Effect of Electrolytes on Discontinuous Cubic Phases - Langmuir (ACS

On the other hand, in DTAC systems, the addition of both NaCl and NaSCN leads to a transition from the discontinuous cubic phase to the hexagonal phas...
0 downloads 0 Views 104KB Size
Download PDF
Langmuir 2000, 16, 8263-8269

8263

Effect of Electrolytes on Discontinuous Cubic Phases Carlos Rodrı´guez and Hironobu Kunieda* Division of Artificial Environments and Systems, Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan Received March 13, 2000. In Final Form: July 18, 2000

We investigated the effect of adding a lyotropic salt (NaCl) and a hydrotropic salt (NaSCN) on the discontinuous cubic phase formed in highly hydrophilic nonionic (C12EO25) and ionic (DTAC) surfactant systems by phase study and small-angle X-ray scattering (SAXS) measurements. In C12EO25 systems, only a small decrease in the thermal stability of the cubic phase was observed upon addition of NaCl or NaSCN. It was also found that NaCl induces a slight reduction on the effective surface area per surfactant molecule (as), whereas in the case of NaSCN, as increases. These results are attributed to changes in the hydration of the poly(oxyethylene) (EO) chain. On the other hand, in DTAC systems, the addition of both NaCl and NaSCN leads to a transition from the discontinuous cubic phase to the hexagonal phase, related to a more pronounced shrinkage of as and to the micellar growth within the cubic phase. Changes in micellar structure resemble that occurring in diluted systems upon addition of salt. The stability of the cubic phase increases when long hydrocarbon-chain oils are added to DTAC/brine systems, because of the formation of swollen micelles with a more positive curvature and a moderate axial ratio. In oil-present systems, as shrinks when the counterion is changed from chloride to bromide. The influence of the degree of counterion dissociation on the phase behavior is discussed.

Introduction Various types of liquid crystals are formed in watersurfactant systems. Discontinuous cubic phases are optically isotropic mesophases, consisting of discrete micellar aggregates packed in a cubic array, that can be found in some systems, including biological ones, between the isotropic aqueous solution and the hexagonal phase.1,2 They are highly viscous and are formed even at relatively low surfactant concentrations, features from which interesting applications, such as cubic-phase-based emulsification,3 can be derived. Much research has been done on the structure of the discontinuous cubic phase,4-13 mainly in binary surfactant-water systems, but studies on the effect of additives such as electrolytes are scarcely reported. Inorganic electrolytes can be classified into two types according to their effect on the water solubility of nonionic compounds:14,15 (1) lyotropic salts, which decrease the * To whom correspondence should be addressed. (1) Fontell, K. Colloid Polym. Sci. 1990, 268, 264. (2) Lindblom, G.; Rilfors, L. Biochim. Biophys. Acta 1989, 988, 221. (3) Rodrı´guez, C.; Shigeta, K.; Kunieda, H. J. Colloid Interface Sci. 2000, 223, 197. (4) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Nature 1969, 222, 1159. (5) Eriksson, P. O.; Khan, A.; Lindblom, G. J. Phys. Chem. 1982, 86, 387. (6) So¨derman, O.; Walderhaug, H.; Henriksson, U.; Stilbs, P. J. Phys. Chem. 1985, 89, 3693. (7) Johansson, L. B.Å.; So¨derman, O. J. Phys. Chem. 1987, 91, 5275. (8) Vargas, R.; Mariani, P.; Gulik, A.; Luzzati, V. J. Mol. Biol. 1992, 225, 137. (9) Auvray, X.; Abiyaala, M.; Duval, P.; Petipas, C. Langmuir 1993, 9, 444. (10) Mariani, P.; Amaral, L. Q.; Saturni, L.; Delacroix, H. J. Phys. II (Paris) 1994, 4, 1393. (11) Sakya, P.; Seddon, J.; Templer, R.; Mirkin, R.; Tiddy, G. Langmuir 1997, 13, 3706. (12) Håkanson, B.; Hansson, P.; Regev, O.; So¨derman, O. Langmuir 1998, 14, 5730. (13) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627.

