Monte Carlo simulations of a single poly(oxyethylene) C12E3 chain headgroup fixed on a bilayer surface in water

Abstract

We report Monte Carlo simulation studies at 298.15, 343.15 and 353.15 K, of a single linear poly(oxyethylene) C₁₂H₂₅(OCH₂CH₂)₃OH surfactant chain head group attached to a hydrophobic wall of a continuum model bilayer. The structural perturbation of water by the head group was investigated by collecting radial distribution functions and densities for different regions in the plane of the bilayer surface. As in a previous study of a single C₁₂H₂₅(OCH₂CH₂)₂OH chain (Y. C. Kong, D. Nicholson, N. G. Parsonage and L. Thompson, J. Chem. Soc., Faraday Trans., 1994, 90, 2375), there is a strong tendency for water to form H-bonded bridges between adjacent oxygen atoms on the same chain and to favour gauche conformations for the O—C—C—O torsional angles. However the tendency to form bridges between non-adjacent oxygen atoms within the chain is diminished. The increase in the gauche/trans ratio, and the stronger chain adsorption found with increasing temperature, can be attributed to weakening of the water–chain interactions. Conformations in which the head group coils towards the wall were more commonly observed in the longer chain. The radial distribution functions for water H atoms around chain O atoms vary along the chain in a manner consistent with the existence of hydrogen-bonded bridging. The corresponding distribution functions for water O vs. chain O and water O and H vs. chain C show maxima that have intensities which vary much more regularly with chain position and which occur at interatomic distances which are independent of chain position. Interpretation of these data indicates that structures additional to those found in pure water must be present. We postulate that in R(OCH₂CH₂)₅OH the head group would form a crown ether-type structure in solution.