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Dependence of Alkyl Chain Asymmetry on the Phase Equilibria of Three Catanionic Surfactant Mixtures Containing Dodecyltrimethylammonium Chloride-Sodium Alkylcarboxylate-Water Malin Byde´n Sjo¨bom and Håkan Edlund* Department of Natural and Environmental Sciences, Chemistry, Mid Sweden University, S-851 70 Sundsvall, Sweden Received April 4, 2002. In Final Form: August 21, 2002 The pseudoternary phase behavior of three aqueous systems with catanionic surfactant mixtures was studied at 20 °C, with emphasis on the effect of the level of asymmetry between the parent surfactants. The three mixtures all contain the cationic surfactant dodecyltrimethylammonium chloride (DoTAC), combined with one of the surfactants sodium hexanoate (SH), sodium octanoate (SO), or sodium decanoate (SD). As could be expected, all three systems form a variety of phases deriving from the binary DoTACwater axis, and the extension of those phases vary with the anionic surfactant chain length. The binary sodium alkylcarboxylate/water systems do not show as rich phase behavior as DoTAC-water at this temperature, and the small hexagonal liquid crystalline phase present in the binary SO-water are destabilized by DoTAC in the pseudoternary system. This can be explained in terms of asymmetry between the surfactant chain lengths, which causes packing problems. Neither SH nor SD forms a hexagonal phase at room temperature, the former due to its short alkyl chain and the latter due to its high Krafft temperature. The DoTAC-SD-water phase diagram is dominated by a lamellar liquid crystalline phase, whereas the range of existence of the lamellar phases in the SO and SH systems are much more limited. The hexagonal phases deriving from the binary DoTAC system can incorporate varying amounts of alkyl carboxylate. The largest extension is found in the SO system, indicating a favorable level of asymmetry between the two surfactants. All three systems form a large isotropic solution phase with water. In the SD system, the solution phase has a discontinuity around the equimolar line at high water content. Up to 13 wt % of total surfactant, there is coacervation resulting in two translucent solutions, one dense and one dilute. In the most dilute region, the solutions within and close to the small two-phase area have a bluish appearance. All solution phases were studied by 1H self-diffusion NMR. The results show that all systems are most likely to form mixed aggregates at anionic/catanionic molar ratios y H2O
κA+X- + yA-X+ 98 κA+A- + (y - κ)A-X+ + κX+X- κ > y H2O
κA+X- + yA-X+ 98 κA+A- + κX+X- κ < y A and X represent the parent surfactants and counterions, respectively, whereas κ and y are the molar concentrations of the components. The simple salt will bring additional electrostatic interactions to the system, like for example screening of surface charges. It is common in this type of system that the phases formed have their origin in the parent systems, although there are exceptions. Examples are the catanionic mixtures DDAB-SDS6 and DDAB-AOT7 that form as many as three and four phases, respectively, that does not originate from any of the binary systems. The systems presented in this work are three asymmetric catanionic surfactant mixtures, all containing the cationic surfactant dodecyltrimethylammonium chloride (DoTAC), combined with one of the anionic surfactants sodium hexanoate (SH), sodium octanoate (SO), or sodium decanoate (SD). The results are compared with other catanionic systems as well as the binary systems of DoTAC and the sodium carboxylates. Besides visual observations, the pseudoternary phase diagrams of this study have been delineated by means of water deuteron NMR studies. SAXS experiments were applied for further characterization in terms of phase structure, and 1H self-diffusion NMR for studying the solution phases. Experimental Section Materials. Sodium hexanoate, sodium octanoate, and sodium decanoate were purchased from Sigma Aldrich Chemie Gmbh, Germany, and DoTAC was purchased from Fluka Chemie AG, Switzerland, all with >99% purity. 2 H2O (99.9% 2H) was purchased from Cambridge Isotope Laboratories, USA. All chemicals were used without further purification. Sample Preparation. The samples were prepared by weighing appropriate amounts of surfactant and 2H2O into 8 mm glass tubes that were flame-sealed. All liquid crystalline samples were mixed by gentle heating, whereas samples in the solution region were mixed by shaking. All samples were then let to stand at the appropriate temperature to attain equilibrium for a couple of weeks before any experimental studies were performed. Samples for the diffusion measurements were prepared in small glass vials and transferred to 5 mm NMR tubes after equilibration. Water Deuteron NMR. The 2H NMR method is a useful tool when constructing phase diagrams of surfactant systems.13 It does not require a macroscopic separation of the individual phases in a mixture and is therefore particularly effective for the detection of small isotropic domains present in an anisotropic phase, which can be (13) Khan, A.; Fontell, K.; Lindman, B.J. Colloid Interface Sci. 1984, 101, 193.
