Langmuir 1993,9,444-448
444
X-ray Diffraction and Freeze-Fracture Electron Microscopy Study of the Cubic Phase in the Cetylpyridinium Chloride/ Formamide and Cetyltrimethylammonium Chloride/ Formamide Systems X. Aumay,' M. Abiyaala, P. Duval, and C. Petipas URA CNRS 808, UFR des Sciences, 76134 Mont Saint Aignan Ckdex, France
I. Rico and A. Lattes Laboratoire des IMRCP, URA CNRS 470, Universitk Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Ckdex, France Received June 22,1992. In Final Form: August 24,1992 A cubic phase (I1)forming between the isotropic micellar and the hexagonal I& phases was detected in the hexadecylpyridinium chloridelformamideand hexadecyltrimethylammonium chloridelformamide systems. The lattice parameters of the phase were determined by X-ray diffraction. Attribution of the space group Pm3n and the size of the ordered micelles was based on the models proposed by Fontell, Charvolin, and Vargas. The structure of thie phase was supported by transmission electron microscopic examination of the freeze-fracture replica.
Introduction
A cubic phase referred to as I1 ( Q or I"1) forms in some ionic surfactant/water systemsbetween the micellar phase I, and the two-dimensional hexagonal phase Ha. It has been observed with the alkyltsimethylammonium(C,,Hh+lN+(CH&, n = 10,12,14),14 chlorides, dodecyl- and Gtradecylpyridinium chlorides? and aqueous solutions of hexadecyltrimethylammoniumsulfate.l0 Structural studies have been carried out on solutions of dodecyltrimethylammonium chloride ((DTA)Cl)by X-ray diffraction,2?5 NMR? or photobleaching.ll The I, I1 c)H, transitions have also been observed with lysolecithins, alkylethylene oxides (C1zEOa),l2lysophosphatidylcholines,and zwitterionic surfactants in water,213J3J4as well as in the ternary systems comprising sodium or potassium soapslorganic solvents/water.2~~ Recent investigations by transmission electron microscopy at low temperature and X-ray diffraction have demonstrated the formation of an I1 phase in the complexnonionicsurfactant HCO-GO/water eysteml5 (this surfactant is a derivative of castor oil with a chain of approximately 20 ethylene oxide groups attached to carbon number 9 of the fatty acid chain (Figure 1 of ref 16)).
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(1)Ekwall, P. Advances in Liquid Crystals; Academic Press: New York, 1975. (2)Tardieu, A.; Luzzati, V. Biochim. Biophys. Acta 1970,219, 11. (3)Fontell, K.; Fox,K.;Haneson, E. Mol. Cryst. Liq. Cryst.Lett. 1986, 1, 9. (4)Fontell, K. Colloid Polym. Sci. 1990,268,264. (5)Mariani, P.;Luzzati, V.; Delacroix, H. J. Mol. Biol. 1988,24,165. (6)Charvolin, J.; Sadoc, J. F. J. Phys. 1988,49, 521. (7)Johaneeon, L. B.; Soderman, 0. J. Phys. Chem. 1987,91,5275. (8) Blackmore, E. S.; Tiddy, G.J. T. J. Chem. SOC.,Faraday Trans. 1988,2,84. (9)Vargae, R.; Mariani, P.; Gulik, A.; Luzzati, V. J. Mol. Biol. 1992, 225,137. (IO)Maciejeweka, D.; Khan, A.; Lindman, B. h o g . Colloid Polym. Sci. 1987,79, 174. (11)Cribier,S.;Bourdieu,I.;Vargae,R.;Gulik,A.;Luzzati,V. J.Phys., CO~~O iOw.. 51. ~ 7 ) .105. (ld Mitchell,'J.;Tiddy, G. J. T.; Waring, L.; Boetock, T.; McDonald, M. P. J . Chem. SOC.,Faraday Trans. 1 1988,79,975. (13)Erihon, P. 0.; Lindblom, G.;Arvidson, G.J. Phys. Chem. 1987, 91. ~.846. . ~ . (14)Amrhar, J.; Chevalier, Y.; Gallot, B.; Le Perchec, P.; Auvray, X.; Petipae, C. To be published. (15)Burns,J. L.;Cohen, Y.; Talmon, Y. J. Phys. Chem. 1990,94,5308.
