Langmuir 1994,10, 1724-1734
1724
Comparison of the Liquid Crystal Phase Behavior of Four Trisiloxane Superwetter Surfactants Randal M. Hill’ Central Research and Development, Dow Corning Corporation, Midland, Michigan 48686-0994
Mengtao He, H. T. Davis, and L. E. Scriven Center for Interfacial Engineering, Department of Chemical Engineering and Materials Science, University of Minnesota, 100 Union Sreet S.E., Minneapolis, Minnesota 55455 Received October 19, 1993. In Final Form: January 18,1994” The patterns of aqueous phase behavior of four trisiloxane superwettersurfactants are generally similar, but there are also substantial differences. All four surfactants readily form bilayer aggregates including vesicles, sponge phase (La),and lamellar liquid crystals. At low concentrations(in the range in which they are used as wetting agents) mixtures are turbid. Both lamellar phase and small particles of uncertain identity are present. Dilute dispersions of these trisiloxane surfactants in water exhibit an unusual macroscopic birefringence. Features of the phase behavior at higher concentration and temperature are similar to the behavior of alkyl ethoxylateorganic Surfactants. The small particlesare the common feature of these four surfactants, indicatinga possible link to superwetting. Severallines of evidence indicate that the small particles are bilayer microstructures, but cry0 transmission electron microscopy has not yet imaged any recognizable bilayer structures. Changing the surfactant structure from branched to linear has little effect on either the wetting or the phase behavior, demonstrating that the ‘T” shape of the molecule is not responsible for superwetting. End-capping groups have a significantly greater impact on the phase behavior than does the structure of the siloxane hydrophobe; changing the end cap of the poly(oxyethy1ene) group from -OH to -OMe or -OAc shifts phase boundaries to substantially lower concentrationsand temperatures. The end-cap effect can be attributed to modification of the attractive potential between the surfactant particles.
Introduction A number of small siloxane surfactants exhibit “superwetting” or “superspreading”-an unusually rapid spreading of a dilute (about 0.1 w t %) aqueous dispersion of the surfactant over a hydrophobic surface (such as paraffin wax film or waxy leaf surfaces).13 Most known superwetters are variations of one molecular structure: a trisiloxane hydrophobe coupled to a poly(oxyethy1ene) hydrophilic group of 7-8 EO units:
((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)7,0P P is an end-capping group. The usual shorthand notation for this structure is M(D’E,)M where M stands for (CH&SiO- and D’ stands for -Si(CH3)(R)-. It has been shown that a monodisperse EO2 and, recently, a quaternary derivative in the presence of
* To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, April 15,1994. (1) Anderson, N. H.; Hall, D. J. The role of dynamic surface tension in the retention of surfactantsprays on pea plants. Adjuvants Agrochem. 1989, 2, 51. (2) Penner,D.;Roggenbuck,F. C.;Burow, R. F.;Petroff,L. J.;Thomaa, B. Determinationof whether dosage of concentrationis the critical factor forefficacyofSylgard309organosiliconeadjuvant.Med.Fac. Landbouww. Rijksuniu. Ghent 1991,56, 827. (3) Roggenbuck, F. C.; Rowe, L.; Penner, D.; Petroff, L.; Burow, R. Increasing postemergence herbicide efficacyand rainfastnesswith silicone adjuvanta. Weed Technol. 1990,4,576. (4) Ananthapadmanabhan,K. P.; Goddard,E. D.; Chandar, P. Colloids Surf. 1990,44,281. (5) Zhu, X. Surfactant Fluid Microstructure and Surfactant Aided Spreading. Ph.D. Thesis, University of Minnesota, 1992. (6) He, M.; Hill, R. M.; Lin, Z.; Davis, H.T.; Scriven, L. E. Phase behavior and microstructure of polyoxyethylene trisiloxane Surfactants in aqueous solution. J.Phys. Chem. 1993, 97, 8820.
