The Journal of Chemical Physics -- December 15, 1995 -- Volume 103, Issue
23, pp. 10252-10266
Computer simulation of liquid/liquid interfaces. I. Theory and application
to octane/water
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Yuhong Zhang and Scott E. Feller
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Biophysics Laboratory, Center for Biologics Evaluation
and Research, Food and Drug Administration, 1401 Rockville Pike, Rockville,
Maryland 20852-1448
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Bernard R. Brooks
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Laboratory of Structural Biology, Division of Computer
Research and Technology, National Institutes of Health, Bethesda, Maryland
20892
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Richard W. Pastor
-
Biophysics Laboratory, Center for Biologics Evaluation
and Research, Food and Drug Administration, 1401 Rockville Pike, Rockville,
Maryland 20852-1448
Statistical ensembles for simulating liquid interfaces at constant
pressure and/or surface tension are examined, and equations
of motion for molecular dynamics are obtained by various extensions
of the Andersen extended system approach. Valid ensembles include:
constant normal pressure and surface area; constant tangential
pressure and length normal to the interface; constant volume
and surface tension; and constant normal pressure and surface tension.
Simulations at 293 K and 1 atm normal pressure show consistent
results with each other and with a simulation carried out at
constant volume and energy. Calculated surface tensions for
octane/water (61.5 dyn/cm), octane/vacuum (20.4 dyn/cm) and water/vacuum
(70.2 dyn/cm) are in very good agreement with experiment (51.6,
21.7, and 72.8 dyn/cm, respectively). The practical consequences
of simulating with two other approaches commonly used for isotropic
systems are demonstrated on octane/water: applying equal normal
and tangential pressures leads to an instability; and applying
a constant isotropic pressure of 1 atm leads to a large positive
normal pressure. Both results are expected for a system of nonzero
surface tension. Mass density and water polarization profiles
in the liquid/liquid and liquid/vapor interfaces are also compared.