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We determine time correlation functions and dynamic structure factors of the number and charge density of liquid water from molecular dynamics simulations. Using these correlation functions we consider dielectric friction and electro-acoustic coupling effects via linear response theory. From charge-charge correlations, the drag force on a moving point charge is derived and found to be maximal at a velocity of around 300 m/s. Strong deviations in the resulting friction coefficients from approximate theory employing a single Debye relaxation mode are found that are due to non-Debye-like resonances at high frequencies. From charge-mass cross-correlations the ultrasonic vibration potential is derived, which characterizes the conversion of acoustic waves into electric time-varying potentials. Along the dispersion relation for normal sound waves in water, the ultrasonic vibration potential is shown to strongly vary and to increase for larger wavelengths. [ABSTRACT FROM AUTHOR]

The experimentally well-known convergence of solvation entropies and enthalpies of different small hydrophobic solutes at universal temperatures seems to indicate that hydrophobic solvation is dominated by universal water features and not so much by solute specifics. The reported convergence of the denaturing entropy of a group of different proteins at roughly the same temperature as hydrophobic solutes was consequently argued to indicate that the denaturing entropy of proteins is dominated by the hydrophobic effect and used to estimate the hydrophobic contribution to protein stability. However, this appealing picture was subsequently questioned since the initially claimed universal convergence of denaturing entropies holds only for a small subset of proteins; for a larger data collection no convergence is seen. We report extensive simulation results for the solvation of small spherical solutes in explicit water with varying solute-water potentials. We show that convergence of solvation properties for solutes of different radii exists but that the convergence temperatures depend sensitively on solute-water potential features such as stiffness of the repulsive part and attraction strength, not so much on the attraction range. Accordingly, convergence of solvation properties is only expected for solutes of a homologous series that differ in the number of one species of subunits (which attests to the additivity of solvation properties) or solutes that are characterized by similar solute-water interaction potentials. In contrast, for peptides that arguably consist of multiple groups with widely disperse interactions with water, it means that thermodynamic convergence at a universal temperature cannot be expected, in general, in agreement with experimental results. [ABSTRACT FROM AUTHOR]

A combined high-energy X-ray reflectivity and molecular dynamics simulation study of the water depletion at a perfluorinated hydrophobic interface with a spatial resolution on the molecular scale is presented. The complex interplay between surface chemistry and topography is found to prevent the existence of a direct and universal relation between the macroscopic contact angle and the nanoscopic water depletion.

In this work, we perform a theoretical study of liquidflow ingraphitic nanopores of different sizes and geometries. Molecular dynamicsflow simulations of different liquids (water, decane, ethanol, andOMCTS) in carbon nanotubes (CNT) are shown to exhibit flow velocities1–3 orders of magnitude higher than those predicted from thecontinuum hydrodynamics framework and the no-slip boundary condition.These results support previous experimental findings obtained by severalgroups that reported exceptionally high liquid flow rates in CNT membranes.The liquid/graphite friction coefficient is identified as the crucialparameter for this fast mass transport in CNT. The friction coefficientis found to be very sensitive to wall curvature: friction is independentof confinement for liquids between flat graphene walls with zero curvature,whereas it decreases with increasing positive curvature (liquid insideCNT), and it increases with increasing negative curvature (liquidoutside CNT). Furthermore, we present a theoretical approximate expressionfor the friction coefficient, which predicts qualitatively and semiquantitativelyits curvature dependent behavior. The proposed theoretical description,which works well for different kinds of liquids (alcohols, alkanes,and water), sheds light on the physical mechanisms at the origin ofthe ultra low liquid/solid friction in CNT. In fact, it is due totheir perfectly ordered molecular structure and their atomically smoothsurface that carbon nanotubes are quasiperfect liquid conductors comparedto other membrane pores like nanochannels in amorphous silica. [ABSTRACT FROM AUTHOR]