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Physical chemistry chemical physics : PCCP

Structure simulations for the 0.22 and 1 molar aqueous dimethylammonium chloride solutions.


PMID 22108699

Abstract

Monte Carlo simulations have been performed for characterizing the 0.22 and 1 molar aqueous dimethylammonium chloride solutions at p = 1 atm and T = 310 K. On the basis of potential of mean force curves for the two systems with increasing concentrations, the N···N separations of about 8.7 and 7.5 Å correspond to a vague and a more pronounced minimum, respectively. Nitrogen separations at the minima are considerably smaller than those what the cations would take if the solutes comprised uniform local solute density on a microscale. The derived N by N coordination numbers predict non-negligible cation association and concomitant inhomogeneity for the studied systems. The calculated N···N distance distribution in the molar solution indicates that about 12% of the N···N separations are shorter than 8.5 Å compared with R(N···N) = 11.84 Å corresponding to the closest distance in a uniform cation local density. Despite a global minimum of -1.79 ± 0.63 kcal mol(-1) at N···Cl separation of 3.24 Å, obtained from the pmf for the 0.22 molar model, the N by Cl coordination number is only 0.14 in the first coordination shell, suggesting frequent disruption of an N-H(+)···Cl(+) hydrogen bond in a relatively dilute solution. The expression for the chemical potential of a solute includes a concentration-dependent activity coefficient, whose varying values are expected to reflect the different degrees of solute association in the 0.2-1 molar range. Theoretical follow-up of the changes in the activity coefficient values is difficult, thus calculation of the K(c) equilibrium constant has been proposed by considering 1 molar solutions as the standard state.

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