(145e) Using Atomistic Simulation to Determine the Optimal Aggregation Number of a Micelle, and Its Equilibrium with Polymer Surface in Aqueous Solution

In waterborne coatings, surfactants in the aqueous phase can either adsorb to polymer particle surfaces, or form micelles above the critical micelle concentration. The balance of surfactants between micelles and polymer surfaces is crucial for the colloidal stability of polymer particles. Here, we describe this balance by calculating the potential of mean force (PMF) to pull a sodium dodecyl sulfate (SDS) surfactant from a polymer surface and from a micelle respectively, using atomistic molecular dynamics simulations. For the PMF of the polymer surface, we investigated the role of salts in solution, other SDS molecules, and bound surface charges on the PMF, all of which influence the electrostatic forces between SDS and the surface. We calculated the PMF for SDS micelles with an aggregation number ranging from 10 to 100, from which the optimal aggregation number can be determined. This provides a practical method to calculate the aggregation number without simulating the exchange of surfactants between multiple micelles, which occurs at the time scale of seconds, and far surpasses the capability of atomistic simulations. Our predicted aggregation number of SDS micelles is in a very good agreement with the experimental data. By comparing the PMF of removing a SDS molecule from a polymer surface and from a micelle, we can predict the equilibrium of SDS molecules in the presence of both micelle and polymer surface without actually simulating both, which reduces computational cost and provides guidance for formulation of coatings.