0743-7463/93/2409-0444(04.00/0
Several models have been suggested for this cubicphase of space group Pm3n. Models consistent with the diffraction diagrams and the NMR findings are based on ordered, spherical or ellipsoidal micelles with 8 micelles in a unit cell,416although Burns16has proposed a structure with 24 spherical micelles. Viscous isotropic phases have recently been observed by polarized optical microscopy between the isotropic micellar phases and the anisotropic H, phase in nonaqueous polar solvents such as formamide (FA) and glycerol (G)with both anionic9 and cationicsurfactants17 such as the alkylpyridinium chlorides and bromides ((ClsP)Cl,(ClsP)Br,and (Cd)Br,only inFA). BYanalogy with the surfactant/water phase diagrams, this phase was assigned to the I1 phase. The diffraction diagrams presented here show that this viscous isotropic phase is a cubic phase of space group PmBn, and that it ale0 forms with cetyltrimethylammoniumchloride ((CTA)Cl)in FA. The lattice parameters and the size of the micelles based on Pm3n phase models were determined. The method used in a previous X-ray diffraction study of the CTAB/polar solvent and (CP)Br/polar solvent systems18J9was employed to study the transitions between the micellar and the ordered lyotropic phases. The 11 H, was observed in the solutions of sequence I, (CP)Cl and (CTA)Cl in formamide. The 11 phase was examined by freeze-fracture electron microscopy.
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Materials and Methods Small-angle X-ray diffraction experimenta were carried out using a conventional setup with a linear localizing detector and a wavelength of 0.154 nm. The solution was placed in a l-mmthick cell with Mylar windowe, which could be heated from 6 to 95 "C in 0.1 O C incrementa. In this way the behavior of the crystalline surfactantlsolvent could be explored along the 80called T,curve separating the micellar phase or ordered lyotropic (16)Jonetromer, M.;Sjoberg, M.;Warnheim, T. J. Phye. Chem. 1990, 94,7549. (17)Bleasdale,T. A.;Tiddy,J. T.;Wyn-Jonea,E.J.Phys. Chem. 1991, 95,5385. (18)Auvray, X.;Perche, T.; Anthore, R.; Petipae, C.; Rico, I.; Latten, A. Langmuir 1991, 7,2385. (19)Auvray, X.;Perche, T.; Anthore, R.; Petipaa, C.; Marti, M.J.; Rico, I.; Lattes, A. to be published in Langmuir.
CP 1993 American Chemical Society
Cubic P h e in the (CP)ClIFA a n d (CTA)ClIFASystems
Langmuir, Vol. 9, No.2, 1993 446
phase regions to the crystal region. The progressive dissolution of the crystalline surfactant was followed from the characteristic diffraction peaks corresponding to distances d between ordered layers with rigid aliphatic chains (d = 2.75 nm for (CP)Cl and 2.78 nm for (CTA)Cl). These peaks overlap with the diffraction diagram of the lyotropic phases. In the presence of formamide,solvationof the cryatal of (CTA)C1was characterized by an increase in the lattice distance to 3.14 nm. Two crystals of (CP)Cl were observed in formamide; they were characterized by lattice distances d = 2.75 nm and d = 2.95 nm for the solvated crystal. The (CP)Cl/formamide system was examined by freezefracture electron microscopy. Sample films with a thickness of eeveral tens of micrometers were wedged between two 3".diameter copper cups. The resulting sandwicheswere maintained at a specifiedtemperature and then quenched into liquid nitrogen. This cooling liquid was found to be adequate for formamide systems. The sandwiches were then fractured under vacuum, and a 2.0-nm layer of carbon-platinum reinforced by a 10.0-nm layer of carbon was evaporated onto the fractures, which were then subjected to transmission electron microscopicexamination.