salt7 also exhibit superwetting. The three commercial examples differ in the end-capping of the poly(oxyethylene) group: -OH, -OMe, or -OAc (see below under Materials). There is, at present, no adequate explanation of what is so unique about this molecular structure. Kanner et al., reported in 1967 that solutions of certain small siloxane surfactants spread rapidly to a thin film (wet out) on polyethylene and polystyrene. They do not use the term superspreading, and it is not clear whether their materials would be considered superwetters or not; the only data they actually reported relate to the extent of spreading not the rate of spreading. The surfactants investigated did not include any based on the MD’M hydrophobe. They concluded that the best wetting agents were based on siloxanehydrophobes containing 2-5 silicon atoms with a very specific solubility balance-a small but finite solubility in water. For example, they found MM’E4.8OMe (M’ stands for (CH&(R)SiO-) to be a good wetting agent, compared with MM’Es.sOMe which was more soluble, but much less effective-a change of less than one EO group! They defined solubility in terms of the cloud point, but we now know that many siloxane surfactants form turbid lamellar-phase dispersions that are not related to the cloud point.@ Ananth et aL4 attributed the rapid spreading to the unique “T” or “umbrella” shape of the trisiloxane hydrophobe. We here report results which demonstrate that this cannot be the explanation. Kanner et al., found the “best”wetting in turbid solutions, and ZhuSshowed a linear (7) Lin,2.; He, M.; Davis, H. T.; Scriven, L. E., Snow, 5. A. Vesicle formation in electrolyte solutions of a new cationic siloxane surfactant. J. Phys. Chem. 1993,97,3571. (8) Kanner, B.; Reid, W. G.; Petersen, 1. H. Znd. Eng. Chem. Rod. Res. Dev.1967,6,88. (9) Hill,R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Lyotropic Liquid Crystal Phase Behavior of Polymeric Siloxane Surfactanta. Langmuir 1993,9, 2789.
0743-7463/94/241Q-1724$04.50/00 1994 American Chemical Society
Four Trisiloxane Superwetter Surfactants correlation between turbidity and rate of spreading; thus, the unusual spreadingdepends in some way on the presence of a dispersed second phase. Zhu5 related the rate of spreadingto the transport rate of surfactant from dispersed particles to the liquid/solid and liquid/gas interfaces at the spreading front. He showed that reducing the particle size accelerates the spreading, but that the extent of spreading is limited by the inventory of surfactant present in the solution. Surfactant transport between the bulk and interface in micellar solutions takes place by dissolution of micelles and diffusion of individual molecules.10 Zhu assumed that a similar particle dissolution/molecular diffusion process occurs in superwetter solutions, but the possibility of convective transport of these much larger particles in the turbulent vicinity of the spreading front should not be ruled out. Trisiloxane surfactants with slightly larger EO groups form micellar solutions with even smaller particle sizes but do not superwet. Spreading of two-phase dispersions is obviously a complex problem which has not yet been adequately studied. While this is a fascinating and challenging problem, it is not the purpose of this paper to attempt a definitive explanation of this unusual spreading, but rather to lay the ground work for such an understanding by clearly delineating the aqueous phase behavior of the surfactants involved. The aqueous phase behavior of these trisiloxane surfactants exhibits a number of new and unusual features worthy of study quite apart from possible links to the spreading problem. Trisiloxane surfactants are an important class of novel surfactants used primarily as wetting agents. This investigation was therefore carried out to determine the liquid crystal phase behavior of the three commercial superwetters. We were looking for unique features of the phase behavior which would give us clues to the nature of the dispersed second phase and perhaps to the origin of the rapid spreading. As we will show, although the phase diagrams are generally similar, they are more different than would be expected for three such similar molecules. The complete phase diagram of the quaternary derivative in the presence of sufficient salt to induce superwetting has not been determined. During the course of this work we prepared an MDM' variant of the trisiloxane structure in which the poly(oxyethy1ene) group is attached terminally to the siloxane group rather than in the middle. Since this surfactant turned out to exhibit similar rapid wetting, we included it in the phase study also. Gradzielski et al." showed that the hydrophobicity of the trisiloxane hydrophobe is comparable to that of the linear dodecyl hydrophobe. The phase behavior of the nonionic alkyl ethoxylate surfactants, CiEj, has been extensively investigated.l"15 The phase diagrams of C12E5
Langmuir, Vol. 10, No. 6,1994 1725 100 lo)
0
1
L,+L2
25
50 C,&
75
100
(-1
Figure 1. (a, top) Phase diagram of C12Ea. Reprinted with permission from ref 14. Copyright 1990 Royal Society of Chemistry. (b, bottom) Phase diagram of C12Ee. Reprinted with permission from ref 12. Copyright 1983 Royal Society of Chemistry. (from ref 14) and ClzEs (from ref 12) are shown in Figure 1. For C12E8, the phase diagram is dominated by a large region of hexagonal phase. The lower consolute temperature (LCT) boundary lies well above the H1 region, and approachesthe temperature axis at about 75 "C. For C135, the phase diagram is dominated by an La region which penetrates through the LCT boundary.l5 The LCT boundary has moved downward about 45 "C compared with that of C12Ea.
