Yura V. Kalyuzhnyi
Institute for Condensed Matter Physics, Svientsitskoho 1, 290011, Lviv, Ukraine.
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.
Ions are often classied as `kosmotropes' or `chaotropes' depending
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.