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Langmuir 2008, 24, 4530-4536
Self-Assembly of Cationic Surfactants That Contain Thioether Groups in the Hydrophobic Tails Dan Lundberg,*,† Lei Shi, and Fredric M. Menger* Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed December 17, 2007. In Final Form: January 29, 2008 Self-assembly in aqueous solutions of cationic surfactants that carry thioether groups in their hydrophobic tails has been investigated. Of particular interest was the identification of possible changes in the aggregate structure due to the presence of sulfur atoms. Solutions of four different compounds [CH3CH2S(CH2)10N(CH3)3+Br- (2-10), CH3(CH2)5S(CH2)6N(CH3)3+Br- (6-6), CH3(CH2)7S(CH2)6N(CH3)3+Br- (8-6), and CH3(CH2)7S(CH2)8N(CH3)3+Br(8-8)] were characterized by 1H NMR, 13C NMR, NMR diffusometry, and conductivity measurements. In addition to investigating aqueous solutions containing each of the thioethers present as the sole solute, mixtures of 2-10 or 6-6 with dodecyltrimethylammonium bromide (DTAB) were studied. The addition of a sulfide group to the hydrophobic tail causes an increase in the critical micelle concentration but has a limited effect on the aggregate structure. Micelles are formed at a well-defined concentration for all of the investigated surfactants and surfactant mixtures. However, a comparison of the behavior of concentrated solutions of 8-8 to that of solutions of hexadecyltrimethylammonium bromide (CTAB) of similar concentrations suggests that the presence of a sulfur atom decreases the tendency for micellar growth. This may be a consequence of a slightly higher preference for the micellar surface of a sulfur atom as compared to that of a methylene group in a similar position, an idea that is also supported by results for the surfactant mixtures.
Introduction Surfactants and related amphiphilic substances commonly show a rich variety of self-assembling structures when dissolved in aqueous media.1,2 The main driving force for aggregation is the propensity to minimize the exposure of the hydrophobic parts to the surrounding solvent, whereas the morphology of the resulting aggregates is governed by an intricate balance between a number of factors: the relative sizes of the hydrophilic and hydrophobic moieties of the surfactant molecule, the degree of repulsive or attractive interactions between the hydrophilic headgroups, the extent of headgroup hydration, the presence of cosolutes, and various interaggregate interactions can all affect the character of the assemblies that are formed by a certain amphiphile. Single-tailed surfactants commonly assemble with high cooperativity into spherical micelles at the critical micelle concentration (cmc). Depending on the surfactant type, the spherical aggregates can grow into elongated micelles and/or arrange themselves into liquid-crystalline phases when the concentration is increased. The introduction of branching or the inclusion of aromatic groups or other rigid structures into the hydrophobic part of a surfactant (while keeping the total number of carbons constant) generally has the effect of increasing the cmc. This tendency can be attributed, at least in part, to steric constraints since the presence of bulky groups in the hydrophobic tails impedes effective packing in the micellar core.3 In a case where an amphiphilic * Corresponding authors. E-mail:
[email protected], menger@ emory.edu. † Present address: Department of Chemistry, University of Coimbra, 3004535 Coimbra, Portugal. (1) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester, England, 2002. (2) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (3) Patist, A. Determining Critical Micelle Concentration. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons: New York, 2001; Vol. 2, p 239.
