Langmuir 1988,4, 1363-1367
1363
Control of Aggregate Structure with Mixed Counterions in an Ionic Double-Chained Surfactant D. D. Miller, J. R. Bellare, T. Kaneko, and D. F. Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received May 19, 1988. I n Final Form: July 25, 1988 The microstructures in dilute aqueous solutions of the ditetradecyldimethylammonium surfactant cation with mixtures of bromide and acetate counterion were studied by using video-enhanced microscopy, video-enhanced microelectrophoresis,cryotransmission electron microscopy, and time-resolved fluorescence quenching. A gradual transformation from multilamellar liposomes to, first, unilamellar vesicles and microtubules and, second, small spherical micelles was seen when the bromide-to-acetate ratio was reduced from large values to zero. Fluorescence quenchmg reveals that micells and larger aggregates (i.e., microtubules and vesicles) coexist at intermediate bromide-to-acetate ratios. Electron micrographs of these systems at low bromide-to-acetate ratios reveal the presence of single-component spherical micelles and confirm the ability to image micelles. The results can be explained in terms of increased head-group and interaggregate repulsions as the bromide ion is replaced by the highly hydrated acetate ion. Introduction Biological and industrial applications of surfactant microstructures depend on the creation of aggregates of specified size and shape. Hence, the ability to design, control, and tune microstructure is an area of active research. Many theoretical have confirmed that interactions between the surfactant head groups play a vital role in defining aggregate size and shape. Thus, by employing strategies designed to change the nature of head-group interactions, we can expect to change aggregate architecture in ways that are entirely predictable from theory. For example, Sackmann, Duwe, and Engelhardt4 have demonstrated that by cross-linking the head groups of a polymerizable surfactant that has been incorporated into the outer leaf of a giant unilamellar vesicle, the area per head group in the outer portion of the vesicle is decreased, and, as a result, a small vesicle is invaginated into the interior of the large vesicle. This phenomenon is predicted by the cell shape theory of Svetina and Z e k ~ . ~ In this paper we present another strategy for controlling head-group interactions. It has been shown that the electrostatic interactions between the head groups of ionic surfactants are significantly affected by the counterion. For example, replacement of the bromide counterion with hydroxide or acetate in the (single-chained) surfactant hexadecyltrimethylammonium bromide results in a substantial decrease in micelle The reason for such behavior is that the highly hydrated hydroxide or acetate counterions must sit further out from the aggregate surface than the bromide ions; thus they are not as effective in screening head-group repulsions. The result is larger areas per head group, increased curvature, and decreased aggregate s i ~ e . ~With ~ ~ double-chained ~' surfactants, the result of this counterion substitution is the transformation of large vesicles and liposomal aggregates to small unilamellar vesicles and (1)Tanford, C.The Hydrophobic Effect, 2nd ed.; Wiley: New York, 1980. (2) Mitchell, D. J.; Ninham, B. W. J. Chem. SOC.,Faraday Trans. 2, 1981,77,601. (3)Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1987,91, 338. (4) Sackman, E.; Duwe, H.-P.; Engelhardt, H. Faraday Discuss. Chem. SOC. 1986,81,281. (5)Svetina, S.; Zeks,B. Biomed. Biochem. Acta 1983,42,86. (6)Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986,90,1853. (7) Miller, D. D.; Evans, D. F.; Magid, L. J. J.Phys. Chem., submitted.
0743-7463/88/2404-1363$01.50/0
By employing mixtures of counterions, we can form aggregates that span a range of head-group areas and curvatures. For example, with the hexadecyltrimethylammonium surfactant cation, mixtures of bromide and acetate counterions produce micelles whose size shows a remarkably linear dependence on the ratio of bromideto-acetate (see Figure 1). Since double-chained surfactants are capable of expressing a wider range of aggregate types? we have applied the same counterion-mixing strategy to the ditetradecyldimethylammonium cation in an attempt to form these structural types in a controlled manner. We report on the aggregate structure in these mixtures below. Experimental Section Materials. Ditetradecyldimethylammonium bromide (2Cl4N2C1Br,Sag6 Pharmaceuticals, Japan) was converted to the corresponding acetate (2C14N2C10Ac)by ion exchange and purified according to Brady et aL6 Video-Enhanced Microscopy (VEM). Surfactant microstructures in mixtures of 2C14N2C1Brand 2CI4N2C10Acwere directly visualized by using VEM. This technique has been described in detail elsewhere.g10 The ability to visualize small, nearly transparent phase objects (such as unilamellar vesicles and small liposomes) allows VEM to be used in some unique microelectrophoreticapplications. By use of the microelectrophoresisflow cell illustrated in Figure 2, the mobilities and { potentials of the aggregates formed by 2C14N2C1bromide and acetate mxtures were measured. The following procedure was employed (1)Egg lecithin (0.5 mM) in 7.5 mM NaCl was injected into the cell, and a dc voltage of 39-45 V was applied. The vertical level within the cell where the velocity of the neutral lecithin particles was zero was established as the stationary level. The video microscope was kept in focus at this level for all subsequent measurements. (2) The lecithin solution was rinsed out, and the solution of interest was injected into the cell, all without removing the cell from the microscope stage or disturbing the focus. (3) The true velocity of the structures in solution was measured at the stationary level. The velocity of at least 40 particles was measured in both directions (by applying forward and reversed polarity). The mobilities, u (m2 s-l), of the particles were calculated from the Helmholtz-Smoluchowski equation u =
v/E
(1)
(8)Miller, D.D.; Bellare, J. R.; Evans, D. F.; Talmon, Y.; Ninham, B. W. J. Phys. Chem. 1987,91,674. (9)Inou6, S. Video Microscopy; Plenum: New York, 1986. (10)Miller, D. D.; Benton, W. J.; Evans, D. F.; Machuga, S. C. AIChE J.,submitted.
