Volker C. Weiss

Macroscopic theory for equilibrium properties of ionic-dipolar mixtures and application to an ionic model fluid

An electrostatic theory for the equilibrium properties of a mixture of equisized ions and dipoles is developed by solving the Debye–Huckel differential equation for the cases of a charge and a dipole, respectively, in a dielectric continuum that contains free charges. This theory yields expressions for the potentials of mean force of ion-ion, ion-dipole, and dipole-dipole interactions. Furthermore, it leads to a formula for the dielectric constant that modifies Onsager’s result by accounting not only for the dipoles, but also for the free ions, and resembles the expression obtained by Adelman and Chen. The theory is applied to calculate thermodynamic properties of the model system of equisized charged hard spheres in a dielectric continuum (restricted primitive model), approximating dipolar ion pairs as spheres. We thus augment the work of Fisher and Levin, who added a shielded dipole-ion interaction term to the traditional Debye–Huckel theory, for a shielded dipole-dipole interaction and a density-dependent dielectric constant. The approximate evaluation of this theory yields a narrowing of the coexistence curve compared to the Fisher–Levin theory along with a considerable decrease of the critical density; however, it does not lead to the dramatic increase of the critical temperature that was observed by Guillot and Guissani who modified the Fisher–Levin theory by including unscreened dipole-dipole interactions adopting the original Onsager theory.

Critical behaviour of ionic solutions in non-polar solvents with a liquid - liquid phase transition

Turbidity measurements showing crossover from mean-field to Ising criticality have been reported by Narayanan and Pitzer for the liquid - liquid phase transition in ionic solutions of alkyl-ammonium picrates in higher alcohols. The Ising region was found to increase with the dielectric permittivity D for solvents with 4 < D < 8. It was conjectured that the Ising region becomes too small to be observed for lower values of D, which is in accordance with the finding of mean-field criticality in the system triethylhexylammonium triethylhexylborate in diphenyl ether , where . In order to check this hypothesis, we investigate solutions of salts in non-protonating solvents with D<2.5. The systems are tetrabutylammonium naphthyl sulphonate in toluene and tributylheptylammonium dodecyl sulphate in cyclohexane. The location of the critical points in the corresponding state diagram is in general agreement with the model system of charged hard spheres in a dielectric continuum, i.e. the restricted primitive model (RPM). However, changes of by minute variations of the salt and of the solvent (toluene, xylene, ethylbenzene) cannot be explained by the RPM. We report measurements of the phase diagram and light-scattering results. The amplitudes of the correlation length are up to an order of magnitude larger than those typically found in non-ionic fluids. For the new systems, but also for the solution of in , Ising criticality is found in the region of .

What makes ionic fluids characteristically ionic? A corresponding-states analysis of the surface tension of an ionic model fluid with variable dispersion interactions

Inorganic molten salts, such as NaCl, are known to show characteristically lower values of Guggenheims corresponding-states surface tension γ(red) at a given reduced temperature T∕T(c) than simple or aprotic polar fluids. Recently, the corresponding values of γ(red) for (some) room temperature ionic liquids (RTILs) were found in the same region as those for weakly polar fluids, that is, markedly above the values typical of inorganic molten salts despite the ionic character of RTILs. Here, we present the results of simulations of an ionic model fluid in which the strength of attractive dispersion interactions among the ions is varied relative to the Coulomb interactions. For weak dispersive interactions, the behavior known for real inorganic molten salts is found. If the attractive dispersion energy of two unlike ions at contact exceeds 20% of the Coulombic attraction in such an isolated ion pair, γ(red) increases markedly and approaches the region of values for simple and polar fluids. Rough theoretical estimates of the relative strengths of dispersive and Coulombic attractions in molten inorganic salts and in RTILs support our conclusion that the dispersion interactions in RTILs are strong enough for their corresponding-states surface tension to behave regularly and, thus, to deviate from the values one would expect for strongly ionic systems.

The liquid-vapour interface of the restricted primitive model (RPM) of ionic fluids

The liquid-vapour interface of the restricted primitive model (RPM) of ionic fluids is investigated within a square-gradient theory. We compute density profiles and interfacial tensions for different temperatures using Debye-Huckel (DH) theory and its recent extension for ion-pair formation and interactions between the dipolar ion pairs and free ions developed by Fisher and Levin. This Fisher-Levin (FL) theory is known to give an accurate description of the coexistence curve of the RPM. To account for the inhomogeneities in the interfacial region, the local free-energy density is expanded in terms of the density gradient. For small gradients, e.g. reasonably close to the critical point, such an expansion can be truncated after the square-gradient term. The coefficient of the latter is calculated from the direct correlation function using an approximate (quadratic) hypernetted-chain (AHNC) relation and, alternatively, from an extended van der Waals approach in conjunction with different approximations to the local density. The results from the AHNC relation and various local density approximations in the thermodynamic framework of DH theory and FL theory, respectively, are compared, and it is asserted that the AHNC relation in conjunction with FL theory predicts reliably the interfacial properties of the RPM even within this simple square-gradient theory. In contrast to the situation for simple fluids, the local density approximation must be chosen carefully for ionic fluids since properties such as the interfacial thickness and the surface tension may vary by a factor of three or four depending on the applied local density approximation.

Vibrational circular dichroism of 3-(trifluoroacetyl)-camphor and its interaction with chiral amines

Vibrational circular dichroism (VCD) spectroscopy and density functional theory (DFT) calculations are used to investigate the keto-enol equilibrium of 3-(trifluoroacetyl)-camphor (TFC) and to study the interaction of TFC with chiral amines in deuterated Chloroform. It is shown that the VCD spectra of the enol- and keto forms of TFC can clearly be distinguished and that the enol form is favored. By deprotonation of the TFC enol with chiral amines, no indication of a mutual diasteriomeric influence on the VCD spectra induced by transfer of stereochemical information between the chiral ionic species is found, neither experimentally nor theoretically.