Relationship between ionic radius and pressure dependence of ionic conductivity in water

Experimental measurements of ionic conductivity in water are analysed in order to obtain insight into the pressure dependence of limiting ionic conductivity of individual ions (λ0) for ions of differing sizes. Conductivities of individual ions, λ0 do not exhibit the same trend as a function of pressure for all ions. Our analysis suggests that the effect of pressure on ionic conductivity depends on the temperature. At low temperatures, the effect of pressure on relatively small ions such as Li+ exhibit an increase in conductivity with pressure. Intermediate sized ions exhibit an increase in conductivity with increase in pressure initially and then at still higher pressures, a decrease in ionic conductivity is observed. Although there are data at low temperatures for ions of large radius, the effect of increased pressure is expected to lower conductivity with increase in pressure over the whole range. At higher temperatures, the dependence of conductivity on pressure changes and these changes are discussed. Divalent ions such as SO2−4 exhibit different trends as a function of pressure at different temperatures. Both the divalent ions (Ca2+ and SO2−4 ) for which experimental data exists, exhibit an increase with pressure at lower temperatures. At slightly higher temperatures, a maximum in conductivity is seen as a function of pressure over the same range of pressure

Diffusive and Superdiffusive Motion of sorbates in Carbon nanotubes

Molecular dynamics simulations of sorbates of different sizes confined to the interior of carbon nanotubes are reported. The mean squared displacement shows gradual change from diffusive for small sorbates to superdiffusive for intermediate sized-sorbates to ballistic for sizes comparable to the channel diameter. We show that this crossover behaviour can be understood on the basis of a gradual decrease of the x-y component of the force with the levitation parameter. The analysis can also help to rationalize some recently published results.Comment: 3 pages, 3 figure

Diffusion of hydrocarbons in confined media: translational and rotational motion

Diffusion of monatomic guest species within confined media has been understood to a good degree due to investigations carried out during the past decade and a half. Most guest species that are of industrial relevance are actually polyatomics such as, for example, hydrocarbons in zeolites. We attempt to investigate the influence of non-spherical nature of guest species on diffusion. Recent molecular dynamics (MD) simulations of motion of methane in NaCaA and NaY, benzene in NaY and one-dimensional channels AlPO4−5, VPI-5 and carbon nanotube indicate interesting insights into the influence of the host on rotational degrees of freedom and orientational properties. It is shown that benzene in one-dimensional channels where the levitation parameter is near unity exhibits translational motion opposite to what is expected on the basis of molecular anisotropy. Rotational motion of benzene also possesses rotational diffusivities aroundC6 and C2axes opposite to what is expected on the basis of molecular geometry. Methane shows orientational preference for 2+2 or 1+3 depending on the magnitude of the levitation parameter

Influence of temperature inhomogeneity on product profile of reactions occurring within zeolites

In zeolites, diffusion is often accompanied by a reaction or sorption which in turn can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported for the presence of a hot zone presumed to be created by a reaction or chemi-or physi-sorption site. These simulations show that the presence of localized hot regions can alter both kinetic and transport properties such as diffusion. Further, we show that enhancement of diffusion constant is greater for systems with larger barrier height, a surprising result that may be of considerable significance in many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of a hot zone in addition to that which normally exists via concentration. Implications of this coupling for the product profile of a reaction are discussed. We also propose a mechanism by which mobility of ions or diffusion of molecular species within biomembranes may take place