Studies on an Ester-Modified Cationic Amphiphile in Aqueous

Oct 9, 2007 - The aqueous behavior of an ester-modified cationic amphiphile with the molecular structure CH3CH2O(C O)(CH2)6(C ...
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Langmuir 2007, 23, 11434-11442

Articles Studies on an Ester-Modified Cationic Amphiphile in Aqueous Systems: Behavior of Binary Solutions and Ternary Mixtures with Conventional Surfactants Dan Lundberg,*,†,‡ Johan Unga,§ Ashley L. Galloway,† and Fredric M. Menger*,† Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322, and Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, 412 96 Go¨teborg, Sweden ReceiVed February 13, 2007. In Final Form: August 15, 2007 The aqueous behavior of an ester-modified cationic amphiphile with the molecular structure CH3CH2O(CdO)(CH2)6(CdO)O(CH2)8N+(CH3)3Br-, in the following referred to as A, has been investigated. Systems with A as the only solute, as well as different aqueous mixtures with conventional cationic surfactants, primarily dodecyltrimethylammonium bromide (DTAB), were included in the study. Isotropic solution samples were characterized using 1H NMR, 13C NMR, NMR diffusometry, and conductivity measurements, whereas liquid crystalline samples were investigated by optical polarization microscopy and small-angle X-ray diffraction. The results are compared to the behavior of the binary system of DTAB and water. A does not exhibit a typical surfactant behavior. When it is present as the only solute in a binary aqueous system, it forms neither conventional micelles nor liquid crystalline phases. However, there is clear evidence that it assembles with lower cooperativity into loose clusters at concentrations above 25-30 mM. When A is mixed with DTAB in solution, the two amphiphiles form mixed assemblies, the structure of which varies with the total amphiphile concentration. In concentrated mixtures with alkyltrimethylammonium surfactants, A can participate in hexagonal liquid crystalline phases even when it constitutes a significant fraction of the total amphiphile content.

Introduction Most water-soluble organic substances show at least some degree of amphiphilicity and, hence, have an inherent driving force to minimize unfavorable exposure of their hydrophobic moieties to water.1 The amphiphilic character of a compound is manifested, for instance, by a decrease in the surface tension of its aqueous solution. Even a small molecule, such as methanol, which has an unlimited miscibility with water, causes a significant decrease in surface tension by exposing its methyl group to the air.1 Another common consequence of the amphiphilicity is aggregation of the molecules in aqueous solutions. Depending on the molecular structure of the solute, particularly the size, type, and distribution of its hydrophobic portions, the aggregation can show a varying degree of cooperativity, and the resulting aggregates can have very different morphologies and sizes, everything from small oligomers such as dimers or trimers to large assemblies consisting of thousands of molecules is possible.1 Amphiphiles with distinctly separated hydrophilic and hydrophobic parts, such as synthetic surfactants and certain natural polar lipids, commonly show self-assembly with a high degree of cooperativity, resulting in, for instance, micelles or bilayer structures. Micelle formation is governed by a delicate balance between several factors, e.g., the relative sizes of the hydrophilic and hydrophobic parts, the presence of charges, and the degree * Corresponding authors. E-mail: [email protected] (D.L.), menger@ emory.edu (F.M.M.). † Emory University. ‡ Present address: Department of Chemistry, University of Coimbra, 3004535 Coimbra, Portugal. § Chalmers University of Technology. (1) Mukerjee, P. J. Pharm. Sci. 1974, 63, 972.

of hydration, but it generally occurs at a well-defined concentration, the critical micelle concentration (cmc). It is critically important for the ability of an amphiphile to form micelles that its hydrophobic moieties can pack closely in order to effectively exclude water from the aggregates. Hence, the hydrophobic parts must be flexible. Surfactant-like molecules with rigid structures, such as bile salts or substances carrying a polycyclic aromatic group as the hydrophobic part do in general not form conventional micelles, even if their hydrophilic and hydrophobic moieties are well-separated.1 In recent years, Menger and co-workers have studied several amphiphiles, whose structures are based on that of the simplest type of surfactants, single-tailed with a small ionic headgroup, but are modified by the inclusion of nonhydrocarbon groups in their hydrophobic parts. One of the main incentives for studying these substances has been to investigate how the presence of different functional groups affect the self-assembly behavior of amphiphiles. Thus, substances having one or two ester,2,3 ether,4 or thioether groups5 inserted at different positions along the hydrocarbon chain have been synthesized and characterized with regard to their basic physicochemical properties in aqueous solution. All of the inserted functional groups can be classified (to use the terminology of Laughlin6) as nonoperative functional groups, i.e., they are by themselves not sufficiently hydrophilic to act as a surfactant head group. On the other hand, the inserted (2) Menger, F. M.; Galloway, A. L. J. Am. Chem. Soc. 2004, 126, 15883. (3) Menger, F. M.; Galloway, A. L.; Chlebowski, M. E. Langmuir 2005, 21, 9010. (4) Menger, F. M.; Chlebowski, M. E. Langmuir 2005, 21, 2689. (5) Menger, F. M.; Shi, L. J. Am. Chem. Soc. 2006, 128, 9338. (6) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: London, 1994.

10.1021/la700430u CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007

An Ester-Modified Amphiphile in Aqueous Systems

Langmuir, Vol. 23, No. 23, 2007 11435 Table 1. The Used Values for the Densities of the Complete Amphiphiles (G) and Their Apolar Parts (Gap), Respectively

Figure 1. The molecular structure of A.

