Static and dynamic correlations in water at hydrophobic interfaces

Abstract

We study the static and dynamic properties of the water-density fluctuations in the interface of large nonpolar solutes. With the help of extensive molecular dynamics simulations of TIP4P water near smooth spherical solutes, we show that for large solutes, the interfacial density profile is broadened by capillary waves. For purely repulsive solutes, the squared width of the interface increases linearly with the logarithm of the solute size, as predicted by capillary-wave theory. The apparent interfacial tension extracted from the slope agrees with that of a free liquid-vapor interface. The characteristic length of local density fluctuations is Ϸ0.5 nm, measured along the arc, again consistent with that of a free liquid-vapor interface. Probed locally, the interfacial density fluctuations exhibit large variances that exceed those expected for an ideal gas. Qualitatively consistent with theories of the free liquid-vapor interface, we find that the water interface near large and strongly nonpolar solutes is flickering, broadened by capillarywave fluctuations. These fluctuations result in transitions between locally wet and dry regions that are slow on a molecular time scale.capillary waves ͉ drying transition ͉ hydrophobic effect ͉ surface tension T he hydration structure and thermodynamics of simple nonpolar solutes in water is central to a molecular understanding of many biological self-assembly processes, including protein folding and the formation of lipid membranes (1-4). To understand the water-induced conformational changes in biopolymers that lead to functional structures, it is critical to study not only the effects of water on these molecules but also the reverse, i.e., the modified behavior of water in the vicinity of these molecules. To avoid the inherent chemical and structural complexities of present in biological molecules, it is instructive to consider model systems with tunable degrees of freedom. Arguably the simplest such model system is a smooth spherical particle immersed in a water bath (5-18). Such a rudimentary solute model captures two key factors in solvation, the size of the solute and the strength of its interactions with water.Small, methane-sized solutes are accommodated by water with only minor disruptions of the bulk hydrogen-bond network. The resulting small-solute hydration thermodynamics has been predicted successfully by approaches that take into account the molecular-scale density fluctuations in bulk water, including scaled-particle theory (5, 19) (see ref. 20 for a recent review), Pratt-Chandler theory (8), the Gaussian field model (21), an information theory model (22), and Lum-Chandler-Weeks (LCW) theory (23). With the addition of a large repulsive solute, it becomes impossible for water molecules to maintain their bulk hydrogen-bond structure (24), which also affects the wetting behavior. At ambient conditions the contact theorem predicts a near-zero water density at a flat hard wall (5). Based on this rigorous limit and an interpolation formula, Stillinger anticipated that...

An improved coarse-grained model of solvation and the hydrophobic effect

“…This fat tail is the hallmark of a soft vapor-liquid interface, in this case a soft interface next to the hydrophobic solute. [73][74][75][76] At higher attractive interactions, this fat tail is correspondingly depressed, but not entirely suppressed. Accordingly, in Ref.…”

“…The thickness parameter d can thus be determined from simulation. A complication due to capillary waves is that d grows logarithmically with simulation box size, 73 94. We choose the smaller value because the instantaneous configuration of n(r) should be blurred only by small-scale fluctuations, not by large-scale capillary waves, which correspond instead to different conformations of n(r).…”

Characterizing hydrophobicity of interfaces by using cavity formation, solute binding, and water correlations

“…A purely repulsive hard-wall-like interface is similar to the V-L interface of water. It will accommodate significant fluctuations consistent with the long-range capillary waves present at that interface (20,54,64). An external field (e.g., a weak field such as gravity) is known to quench the range of these correlations (65,66).…”

Activated drying in hydrophobic nanopores and the line tension of water

“…The formation of cavities able to bridge hydrophobic units provides a driving force for protein folding and supermolecular aggregation (9). Simulation examples of such drying-induced phenomena include the collapse of a polymer chain, multidomain proteins, and hydrophobic particles (9)(10)(11)(12)(13).…”