Conformational behaviour of amphiphilic molecules in aqueous solution and at a water/air interface: computational studies at the molecular level

Abstract

Previous experimental studies have indicated that the amphiphilic graft co-polymer polynorbornene-g-poly(ethylene oxide) (PNB-g-PEO) undergoes interesting conformational behaviour when placed at a water/air interface. This polymer adopts different conformations depending upon surface concentration, as elucidated through neutron reflectometry measurements. The work in this thesis details the preparation for, and execution of atomistic molecular dynamics simulations of this system at a range of surface concentrations. Three commonly used water models were assessed for computational expense and accuracy in the reproduction of key experimental properties, particularly density. It was found that the TIP4P water model was the most appropriate, and was therefore used to generate a water/vapour interface configuration. The OPLS-AA force field was then examined in detail on the basis of ab initio structural optimisation calculations on 1,2-dimethoxyethane (DME), a model molecule for poly (ethylene oxide) (PEO). Torsion parameters for the 0-C-C-O and C-O-C-C dihedral potentials were fitted to these ab initio data in an attempt to obtain a force field capable of reproducing the conformational behaviour of DME in the bulk liquid as measured previously by experiment. Using this fitted force field, fully atomistic simulations of PNB-g'-PEO at the water/vapour interface were performed at a range of surface concentrations coinciding with the experimental study. Excellent agreement was found between simulated and experimental neutron reflectivity profiles for low surface concentrations. Agreement at higher concentrations was slightly poorer, but still much better than that obtained in a previous simulation study without explicit water. Four force fields were then compared in "simulations of a PEO chain in aqueous solution. Dihedral angle analysis was performed on these PEO chains and compared to the behaviour of the PEO side chains in PNB-g-PEO. Agreement with conformational populations was confirmed between the two studies, however the frequency of conformational transitions was found to differ significantly between the two sets of simulation.