mutual solubility between water and nonionic surfactant, such as chlorides (salting-out phenomenon); and (2) hydrotropic salts, which increase the mutual solubility between water and nonionic surfactant, such as thiocyanates (salting-in phenomenon). The salting-out strength of anions follows the Holfmeister series:16 SO42- ≈ HPO42> F- > Cl- > Br- > I- > SCN-. This behavior has been attributed to two types of mechanisms. In one of them,17-19 the salt modifies water-solution properties; in the other,20 the salt effect is driven by ion adsorption/depletion at the surfactant monolayer. The effect of added electrolytes on the phase behavior or self-organization of nonionic surfactants in water is highly dependent on the types of ions, especially anions. For example, salting-out electrolytes, e.g., NaCl, reduce the repulsion between EO chains of nonionic surfactants, resulting in shrinkage of the effective cross-sectional area per surfactant molecule in the hexagonal phase, whereas the opposite effect is induced by salting-in electrolytes such as NaSCN.21 However, the effect of added salts on the structural change in the cubic phase has not been studied. On the other hand, in the case of ionic surfactants, added inorganic salts reduce the electrostatic repulsion of the headgroups in surfactant aggregates. As a result, the aggregation number of micelles is increased or the critical micellar concentration is reduced in the presence of salts.22,23 Added electrolyte may also influence the structures of liquid crystals, particularly cubic phases, at high (14) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 1937. (15) Kahlweit, M.; Strey, R.; Haase, D. J. Phys. Chem. 1985, 89, 163. (16) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247. (17) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 4, 323. (18) Von Hippel, P. H.; Wong, K. Y. Science 1964, 145, 577. (19) Franks, F. In WatersA Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2, p 1. (20) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. J. Phys. Chem. 1995, 99, 6220. (21) Iwanaga, T.; Suzuki, M.; Kunieda, H. Langmuir 1998, 14, 5775.

10.1021/la000388b CCC: $19.00 © 2000 American Chemical Society Published on Web 10/03/2000

8264

Langmuir, Vol. 16, No. 22, 2000

Rodrı´guez and Kunieda

surfactant concentration, but little has been published on this subject. The solubilization of oil also has an effect on the surfactant layer curvature in aggregates. It has been found in nonionic systems24,25 that short alkanes or aromatic hydrocarbons tend to penetrate in the surfactant palisade layer and make the curvature less positive or even negative (the curvature is defined as positive when the surfactant layer is convex toward the aqueous phase). On the other hand, long-chain alkanes are solubilized in the deep core of the aggregates, causing a change to more positive surfactant curvatures, which is often accompanied by a phase transition. However, the influence of oil solubilization has not been systematically investigated in ionic cubic phases. In this context, we investigated the phase behavior and self-organizing structures in the discontinuous cubic phase with added salt for nonionic (poly(oxyethylene) dodecyl ether) and ionic surfactant (dodecyl trimethylammonium chloride) systems. The effect of different kinds of added oil was also studied. Experimental Section Materials. Poly(oxyethylene) dodecyl ether containing an average of 25 oxyethylene units per molecule (designated C12EO25), dodecyl trimethylammonium chloride (DTAC, 98%), dodecyl trimethylammonium bromide (DTAB, 99%), n-decane (99%), n-hexadecane (98%), and squalane (2,6,10,15,19,23hexadecyl methyl tetracosane, 98%) were supplied by Tokyo Kasei Kogyo Co. Ltd. (Japan). Sodium chloride (99%) and Sodium thiocyanate (99%) from Junsei Chemical Co. (Japan) were also used. DTAC and DTAB were recrystallized three times from acetone. Distilled water was used in the preparation of the samples. Determination of Phase Diagrams. All chemicals were weighed and sealed in test tubes with a narrow constriction. Samples were mixed by a vortex mixer and centrifuged. The phase change was detected by direct visual inspection of the samples and with crossed polarizers for birefringence. The type of liquid crystal was determined by optical microscopy and smallangle X-ray scattering. Calculation of Volume Fractions. Volume fractions of surfactant (φs) and its lipophilic moiety (φL) were calculated by

