Hydrophobicity at small and large length scales

The hydration of small apolar species is qualitatively different from that of large apolar species. In the former, hydrogen bonding of water is hindered yet persists near the solutes. In the latter, hydrogen bonding is depleted, leading to drying at extended apolar surfaces. Hydrophobic effects of small apolar molecules are weak and short-ranged entropic effects, and have been understood for more than two decades by Pratt-Chandler theory. Hydrophobic effects of extended apolar surfaces are strong energetic effects. Large forces of attraction and hysteresis have been observed experimentally. Until now, large and moderate length scale hydrophobicity has been poorly understood.; We study the hydrophobic effect in the large length scale regime through a lattice gas model confined between two extended drying surfaces. Using mean field theory, we describe the pertinent phases and the corresponding density profiles. In one of the three phases we identify, vapor films form between the liquid and the drying surfaces. The growth of these vapor films is driven by capillary wave fluctuations in the associated liquid-vapor interfaces. Evaporation of the fluid can occur when two such surfaces are in close enough distance. Analytical estimates and Monte Carlo simulations indicate that the pathway to phase transition involves cooperation between large-amplitude fluctuations of the liquid-vapor interfaces and the formation of vapor tubes that bridge these interfaces.; A unified theory for hydrophobicity at small and large length scales is presented. We find that at intermediate length scales, microscopic scale entropic effects compete with drying, and the strength and nature of hydrophobicity is a sensitive function of sizes and arrangements of apolar units. The cross over to drying is closely related to the nucleation of vapor in the phase transition between liquid water and vapor, or to the nucleation of oil in the phase separation of water and oil. It occurs when the local concentration of apolar units is sufficiently high, or when an apolar surface is sufficiently large. The cross over occurs on nanometer length scales, often of pertinence to structural biology. Our theory of the cross over has significant implications concerning the stability of protein assemblies and protein folding.