Charge/mass dynamic structure factors of water and applications to dielectric friction and electroacoustic conversion.

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.

Solvation thermodynamics and heat capacity of polar and charged solutes in water.

The solvation thermodynamics and in particular the solvation heat capacity of polar and charged solutes in water is studied using atomistic molecular dynamics simulations. As ionic solutes we consider a F(-) and a Na(+) ion, as an example for a polar molecule with vanishing net charge we take a SPC/E water molecule. The partial charges of all three solutes are varied in a wide range by a scaling factor. Using a recently introduced method for the accurate determination of the solvation free energy of polar solutes, we determine the free energy, entropy, enthalpy, and heat capacity of the three different solutes as a function of temperature and partial solute charge. We find that the sum of the solvation heat capacities of the Na(+) and F(-) ions is negative, in agreement with experimental observations, but our results uncover a pronounced difference in the heat capacity between positively and negatively charged groups. While the solvation heat capacity ΔC(p) stays positive and even increases slightly upon charging the Na(+) ion, it decreases upon charging the F(-) ion and becomes negative beyond an ion charge of q = -0.3e. On the other hand, the heat capacity of the overall charge-neutral polar solute derived from a SPC/E water molecule is positive for all charge scaling factors considered by us. This means that the heat capacity of a wide class of polar solutes with vanishing net charge is positive. The common ascription of negative heat capacities to polar chemical groups might arise from the neglect of non-additive interaction effects between polar and apolar groups. The reason behind this non-additivity is suggested to be related to the second solvation shell that significantly affects the solvation thermodynamics and due to its large spatial extent induces quite long-ranged interactions between solvated molecular parts and groups.

The spontaneous curvature of the water-hydrophobe interface.

The temperature-dependent solvation of hydrophobic solutes in water is investigated by large-scale molecular dynamics simulations. A simultaneous fit of solvation free energies for spheres and cylinders with radii up to R = 2 nm yields a negative Tolman length on the order of 1 Å at room temperature, equivalent to a spontaneous curvature that favors water droplets over cavities. Pronounced crossover effects of the surface free energy are analyzed in terms of higher-order curvature corrections and water-discreteness effects.

Ultralow liquid/solid friction in carbon nanotubes: comprehensive theory for alcohols, alkanes, OMCTS, and water.

In this work, we perform a theoretical study of liquid flow in graphitic nanopores of different sizes and geometries. Molecular dynamics flow simulations of different liquids (water, decane, ethanol, and OMCTS) in carbon nanotubes (CNT) are shown to exhibit flow velocities 1-3 orders of magnitude higher than those predicted from the continuum hydrodynamics framework and the no-slip boundary condition. These results support previous experimental findings obtained by several groups that reported exceptionally high liquid flow rates in CNT membranes. The liquid/graphite friction coefficient is identified as the crucial parameter for this fast mass transport in CNT. The friction coefficient is found to be very sensitive to wall curvature: friction is independent of confinement for liquids between flat graphene walls with zero curvature, whereas it decreases with increasing positive curvature (liquid inside CNT), and it increases with increasing negative curvature (liquid outside CNT). Furthermore, we present a theoretical approximate expression for the friction coefficient, which predicts qualitatively and semiquantitatively its 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 of the ultra low liquid/solid friction in CNT. In fact, it is due to their perfectly ordered molecular structure and their atomically smooth surface that carbon nanotubes are quasiperfect liquid conductors compared to other membrane pores like nanochannels in amorphous silica.

Hydration repulsion between biomembranes results from an interplay of dehydration and depolarization.

Hydration repulsion dominates the interaction between polar surfaces in water at nanometer separations and ultimately prevents the sticking together of biological matter. Although confirmed by a multitude of experimental methods for various systems, its mechanism remained unclear. A simulation technique is introduced that yields accurate pressures between solvated surfaces at prescribed water chemical potential and is applied to a stack of phospholipid bilayers. Experimental pressure data are quantitatively reproduced and the simulations unveil a rich microscopic picture: Direct membrane-membrane interactions are attractive but overwhelmed by repulsive indirect water contributions. Below about 17 water molecules per lipid, this indirect repulsion is of an energetic nature and due to desorption of hydration water; for larger hydration it is entropic and suggested to involve water depolarization. This antagonistic nature and the presence of various compensating contributions indicate that the hydration repulsion is less universal than previously assumed and rather involves finely tuned surface-water interactions.

