Ion Pairing in Molecular Simulations of Aqueous Alkali Halide Solutions

Christopher J. Fennell*, Alan Bizjak, Vojko Vlachy and Ken A. Dill
Department of Pharmaceutical Chemistry, University of California at San Francisco, San Francisco, California 94143, and Faculty of Chemistry and Chemical Technology, University of Ljubljana, 1000 Ljubljana, Slovenia
J. Phys. Chem. B, 2009, 113 (19), pp 6782–6791
DOI: 10.1021/jp809782z
Publication Date (Web): February 10, 2009
Copyright © 2009 American Chemical Society
* Corresponding author. E-mail: cfennell@maxwell.compbio.ucsf.edu., †

University of California at San Francisco.

, ‡

University of Ljubljana.

Abstract

Abstract Image

Using classical molecular dynamics simulations, we study ion−ion interactions in water. We study the potentials of mean force (PMF) for the full set of alkali halide ion pairs, and in each case, we test different parameter sets for modeling both the water and the ions. Altogether, we compared 300 different PMFs. We also calculate association equilibrium constants (KA) and compare them to two types of experiments. Of additional interest here was the proposition of Collins called the “law of matching water affinities”, where the relative affinity of ions in solution depends on the matching of cation and anion sizes. From observations on the relative depths of the free energies of the contact ion pair (CIP) and the solvent-shared ion pair (SIP), along with related solvent structure analyses, we find a good correlation with this proposition: small−small and large−large should associate in water, and small−large should be more dissociated.

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History

  • Published In Issue May 14, 2009
  • Article ASAPFebruary 10, 2009
  • Received: November 6, 2008
    Revised: December 5, 2008

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