functionalities can all be expected to show differences in their interactions with water, both in magnitude and type, as compared to the ordinary hydrocarbon segment that they replace. Since hydrophobic and hydrophilic interactions are interdependent, and hence not additive,7 it is difficult to predict the degree of tail modification that can be sustained before an amphiphile loses its typical surfactant behavior, e.g., the ability to form micelles at a distinct concentration. It is also difficult to predict the character of nonmicellar assemblies that may possibly be formed by “overly modified” surfactants. If an inserted functional group shows a significant attraction to water, this may severely impede close packing of the hydrophobic parts, and hence decrease the propensity for micelle formation. On the other hand, if the interactions with the solvent are weaker, the chain modification may have a minor, or even negligible, effect on the qualitative behavior of the amphiphile. Not surprisingly, the behavior of the modified surfactants varies widely depending on the type, number, and positions of the inserted functional groups. However, some general trends are observed. For instance, among the studied compounds that are modified with ester groups, which can act as hydrogen-bond donors and thus show notable interactions with water, only amphiphiles having a terminal segment of C10H21 or larger form normal micelles.2,8 For related compounds that lack a sufficiently large terminal hydrocarbon segment, the aggregation seems to occur with lower cooperativity, resulting in smaller assemblies with a looser structure than in a conventional micelle. Consequently, it is in a sense possible to view the hydrocarbon portion between the ester group and the polar head group effectively as an extension of the latter. This observation is consistent with previous reports on modified surfactants.6 The objective of this work was to make a more detailed investigation of the aqueous behavior of one of the ester-modified amphiphiles that were previously synthesized and studied in our laboratories, namely the compound A depicted in Figure 1.2,3,9 One of the main aims was to obtain a better picture of its aggregates in dilute solutions, but we also extended the composition range from that in previous work to include much higher concentrations (up to 90 wt % of amphiphile). Furthermore, in addition to studies on solutions of A by itself, we present data on the aggregation and phase behavior of mixtures of A and conventional cationic surfactants of the alkyltrimethylammonium bromide type, primarily dodecyltrimethylammonium bromide (DTAB). A multimethod approach was adopted: isotropic solution samples were characterized using 1H NMR, 13C NMR, NMR diffusometry, and conductivity measurements, whereas liquid crystalline samples were investigated by optical polarization microscopy and small-angle X-ray diffraction. Throughout the text, the results are discussed in relation to the binary system of DTAB and water, which is taken to represent “typical” surfactant behavior. Experimental Procedures Materials. A was synthesized using previously described procedures.2 DTAB (99%) and tetradecyltrimethylammonium bromide (TTAB, 99%) were purchased from Sigma, decyltrimethy(7) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (8) As judged from surface tension vs concentration plots and solubilization studies. (9) A was designated E in ref 2 and A in ref 3.

amphiphile

F (g/cm3)

Fap (g/cm3)

amphiphile

F (g/cm3)

Fap (g/cm3)

A DeTAB

1.12 -

0.98 0.80

DTAB TTAB

1.04 -

0.81 0.82

lammonum bromide (DeTAB, >98%) and hexadecyltrimethylammonium bromide (CTAB, >99%) were from Fluka, and hexamethyldisilane (HMDS, 98%) was from Aldrich. Deuterium oxide (99.9% D) was obtained from Cambridge Isotope Labs. All commercial chemicals were used as received. The aqueous solutions used in the conductivity experiments were prepared using water that had been purified with a Millipore Milli-Q system. Samples. Isotropic Solutions. Solution samples were prepared by mixing and/or diluting concentrated solutions of A or DTAB prepared with either D2O or H2O. The dilution was made by volume, but the aliquots were also weighed for verification. In samples containing HMDS, a small amount of this substance (on the order of one molecule of HMDS per 1000 surfactant molecules, verified by 1H NMR) was added to the sample solutions using a microliter syringe. To ensure that the added HMDS did not cause any significant change in the solution behavior, the selfdiffusion coefficient of the amphiphile was measured both before and after the addition. When the same samples were used in multiple experiments and more than 2 days passed between these, the samples were stored at 5 °C in order to avoid hydrolysis of the ester groups. In some of the most concentrated solutions of A, a detectable degradation of the substance was observed (on the order of a few percent). However, repeated experiments on these samples showed that the hydrolysis products did not cause any appreciable changes in the behavior of the samples. Liquid Crystalline Phases. Liquid crystalline samples were prepared by weighing the appropriate amounts of surfactants and D2O into screw-capped vials. The surfactant components (which are all dry powders) were mixed by shaking the vials before the solvent was added. After preparation, the samples were allowed to rest for a few days, centrifuged, and finally left for equilibration for at least 1 week before characterization. To monitor sample stability, 1H NMR spectra were run on samples prepared by diluting small portions of the liquid crystals. No appreciable degradation was detected during the relevant storage times. Only samples that by visual observation were deemed to be singlephase were examined by optical microscopy and SAXD. Calculation of the Volume Fraction of Aggregates. The volume fractions of aggregates, Φ, used in the calculations involving eq 3 and in Figure 6, and the volume fractions of the apolar domains, Φap, shown in Table 2, were estimated using the densities presented in Table 1. These densities were calculated from the molecular weights (or fractional molecular weights) and the following values of group volumes: 26.99 Å3 for -CH2-, 54.19 Å3 for -CH3, 35.00 Å3 for -COO-, and 141.0 Å3 for -N(CH3)3Br.10 For A, the whole chain, including the ester groups, is assumed to contribute to the apolar volume. When calculating the volume fraction of aggregates in the isotropic solution samples, we assumed that the concentrations of aggregated amphiphile are c - 30 mM for A and c - 16 mM for DTAB, where c denotes the total concentration of amphiphile. In the hexagonal phases, the head group region is considered as part of the polar domains. NMR. All NMR experiments were performed at 25 °C on a Varian INOVA 600 spectrometer equipped with a pulsed field gradient (PFG) generator and a PFG amplifier. The samples were inserted to the probe at least 20 min prior to the experiments to allow for thermal equilibrium to be attained. The 1D experiments were performed at 599.7 and 150.8 MHz for the 1H and 13C experiments, respectively. The 13C NMR spectra (10) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Liq. Cryst. 1992, 12, 667.

