Langmuir 1992,8, 2611-2619
2671
Influence of Solvent-Headgroup Interactions on the Formation of Lyotropic Liquid Crystal Phases of Surfactants in Water and Nonaqueous Protic and Aprotic Solvents X. Auvray,. T. Perche, C. Petipas, and R. Anthore U.R.A. CNRS 808, Facultb des Sciences et des Techniques, 76134 Mont Saint Aignan Chdex, France
M. J. Marti, I. Rico, and A. Lattes Laboratoire des I.M.R.C.P. U.R.A. CNRS 470, Universitk Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cbdex, France Received April 22, 1992. In Final Form: July 27, 1992 A fast method for the analysis of the phase diagrams of lyotropic compounds was employed to study the formation of liquid crystala from two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPBr) in water. Studies were also carried out in the protic solvents, glycerol (G), formamide (FA), ethylene glycol (EG), and N-methylformamide (NMF), and the aprotic solvents, dimethylformamide(DMF) and N-methylsydnone(NMS). Wile the normal successionof ordered phases appeared to be governed by geometricconstraints of interface curvature, the differences in behavior were accounted for by the differences in cohesion energy of the solvents and the different natures of the polar heads of the two surfactants. In DMF, a solvent with low cohesion energy, both surfactants showed only lamellar phases, whereas CPBr with a highly delocalized charge on the polar head displayed a succession of conventionalphases in all the other solvents. CTAB with a localizedcharge formed only lamellar phases in NMF and NMS. This behavior was interpreted aa resulting from headgroup solvation due to dipoledipole interactionsor hydrogen bonding. The particular case of NMS was accounted for by better stacking between the planar molecules of this solvent and the pyridinium rings of CPBr.
Introduction The formation of lyotropic liquid crystals is generally observed in aqueous solutions, and a large number of studies have been devoted to the succession of different phases. The formation of 1D lamellar phases in nonaqueous systems was first demonstrated with lecithins in ethylene glycol.' Recently, it has been reported that conventional ionic and nonionic surfactanta can also associate to form lyotropic phases in various nonaqueous polar solvents,2+ and in particular in formamide (FA). This structured liquid is a good solvent for both hydrogenated'~~ and fluorinated surfactanta~Jo and numerous studies have been carried out with allrylammoniumhalides such as cetyltrimethylammoniumbromide (CTAB).26J1*12 Other nonaqueous polar solventa have also been studied including ethylene glycol (EG), glycerol (G) N-methyl(1) (a) Mouchardieh, N.;Friberg, S. E. Mol. Cryst. Liq. Cryst. 1979, 49,231. (b) Larsen, D. W.; Friberg, S. E.; Christenson, H. J. Am. Chem. SOC.1980, 102, 6565. (2) Belmajdoub, A.; Marchall, J. P.; Canet, D.; Rico, I.; Lattes, A. New J. Chem. 1987,11,415. (3) Bergenstahl, B. A.; Stenius, P. J. Phys. Chem. 1987,91,5944. (4) Warnheim, T.;Jonsson, A. J. Colloid Interface Sci. 1988,125,627. J.Phys. (5) (a)Auvray,X.;Anthore,R.;Petipas,C.;Rico,L;Lattes,A. Chem. 1989,93,7458. (b) Auvray, X.;Anthore, R.; Petipas, C.; Rico, I.; L a t h , A. C. R. Acad. Sci. Pans, Ser. 2 1988,306,695. (6) Auvray, X.;Danoix, F.; Perche, T.; Duval, P.; Petipas, C.; Rico, I.; L a t h , A. C. R. Acad. Sci. Paris, Ser. 2 1990,310,471. (b) Auvray, X.; Perche, T.; Anthore, R.; Petipas, C.; Rico, I.; Lattes, A. Langmuir 1991, 7 (lo), 2385. (7) Escoula, B.; Hajjaji, N.; Rico, I.; Lattes, A. J. Chem. SOC.,Chem. Commun. 1984,1233. (8) Marti, M. J.; Rico, 1.; Ader, J. C.; De Savignac, A.; Lattes, A. Tetrahedron Lett. 1989,30, 245. (9) Rico, I.; Lattes, A.; Das, K. P.; Lindman, B. J. Am. Chem. SOC. 1989,111,7266. (10) Cautier, M.;Rico, I.; Lattes, A. J. Org. Chem. 1990,55, 1900. (11) Sjoberg, M.; Henrikseon, U.; Wemheim, T. Langmuir 1990, 6, 1205.