substance carries a very rigid hydrophobic part, for instance, a polycyclic aromatic group, the compound may not show typical surfactant behavior. Another less common type of surfactant tail modification is the inclusion of non-hydrocarbon substituents along the hydrocarbon chain. It has been shown that, depending on the type and position of the inserted atom(s) or group(s), the consequences of such a modification can vary widely.4-9 If an insert is too polar, then it can inhibit micelle formation even if the amphiphile carries a tail of significant size. For instance, if a surfactant tail is modified with an ester or ether group, both of which can act as hydrogen bond acceptors and thus show significant interactions with water, then the compound must have an uninterrupted terminal hydrocarbon chain of at least 8-10 carbons to form conventional micelles.5,7 In a recent study, it was shown that an amphiphile carrying two ester groups along its tail [CH3CH2O(CdO)(CH2)6(CdO)O(CH2)8N+(CH3)3Br-] forms only loose clusters in aqueous solutions all the way up to a concentration of approximately 75 wt %, above which the solution coexists with a solid.9 If, however, a group (or groups) with a less pronounced interaction with water, such as a thioether (sulfide) group, is included in a surfactant tail, then this may have a much smaller influence on the behavior of the surfactant. In fact, it has been shown that a surfactant modified by the inclusion of one or two thioether groups along its tail retains typical surfactant behavior, regardless of where the groups are inserted.4,8 A methylene group and a sulfide group are, however, clearly not directly interchangeable. Generally, the addition of a methylene group to a (4) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: London, 1994. (5) Menger, F. M.; Galloway, A. L. J. Am. Chem. Soc. 2004, 126, 15883. (6) Menger, F. M.; Galloway, A. L.; Chlebowski, M. E. Langmuir 2005, 21, 9010. (7) Menger, F. M.; Chlebowski, M. E. Langmuir 2005, 21, 2689. (8) Menger, F. M.; Shi, L. J. Am. Chem. Soc. 2006, 128, 9338. (9) Lundberg, D.; Unga, J.; Galloway, A. L.; Menger, F. M. Langmuir 2007, 23, 11434.
10.1021/la7039465 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008
Self-Assembly of Cationic Surfactants Scheme 1. Molecular Structures of the Investigated Thioethers
micelle-forming, single-tailed surfactant decreases the cmc by roughly one-half.1 The insertion of a sulfide group, on the other hand, causes an increase in the cmc, the magnitude of which depends on the position of the sulfur.8 The aim of the present work was to investigate thioether surfactant micelles in more detail. Of particular interest was to identify possible changes in the aggregate structure due to the presence of the sulfide groups. To this end, we adopted a multitechnique approach in which the surfactant solutions were characterized by 1H NMR, 13C NMR, NMR diffusometry, and conductivity measurements. Four different thioether surfactants, depicted in Scheme 1, were studied. As can be seen in the Scheme, all compounds carry a straight-chain hydrophobic tail but vary in the total number of carbons and/or the position of the sulfide groups. In addition to investigating aqueous solutions containing each of the thioethers present as the sole solute, we examined solutions where 2-10 or 6-6 was admixed with the conventional cationic surfactant dodecyltrimethylammonium bromide (DTAB). Experimental Procedures Materials. The four thioether surfactants were synthesized and purified using previously described procedures.8 Dodecyltrimethylammonium bromide (DTAB, 99%) was purchased from Sigma, hexadecyltrimethylammonium bromide (CTAB, >99%) from Fluka, and deuterium oxide (99.9% D) from Cambridge Isotope Labs. All commercial substances 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. Samples were prepared by diluting and/or mixing stock solutions of the different surfactants prepared with either D2O or H2O. The dilution was made by volume, but the aliquots were also weighed for verification. NMR. All NMR experiments were performed at 25 °C (except 1H NMR experiments on concentrated solutions of 8-8 and CTAB, which were performed at 28 °C) on a Varian INOVA 600 spectrometer equipped with a pulsed field gradient (PFG) generator and a PFG amplifier. The samples were inserted into the probe at least 20 min prior to the experiments to allow thermal equilibration. Selected experiments were repeated for verification. One-dimensional experiments were performed at 599.7 and 150.8 MHz for the 1H and 13C experiments, respectively. The 13C NMR spectra were recorded in 1H-decoupled mode. It was assumed that the chemical shift of the 2H lock signal was independent of the amphiphile concentration (which has been shown to be a good approximation for other ionic amphiphiles in a similar concentration range).10 Because the frequency offset was kept constant, the changes in the 13C NMR chemical shifts with concentration could be calculated directly from the measured frequencies of the respective resonances. (10) O ¨ dberg, L.; Svens, B.; Danielsson, I. J. Colloid Interface Sci. 1972, 41, 298.