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were recorded in a 1H-decoupled mode. It was assumed that the chemical shift of the 2H lock signal was independent of the amphiphile concentration (this has been shown to be a good approximation for other ionic amphiphiles11), and since the frequency offset was kept constant, the changes in the 13C NMR chemical shifts with concentration could be calculated directly from the “absolute” frequencies of the respective resonances. In the 13C NMR spectra, one peak is observed for each nonequivalent carbon. For A it was possible to assign the following resonances with good certainty (as counted from the headgroup): the methylene carbon neighboring the head group (C1), the methylene carbons next to the ester oxygens (C8 and C17), the methylene carbons next to the carbonyl groups (C10 and C15, as a pair), the carbonyl carbons (C9 and C16, as a pair), the terminal methyl carbon (C18), and the head group methyl carbons. For DTAB, C1, C3, C10, C12, and the headgroup methyl carbons were assigned. The carbons that are not specifically assigned are referred to as midchain methylene carbons. The diffusion experiments were run using a Hahn-echo sequence with intervening pulsed field gradients (PG), resulting in a complete pulse sequence of 90°-PG-180°-PG. The delays between the gradient pulses (∆) and the width of the pulsed gradients (δ) was kept constant at 140 and 7 ms, respectively, while the strength of the pulsed gradient (G) was linearly incremented from 0.01 up to 0.6 T/m (maximum varied among experiments and samples) in 16 steps. The gradient strength was calibrated by making a measurement on a trace amount of H2O in D2O (D ) 1.902 × 10-9 m2 s-1), while linearity of the gradient amplifier in the applied gradient strength interval was verified by measurements on poly(ethylene glycols) with known D.12 The self-diffusion coefficients (D) of A and water were obtained from the attenuation of the relevant echo peaks by linear leastsquare fits to the Stejskal-Tanner equation (eq 1):13 1n(I/I0) ) -(γGδ)2 D(∆ - δ/3)

(1)

where I is the measured signal intensity, I0 the signal intensity in the absence of gradient pulses, γ the magnetogyric ratio of protons, and the rest of the parameters are as defined above. In all experiments the observed echo-decays gave good fits to eq 1, which shows that they represent single self-diffusion coefficients. Conductivity Measurements. Electrical conductivity measurements were performed using a Fisher Scientific Traceable conductimeter, which was calibrated to standard solutions with known conductivities (Fisher Scientific Traceable One-Shot). The measurements were carried out at 25 °C, and the electrode was immersed in stirred sample solutions until a stable reading was achieved. Optical Microscopy. The liquid crystalline samples were examined using a Nikon Diaphot-TMD microscope, with the samples placed between crossed polarizers, in order to identify characteristic birefringence patterns. Small-Angle X-ray Diffraction (SAXD). The SAXD experiments were performed on a Hecus X-ray Systems Kratky camera equipped with a MBraun linear position-sensitive detector. CuKR X-rays with a wavelength of 0.1542 nm were produced by a Philips PW 1830/40 X-ray generator operated at 50 kV and 40 mA, and a tungsten beam stop was used to protect the detector from the primary beam. In each experiment, data were collected for 30 min in a vacuum.

Results and Discussion Overview of Phase Behavior. The modified surfactant A exhibits a significant aqueous solubility. It forms an isotropic solution when present at up to approximately 75 wt % (1850 mM) in a binary mixture with water (D2O), while at higher (11) O ¨ dberg, L.; Svens, B.; Danielsson, I. J. Colloid Interface Sci. 1972, 41, 298. (12) Nyde´n, M. Measuring Micelle Size and Shape. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons: New York, 2001; Vol. 2, p 281. (13) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288.

Figure 2. Phase diagram of the system A/DTAB/D2O at 25 °C. The accuracy of the phase boundaries is within 5 wt %. The shaded region represents compositions not characterized. L ) isotropic liquid phase, Hex ) Hexagonal liquid crystalline phase.

concentrations the solution coexists with a solid, presumably hydrated amphiphile. When the concentration of A is higher than about 1000 mM (approximately 40 wt %), there is a visually detectable increase in the viscosity of the solutions. Contrary to what is common for conventional surfactants, the A system shows no liquid crystalline phases within the whole investigated composition range (up to 90 wt %). This departure from typical surfactant behavior does in itself indicate that aggregates of A probably do not have well-defined hydrophobic domains. If such assemblies are formed, these are generally expected to be forced, by geometrical constraints, into an ordered packing at higher concentrations. For comparison, the maximum volume fraction of randomly close-packed hard spheres is about 0.64.14 However, in the case of spherical surfactant micelles, the presence of aggregate-aggregate repulsions (due to electrostatic interactions and/or hydration) generally causes the formation of liquid crystalline phases at significantly lower volume fractions.15 The aqueous phase behavior of A can be compared to that of our reference system, i.e., the aqueous system of the conventional cationic surfactant DTAB, which shows aggregate growth at surfactant concentrations above approximately 450 mM (13 wt %)16 and a hexagonal liquid crystalline phase above about 50 wt %.17 It can also be valuable to relate the behavior of the concentrated, viscous solutions of A to that of solutions of CTAB, a surfactant that is known to form large, wormlike micelles at concentrations above 10 wt %. A visual comparison of a 60 wt % (1500 mM) solution of A and a 16 wt % solution of CTAB reveals that the viscosity of the latter is dramatically higher. This finding suggests that if any aggregate growth occurs in solutions of A, this growth is much less pronounced than that in solutions of CTAB. When A is present in concentrated aqueous mixtures with surfactants of the XTAB type, it can participate in hexagonal liquid crystalline phases even when it is present in quite high proportions. The phase diagram of the system A/DTAB/D2O is shown in Figure 2. Hexagonal phases are also formed in (14) Scott, G. D.; Kilgour, D. M. J. Phys. D: Appl. Phys. 1969, 2, 863. (15) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester, 2002. (16) Minardi, R. M.; Schulz, P. C.; Vuano, B. Colloids Surf. A 2002, 197, 167. (17) McGrath, K. M. Langmuir 1995, 11, 1835.

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Figure 3. Partial 1H NMR spectra of A in D2O at three different concentrations. 0.1 ppm ) 60 Hz.

