(127b) Theoretical Insights Into the Multiscale Nature of the Hydrophobic Force

            Despite much research in the field, we are still lacking a complete understanding of water-mediated hydrophobic interactions, one of biology’s most elementary forces, responsible for folding of proteins, forming of membranes, and numerous other self-assembly mechanisms.  At one basic level, the hydrophobic force is manifested as an effective attraction between a pair of nonpolar surfaces immersed in water [1].  Recent corresponding measurements with the surface-force apparatus suggest that the (pure) hydrophobic force has two separate regimes (in terms of separation distance): a short-range spanning ~ 0.1-1 nm and a long-range spanning ~ 1-10 nm [2].  While several studies have addressed the short-range regime, no definitive theory yet exists regarding the long-range one, which extends to unexpectedly far distances. 

            In this work, we address the multiscale nature of the hydrophobic force using molecular simulations.  We explore several new hypotheses regarding the origin of the long-range force.  In particular, we consider the magnitudes of subtle electrostatic effects in this regime.  Namely, we examine water’s dilute ions (hydronium and hydroxide) and collective dipole configurations (many cooperatively correlated molecules), specifically investigating the disturbance of such properties by a low-dielectric hydrophobic surface, and how in turn, these electrostatic effects might emerge as nano-scale attractive forces in surface-force apparatus experiments.  We pay particular attention to how these effects suggest a novel and simple interpretation of the relevant physics of hydrophobicity at these scales.  Finally, we discuss the transition between these distinct driving forces to the well-studied mechanisms of hydrophobicity in the short-range regime.   

1.         Israelachvili, J. and R. Pashley, The hydrophobic interaction is long range, decaying exponentially with distance. Nature, 1982. 300(5890): p. 341-342.

2.         Hammer, M.U., et al., The Search for the hydrophobic force law. Faraday Discussions, 2010. 146: p. 299-308.