Comparison of these two diagrams aptly illustrates two of the factors which influence surfactant phase behavior: the tendency to form a particular phase depends largely on molecular shape (the relative sizes of the hydrophobic and hydrophilic groups), while the location of the LCT boundary depends on solute-solvent interactions. De(10) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; creasing the EO chain length from 8 to 5 units shifts the Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. Theory of the kinetics of micellar equilibria and quantitative interpretation of chemical molecular shape from hexagonal to lamellar packing, but relaxationstudiesof micellarsolutionsof ionicsurfactants. J.Phys. Chem. also decreases the interaction with water, causing the LCT 1976, 80,905. (11) Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbricht, W. The to shift downward. Molecular packing calculations of the aggregationbehavior of silicone surfactants in aqueoussolutions. Tenside, type illustrated by Mitchell et al.12indicate that M(D'E8)M Surfactants, Deterg. 1990, 27, 366. (12) Mitchell,J.D.;Tiddy,G. J.T.;Waring,L.;Bostock,T.;McDonald, should favor lamellar packing similar to that of C12E5; the M. P. Phase behavior of polyoxyethylene surfactants with water. MD'M trisiloxane hydrophobe occupies approximately Mesophase structures and partial miscibility (cloud points). J . Chem. twice the cross-sectional area of the Clz hydrophobe. SOC.,Faraday Trans. 1 1983, 79,975. (13) Bleasdale,T.A.;Tiddy,G. J. T. SurfactantLiquidCrystals. NATO However, the location of the LCT boundary should be ASI Ser., Ser. C 1990,324, 397. similar to that of C12Ea. Thus, we might expectthe aqueous (14) Strey, R.; Schomacker, R.; Roux, D.; Nallet, F.; Olsson, U. Dilute phase behavior of M(D'E8)M to be similar to that of lamellar and L3 phases in the binary water-C12E5 system. J.Chem. SOC., Faraday Trans. 1990,86,2253. in regard to the location of the lamellar phase region, but (15) Lang, J. C.; Morgan,R. D. Nonionic surfactant mixtures. I. Phase similar to that of Cl2E8 in regard to the location of the equilibria in ClOE4 - H20 and closed loop coexistence. J.Chem. Phys., LCT boundary. 1980, 73, 5849.
Hill et al.
1726 Langmuir, Vol. 10, No. 6, 1994 Table 1. Trisiloxane Surfactant Nomenclature and Structures. name origin structure M(D’E80H)M Dow Corning Corp. (Me3SiO)#i(Me)RlOH M(D’E8OAc)M Dow Corning Corp. (Me&3iO)2Si(Me)RlOAc MDM’EsOH Dow Coming Corp. MeaSiOSi(Me)lOSi(Me)zRlOH M(D’E8OMe)M Union Carbide Corp. (MeaSiO)zSi(Me)RPOMe
50
r j
0
\
I
I
I
-5.00
4.00
-3.00
a Me stands for CHa, R1 for - C H ~ C H ~ C H Z ( O C H ~ C R2Hfor ~)~, -CHzCHzCHz(OCH&H2)7.6-, M for MeaSiO-, D for -Si(Me)ZO-, M’ for RME2SiO-, D‘ for -Si(Me)(R)O-, E or EO for -OCH&Hr, and Ac for -CH&(O)OH.
