Monte Carlo simulations of a polyoxyethylene C₁₂E₂ lamellar bilayer in water

Abstract

The polyoxyethylene C₁₂E₂/ water lamellar phase has been modelled as an aqueous region sandwiched by two bilayers. The bilayers were represented as ethoxy chains attached to a hydrocarbon continuum. Initially, the bilayer separation distance was fixed at 24 Å, which is a typical experimental value for the surfactant weight fraction of 71%. Both canonical and Gibbs ensemble simulations have been performed on the system at different temperatures ranging from 298⋅15 K to 353⋅15 K. In the Gibbs simulations the bilayer separation was allowed to readjust to an equilibrium value. The results are compared and contrasted with previous studies in which single chains (Kong, Y. C., Nicholson, D., Parsonage, N. G., and Thompson, L., 1994, J. chem Soc. Faraday Trans, 90, 2375) and charged OH groups (Cracknell, R. F., Nicholson, D., and Parsonage, N. G., 1992, Molec. Phys., 75, 1023), attached to a hydrocarbon substrate, were simulated. As in the previous work with single chains, there is evidence of water bridging which stabilizes certain chain conformations, but in these multichain systems there is competition between intra- and inter-chain bridging. The water structure is substantially modified, in comparison with bulk water, by the presence of the chains, as evidenced by positional and orientational distributions. In the Gibbs ensemble simulations at 298 K, the bilayer separation contracted by about 1⋅5 Å and appeared to have reached equilibrium at this separation after about 4 x 10⁷ configurations. At 343⋅15 K the bilayer separation was still decreasing when the simulation was terminated at 6 x 10⁷ configurations. It is concluded that water restructuring plays an important part in stabilizing the lamellar phase, and that a flexible chain model, with multiple binding sites, is necessary in order to account for bilayer stability.