J. Phys. Chem. 1994, 98, 6148-6157
6148
Aggregation Behavior and Microstructure of Cationic Trisiloxane Surfactants in Aqueous Solutions M. He, Z. Lin, L. E. Scriven, and H. T. Davis’ Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. S. E., Minneapolis, Minnesota 55455
S. A. Snow Central Research and Development, Dow Corning Corporation, Mail No. C042A1, Midland, Michigan 48686 Received: October 20, 1993; In Final Form: February 19, 1994’
Aqueous aggregation behavior of a family of novel cationic siloxane surfactants (Me3SiO)zSi(Me)(CHz)3N+Mez(CH2)zOH X- (denoted as BisX, X = C1, Br, and I) is reported. At 20 OC, BisCl forms micelles between 1.3 and 20 wt %, but both BisBr and BisI assemble into vesicles at surfactant concentrations even below 1 wt %. At the midrange of surfactant concentrations, BisCl and BisBr form a lamellar liquid crystal phase. All of the three BisX surfactants form a solid phase on the surfactant-rich side of the lamellar liquid crystal phase, in which BisX molecules pack in lamellar bilayers. Differences in aqueous aggregation behavior exhibited by the three BisX surfactants are attributed to the variable degrees of halide counterion binding to the quaternary ammonium head group. The correlation between the microstructures of BisX surfactants in diluteconcentrations and “superspreading” is the same as that found previously in nonionic siloxane surfactant solutions.
Introduction Silicone surfactants have a wide range of technological applications,including the rapid wetting of low-energy surfaces, the stabilization of polyurethane foam, surface activity in organic media, and their utility as coating additives. Generally, these applicationsexploit the high surface activity of these surfactants, including their ability to reduce the surface tension of liquids to approximately 20 mN/M. This extraordinary surface activity is a result of the intrinsic low surface energy of the siloxane backbone (Owen, 1980). The silicone surfactant structure includes a wide range of organic moieties linked to that backbone in order to solubilize the surfactant. These moieties include nonionic, anionic, cationic, and zwitterionic organic groups. In the past few years, there has been a growing awareness of the importanceof understandingthe bulk solution phase properties of these surfactants. An early report was that a cationic silicone surfactant formed pseudoplasticand elastic aqueous solutionsat high (approximately 80%) surfactant levels (Rosen and Prokai, 1972). More recently, L. J. Petroff and R. M. Hill (1993) discovered that silicone surfactant solutions that were exceptional herbicide adjuvants were turbid in appearance. This turbidity was linked to the presence of a lamellar liquid crystalline (LLC) phase in the mixture. The presence of LLC phase in solutions of silicone “superwetting” surfactants was suggested by Zhu (1992). Gradzielski et al. (1990) studied the aggregation properties of a variety of nonionic, anionic, and cationic silicone surfactants. They reported that some of these species formed liquid crystalline phases in solution. These included lamellar and hexagonal LC phases, similarto those extensively reported for organic surfactant solutions. He et al. (1993) investigated the aggregation of a series of nonionic (polyether-based) silicone surfactants. A common feature of the aqueous solutions of these surfactants was the presence of aggregates with the fundamental structural unit of the surfactant bilayer. There included LLC phase, vesicles, tubules, and possibly “sponge” (L3) phases. 0
Abstract published in Advance ACS Abstracts, May 1, 1994.
