Dynamic light scattering and viscosity studies on the association behavior of silicone surfactants in aqueous solutions

The micellar structure of three nonionic amphiphilic silicone surfactants based on polyether-modified poly(dimethylsiloxane) has been investigated by dynamic light scattering (DLS) and viscosity measurements. Out of these three, two surfactants have a comblike structure, and one has a trisiloxane-type structure with a linear poly(dimethylsiloxane) backbone chain and a grid containing block oligomers of oxyethylene and/or oxypropylene attached to one of the units as grafts. The dilute-solution phase diagram of the trisiloxane surfactant has been constructed and explained. The hydrodynamic radius, Rh, and the size distribution in terms of the translational diffusion coefficient of the micelles were obtained from the analysis of the time correlation function of the scattering intensity measured at a fixed scattering angle of 90degrees and for different concentrations and temperatures (30, 45, and 60 degreesC). The micellar systems under study were found to have a less polydisperse size distribution. The analysis of DLS data revealed the presence of oblate ellipsoidal micelles that showed continuous growth along the semimajor axis, b, with the rise in temperature. The values of the mass average association number, the hydrodynamic expansion factor, and the intrinsic viscosity of the micelles exhibited a similar temperature dependence, but the dominant repulsive solute (hydrophobic part)-solvent interactions noticed at 30 degreesC were found to get weaker. The combination of small-angle neutron scattering (SANS), DLS data, and intrinsic viscosities of micellar solutions were used to estimate the hydration of the micelles. The observed unusual temperature dependence of the micellar hydration has been explained.