Computational predictions of interfacial tension, surface tension, and surfactant adsorption isotherms

Abstract

All-atom (AA) molecular dynamics (MD) simulations are employed to predict interfacial tensions (IFT) and surface tensions (ST) of both ionic and non-ionic surfactants. The general AMBER force field (GAFF) and variants are examined in terms of their performance in predicting accurate IFT/ST, γ, values for chosen water models, together with the hydration free energy, ΔGhyd, and density, ρ, predictions for organic bulk phases. A strong correlation is observed between the quality of ρ and γ predictions. Based on the results, the GAFF-LIPID force field, which provides improved ρ predictions is selected for simulating surfactant tail groups. Good γ predictions are obtained with GAFF/GAFF-LIPID parameters and the TIP3P water model for IFT simulations at a water-triolein interface, and for GAFF/GAFF-LIPID parameters together with the OPC4 water model for ST simulations at a water-vacuum interface. Using a combined molecular dynamics-molecular thermodynamics theory (MD-MTT) framework, a mole fraction of C12E6 molecule of 1.477 × 10−6 (from the experimental critical micelle concentration, CMC) gives a simulated surface excess concentration, ΓMAX, of 76 C12E6 molecules at a 36 nm2 water-vacuum surface (3.5 × 10−10 mol cm−2), which corresponds to a simulated ST of 35 mN m−1. The results compare favourably with an experimental ΓMAX of C12E6 of 3.7 × 10−10 mol cm−2 (80 surfactants for a 36 nm2 surface) and experimental ST of C12E6 of 32 mN m−1 at the CMC.