concentrated mixtures of A and DeTAB or TTAB (see the SmallAngle X-ray Diffraction section). The extension of the liquid crystalline region toward the binary D2O-A axis increases with an increasing alkyl chain length of the XTAB molecule. Characterization of Isotropic Solution Samples. 1H NMR. Figure 3 shows the 1H NMR spectra of three solutions of A at different concentrations. When comparing the three spectra, one can clearly see that the peaks become gradually broader with increasing concentration. A pronounced “smearing” of the resonances occurs for concentrations of A above 750 mM, which means that the change in peak width probably reflects the increase in solution viscosity that was mentioned above. Both an increase in solution viscosity and a broadening of NMR peaks can be signs of aggregate growth. However, in comparison to the peak broadening that is observed when an amphiphile of similar size as A forms large micelles, the change in width for the resonances in Figure 3 is almost negligible. For instance, the half-height bandwidths of the 1H NMR peaks from CTAB in threadlike micelles are on the order of hundreds of hertz.18 Furthermore, the broad resonances do not show the typical band shape, characterized by a broad base combined with a sharp apex, that is expected in the 1H NMR spectrum of an amphiphile participating in large micelles.18,19 These findings are consistent with the difference in viscosity of solutions of A or CTAB and give further support to the notion that A does not form typical micelle-like aggregates. In Figure 3 it can also be seen that the resonances shift downfield with increasing concentration. This phenomenon will be further discussed in the 13C NMR section. For the mixtures of A and DTAB, there is no qualitative difference in the appearance of the 1H NMR spectra as compared to those of samples containing A as the lone solute. NMR Diffusometry. The self-diffusion coefficients of the different species present in an aqueous surfactant solution depend on the formation of self-assemblies and on other interactions between the components. Thus, in order to obtain information on the aggregate sizes and the degrees of hydration in the presently investigated systems, the PGSE-NMR technique20-23 was used (18) Ulmius, J.; Wennerstro¨m, H. J. Magn. Reson. 1977, 28, 309. (19) Olsson, U.; So¨derman, O.; Guering, P. J. Phys. Chem. 1986, 90, 5223. (20) Furo, I. J. Mol. Liq. 2005, 117, 117. (21) So¨derman, O.; Stilbs, P.; Price, W. S. Concepts Magn. Reson. A 2004, 23A, 121. (22) Price, W. S. Concepts Magn. Reson. 1997, 9, 299. (23) Price, W. S. Concepts Magn. Reson. 1998, 10, 197.

Figure 4. The observed self-diffusion coefficients of A (O) or DTAB (×) when each substance is present as single amphiphiles, of A (4) or DTAB (+) when present in an equimolar mixture of the two, and HMDS present in trace amounts in solutions of A (0) or DTAB (]) as a function of the total surfactant concentration. For the surfactants, Dobs values were measured for A alone, DTAB alone, as well as for both compounds in the mixture, at all studied concentrations. DHMDS values were determined in solutions of 50-500 mM A or 50-400 mM DTAB.

to determine the self-diffusion coefficients of the amphiphiles, the water, and the highly hydrophobic probe molecule HMDS in samples with different concentrations of A and/or DTAB. When HMDS is added to a micellar solution, it is heavily partitioned to the hydrophobic regions of the aggregates. Thus, under the assumption that HMDS resides only in the aggregates, its self-diffusion coefficient can be taken to represent the diffusion coefficient of the aggregates. Figure 4 shows the self-diffusion coefficients for A and DTAB in aqueous solutions where the compounds are present either as the sole amphiphiles or in equimolar aqueous mixtures of both, as well as the diffusion coefficients for HMDS that is present in trace amounts in solutions of A or DTAB. In addition to the data shown in Figure 4, the self-diffusion coefficient of A was also measured in samples with concentrations between 500 and 1500 mM. In this interval, the observed diffusion coefficient decreases essentially linearly down to 1.4 × 10-11 m2 s-1 at 1500 mM. The observed decrease in the amphiphile self-diffusion

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generally be described by a first-order expression (eq 3)12,21

Dagg ) D0,agg(1 - kΦ)

Figure 5. The observed self-diffusion coefficients of A (O) or DTAB (×) as a function of the reciprocal of the total surfactant concentration. The dashed line is a prediction of the observed diffusion coefficients obtained using eq 2 with a Dmono of 4.7 × 10-10 m2 s-1, a Dagg of 8.7 × 10-11 m2 s-1, and a cmc of 15.6 mM.

coefficients with increasing concentration is consistent with assembly into some sort of aggregate. A comparison between the diffusion coefficients for the two amphiphiles show that the aggregates of A have an apparent size on the same order as that of DTAB micelles. Under the assumptions that only monomers are present below the cmc and that the micelles can be approximated as monodisperse, discrete entities, the observed self-diffusion coefficient, Dobs, is the weighted average of the diffusion coefficients for the monomers, Dmono, and the aggregates, Dagg, according to eq 2

Dobs )

cmono cagg Dmono + D c c agg

(2)

where cmono and cagg are the concentrations of surfactant molecules at the respective sites, and c is the total surfactant concentration.15 For a conventional, micelle-forming surfactant, eq 2 gives a good approximation of reality. It follows from eq 2 that, for a conventional surfactant, a plot of Dobs versus the reciprocal of the total surfactant concentration should show two approximately straight lines intersecting at the cmc, a horizontal line Dobs ) Dmono for concentrations up to the cmc and a line with a slope of approximately (Dmono - Dagg) × cmc for higher concentrations. Plots of Dobs of A and DTAB versus c-1, see Figure 5, show approximately linear sections at both high and low c-1 for both the amphiphiles. However, the plots also indicate a higher curvature around the intersection of the two lines in the case of A than in the case of DTAB. A smoother change in Dobs with c-1 indicates a less cooperative self-assembly and/or a gradual growth in aggregate size with increasing concentration.24 Note that Dobs of A is lower than Dmono at values of c-1 higher than 0.033 mM-1.3 This finding indicates that A begins to assemble at concentration lower than 30 mM, i.e., the concentration where a break in a surface tension curve for A has been previously observed.3 To make a more detailed analysis of the aggregation behavior of A, we took a closer look at the observed self-diffusion coefficients of HMDS, DHMDS. One can see in Figure 4 that DHMDS decreases slowly with increasing concentration. A decrease with increasing volume fraction of aggregates in a solution is expected, due to obstruction effects, but it is also possible that there is a contribution from an increase in aggregate size. In the case of monodisperse spherical aggregates of constant size, the decrease in the diffusion coefficient, Dagg, as a function of the volume fraction of aggregates, Φ, can, at low volume fractions, (24) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, 2nd ed.; Wiley-VCH: New York, 1999.