Friction contribution to water-bond breakage kinetics.

Based on the trajectories of the separation between water molecule pairs from MD simulations, we investigate the bond breakage dynamics in bulk water. From the spectrum of mean first-passage times, the Fokker-Planck equation allows us to derive the diffusivity profile along the separation coordinate and thus to unambiguously disentangle the effects of free-energy and local friction on the separation kinetics. For tightly coordinated water, the friction is six times higher than in bulk, which can be interpreted in terms of a dominant reaction path that involves additional orthogonal coordinates.

Entropy and enthalpy convergence of hydrophobic solvation beyond the hard-sphere limit.

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.

Spatial correlations of density and structural fluctuations in liquid water: a comparative simulation study.

We use large-scale classical simulations employing different force fields to study spatial correlations between local density and structural order for water in the liquid temperature range. All force fields investigated reproduce the main features of the experimental SAXS structure factor S(q), including the minimum at small q, and the recent TIP4P/2005 parametrization yields almost quantitative agreement. As local structural order parameters we consider the tetrahedrality and the number of hydrogen bonds and calculate all pure and mixed spatial two-point correlation functions. Except for the density-density correlation function, there are only weak features present in all other correlation functions, showing that the tendency to form structural clusters is much weaker than the well-known tendency of water to form density clusters (i.e., spatially correlated regions where the density deviates from the mean). In particular, there are only small spatial correlations between local density and structural fluctuations, suggesting that features in density-density correlations (such as measured by the structure factor) are not straightforwardly related to spatial correlations of structure in liquid water.

Molecular origin of fast water transport in carbon nanotube membranes: superlubricity versus curvature dependent friction.

In this paper, we study the interfacial friction of water at graphitic interfaces with various topologies, water between planar graphene sheets, inside and outside carbon nanotubes, with the goal to disentangle confinement and curvature effects on friction. We show that the friction coefficient exhibits a strong curvature dependence; while friction is independent of confinement for the graphene slab, it decreases with carbon nanotube radius for water inside, but increases for water outside. As a paradigm the friction coefficient is found to vanish below a threshold diameter for armchair nanotubes. Using a statistical description of the interfacial friction, we highlight here a structural origin of this curvature dependence, mainly associated with a curvature-induced incommensurability between the water and carbon structures. These results support the recent experiments reporting fast transport of water in nanometric carbon nanotube membranes.

On the origin of the hydrophobic water gap: An X-ray reflectivity and MD simulation study.

The density deficit of water at hydrophobic interfaces, frequently called the hydrophobic gap, has been the subject of numerous experimental and theoretical studies in the past decade. Recent experiments give values for the interfacial depletion that consistently correspond to less than a monolayer of water. The main question which remained so far unanswered is its origin and the mechanisms affected by the chemistry and molecular geometry of a particular hydrophobic coating. In this work, we present 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. A comparison of our experimental and computational results elucidates the underlying mechanisms that affect the extent of the interfacial depletion. The complex interplay between surface chemistry and topography precludes the existence of a direct and universal relation between the macroscopic contact angle and the nanoscopic water depletion.

Nanoroughness, intrinsic density profile, and rigidity of the air-water interface.

From large-scale molecular dynamics simulations of the free water surface the capillary wave spectrum is obtained and analyzed in terms of a wave vector dependent surface tension. The bending rigidity is positive but suggested to change sign in the presence of long-range dispersive interactions. Nanoroughness and capillary waves give rise to a scale-dependent density profile that displays minimal broadening at a nanometer scale which defines the intrinsic density profile.

Water at polar and nonpolar solid walls.

Recent progress in simulating the properties of interfacial water at hard hydrophobic and hydrophilic surfaces is reviewed and compared to results for the air/water interface. The authors discuss static properties such as the equilibrium contact angle, the depletion layer thickness, and the orientation of interfacial water molecules. Relations between these properties, e.g., the relation between the contact angle and the thickness of the depletion layer which is experimentally observed on hydrophobic surfaces, are emphasized. For a hydrophilic sapphire surface, the authors discuss the influence of geometry and density of polar surface groups on the interfacial water structure. They discuss nonequilibrium effects arising in laminar shear flows, where the classic no-slip hydrodynamic boundary condition is violated at hydrophobic interfaces. They discuss the arising slip and relate it to static properties of the solid hydrophobic/water interface.