Langmuir, Vol. 24, No. 9, 2008 4531 The 13C NMR ppm scale was calibrated using an external sample of methanol in D2O; the shift of the methyl carbon peak was set equal to 50 ppm.11 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 diffusion time (∆) and the width of the gradient pulses (δ) were kept constant at 140 and 7 ms, respectively, whereas the strength of the pulsed gradient (G) was linearly incremented from 0.01 up to 0.6 T/m (with the maximum varying among experiments and samples) in 16 steps. Calibration of the gradient strength and verification of gradient amplifier linearity in the applied gradient strength interval were carried out by measurements on a trace amount of H2O in D2O (D ) 1.90 × 10-9 m2 s-1) and on poly(ethylene glycols) with known self-diffusion coefficients.12 The self-diffusion coefficients (D) of the amphiphiles were obtained from the attenuation of relevant echo peaks by linear leastsquares fits to the Stejskal-Tanner equation13 ln
()
( )
I δ ) -(γGδ)2D ∆ I0 3
(1)
where I is the measured signal intensity, I0 is the signal intensity in the absence of gradient pulses, and γ is the magnetogyric ratio of protons and the rest of the parameters are 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 electrode was immersed in stirred sample solutions at 25 °C until a stable reading was achieved.
Results and Discussion Solutions of Single Surfactants. In this section, results on aqueous solutions of 2-10, 6-6, 8-6, and 8-8 are presented, discussed, and compared to the behavior of DTAB or CTAB in water within the same concentration range. Viscosity and 1H NMR. All samples except solutions of 8-8 in concentrations of about 500 mM and above have low viscosity and show high-resolution peaks in the 1H NMR spectra. These features indicate that no or very limited micellar growth occurs in the samples. In concentrated solutions of 8-8, however, there is a notable increase in viscosity with increasing concentration. Solutions of large micelles exhibit behavior that is analogous to that of solutions of linear polymers. In fact, large micelles can in many senses be regarded as “living polymers” (since the “degree of polymerization” shows a strong dependence on the conditions), and concepts and theories developed for polymer solutions can successfully be applied when analyzing their behavior.1 At concentrations above the so-called overlap concentration, polymerlike micelles form a transient entangled network, and the zero-shear viscosity of the solution, η, is expected to depend on the micellar aggregation number, N, and the volume fraction of aggregates, Φ, according to1
η ) constant N 3Φ3.75
(2)
If the concentrated 8-8 samples are compared to solutions of CTAB, the micelles of which are known to show notable growth (11) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 75127515. (12) Håkansson, B.; Nyde´n, M.; So¨derman, O. Colloid Polym. Sci. 2000, 278, 399. (13) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288.
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Figure 1. Partial 1H NMR spectra of 16 wt % solutions of CTAB (top) and 8-8 (bottom) at 28 °C. The peaks at 0.9 ppm arise from the terminal methyl group, and the signals between 1.2 and 2.0 ppm arise from the majority of the tail methylene groups.
Figure 2. Observed self-diffusion coefficients, Dobs, of (a) 2-10 (b), 6-6 (O), and DTAB (×) and (b) 8-6 (2) and 8-8 (4) vs the inverse normalized surfactant concentration. The concentration, c, is normalized to the cmc of the respective surfactants.
into threadlike aggregates with increasing concentration,14 solutions of the former consistently have a significantly lower viscosity than those of the latter at comparable concentrations. Considering the similarities between 8-8 and CTABsthe compounds have the same type of headgroup and hydrophobic tails of similar sizesthis observation gives a strong indication (14) Ulmius, J.; Wennerstro¨m, H. J. Magn. Reson. 1977, 28, 309.
