Liquid–vapor criticality in a fluid of charged hard dumbbells

Thermo-molecular orientation effects in fluids of dipolar dumbbells

Plots of first-order (left) and novel second-order (right) thermomolecular orientation effects in fluids of dipolar dumbbells.

Phase coexistence for charged soft dumbbell and ionic soft sphere systems via molecular dynamics simulation

Gas-liquid critical parameters of charged soft dumbbells (CSDs) as function of the site-to-site separation on the dumbbells, d, obtained on the basis of molecular dynamics computer simulation are presented. A mean field theoretical description is developed, which explains the results at small and large d, respectively. We note that in the limit d→0 the CSD system exhibits gas-liquid phase separation solely driven by repulsion and dipole-dipole interaction. If the dumbbell bond is eliminated the CSD system becomes similar to the restricted primitive model of ionic fluids with hard core repulsion replaced by soft repulsion. We obtain the gas-liquid critical parameters for this model and discuss their relation to the CSD system.

Microscopic approach to the kinetics of pattern formation of charged molecules on surfaces

A microscopic formalism based on computing many-particle densities is applied to the analysis of the diffusion-controlled kinetics of pattern formation in oppositely charged molecules on surfaces or adsorbed at interfaces with competing long-range Coulomb and short-range Lennard-Jones interactions. Particular attention is paid to the proper molecular treatment of energetic interactions driving pattern formation in inhomogeneous systems. The reverse Monte Carlo method is used to visualize the spatial molecular distribution based on the calculated radial distribution functions (joint correlation functions). We show the formation of charge domains for certain combinations of temperature and dynamical interaction parameters. The charge segregation evolves into quasicrystalline clusters of charges, due to the competing long- and short-range interactions. The clusters initially co-exist with a gas phase of charges that eventually add to the clusters, generating "fingers" or line of charges of the same sign, very different than the nanopatterns expected by molecular dynamics in systems with competing interactions in two dimensions, such as strain or dipolar versus van der Waals interactions.

Vapor-liquid coexistence in fluids of charged hard dumbbells

Vapor-liquid coexistence in fluids of charged hard dumbbells, each made up of two oppositely charged hard spheres with diameters sigma and separation d, has been studied using grand-canonical Monte Carlo simulations. In the limit d/sigma-->0, and with the temperature scaled accordingly, the system corresponds to dipolar hard spheres. For separations in the range 0.3<d/sigma<or=1 the coexisting vapor phase contains compact clusters. For separations in the range 0.1<or=d/sigma<0.3 the coexistence is between a chainlike vapor and a networklike liquid. Finite-size effects preclude the simulation of the coexistence in systems with d/sigma<0.1, but extrapolations of the results to d/sigma-->0 yield estimates of the apparent critical parameters for dipolar hard spheres.

Criticality in charge-asymmetric hard-sphere ionic fluids

Phase separation and criticality are analyzed in z:1 charge-asymmetric ionic fluids of equisized hard spheres by generalizing the Debye-Hückel approach combined with ionic association, cluster solvation by charged ions, and hard-core interactions, following lines developed by Fisher and Levin for the 1:1 case (i.e., the restricted primitive model). Explicit analytical calculations for 2:1 and 3:1 systems account for ionic association into dimers, trimers, and tetramers and subsequent multipolar cluster solvation. The reduced critical temperatures, Tc* (normalized by z), decrease with charge asymmetry, while the critical densities increase rapidly with . The results compare favorably with simulations and represent a distinct improvement over all current theories such as the mean spherical approximation, symmetric Poisson-Boltzmann theory, etc. For z not equal to 1, the interphase Galvani (or absolute electrostatic) potential difference, Deltaphi(T), between coexisting liquid and vapor phases is calculated and found to vanish as absolute value (T-Tc) beta when T-->Tc-with, since our approximations are classical, beta = (1/2). Above Tc, the compressibility maxima and so-called k-inflection loci (which aid the fast and accurate determination of the critical parameters) are found to exhibit a strong z dependence.

Phase behavior of the restricted primitive model of ionic fluids with association in slitlike pores. Density-functional approach

We present results of investigations of condensation of restricted primitive model of electrolyte solutions with association between oppositely charged ions confined to slitlike pores. The associative interaction leads to the formation of ionic pairs. It is accounted for by incorporating the first-order thermodynamic perturbation theory into the free energy functional. In order to elucidate the role of association, the phase diagrams are compared with those obtained by us recently [O. Pizio et al., J. Chem. Phys. 121, 11957 (2004)] for the restricted primitive model. The inclusion of the association into the theory leads to lowering the critical temperature for the fluid confined to pores with uncharged and with charged walls. We have observed that the average fraction of bonded ions is high along the coexistence envelope.

Phase diagram of charged dumbbells: a random phase approximation approach

The phase diagram of the charged hard dumbbell system (hard spheres of opposite unit charge fixed at contact) is obtained with the use of the random phase approximation (RPA). The effect of the impenetrability of charged spheres on charge-charge fluctuations is described by introduction of a modified electrostatic potential. The correlations of ions in a pair are included via a correlation function in the RPA. The coexistence curve is in good agreement with Monte Carlo simulations. The relevance of the theory to the restricted primitive model is discussed.