Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
J Phys Chem B. 2010 Oct 14;114(40):12875-82. doi: 10.1021/jp1045645.
We observe a surprisingly sharp increase in the pair hydrophobicity in the water-dimethylsulfoxide (DMSO) binary mixture at small DMSO concentrations, with the mole fraction of DMSO (x(D)) in the range 0.12-0.16. The increase in pair hydrophobicity is measured by an increase in the depth of the first minimum in the potential of mean force (PMF) between two methane molecules. However, this enhanced hydrophobicity again weakens at higher DMSO concentrations. We find markedly unusual behavior of the pure binary mixture (in the same composition range) in the diffusion coefficient of DMSO and in the local composition fluctuation of water. We find that, in the said composition range, the average coordination number of the methyl groups (of distinct DMSO) varies between 2.4 and 2.6, indicating the onset of the formation of a chain-like extended connectivity in an otherwise stable tetrahedral network comprising of water and DMSO molecules. We propose that the enhanced pair hydrophobicity of the binary mixture at low DMSO concentrations is due to the participation of the two methane molecules in the local structural order and the emerging molecular associations in the water-DMSO mixture.
我们观察到在水-二甲基亚砜（DMSO）二元混合物中，在 DMSO 浓度较小的情况下，对疏水相互作用具有惊人的急剧增加，其中 DMSO 的摩尔分数（x(D)）在 0.12-0.16 范围内。对疏水相互作用的增强是通过增加两个甲烷分子之间的平均力势能（PMF）的第一个最小值的深度来测量的。然而，这种增强的疏水性在更高的 DMSO 浓度下再次减弱。我们发现纯二元混合物（在相同组成范围内）在 DMSO 的扩散系数和水的局部组成波动方面表现出明显异常的行为。我们发现，在所提到的组成范围内，甲基（不同 DMSO 的）的平均配位数在 2.4 到 2.6 之间变化，表明在由水和 DMSO 分子组成的稳定四面体网络中，形成链状扩展连接的开始。我们提出，在低 DMSO 浓度下二元混合物中增强的对疏水相互作用是由于两个甲烷分子参与了局部结构有序和水-DMSO 混合物中出现的分子缔合。
