MONTE-CARLO SIMULATIONS OF A POLYOXYETHYLENE C(12)E(2) LAMELLAR BILAYER IN WATER

The polyoxyethylene C(12)E(2)/water lamellar phase has been modelled a s an aqueous region sandwiched by two bilayers. The bilayers were repr esented as ethoxy chains attached to a hydrocarbon continuum. Initiall y, the bilayer separation distance was fixed at 24 Angstrom, 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 read just 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 Sec. Faraday Tra ns, 90, 2375) and charged OH groups (Cracknell, R. F., Nicholson, D., and Parsonage, N.G., 1992, Molec. Phys., 75, 1023), attached to a hydr ocarbon 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 competit ion between intra- and inter-chain bridging. The water structure is su bstantially modified, in comparison with bulk water, by the presence o f the chains, as evidenced by positional and orientational distributio ns. In the Gibbs ensemble simulations at 298 K, the bilayer separation contracted by about 1.5 Angstrom and appeared to have reached equilib rium at this separation after about 4 x 10(7) configurations. At 343.1 5 K the bilayer separation was still decreasing when the simulation wa s terminated at 6 x 10(7) configurations. It is concluded that water r estructuring plays an important part in stabilizing the lamellar phase , and that a flexible chain model, with multiple binding sites, is nec essary in order to account for bilayer stability.