287 research outputs found
Effects of translational and rotational degrees of freedom on the properties of model water
Molecular dynamics simulations with separate thermostats for rotational and
translational motions were used to study the effects of these degrees of
freedom on the structure of water at a fixed density. To describe water
molecules, we used the SPC/E model. The results indicate that an increase of
the rotational temperature, , causes a significant breaking of
the hydrogen bonds. This is not the case, at least not to such an extent, when
the translational temperature, , is raised. The probability of
finding an empty spherical cavity (no water molecule present) of a given size,
strongly decreases with an increase of , but this only marginally
affects the free energy of the hydrophobe insertion. The excess internal energy
increases proportionally with an increase of , while an increase
of yields a much smaller effect at high temperatures. The
diffusion coefficient of water exhibits a non-monotonous behaviour with an
increase of the rotational temperature.Comment: 9 pages, 9 figure
Primitive model electrolytes. A comparison of the HNC approximation for the activity coefficient with Monte Carlo data
Accuracy of the mean activity coefficient expression
(Hansen-Vieillefosse-Belloni equation), valid within the hypernetted chain
(HNC) approximation, was tested in a wide concentration range against new Monte
Carlo (MC) data for +1:-1 and +2:-2 primitive model electrolytes. The
expression has an advantage that the excess chemical potential can be obtained
directly, without invoking the time consuming Gibbs-Duhem calculation. We found
the HNC results for the mean activity coefficient to be in good agreement with
the machine calculations performed for the same model. In addition, the
thermodynamic consistency of the HNC approximation was tested. The mean
activity coefficients, calculated via the Gibbs-Duhem equation, seem to follow
the MC data slightly better than the Hansen-Vieillefosse-Belloni expression.
For completeness of the calculation, the HNC excess internal energies and
osmotic coefficients are also presented. These results are compared with the
calculations based on other theories commonly used to describe electrolyte
solutions, such as the mean spherical approximation, Pitzer's extension of the
Debye-H\"uckel theory, and the Debye-H\"uckel limiting law.Comment: 15 pages, 6 figure
Modelling the ion-exchange equilibrium in nanoporous materials
Distribution of a two component electrolyte mixture between the model
adsorbent and a bulk aqueous electrolyte solution was studied using the replica
Ornstein-Zernike theory and the grand canonical Monte Carlo method. The
electrolyte components were modelled to mimic the HCl/NaCl and HCl/CaCl_2
mixtures, respectively. The matrix, invaded by the primitive model electrolyte
mixture, was formed from monovalent negatively charged spherical obstacles. The
solution was treated as a continuous dielectric with the properties of pure
water. Comparison of the pair distribution functions (obtained by the two
methods) between the various ionic species indicated a good agreement between
the replica Ornstein-Zernike results and machine calculations. Among
thermodynamic properties, the mean activity coefficient of the invaded
electrolyte components was calculated. Simple model for the ion-exchange resin
was proposed. The selectivity calculations yielded qualitative agreement with
the following experimental observations: (i) selectivity increases with the
increasing capacity of the adsorbent (matrix concentration), (ii) the adsorbent
is more selective for the ion having higher charge density if its fraction in
mixture is smaller.Comment: 12 pages, 9 figure
Modeling Amphiphilic Solutes in a Jagla Solvent
Methanol is an amphiphilic solute whose aqueous solutions exhibit distinctive
physical properties. The volume change upon mixing, for example, is negative
across the entire composition range, indicating strong association. We explore
the corresponding behavior of a Jagla solvent, which has been previously shown
to exhibit many of the anomalous properties of water. We consider two models of
an amphiphilic solute: (i) a "dimer" model, which consists of one hydrophobic
hard sphere linked to a Jagla particle with a permanent bond, and (ii) a
"monomer" model, which is a limiting case of the dimer, formed by
concentrically overlapping a hard sphere and a Jagla particle. Using discrete
molecular dynamics, we calculate the thermodynamic properties of the resulting
solutions. We systematically vary the set of parameters of the dimer and
monomer models and find that one can readily reproduce the experimental
behavior of the excess volume of the methanolwater system as a function of
methanol volume fraction. We compare the pressure and temperature dependence of
the excess volume and the excess enthalpy of both models with experimental data
on methanol-water solutions and find qualitative agreement in most cases. We
also investigate the solute effect on the temperature of maximum density and
find that the effect of concentration isorders of magnitude stronger than
measured experimentally
Influence of solvent granularity on the effective interaction between charged colloidal suspensions
We study the effect of solvent granularity on the effective force between two
charged colloidal particles by computer simulations of the primitive model of
strongly asymmetric electrolytes with an explicitly added hard sphere solvent.