that, at a given concentration (at concentrations where aggregate growth occurs), the micelles of 8-8 are smaller than those of CTAB. This notion is also supported by a comparison of the 1H NMR spectra of the two compounds at different concentrations. The peaks in the 1H NMR spectrum of a surfactant that reside in large micelles are broad and show a characteristic band shape with a broad base and a narrow apex. This appearance of the peaks is due to the presence of slow-motion components.14,15 Figure 1 shows the spectra of 8-8 and CTAB, both at a concentration of 16 wt % (i.e., approximately 440 and 480 mM 8-8 and CTAB, respectively). It is clear that although the observed peak broadening for 8-8 is significant (the peaks are tens of Hz in half-height width) it is much smaller than for CTAB. This observation is consistent with a smaller micelle size for 8-8. Taken together, the results from 1H NMR and the differences in the viscosity of solutions of 8-8 or CTAB clearly indicate that the presence of a sulfide group in the hydrocarbon tail of a surfactant decreases the tendency for micellar growth. In other words, the inclusion of the sulfur renders the surfactant a smaller effective packing parameter and the surfactant film a higher spontaneous curvature.1 From a molecular point of view, this observation may be rationalized by the sulfide group having a slightly higher propensity to reside close to the aggregate surface as compared to a methylene group in the same position. This would give a surfactant containing a sulfide group a slightly larger area per molecule that is exposed to the aqueous surroundings as well as a decreased effective hydrophobe volume and hence a somewhat lower spontaneous packing parameter as compared to that of an all-methylene counterpart.2 NMR Diffusometry. The translational mobility of a surfactant in solution depends on the formation of micelles and other aggregates, and self-assembly is manifested by a decrease in the self-diffusion coefficient of the surfactant as the effective size of the diffusing entities increases. Hence, by determining the concentration dependence of a surfactant’s self-diffusion coefficient, it is possible to estimate the size of its micelles as well as to probe the process by which these are formed. In this work, the diffusion coefficients were measured using the pulsed gradient spin-echo (PGSE) NMR technique, a well-established method for studying surfactant self-assembly.16,17 (15) Olsson, U.; So¨derman, O.; Guering, P. J. Phys. Chem. 1986, 90, 5223. (16) So¨derman, O.; Stilbs, P.; Price, W. S. Concepts Magn. Reson. 2004, 23A, 121. (17) Furo, I. J. Mol. Liq. 2005, 117, 117.
Self-Assembly of Cationic Surfactants
Langmuir, Vol. 24, No. 9, 2008 4533
Table 1. Data on the Studied Surfactants and Surfactant Mixtures
cmc (mM) from NMR diff cmc (mM) from 13C-NMR cmc (mM) from conductivity degree of ionization, R cmc (mM) from STb
2-10
6-6
8-6
8-8
DTAB
39 42 46 0.35 16
30 31 33 0.40 28
6.8 7.1 8.2 0.28 8.0
2.2 2.0 2.8 0.28 2.6
16 15 16 0.30
2-10+ DTABa
6-6+ DTABa
18 18
17 18
a Equimolar mixtures. b ST ) surface tension. The values were obtained by reinterpreting the raw data from measurements made for a previous publication.8
Generally, the exchange between surfactant monomers and surfactant molecules that reside in aggregates occurs at a rate much faster than the time scale of the NMR diffusion experiment. Hence, the observed self-diffusion coefficient, Dobs, is a population-weighted average of the self-diffusion coefficients at the different sites where the surfactant resides. Under the assumption that the micellization can be described by the phase-separation model1 and that the micelles can be approximated as discrete monodisperse aggregates, Dobs can be expressed as follows
Dobs )
cmono cmic + D D c mono c mic
(3)
where Dmono and Dmic are the diffusion coefficients for the surfactant monomers and the micelles, respectively, cmono is the concentration of surfactant monomers, cmic is the concentration of surfactant molecules that reside in the aggregates, and c is the total surfactant concentration. It follows from eq 3 that a plot of the experimental values of Dobs versus c-1 should give two straight lines that intersect at the cmcsa horizontal line with Dobs ) Dmono for concentrations up to the cmc and a line with a slope of approximately (Dmono - Dagg)cmc for higher concentrations. For a conventional micelle-forming surfactant, this is generally a good approximation of reality. Figure 2 presents the results from diffusion measurements on the four studied thioether surfactants, along with the corresponding data for DTAB. One can see that all surfactants indeed give two sharply intersecting straight lines. To simplify the comparison between the results for the different surfactants, the concentrations are normalized to the cmc