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       SEMINAR OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS
        Institute for Computational Sciences and Informatics
                              CSI 898-Sec 001  
              (http://www.csi.gmu.edu/lcdm/seminar/schedule.html)
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     The Friction Coefficients of Ions in Aqueous Solutions

                         J.C Rasaiah

Departments of Chemistry and Physics, University of Maine, Orono, and
        Biotechnology Division, N.I.S.T., Gaithersburg

The computation of the diffusion coefficients of ions in aqueous
solution at infinite dilution by Molecular Dynamics (MD) simulations
using a simple-point charge(SPC/E) model for water will be discussed.
They exhibit a maximum as a function of ion size with distinct curves
for positive and negative ions at 25C in qualitative agreement with 
experiment.
I will discuss explanations of this at a microscopic level in terms
of the structure and dynamics of water and the friction coefficient
of the ion divided into hydrodynamic and dielectric components studied
initially by Max Born followed by Robert Zwanzig, Lars Onsager and
Joseph Hubbard using continuum models for the solvent.
Molecular theories due to Peter Wolynes and Biman Bagchi calculate
the friction from the integral of the random-force auto correlation
function by separating this into hard and soft components with the 
neglect of cross terms.
However, MD simulations show that the cross terms are significant,
suggesting that the dielectric and hydrodynamic friction calculated
with this separation are less accurate than previously supposed.
A new method to determine, in an MD simulation, the free energy and
entropy of solvation as a continuous function of the size and charge
of the solute by using an extended Lagrangian will be described.
Using this, it is found that the solvation entropy and dynamics are
correlated with ion mobolity in aqueous solutions. Supercritical
conditions and confinement in channels changes this correlation.
The difficulties of formulating a molecular theory of ion mobolity
in hydrogen bonded solvents will be briefly discussed.

	               Monday , November 23 1998
                               4:30 pm
                       Room 206, Science & Tech. I
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