12D1SE12, compared with extended molecular lengths of 94 and 113 A. Monolayer molecular areas derived from Langmuir film balance results are 200 A2 for E&& and 251 A2 for E12D15E12. In order to accommodate the larger interfacial area required by the EO12 group, the siloxane chain expandslaterally causing the overall film thickness to not increase proportionately to the molecular length. The ability of the siloxane backbone to accommodate in an interface to the area requirements of varying headgroups has been noted before from data on a variety of siloxanecontaining copolymer^.^^ Vesicle Surface Roughness. All of the cryo-TEM micrographs presented above show examples of vesicles with rough surfaces. In most cases, the wavelength and amplitude of the roughness correspond reasonably with the visible bilayer spacing. Thus, vesicle surface roughness could be due to thermal undulations frozen during the sample preparation (the time scale of freezing is about 10 ps), but it could also be due to the sample being below ita gel point. In the latter case the irregularities should be polygonal (see photographs in ref 66) and the transition should be detectable by DSC. Since the surfaceroughness is not polygonal, and since no gel transition could be detected in these systems by DSC, we conclude that the roughness is due to thermal undulations. Several lines of evidence support the hypothesis that the vesicle surface roughness we observed in our cryoTEM photographs is due to thermal undulations. First,
(60) The following unit lengths were calculated from publiihed bond lengths and bond angles given in refs 64 and 65: Si-C-H, 2.41A; Si-0-Si, 2.97 A; C-C-C, 2.51 A. (56) Israelachvili,J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. SOC., (61) Thisisactuallyacontourlength.BecausetheSi-O-SiandOSi4 Faraday Trans. 2 1976, 72, 1525. bond angles are not equal and because of the small bending energy of (57) Ninham, B. W.; Evans, D. F. Faraday Discuss.Chem. SOC.1986, these bonds,this length only representaan approximationof the extended 81, 1. of the molecule. See Grigoras and Laneu for further discussion. (58) Mitchell,D.J.;Tiddy,G.J.T.;Waring,L.;Bostock,T.;McDonald,length (62) The additional factor of 2 arisesfrom consideration of the average M. P. J . Chem. SOC.,Faraday Trans. 1 1983, 79,975. conformation of such a comb-type copolymer at an interface: (59) The values are: SiH polymer
MW
density, cm3
v,A3
HD15H MD22D'zM
1098 1914
0.941 0.953
1050 1749
$3
(63) Owen, M. J. MMZ Press Symp. Ser. 1983, 3, 129.
2798 Langmuir, Vol. 9, No. 11, 1993
Hill et al.
we measured large d-spacingswhich should correlate with large bilayer undulations. Using the procedure described above, we estimated the bilayer persistence length of the E D 1 8 8 system to be about 200 A between 30 and 40 w t % surfactant (note that this only 2-3 times the bilayer thickness) and the bilayer bending constant to be about 0.15kT,both indicative of a very flexible bilayer. Second, these siloxane surfactants are polydisperse and mixing short chain and long chain species generally decreases the bending ~ o n s t a n t . ~Third, ~*~~ the siloxane backbone is extremely flexible and methylated siloxanes have a very low cohesive energy.l2Ia Since part of the elastic constant of the bilayer is due to interactions between adjacent molecules in the a small bending constant is consistent with the known properties of siloxane surfactants. Large d spacings, broad scatteringpeaks, and vesicle surface roughness all point to a low bilayer bending constant for these polymeric siloxane surfactants. Conclusions The polymeric ABA-type and comb-type siloxane surfactants in this study follow the usual sequence of LC
phases, progressing from lamellar phase to inverse hexagonal phase upon increasing temperature or decreasing the hydrophilic head group size. The relationshipbetween molecular structure and phase behavior can be explained qualitativelyin terms of molecular packing considerations. Bilayer thicknessesare consistentlyshorter than extended molecular lengths, indicatingthat the moleculesare packed into the bilayer in a relatively coiled and interdigitated conformation. Polydiepersity appears to stabilizevesicles, allowing the curvature requirements of all species to be met while maintaining zero net curvature. At low concentrations, the different species segregate into wormlike micelles coexisting with vesic1es.l Three of the four surfactants in this study form vesicles spontaneously on contact with water. Cryo-T'EM micrographs show a diverse array of vesicle shapes, including round and tubular shapes, as well as possible instances of vesicle fission. Vesicle surfaces appear rough due to thermal undulations frozen during sample preparation. This roughness, combined with large interlayer spacings and broad SAXS peaks, points to small bilayer bending constants.
(64) Grigoras, S.; Lane, T. H. J. Comput. Chem. 1988,9,25. (66)CRC Handbook of Chemistry and Physics, 72nd ed.; CRC Press: Boca Raton, FL, 1991. (66)Frederik, P. M.; Burger, K. N. J.; Stuart, M. C. A.; Verkleij, A. J. h o c . XZZth Znt. Cong. Electron Microsc. 1990, 49.
Acknowledgment. We gratefully acknowledge the support for this research which we received from the DowCorning Corporation and from the Center for Interfacial Engineering at the University of Minnesota.
