Orientational fluctuations of liquid water at hydrophobic and hydrophilic interfaces

Abstract

Being ubiquitous in nature, liquid water interfaces host many important physical and chemical processes. Most of these processes are affected by the properties of water interfaces which depend on molecular details of the interfacial environments. It is thus fundamental to understand microscopic features of a water interface for studying the interfacial phenomena. In this work, we explore structural characteristics of various water interfaces using the methods of molecular simulations and statistical mechanics. Specifically, we describe the intrinsic molecular structure of a water interface in terms of the anisotropic, depth-dependent distribution of water's molecular orientation. We also analyze the collective hydrogen bond network at the interface which determines the orientational fluctuations of interfacial water molecules. This dissertation consists of three separate studies on related but distinct subtopics. First, we focus on the molecular structure and dielectric properties of the liquid water-vapor interface. We present a statistical mechanical model of interfacial hydrogen bonding which is capable of predicting the orientational distribution at the interface. Using this model along with the atomistic simulations, we reveal that the hydrogen bonding interactions of non-ideal geometries are responsible for the characteristics of the interfacial dielectric properties. Then, we introduce a general computational framework that can characterize the interfacial properties of a hydrated solute from its atomistic simulation. This method utilizes the orientational structure of interfacial water molecules which reflects the microscopic informations about the solute's surface. We demonstrate that our method is applicable with both temporal and spatial resolutions to a chemically heterogenous surface as well as an irregular surface. Therefore, it is useful especially for probing the local hydration dynamics of a protein which is correlated with the conformational fluctuations. Finally, we study the effects of surface-water interactions on water's interfacial hydrogen bonding structure using a disordered model hydrophilic surface with tunable polarity. We investigate the relationship between the surface polarity and interfacial molecular structure using the characterization method previously introduced. Based on a mean-field model of interfacial hydrogen bonding, we also quantify the energetic component of surface-water interactions that specifically contribute to modifying the interfacial hydrogen bond network. We identify this specific energetic component as a new measure for hydrophilicity.