φs )

1 1 + FsWa/FaWs

(1)

VL φ Vs s

(2)

φL )

where Ws and Wa are the weight fractions of surfactant and the aqueous phase (water or brine). Fs and Fa denote the densities of surfactant and the aqueous phase (water or brine).VS and VL are the molar volumes of surfactant and its lipophilic moiety. Small-Angle X-ray Scattering Measurements. The interlayer spacing of liquid crystals was measured using small-angle X-ray scattering (SAXS), performed on a small-angle scattering goniometer with a 15 kW Rigaku Denki rotating anode generator (RINT-2500) at 25 °C. The samples were covered with plastic films (Mylar seal method) for the measurement. Hexagonal (H1) Phase. For this liquid-crystal phase, the peak ratios are 1:1/x3:1/2. The radius of the lipophilic cylinder, rH, (22) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (23) Porte, G.; Appell, J. J. Phys. Chem. 1981, 85, 2511. (24) Kunieda, H.; Ozawa, K.; Huang, L. J. Phys. Chem. B 1998, 102, 831. (25) Kunieda, H.; Shigeta, K.; Suzuki, M. Langmuir 1999, 15, 3118.

and the effective cross-sectional area per surfactant molecule, as, are given by

rH )

{

}

2 φL x3π

as )

1/2

d

2VL rHNA

(3)

(4)

where d is the interlayer spacing and NA is Avogadro’s constant. Assuming that no water penetrates into the hydrophilic shell of micelles, the separation between lipophilic cylinders, dw, can be estimated from

dw )

2 d - 2rH x3

(5)

Discontinuous Cubic (I1) Phase. The crystallographic space group of the discontinuous cubic phase was assigned following the relative position of the peaks and their relative intensities. For a valid assignment, the plot of the reciprocal spacings (1/ dhkl) versus m ) (h2+k2+l2)1/2 (where h, k, and l are the Miller indices) should give a straight line passing through the origin with a slope of 1/p, where p is the lattice parameter. Notation. We use the following nomenclature in the phase diagrams: Wm ) aqueous phase containing surfactant aggregates; I1 ) discontinuous-type cubic phase (water-continuous); H1 ) hexagonal liquid crystal; S ) solid-present region.

Results and Discussion Phase Behavior and Self-Organization in C12EO25/ Brine Systems. The phase diagram of the C12EO25/water system shows a wide discontinuous cubic phase (I1) region ranging from approximately 30 to 70 wt % of surfactant.3 A lyotropic salt (NaCl) and a hydrotropic salt (NaSCN) were added to this system, and the effect on the phase behavior is shown in Figure 1. Although a two-phase region is expected to exist between the cubic phase and the isotropic solution, the region seemed to be very narrow, and it is not shown in the phase diagram. It should be pointed out that for the binary system surfactant/water,3 the azeotropic point of the cubic phase is located at a surfactant/water ratio equal to 50/ 50, which may explain the narrowness of the two-phase region in systems with added salt. Both the melting point of the cubic phase and the cloud point monotonically decrease upon addition of NaCl (Figure 1a), as a result of the dehydration of the EO chain by the salting-out effect. It is known that the hydrated structure plays an important role in the stability of mesophases. In the case of NaSCN, no cloud point phenomenon was observed up to 100 °C. Also, the melting point of the cubic phase shows almost no change at low salt concentrations and then decreases at high salt concentrations (Figure 1b). It has been reported21 that in nonionic surfactant systems of moderate EO chain length, NaSCN increases the cloud point due to the salting-in effect and also causes the disappearance of the hexagonal phase at high salt concentrations. The interpretation of NaSCN’s effect on the melting point of the cubic phase is not straightforward. It is possible that the modification of the solvent properties of water by NaSCN leads to a reduction in the intermicellar forces and therefore destabilizes the cubic structure. SAXS measurements were carried out on the cubic phase in C12EO25/brine systems to investigate the influence of added salt on the structure of the I1 phase. A representative

