The shape, size, aggregation, hydration, correlation times, and thermodynamic studies on macromonomer micelles

Abstract

The hydration of the macromonomer micelles has been directly determined from the measurements of conductance of micellar solutions of macromonomer in 5 mM NaCl using the principle of the obstruction of electrolyte migration by the surfactants. This coupled with the intrinsic viscosity has helped in evaluating asymmetry of the micellar particles and their average axial ratios. The hydration has also been determined by Einstein and Vand's equations, and are in good agreement with the conductivity results. Hydration number and micellar sizes are variable with temperature. The shape of the macromonomer micelles has been observed to be more or less spherical at 15°C. However, above 15°C the shape of the micelles are ellipsoids. From the absolute values of the axes, the micellar volume, hydrodynamic radius, radius of gyration, frictional, and diffusional coefficients as well as translational (τ_D), rotational (τ_r), and effective correlation (τ_a) times have been calculated. τ_r values for Triton X-100, Tween-20, Tween-80, and macromonomer at 25°C are 45, 28, 25, and 8 ns, respectively. The aggregation number N of the macromonomer in an aqueous solution was determined by fluorescence spectroscopic technique. Blueshift of ANS emission was observed in the presence of macromonomer micelles. Both N and the Stern-Volmer rate constant (K_SV) are variable with temperature and found them to be maximum at 25°C indicating the macromonomer micelles attained a state of fully extended conformation at 25°C. Diffusion energy (E_D) of the macromonomer micelles in water has been calculated and found to be 21.0 and 21.1 kJmol^-1 for oblate and prolate models, respectively. The partial molal volume of the macromonomer micelles has also been determined and its comparison with the molar volume of pure macromonomer suggested a volume contraction due to immobilization of the water phase by the hydrophilic head groups of macromonomer. Thermodynamic activation parameters for viscous flow are in favor of the formation of structured water by the presence of macromonomer micelles. Enthalpy (ΔH∗ ) and entropy (ΔS∗ ) values show a very good compensation between them.