Experimental Section Methods. All samples were prepared by weight in glassscrewtop test tubes using Milli-Q deionized water and mixed on a rotatingmixerfor at least 12hat 22.5OC before initialobservations were made. The presence of liquid crystals was detected by observingsamplesbetween two crmsed sheets of polarizingplastic film. This was especially useful for these systems because they tended to give faint birefringence on a polarized light microscope. (Birefringence means that the sample appears bright or colored when viewed between crossed polarizers. See photograph later in this paper for examples. Birefringence indicates the presence of anisotropic long-range order in the sample.) It also allowed us to observe flow birefringence, and to determine when two layers (indicating two phases) were present. Particular liquid crystal phases were identified by observing the texture of the birefringence pattern on a polarized light microscope. The only birefringent phase actually observed in this study was the lamellar phase. A polarized light microscope equipped for differential interference contrast (DIC) was used to observe vesicles. Further (analog) contrast enhancement by a Hamamatsu C2400 CCD video camera often revealed the presence of small particles, or faint, rapidly fluctuating vesicle walls, that were difficult to see with the unaided eye. We located two-phase to one-phase transitions by observing the temperature at which mixtures changed from turbid to translucent (isoplethalmethod). This appeared to be satisfactory in regions where water-rich isotropic solution and lamellar phase were in equilibrium. (Isotropic means that the samples are not birefringent between crossed polars. This can mean either no long-range order in the solution or ordering which is spherically symmetric.) However, mixtures of lamellar phase (L,) and surfactant-rich isotropic solution (L2) were usually clear, so this criteria should be used with caution. The LCT boundary was determined by heating samples using a heat gun until they became cloudy-gray and isotropic. As the samples cooled, they went through a sequence of transitions: cloudy to clear (isotropic) to birefringent. The temperatures of these transitions were noted and are plotted on the phase diagrams. The temperatures on the cooling cycle were generally used rather than the heating cycle because many of the samples were quite viscous and difficult to heat uniformly but became much more fluid on crossing the LCT boundary. Thus, temperature homogeneity was best on the cooling cycle. A few water bath measurements carried out to confirm the transition temperatures obtained from this procedure gave essentially the same values. Materials. The nomenclature and structures of the four surfactants investigated are given in Table 1. The first three surfactants were prepared at Dow Corning Corp., Midland, MI. MDM’EBOH was made by hydrosilylation of 1,1,1,3,3,5,5-heptamethyltrisiloxane and H ~ C ~ H ~ C H ~ E Ousing E O chloroplaH tinic acid catalyst and standard conditions. The reaction was rapid and complete within 60 min. M(D’E8OMe)M was the commercial surfactant L77 obtained from Union Carbide Corp. As studied, all four surfactants were 95% “pure”, the balance being unreacted poly(oxyethy1ene). The poly(oxyethy1ene)portion of all four was polydisperse (M,IM, = 1.2 by gel permeation chromatography (GPC)). The point of attachment of the polar group to the hydrophobe for the first three surfactants is to the middle silicon atom; hence, these surfactants have a T or “branched” structure.‘ The point of attachment for the fourth surfactant is to the terminal silicon; this surfactant therefore has a “linear” structure. We put the
10
-6.00
-2.00
log (molkg)
Figure 2. Surface tensions of aqueous solutionsof M(D‘Q0H)M at 25 OC. branched and linear in quotes because the methylsiloxane structure is itselfbranched; we are referring to the superstructure of the molecule.
