(592g) Effects of Surfactant Transport On High Internal Phase Emulsions Under Shear: A Combined Rheological and Structural Study

We report the change in rheological behavior of high-internal phase emulsions (HIPEs) under shear of a polyisobutylene-based (PIBSA) oil-soluble surfactant and with and without the addition of a water-soluble polyacrylamide (PAM) co-surfactant. We used a series of contrast-matched small-angle and ultra-small angle neutron scattering (SANS and USANS) coupled with in situ rheological measurements to track the locations of the surfactant and co-surfactant as a function of shear. This work follows a series of papers analyzing the structural variation and stability of emulsions stabilized with PIBSA and various mixtures of PIBSA/PAM under static and shear conditions.

The emulsions’ sensitivity to aqueous/oil phase ratios, surfactant concentration, surfactant molecular weight, and polydispersity has been defined. The emulsions consist of almost spherical micron scale, highly polydisperse, aqueous droplets dispersed in a continuous oil phase with aqueous/oil phase ratios of about 9:1. The emulsions are rheologically unexceptional and follow previously established predictions and theory. The emulsions show refinement to higher viscosity after high shear, and shear thinning. The structural basis for its rheological behavior however does not follow theory. Shear dependent changes observed in the SANS data were tracked by SANS model parameters using a convolution of two well-established models. Shear thinning is explained by SANS observed shear disruption of inter-droplet bilayer links, causing deflocculation to more spherical, less linked, aqueous droplets. Refinement to higher viscosity is accompanied by droplet size reduction and loss of surfactant from the oil continuous phase. Refinement occurs because of shear-induced droplet anisotropy, which we have also observed in the SANS experiment. The observed anisotropy and emulsion refinement cannot be reproduced by either isolated molecule or mean-field models and require a more detailed consideration of interdroplet forces in the sheared fluid. Even at concentrations by the stability limit, a large percentage of surfactant(s) is dissolved as small n-mers or as larger reverse micelles that play an important part in stabilization of the emulsion under shear.

Steady shear on PIBSA/PAM emulsions reduces the number of reverse surfactant micelles present in the both the continuous oil phase to provide the surfactant needed to cover the newly formed surface area as the emulsion refines to smaller droplet sizes. Surfactant adsorption to droplet interfaces is accomplished by two processes. The first draws soluble surfactant from the oil phase to the interface and lowers the concentration of dissolved n-meric surfactant. In parallel, oil-soluble reverse micelles begin to break up, allowing a shear rate dependent steady state to establish between the surfactant reservoirs and aqueous droplets. The application of low-shear rate recovery intervals allowed the recovery dynamics of the surfactant distribution to be observed. The results showed little reduction of the emulsion interfacial area upon return to its quiescent state, but a large recovery of the reverse micelle volume fraction that indicates the continuous phase acts as the reservoir of surfactant when the emulsion is under shear. Drastic changes in the rheological behavior of emulsions with PAM co-surfactant indicate different kinetics dictate surfactant and co-surfactant droplet adsorption. We report the effect of altering the chain length and concentration of PAM based co-surfactants (C₁₂-PAM, C₁₄-PAM, and C₁₆-PAM) on the properties of the high-internal phase PIBSA emulsions under shear.