Sjo¨ bom and Edlund
difficult to achieve with an optical method. 2H is a quadrupolar nucleus (I ) 1), and in anisotropic liquid crystals, such as hexagonal or lamellar phases, its spectrum is a symmetrical doublet. In optically isotropic phases, like micellar solutions or cubic liquid crystalline phases, the 2H NMR spectra show only a singlet, i.e., a single peak. The formation of a singlet is due to the interaction between the quadrupolar moment and the electric field being averaged to zero. In a heterogeneous system with at least one liquid crystalline phase, where the deuteron exchange between phases is normally slow on the NMR time-scale, the 2H NMR spectrum is a superposition of the spectra of the individual phases. This means that two liquid crystalline phases will give two splittings, whereas a liquid crystal and a solution phase will give a splitting and a singlet. The analysis of 2H NMR spectra can also give some information on the water binding effects.14,15 If we assume that there are only two types of water, i.e., either bound to the aggregate surface or free, we can apply a two-site model.15 The width of the 2H NMR quadrupolar splitting, ∆, obtained for the liquid crystalline phase may then be related to the molar ratio between the total amount of surfactant and water, 1 - XW/XW, as
k
1 - XW XW
(1)
where k is a constant. The NMR experiments were performed using a Bruker AVANCE DPX 250 with a variable temperature control unit. The quadrupolar splitting, ∆, was measured as the peak-to-peak distance and given in frequency units. 1H Self-Diffusion NMR. The 1H NMR self-diffusion measurements were performed using the Fourier transform pulsed-gradient spin-echo method (FT-PGSE). This method can be found described in detail in the literature.16,17 The amplitude of the stimulated spin-echo signal was evaluated using the following equation
A ) A0 exp[-(gδγ )2D(∆ - (δ/3)]
(2)
where A is the amplitude of the signal with applied gradient and A0 the signal without gradient. g and δ are the strength and the width of the gradient pulse, respectively, ∆ is the time between the gradient pulses, and D is the diffusion coefficient of the compound in question. In the cases when the evaluation did not fit well to a single exponential, a biexponential fit was used and eq 2 rewritten as below:
A ) A1 exp[-(gδγ )2D1(∆ - (δ/3)] + A2 exp[-(gδγ )2D2(∆ - (δ/3)] (3) A1 and A2 are the respective contributions of sodium carboxylate and DoTAC, and D1 and D2 are their respective diffusion coefficients. To simplify characterization of the different peaks in the NMR spectra, diffusion measurements were made on pure sodium carboxylate and DoTAC solutions. Both surfactants have protons giving rise to characteristic (14) Wennerstro¨m, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, 97. (15) Wennerstro¨m, H.; Persson, N.-O.; Lindman, B. Am. Chem. Soc. Symp. Ser. 1975, 9, 253. (16) Stilbs, P. Prog. Nucl. Magn. Reson. 1987, 19, 1. (17) Lindblom, G. In Advances in Lipid Methodology; Christie, W. W., Ed.; Oily Press: Dundee, 1996.
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peaks; for DoTAC, it is the -N(CH3)3 protons, and for sodium carboxylate, it is the methylene protons in R-position. The largest, most intense peak is the signal from the protons of the methylene groups of both DoTAC and sodium carboxylate. The diffusion studies of the isotropic solution phases were performed using the same spectrometer as above, equipped with a modified Bruker probe for self-diffusion capable of providing magnetic field strengths of maximum 3 T/m. Small Angle X-ray Scattering (SAXS). For a twodimensional hexagonal symmetry, the first three reflections correspond to (h, k) ) (1, 0), (1, 1), and (2, 0 ) and give the characteristic pattern 1:x3:x4 . . . h and k are the Miller indices. If the aggregates in the hexagonal phase can be considered as infinite cylinders, the hydrocarbon radius, rHC, can be calculated from the equation
rHC ) d
x
2φHC πx3
(4)
where d is the first-order Bragg spacing and φHC is the hydrocarbon volume fraction. Further, the surfactant headgroup area, S, can be estimated from the following relation:
Vs S)2 rHC
(5)
where Vs is the volume of the hydrocarbon chain of a single surfactant molecule. For a lamellar phase (the bilayers are assumed to be classical bilayers and the headgroups are in contact with the water layer), the reflections show a 1:2:3 . . . relationship. For a cubic liquid crystalline phase, there are a variety of different types of cubic structures, giving different SAXS diffraction patterns. For a specific cubic structure, a plot of the reciprocal spacings, (1/dhkl), of the Bragg reflections, versus m ) (h2 + k2 + l2)1/2, should give a straight line passing through the origin. h, k, and l are all Miller indices. From the slope of such a plot, the cubic cell lattice parameter, a, can be obtained, as the slope is equal to 1/a. (1/dhkl) is obtained by the equation
(1/dhkl) )
q 2π
(6)
where q is the Bragg reflection. The SAXS experiments in this work were performed on a Kratky compact small angle system equipped with a position sensitive detector with 1024 channels. The sample-to-detector distance is 277 mm, and the wavelength is 1.54 Å, allowing measurements down to a scattering vector of q ) 0.01 Å-1. The samples for the measurements were filled into a paste holder with thin mica windows. Construction of the Pseudo-Ternary Phase Diagrams. All samples were first examined for birefringence between crossed polarizers. 2H NMR spectra were then recorded for samples within the whole composition range at regular intervals, until no noticeable changes could be detected. The 2H NMR results can help outlining both one-phase and multiphase regions. From these results, the pseudoternary phase diagrams could be outlined. The structures of the liquid crystalline phases were further verified by SAXS studies.