Apart from molecular oscillating forces for nearly touching colloids which
arise from solvent and counterion layering, the counterions are attracted
towards the colloidal surfaces by solvent depletion providing a simple
statistical description of hydration. This, in turn, has an important influence
on the effective forces for larger distances which are considerably reduced as
compared to the prediction based on the primitive model. When these forces are
repulsive, the long-distance behaviour can be described by an effective Yukawa
pair potential with a solvent-renormalized charge. As a function of colloidal
volume fraction and added salt concentration, this solvent-renormalized charge
behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell
model but there are quantitative differences. For divalent counterions and
nano-sized colloids, on the other hand, the hydration may lead to overscreened
colloids with mutual attraction while the primitive model yields repulsive
forces. All these new effects can be accounted for through a solvent-averaged
primitive model (SPM) which is obtained from the full model by integrating out
the solvent degrees of freedom. The SPM was used to access larger colloidal
particles without simulating the solvent explicitly.Comment: 14 pages, 16 craphic
Disaccharide topology induces slow down in local water dynamics
Molecular level insight into water structure and structural dynamics near proteins, lipids and
nucleic acids is critical to the quantitative understanding of many biophysical processes. Un-
fortunately, understanding hydration and hydration dynamics around such large molecules is challenging because of the necessity of deconvoluting the effects of topography and chemical heterogeneity. Here we study, via classical all atom simulation, water structure and structural dynamics around two biologically relevant solutes large enough to have significant chemical and topological heterogeneity but small enough to be computationally tractable: the disaccharides Kojibiose and Trehalose. We find both molecules to be strongly amphiphilic (as quantified from normalized local density fluctuations) and to induce nonuniform local slowdown in water translational and rotational motion. Detailed analysis of the rotational slowdown shows that while the rotational mechanism is similar to that previously identified in other aqueous systems by Laage, Hynes and coworkers, two novel characteristics are observed: broadening of the transition state during hydrogen bond exchange (water rotation) and a subpopulation of water for which rotation is slowed because of hindered access of the new accepting water molecule to the transition state. Both of these characteristics are expected to be generic features of water rotation around larger biomolecules and, taken together, emphasize the difficulty in transferring insight into water rotation around small molecules to much larger amphiphilic
solutes.This work is part of the research program of the “Stichting voor Fundamenteel Onderzoek der
Materie (FOM)” which is financially supported by the “Nederlandse organisatie voor Wetenschap-
pelijk Onderzoek (NWO)”. Further financial support was provided by a Marie Curie Incoming
International Fellowship (RKC). We gratefully acknowledge SARA, the Dutch center for high-
performance computing, for computational time and Huib Bakker and Daan Frenkel for useful
critical reviews on an earlier version of this work. We thank two anonymous reviewers for their
excellent work, especially for bringing to our attention calculations done on the transition state geometry of dimers and the overstructuring of the O-O radial distribution function of SPC/E water
CEO Profile and Earnings Quality
This paper introduces the PSCORE, which aggregates nine personal characteristics of chief executive officers (CEOs), to signal the quality of earnings. The PSCORE is a composite score based on publicly available data on CEOs. The study reports strong positive relationships between the PSCORE and two different proxies for earnings quality, (i) discretionary accruals and (ii) financial statement errors, measured by deviations of the first digits of figures reported in financial statements from those expected by Benford’s Law. Further analyses indicate that the relationships between the PSCORE and the proxies for earnings quality become more pronounced when CEOs have high equity-based compensation incentives. The findings have some implications for practitioners
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