Molecular Hydrophobic Attraction and Ion-Specific Effects Studied by Molecular Dynamics

Abstract

Much is written about "hydrophobic forces" that act between solvated molecules and nonpolar interfaces, but it is not always clear what causes these forces and whether they should be labeled as hydrophobic. Hydrophobic effects roughly fall in two classes, those that are influenced by the addition of salt and those that are not. Bubble adsorption and cavitation effects plague experiments and simulations of interacting extended hydrophobic surfaces and lead to a strong, almost irreversible attraction that has little or no dependence on salt type and concentration. In this paper, we are concerned with hydrophobic interactions between single molecules and extended surfaces and try to elucidate the relation to electrostatic and ion-specific effects. For these nanoscopic hydrophobic forces, bubbles and cavitation effects play only a minor role and even if present cause no equilibration problems. In specific, we study the forced desorption of peptides from nonpolar interfaces by means of molecular dynamics simulations and determine the adsorption potential of mean force. The simulation results for peptides compare well with corresponding AFM experiments. An analysis of the various contributions to the total peptide-surface interactions shows that structural effects of water as well as van der Waals interactions between surface and peptide are important. Hofmeister ion effects are studied by separately determining the effective interaction of various ions with hydrophobic surfaces. An extension of the Poisson-Boltzmann equation that includes the ion-specific potential of mean force yields surface potentials, interfacial tensions, and effective interactions between hydrophobic surfaces. There, we also analyze the energetic contributions to the potential of mean force and find that the most important factor determining ion-specific adsorption at hydrophobic surfaces can best be described as surface-modified ion hydration.

Analytical insights on ion behaviour at interfaces

“…Moreover, these attractive contributions are longer ranged and they are linear in dipole moment, the dipole-dipole one being quadratic. The new analytical results that we have obtained are in agreement with molecular dynamics simulations [6,7,9].…”

“…The specific propensities of ions for interfaces are relevant not only for understanding the physical basis of Hofmeister effects but also in the chemistry of atmospheric aerosols [8]. The usual theoretical approach to surface behaviour of ions is based on extensive molecular dynamics simulations [6,7,9]. In this letter we report analytical results which include specific ion effects.…”

Ion Specific Protein Assembly and Hydrophobic Surface Forces

“…Mean field studies [25] of the salt induced attraction between planar hydrophobic surfaces show the opposite effect: iodide leads to more repulsion between the surfaces than does chloride. This is attributed to an effective charging of the surfaces due to anion adsorption.…”

Ion-specific thermodynamical properties of aqueous proteins

“…However, it is clear that this approach does not fully account for the liquid molecular structure and hydration effects. This has recently been improved by using parameterized ionic potentials deduced from recent non-primitive model MD simulations in a generalized Poisson-Boltzmann equation (Horinek et al 2008, Lima et al 2008a, b, Horinek and Netz 2007. We investigated how ion distributions and double layer forces depend on the choice of the background salt.…”