Light scattering and small-angle X-ray scattering on micellar benzethonium chloride (bromide) solutions III: Micellar structure of BZCl at different solvent electron densities

摘要

The micellar structure of the cationic surfactants benzethonium chloride (BZCl) 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℃ (CMC = 1.78 mM) by changing the electron density of the solvent by addition of sucrose or glycerol. The morphological parameters, i.e. average election 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_2O g_(mic)~(-1)) were determined using scattering data in the range 0 < h < 0.08 A~(-1). The micellar shape of BzCl 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))= 16000, apparent aggregation number (N_(agg)) = 36, CMC = (). 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 BZCl 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℃).