values of the respective surfactants (i.e., the x axis shows (c/cmc)-1 rather than c-1). The cmc values, which were obtained from the intersection of the horizontal and sloping lines in plots of Dobs versus c-1, are presented in Table 1. With the exception of the value for 2-10,18 these diffusometrybased cmc values correspond well with those obtained from surface tension measurements. Another important observation that can be made in Figure 2a is the fact that the variation of Dobs with (c/cmc)-1 for 2-10 and 6-6 practically overlaps with that for DTAB. The radius of a spherical micelle, which is inversely proportional to Dmic, is forced by geometrical constraints to be approximately the length of one extended surfactant molecule, lmax, for an effectively packed micelle.2 Thus, because the lmax values of 2-10 and 6-6 are similar to that of DTAB, which is known to form spherical micelles up to a concentration of about 450 mM,19 the overlapping data give direct support that these two thioether surfactants form essentially normal, spherical micelles with a degree of cooperativity similar to that in the assembly of a conventional analog. (18) We cannot easily explain the discrepancy in the cmc values for 2-10 obtained from the surface tension measurements and the other used techniques. However, since the NMR techniques and the conductivity measurements all investigate the bulk and hence are more direct than a surface tension study and give results that are in reasonable agreement with one another, we believe that the values obtained from these methods are closer to the true cmc. (19) Minardi, R. M.; Schulz, P. C.; Vuano, B. Colloids Surf., A 2002, 197, 167.
Before continuing on to the next section, it is valuable to comment on the difference in cmc between the thioether surfactants and their conventional counterparts. If one compares the cmc values for 2-10, 6-6, and DTAB (all with 12 carbons in their hydrophobic parts), then one can see that the insertion of a sulfide group in the chain raises the cmc by 2- to 3-fold. A decreased propensity for micellization (as manifested by an increased cmc) might reflect a perturbation on either side of the monomer-micelle equilibrium (or both). On the monomer side, an increased cmc may be explained by attractive interactions between the thioether groups and water. A sulfide group can act as a weak hydrogen bond acceptor (for instance, the hydrogen basicity of diethyl sulfide is 0.32 whereas it is 0 for butane20) and hence can be expected to contribute to increased monomer solubility. It is also conceivable that the inserted group causes a change in the water structure around the monomer into a less ordered one and hence decreases the entropic gain on micellization. On the micelle side, an elevated cmc can be rationalized by steric effects. Packing constraints can arise from either the mere bulkiness of the inserted group or from alterations in the average tail conformation due to interactions between the insert and the solvent. In the case of the thioether surfactants, it is likely that their higher cmc values, as compared to the values for conventional surfactants carrying the same number of carbons, are caused by a combination of the effects discussed above. 13C NMR. The chemical shifts in NMR spectra of surfactants in aqueous solution commonly show pronounced concentration dependencies. This phenomenon can be explained by differences between monomers and micellized surfactant in both the direct effects of the environment (medium effects) and in the average conformation of the molecules (conformation effects). Whereas medium and conformation effects generally have comparable influence on 1H NMR shifts, it has been shown that changes in 13C NMR shifts for carbon atoms along an alkyl chain show only a very weak dependence on medium effects and to good approximation can be ascribed exclusively to changes in the average chain conformation.21-23 Thus, a downfield 13C NMR shift can be related to an increase in the average ratio of trans to gauche conformations in the chains. Because a rather small change in molecular conformation is accompanied by significant changes in chemical shifts, a concentration-dependence study of the 13C NMR shifts of a surfactant is valuable for revealing structural changes in its aggregates with concentration. If only one peak for each nonequivalent carbon in a molecule appears at all concentrations, as is the case for all of the herein (20) Abraham, M. H.; Andonian-Haftvan, J.; Whiting, G. S.; Leo, A.; Taft, R. S. J. Chem. Soc., Perkin Trans. 2 1994, 1777. (21) Batchelor, J. G.; Prestegard, J. H.; Cushley, R. J.; Lipsky, S. R. Biochem. Biophys. Res. Commun. 1972, 48, 70. (22) Persson, B. O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (23) Stothers, J. B. Carbon-13 NMR Spectroscopy; Academic Press: New York, 1972.