Results M(D’EBOH)M/WaterSystem. Figure 2 shows a plot of the surface tension vs log concentration for M(D’E80H)M. There is a sharp break in the curve at 1.24 X lW mol/kg (=0.007wt %) which marks the onset of aggregation. The aggregates may be micelles or bilayer structures, or both may coexist.16 The area per molecule is 59 A2,and the minimum surface tension is 21.0dyn/cm. There is no minimum in the surface tension data in the vicinity of the break, indicating that no significant level of surface-active impurities is present. We did not determine whether the onset of turbidity coincided with the break in the surface tension curve. Ananth et found that the onset of turbidity for the -0Me variant (discussed in the next section) occurs at about 0.007 wt % ’ , which is very close to the break in the surface tension curve for both the -0Me and the -OH variants. The phase diagram of M(D’E80H)M is shown in Figure 3. The center of the phase diagram is dominated by a region of lamellar phase, La,which leans toward lower concentrations a t higher temperatures. No hexagonal or cubic phases are found. Between 4% and 40% surfactant, the behavior is complex. At room temperature, mixtures in this range are cloudy and birefringent, indicating the presence of liquid crystals. At higher temperatures, a clear isotropic phase appears which is identified as the so-called “anomalous” or sponge phase, L3,17 by analogy with the phase diagram of C12E6.12J4 Small-angle neutron scattering (SANS)and cryo transmission electron microscopy (cryo-TEM)work is in progress to positively identify the microstructures present in this region. Above the L3 region, mixtures again become cloudy but isotropic. This (16)Lin, Z.; Hill, R. M.; Scriven, L. E.; Davis, H. T.; Talmon, Y. Langmuir, in press. (17) Strey, R.; Winkler, J.; Magid, L. Small angle neutron scattering from diffuse interfaces. 1.Mono- and bilayers in the water-octane-Cl2ES system. J. Phys. Chem. 1991,95, 7502.
Langmuir, Vol. 10, No. 6, 1994 1727
Four Trisiloxane Superwetter Surfactants 100
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o z
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??
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L
!
0
n I-
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Weight % Surfactant
Figure 3. Phase diagram of the M(D’EBOH)M/water system. The dashed line marks part of the boundary of the L, region as uncertain. The squares mark the concentrations actually prepared.
liquid-liquid insolubility boundary is the LCT boundary, or cloud point.’8 The water-rich isotropic phase in equilibrium with L3 or L2 above the LCT is very dilute, and its composition has not been determined. Between 40% and 45%, no clear isotropic region is found directly above the L, region. Since both L3 and L, appear to show a maximum, we presume that a three-phase discontinuity touches the top of both of these regions, but we did not experimentally resolve the feature. The upper boundary of the L, region in this vicinity was not well-resolved and is drawn as a dotted line for this reason. Above 45 76, the clear isotropic surfactant-rich phase, L2, is found above and to the right of L,. Although this phase is optically isotropic, small-angle X-ray scattering (SAXS) results6 indicate that it, like L3, contains shortrange ordering, perhaps related to the structure of the L3 phase. Recent SAXS and SANS measurements indicate that anhydrous M(D’E80H)M and some other siloxane surfactants form ordered liquids, presumably with alternating poly(oxyethy1ene)and (methy1)siloxanedomains,lS as do many other block copolymers.20p21A number of phase diagrams including those of C12E3,12C12E8,12and M(D’E12)Me show clear isotropic solutions connecting regions usually labeled L1, L2, and L3. The evolution of surfactant aggregate structures within and between such regions, (18) For hydrocarbon nonionic surfactants, this liquid-liquid immiscibility involves the equilibrium of two isotropic liquid phases (for example, L1 and Lz). Thus, the upper boundary of the L3 region, where an isotropic liquid phase is in equilibrium with the bilayer-containing Ls phase should probably not be referred to as the LCT. However, as this paper makes clear, there is a fundamental difference in the relative locations of the LCT and the liquid crystal phases for trisiloxane surfactants. In any case it should be obvious from the phase diagram which phases are in equilibrium. (19)He, M.; Hill, R. M. Results to be reported. (20) Almdal, K.;Bates, F. S.; Mortensen, K. J. Order, disorder, and fluctuation effects in an asymmetric poly(ethy1ene-propylene)-poly(ethylethylene) diblock copolymer. Chem. Phys. 1992, 96,9122. (21) Galin, M.; Mathis, AStructural and thermodynamic study of dimethylsiloxane-ethyleneoxide PDMS-PEO-PDMS triblock copolymers. Macromolecules 1981, 14, 677.
particularly when they are contiguous,has not been studied much.6 Strey et al.14 have shown that the lower and leftward boundaries of the L, region are sensitive to agitation and sample purity. We had considerable difficulty resolving these boundaries with precision; hence, we mark them as dotted lines in Figure 3 to reflect this uncertainty. We intend to address this issue in a future paper. Dilute mixtures (