Cationic siloxane surfactants have applications in fabric softening, foam stabilization, wetting, bacterial destruction, and corrosion resistance (Snow et al., 1990, 1991, 1992; Gruning et al., 1989;Maki et al., 1970). Snow (1993) recently reported the synthesis, characterization, and aqueous surface activity of a family of novel trisiloxane quaternary ammonium salt surfactants: (M~~S~O)ZS~(M~)(CH~)~N+M~Z(CH~)~OR X- (R = H, C(O)Me, C(O)NH(Ph); X = C1, Br, I, NO3, MeOS03). From surface activity data, he concludes that a number of these surfactants may form micelles at low surfactant concentrations. Following his research, Lin et al. (1993) investigated the vesicle and cubic liquid crystal formation by the chloride salt surfactant of this family (R = H, X = Cl; named BisC1) upon addition of various sodium salts (NaCl, NaBr, NaI, Na salicylate, and Na dodecylsulfate). They found that thecritical molar concentrations of the salts that induced the micelle-to-vesicle transition follow the sequence NaI < NaBr < NaCl. The crystal structure of certain cationictrisiloxane surfactants has also been studiedusing X-ray scattering. Ramm et al. (1990) and G. Schmaucks et al. (1992) reported that the solid crystals of trisiloxane ammonium salt surfactants (Me3SiO)&Me(CH2)3N+MqRBr (R = n-C,H,-, n-C4H9-) form lamellar-packed double layers. The goal of the research reported herein is to measure the effects of surfactant concentration,temperature,and counteranion (Cl, Br, I) on the aggregation and resultant phase behavior of the “Bis” [(Me$30)2Si(Me)-(CH2)3N+Me2(CH2)20H] cation. The phase diagrams of many single- or double-chainalkyl quaternary ammonium halide salts in water have been documented in the literature (Laughlin, 1991; Lindman et al., 1980; Blackmore et al., 1988;Tiddy et al., 1980). We are also interested in the effect of the siloxane moiety (versushydrocarbon) in terms of molecular aggregation. Finally, we were interested in discovering if the aqueoussolutions of the BisX surfactants “supenvet”hydrophobic surfaces. This phenomenon has been previously reported for certain nonionic trisiloxane surfactants with “Bis” backbone (Ananthapadmanabhan et al., 1990; Zhu, 1992; He et al., 1993; Hill et al., 1993). In particular, we were concerned with a correlation between surfactant aggregate microstructure and superwetting.
0022-365419412098-6148%04.50/0 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6149
TABLE 1: Name and Molecular Formula of BisX Fourier transform infrared spectroscopy (FT-IR) was obtained on a Nicolet Magana-IR spectrometer (Model 550, Nicolet name formula Instrument Corp., Madison, WI) equipped with a liquid nitrogen BisCl (M~~S~~)ZS~(M~)-(CH~)~-N+M~~(CH~)~OH CIcooled MCT detector and supported by Nicolet data control and BisBr (M~~S~O)ZS~(M~)-(CH~)~-N+M~~(CH~)~OH Br data BisI ( M ~ $ ~ O ) Z S ~ ( M ~ ) - ( C H ~ ) ~ - N + M I~ ~ ( C H ~ ) ~ ~ H processing software. The aqueous samples were held at 20 OC between twoCaFzcrystalwindow pieceswithaO.1-"Teflon spacer in between. The IR light beam was designed to transmit Materials through the aqueous surfactant sample, and the absorption The three siloxane surfactants, Table 1, studied in this report spectrums were collected thereafter. Between wavenumbers of 1000 and 4000 cm-1,500 scans at 2-cm-1 resolution provided the were synthesized as previously described (Snow, 1993). The averaged spectrum for each run. The spectrum of water was cation, (M~~S~O)~S~(M~)(CHZ)~N+(M~)Z(CH~)~OH, is denoted used as the reference. "Bis". NMR measurements were made using a 300-MHz Nicolet spectrometer (Model NMC 1180), operating at 60-MHz 29Si Experimental Methods resonances. Samples were sealed in 5-mm glass tubes. For each Small-angle X-ray scattering (SAXS) experiments were run, the spectrum was averaged by 2340 scans. performed on a modified Kratky camera from Anton Paar KG, Aqueous surfactant samples were prepared with distilled, Graz, Austria, equipped with an extended flight tube and a deionized water. The samples of known composition were movable beam stop (Kaler, 1982). The X-ray generator was a contained in 7-mL 1-cm-ID sealed glass tubes with 0.1-mL rotating anode ("ROTAFLEX" Model RU-200B, Rigaku Corp., volumetric tick-marks, which were held in constant-temperature Japan) operating at 10 Kw, with a copper target. The Ka (fO. 1 "C) water baths. Submerged sample tubes were gently wavelength of 1.54 A was selected by means of Nichol filters. hand-shaken for about a minute and allowed to settle from 30 The energy window on a Model MBRAUN OED-100-M 10-cm min to a few hours. Then the samples were inspected under linear position sensitive detector (Innovative Technology, Inc., transmitted visible light for turbidity and between crossed polars Newburyport, MA) was set to accept only the scattering photons (Bausch & Lomb, Rochester, NY) for birefringence. The with wavelength close to 1.54 A. The Kratky linear collimation viscosity change