(3)

where D0,agg is the diffusion coefficient of the micelles at infinite dilution and k is a constant that usually varies between 1.7 and 2.5, depending on the type and degree of interactions between the aggregates.12,25,26 DHMDS for DTAB shows an essentially linear decrease with concentration, and a fit of these diffusion coefficients versus Φ (see Experimental Procedures for calculation of Φ) to eq 3 gives a value of k of 2.2, which is consistent with spherical aggregates, and a D0,agg of 8.7 × 10-11 m2 s-1. The latter is related to the hydrodynamic radius of the aggregates, RH, via the StokesEinstein equation (eq 4)

D0,agg )

kBT 6πηRH

(4)

where kB is the Boltzmann constant and η the viscosity of the solvent at the experimental temperature T. Setting η ) 1.132 mPa s for D2O in eq 4 gives a hydrodynamic radius of the DTAB micelles of 2.2 nm. This value approximates the extended length of a DTAB molecule, i.e., 2.1 nm, and is hence close to the expected micellar radius.7 In the A system, DHMDS show a nonlinear decrease with increasing amphiphile concentration that cannot be fit directly to eq 3. This nonlinearity supports the idea that the assemblies of A grow continuously with increasing concentration. It is important to keep in mind, however, that since the aggregates may have a loose structure, e.g., contain water due to the presence of the more polar ester groups, it is likely that partitioning of HMDS into these assemblies is weaker than it is into the more well-defined hydrophobic domains of conventional micelles, e.g., those formed by DTAB. In this case, the measured DHMDS can, particularly at lower concentrations, be expected to be higher than the true Dagg and may hence give an underestimation in the effective size of the A aggregates when this is calculated using eq 4. If, however, the discrepancy is assumed to be negligible at 500 mM A (where Φ ) 0.19) and obstruction as an approximation is accounted for via eq 3 with a k of 2 (which is roughly the average of the k values generally observed for spherical aggregates), one obtains an RH of 3.1 nm. This value approximates the extended length of a molecule of A. Hence, the diffusometry results for HMDS suggest that the assemblies of A at higher amphiphile concentrations have an apparent size similar to that expected had it formed conventional micelles. The estimates of Dagg for A and DTAB together with the experimental values of Dmono and the cmc values15 were inserted into eq 2 to calculate the corresponding Dobs vs c-1. As can be seen in Figure 5, the experimental values of Dobs for DTAB show a good fit to the calculated concentration dependence, except at the lowest c-1. This discrepancy is probably caused by obstruction effects and/or minor micellar growth. However, regardless of the cmc value used, the measured Dobs values of A will not fit eq 2, which is consistent with our interpretation of the diffusion data. Returning to Figure 4, one can see that the values of Dobs for A and DTAB in the equimolar mixtures are very close to the values obtained for the individual amphiphiles at equivalent total concentrations. This observation gives clear evidence for the formation of mixed assemblies. If the onset of aggregation of (25) Evans, G. T.; James, C. P. J. Chem. Phys. 1983, 79, 5553. (26) Ohtsuki, T.; Okano, K. J. Chem. Phys. 1982, 77, 1443.

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Figure 6. Normalized self-diffusion coefficients of water as a function of the volume fraction of aggregated A (O) or DTAB (b). D0,water, i.e., the self-diffusion coefficient for water in bulk, is 1.902 × 10-9 m2 s-1. In terms of molarity, the concentrations range from 50 to 1000 mM A and from 25 to 1400 mM DTAB. The calculation of Φ is detailed in the Experimental Procedures. The dashed line represents a prediction of the contribution from obstruction to the decrease in the water diffusion (see the text).

one amphiphile was unaffected by the presence of the other, signs of aggregation would not be observed until each of the components reached their respective aggregation concentrations (i.e., the cmc in the case of DTAB). Another important observation is that Dobs of A is notably higher than that of DTAB in a significant concentration range above the onset of assembly. Since Dmono of DTAB is higher than Dmono of A, this observation directly shows that the first formed micelles (with increasing concentration) consist mainly of DTAB and that the fraction of A in the micelles increases with increasing total concentration (refer to eq 2). Figure 6 shows the concentration dependence of the selfdiffusion coefficients of water, Dwater, in solutions of either A or DTAB. The data are normalized to the self-diffusion coefficient of water in bulk (i.e., D of a trace amount of HDO in pure D2O), D0,water, and plotted versus the volume fraction of aggregated amphiphile, Φ. The self-diffusion coefficient of water in a micellar solution is reduced due to the combination of two effects. First, the aggregates exclude a fraction of the total volume for the diffusing water and thereby obstruct the diffusion pathway. Second, a fraction of the water hydrates the surfactant molecules. If the aggregates in a solution can be approximated as hard spheres, the decrease in the diffusion coefficient of the solvent due to obstruction can be predicted by eq 527

Dwater )

D0,water 1 + Φ/2

(5)

where all parameters are defined above. Such an approximation is relevant for spherical micelles and hence applicable at least for the DTAB system. A plot of a prediction from eq 5 is included in Figure 6. It is evident from Figure 6 that hydration of the solutes provides a major contribution to the decrease in water mobility in both of the studied systems. One can also see that the decrease in Dwater with amphiphile concentration is significantly larger in solutions of A than in solutions of DTAB at a given concentration. As can be understood from eq 5, the obstruction of solvent diffusion exerted by spherical aggregates depends on the volume (27) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77.

