Molecular Studies of Forces at Liquid-Solid Interfaces

We have combined computer simulation and theory to study the forces at liquid-solid (L-S) interfaces, including the friction and solvation forces. We model the energy dissipation and momentum transfer at vibrating solid-water interfaces with a wide range of wettabilities and demonstrate the effect of L-S friction on the mechanical response of the high frequency resonators. To overcome the difficulty of measuring L-S friction at experimentally relevant time scales, we have developed a Green-Kubo (GK) relation that enables accurate and efficient calculations of friction at L-S interfaces directly from equilibrium molecular dynamics (MD) simulations. Using our GK relation and mus-long large-scale MD simulations, we further investigate the effect of L-S interfaces on the nearby Brownian motion. Our computer experiment unambiguously reveals that the t--3/2 long-time decay of the VAF of a Brownian particle in bulk liquid is replaced by a t --5/2 decay near a boundary. We discover a general breakdown of traditional no-slip boundary condition at short time scales and we show that this breakdown has a profound impact on the near-boundary Brownian motion.;To better understand hydrophobically driven self-assembly, we have also studied the hydrophobic solvation force with a specific focus on the effects of ions. Using nano-rods models of beta-peptides, we investigate the effects of both free ions (dissolved salts) and proximally immobilized ions on hydrophobic interactions. We find that the free ion effect is correlated with the water density fluctuation near a non-polar molecular surface, showing that such fluctuation can be an indicator of hydrophobic interactions in the case of solution additives. In the case of immobilized ion, our results demonstrate that hydrophobic interactions can be switched on and off by choosing different spatial arrangements of proximal ions on a nano-rod. For globally amphiphilic nano-rods, we find that the magnitude of the interaction can be further tuned using proximal ions with varying ionic sizes. Interestingly, immobilized anions of increasing ionic size do not follow the same ordering (Hofmeister-like ranking) as free ions when it comes to their impact on hydrophobic interactions. We propose a molecular picture that explains the contrasting effects of immobilized versus free ions.