Figure 1. Isothermal pseudoternary phase diagrams of the three aqueous catanionic surfactant mixtures at 20 °C. (a) DoTAC-SH-2H2O; (b) DoTAC-SO-2H2O; and (c) DoTACSD-2H2O. Phase abbreviations are as follows: L1, normal micellar solution; I, micellar cubic; H1, DoTAC-rich normal hexagonal; H2, sodium alkanoate-rich normal hexagonal; LR, lamellar; V, bicontinuous cubic liquid crystalline phase; and C, crystals.
The pseudoternary phase diagrams of the three systems DoTAC-SH-2H2O, DoTAC-SO-2H2O, and DoTAC-SD-2H2O constructed from the analysis of experimental studies are shown in Figure 1.
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Figure 2. Binary phase diagrams of the aqueous parent surfactant systems at 20 °C. (a) Dodecyltrimethylammonium chloride (DoTAC);18 (b) sodium hexanoate (SH); (c) sodium octanoate (SO);19 and (d) sodium decanoate (SD). Phase abbreviations as in Figure 1.
Results and Discussion Parent Systems. The binary phase diagrams of DoTAC18 and SO19 at 20 °C were redrawn from the literature and shown in Figure 2 parts a and c. The binary phase diagrams of SH and SD are based on experimental results and shown in parts b and d of Figure 2, respectively. As can be seen in Figure 2a, the binary DoTAC-water system shows a variety of phases at 20 °C, i.e., a large isotropic solution phase and three liquid crystalline phases; a micellar cubic, a hexagonal, and a bicontinuous liquid crystalline phase.18 It is known that short-chained surfactants possess few amphiphilic properties.20,21 It is therefore natural that SH only forms an isotropic solution phase with water, up to 42 wt % surfactant, and no liquid crystalline phase. There is no liquid crystal present in the binary SD-water system at 20 °C either, because of its high Krafft-temperature, but an isotropic solution up to 26 wt %. SO, on the contrary, forms both an isotropic solution phase below 41 wt % and a hexagonal liquid crystalline phase between 46 and 52 wt %. The Pseudo-Ternary Phase Diagrams of the DoTAC-Sodium Carboxylate-Water Systems. Global Phase Behavior. At the studied temperature, all three systems in this study form four different phases deriving from the binary DoTAC-water axis, namely, an isotropic solution phase followed by a micellar cubic, an hexagonal, and a bicontinuous cubic phase. A similar phase sequence, although at higher temperature, has been found in the DoTAC-sodium nonanoate-water system.4 However, the extension of phases varies between the systems. In the anionic surfactant-rich part of the phase diagram, along the binary sodium carboxylate-water axis, the phase equilibria differ more between the different systems. The DoTAC-SO system is the only system in this study that forms a hexagonal phase deriving from this axis, whereas the SH and SD systems do not. The sodium hexanoate does not form any liquid crystalline phases, as its alkyl chain is too short, whereas the reason for the lack of liquid (18) Balmbra, R.; Clunie, J.; Goodman, J. Nature 1969, 222, 1159. (19) Ekwall, P. In Advances in Liquid Crystal; Brown, G. H., Ed.; Academic Press: New York, 1975. (20) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (21) Khan, A.; Mendonca, C. J. Colloid Interface Sci. 1995, 169, 60.
