Pal Subrata, Bagchi Biman, Balasubramanian Sundaram
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India.
J Phys Chem B. 2005 Jul 7;109(26):12879-90. doi: 10.1021/jp0510793.
We report a theoretical study of the structure and dynamics of the water layer (the hydration layer) present at the surface of the cationic micelle decyltrimethylammonium bromide (DeTAB) by using atomistic molecular dynamics simulations. The simulated micelle consisted of 47 surfactant molecules (and an equal number of bromide ions), in good agreement with the pioneering light scattering experiments by Debye which found an aggregation number of 50. In this micelle, three partially positively charged methyl groups of each surfactant headgroup face the surrounding water. The nature of the cationic micellar surface is found to play an important role in determining the arrangement of water which is quite different from that in the bulk or on the surface of an anionic micelle, like cesium perfluorooctanoate. Water molecules present in the hydration layer are found to be preferentially distributed in the region between the three partially charged methyl headgroups. It is found that both the translational and rotational motions of water exhibit appreciably slower dynamics in the layer than those in the bulk. The solvation time correlation function (TCF) of bromide ions exhibits a long time component which is found to originate primarily from the interaction of the probe with the micellar headgroups. Thus, the decay of the solvation TCF is controlled largely by the residence time of the probe in the surface. The residence time distribution of the water molecules also exhibits a slow time component. We also calculate the collective number density fluctuation in the layer and find a prominent slow component compared to the similar quantity in the bulk. This slow component demonstrates that water structure in the hydration layer is more rigid than that in the bulk. These results demonstrate that the slow dynamics of hydration layer water is generic to macromolecular surfaces of either polarity.
我们通过原子分子动力学模拟，对阳离子胶束溴化癸基三甲基铵（DeTAB）表面存在的水层（水化层）的结构和动力学进行了理论研究。模拟的胶束由47个表面活性剂分子（以及相同数量的溴离子）组成，这与德拜开创性的光散射实验结果非常吻合，该实验发现聚集数为50。在这个胶束中，每个表面活性剂头基的三个部分带正电的甲基面向周围的水。发现阳离子胶束表面的性质在决定水的排列方面起着重要作用，这与在本体中或阴离子胶束（如全氟辛酸铯）表面的水排列有很大不同。发现在水化层中的水分子优先分布在三个部分带电荷的甲基头基之间的区域。结果发现，与本体中的水相比，水层中水分子的平动和转动动力学都明显更慢。溴离子的溶剂化时间关联函数（TCF）表现出一个长时间分量，发现这主要源于探针与胶束头基的相互作用。因此，溶剂化TCF的衰减在很大程度上由探针在表面的停留时间控制。水分子的停留时间分布也表现出一个缓慢的时间分量。我们还计算了该层中的集体数密度涨落，发现与本体中的类似量相比，有一个显著的缓慢分量。这个缓慢分量表明水化层中的水结构比本体中的更刚性。这些结果表明，水化层水的缓慢动力学对于任何极性的大分子表面都是普遍存在的。
