دانلود رایگان ترجمه مقاله ترمودینامیک سطح مشترک آب حبس شده نزدیک سطوح مولکولی زبر (نشریه RSC 2010)

ترمودینامیک سطح مشترک آب حبس شده نزدیک سطوح مولکولی زبر

Interfacial thermodynamics of confined water near molecularly rough surfaces

Abstract

We study the effects of nanoscopic roughness on the interfacial free energy of water confined between solid surfaces. SPC/E water is simulated in confinement between two infinite planar surfaces that differ in their physical topology: one is smooth and the other one is physically rough on a nanometer length scale. The two thermodynamic ensembles considered, with constant pressure either normal or parallel to the walls, correspond to different experimental conditions. We find that molecular-scale surface roughness significantly increases the solid-liquid interfacial free energy compared to the smooth surface. For our surfaces with a water-wall interaction energy minimum of −1.2 kcal/mol, we observe a transition from a hydrophilic surface to a hydrophobic surface at a roughness amplitude of about 3 Å and a wave length of 11.6 Å, with the interfacial free energy changing sign from negative to positive. In agreement with previous studies of water near hydrophobic surfaces, we find an increase in the isothermal compressibility of water with increasing surface roughness. Interestingly, average measures of the water density and hydrogenbond number do not contain distinct signatures of increased hydrophobicity. In contrast, a local analysis indicates transient dewetting of water in the valleys of the rough surface, together with a significant loss of hydrogen bonds, and a change in the dipole orientation toward the surface. These microscopic changes in the density, hydrogen bonding, and water orientation contribute to the large increase in the interfacial free energy, and the change from a hydrophilic to a hydrophobic character of the surface.

1 Introduction

Water plays a central role in many of the biomolecular self-assembly processes, including the folding of proteins and the formation of lipid membranes.1–10 In key steps of these biomolecular processes, water is often highly confined,11 reduced for instance to a few layers of water molecules between the extended surfaces of large macromolecules. Considering the diversity in the chemical and physical topology of these surfaces, the interfacial behavior of water in a cellular environment will depend sensitively on the details of the confining surfaces. The wetting behavior of surfaces and its dependence on the physical and chemical nanostructure are also technologically relevant, for instance in the development of low-friction fluid flow channels.11–13

Quantitative experimental studies of water at interfaces are highly challenging,14–17 and roughness can be a relevant parameter.18 Recent advances in both theory and experiments9 have re-invigorated the interest in the molecular underpinning of the hyydrophobic effect and the behavior of water near surfaces. Water confined between a “Janus interface” of adjoining hydrophobic and hydrophilic surfaces was found to fluctuate significantly during shear deformations.19 This observation raises several interesting questions about water near heterogeneous surfaces and how local patchiness (wetting versus nonwetting) may affect hydrophobicity and microscopic density fluctuations.20 To find answers to at least some of these questions, molecular simulations can complement the laboratory experiments because in the simulations surface details can be controlled precisely and molecular information can be obtained directly.

Previous theoretical and simulation studies have typically focused on the structure, thermodynamics, and dynamics of water confined between idealized smooth surfaces,21–32 between atomistic surfaces,33–37 within carbon nanotubes,12 and between realistic proteinlike surfaces.38,39 These studies have vastly improved our molecular-level understanding of the expected changes in the water behavior due to confinement and the presence of hydrophilic versus hydrophobic surfaces.

In a laboratory experiment, one way to characterize surfaces as hydrophilic or hydrophobic is to measure the macroscopic contact angle of small water droplets on these surfaces. Recent molecular simulation studies have used a microscopic analog of the macroscopic contact angle to define the hydrophobicity of surfaces.40–42 Giovambattista et al. 41 used contact angle data to show that that water behavior can be a non-trivial function of the surface polarity. Godawat et al. 42 studied the behavior of water near self-assembled monolayer structures with a wide range of chemistries and showed that the hydrophobicities of such surfaces as measured by contact angles can be characterized well by measures related to the water density fluctuations. We similarly saw enhanced water-density fluctuations near extended non-polar surfaces,43 associated with transitions between locally wet and dry regions and resulting in a broadened liquid-vapor like interfacial density profile.

It is well known in the surface wetting literature that the physical roughness of the surface can cause the contact angle to change significantly.44 This change is typically described by either the Cassie45 or the Wenzel46 model depending on the wetting properties of the smooth surface. These models are widely used to describe the effects of the mesoscopic (micron-scale) roughness of superhydrophobic surfaces, such as Lotus leafs. Much less is known on how surface roughness of the order of few molecular diameters,47–49 which is relevant for instance to characterize protein surfaces, will affect the interfacial behavior of water.

This poor understanding of surface roughness effects on the interfacial thermodynamics also severely limits the applicability of surface-area based solvation models. Many widely used models for ligand binding and self-assembly rely on the interfacial free energy of surfaces to predict their solvation free energy.50 However, at the molecular scale corrections should be made to the interfacial free energy to account for curvature and physical roughness.51,52 Examples of such curvature corrections rely on the macroscopic concept of the Tolman length,53 or on the microscopic observation of curvature dependent “cavity expulsion” of water.54

To explore how molecular-scale physical roughness affects the interfacial free energy, we perform molecular dynamics simulations of water confined between walls. One of the major difficulties in performing simulations of confined water is to define the thermodynamic state of the system and to make sure that water is present in a thermodynamically stable equilibrium. Here, we consider two thermodynamic ensembles, defined by constant normal and parallel pressures (Fig. 1) at a constant temperature and with a constant number of particles. We also monitor individual components of the pressure tensor to make sure that the system is in a stable state during the entire simulation time.

In our model system, the wall roughness can be changed in a controlled manner by modulating the local wall position with periodic functions of given amplitudes and wavelengths (see Fig. 2). We find that increasing wall roughness, as characterized by increasing roughness amplitude or decreasing roughness wavelength, can significantly increase the solid-liquid interfacial free energy. Roughness can even change the sign of the interfacial free energy from negative (hydrophilic) to positive (hydrophobic). We examine the microscopic origins of this change in wetting behavior in the context of hydration structure near rough walls and associated density and hydrogen bond distributions.