Sjo¨ bom and Edlund
Figure 3. L1 solution phase of the DoTAC-SD-2H2O system with the two-phase area boundary at different temperatures. Notations are (s) 20 °C and (s s) 45 °C.
crystals coming from the SD-water axis is the high Krafft temperature. In all three systems, being most dominant in the DoTAC-SD-water system, there is a lamellar liquid crystalline phase coming from the DoTAC-rich corner on addition of anionic surfactant. The range of existence of this lamellar phase is limited in the two systems with shorter alkylcarboxylates, but dominating the phase diagram in the DoTAC-SD-water system. All three systems naturally show a variety of multiphase regions, which have not been studied extensively in this work. Isotropic Solution Phases. The DoTAC-SH-2H2O and DoTAC-SO-2H2O systems both show a single isotropic solution phase in the water rich corner, up to 41 wt % DoTAC and 42 and 41 wt % SH and SO, along the respective binary axes. The first can take up to about 41 wt % of total surfactant at the equimolar line and the latter about 35 wt % at 20 °C. On increasing the temperature to 45 °C, the boundary of the solution phases moves slightly toward higher concentrations of total surfactant, i.e., 43 and 36 wt %, respectively. When looking at a series of samples with constant water content, there is no apparent change in the solution viscosity on changing the molar ratio between the two surfactants in either of these two systems. The system DoTAC-SD-water shows a different phase behavior in this area (Figure 3). It forms a solution phase with its origins in the binary DoTAC-water and SDwater axes, just like the other two systems. At equimolar amounts of the surfactants, the isotropic solution can take up to 20 wt % at 20 °C and up to 30 wt % of total surfactant at 45 °C. However, within a narrow area around the equimolar line, up to 13 wt % of total surfactant, phase separation occurs. Associative phase separation at equimolar amounts of two oppositely charged surfactants in aqueous solution is a common phenomenon, which often results in precipitation.1 This is not the case in this system where there is coacervation, i.e., phase separation into two solutions. The resulting primary solution has a cloudy appearance and finally separates into two translucent solution phases, one dense and viscous and one dilute. The coacervation phenomenon is universal for nonionic surfactants but can also been seen in other mixed systems, for example, cetyltrimethylammonium bromide-sodium deoxycholate.2 The CTAB-deoxycholate system shows the same behavior as DoTAC-SD, as the solution phases originating from the respective binary axis are connected at surfactant concentrations above the equimolar coacervation area.
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Figure 4. Results of diffusion measurements in the isotropic solution phases at 20 °C. The surfactant self-diffusion coefficient is shown as a function of the sodium alkanoate/DoTAC molar ratio. The shaded region in d is the approximate area of coacervation. (a) DoTAC-SH-2H2O (95 wt % 2H2O); (b) DoTAC-SO-2H2O (95 wt % 2H2O); (c) DoTAC-SD-2H2O (85 wt % 2H2O); and (d) DoTAC-SD-2H2O (95 wt % 2H2O). Notations are (b) the diffusion coefficient obtained from the N-CH3 signal; (4) and (0) the fast and slow diffusion coefficient, respectively, obtained from the large -CH2- signal.
The maximum width of the DoTAC-SD two-phase area, in terms of SD, is about 1 wt % around the equimolar line. The coacervation area diminishes upon heating, and Figure 3 shows the difference in its range of existence between 20 and 45 °C. In a series of samples of the DoTACSD-2H2O system, at a constant water content of 85 wt %, the viscosity appears to be a little higher for the samples close to the equimolar line. However, at a water content of 95 wt % the samples are of low viscosity at all cationic/ anionic surfactant molar ratios on both sides of the coacervation area. At very high water content, i.e., above about 98 wt %, the solutions within the small two-phase area mentioned above start to get a bluish appearance. Some of these solutions are slightly birefringent, and others show flow birefringence; that is, they turn birefringent after shear. However, the structure of the phase and its aggregates in this region has not been investigated in detail, and more work is required. It is apparent that the solution phase behavior is very much dependent on the level of asymmetry in these systems. None of the shorter alkylcarboxylates (SH and SO) show any tendencies of associative phase separation at equimolarity. Regev and Khan have studied the asymmetric system DoTAC-sodium nonanoate-water, and there is no phase separation within the solution phase in that system either.3 However, the symmetric system DoTAC-sodium dodecanoate-water gives rise to precipitation, which is another type of associative phase separation common for catanionic surfactant mixtures.3 1 H self-diffusion measurements were performed in the
solution phases of all three systems at constant water content and varying sodium carboxylate/DoTAC molar ratios. The results are presented and discussed in the following section. 1 H Self-Diffusion in the Isotropic Solution Phases. The 1H signal of DoTAC (-N(CH3)3) as well as the large CH2 signal were followed, the latter with contribution from both DoTAC and sodium carboxylate. Because of unsatisfactory resolution (the peak being too small), the sodium carboxylate diffusion has not been evaluated at the lowest molar ratios. DoTAC-SH-Water and DoTAC-SO-Water. The DoTAC-SH and DoTAC-SO solutions were studied at a constant water concentration of 95 wt %. The pure SH and SO solutions give diffusion constants of 610 and 530 × 10-12 m2 s-1, respectively. The results are presented in Figure 4 parts a and b. It can be seen for both systems that, at sodium carboxylate/ DoTAC molar ratios