Effect of Electrolytes on Cubic Phases

Langmuir, Vol. 16, No. 22, 2000 8265

Figure 2. Representative SAXS pattern for the I1 phase in a C12EO25/brine system (Im3m space group). q is the scattering vector. The positions of the higher-order reflections with respect to that of the first peak, q*, are indicated in the upper X axis. Composition: 50% C12EO25, 50% aqueous solution (2% NaSCN).

and the effective cross-section per surfactant molecule, as, were calculated by using the following equations:

Vmic ) φLp3/nm

)

(7)

l ) R(2 + b)/2

(8)

R)

(

as )

Figure 1. Effect of salts on the cubic phase in C12EO25/water systems. The C12EO25/brine weight ratio is fixed at 50/50. XE is the weight fraction of salt in the salt + water mixture. I and II denote one-phase and two-phase region, respectively. (a) NaCl; (b) NaSCN.

SAXS pattern is shown in Figure 2. A total of four Bragg peaks are identified; they can be indexed as hkl ) 110, 200, 211, and 220 reflections of a body-centered structure. The relative position of the peaks (1:1/x2:1/x3:1/2) and the very low intensity of high-order reflections are characteristic of the Im3m space group, the same of that corresponding to the system in the absence of salt.3 In a previous study3 it was found that micelles in the binary system C12EO25/water might not be completely spherical. To take into account this deviation from spherical shape, the cubic phase was considered to be built of rodlike micelles, as depicted in Figure 3a, with a minor axis R equal to the fully extended length of the dodecyl chain according to Tanford’s equation (1.67 nm).26 Under this assumption, the half-length of the lipophilic core, l, (26) Tanford, C. J. J. Phys. Chem. 1972, 76, 3020.

(6)

Vmic

π(4/3 + b)

1/3

(4 + 2b) V (4/3 + b)R L

(9)

where Vmic is the volume of one micelle and nm is the number of micelles per unit cell (nm ) 2 for a body-centered structure). b can be regarded as an index of anisotropy; b ) 0 for spheres and b ) ∞ for infinite cylinders. Figure 4a shows that NaCl induces a small reduction in the specific surface area per surfactant molecule, whereas NaSCN (Figure 4b) shows the opposite effect. However, the effect of salt on as in the cubic phase is smaller than in the case of the hexagonal phase in C12EO7 systems.21 If these results are interpreted in terms of a change in the hydration of the EO chain, it can be inferred that NaCl decreases the hydrophilicity of the EO group whereas NaSCN increases it, and this effect is expected to be smaller as the EO chain increases, because hydration bonds become stronger. Because NaSCN decreases the stability of the cubic phase irrespective of an increase in as, the destabilizing effect may be attributed to a change in intermicellar, rather than intramicellar, interactions. Phase Behavior and Self-Organization in DTAC/ Brine Systems. The discontinuous cubic phase I1 is also found in the DTAC/water system.4 NaCl and NaSCN were added to this system, and the effect on the phase behavior at 25 °C is shown in Figure 5. At relatively low salt concentrations, the I1 phase disappears, and the hexagonal phase region expands toward low surfactant concentrations. However, in the case of NaSCN (Figure 5b), further

8266

Langmuir, Vol. 16, No. 22, 2000

Figure 3. Schematic representation of the unit cell in I1 phases. l is the half-length of the micellar lipophilic core. (a) Im3m space group. (b) Pm3n space group (Fontell’s model).

addition of salt leads to a destabilization of the hexagonal phase, and an isotropic solution phase is obtained. The destabilization effect of salt on cubic phases has been reported previously,27 although there have been few of detailed phase behavior studies. The addition of both NaCl and NaSCN dramatically decreases the stability range of the cubic phase and induces a phase transition from the cubic phase I1 to the hexagonal H1 phase. The concentration of salt required to obtain such a transition is lower in the case of NaSCN. A representative SAXS pattern for the cubic phase in a DTAC /brine system is given in Figure 6. A total of six Bragg peaks are identified and can be indexed as hkl ) 200, 210, 211, 220, 310, and 222 reflections of a primitive cubic structure. The diffraction pattern is characteristic of the Pm3n space group, the same of that corresponding to the system in the absence of salt.4 The Pm3n cubic phase has also been found in other cationic surfactants dissolved in nonaqueous polar solvents.9 (27) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1115.

