Computer simulation of topological and spatial structure in water and aqueous solutions

University dissertation from Stockholm : Stockholm University

Abstract: The present thesis concerns molecular-dynamics and expanded-ensemble simulation of topological and spatial structure in water and aqueous solutions.First, methods for analysis of molecular pair configurations using angularly resolved density distributions were considered. It was shown that these distributions expose major features of liquid structure and that they can be used to define the hydrogen bond. Second, a method was developed for analysis of hydrogen-bond network topology. It characterizes networks in terms of the local bond patterns surrounding the water molecules and in terms of loops and chains of directed bonds.The methods developed have been applied to pure water, to water--acetonitrile mixtures and to charged Lennard-Jones spheres dissolved in water. Solvation structures in aqueous solutions of methylamine and tert-butyl alcohol have also been considered. The main results are: (a) In pure water, there is proton ordering around short loops, but not along chains. (b) The hydrogen-bond network in water--acetonitrile mixtures with acetonitrile mole fractions x=0.1, 0.5 and 0.9 have been characterized. As x increases from 0.1 to 0.9, the network is depleted of crosslinks, the proton ordering along chains increases and the most probable loop length decreases. For x=0.5 and 0.9 large water clusters form. (c) The hydration structures surrounding charged Lennard-Jones solutes corresponding to extrema in the solvation entropy have been characterized. Of the two solutes corresponding to the maxima, one acts as a double proton-donor and the other as a double proton-acceptor. The solute corresponding to the minimum enhances the water--water correlations. Further, the orientational and the radial parts of the two-body solute--water entropy have been calculated. The orientational part has a single maximum, whereas the radial part maintains the bimodal form of the full solvation entropy.

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