Results (1) X-ray Diffraction. The presence of more or less structured aggregatesor micellesis characterized by a peak in the small-anglescattered intensity at wavelength vector 8 (CI = 28/X, 28 being the angle between the incident and scattered beams). On formation of the cubic I1 phase, the micellar concentration is elevated, and intermicellar interferencepredominates. The positionssm= of the peak in the scattered intensity Z(s) is associated with a length d, = l/s, relating the distances between the centers of the micelles. This distance depends on temperature along the T,curve. The I, 11 H, transitions were indicated by the appearance of Bragg diffraction peaks. In the (CP)Cl/FA solution, the formation of aggregates or micelles was observed between 13 and 24 OC in the presence of excess surfactant crystals (Figure 1). The characteristic peaks of a cubic phase of space group Pm3n were seen between 25,and 26 OC (Figure l), although their relative intensities were disturbed by the formation of large crystals of marked texture. Between 24 and 25 OC, the scattering peak sharpened just prior to the appearance of the cubic phase diffraction peaks. The 11 monophasic region along the T, curve was determined in experiments carried out with solutions of fired concentration: from 25.5 to 34.5 "C and from 42% to 52 % ,there was a change in the lattice parameter from 9.36 to 9.10 nm. In this concentration range, the 11phase could be maintained in the absenceof crystallinesurfactant up to 64 O C when it turned into the micellar phase. On cooling,this phase could be maintained down to 15 "C; the diffraction peaks of crystalline surfactant appear at this temperature. The 11 monophasic region of thermodynamic stability could not be determined accurately due to surfusion. Its region of existence as determined by X-ray diffraction overlapped that observed by Tiddy by optical microscopy for the viscous isotropic phase." In the presence of crystalline surfactant, the intensities of peaks 200 and 210 fell abruptly at 34.5 OC, while the peak at 211 whose position coincided with peak 10 of the Haphase increased in intensity (Figure 2). At a concentration of 53 % ,the parameter ah of H, was 4.398 nm at 36 OC. Identical experimentswere carried out with the (CTA)CUFA solution. Along the T,curve, the 11phase formed at 25 OC at a concentration of 42%,and was observed up to 35 OC for a concentration of 50%. Its lattice parameter fell from 10.3 to 10.0 nm (Figure 3) away from the equilibrium line. This phase was observed up to 48 OC,
210
I'
11 1'
--
T.24O
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Figure 1. X-ray scattering and diagrams of the micellar phase I, and diffraction diagram of the Pm3n phase showing the I,, I1phase transition along the T,curve for the (CP)Cl/FA system. The arrows show the diffraction peak of the surfactant crystals.
and it could be maintained down to 12 "C on cooling. The H, phase formed in the presence of crystals of surfactant at 36 OC for a concentration of 51 % ,with the parameter ah equal to 4.73 nm. Similar experiments along the Tocurve with solutions of (CP)Br/FA and CTAB/FA systems indicated a direct I, H, transition. The diffraction peak 10 appeared close to the peaks in scattered intensity of the micellar solutions at 34 and 52 OC, respectively, the parameter a h being 4.68 and 4.62 nm.19 (2) Freeze-Fracture Electron Microscopy. The micrographs in Figure 4 show replicas of freeze fractures of the (CP)Cl/formamide system at a concentration of 42 % that was previously investigatedby X-ray diffraction. The samples were maintained for 5 min at 30 "C before freezing. They show the existence of varying lamellar structures. We observed three different periodicities (measured by light diffraction) of 6.2, 4.0, and 3.4 f 0.2 nm that were consistent withlattice distances (1111, (2111, and (310) of the Pm3n phase attributed from the X-ray diffraction peaks. The notable feature was the small size of the platinum particles (around 1.0 nm in diameter as opposedto 3.0nm normally observed). Thie was attzibuted to the low mobility of platinum over the surface of these fractures. Clusters of particles at certain sites were thus thought to be indicative of a particular morphology. The granular appearance (cf. photomicrographs a and b of
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Auvray et al.
446 Langmuir, Vol. 9, No. 2,1993
0.6
snm-1
arrows show the two diffraction peaks of the surfactant crystals.
a
‘t
210
I
b
T=34’
I
0
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0:2
0.4
s nm-1
Figure 2. Diffraction diagrams of the Pm3n and Ha phases showing the 11 Haphase along the T,curve for the (CP)Cl/FA system. The arrows show the diffraction peak of the surfactant crystals. Figure 4) probably corresponds to a granular structure of
the fractures. Discussion To date, three models of the Pm3n phase have been described. The structure of this phase consists of disjointed micelles. Burns15 reported 24 micelles per unit cell (position “k” of PmSn), although the ratios of intensities of the main diffraction rings he reported do not fit the observations reported here. Ring 110 is particularly strong in the Burns model. F0ntell39*described a network of eight identical ellipsoidalmicellesin positions “a”and “d”,whereas Charvolin6
Figure 4. Electron micrographs of freeze fractures of the (CP)Cl/FA system at 42% surfactant concentration: (a) periodicity 6.2 f 0.2 nm correspondingto direction [ill] of the Pm3n phase, bar = 50 nm; (b) periodicity 3.4 f 0.2 nm corresponding to direction [122] of the Pm3n phase, bar = 50 nm.
