A molecular dynamics model for clays and the oxide minerals is
desirable for studying the kinetics and
thermodynamics of adsorption processes. To this end, a valence
force field for aluminous, dioctahedral clay
minerals was developed. Novel aspects of this development include
the bending potential for octahedral
O−Al−O angles, which uses a quartic polynomial to create a
double-well potential with minima at both 90°
and 180°. Also, atomic point charges were derived from
comparisons of ab initio molecular electrostatic
potentials with X-ray diffraction-based deformation electron densities.
Isothermal−isobaric molecular dynamics
simulations of quartz, gibbsite, kaolinite, and pyrophyllite were used
to refine the potential energy parameters.
The resultant force field reproduced all the major structural
parameters of these minerals to within 1% of
their experimentally determined values. Transferability of the
force field to simulations of adsorption onto
clay mineral surfaces was tested through simulations of
Na+, Ca2+, and
hexadecyltrimethylammonium
(HDTMA+) in the interlayers of beidellite clays. The
new force field worked rather well with independently
derived nonbonded parameters for all three adsorbates, as indicated by
comparisons between experimental
and molecular-dynamics-predicted d
(001) layer
spacings of the homoionic beidellites.
We present simulation results for long (N ≤ 4000) self-avoiding walks in four dimensions. We find definite indications of logarithmic corrections, but the data are poorly described by the asymptotically leading terms. Detailed comparisons are presented with renormalization group flow equations derived in direct renormalization and with results of a field theoretic calculation.
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