LIGHT-SCATTERING AND SMALL-ANGLE X-RAY-SCATTERING ON MICELLAR BENZETHONIUM CHLORIDE (BROMIDE) SOLUTIONS .3. MICELLAR STRUCTURE OF BZCL AT DIFFERENT SOLVENT ELECTRON-DENSITIES

The micellar structure of the cationic surfactants benzethonium chlori de (BZCl) and bromide (BZBr) was analyzed by small-angle X-ray scatter ing and light scattering techniques. The shape, size and characteristi cs of the internal structure of BZCl was studied by the contrast-varia tion method at 20 degrees C (CMC = 1.78 mM) by changing the electron d ensity of the solvent by addition of sucrose or glycerol. The morpholo gical parameters, i.e. average electron density rho(1), radius of gyra tion of micellar shape, R(v), apparent molecular weight, aggregation n umber (52) and degree of hydration (0.24 g H2O g(mic)(-1)) were determ ined using scattering data in the range 0 < h < 0.08 Angstrom(-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 Angstrom), and is consistent with an oblate el lipsoid of revolution (a = 27.8 Angstrom, b = 25.1 Angstrom). A three- shell structure ellipsoid of revolution model was found to be an appro priate model, consisting of a polar group layer L(pol) = 9.65), a laye r consisting partly of the ether residues (L' = 9.20) and the hydrocar bon core (R(HC) = 17.4 Angstrom). BZBr revealed a much lower aggregati on number and different hydrodynamic properties, i.e. R(g) = 13.5 Angs trom, D-max = 28.4 Angstrom, apparent molecular weight (MW(app)) = 160 00, 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 effectiv e charge (Z(eff) = 19.5), the S-pp(h, rho(0)) factor revealed no diffe rences between the two systems. The polydispersity in micellar size fo r both BZCl and BZBr was found to be small (approximate to 8%) at cons tant chemical potential. The CMC and N-app are independent of the dens ity of the solvent when using glycerol or sucrose as additives in the presence of 0.01 M NaCl for BZCl (20 degrees C).