The Role of Solvent Fluctuations in Hydrophobically Driven Assembly

Water-mediated hydrophobic interactions play a central role in a wide range of complex phenomena, including biological processes like protein assembly and the design of synthetic nanoscale drug delivery systems. While classical descriptions of these processes typically attempt to incorporate only the averaged effects of the solvent, recent work has established that fluctuations in water density near hydrophobic surfaces are significantly enhanced and are of paramount importance to hydrophobic effects. However, the thermodynamics of these solvent fluctuations depend sensitively on the size and chemistry of the solutes of interest. Using molecular simulations, we provide a detailed characterization of the role of the aqueous solvent in mediating the assembly of particles of varying size and hydrophobicity. In analogy to the length-scale dependence of hydrophobic solvation, several regimes of hydrophobic association emerge; the associated state can be separated from the dissociated state by either an entropic or enthalpic barrier, or assembly can be completely barrier-less. Our findings indicate the importance of water density fluctuations in determining the thermodynamics and kinetics of hydrophobically driven assembly processes.