136 research outputs found
Two-Gaussian excitations model for the glass transition
We develop a modified "two-state" model with Gaussian widths for the site
energies of both ground and excited states, consistent with expectations for a
disordered system. The thermodynamic properties of the system are analyzed in
configuration space and found to bridge the gap between simple two state models
("logarithmic" model in configuration space) and the random energy model
("Gaussian" model in configuration space). The Kauzmann singularity given by
the random energy model remains for very fragile liquids but is suppressed or
eliminated for stronger liquids. The sharp form of constant volume heat
capacity found by recent simulations for binary mixed Lennard Jones and soft
sphere systems is reproduced by the model, as is the excess entropy and heat
capacity of a variety of laboratory systems, strong and fragile. The ideal
glass in all cases has a narrow Gaussian, almost invariant among molecular and
atomic glassformers, while the excited state Gaussian depends on the system and
its width plays a role in the thermodynamic fragility. The model predicts the
existence of first-order phase transition for fragile liquids.Comment: 12 pages, 12 figure
Model Energy Landscapes of Low-Temperature Fluids: Dipolar Hard Spheres
An analytical model of non-Gaussian energy landscape of low-temperature
fluids is developed based on the thermodynamics of the fluid of dipolar hard
spheres. The entire excitation profile of the liquid, from the high
temperatures to the point of ideal-glass transition, has been obtained from the
Monte Carlo simulations. The fluid of dipolar hard spheres loses stability when
reaching the point of ideal-glass transition transforming via a first-order
transition into a columnar liquid phase of dipolar chains locally arranged in a
body-centered tetragonal order.Comment: 4 pages, 3 figure
Non-Gaussian statistics of electrostatic fluctuations of hydration shells
We report the statistics of electric field fluctuations produced by SPC/E
water inside a Kihara solute given as a hard-sphere core with a Lennard-Jones
layer at its surface. The statistics of electric field fluctuations, obtained
from numerical simulations, are studied as a function of the magnitude of a
point dipole placed close to the solute-water interface. The free energy
surface as a function of the electric field projected on the dipole direction
shows a cross-over with the increasing dipole magnitude. While it is a
single-well harmonic function at low dipole values, it becomes a double-well
surface at intermediate dipole moment magnitudes, transforming to a single-well
surface, with a non-zero minimum position, at still higher dipoles. A broad
intermediate region where the interfacial waters fluctuate between the two
minima is characterized by intense field fluctuations, with non-Gaussian
statistics and the variance far exceeding the linear-response expectations. The
excited state of the surface water is found to be lifted above the ground state
by the energy required to break approximately two hydrogen bonds. This state is
pulled down in energy by the external electric field of the solute dipole,
making it readily accessible to thermal excitations. The excited state is a
localized surface defect in the hydrogen-bond network creating a stress in the
nearby network, but otherwise relatively localized in the region closest to the
solute dipole
Solvated dissipative electro-elastic network model of hydrated proteins
Elastic netwok models coarse grain proteins into a network of residue beads
connected by springs. We add dissipative dynamics to this mechanical system by
applying overdamped Langevin equations of motion to normal-mode vibrations of
the network. In addition, the network is made heterogeneous and softened at the
protein surface by accounting for hydration of the ionized residues. Solvation
changes the network Hessian in two ways. Diagonal solvation terms soften the
spring constants and off-diagonal dipole-dipole terms correlate displacements
of the ionized residues. The model is used to formulate the response functions
of the electrostatic potential and electric field appearing in theories of
redox reactions and spectroscopy. We also formulate the dielectric response of
the protein and find that solvation of the surface ionized residues leads to a
slow relaxation peak in the dielectric loss spectrum, about two orders of
magnitude slower than the main peak of protein relaxation. Finally, the
solvated network is used to formulate the allosteric response of the protein to
ion binding. The global thermodynamics of ion binding is not strongly affected
by the network solvation, but it dramatically enhances conformational changes
in response to placing a charge at the active site of the protein
Electrostatic fluctuations in cavities within polar liquids and thermodynamics of polar solvation
We present the results of numerical simulations of fluctuations of the
electrostatic potential and electric field inside cavities created in the fluid
of dipolar hard spheres. We found that the thermodynamics of polar solvation
dramatically changes its regime when the cavity size becomes about 4-5 times
larger than the size of the liquid particle. The range of small cavities can be
reasonably understood within the framework of current solvation models. On the
contrary, the regime of large cavities is characterized by a significant
softening of the cavity interface resulting in a decay of the fluctuation
variances with the cavity size much faster than anticipated by both the
continuum electrostatics and microscopic theories. For instance, the variance
of potential decays with the cavity size approximately as
instead of the scaling expected from standard electrostatics. Our
results suggest that cores of non-polar molecular assemblies in polar liquids
lose solvation strength much faster than is traditionally anticipated.Comment: 10 pp, 10 fig
Electric field inside a "Rossky cavity" in uniformly polarized water
Electric field produced inside a solute by a uniformly polarized liquid is
strongly affected by dipolar polarization of the liquid at the interface. We
show, by numerical simulations, that the electric "cavity" field inside a
hydrated non-polar solute does not follow the predictions of standard Maxwell's
electrostatics of dielectrics. Instead, the field inside the solute tends, with
increasing solute size, to the limit predicted by the Lorentz virtual cavity.
The standard paradigm fails because of its reliance on the surface charge
density at the dielectric interface determined by the boundary conditions of
the Maxwell dielectric. The interface of a polar liquid instead carries a
preferential in-plane orientation of the surface dipoles thus producing
virtually no surface charge. The resulting boundary conditions for
electrostatic problems differ from the traditional recipes, affecting the
microscopic and macroscopic fields based on them. We show that relatively small
differences in cavity fields propagate into significant differences in the
dielectric constant of an ideal mixture. The slope of the dielectric increment
of the mixture versus the solute concentration depends strongly on which
polarization scenario at the interface is realized. A much steeper slope found
in the case of Lorentz polarization also implies a higher free energy penalty
for polarizing such mixtures.Comment: 9 pages, 8 figure
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