Ion Pair Potentials-of-Mean-Force in Water

Recent molecular simulation and integral equation results alkali-halide ion pair potentials-of-mean-force in water are discussed. Dielectric model calculations are implemented to check that these models produce that characteristic structure of contact and solvent-separated minima for oppositely charged ions in water under physiological thermodynamic conditions. Comparison of the dielectric model results with the most current molecular level information indicates that the dielectric model does not, however, provide an accurate description of these potentials-of-mean-force. We note that linear dielectric models correspond to modelistic implementations of second-order thermodynamic perturbation theory for the excess chemical potential of a distinguished solute molecule. Therefore, the molecular theory corresponding to the dielectric models is second-order thermodynamic perturbation theory for that excess chemical potential. The second-order, or fluctuation, term raises a technical computational issue of treatment of long-ranged interactions similar to the one which arises in calculation of the dielectric constant of the solvent. It is contended that the most important step for further development of dielectric models would be a separate assessment of the first-order perturbative term (equivalently the {\it potential at zero charge} ) which vanishes in the dielectric models but is generally nonzero. Parameterization of radii and molecular volumes should then be based of the second-order perturbative term alone. Illustrative initial calculations are presented and discussed.Comment: 37 pages and 8 figures. LA-UR-93-420

Ion Sizes and Finite-Size Corrections for Ionic-Solvation Free Energies

Free energies of ionic solvation calculated from computer simulations exhibit a strong system size dependence. We perform a finite-size analysis based on a dielectric-continuum model with periodic boundary conditions. That analysis results in an estimate of the Born ion size. Remarkably, the finite-size correction applies to systems with only eight water molecules hydrating a sodium ion and results in an estimate of the Born radius of sodium that agrees with the experimental value.Comment: 2 EPS figure

The heat flux from a relativistic kinetic equation with a simplified collision kernel

We show how using a special relativistic kinetic equation with a BGK- like collision operator the ensuing expression for the heat flux can be casted in the form required by Classical Irreversible Thermodynamics. Indeed, it is linearly related to the temperature and number density gradients and not to the acceleration as the so-called "first order in the gradients theories" contend. Here we calculate explicitly the ensuing transport coefficients and compare them with the results obtained by other authors.Comment: 4 pages, one figure. To appear in the AIP conference proceedings of the XXXI Spanish Relativity Meeting: Physics and Mathematics of Gravitation. Salamanca Spain, 15-19 September 200

D-brane instantons and the effective field theory of flux compactifications

We provide a description of the effects of fluxes on euclidean D-brane instantons purely in terms of the 4d effective action. The effect corresponds to the dressing of the effective non-perturbative 4d effective vertex with 4d flux superpotential interactions, generated when the moduli fields made massive by the flux are integrated out. The description in terms of effective field theory allows a unified description of non-perturbative effects in all flux compactifications of a given underlying fluxless model, globally in the moduli space of the latter. It also allows us to describe explicitly the effects on D-brane instantons of fluxes with no microscopic description, like non-geometric fluxes. At the more formal level, the description has interesting connections with the bulk-boundary map of open-closed two-dimensional topological string theory, and with the \NN=1 special geometry.Comment: 33 page

In Vitro Assessment of Fecal Inocula From Horses Fed on High-Fiber Diets With Fibrolytic Enzymes Addition on Gas, Methane, and Carbon Dioxide Productions as Indicators of Hindgut Activity

The aim of this study was to assess the effect of fecal inocula from horses fed on concentrate (restricted amount daily) and oat straw (ad libitum) supplemented with fibrolytic enzymes on in vitro hindgut activity. Cellulase (CE), xylanase (XY), and CE þ XY (1:1 vol/vol; CX) were tested at three levels (mL/g dry matter [DM]): 0, 1, and 3, in addition to control without enzyme addition. Fecal inocula were collected from 16 Quarter Horse mares supplemented with enzyme at 0 (without enzyme), or fed 5-mL enzyme/mare/d of CE (FCE), XY (FXY), or CE þ XY (1:1 vol/vol; FCX) for 15 days. The fecal content mixed with the culture media were used for incubation in bottles containing 1-g DM of substrate (a mixture of concentrate and oat straw [1:1 DM]). Gas (GP), methane (CH4), and carbon dioxide productions were measured at 2, 4, 6, 8, 10, 12, 24, and 48 hours after incubation. Interactions occurred (P < .05) between fecal source enzyme product for the asymptotic GP, the rate of GP, CH4 production, and fermentation kinetic parameters. Moreover, interactions were observed (P < .05) between fecal source enzyme product enzyme dose for the rate of GP, CH4 production, and DM digestibility. Xylanase at 3-mL/g DM with FXY fecal increased (P < .05) the asymptotic GP, short-chain fatty acids, and microbial protein productions with lowering (P < .05) partitioning factor. At 24 and 48 hours and without enzyme, FCX and FXY, had the highest (P < .05) CH4 production. It can be concluded that XY enzyme at 3-mL/g DM was the most effective compared with other treatments