Rodrı´guez and Kunieda

Figure 4. SAXS data for the cubic phase in C12EO25/brine systems. The C12EO25/brine weight ratio is fixed at 50/50. 0, interlayer spacing from the most intense peak, d; O, half-length of the micellar lipophilic core in the I1 phase, l; b, radius of the lipophilic core in the H1 phase, r; 4, effective cross-sectional area per surfactant molecule, as. (a) NaCl; (b) NaSCN.

The structure of the I1 phase in DTAC/water systems has been extensively studied. The model proposed by Fontell is considered to be correct, and it is depicted in Figure 3b. It consists of eight hemisphere capped rods (nm ) 8 in eq 6). The values calculated using eqs 6-9 are shown in Figure 7. It can be seen that both NaCl and NaSCN induce a considerable reduction in the specific surface area per surfactant molecule that finally leads to a transition to the hexagonal phase. The addition of salt also promotes the growth of micelles forming the cubic structure. However, this growth is not very rapid, as observed in dilute surfactant solutions, so the transition to the very long cylinders in the hexagonal phase may occur by coalescence of the rods forming the cubic phase. Nevertheless, some authors have proposed a hexagonal structure composed of finite cylinders.10,28 SAXS patterns in NaSCN systems showed that the (211) spacing in the I1 phase system decreases continuously toward the H1 phase and comes to almost coincide with the (10) spacing of the hexagonal phase, indicating the same epitaxial relation(28) Taylor, M. P.; Herzfeld, J. Phys. Rev. A 1992, 45, 1892.

Effect of Electrolytes on Cubic Phases

Langmuir, Vol. 16, No. 22, 2000 8267

Figure 6. Representative SAXS pattern for the I1 phase in a DTAC/brine system (Pm3n space group). q is the scattering vector. The positions of the higher-order reflections with respect to that of the first peak, q*, are indicated in the upper X axis. Composition: 50% DTAC, 50% aqueous solution (0.3% NaSCN).

Figure 5. Phase behavior of DTAC/brine systems at 25 °C. Ws is the weight fraction of surfactant in the system. Other notations as in Figure 1. (a) NaCl; (b) NaSCN.

ship proposed by Mariani et al.10 for the binary system DTAC/water. According to Israelachvili,22 the optimal area per amphiphile, ao, determined from repulsive electrostatic forces can be estimated by means of a capacitance model

ao )

x

2πe2D γ

(10)

where  is the dielectric constant of the medium, γ is the interfacial tension, and e is the charge of an equivalent capacitor, the planes of which are separated by a distance D that is related to headgroup separation. It is well-known that the screening length between charged surfaces decreases with salt concentration; therefore, ao is expected to decrease if eq 10 applies. In the binary system DTAC/water, a transition occurs, from hexagonal phase to the bicontinuous cubic phase (V1), with increasing surfactant concentration (i.e., decreasing as) at constant temperature. The addition of NaCl does not induce such a transition because the decrease in

