Yura V. Kalyuzhnyi
Institute for Condensed Matter Physics, Svientsitskoho 1, 290011,
Lviv, Ukraine.
Vojko Vlachy
Faculty of Chemistry and Chemical Technology, University of Ljubljana,
Aškerčeva 5, 1000 Ljubljana, Slovenia.
Ken A. Dill
Department of Pharmaceutical Chemistry and Graduate Group in Biophysics,
University of California, San Francisco, California 94143-1204,
U.S.A.
Abstract
Ions are often classied as `kosmotropes' or `chaotropes' depending
on whether
they order or disorder water, as determined by how much they change
the vis-
cosity ofwater. Collins has proposed that water ordering by an ion
depends on
its charge density, using a simple fused-sphere model (K. D. Collins,
Biophys.
J., 1997 , 72, 65-76). To make his idea more quantitative, we have
applied
the site-site integral equation of Chandler, Silbey and Ladanyi to
obtain the
potential of the mean force between ions and water molecules. From
the po-
tential of mean force curves we obtain the activation energy for transferring
a water molecule from near the ion to the bulk. The results for this
quantity
are in semi-quantitative agreement with experimental data for alkali
halides
and with theoretical results based on the TIP3P model of water.