and Vargasg proposed a structure of two quasi-spherical micelles in position a, and six ellipsoidalmicellesin position d in line with the difference in symmetry of these two sites. Thisdifference in symmetry is consideredby Fontell to lead only to differences in rotation and orientation. The diffraction diagrams are comparable for the two models.
Langmuir, Vol. 9, No. 2, 1993 447
Cubic Phoree in the (CP)Cl/FAand (CTA)Cl/FAS y s t e m Table I CPCl
4
c, % ac,nm
11 (F)
11 (C)
11 (VI
Ha
r, nm u,nm2 N r,,nm r, nm be, nm u, nm2 rrnm be u, nm2
c, % ah,nm n,, nm u, nm2
42 9.36 1.60 0.73 77 1.93 1.69 3.07 0.71 2.35
1.56 0.75 53 4.39 1.54 0.60
CTACl 52 9.10 1.68 0.70 87 2.02 1.74 3.29 0.68 2.46 1.64 0.72
42 10.3 1.80 0.65 108 2.17 1.90 3.42 0.63 2.64 1.76 0.66 51 4.73 1.65 0.57
50
10.0 1.85 0.63 117 2.22 1.95 3.51 0.61 2.70 1.80 0.64
0 11 (F), 11 (C), and I1 (V) are models of 11 phases deacribed by Fontell (F),Charvolin (C), and Vargae (V). acand ah are parameters of cubic and hexagonal lattices. r, ia the radius of a quasi-spherical micelle in site a (Pm3n) (Charvolin and Vargaa models). r is the radius of ellipsoidal micelles (prolate or oblate) in site d (Pm3n). be ia the half-length of the axis of revolution of ellipsoidal micelles (prolate or oblate) in site d (Pm3n). u is the mean surface area per polar head. fi is the radius of cylindrical micellea. N is the number of molecules per micelle.
The model described by Fontell is based on an axial ratio of approximately 2 1 for the eight elliptical micelles, and this model is a good agreement with results obtained by Ericksson.13 On this assumption and assuming that the concentrations (by weight and volume) are comparablem with an axial ratio b/r = 2 for the aliphatic core, we calculatedthe radii of the micelles as well as the surface area per polar head at the core/solvent interface (Table I, 11Fontell model). In the model deacribed by Charvolin and Vargas, the quasi-spherical micelles in position a occupy the two dodecahedral units and the elliptical micelles in position d the six tetrakaidecahedral units, the volume of each tetrakaidecahedron being 1.2 times that of the dodecahedron. The dimensions of these two types of micelles can be calculated on the assumption that they have the same mean surface area per polar head and that the ratio of their volume is 1.2. The values obtained are listed in Table I for prolate ellipsoids (I1Charvolinmodel). We found axial ratios of ellipticalmicellesof 1.91 for (CP)C1(52%) and 1.81for the others. The electron density mapss show that the micelles centered at point d are disk-shaped, with an axial ratio of approximately 1:l.k the values obtained are listed in Table I (I1 (V)). The mean surface area per polar head of the spherical and disk-shaped micelles differed by 0.04 nm2; the curvature of the interface21and the localization of free counterionss could explain this difference. The characteristic dimensions of the micelles were compared to those calculated in the Haphase at the 11 H, transition on the assumption of infinite cylinder micelles. We observed a s m d change in parameter a h in the domain of existence of H, in the CTAB/FA and (CP)Br/FA systems,19@which was indicative of an increase in the length of micelles with an increase in the surfactant concentration. The values of radius n, of the cylindrical micelles given in Table I are thus minimal values. The dimensions of the corresponding micelles in the three models of the I1 phase are quite comparable. The choice of model could not therefore be decided on purely
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(20)Auvray, X.; Anthore, R.; Petipae, C.; Rico, I.; L a t h , A. J.Phys. Chem. 1989,93,7460. (21) Hyde, 5.T.; J. Phys., Colloq. 1990,61 (C7), 209.