as is small in the hexagonal phase, and further addition of salt causes the salting-out of the surfactant. In the case of NaSCN, the decrease in as is more pronounced, but the separation between lipophilic cylinders dw (dw ≈ 1.1 nm) as calculated from eq 5 is larger than the corresponding value for the binary system DTAC/water (dw ≈ 0.82 nm using the values reported by Vargas et al.8). The result is that the intermicellar interactions are not strong enough and a hexagonal phase f isotropic solution transition takes place. Looking at the results, it seems that NaSCN more efficiently inhibits the dissociation of the counterion from the surfactant molecule. In fact, addition of NaSCN leads to a dramatic increase in the aggregation number of micelles in alkyltrimethylammonium diluted systems,29 but the mechanism is still unclear. Effect of Adding Oil to DTAC/Brine Systems. Decane and squalane were added to the cubic phase at a fixed salt concentration, and the results are shown in Figure 8. The stability of the cubic phase increases upon addition of oil. This effect has been found previously in cubic phases formed from nonionic surfactants; however, in this case it is larger. It can be also observed that a transition from the cubic phase to the hexagonal phase occurs with increasing temperature. The hexagonal-phase domain shrinks when the oil is changed from decane to squalane. SAXS measurements were carried out in the systems with added oil. The SAXS patterns indicated no major change in the cubic structure upon addition of oil. As can be seen in Figure 9, interlayer spacings increase as oil is added and then become constant at the solubilization limit. The lattice parameter, p, is expected to increase with the interlayer spacing, d, and therefore, the micellar volume, Vmic, given by eq 6 also increases. The increment in interlayer spacingsand, consequently, in the micellar volumesis larger as the hydrocarbon chain length increases from decane to squalane, indicating that micelles become more swollen because the oil tends to incorporate deep in the micellar core. From the position of the inflection (29) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1987, 117, 243.

8268

Langmuir, Vol. 16, No. 22, 2000

Figure 7. SAXS data for DTAC/brine systems. The DTAC/ brine weight ratio is fixed at 50/50. 0, interlayer spacing from the most intense peak, d; O, half-length of the micellar lipophilic core in the I1 phase, l; b, radius of the lipophilic core in the H1 phase, r; 4, effective cross-sectional area per surfactant molecule, as. (a) NaCl; (b) NaSCN.

points in Figure 9, it can also be inferred that oil solubility decreases as the hydrocarbon chain length becomes longer. It has been pointed out that the incorporation of oil in the micellar core increases the repulsion between the headgroups of surfactant,30,31 so that the surfactant layer becomes less flexible toward changes in curvature, which are induced by increasing temperature. The rigidity of the surfactant layer may promote the stability of the cubic phase over that of the hexagonal phase and also may be a cause for a decreased solubilization capability. The same tendency has been observed in nonionic surfactant systems.30,31 It is also expected that the axial ratio of micelles may decrease (i.e., their sphericity increases) as oil is incorporated in the micellar core, such that the curvature increases. The model of Safran and co-workers32 predicts an oil-phase separation (emulsification failure) when the (30) Kunieda, H.; Umizu, G.; Aramaki, K. J. Phys. Chem. B 2000, 104, 2005. (31) Shigeta, K.; Rodrı´guez, C.; Kunieda, H. Submitted for publication. (32) Safran, S. A.; Turkevich, L. A.; Pincus, P. A. J. Phys. Lett. 1984, 45, L69.

Rodrı´guez and Kunieda

Figure 8. Phase behavior of DTAC/NaCl 3%(aq)/oil systems. The DTAC/NaCl 3%(aq) weight ratio is fixed at 50/50. φο is the volume fraction of oil in the system. (a) decane; (b) squalane.

spontaneous mean curvature, Ho, exceeds a value given by

Ho >

1 κj 1+ rs κ

(

)

(11)

where rs is the radius of a stoichiometric sphere, dictated by the surfactant-oil ratio, and κ and κj are bending moduli associated with the mean curvature and the Gaussian curvature, respectively. It was found that the addition of NaCl increases the solubilization of decane in the cubic phase of DTAC by almost twice compared to the system without electrolyte. It is known33 that the addition of salt leads to a decrease in κ and an increase in κj; therefore, the critical curvature for emulsification failure calculated from eq 11 increases and more oil can be incorporated in micelles. This finding is also consistent with the concept that long-hydrocarbon(33) Maugey, M.; Bellocq, A. M. Langmuir 1999, 15, 8602.