0 1988 American Chemical Society
1364 Langmuir, Vol. 4 , No. 6, 1988
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Figure 1. Aggregation number of hexadecyltrimethylammonium micelles with mixed (Br- and OAc-) counterions. Reprinted from ref 7 [TOP plate)
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F i g u r e 3. Plots of A4/A4m vs 7 for computer-generated fluorescence decay data simuyating mixtures of micelles and vesicles. F, represents the fraction of surfactant in micellar form. Data for 100% hexadecyltrimethylammonium acetate micelles are given for comparison. Note the increasing dependence of A4 on quencher concentration with vesicle population. Reprinted from ref 15. aggregation number and 7 is the scaled quencher concentration [Q]/([surfl - cmc). Here [Q] is the actual quencher concentration, [surf] is the surfactant concentration, and cmc is the critical micelle concentration. With only micellar aggregates, A4 is independent of quencher concentration. When the system contains both micelles and vesicles, the data are forced to fit eq 3. In this case, the value of A4 is dependent of quencher concentration. The strength of the dependence of A4 on 7 gives some indication of the ratio of surfactant in large aggregates versus that in micelles (see Figure 3). Data from dispersions containing only large aggregates appear monoexponential and are fitted to eq 4:
\
i
'I
I ( t ) = Z(0) exp(-Azt
55"
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Figure 2. Microelectrophoretic flow cell. The cell maintains the 0.17-mm coverslip-on-top and 0.9-1.4-mm microscope slide-onbottom arrangement, which is essential for optimum resolution. where u is the true velocity (m/s) and E is the electric field in V/m. The { potentials were determined from the Huckel and Helmholtz-Smoluchowski equation = 3~u/2c (2)
Time-Resolved Fluorescence Quenching (TRFQ). The use of TRFQ to measure micelle aggregation numbers is well estab1 i ~ h e d . lA ~ new extension of this technique, described in detail elsewhere>15 allows dispersions with mixtures of micelles and large aggregates (such as vesicles and liposomes) to be studied. Fluorescence decay data from solutions containing micelles appear biexponential14J5 and are fitted to eq 3: Z ( t ) = Z(0) exp[-A2t - A3(l - e-A4t)] (3) where I ( t ) is the fluorescence intensity emitted at time t. When the system contains only micelles, Az represents the unquenched decay constant of the fluorophore, A4 represents the decay constant due to quenching, and A3 is equal to +, where i is the micelle (11) Adrian, M.; Dubochet, J.; Lepault, J.; McDowell, A. W. Nature (London) _. 1984. ,308. 32. -(12) Talmon, Y. Colloids Surf. 1986, 19, 237. I_.
-I
- - - - I
(4)
Here, k , and kD are quencher decay constants, and [Q,] is the quencher concentration based on the total aggregate volume, not the bulk solution v01ume.I~ The diffusion coefficient of the quencher molecules in the vesicles and liposomes is determined by using eq 5:15
r
where p is the viscosity in kg mw1s-l and t is the permitivity in C2 J-1 ,-I Cry0 Transmission Electron Microscopy (Cryo-TEM). High-resolution, direct images of the microstructure in 2C14N2C1 (Br/OAc) were obtained by using cryo-TEM. Cryofixation of colloidal dispersions with the controlled environment vitirification system allows microstructural detail of surfactant aggregates as small as 4 nm to be clearly seen. Thus bilayers as well as micelles can be visualized with this technique.21 Details are discussed
- k,[Q,]t - k~[Q,]t'/')
(5) where L'is 6.02 X lp,k, is in cm3mol-' s-l, and kD is in cm3m o r s-112.
Fluorescence decay data were obtained by using the single photon counting technique.16 Pyrene (fluorophore) and dibutylaniline (quencher) were solubilized in mixtures of 2C14N2C1Brand 2C14N2C10Acas described e l ~ e w h e r e . 7All ~~~ experiments were performed a t 37 "C. Further experimental details can be found el~ewhere.~?'~
Results Visual Observation. At room temperature, crystals of 2Cl,N2C1Br did not dissolve in water (the chain-melting transition temperature of 2CI4N2ClBr, T,,is 31 OC1.l' At slightly elevated temperatures (31-35 "C),aqueous dispersions of 2C14N2C1Br were turbid even at very low concentrations (
