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

Much is written about "hydrophobic forces" that act between solvated mols. 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 addn. of salt and those that are not. Bubble adsorption and cavitation effects plague expts. 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 concn. In this paper, we are concerned with hydrophobic interactions between single mols. 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 mol. dynamics simulations and det. the adsorption potential of mean force. The simulation results for peptides compare well with corresponding AFM expts. An anal. 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 sep. detg. 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 detg. ion-specific adsorption at hydrophobic surfaces can best be described as surface-modified ion hydration.