fraction of dispersed matter but not on the radius of the spheres.28 Thus, if we for a moment assume the hypothetical situation where A forms conventional micelles, a difference in Dwater at a given Φ between solutions of A or DTAB can be attributed to a difference in the degree of hydration of the respective compounds. Since the two amphiphiles both carry trimethylammonium groups as their hydrophilic head groups and since it hence is reasonable to assume that the hydration number per head group is similar in the two cases, one would then expect the decrease in Dwater to be larger in the sample with the higher molar concentration, i.e., the DTAB solution (due to its lower molecular weight). However, the opposite is observed. This finding gives further strong support to the proposal that A forms loosely assembled clusters. For instance, the degree of hydrophobic hydration per amphiphile molecule can be expected to increase with an increased area of exposure to the surrounding water,29 which is one of the consequences of “looseness”. In addition, confinement of water within the clusters, as well as attractive interactions between the ester groups and the solvent, may contribute to the decrease in Dwater in the A system relative to that in the DTAB system. 13C NMR. It could be seen in Figure 3 that the peaks in the 1H NMR spectra of A in D O shifted downfield with increasing 2 concentration. A concentration dependence of NMR chemical shifts is commonly observed for amphiphiles in aqueous solution and can in general be attributed to a combination of differences in the direct effects of the environment (“medium effects”) and differences in the average conformation of the molecules (“conformation effects”) between the monomeric and aggregated states. The phenomenon has been used to monitor micellization of surfactants for several decades.30-32 It has been shown that the changes in 13C NMR shifts for an alkyl chain show only a very weak dependence on medium effects, and thus to a good approximation can be ascribed exclusively to changes in chain conformation,33-35 and that a downfield shift can be related to an increase in the average ratio of trans to gauche conformations in the chains.34,35 In contrast to the case of alkyl carbons, the chemical shift of a carbonyl carbon can show a significant dependence on direct interactions with the surrounding solvent.33 This is particularly true in a protic solvent, as water, where the degree of solvent-solute hydrogen bonding have a pronounced influence on the observed chemical shift. Hence, the change in shift for the carbonyl carbons of A can to a significant extent be taken to reflect the degree of exposure of the ester bonds to water. When only one peak for each nonequivalent carbon in a molecule appears in the spectra, the observed chemical shift, δobs, for each atom is the population-weighted average of the values at the different sites where the molecules reside. Accordingly, in the case of a conventional, micelle-forming surfactant, one obtains an expression for δobs of the same form as eq 2, given in eq 6 (28) This is true as long as the aggregate dimensions are much larger than the size of a water molecule, a condition that is fulfilled for conventional surfactant micelles. (29) Lindman, B.; Wennerstro¨m, H.; Gustavsson, H.; Kamenka, N.; Brun, B. Pure Appl. Chem. 1980, 52, 1307. (30) Clifford, J.; Pethica, B. A. Trans. Faraday Soc. 1964, 60, 1483. (31) Muller, N.; Birkhahn, R. H. J. Phys. Chem. 1967, 71, 957. (32) Drakenberg, T.; Lindman, B. J. Colloid Interface Sci. 1973, 44, 184. (33) Stothers, J. B. Carbon-13 NMR Spectroscopy; Academic Press: New York, 1972. (34) Batchelor, J. G.; Prestegard, J. H.; Cushley, R. J.; Lipsky, S. R. Biochem. Biophys. Res. Commun. 1972, 48, 70. (35) Persson, B. O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124.

11440 Langmuir, Vol. 23, No. 23, 2007

δobs )

cmono cagg δmono + δ c c agg

Lundberg et al.

(6)

where δmonoand δmic are the chemical shifts of amphiphile molecules present as monomers or residing in micelles, respectively, while cmono, cmic, and c are as defined above. Figure 7 shows the change in δobs with c-1 for selected carbons in A and DTAB. The data for DTAB show the expected dependence on c-1, i.e. a distinct kink at cmc-1, a downfield shift on micelle formation, and a larger change in shift for the midchain carbons than for those in either end of the alkyl chain.32 For A, the change in chemical shift for all alkyl carbons, except those neighboring the ester oxygens, is also downfield, but the change is significantly smaller than that observed for DTAB. Furthermore, the shifts change with c-1 in a more gradual fashion. If the chemical shifts are assumed to exclusively reflect the average chain conformation, these observations strongly support an assembly structure for A that differs significantly from that of conventional micelles and that the structure of the assemblies changes gradually with concentration. Consistent with the results from the NMR diffusion experiments, the 13C NMR data also indicate that self-assembly of A begins at a concentration well below 30 mM, i.e., the previously suggested aggregation concentration (30 mM corresponds to a c-1 of 0.033 mM-1).3 The signals from the carbonyl carbons, as well as those from the carbons adjacent to the ester groups, show upfield shifts with increasing concentration. An upfield shift can be related to an average decrease in hydrogen bonding to the oxygen atoms,33 indicating at least a partial loss of ester-water interactions when A form assemblies. However, the change in shift from the lowest to the highest concentration of A, i.e. approximately -2 ppm, is significantly smaller than, for instance, the difference of more than 11 ppm in the carbonyl carbon shift of acetone in water and cyclohexane.33 Hence, these results do not suggest that the ester bonds are transferred to a “dry” hydrocarbon environment when A assembles and thus support the interpretation of the diffusion data. Interestingly, the carbonyl peaks (4 in Figure 7) begin to shift from δlow at a lower concentration (i.e., a higher c-1) than the other signals. This observation suggests that there is a loss of hydrogen bonding at a concentration lower than where the average molecular conformation is affected and further supports that the character of the aggregates changes with concentration. Figure 8 compares the changes in δobs for selected peaks of A or DTAB when the compounds are present as either the only solute or in an equimolar mixture of the two. The observed changes in δobs with c-1 are consistent with the results from the NMR diffusion measurements in that they support that mixed aggregates are formed (by equivalent arguments). One can also see that at higher concentrations (i.e., lower c-1) the average chain conformations for the two compounds seem to be similar to those experienced in the respective single-amphiphile aggregates. The most interesting observation is that for some of the midchain carbons in A (9 and 0 in the plot), at intermediate total amphiphile concentrations, the shifts deviate from δlow to upfield shifts when the substance is present in the mixture. This finding suggests a decrease in the ratio of trans to gauche conformations at certain carbon positions and can be explained by the tails of A being, on average, more bent in an intermediate concentration range but adopting a more extended conformation at higher concentrations. In terms of aggregate structure, the observed data are consistent with a picture in which the first molecules of A that participate in assemblies are oriented with their ester groups close to the surface of the clusters, but are forced into the core of the assemblies with an increasing total concentration, when also the fraction of A in the aggregates increases.

Figure 7. Change in 13C NMR shifts relative to the shift for monomeric amphiphile versus the reciprocal of the concentration for selected peaks from A or DTAB. The peaks are from the terminal methyl (O), two different “midchain” methylene groups (0, ]), the methylene groups neighboring the ester oxygens (+), and the carbonyl carbons (4) of A, or the terminal methyl (b), the midchain methylene groups (9), or the C10 methylene group (2) of DTAB. ∆δ ) δobs - δlow, where δlow is the observed chemical shift at the lowest investigated concentration, i.e., 5 mM (not shown). The ∆δ values of the carbonyl carbons (4) of A for the four most concentrated samples (i.e., at low c-1) have been omitted from the plot. These decrease monotonically with increasing concentration to a value of -1.9 for the 100 mM sample (i.e., at c-1 ) 0.01 mM-1).

