Structure and criticality of ionic fluids
W. Schr�er and H. Weing�rtner
Institut für Anorganische und Physikalische Chemie
der Universität Bremen, D-28359 Bremen, Germany;
Physikalische Chemie II, Ruhr-Universität Bochum, D-44780 Bochum,Germany
Abstract:
Two properties render electrolyte theories difficult, namely the long-range
nature of the Coulomb interactions and the high figures of the Coulomb
energy at small ion separations. In solvents of low dielectric constant,
where the Coulomb interactions are particularly strong, electrical conductance
and dielectric spectra suggest that the ion distribution involves dipolar
ion pairs, which then interact with the free ions and with other dipolar
pairs. The dipole-dipole interactions between ion pairs lead to an increase
of the dielectric constant, which in turn stabilizes the free ions,
thus leading to redissociation at high salt concentrations. An equation
of state that accounts for ion pairing, ion-ion pair, and ion pair-ion
pair interactions rationalizes the basic features of the ion distribution.
It also predicts a fluid-phase transition at low reduced temperatures,
which closely corresponds to simulation results and to experimentally
observed liquid-liquid phase transitions. The long-range nature of the
Coulomb potential driving these transitions raises questions concerning
their universality class. Experiments suggest that the Ising universality
class applies, but there is cross-over to mean-field behavior rather
close to the critical, not yet well explained by theory.
*Lecture presented at the European Molecular Liquids
Group (EMLG) Annual Meeting on the Physical Chemistry of Liquids: Novel
Approaches to the Structure, Dynamics of Liquids: Experiments, Theories,
and Simulation,Rhodes, Greece, 7-15 September 2002. Other presentations
are published in this issue, pp. 1-261.
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