of the samples was roughly estimated by the produced a 15 X 0.13 mm2 X-ray area on the sample sealed in drainage time upon inverting the glass tubes. a 1.5-"-ID glass capillary (Charles Super Co., Natick, MA). The phase boundaries for each single phase region were The sample-to-detector distance was set at 68.2 cm. The estimated according to the following criteria: (1) a sharp interface detectable wave vector q range was from 0.02 to 0.3 A-l, where that totally reflected light below some angle of incidenceindicates q = (4r/X) sin(0) and 20 is the scattering angle. The scattering that two phases were definitely present, and the onset or data accumulatedover 30-240 min were corrected for background disappearance of turbidity may indicate the boundary of singlescattering by subtracting the scattering intensity of water and to-multi-phase transition; (2) birefringence observed under the empty capillary. Then the slit-smeared SAXS intensities polarized light suggests that liquid crystal phase is present. were converted numerically to pinhole, or unsmeared, intensities By repeating the visual observationsat different temperatures by means of Vonk's method (Vonk, 1971). 10 OC apart, and then 2 "C apart near cloud points, we recorded Crystal structures with unit cell dimensions smaller than 20 the temperature at onset and disappearance of cloudiness or 8, were determined by wide-angle X-ray scattering (WAXS) birefringence with f 2 OC uncertainty by crossing the phase with a Model D-500 Siemens diffractometer (Siemens Corp., boundary from below and above the phase transition temperature. Iselin, NJ). The hysteresis differences of the temperatures of onset or Cryo-transmission electron microscopy (Cryo-TEM) experidisappearance of cloudiness determined from the heating and ments were carried out as previously described (Bellare, 1988). cooling processes were always smaller than 2 OC, and so the Samples were prepared in the Controlled Environment Vitrificahysteresis effect was negligible in comparison to the uncertainty tion System (CEVS). Before introducing the sample into the fixed by the step size of temperature variation (2 "C). CEVS, the environmentalchamber was brought to a steady state at the desired temperature with saturation of water. A 3-mL Results drop of sample was placed on a carbon-coated holey polymer Aggregation Behavior and Microstructure of BisCl/Water. support film mounted on a standard 200-mesh TEM grid (Vinson, 1987). Blotting the liquid with filter paper caused a thin film Inspection of the BisCl/water samples led to the phase diagram of the sample that spanned the holes to be created. Then the grid shown in Figure 1. In the concentration range from 0 to 20 wt was plunged into liquid ethane and transferred under liquid % BisCl and in the temperature range between 5 and 75 OC, nitrogen into the cryotransfer stage (Gatan 626, Gatan Inc., PA), BisCl in water formed a clear, colorless, isotropic liquid phase LI which was inserted into the TEM (JEOL 1010, Japan Electron with low viscosity. A Cryo-TEM image shows that 5 wt % BisCl Optical Laboratory, Japan). The specimen was imaged at 100 in water forms spherical micelles at 20 OC (Lin et al., 1993). The critical micelle concentration (CMC) of the BisCl aqueoussolution kV and an underfocus of 4 mm. TEM images were recorded on was previously reported to be 0.033 M or 1.3 wt % (Snow, 1993). SO-163 film (Eastman Kodak, Rochester, NY) and were developed with full-strength D-19 developer (Eastman Kodak) Between approximately 20 and 50 wt % BisC1, samples had two for 12 min. layersseparated by a sharp reflective interface. The bottom layer displayed birefringence and had high viscosity, and the top layer The video-enhanced optical microscope (VEM) used to image was isotropic liquid. When examined by SAXS, the bottom layer some of the systems consisted of a Nikon Optiphot-Polmicroscope appeared to be a lamellar liquid crystal (LLC) phase. The upper (Nikon, Inc., Japan) fitted with rectified differential interference phase might be a coexisting L1 phase. contrast (DIC) optics. The VEM also had phase contrast and Between approximately 50 and 90 wt 75 BisC1, a single, highpolarization microscopy capabilities. The microscope was connected to a Dage Model NCG8 black and white television camera viscosity phase was found which exhibited bright yellow birefringence between crossed polars. Figure 2 shows the SAXS and equipped with a newvicon imaging tube via a Nikon 0.9-2.25X WAXS spectra of a 65 wt % BisCl sample at 20 "C. From the zoom lens (Nikon, Inc., Japan). Improved image contrast was SAXS spectrum, the wave vectors of the first- and second-order obtained through video and digital image processing by a Psicom 327 system (Perceptive Systems, Houston, TX), which allows a diffraction peaks (ql and 42) are in the ratio 1:2, which is a resolution as low as 0.025 mm. signature of a lamellar liquid crystal phase. The interbilayer
He et al.