and N,N-dimethylformamide (NMF and DMF),and N-methylsydnone (NMS).13J4The latter is an aprotic polar compound in which aggregates can form, despite the absenceof hydrogen bonding. This property depends essentially on the cohesion energy density (CEDI. It appears from the values of various solvent parametera listed in Table I that the ability to form micelles increases with CED. In the formation of lyotropic phases, the two structural sequences of ordered phases most frequently found with decreasing surfactant concentration in binary ionic surfactant/nonaqueous polar solvent systems are as follows: sequence 1,lamellar phases L, *i* cubic Q, (space group Ia3d) two-dimensional hexagonal phase H, @6m) c) micellar phase I,; sequence 2, L, t;* isotropic phase. The transitions between ordered phases correspond to a change in the curvature of the interfaces between the solvent and the aliphaticmedium. Sequence 1corresponds to the following transitions: zero curvature (La) mean curvature of zero (Bp is an infinite periodic minimal surface (IPMSof Schwartztype G)) one zero principalcurvature (H,)tr* devoid of zero curvature. Charvolin and S a d o P have accounted for these transitionsin geometricterms, and the universalityof sequence 1in nonaqueous polar solventa appears to bear this out. The isotropicphase I, followingphase H, generally consista of disordered cylindrical micelles with a diameter similar
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(12) Perche, T.;Auvray, X.; Anthore, R.; Petipae, C.; Rico, I.; Lattea, A. J. Phys. I2 (Paris), in press. (13) Beesley, A. H.; Evans, D. F.; Laughlin, R. G. J. Phys. Chem. 1988, 92, 791. (14) Auvray, X.;Perche, T.; Anthore, R.; Petipas, C.; Marti, M. J.; Rico, I.; Lattes, A. J. Phys. Chem. 1990,94,8604. (15) Becher, P. J. of Disp. Sci. Technol. 1984,5 (11, 81. (16) Charvolin, J.; Sadoc, J. F. J. Phys. I2 (Paris) 1987,48, 1559.
0143-1463/92/2408-2611$03.00/0Q 1992 American Chemical Society
Auuray et al.
2672 Langmuir, Vol. 8, No. 11, 1992 Table I. Properties of Solvents at 26 OC (Except for NMS Which Are Given at 40 "C)*
water FA
G NMS*
EG NMF DMF
I8 109 42.9 144 31.1 182.4 31
6.2 12.5
72.8 58.2 63.4 51 47.1 40 35.2
24 1.6 12.8 12.8
50 21.3 29.1 11.2 12.5
21 11 17 14 12 10 8
+ + + + + -
0 Key: G, glycerol; FA, formamide; EG, ethylene glycol; NMF, N-methylformamide; NMS, N-methylsydnone;DMF, dimethylformamide. At 40 "C. cI = relative dielectric constant. P = dipole moment. e ro= surface tension of solvent. f roIw = interfacialtension of water/hexadecane and other solventa/dodecane. 8 CED = cohesion ita molar energy density of solvent calculated from r ~ / u l /u~being , volume. Note that from experimental determination of the AU of vaporization, formamide is identical to that of glycerol, as indicated in thetable.I6 Datataken fromHandbook ofChemistryandPhysics, 51st ed.; The Chemical Rubber Co.: Cleveland, OH, 1970,except for NMS, ref 13.
