We have studied mixtures of alcohol and water in an extensive series of 465 molecular-dynamics simulations with an aggregate length of 713 ns, in order to study excess properties of mixing, in particular the relation between mobility and viscosity. Methanol/water, ethanol/water, and 1-propanol/water mixtures were simulated using an alcohol content of 0-100 mass % in steps of 10%, using the OPLS ͑optimized potential for liquid simulations͒ force field for the alcohol molecules and the TIP4P ͑transferable intermolecular potential with four particles͒ water model. Computed densities and energies show very good agreement with experimental data for bulk simulations and the mixtures are satisfactory as well. The shear viscosity was computed using nonequilibrium molecular-dynamics simulations. Other properties studied include diffusion constants and rotational correlation times. We find the mobility to correlate well with the viscosity data, i.e., at intermediate alcohol concentrations the viscosity is maximal and the mobility is minimal. Furthermore, we have combined the viscosity and diffusion calculations in order to compute an effective hydrodynamic radius of the particles in the mixtures, using the Stokes-Einstein relation. This analysis indicates that there is no collective diffusion of molecular clusters in these mixtures. For all properties we find that the excess values are underestimated in the simulations, which, given that the pure liquids are described rather well, raises the question whether the potential function is too simplistic to describe mixtures quantitatively. The set of simulations presented here can hence be regarded as a force-field benchmark.

The potential of molecular dynamics (MD) simulation for the study and prediction of particle/particle and particle/wall interaction in the wide context of technology has been explored. The present study concerns the nature of adsorbed water and its effect on the interaction between two surfaces. Computer models of two opposing (1,0,-1) crystal surfaces of R-quartz (dimensions 5.49 × 4.91 nm) were constructed and up to 1500 water molecules positioned between the surfaces. The simulations were performed in the NVT ensemble in "math mode" at a temperature of 300 K. The axial profiles of density and mobility (the latter resolved in planar and axial components) in the adsorbed layers were studied. The separation between the crystal surfaces was varied, monitoring the adsorbed layer morphology and the forces acting on the crystals. Most of the simulations shown are with 1500 molecules between the plates, giving around 3.1 adsorbed monolayers, corresponding to a relative saturation (humidity) of 67% according to the BET isotherm. The density profiles show an ordered packing of molecules in the first two adsorbed layers with density peaks considerably higher than in bulk water and a low molecular mobility. The density tails off to zero, and the mobility rises to above that of bulk water at the surface of the adsorbed layer, which was clearly defined but undulating. Determination of the forces acting on the crystals was difficult due to strong fluctuations on a short time scale, so only simulations for long times yielded statistically significant average forces. At a surface separation of 3 nm, spontaneous bridge forming took place, paired with significant attractive forces between the crystals. The nature of the bridge is discussed. The observed bridging and resulting surface/surface force are shown to be roughly consistent with expectations based on macroscopic theory represented by the BET isotherm, the Kelvin equation (using the surface tension of bulk water), and a bridging force calculated from pressure-deficiency and surface tension contributions.

Why is it so hard to lift a wet glass from a table? Is it easier when there is whiskey between the glass and the table? Macroscopically, the picture is quite simple: two surfaces have to be disrupted that are connected indirectly through hydrogen bonds and/or van der Waals forces. In the beginning, a surface has to be created leading to surface tension, and after that a liquid bridge has to be broken. Here we study the phenomenon at the microscopic level using molecular dynamics simulations. The effective force between two quartz plates is measured at different distances and with different alcohol/water mixtures between them. This allows us to compute the total work necessary to "lift the glass from the table". Different aspects of the process, such as clustering and liquid ordering are discussed. We compare the structure of the liquid/glass interface to that of a liquid/vapor interface, for which we present simulation results, like surface tension, as well. On the basis of the simulations, we are able to provide a detailed description of the energetics during the separation process as a function of alcohol concentration. It is shown that there is a net entropy loss upon separating two plates with water or a 10% MeOH solution between them, whereas for higher alcohol concentrations, there is net entropy gain. These findings increase our understanding of the properties of colloid suspensions which is important for process technology.

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