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Figure 3. Maximum observed change in the chemical shifts, ∆δ ) δobs - δmono, for peaks from methylene carbons in the 13C NMR spectra of 2-10 (b), 6-6 (O), 8-6 (2), 8-8 (4), and DTAB (×) with the inverse normalized surfactant concentration. The concentration, c, is normalized to the critical micelle concentration, cmc, of the respective surfactants. The shifts of the respective peaks in samples with concentrations below the cmc, δmono, are 29.5 ppm for 2-10, 29.6 ppm for 6-6, 29.1 ppm for 8-6, 29.0 ppm for 8-8, and 29.8 ppm for DTAB.
discussed compounds,24 then the observed chemical shift, δobs, for an atom at a given position in the studied molecule is, in a similar way to Dobs, a population-weighted average of the values at the different sites where the molecule can reside. Accordingly, one expects that δobs can be described by an expression of the same form as eq 3, given as eq 4
δobs )
cmono cmic δmono + δ c c mic
(4)
where δmono and δmic are the average chemical shifts of surfactant molecules present as monomers or residing in micelles, respectively, and cmono, cmic, and c have the same meanings as above. Since a majority of the peaks in the 13C NMR spectra of the studied compounds show quite similar chemical shifts (which is also true for those from the carbons next to the sulfide groups), it is difficult to assign all carbons along the hydrophobic tails accurately. However, as is also the case for surfactants with a normal hydrocarbon tail,25 signals from most of the midchain methylene groups in the thioether surfactants show similar changes in shift with concentration; these are all larger than the shift changes for signals from methylene groups close to either the terminal methyl groups or the head groups.26 Figure 3 shows the change in chemical shift, ∆δ, where ∆δ ) δobs - δmono, for the methylene signal that gives the largest shift change with increasing concentration (which, as mentioned above, is representative of the change in shift of a majority of the methylene peaks from each surfactant). Plots of ∆δ versus (c/cmc)-1 are similar for all of the studied surfactants. Most importantly, the plots for 2-10 and 6-6 closely resemble that for DTAB. These findings suggest that the inclusion of a sulfide group into the tail of a surfactant does not cause any major changes in the average conformation of the molecule when it resides in spherical single-surfactant micelles at low concentration. (24) Although not all methylene carbons could be unambiguously assigned, the separation in shift of most of the peaks is larger than the change in shift between the different concentrations. (25) Drakenberg, T.; Lindman, B. J. Colloid Interface Sci. 1973, 44, 184. (26) 13C-NMR data for 2-10, 6-6, and DTAB are provided in Supporting Information.
Figure 4. Specific conductivity as a function of the concentration of 8-6.
