11,927 research outputs found
Scaled-Particle Theory and the Length-scales Involved in Hydrophobic Hydration of Aqueous Biomolecular Assemblies
Hydrophobic hydration plays a crucial role in self-assembly processes over
multiple length-scales, but the extrapolation of molecular-scale models to
larger length-scale hydration phenomena is sometimes not warranted.
Scaled-particle theories are based upon an interpolative view of that issue. We
revisit the scaled-particle theory proposed thirty years ago by Stillinger,
adopt a practical generalization, and consider the implications for hydrophobic
hydration in light of our current understanding. The generalization is based
upon identifying a molecular length, implicit in previous applications of
scaled-particle models, that provides an effective radius for joining
microscopic and macroscopic descriptions. We demonstrate that the generalized
theory correctly reproduces many of the anomalous thermodynamic properties of
hydrophobic hydration for molecularly sized solutes, including solubility
minima and entropy convergence, successfully interpolates between the
microscopic and macroscopic extremes, and provides new insights into the
underlying molecular mechanisms. The results are discussed in terms of
length-scales associated with component phenomena; in particular we first
discuss the micro-macroscopic joining radius identified by the theory, then we
discuss in turn the Tolman length that leads to an analogous length describing
curvature corrections of a surface area model of hydrophobic hydration free
energies, and the length-scales on which entropy convergence of hydration free
energies are expected.Comment: 19 pages, 14 figures, one figure added, submitted to Rev. Mod. Phy
Balancing Local Order and Long-Ranged Interactions in the Molecular Theory of Liquid Water
A molecular theory of liquid water is identified and studied on the basis of
computer simulation of the TIP3P model of liquid water. This theory would be
exact for models of liquid water in which the intermolecular interactions
vanish outside a finite spatial range, and therefore provides a precise
analysis tool for investigating the effects of longer-ranged intermolecular
interactions. We show how local order can be introduced through quasi-chemical
theory. Long-ranged interactions are characterized generally by a conditional
distribution of binding energies, and this formulation is interpreted as a
regularization of the primitive statistical thermodynamic problem. These
binding-energy distributions for liquid water are observed to be unimodal. The
gaussian approximation proposed is remarkably successful in predicting the
Gibbs free energy and the molar entropy of liquid water, as judged by
comparison with numerically exact results. The remaining discrepancies are
subtle quantitative problems that do have significant consequences for the
thermodynamic properties that distinguish water from many other liquids. The
basic subtlety of liquid water is found then in the competition of several
effects which must be quantitatively balanced for realistic results.Comment: 8 pages, 6 figure
Structure/permeability relationships of silicon-containing polyimides
The permeability to H2, O2, N2, CO2 and CH4 of three silicone-polyimide random copolymers and two polyimides containing silicon atoms in their backbone chains, was determined at 35.0 C and at pressures up to about 120 psig (approximately 8.2 atm). The copolymers contained different amounts of BPADA-m-PDA and amine-terminated poly (dimethyl siloxane) and also had different numbers of siloxane linkages in their silicone component. The polyimides containing silicon atoms (silicon-modified polyimides) were SiDA-4,4'-ODA and SiDA-p-PDA. The gas permeability and selectivity of the copolymers are more similar to those of their silicone component than of the polyimide component. By contrast, the permeability and selectivity of the silicon-modified polyimides are more similar to those of their parent polyimides, PMDA-4,4'-ODA and SiDA-p-PDA. The substitution of SiDA for the PMDA moiety in a polyimide appears to result in a significant increase in gas permeability, without a correspondingly large decrease in selectivity. The potential usefulness of the above polymers and copolymers as gas separation membranes is discussed
Final state interactions in two-particle interferometry
We reconsider the influence of two-particle final state interactions (FSI) on
two-particle Bose-Einstein interferometry. We concentrate in particular on the
problem of particle emission at different times. Assuming chaoticity of the
source, we derive a new general expression for the symmetrized two-particle
cross section. We discuss the approximations needed to derive from the general
result the Koonin-Pratt formula. Introducing a less stringent version of the
so-called smoothness approximation we also derive a more accurate formula. It
can be implemented into classical event generators and allows to calculate FSI
corrected two-particle correlation functions via modified Bose-Einstein
"weights".Comment: 12 pages RevTeX, 2 ps-figures included, submitted to Phys. Rev.
Quasi-chemical theory with a soft cutoff
In view of the wide success of molecular quasi-chemical theory of liquids,
this paper develops the soft-cutoff version of that theory. This development
has important practical consequences in the common cases that the packing
contribution dominates the solvation free energy of realistically-modeled
molecules because treatment of hard-core interactions usually requires special
purpose simulation methods. In contrast, treatment of smooth repulsive
interactions is typically straightforward on the basis of widely available
software. This development also shows how fluids composed of molecules with
smooth repulsive interactions can be treated analogously to the molecular-field
theory of the hard-sphere fluid. In the treatment of liquid water,
quasi-chemical theory with soft-cutoff conditioning doesn't change the
fundamental convergence characteristics of the theory using hard-cutoff
conditioning. In fact, hard cutoffs are found here to work better than softer
ones.Comment: 5 pages, 2 figure
Molecular Realism in Default Models for Information Theories of Hydrophobic Effects
This letter considers several physical arguments about contributions to
hydrophobic hydration of inert gases, constructs default models to test them
within information theories, and gives information theory predictions using
those default models with moment information drawn from simulation of liquid
water. Tested physical features include: packing or steric effects, the role of
attractive forces that lower the solvent pressure, and the roughly tetrahedral
coordination of water molecules in liquid water. Packing effects (hard sphere
default model) and packing effects plus attractive forces (Lennard-Jones
default model) are ineffective in improving the prediction of hydrophobic
hydration free energies of inert gases over the previously used Gibbs and flat
default models. However, a conceptually simple cluster Poisson model that
incorporates tetrahedral coordination structure in the default model is one of
the better performers for these predictions. These results provide a partial
rationalization of the remarkable performance of the flat default model with
two moments in previous applications. The cluster Poisson default model thus
will be the subject of further refinement.Comment: 5 pages including 3 figure
Towards the 3D-Imaging of Sources
Geometric details of a nuclear reaction zone, at the time of particle
emission, can be restored from low relative-velocity particle-correlations,
following imaging. Some of the source details get erased and are a potential
cause of problems in the imaging, in the form of instabilities. These can be
coped with by following the method of discretized optimization for the restored
sources. So far it has been possible to produce 1-dimensional emission source
images, corresponding to the reactions averaged over all possible spatial
directions. Currently, efforts are in progress to restore angular details.Comment: Talk given at the Int. Workshop on Hot and Dense Matter in
Relativistic Heavy Ion Collisions, March 24-27, 2004, Budapest; 10 pages, 6
figure
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