Figure 8. Change in 13C NMR shifts relative to the shift for monomeric amphiphile versus the reciprocal of the total concentration for selected peaks from A or DTAB when present as either the only amphiphile (solid symbols) or in an equimolar mixture of A and DTAB (open symbols). For A, the peaks are from a midchain methylene group (9, 0) and the carbonyl carbons (2, 4), and for DTAB, the peaks are from the midchain methylene groups (b, O). ∆δ values for the carbonyl carbons for A (2, 4) for the most concentrated samples (i.e., at low c-1) have been omitted from the plot. These decrease monotonically with increasing concentration to values of -1.9 and -2.0 for the 100 mM samples (i.e., at c-1 ) 0.01 mM-1) where A is present as the lone solute and where it is admixed with DTAB, respectively.

ConductiVity Measurements. Determining the concentration dependence of the specific conductivity, κ, of an aqueous solution of an ionic surfactant gives values of both the cmc and the degree of counterion dissociation, R, for the micelles. The cmc is generally obtained from the intersection of two essentially straight lines and a good estimate of R can often be obtained by simply calculating the ratio of the slopes above and below the cmc. The results from a conductivity study on A and DTAB are shown in Figure 9. The plot for DTAB shows the expected appearance with a distinct break at about 16 mM and a change in slope that corresponds to an R of about 0.3. Both the cmc and the counterion dissociation are in good agreement with values found in the

An Ester-Modified Amphiphile in Aqueous Systems

Langmuir, Vol. 23, No. 23, 2007 11441 Table 2. Values of the Lattice Parameter (a) Obtained from the SAXD Experiments Together with Values of the Apolar Volume Fraction (Φap) and Cylinder Radii (R) Calculated for the Hypothetical Cases Where All (ΦXTAB+A, RXTAB+A) or None (ΦXTAB, RXTAB) of the Fraction of A Is Included in the Cylindrical Aggregates That Make Up the Hexagonal Phase (see Text for Further Details)a

Figure 9. Specific conductivity as a function of concentration of A (a) or DTAB (b).

literature.15,17,36 For A, the dependence of κ on concentration is much weaker. The slight change in slope at about 25 mM corresponds to an R of about 0.9, which indicates a very limited condensation of ions when the assemblies are formed. The plot for A can be compared to the results from conductivity studies on surfactant aggregation in situations where the cooperativity is impaired, for instance in aqueous solutions containing ethylene glycol37 or urea.38 Under such conditions one obtain plots that are similar to that obtained for A. In conclusion, the findings from the conductivity measurements on A are consistent with those from the previously discussed experiments, in that they suggest that loosely assembled aggregates are formed at a concentration around 25-30 mM. Structure in Highly Concentrated Solutions of A. Several of the results presented above suggest that the assemblies of A in dilute solutions have a “looser” structure than conventional micelles. At higher concentrations (500 mM, Φ ) 0.19), DHMDS data correspond to an apparent aggregate size similar to that expected for conventional spherical micelles. When the amphiphile concentrations is increased above 500 mM, the decrease in D of A is quite limited and can be most probably accounted for by obstruction effects; any notable growth of the aggregates should show a larger effect on the observed self-diffusion coefficient.39 Hence, it is reasonable to assume that A remains in roughly spherical clusters up to the point (with regard to increasing concentration) where these are forced into close contact. At very high amphiphile volume fractions, the self-diffusion data are difficult to evaluate in terms of aggregate size and shape, because when the aggregates are in close contact, there is a significant contribution from molecular diffusion between the assemblies to the observed diffusion coefficient.39 However, the simple fact that no liquid crystalline phases are formed at any (36) Carpena, P.; Aguiar, J.; Bernaola-Galvan, P.; Ruiz, C. C. Langmuir 2002, 18, 6054. (37) Carnero Ruiz, C. Colloid Polym. Sci. 1999, 277, 701. (38) Carnero Ruiz, C. Colloids Surf. A 1999, 147, 349. (39) Jonstro¨mer, M.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293.

sample

a (nm)

ΦXTAB+A

RXTAB+A (nm)

ΦXTAB

RXTAB (nm)

DTAB 60/0 DTAB 60/10 DTAB 60/20 DTAB 60/30 DTAB 60/40 DTAB 70/0 DTAB 70/10 DTAB 70/20 DTAB 70/30 DTAB 70/40 DTAB 70/50

4.16 4.11 4.14 4.19 4.21 3.96 3.95 3.98 4.00 4.04 4.13

0.44 0.44 0.45 0.45 0.45 0.51 0.51 0.52 0.52 0.53 0.53

1.45 1.43 1.45 1.47 1.49 1.48 1.48 1.50 1.51 1.54 1.58

0.44 0.40 0.35 0.31 0.27 0.51 0.46 0.41 0.36 0.31 0.26

1.45 1.36 1.29 1.23 1.14 1.48 1.40 1.34 1.26 1.18 1.11

DeTAB 60/0 DeTAB 60/10 DeTAB 60/20

3.53 3.60 3.64

0.41 0.42 0.43

1.19 1.22 1.25

0.41 0.37 0.33

1.19 1.15 1.10

TTAB 60/0 TTAB 60/10 TTAB 60/20 TTAB 60/30 TTAB 60/40 TTAB 60/50 TTAB 60/60

4.74 4.62 4.61 4.65 4.66 4.61 4.64

0.46 0.46 0.46 0.46 0.46 0.47 0.47

1.68 1.64 1.64 1.66 1.67 1.65 1.66

0.46 0.42 0.37 0.33 0.28 0.24 0.19

1.68 1.56 1.47 1.40 1.30 1.17 1.06

a The samples are named according to the following system: in a sample XTAB x/y, the total concentration of amphiphiles, i.e., of A + XTAB, is x wt%, while A constitutes y wt% of the total amount of amphiphiles.