The cmc values that are obtained from the data underlying Figure 3 are shown in Table 1. As can be seen in the Table, these cmc values correspond well to those obtained from the NMR diffusometry experiments. ConductiVity Measurements. By determining the concentration dependence of the specific conductivity, κ, of aqueous solutions of an ionic surfactant, it is possible to obtain both the cmc and the degree of counterion dissociation, R, for its micelles. The cmc is generally revealed by the intersection of two essentially straight lines, whereas an estimate of R can be obtained by calculating the ratio of the slopes above and below the cmc. Plots of κ versus c for the different thioether surfactants all show a conventional appearance, as exemplified by the data for 8-6 in Figure 4. The obtained cmc and R values for all compounds are shown in Table 1. As can be seen in the Table, no dramatic difference in R among the five surfactants was observed. Mixtures of 2-10 or 6-6 with DTAB. As discussed above, there are indications that a sulfide group included in a surfactant tail may have a higher propensity to reside at the micellar surface as compared to a methylene group. However, the conformational perturbations, and hence the packing constraints, induced by the surfur-solvent interactions are not large enough to impair the cooperativity of the self-assembly significantly or to cause a notable deviation from a normal micellar structure (in contrast to the case with the ester-modified amphiphile discussed in the Introduction). If, however, a fraction of a thioether surfactant is added to an existing micelle of a conventional surfactant, one can expect a larger freedom to accommodate a higher degree of sulfur-solvent interactions with less disturbance of the overall aggregate structure. An increase in the contact between sulfide groups and water can be expected to require a larger degree of bending of the thioether surfactant tails and, hence, a higher ratio of gauche to trans conformations along the chain. To investigate the behavior of the thioether surfactants in mixtures with a conventional surfactant, in particular, the possible occurrence of conformational changes in the surfactant chains with changing micelle composition, equimolar mixtures of 2-10 or 6-6 with DTAB at different total surfactant concentrations were studied by visual inspection, 1H NMR, 13C NMR, and NMR diffusometry. Viscosity and 1H NMR. Judging from the visual inspection, there is no appreciable increase in viscosity with increasing total surfactant concentration for either of the two studied systems. In agreement with the appearance of the samples, 1H NMR spectra of the samples show only narrow peaks. NMR Diffusometry. Figure 5 shows the results from a diffusion study on equimolar aqueous mixtures of 2-10 and DTAB at
Self-Assembly of Cationic Surfactants
Figure 5. Observed self-diffusion coefficients, Dobs, of 2-10 (b) and DTAB (O) in equimolar aqueous mixtures of the two vs the inverse normalized total surfactant concentration. The total concentration, c, is normalized to the critical micelle concentration, cmc, of the surfactant mixture.
different total surfactant concentrations. The fact that Dobs for both surfactants departs from Dmono at the same total concentration suggests that the two surfactants form true mixed micelles. Furthermore, the observed difference in Dobs for the compounds at concentrations above the cmc (i.e., at (c/cmc)-1 < 1) can be explained by the difference in the cmc values of the two surfactants (about 42 and 16 mM for 2-10 and DTAB, respectively). Because of this difference, the fraction of DTAB in the first formed micelles (with increasing concentration) is higher than the global fraction but approaches the bulk composition at high total surfactant concentration (i.e., with decreasing (c/cmc)-1). Consequently, Dobs values for the two surfactants approach each other at higher concentration. The effect of the cmc difference and the fact that the cmc of the mixture is similar to that of the surfactant with the lower cmc (i.e., DTAB (Table 1)) are in line with what is expected for the formation of ideal mixed micelles.1 A corresponding study on a mixture of 6-6 and DTAB gives essentially the same result, except that the difference in Dobs for the components at concentrations above the cmc is somewhat smaller. Since the difference in cmc for the pure 6-6 and DTAB is smaller than the difference in cmc for 2-10 and DTAB, this result is in line with expectations. 13C NMR. Figure 6 presents the change in the chemical shift of methylene peaks from 2-10 and DTAB with (c/cmc)-1 when they are present in equimolar mixtures of the two. The signals underlying Figure 6 were selected mainly because they were clearly distinguishable, and too much attention should not be directed to the absolute values of ∆δ; what is most important in this context is the functional form of ∆δ versus (c/cmc)-1. A corresponding plot of the 13C NMR data on a mixture of 6-6 and DTAB has a very similar appearance. It follows from eq 4 and the above discussion on 13C NMR shifts that, in a situation where the average conformation of the hydrophobic tails in micelles does not change with the total surfactant concentration, ∆δ for component i in a surfactant mixture should show a roughly linear dependence on the fraction p of i that resides in micelles (i.e., p ) cmic,i/ci, where cmic,i is the concentration of micellized i and ci is the total concentration of i). If one assumes that the micelle size is constant with changing total surfactant concentration, then p can be estimated from eq 3 and the experimental values of Dmono and Dmic for each component (in which the latter is approximated by extrapolating Dobs to c-1 ) 0). Because ∆δ is sensitive to small changes in the average chain conformation and it is unlikely that a mixture
Langmuir, Vol. 24, No. 9, 2008 4535
Figure 6. Observed change in the chemical shifts, ∆δ ) δobs δmono, for peaks from methylene carbons in the 13C NMR spectra of 2-10 (b) and DTAB (O) in equimolar aqueous mixtures of the two with the inverse normalized total surfactant concentration. The total surfactant concentration, c, is normalized to the critical micelle concentration, cmc, of the surfactant mixture. The shifts of the respective peaks in samples with concentrations below the cmc, δmono, are 26.5 ppm for 2-10 and 26.6 ppm for DTAB.
