All-Atom Simulations of Surfactant Systems: Interfacial Tension, Surface Tension, Adsorption Isotherms and Self-Assembly

Abstract

The primary objective of this thesis is to establish and validate force field-based all-atom (AA) molecular dynamics (MD) simulations capable of accurately replicating and elucidating the physical properties of surfactants in both bulk water and interfacial regions.
AA MD simulations are employed to simulate 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, ∆Ghyd, 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 together with 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.
A combined molecular dynamics-molecular thermodynamics theory (MD-MTT) framework for non-ionic surfactants is tested using calculated ST values, together with adsorption free energies (∆Gads) obtained from calculations of the potential of mean force potential (PMF) and experimental critical micelle concentrations (CMC). The methodology provides excellent predictions for the simulated ST at the CMC, and as a function of bulk surfactant concentration for the non-ionic surfactant C12E6. This gives a 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.
GAFF and GAFF-derived parameters (combined with the TIP3P water model) are employed to simulate surfactant micelles using AA MD. Results show that C12E5 surfactants micelle simulated using GAFF-LIPID showed a similar structure to ones simulated using OPLS-AA. Simulations of SD6BS micelles, together with polyvinyl acetate (PVA) chains and 5-chloro-2-(4-chlorophenoxy)-phenol (Diclosan) molecules reveal that center-of-mass (COM) diffusion coefficients of PVA chains decrease with an increasing number of chains, reflecting the incorporation of PVA chains into micelles. The COM diffusion coefficients of Diclosan molecules show large fluctuations with an increasing number of Diclosan molecules. However, the trend also appears to indicate that diffusion decreases with increasing numbers of Diclosan molecules as surfactant micelles start to swell.
Amended GAFF parameters are employed to explore the adsorption of surfactants and polymer actives on model polyethylene terephthalate (PET) and cellulose surfaces. Results from simulations of SDS suggest that a hemispherical aggregate and a partially deformed micelle aggregate were obtained on a PET surface and a cellulose surface respectively. ∆Gads values (from bulk water to PET surface) for polymer actives were obtained from MM-GBSA and PMF calculation methods. These calculations considered both globular and stretched-out structures representing the hydrophobic blocks of a series of newly synthesised soil-release polymers. All oligomers studied were found to fold in water to produce a globular structure, which was attracted to the PET surface. PER (a 3,5-substituted pyridine analogue of a PET-based oligomer) was found to form the most strongly surface-attached globular oligomer structure, with a value of ∆Gads of -220.98 ± 3.77 kJ mol-1, but PEY (a PET-based oligomer) was the most strongly-attached stretched-out oligomer, with a value of ∆Gads of -406.68 ± 1.18 kJ mol-1.