Sun Li, Siepmann J Ilja, Schure Mark R
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, USA.
J Phys Chem B. 2006 Jun 1;110(21):10519-25. doi: 10.1021/jp0602631.
Configurational-bias Monte Carlo simulations in the isobaric-isothermal ensemble (T = 323 K and p = 10 atm) were carried out to probe structural properties of an isolated n-octadecane chain solvated in water, methanol, water-rich, or methanol-rich mixtures and, for comparison, of an isolated chain in the gas phase and for neat liquid n-octadecane. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to represent n-octadecane and methanol and the TIP-4P model was used for water. In all six environments, broad conformational distributions are observed and the n-octadecane chains are found to predominantly adopt extended, but not all-trans conformations. In addition, a small fraction of more collapsed conformations in which the chain ends approach each other is observed for aqueous hydration, the water-rich solvent mixture and the gas phase, but the simulation data do not support a simple two-state picture with folded and unfolded basins of attraction. For chains in these three "poor" solvent environments, the dihedral angles near the center of the chain show an enhancement of the gauche population. The ensemble of water-solvated chains with end-to-end contacts is preferentially found in a U-shaped conformation rather than a more globular state. An analysis of the local solvation structures in the water-methanol mixtures shows, as expected, an enrichment of the methyl group of methanol near the methylene and methyl segments of the n-octadecane chain. Interestingly, these local bead fractions are enhanced by factors of 2.5 and 1.5 for methyl and methylene segments reflecting the more hydrophobic nature of the former segments.
在等压等温系综（T = 323 K，p = 10 atm）中进行了构型偏置蒙特卡罗模拟，以探究溶解于水、甲醇、富水或富甲醇混合物中的孤立正十八烷链的结构性质，作为对比，还研究了气相中孤立链以及纯液态正十八烷的结构性质。采用TraPPE（相平衡可转移势）力场的联合原子版本来描述正十八烷和甲醇，水则采用TIP - 4P模型。在所有六种环境中，均观察到了宽泛的构象分布，并且发现正十八烷链主要采取伸展构象，但并非全反式构象。此外，对于水合、富水溶剂混合物和气相，观察到一小部分链端相互靠近的更为折叠的构象，但模拟数据并不支持存在折叠和未折叠吸引盆地的简单两态图景。对于处于这三种“不良”溶剂环境中的链，链中心附近的二面角显示出gauche构象的比例增加。具有端到端接触的水合链系综优先呈现U形构象，而非更球状的状态。对水 - 甲醇混合物中局部溶剂化结构的分析表明，正如预期的那样，甲醇的甲基在正十八烷链的亚甲基和甲基片段附近富集。有趣的是，甲基和亚甲基片段的这些局部珠子分数分别增加了2.5倍和1.5倍，这反映了前者片段更强的疏水性。