Figure 7. Plot of ∆δ vs the fraction of micellized surfactant, p, for 2-10 (b) and DTAB (O) in equimolar aqueous mixtures of the two substances. The values of p are calculated from the data presented in Figure 5, and the ∆δ data are the same as those presented in Figure 6 (i.e., δmono is 26.5 ppm for 2-10 and 26.6 ppm for DTAB).
of two cationic surfactants that both give spherical micelles when present as the lone solutes would form elongated micelles, a plot of ∆δ versus p, as calculated from the diffusion data, can be expected to reveal possible bending of the thioether surfactant tails in the mixed micelles. It can be noted that the difference in the cmc values of the thioethers and DTAB makes it possible to monitor changes in tail conformation with a change in micellar composition. As discussed above, the difference in cmc values causes a gradual increase in the micellar fraction of the thioethers from a low value up to the bulk compositions (i.e., equimolarity) as the total surfactant concentration is increased. Figure 7 shows a plot of ∆δ versus p for the respective surfactant components of equimolar mixtures of 2-10 and DTAB. One can see that for DTAB ∆δ does indeed show a roughly linear dependence of p, whereas for 2-10 it shows a slight curvature. This curvature indicates that, on average, there is a more gradual changeover from gauche to trans conformation in the chain with an increasing p value for the thioether surfactant. This observation is consistent with the chains of 2-10 being more bent at low p and therefore does support the idea that a sulfur atom has a slightly higher preference for the micellar
4536 Langmuir, Vol. 24, No. 9, 2008
surface as compared to a methylene group. (Note, as is discussed above, that the absolute values of ∆δ should not be directly compared.) Again, corresponding results are obtained for the mixtures of 6-6 and DTAB.
Conclusions Self-assembly in aqueous solutions of cationic surfactants that carry thioether groups in their hydrophobic tails has been investigated. The addition of a sulfur atom to the hydrophobic tail causes an increase in the critical micelle concentration but has a rather limited effect on the aggregate structure, and micelles are formed at a well-defined concentration for all of the investigated surfactants. This is true for both the single-surfactant systems and the mixtures of thioether surfactants with DTAB. However, the presence of sulfide groups in a surfactant tail causes a decreased tendency for micellar growth with increasing
Lundberg et al.
concentration. This observation can probably be explained by attraction between the sulfur and the solvent, which may lead to a slightly higher probability of finding a sulfide group close to the micellar surface as compared to that for a methylene group in a similar position and consequently gives the thioether a slightly lower effective packing parameter. This idea also finds support from the results on the surfactant mixtures. Acknowledgment. This work was financially supported by an NIH grant to F.M.M. We are grateful to Dr. Shaoxiong Wu for technical assistance with the setup of the NMR spectrometer. Supporting Information Available:
13
C NMR data for 2-10, 6-6, and DTAB are provided as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. LA7039465
