We manifest a significant influence of field direction and polarity on surface wetting, when the latter is tuned by application of an external electric field. Thermodynamics of field-induced filling of hydrocarbon-like nanopores with water is studied by open ensemble molecular simulation. Increased field strength consistently results in water-filling and electrostriction in hydrophobic nanopores. A threshold field commensurate with surface charge density of about one elementary charge per 10 nm2 suffices to render prototypical paraffin surfaces hydrophilic. When a field is applied in the direction perpendicular to the confining walls, the competition between orientational polarization and angle preferences of interfacial water molecules relative to the walls results in an asymmetric wettability of opposing surfaces (Janus interface). Reduction of surface free energy observed upon alignment of confinement walls with field direction suggests a novel mechanism whereby the applied electric field can operate selectively on water-filled nanotubes while empty ones remain unaffected.

We use atomistic simulations to address the question when capillary evaporation of water confined in a hydrocarbonlike slit is kinetically viable. Activation barriers and absolute rates of evaporation are estimated using open ensemble Monte Carlo-umbrella sampling and molecular dynamics simulations. At ambient conditions, the evaporation rate in a water film four molecular diameters thick is found to be of the order 10(5)(nm(2) s)(-1), meaning that water readily evaporates. Films more than a few nanometers thick will persist in a metastable liquid state. Dissolved atmospheric gas molecules do not significantly decrease the activation barrier.

Using molecular simulations of nanosized aqueous droplets on a model graphite surface, we demonstrate remarkable sensitivity of water contact angles to the applied electric field polarity and direction relative to the liquid/solid interface. The effect is explained by analyzing the influence of the field on interfacial hydrogen bonding in the nanodrop, which in turn affects the interfacial tensions. The observed anisotropy in droplet wetting is a new nanoscale phenomenon that has so far been elusive as, in current experimental setups, surface molecules represent a very low fraction of the total number affected by the field. Our findings may have important implications for the design of electrowetting techniques in fabrication and property tuning of nanomaterials.

Molecular simulation is used to elucidate hydrophobic interaction at atmospheric pressure where liquid water between apolar walls is metastable with respect to capillary evaporation. The steep increase of the estimated activation barrier of evaporation with surface-surface separation explains the apparent stability of the liquid at distances more than an order of magnitude below the thermodynamic threshold of evaporation. Solvation by metastable liquid results in a short-ranged oscillatory repulsion which gives rise to an irreversible potential barrier between approaching walls. The barrier increases with external pressure in accord with measured pressure-induced slowing of conformational transitions of biopolymers with strong hydrophobic interactions. At a sufficiently small separation, the force abruptly turns attractive signaling nucleation of the vapor phase. This behavior is consistent with the cavitation-induced hysteresis observed in a number of surface-force measurements for strongly hydrophobic surfaces at ambient conditions.

In an aqueous electrolyte solution, the potential of mean force (PMF) for two macroions is affected not only by the size and charge of each electrolyte ion but also by the ion's polarizability. The Lifshitz theory provides a basis for calculating the van der Waals interaction between cation-colloid, anion-colloid, cation-cation, and anion-anion pairs. Monte Carlo simulations are used to determine how salt identity affects the PMF between colloidal particles or globular proteins in a saline solution, a phenomenon observed experimentally by Hofmeister for aqueous proteins more than 100 years ago. The calculations show that the PMF and, hence, solution phase behavior are sensitive to the van der Waals interaction between an ion and a macroion. The calculations described here may be useful for interpretation of experimental phase diagrams and for guiding design of separation processes where a salt is used to induce colloid or protein precipitation.

How colloidal particles interact with each other is one of the key issues that determines our ability to interpret experimental results for phase transitions in colloidal dispersions and our ability to apply colloid science to various industrial processes. The long-accepted theories for answering this question have been challenged by results from recent experiments. Herein we show from Monte-Carlo simulations that there is a short-range attractive force between identical macroions in electrolyte solutions containing divalent counterions. Complementing some recent and related results by others, we present strong evidence of attraction between a pair of spherical macroions in the presence of added salt ions for the conditions where the interacting macroion pair is not affected by any other macroions that may be in the solution. This attractive force follows from the internal-energy contribution of counterion mediation. Contrary to conventional expectations, for charged macroions in an electrolyte solution, the entropic force is repulsive at most solution conditions because of localization of small ions in the vicinity of macroions. Both Derjaguin-Landau-Verwey-Overbeek theory and Sogami-Ise theory fail to describe the attractive interactions found in our simulations; the former predicts only repulsive interaction and the latter predicts a long-range attraction that is too weak and occurs at macroion separations that are too large. Our simulations provide fundamental ''data'' toward an improved theory for the potential of mean force as required for optimum design of new materials including those containing nanoparticles.The potential of mean force between colloidal particles in electrolyte solutions plays a central role in describing the phase behavior and the kinetics of agglomeration in colloidal dispersions (1, 2). It is of fundamental importance for understanding the properties of inorganic materials (e.g., ceramics composed of nanoparticles), foods such as milk, and solutions of biomacromolecules including globular proteins (3-5). After decades of theoretical and experimental efforts, some aspects of colloidal interactions remain puzzling, in particular the issue of attractive electrostatic forces between like-charged colloidal particles in an electrolyte solution. Experimental evidence for such attraction has been indirect and largely complicated by boundary or polydispersity effects (6-8). Classical theories are not satisfactory because they are based on the mean-field approximation that neglects the effects of excluded volume and Coulombic correlations among small ions. Depending on simplifying assumptions, these theories have led to qualitatively different results. For example, theories based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) approximation describe the electrostatic interaction between macroions of the same charge as screened repulsion (9-10, 12, 13), and others, represented by Sogami-Ise (SI) theory, predict a universal long-range attractive interaction (11,14). Likewise, the possibil...