6150 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
Phase Diagram of BisCl / H20
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Figure 2, SAXS and WAXS of a 65 wt % BisCl sample (LLC phase) at 20 O C .
distance calculated from 27r/ql is 35.1 A. The WAXS of the same 65 wt % sample, at wave vector q above 0.6 A-1, shows only a broad correlationmaximum around 6.5 A, suggesting significant
50
60
70
80
90
100
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BisCl 3'% (W/W) Figure3. Inter-bilayerspacingoftheBisClLLCsamplesatconcentrations over 50-80 wt % ' us BisCl concentrationand the bilayer thickness of the BisCl solid lamellar phase samples us BisCl concentration.
molecular disorder within BisCl bilayers. The WAXS peaks at q below 0.6 A-l are consistent with a lamellar liquid crystal phase suggested by the SAXS results reported above. This sample displayed focal conic texture under cross polarization microscopy-also a signature of LLC phase. The interbilayer distance (d-spacing) deduced from SAXS results for the LLC samplesfrom 5 5 to 80 wt 5% is plotted vs BisCl concentration in Figure 3. The d-spacing appears to be a linear function of the BisCl concentration. The linear extrapolation of the plot to 100 wt % BisCl provides an estimated bilayer thickness of 22 A. At surfactant concentrations above 90 wt % and temperatures between 5 and 75 OC, BisCl samples were solid and showed faint birefringence. Under crosspolarizationoptical microscopy, these samples showed fiber-like texture (Figure 4a) different from the focal conic texture of LLC shown in Figure 4b. Freshly precipitated (from warm hexane) and rigorously dried BisCl was a white, waxy, slightly hygroscopic solid. The microstructure of this sample was studied by WAXS at 20 OC (Figure 5 ) . The wave vectors q1 and 42 of the first- and the second-order diffraction peaks are in the ratio 1:2, consistent with lamellar packing with an interlayer distance of 23.2 A. The diffraction peaks at d-spacing of 7.8 and 5.7 A are presumably from the crystalline ordering of BisCl molecules. The WAXS result suggests that the solid phase formed lamellar bilayersin which BisCl molecules packed in long-range crystalline order. Thus, the solid phase is named as solid lamellar phase S. Ramm et al. (1990) also reported that the trisiloxane ammonium salt surfactant (Me3SiO)zSiMe(CHZ)sN+Me2RBr (R = n-C3H7-, n-C4H9-) formed a similar solid lamellar phase, wherein trisiloxane moieties and the charged head groups were segregated in different layers alternatively. Also shown in Figure 3, the bilayer spacing of BisCl solid from WAXS data, 23.2 A, agrees well with the extrapolation of the SAXS d-spacing of the L, samples to 100 wt % BisCl (22. A). The bilayer thicknesscalculated from twice the "all-trans" length
Aqueous Cationic Trisiloxane Surfactants
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The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6151
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Figure 4. VEM under DIC of (a, left) dried solid BisCl sample (S phase) and (b, right) 65 wt % BisCl in water (LLC phase) at 20 OC. Scale: 10 = 45 mm.
