SIPS2022 Volume 4 Kipouros Intl. Symp. Molten Salt, Ionic & Glass-forming Liquids & Powdered Materials

Editors: | F. Kongoli, R. Fehrmann, V. Papangelakis, I.Paspaliaris, G. Saevarsdottir. |

Publisher: | Flogen Star OUTREACH |

Publication Year: | 2022 |

Pages: | 100 pages |

ISBN: | 978-1-989820-40-7(CD) |

ISSN: | 2291-1227 (Metals and Materials Processing in a Clean Environment Series) |

The interionic forces in molten electrolytes can be classified according to the number of particles entering to the interaction energy. The first group includes the pair interactions: the repulsions at short distances, the London dispersion forces and the Coulomb interactions. The most difficulty is the consideration of the second group, namely, induction interactions, since the charge of a given ion induces dipole moments on neighboring ones, which will not only interact with each other, but also induce dipole moments on other ions. The charge-induced dipole part of the energy can be calculated for molten salts by introducing the dielectric constant to avoid the many-body problem [1]. The difficulty of statistical-thermodynamic calculations is the absence of exact solutions for complicated interaction models (for example, taking into account the induction interactions).

From a physical standpoint, the problem of the thermodynamics of molten alkali halides can be usefully considered in terms of thermodynamic perturbation theory [2], which allows extra terms to pair potentials to be taken into consideration based on analytical statistical mechanics models. Therefore, objectives of this report are to propose the model based on the thermodynamic perturbation theory, which provides the possibility of reducing the charge-induced dipole contribution to the relatively simple pair potential, using the reference system of charged hard spheres and to present the results of calculations for this term to the thermodynamics of alkali halide melts.

The Helmholtz free energy for the reference system of liquid melts included the standard contribution of the ideal-gas mixture [3], the hard-sphere contribution within the Mansoori-Carnahan-Starling-Leland approximation [4], and the Coulombic contribution within the mean spherical approximation [5]. The perturbation to the free energy due to the charge-induced dipole interactions was taken into account through Gibbs-Bogoliubov approach of the thermodynamic perturbation theory.

We will demonstrate, that the fraction of the ion-dipole contribution to the free energy is not more than 10 percent, while its absolute values for alkali halides lies in the range of 30–60 kJ/mol. The discrepancy between the calculated and available experimental enthalpies do not exceed 10 percent for most considered halides. Moreover, the calculated temperature dependences of enthalpy almost completely correspond to the experimental data on its base trend, when the enthalpy of the melts slightly increases with heating. The report will present and analyze the results of modeling the temperature dependences of the free energy, the enthalpy and other thermodynamic characteristics for molten alkali halides.

The reported study was funded by RFBR, projects number 19-33-90180 and number 18-03-00606.

[2] R.W. Zwanzig, J. Chem. Phys. 22 (1954) 1420-1426.

[3] K.G. Peshkina, N.K. Tkachev, J. Mol. Liq. 216 (2016) 856-861.

[4] G.A. Mansoori, et al., J. Chem. Phys. 54 (1971) 1523-1525.

[5] K. Hiroike, Mol. Phys. 33 (1977) 1195-1198.