to that of the ordered micelles.12 Recently, a cubic phase has been observed by polarized light microscopy between the I, and Haphases in the C#Br and C2oPBr/FA and CuPCl/FA and G systems17 (CleP, ClsP, and C20P are cetyl-, octodecyl-, and eicosadecylpyridinium groups, respectively). By analogy with certain surfactant/water systems, this phase was identified as an I1 cubic phase formed from almost spherical micelles or rodlike aggregates with an axial ratio of approximately 2:1.18J9 The Ha- I1transition thus corresponds to the disappearance of the zero principal curvature, and the I1 I, transition to the change from an ordered to a disordered phase. The phase transition can be accounted for in terms of a competition between interface curvature and chain stacking. Curvature is essentially a result of tangential forcesbetween polar heads linked by solvent molecules. The size of the polar head and its charge distribution are the main factors affecting interactions with solvent molecules. Solvents can be characterized by their dipole moment, steric hindrance, and their capacity to form hydrogen bonds. Formation of the I1phase with C1- as counterion indicatesthe importance of the counterion. Increasing concentration leads to a change in solvation and which in turn induces the elongation of micelles in the I, phase. All these factors affect the intermicellar repulsive forces and the surface area per polar head.17 On the other hand, the counterion appears to have little influence on the L, 0 Q, 0 Ha transitions in nonaqueous polar solvents.20 If the elastic energy required to bend the lamelles is too great, they break up, giving an isotropic phase (sequence 2). The presence of structured aggregates has yet to be demonstrated in the isotropic phase, although various techniques In aqueous have detected micelles in sequence 1.11J2t21 systems,there is a much greater variety in phase behavior, and numerous ordered phases have been described.22The determination of certain diagrams, such as that of sodium dodecyl sulfate (SDS),has required much patient
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(17) Bleasdale,T.A.;Tiddy, J. T.; Wyn-Jones, E.J.Phys. Chem. 1991, 95,5385. (18)(a) Charvolin, J.; Sadoc,J. F. J.Phys. ZZ (Paris)1988,49,521. (b) Charvolin, J.; Sadoc, J. F. J. Phys. Chem. 1988, 92,5787. (19) Fontell, K. Colloid Polym. Sei. 1990,268, 264. (20) Auvray, X.;Perche, T.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A. To be submitted for publication. (21) Sjoberg, M.; Jansson, M.; Henriksson, U. Langmuir 1992,8,409. (22) Ekwall, P. Advances in Liquid Crystals; Academic Press: New York, 1976.
~ 0 r k , 6while ~ * many ~ ~ ~others ~ remain to be completely elucidated. Sequence 1 is rarely observed in these diagrams, as with increasing concentration the H, phase is transformed into a monoclinic or centered rectangular phase. This occurs prior to the formation of the Q, phase in the CTAB/watersystem394*26 or before the succession of three-dimensionalphases.6b*25 To investigate polar headsolvent interactions, we compared the behavior of two surfactants, CPBr and CTAB, which differ essentially in the nature of their polar heads: diffuse (CPBr)or localized charge (CTAB);different shapes, although both are bulky. The relevant characteristics of the various solventa examined are listed in Table I. The two surfactantsCPBr and CTAB at concentrations (by weight) of several percent are known to produce highly elongated micelles in water even in the absence of ~ a l t . ~ ~ ? ~ Various binary diagrams have been drawn up for these two surfactants in water,3~26~29 and we have described the behavior of the CPBr, CTAB/NMS4 and CTAB/water, FA, and G systems in previous publication^.^ In the present study, we show for the first time the unambiguous presence of ordered lyotropic phases Q, and H a in N-methylformamide. In addition to polariid optical microscopy, we employed a fast method of X-ray diffraction adapted for rapid analysisof equilibrium diagrams. The diagramsobtained are compared to those reported by Laughlin30 and Keki~heff.~~
Materials and Methods Materials. Cetyltrimethylammonium bromide (Merck,99% minimum purity) was used as supplied. CPBr (Fluka) was recrystallized 4 times from ethyl acetate/acetone (50/50 (v/v)) and dried in avacuum oven at 30 "C. Elemental analysisindicated a purity of >99 % . Formamide (Aldrich,99% spectrophotometric grade), glycerol (Aldrich, 99+ % , spectrophotometric grade), ethylene glycol (Aldrich,99+ % ,spectrophotometricgrade),N-methylformamide (Aldrich, 99%), and N,N-dimethylformamide (Aldrich, 99+ %, A.C.S. spectrophotometric grade) were kept over molecular sieves (3 A) and used as supplied. N-Methylaydnone was syntheeized using a procedure described el~ewhere.~'.~~ The water content (0