Monte Carlo simulations of symmetric and asymmetric angular model liquids J. Chem. Phys. 114, 9075 (2001); 10.1063/1.1353551From polypeptide sequences to structures using Monte Carlo simulations and an optimized potential A new technique for Monte Carlo sampling of the hard-sphere collision force has been applied to study the interaction between a pair of spherical macroions in primitive-model electrolyte solutions with valences 1:2, 2:1, and 2:2. Macroions of the same charge can attract each other in the presence of divalent counterions, in analogy with earlier observations for planar and cylindrical geometries. The attraction is most significant at intermediate counterion concentrations. In contrast to the entropic depletion force between neutral particles, attraction between macroions is of energetic origin. The entropic contribution to the potential of mean force is generally repulsive at conditions corresponding to aqueous colloids with or without salt. For systems with divalent counterions, the potentials of mean force predicted by mean-field approximations like the Derjaguin-LandauVerwey-Overbeek ͑DLVO͒ theory or the Sogami-Ise ͑SI͒ theory are qualitatively different from those observed in the simulations. However, for systems with monovalent counterions, predictions of DLVO theory are in fair agreement with simulation results.

Surface free energy of a chemically heterogeneous surface is often treated as an approximately additive quantity through the Cassie equation [Cassie ABD (1948) Discuss Faraday Soc 3:11-16]. However, deviations from additivity are common, and molecular interpretations are still lacking. We use molecular simulations to measure the microscopic analogue of contact angle, θ c , of aqueous nanodrops on heterogeneous synthetic and natural surfaces as a function of surface composition. The synthetic surfaces are layers of graphene functionalized with prototypical nonpolar and polar head group: methyl, amino, and nitrile. We demonstrate positive as well as negative deviations from the linear additivity. We show the deviations reflect the uneven exposure of mixture components to the solvent and the linear relation is recovered if fractions of solvent-accessible surface are used as the measure of composition. As the spatial variations in polarity become of larger amplitude, the linear relation can no longer be obtained. Protein surfaces represent such natural patterned surfaces, also characterized by larger patches and roughness. Our calculations reveal strong deviations from linear additivity on a prototypical surface comprising surface fragments of melittin dimer. The deviations reflect the disproportionately strong influence of isolated polar patches, preferential wetting, and changes in the position of the liquid interface above hydrophobic patches. Because solvent-induced contribution to the free energy of surface association grows as cos θ c , deviations of cos θ c from the linear relation directly reflect nonadditive adhesive energies of biosurfaces. wetting free energy | surface functionalization | nanopatterning | Cassie relation | biointeractions W etting phenomena on chemically heterogeneous surfaces are important in material sciences and biology, in examples ranging from inkjet printing to protein hydration (1, 2). Conventional metrics of surface interactions, designed for homogeneous systems, can often be applied to mixed surfaces characterized by averaged properties of multiple ingredients. The design of composite surface materials and characterization of biosurfaces benefit from combining rules predicting the interfacial free-energy change of wetting, Δγ, from the knowledge about individual constituents and surface composition. In view of the Young equation, Δγ ¼ −γ cos θ c , the strength of intersolute adhesion, W a ∼ −2ðΔγ þ γ sg Þ, relates to contact angle θ c ; here, γ and γ sg denote surface tensions of the solvent and dry solute, respectively (3). Contact angles on macroscopic heterogeneous surfaces are commonly estimated by the Cassie equation (4, 5),[1]developed by assuming linear additivity of the wetting free energies, Δγ. Here, f α is the projected fractional area occupied by component α, θ α is the contact angle on a homogeneous surface of type α, and r α is the Wenzel roughness factor (6), which can be defined as the ratio of solvent-exposed areas of patch of type α in the mixture and that of a ...