Prediction of Surface and Bulk Partition of Nonionic Surfactants Using Free Energy Calculations

The surface-bulk partition of nonionic surfactants was predicted by calculating chemical potential differences using two simulation boxes representing the surface and the bulk phases separately. A published coarse grained force field was modified and validated for this application. Thermodynamic integration (TI) was applied to compute excess chemical potentials. The high concentration surface was stabilized by applying an external harmonic potential, and the bulk was treated as ideal solution, which was confirmed by simulation using a lattice model at conditions near the critical micelle concentration (∼10(-5) mol/L). Based on the calculated chemical potential differences with precision of ca. 1 kJ/mol, the equilibria of surface-bulk concentration was predicted well in comparison with the experimental data.