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The micellar structure of the cationic surfactants benzethonium chloride (BZCI) and bromide (BZBr) was analyzed by small-angle X-ray scattering and light scattering techniques. The shape, size and characteristics of the internal structure of BZCl was studied by the contrast-variation method at 20°C (CMC = 1.78 mM) by changing the electron density of the solvent by addition of sucrose or glycerol. The morphological parameters, i.e. average electron density ρ 1 , radius of gyration of micellar shape, R v , apparent molecular weight, aggregation number (52) and degree of hydration (0.24 g H 2 O g -1 mic ) were determined using scattering data in the range 0 < h < 0.08 A -1 . The micellar shape of BzCI was computed from R v ; the maximum dimension of the micelle in solution was obtained from the distance distribution function (D max = 56.0 A), and is consistent with an oblate ellipsoid of revolution (a = 27.8 A, b = 25.1 A). A three-shell structure ellipsoid of revolution model was found to be an appropriate model, consisting of a polar group layer (L pol = 9.65), a layer consisting partly of the ether residues (L' = 9.20) and the hydrocarbon core (R HC = 17.4 A). BZBr revealed a much lower aggregation number and different hydrodynamic properties, i.e. R g =13.5 A, D max = 28.4 A, apparent molecular weight (MW app ) = 16 000, apparent aggregation number (N agg ) = 36, CMC = 0.17 mM. The BZBr micelles show a very narrow size distribution and are almost spherical in shape when analyzed in terms of a structural system (Z str = 30), but by decreasing the charge of the structural system to the effective charge (Z eff = 19.5), the S pp (h, ρ 0 ) factor revealed no differences between the two systems. The polydispersity in micellar size for both BZCI and BZBr was found to be small (8%) at constant chemical potential. The CMC and N app are independent of the density of the solvent when using glycerol or sucrose as additives in the presence of 0.01 M NaCl for BZCl (20°C).

Zwitterionic N,N'-butyl-n-alkyl-1,4-diazabicyclo-[2,2,2]-octane derivatives of 3'-O-acylthymidine-5'-phosphates (DNO-Cl 2 ) are known to inhibit the growth of resistant neoplasm, and are superior to antineoplastic purine and pyrimidine bases, and nucleoside analogs. We studied both the intensity and the autocorrelation function of light scattered by these zwitterionic micelles for n = 4, 5 and 6. The biologically active surfactants have a high critical micellar concentration (CMC) (0.20-0.25 M), and the aggregates have low aggregation numbers which depend on pH, and on salt and temperature to a lesser degree. The dynamic behavior of the monomers of these zwitterionic surfactants is strongly dependent on surfactant concentration. The dynamics of the DNO-Cl 2 solutions are influenced by the monomer diffusion owing to the high CMC. Close to the CMC, premicellization exists at least for DNO-Cl 2 , n = 6, and no significant effect on the dynamic properties of the solution is observed, although a sharp crossover on the scattered intensity is seen. From the scattered intensity measured above the CMC, and at large volume fractions, the aggregation numbers (N agg ) including sizes of the micellar aggregates were determined. The sizes remain constant with R H = 12.8 A (n = 4), and vary between 15 A (n = 5) and 20 A (n = 6). Above the CMC classical hard-sphere behavior is observed from the volume fraction dependence. DNO-Cl 2 micelles with n = 6 have a strong attractive potential and must be considered for analyzing the experimental results. The effective diffusion coefficient decreases from the CMC, passes through a minimum and then varies linearly with increasing micelle concentration. This system of DNO-Cl 2 (n = 4-6) reveals a heterogeneous solution : pure electrolyte behavior by the monomers, interactions between monomers and counterions, and micelles, showing hard-sphere behavior. The experimental data for DNO-Cl 2 micelles are consistent with a model of chemical exchange between monomers and micelles, and for a mixture of monomers and micelles.