Interactions of DL-serine and L-serine with NaCl and KCl in aqueous solutions

The activity coefficients at 25‡C of DL-serine and L-serine in aqueous solutions of NaCl and KC1 were measured. This study examines the effect of the nature of the cation of the electrolyte on the activity coefficients of the optical-isomers of serine in aqueous solutions for molality of serine up to 0.4 and molality of electrolyte up to 1. An electrochemical cell with two ion-selective electrodes, a cation, and an anion ion selective electrode,vs. a double-junction reference electrode was used to measure the activity coefficients of the electrolyte and the results were converted to the activity coefficients of serine in the aqueous electrolyte solution. The comparison of the results obtained for DL- and L-serine indicates that the two optical isomers have identical interactions with electrolytes in aqueous solutions and that for this amino acid the effect of the cation of the electrolyte is not significant. Comparison of these results with previous measurements for DL-alanine in aqueous solutions of the same electrolytes show the notable effect of the backbone of the amino acid.     

Statistical Geometry and Lattices

Statistical geometry furnishes the tool that allows the transfer of results from a lattice with finite lattice parameter to the continuum. Since lattice simulations are simpler than continuum ones, this suggests that larger scale simulations for the continuum might be more effectively carried out on a lattice with finite lattice parameter followed by the indicated transfer. We also show that a statistical geometry, peculiar to hard particles on a lattice, can be developed. Among other things, this opens the possibility that a scaled particle theory on a lattice might be derived.     

Asymptotic results for a persistent diffusion model of Taylor dispersion of particles

We study Taylor diffusion for the case when the diffusion transverse to the bulk motion is a persistent random walk on a one-dimensional lattice. This is mapped onto a Markovian walk where each lattice site has two internal states. For such a model we find the effective diffusion coefficient which depends on the rate of transition among internal states of the lattice. The Markovian limit is recovered in the limit of infinite rate of transitions among internal states; the initial conditions have no role in the leading-order time-dependent term of the effective dispersion, but a strong effect on the constant term. We derive a continuum limit of the problem presented and study the asymptotic behavior of such limit.