(605g) Molecular Dynamics Simulations of Polymer-Surfactant Interactions

The self-assembly of surfactants in the aqueous media is an area of active research. The presence of the polymer molecules gives rise to changes in the solution and the interfacial properties of the surfactant compared to the corresponding polymer-free systems. In this work, we have analyzed the polymer-surfactant aggregation phenomenon by looking at the interactions between the anionic sodium dodecyl sulfate (SDS) surfactant and two non-ionic polymers – poly-(ethylene oxide) (PEO) and poly-(vinylpyrolidone) (PVP) using molecular dynamics (MD) simulations in water medium. From the simulation results, it is observed that the polymer chains adsorb on the hydrophobic regions of the SDS micelle surface in a pattern similar to the beaded ­­necklace model as it was observed in experimental studies. The association of hydrophobic regions in the polymer and micelle suggest that the hydrophobic effect is the driving force for association. As hydrophobic effect is a function of temperature, our research is concentrated on studying the polymer-micelle interactions at various temperatures in the range of 280-360K. In all the temperature studies, the nonionic polymer displayed interaction with the anionic surfactant by adsorbing on the surface region of the micelle. The Surface Accessible Surface Area (SASA) of the micelle adsorbed by polymer (PEO and PVP) was small compared to the surface area of the polymer-free micelle, proving that the polymer shields the micelle from water promoting hydrophobic interactions between the polymer and micelle. The Potential of Mean Force (PMF) between the polymer and the surfactant assembly was also evaluated at various temperatures by pulling short chains of PEO and PVP from the surface of the micelle. The strength of interactions between micelle and polymer per unit surface area of the polymer was analyzed as a function of temperature. These results provide valuable insight into the temperature dependent hydrophobic interactions between nonpolar polymers with the anionic surfactants.