An extensive series of neutron diffraction experiments and molecular dynamics simulations has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol mole fraction between 0.27 and 0.54) where both methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin.
We have investigated the hydrogen-bonded structures in liquid methanol and a 7:3 mole fraction aqueous solution using classical Molecular Dynamics simulations at 298K and ambient pressure. We find that, in contrast to recent predictions from X-ray emission studies, the hydrogen-bonded structure in liquid methanol is dominated by chain and small ring structures. In the methanol-rich solution, we find evidence of micro-immiscibility, supporting recent conclusions derived from neutron diffraction data.
Monte Carlo simulations are used to predict the adsorption isotherms at 300 and 600 K for binary and ternary mixtures of linear, branched, and cyclic alkanes in silicalite-1, AlPO 4 -5, and the recently synthesized ITQ-22. The theoretical binary and ternary adsorption isotherms predicted by Ideal Adsorption Solution Theory (IAST) agree well with the simulated isotherms. Increasing the temperature altered the adsorption hierarchy, with the adsorption of cyclic molecules increasing as the linear and, to a greater extent, branched molecules decreased. A microscopic analysis of the adsorption locations and molecular conformations provides an explanation for the change in the selectivity of adsorption with temperature.
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