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Theoretical aspects (365-369)
COLL
365: Calculation of electro-optical properties of rigid particles
and flexible macromolecules in solution
J. Garcia de la Torre, H.E. Perez Sanchez, A. Ortega, M.X.
Fernandes, M.C. Lopez Martinez, and F.G. Diaz, Departamento de Quimica
Fisica, Universidad de Murcia, 30071 Murcia, Spain, Fax: 34 968 364148,
jgt@um.es
Abstract
For rigid macromolecules of arbitrary shape, the time-dependence of
transient electro-optical properties depends on the rotational diffusion
tensor of the particle. We have developed computational methodologies,
based on hydrodynamic theory of dilute particles, that allow the
prediction of rotational coefficients and relaxation (re-orientational)
times that can be employed to predict electro-optical transients. These
methods are implemented in various public-domain programs that consider
different formats for the structural data. For flexible macromolecules, we
employ Brownian dynamics simulation and Monte Carlo procedures.
Steady-state simulations permit the calculation of equilibrium properties
in fields of arbitrary strength, and dynamics simulations add more
information concerning the dependence of the relaxation rate on field
strength and flexibility. Furthermore, these simulations can be used to
differentiate possible flexibility mechanisms: for instance, it may be
possible to ascertain whether the a flexible rodlike macromolecule behaves
as according to either the wormlike model or the broken-rod model.
COLL
366: Electrical polarizability of polyelectrolytes by Monte Carlo
simulation
Kazuo Kikuchi, Department of Life Sciences (Chemistry), Graduate
School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku,
Tokyo 153-8902, Japan, Fax: +81-3-5454-6998, ckikuch@mail.ecc.u-tokyo.ac.jp
Abstract
Anisotropy of the electrical polarizability of model DNA fragments in
salt-free aqueous solutions was determined by Monte Carlo simulation.
According to the fluctuation-dissipation theorem, the electrical
polarizability of polyelectrolytes is related to the fluctuations of the
dipole moment generated in the counterion atmosphere around the polyion in
the absence of an applied electric field. We could give a definition of
condensed counterions for charged oligomers based on the simulation and
the fraction of condensed counterions so determined approached Manning's
theoretical value as the molecular weight of polyelectrolytes increased.
Characteristic features of the electric properties of polyelectrolytes
were reproduced. The anisotropy of the electrical polarizability of DNA in
salt-free solution increased on dilution of the polymer concentration and
was proportional to the second or higher power of the molecular weight
consistent with experiment.
COLL
367: Microion disposition in colloidal systems
Kenneth S. Schmitz, Department of Chemistry, University of Missouri
- Kansas City, 5105 Rockhill Road, Kansas City, MO 64110, schmitzks@umkc.edu
Abstract
Brownian dynamics simulations of the microions were performed for two
similar macroion clusters (a 7-particle diamond shape array or an
8-particle simple cubic array) at the volume fraction of 0.01. It was
found that as the two clusters approached each other the diamond cluster
system became less stable whereas the simple cubic cluster system became
more stable. This difference in behavior is attributed to the relative
abilities of these structures to "share" counterions.
Implications regarding the dynamics of such systems are discussed.
COLL
368: Hydration interactions between apoferritin molecules and the
phase behavior of the solution
Peter G. Vekilov, Dimiter N. Petsev, Simon Brandon, and Panagyotis
Katsonis, Department of Chemical Engineering, University of Houston,
Houston, TX 77204-4004, Fax: 713-743-4323, vekilov@uh.edu
Abstract
We link the specificity of phase behavior – lack of liquid-liquid
separation and temperature-independent solubility – to intermolecular
interactions in the solution characterized using light scattering. Results
indicate in the presence of Na+ and Cd2+ ions the
intermolecular interaction potential has a repulsive part due to Na+
assisted hydration sphere build-up at separations between 0.5 and 3 nm,
and a short range (<0.2 - 0.1 nm) attractive part due to Cd2+
mediated bonds. To link the found structured potential to the phase
diagram we carried out Monte Carlo simulations, using Gibbs-Duhem and
Gibbs Ensemble techniques, with an intermolecular potential having a
minimum and a local maximum at separations longer than those of the
minimum. Increasing the height of the maximum resulted in steeper liquidus
lines, and eventually, in temperature-independent solubility. Another
consequence of the increasing maxima was the shift of the liquid-liquid
separation line to lower temperatures, not accessible in an experiment.
COLL
369: Alpha-dispersion of induced dipole moment of the spheroidal
particle
Vladimir N. Shilov, Macrokinetic of the Disperse System, Institute
of Biocolloid Chemistry, Ukrainian Academy of Sciences, Kiev, Ukraine, 42
Vernadskogo, Kiev 03142, Ukraine, shilov@i.kiev.ua
Abstract
Alpha-dispersion of electrooptic effect is caused by concentration
polarization of the double layers of colloid particles, and its
theoretical description requires the solution of non-stationary diffusion
equations. Since these equations do not separate in spheroidal coordinates
no theoretical results exist for suspensions of spheroid-shaped particles.
We sidestepped this problem by relating the characteristics of a
stationary particle's polarization with the characteristic time of
low-frequency relaxation of the induced dipole moment. The relations were
derived from the comparison of two expressions for suspension dielectric
permittivity: one obtained from the summation of dipole fields and the
other from the integration of the density of stored energy. Application of
this method permits analytical calculation of the low-frequency dependence
of the anisotropy of induced dipole moment, controlled by the volume
diffusion of ions in the vicinity of charged spheroids with arbitrary
axial ratio and with arbitrary magnitude of the surface conductivity.
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