Water structuring above solutes with planar hydrophobic surfaces

Abstract

Many important biological solutes possess not only polar and hydrogen bonding functionalities, but also weakly-hydrating, or hydrophobic, surfaces. Theories of the hydration of such surfaces predict that their solvent interactions will change from a wetting type interaction to a dewetting regime as a function of the solute size, with a gradual transition in behavior taking place around characteristic lengths of ~ 1 nm. Aggregations of non-polar species over this size range will undergo a transition from being dominated by entropy to being dominated by enthalpy. These transitions can be understood in part in terms of the geometries required of the solvating water molecules. We report here a series of simulations in aqueous solution of organic molecules with planar faces of increasing size, ranging from cyclopropane to circumcircumcoronene, in order to explore the transition in behavior for such solutes as their size increases. For this series, the dewetting transition occurred gradually, converging asymptotically to a limiting separation value for first layer water molecules of around 3.3 Å, while the transition in hydrogen bonding orientational structure occurred between cyclopropane and cyclopentadene. Water immediately adjacent to the largest planar hydrophobic surfaces oriented in ways that resembled on average the structural organization of the basal planes of ice.

Effects of the Chemical and Structural Properties of Silane Monolayers on the Organization of Water Molecules and Ions at Interfaces, from Molecular Dynamics Simulations

“…Previous experimental 10,11,43 and MD simulation 15 studies have shown that up to 1 nm above positively charged flat surfaces, water molecules are oriented with hydrogen atoms pointing toward outside the surface, while they are oriented with hydrogen atoms pointing toward the surface above negatively charged flat surfaces. Also, flat extended hydrophobic nonpolar surfaces were shown to orient water molecules, up to 1 nm above the surface, with hydrogen atoms pointing toward the surface, 16,17,44,45 while small hydrophobic solutes were shown to orient water molecules with hydrogen atoms pointing toward outside the surface. 46 This organization results from the optimization of both the number of hydrogen bonds and the packing density of water molecules.…”

“…Therefore, at the interface with a CH 3 -short silane monolayer, water molecules orient themselves like at the vicinity of small hydrophobic solutes 46 and not like at the interface with a flat extended hydrophobic surface. 16,17,44,45 Water molecules orient themselves in the vicinity of the methyl group independently from other silane molecules, which are well separated on the surface due to their short alkyl chains. Regarding the CH 3 -long monolayer, we do not observe any global orientation of water molecules at the interface (⟨cos θ⟩ = 0).…”

“…2 While to the best of our knowledge, the interfaces between a liquid and a solid modified with an amorphous chemical functionalization have never been studied by MD simulations, several studies are focused on crystalline and idealized charged surfaces. 15,17 In particular, silica/water interfaces with discrete or homogeneously distributed charge density were studied. 15 The interface was shown to consist of a compact layer of oriented water molecules, and the ion distribution could not be described with the classical electrical double layer/diffuse layer theories.…”

“…Other examples show that the hydration layer plays a decisive role in the adsorption of peptides, proteins, DNA, or RNA . Owing to the crucial role of the hydration layer, several authors have focused on understanding the effect of surface chemistry, and of the surface structure, on the hydration layer. − …”

Density depletion and enhanced fluctuations in water near hydrophobic solutes: identifying the underlying physics

“…Beyond solubility, one is interested in the perturbations to the water structure that occur in the solute's vicinity, see eg. [9,[11][12][13].…”