Romero-Vargas Castrillón Santiago, Giovambattista Nicolás, Aksay Ilhan A, Debenedetti Pablo G
Department of Chemical Engineering, Princeton University, Princeton, NJ 08544-5263, USA.
J Phys Chem B. 2009 Feb 5;113(5):1438-46. doi: 10.1021/jp809032n.
We present a molecular dynamics simulation study of the structure and dynamics of water confined between silica surfaces using beta-cristobalite as a model template. We scale the surface Coulombic charges by means of a dimensionless number, k, ranging from 0 to 1, and thereby we can model systems ranging from hydrophobic apolar to hydrophilic, respectively. Both rotational and translational dynamics exhibit a nonmonotonic dependence on k characterized by a maximum in the in-plane diffusion coefficient, D||, at values between 0.6 and 0.8, and a minimum in the rotational relaxation time, tauR, at k=0.6. The slow dynamics observed in the proximity of the hydrophobic apolar surface are a consequence of beta-cristobalite templating an ice-like water layer. The fully hydrophilic surfaces (k=1.0), on the other hand, result in slow interfacial dynamics due to the presence of dense but disordered water that forms strong hydrogen bonds with surface silanol groups. Confinement also induces decoupling between translational and rotational dynamics, as evidenced by the fact that tauR attains values similar to that of the bulk, while D|| is always lower than in the bulk. The decoupling is characterized by a more drastic reduction in the translational dynamics of water compared to rotational relaxation.
我们使用β-方石英作为模型模板，对限制在二氧化硅表面之间的水的结构和动力学进行了分子动力学模拟研究。我们通过一个无量纲数k（范围从0到1）来缩放表面库仑电荷，从而可以分别模拟从疏水非极性到亲水的系统。旋转和平动动力学对k均表现出非单调依赖性，其特征在于面内扩散系数D||在0.6至0.8之间的值处达到最大值，而旋转弛豫时间tauR在k = 0.6时达到最小值。在疏水非极性表面附近观察到的缓慢动力学是β-方石英形成类似冰的水层的结果。另一方面，由于存在与表面硅醇基团形成强氢键的致密但无序的水，完全亲水的表面（k = 1.0）导致缓慢的界面动力学。限制还会导致平动和旋转动力学之间的解耦，这一事实证明tauR达到了与本体相似的值，而D||始终低于本体中的值。这种解耦的特征是，与旋转弛豫相比，水的平动动力学有更显著的降低。