geometrical criteria. However, it should be noted that the radii of the spherical micelles of (CTA)Cl in the the Charvolin and Vargas models was slightlyhigher than the length of the stretched out chain, and also higher than the radii observed in water. The spherical micelles of CTAB in FA have a radius of 1.5 nm.22v23The micelles centered at point a are quasi-spherical;the length of the rotation axis of disk-shaped micelles in the Vargas model and the radius r in ellipsoidal micelles in the Fontell model are smaller than the length of the stretched out chain and slightly higher than the radius of rodlike micelles in Ha. In light of these two considerations,the Fontell model or disk-shaped micelles centered at point d appeared to be more applicable. The parameter (8.6 nm) of the I1 phase formed by (DTA)Cl in water was relatively insensitive to concentration over a range of 43-57 9%. Fontell3reported a micelle radius of 1.5 nm, equal to the length of the stretched out chain, and a surface area per polar head u of 0.55 nm2. These results are in line with the comment of O l o f ~ s o n ~ ~ that the chains containing n carbon atoms behave in formamide like those with n - 4 carbon atoms in water. Although the size of the micelles cannot be safely deduced from the micrographs of the freeze fractures (Figure 4)) the periodicities can be compared to those calculated from the diffraction diagrams. If the fractures do correspond to the reticular planes, two-dimensional networks should be observed. However, two processes are involved in the deposition and distribution of the platinum particles: (a) the direction of flow during evaporation, which can produce shadows, either masking or enhancing periodicities, and (b) the existence of dense directions in the structure. In our systems,the directions are of type 11121. Therefore, such planes as, e.g., (llO), containing [1111and 11121directions,are possible fracture planes. This type of fracture plane is illustrated, according to us, on the micrograph in Figure 4a, where dense directions are vizualized as black stripes, and the 11113 direction is perpendicular to these stripes. A fracture plane such as (100) could not be observed, possibly because this type of plane does not contain dense directions and consequently can hardly be a fracture plane. It should be borne in mind that the Pm3n phases are observed during growth of micelles at low surfactant concentration. The micelles thus depart little from a spherical shape, as elongation is essentially due to intermicellar interactions. In the absence of electrolyte, these interactions are dominated by the counterion-micelle bond, which depends on solvation of the counterion and the length of the chain. The effect of the chain length on I1 phase formation is due to both the critical micelle concentration (cmc) value and the dissociation counterion-micelles: The cmc value increases with decreasing chain length, or a higher cmc value brings a large increase in free counterions and intermicellar repulsion forces are sufficiently low to allow elongation of micelles, preventing the formation of the Pm3n phase." Since cmc values are higher in FA than in ~ a t e r , 2 la- ~ longer ~ chain length is required. The shorter the chain, the greater the dissociationbetween the counterion and micelles, increasing the separation between polar heads and enhancing repulsion between the solvated anion and the solvophobic ~~
(22) Perche, T.; Auvray, X.; Petipae, C.; Anthore, R.; Rico, I.; Lath, A.; Bellbent, M. C. J. Phys. Z 1992, 2,923. (23) Sjoberg, M.; Henrikseon, U.; Warnheim, T. Longmuir 1990, 6, 1206. (24) Olofseon, G.J . Chem. SOC.,Faraday T ~ o M1991,87 . (18), 3037.
448 Langmuir, Vol. 9, No. 2, 1993
part of the micelle.26 Since the micellar radius is lower in formamide than in water (Table I and refs 22 and 24), this effect can account for the lack of observation of a Pm3n phase in both solvents with identical chain length surfactanta.
Auvray et al.
Conclusion The formation of the Pm3n phase with cationic surfactants in formamideshows that association of molecules of surfactant into micelles, which in turn form ordered
structures, can take place in this solvent as in water. The nature of the polar head, ita size, and steric hindrance affect the value of the lattice parameter in the cubic phase rather than the overall structure. Demonstration of the 11 phase in the (CTA)Cl/FA system is noteworthy as it provides further evidence for the formation of micelles in this system, as association at low concentration does not occur spontaneously in this system.2s Micelles may thus form even in the absence of abrupt associationof monomers at a given concentration.
(25) Bacaloglu, R.; Blaeko, A.; Clifford, A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990,94,5062.
(26) Beanana-Limbele, W.; Zana, R. 440.
Colloid Polym. Sci. 1989, 267-