concentration of A suggests that whatever the actual structure of the assemblies, these are not well-defined enough to be forced into an ordered structure, even at very high volume fractions. Clusters of A probably begin to overlap at about 40 wt % of amphiphile, the concentration above which an increase in viscosity is observed. At the highest concentrations, the assemblies may merge into an essentially homogeneous solution, and it is possible that hydrophobic interactions and/or water-mediated hydrogen bonds between the monomers contribute to the increase in viscosity. Characterization of Liquid Crystalline Samples. Optical Microscopy. Anisotropic liquid crystalline phases (e.g., lamellar and hexagonal phases) are optically birefringent and show characteristic textures when placed between crossed polarizers and viewed in an optical microscope.6 Hence, this technique provides a convenient method for an initial characterization of the liquid crystalline phases in an aqueous surfactant system. A screening of the concentrated aqueous mixtures of A with DeTAB, DTAB, or TTAB shows that all liquid crystalline samples give birefringence patterns typical of hexagonal structures. Small-Angle X-ray Diffraction (SAXD). In order to obtain quantitative information on the liquid crystalline samples, the SAXD technique was employed. The diffractograms of all liquid crystalline samples indicate hexagonal structures, consistent with the optical microscopy results. For hexagonal structures, the relative positions of the diffraction peaks obey the relationship 1:x3:2:x7:3..., and the lattice constant a, i.e., the distance between the centers of adjacent cylinders, can be obtained from the position of the first diffraction peak, q*, via eq 7:

q* )

4π ax3

(7)

11442 Langmuir, Vol. 23, No. 23, 2007

Lundberg et al.

It can be seen in Table 2 that when an increasing amount of DTAB is exchanged for A, at a constant total amphiphile concentration, a shows a shallow minimum for both the DTAB 60/y and the DTAB 70/y series. For the DeTAB 60/y series, a increases with an increasing fraction of A, whereas it is essentially constant or decreases slightly for TTAB 60/y. Given the differences in the aqueous behavior of A as compared to that of a conventional surfactant, it is difficult to predict the degree of inclusion of A in the cylindrical aggregates that constitute the hexagonal phase, as well as its average location and conformation in these. In order to obtain some clues on the role of A in the hexagonal structures, one can consider the two hypothetical cases where (1) all of the present A is included in the cylinders and (2) all A resides in the continuous aqueous domains, i.e., that the cylinders are made up solely of DTAB. For the two cases, the volume fractions of apolar domains, Φap, are denoted ΦXTAB+A and ΦXTAB, respectively. The procedure for estimating the Φap values is given in the Experimental Procedures, and the obtained values are presented in Table 2. The hypothetical values of Φap can then, together with the experimental values of a, be related to their corresponding aggregate radii, R, via eq 8, which can be derived by geometric reasoning:

R)a

(

x3 Φ 2π ap

)

1/2

(8)

The calculated values of R are presented in Table 2. In samples containing DTAB as the only amphiphile, R is 1.45 and 1.48 nm, respectively, for amphiphile concentrations of 60 and 70 wt %. These values are, as expected, slightly smaller than the extended length of a C12H25 chain, which is 1.67 nm.7 When a fraction of DTAB is exchanged for A, and all of this is included in Φap, the change in R essentially reflects the change in a, since ΦDTAB+A is practically constant within each sample series. If A is excluded from Φap, on the other hand, there is, as expected, a considerable decrease in R with an increasing fraction of A. For instance, when 40% of the DTAB is exchanged for A, cylinder radii of 1.49 and 1.54 are obtained for 60 and 70 wt % total amphiphile concentration, respectively, if ΦDTAB+A is used, whereas the corresponding values with ΦDTAB are 1.14 and 1.18. It is highly unlikely that DTAB would arrange itself into cylinders with such small radii as the two latter, since this would, due to geometrical constraints, require substantial exposure of its hydrophobic tails to water. For this reason we conclude that, most likely, a significant fraction of the A participates in the cylinders that form the hexagonal liquid crystalline phase. The same conclusions hold for the DeTAB and TTAB systems. With the above presented calculations on hand it is fair to exclude the possibility that a major part of A is molecularly dissolved in the aqueous domains of the liquid crystals. This means that the differences in the variation in a with composition

among the sample sets mainly reflect a difference in the average conformations of A and the XTAB compounds when they participate in the cylinders. If a conventional surfactant of the same size as A was used instead of this, it should swell the cylinders in hexagonal phases of all the XTAB surfactants that are used in this study. As can be seen in Table 2, A do swell the cylinders in a hexagonal phase of DeTAB, but have a very limited effect on the cylinder radii for DTAB or TTAB, both of which have significantly shorter tails than A. This observation can be explained by A having, on average, a larger surface exposed to the polar surroundings per unit volume when present in the hexagonal phase than that expected for a conventional surfactant of similar size. Such a difference in exposed surface per volume can be rationalized by a larger degree of chain bending in A, most probably in order to allow for better contact between the ester groups and the solvent. This means that the findings from the SAXD study are in line with the above-discussed results on the mixed aggregates that form in isotropic solution and hence give further evidence for the strong influence on the aggregation behavior of A that is exerted by the driving force for ester bondwater interactions.

Conclusions The ester-containing amphiphile A does not exhibit typical surfactant behavior. When it is present as the only solute in a binary aqueous system, it forms neither conventional micelles nor liquid crystalline phases. However, there is clear evidence that A assembles with lower cooperativity into some kind of loose clusters at concentrations above 25-30 mM that cannot be characterized in greater molecular detail at this time. The apparent hydrodynamic radius of the assemblies, as determined from NMR diffusometry, increases with concentration up to the order of what would be expected had they been conventional spherical micelles, but due to the uncertainty in their structure and the indications of strong hydration, the apparent size may not directly reflect the physical size. At concentrations of A above approximately 1000 mM (40 wt %), there is a notable increase in the viscosity of the solutions. The increased viscosity might be explained by the formation of a network-like structure. When A is mixed with DTAB in solution, the two amphiphiles form mixed assemblies, the structure of which varies with the total amphiphile concentration. In concentrated mixtures with alkyltrimethylammonium surfactants, A participates in the formation of hexagonal liquid crystalline phases, even when it constitutes a significant fraction of the total amphiphile content. Acknowledgment. This work was financially supported by an NIH grant to F.M.M. (D.L., A.L.G., F.M.M.) and the Chalmers Foundation (J.U.). The authors are grateful to Dr. Shaoxiong Wu for technical assistance with the setup of the NMR spectrometer. Prof. Bjo¨rn Lindman is acknowledged for valuable discussions. LA700430U