Effect of Electrolytes on Cubic Phases

Langmuir, Vol. 16, No. 22, 2000 8269 Table 2. Effect of Adding HCl on the Phase Behavior of DTAB/Water System at 25 °Ca

a

Figure 9. Interlayer spacings from the most intense SAXS peak for the cubic phase in DTAC/NaCl 3%(aq)/oil systems. The DTAC/brine weight ratios are fixed at 50/50. Lines are included only as a guide to the eyes. 0, n-decane; O, hexadecane; 4, squalane.

HCl concn (M)

phases

0 0.1 0.6 1.2 3 6

H1 H 1 + Wm H 1 + Wm H 1 + Wm Wm I1

The surfactant/aqueous phase weight ratio is fixed at 50/50.

i.e., the degree dissociation of the bromide ion is lower.27,34 As a result, the local electrolyte concentration in the vicinity of the interface is higher, which, according to eq 10, leads to small values of ao. Values of as in the hexagonal phase for the DTAB/water35 system are also smaller than those of the DTAC/water system calculated from the results of Vargas et al.8 To confirm the hypothesis that the degree of dissociation may determine the value of as, we added HCl to the binary system. The effect on phase behavior is shown in Table 2. The addition of HCl drives a transition from the H1 to the I1 phase, the inverse of that obtained when salt is added to the DTAC/water system. Hence, it seems that the counterion binding is an important factor in the mechanism of phase transitions.

Table 1. Interlayer Spacing, d, Corresponding to the Most Intense SAXS Peak in the Cubic Phase of Cationic Surfactant/Water/Decane Systemsa surfactant

φo

d/nm

DTAC DTAC DTAB DTAB

0.04 0.05 0.04 0.05

4.55010 4.57367 5.41538 5.51691

a The surfactant/water weight ratio is fixed at 45/55. φ is the ο volume fraction of oil in the system.

chain oils increase the repulsion between headgroups so that the salt effect on as vanishes. Effect of Changing Counterion on Occurrence of the Cubic Phase in Dodecyltrimethylammonium Systems. It is known that I1 phases occur with chloride, but not with bromide derivatives, in dodecyltrimethylammonium systems. It was interesting to investigate this behavior. Decane was added to a concentrated DTAB micellar solution in the vicinity of the hexagonal phase, and the cubic phase appeared. Interlayer spacings were measured in DTAB/water systems in the presence of decane, and the values corresponding to the most intense SAXS peak are shown in Table 1. The SAXS peaks were somewhat diffuse, but the relative positions of the peaks were similar to those of DTAC systems. Table 1 shows that d (and therefore the lattice parameter) values are larger in the case of DTAB. Although the exact structure of the I1 phase in DTAB systems could not be elucidated, it can be inferred that differences in d values arise from a decrease in as when bromide acts as the counterion. It has been pointed out that the binding of bromide to the amphiphile molecule is stronger than that of chloride,

Conclusions The effect of salts on the cubic phase is more marked in the case of ionic systems than in the case of hydrophilic nonionic systems. In the former case, a transition from the discontinuous cubic phase to the hexagonal phase takes place upon addition of salt. Hydrotropic salts such as NaSCN induce an increase in the effective surface area per surfactant molecule in nonionic surfactants, whereas in ionic systems, they reduce the effective surface area. Lyotropic salts such as NaCl promote a reduction in the surface area in both ionic and nonionic systems. The stability of the cubic phase is enhanced upon addition of oil in ionic systems. The hexagonal-phase domain shrinks when the alkyl chain length of the oil is increased from decane to squalane. The swelling of micelles seems to be more pronounced in the last case. The addition of NaCl increases the solubilization of oil in the cubic phase. The binding strength, and therefore the degree of dissociation of the counterion, in ionic surfactant systems seems to influence the effective surface area. The binding strength then plays an important role in the phase behavior and stability of the cubic phase. Acknowledgment. Financial support from Noevir Co. Ltd. is gratefully acknowledged. LA000388B (34) Bleasdale, T. A.; Tiddy, G. J. T.; Wyn-Jones, E. J. Phys. Chem. 1991, 95, 5385. (35) Li, X.; Kunieda, H. Submitted for publication.