Micellar structure of silicone surfactants in water from surface activity, SANS and viscosity studies

The aqueous solutions of two nonionic silicone surfactants based on polyether modified poly(dimethylsiloxane)s have been analyzed by cloud point, surface tension, small angle neutron scattering (SANS), and viscosity measurements. These surfactants have a comb-like structure with a linear poly(dimethylsiloxane) backbone chain and a grid containing block oligomers of ethylene oxide and/or propylene oxide as branches. The critical micelle concentrations (CMCs) were obtained from break points in surface tension vs log concentration plots and, as expected, decreased systematically with the increase in the hydrophobicity. Small angle neutron scattering (SANS) measurements have been made in aqueous solutions of silicone surfactants at different concentrations (1, 2, and 5 % w/v) and temperatures (30, 45, and 60 degreesC) to elucidate the structural information on micelles. The analysis of the SANS curves revealed the presence of oblate ellipsoidal micelles in three selected concentrations and temperatures. The size of micelles has strikingly showed a nonvariance in the concentration range of 1-5 % w/v, whereas an increase of 30-60% hi the seminor major axis b values was noted when the temperature was raised from 30 to 60 degreesC. Under the same temperature interval, an increase in the aggregation number and a decrease in the number density of micelles were noted. Viscosity measurements also support the presence of aggregates of nonspherical shape. The shape factor for the aggregates calculated from intrinsic viscosities has been found to be in close agreement with the values for the same obtained independently from the SANS measured axial ratios. The micellization of silicone surfactants has been found to be described adequately by closed association process, which was used to obtain the thermodynamic parameters viz. standard free energies (DeltaGdegrees(Mic)), enthalpies (DeltaHdegrees(Mic)), and entropies (DeltaSdegrees(Mic)) of micellization. The micellization process has always been characterized by endothermic enthalpies and thus is driven mainly by entropy factor. The stretching of the core part at elevated temperatures has been attributed to the dehydration phenomenon. as a result of which the water is expelled from the interior of the micelles facilitating the addition of more surfactant molecules into aggregates.