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q BisBr > BisI. The degree of counterion binding is proportional to the radius of the counterions in the order I- > B r > C1- (Berr et al., 1992; Lindman et al., 1980). Therefore, the
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microstructures spherical micelles vesicles, bilayers vesicles, bilayers
Coulomb repulsive forces among the positively charged ammonium head groups are reduced by the counterion binding effect, which results in lowered effective head group area. This conclusion has been further supported by the previous studies. In Snow's study (1993), the averaged packing area of surfactantmolecules at the air-aqueous solution interface, derived from surface tension measurements, is 102 and 81 A2 for BisCl and BisBr, respectively. Lin et al. (1993) added sodium salts NaCl, NaBr, and NaI to 5 wt % BisCl spherical micellar solution. The molar concentrations at which an added salt transformed the BisC1 spherical micelles to bilayer vesicles were found be in the order NaI > NaBr > NaCl. This can be explained by the counterion exchange from C1- to B r and I-, which results in increased counterion/head group binding, lowered head group repulsion, and depressed effective head group area. BisBr and BisI vesicles and bilayers may facilitate the "superwetting" of dilute surfactantsolutionson some hydrophobic solid surfaces such as paraffin wax film. This observation supports the results reported previously for nonionic trisiloxane surfactants (He et al., 1993; Hill et al., 1993; Zhu, et al., 1993). For all of these salts the phase boundaries were close to vertical. This means that temperature effects on aggregation wree not significant. Studies at higher temperatures were impeded by concerns that the surfactants might hydrolyze. The Kraft boundaries and Tu for BisBr were below room temperature. It is difficult to make an unambiguous conclusion about this for BisI.
Aqueous Cationic Trisiloxane Surfactants Conclusions
The aggregation and resultant thermodynamic phase behavior of the cationic BisX (X = C1, Br, I) surfactants in water are a function of BisX concentration and the nature of X. The patterns of aggregation are not strongly temperature-dependent. For all of these compounds, the primary structural unit in the various aggregations is the bilayer. Formatiorrof bilayers is favored by the large (relative to linear alkyl surfactants) area of the Bis cation. As surfactant concentration increasesthe degree of order in thestructureof theaggregate increases, as expected. Typically, vesicles are present at low concentrations, but they transform into liquid crystal sheet structures (LLC) and finally a lamellar solid as the BisX concentration is increased. For BisC1, micelles, instead of vesicles, form at low concentrations,reflecting the lower polarizability of the chloride anion, versus bromide or iodide. On the other hand, BisI does not appear to form a LLC phase, probably because of negligible hydration of BisI solid. Dilute solutions of BisBr and BisI (bilayer structures) superwet Parafilm, whereas BisCl (micelles) did not. This connection of bilayers to superwetting is consistent with previous studies of nonionic siloxane surfactants. Acknowledgment. This research project is supported by the
NSF Center for Interfacial Engineering (CIE) at University of Minnesota. The financial and technical support of the Dow Corning Corp. is greatly acknowleged. The authors gratefully appreciate CIE staff B. Trend, L. Sauer, and Ph.D candidate H. A. Doumaux’s expertise and support in characterization facility. M. He thanks undergraduate researcher L. Yung for his lab assistance. Dr.R.M. Hill and Dr. M. Owen provided insightful comments to this work. References and Notes (1) Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. Colloids
Surf.1990,44, 28 1. (2) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talman, Y. J. Electron Microsc. Tech. 1988, 10, 87. (3) Berr, S.; Jones, R. R. M.; Johnson, J. S.; Jr. J . Phys. Chem. 1992, 96, 5611. (4) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 11 1988,84, 1115. ( 5 ) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G.; Scriven, L. E. J . Phys. Chem. 1992, 96,474. ( 6 ) Conroy, J. P.; Hall, C.; Leng, C. A,; Rendall, K.;Tiddy, G. J. T.; Walsh. J.: Lindblom. G. Prom. Colloid Polvm. Sci. (7) Eaborn, C. ’ Organohcon Compdunds; Butterworths Scientific Publications, 1960; p 465. ( 8 ) Fabre, H.; Kamenka, N.; Khan, A.; Lindblom, G.; Lindman, B.; Tiddy, G. J. T. J. Phys. Chem. 1980,84, 3428. (9) Gradzielski. M.: Hoffman. H.: Robisch.. P.:. Ulbricht.. W.:. Gruninn. -. 8. Tekide Surf. Der. 1990, 27, 366. (10) Gruning, B.; Koerner, G. Tensuide Surf.Der. 1989, 26, 312. (11) He, M.; Hill, R. M.; Lin, Z.; Scriven, L. E.; Davis, H. T. Phase Behavior and Microstructure of Polyoxyethylene Trisiloxane Surfactants in Aqueous Solution. J . Phys. Chem., in press. (12) Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Comparison of the Liquid Crystal Phase Behavior of Four Super-wetters, to be submitted. ’
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