Long-distance correlations in molecular orientations of liquid water and shape-dependent hydrophobic force

Liquid water, at ambient conditions, has short-range density correlations which are well known in literature. Surprisingly, large scale molecular-dynamics simulations reveal an unusually long-distance correlation in "longitudinal" part of dipole-dipole orientational correlations. It is nonvanishing even at 75 A and falls off exponentially with a correlation length of about 24 A beyond solvation region. Numerical evidence suggests that the long-range nature of dipole-dipole correlation is due to underlying fluctuating network of hydrogen bonds in the liquid phase. This correlation is shown to give a shape dependant attraction between two hydrophobic surfaces at large distances of separation and the range of this attractive force is in agreement with experiments. In addition it is seen that quadrupolar fluctuations vanish within the first solvation peak (3 A) .