Ionic Vapor Composition in Critical and Supercritical States of Strongly Interacting Ionic Compounds

Abstract

The critical point, CP (T, P), of the phase diagram quantifies the minimum amount of kinetic energy needed to prevent a substance from existing in a condensed phase. Therefore, the CP is closely related to the properties of the fluid far below the critical temperature. Approaches designed to predict thermophysical properties of a system necessarily aim to provide reliable estimates of the CP. Vice versa, CP estimation is impossible without knowledge of the vapor phase behavior. We report ab initio Born-Oppenheimer molecular dynamics (BOMD) simulations of sodium and potassium chlorides, NaCl and KCl, at and above their expected CPs. We advance the present knowledge regarding the existence of ionic species in the vapor phase by establishing significant percentages of atomic clusters: 29-30% in NaCl and 34-38% in KCl. A neutral pair of counterions is the most abundant cluster in the ionic vapors (ca. 35% of all vaporized ions exist in this form). Unexpectedly, an appreciable fraction of clusters is charged. The ionic vapor composition is determined by the vapor density, rather than the nature of the alkali ion. The previously suggested CPs of NaCl and KCl appear overestimated, based on the present simulations. The reported results offer essential insights into the ionic fluid properties and assist in development of thermodynamic theories. The ab initio BOMD method has been applied to investigate the vapor phase composition of an ionic fluid for the first time.

Anticrossing of Longitudinal and Transverse Modes in Simple Fluids

“…In contrast to crystals, anharmonicity and structural disorder play a defining role in collective dynamics of fluids. ,− There is no long-range order in a fluid state, but (strongly damped) longitudinal and transverse excitations still exist. ,− Moreover, unlike crystals whose dynamics is explained by analysis of excitations in the (anisotropic) first Brillouin zone, (isotropic) pseudozones in fluids are not equal to each other and excitations have to be analyzed in a broad range of wavelengths. Interest in excitations is strongly supported in recent time by a broad discussion of the relation among structure, dynamics, and thermodynamics, as well as the crossover between liquid- and gas-like behavior of various fluids. ,− For instance, longitudinal and transverse modes with the same frequency are known to be mixed in fluids due to structural disorder and anharmonicity. − ,− At the same time, modes in fluids are spatially isotropic, and thus, they cannot cross . Instead of crossing, modes should repel as they approach each other in frequency; this phenomenon is called anticrossing .…”

Ab Initio Molecular Dynamics of Dimerization and Clustering in Alkali Metal Vapors

“…This number is sufficient to observe all possible compositions, because it is several times larger than the number of atoms in the biggest vapor phase structure. The cluster distribution was investigated as a function of the simulated system size in the recent work 31 focusing on alkali metal chlorides. No undesirable deviations for the smaller systems were found.…”

Direct Experimental Evidence of Longitudinal and Transverse Mode Hybridization and Anticrossing in Simple Model Fluids

“…Consequently, we believe that detailed analysis of excitations opens up exciting prospects for elaboration of transport phenomena theory and establishing (as yet unknown) relations between individual, collective dynamics, and thermodynamics of fluids. 2,[56][57][58][59][60][61][62][63][64][65] Furthermore, redistribution of the excitation spectra (as result of their anticrossing), can affect the interaction between longitudinal, transverse, and polarization modes in ionic and polar molecular liquids. We leave these interesting problems for future work.…”

“…We show that the anti-crossing-based method for the analysis of excitations allows us to obtain frequencies and damping rates. Consequently, we believe that a detailed analysis of excitations opens up exciting prospects for the elaboration of the transport phenomena theory and establishing (as yet unknown) relations between individual and collective dynamics and thermodynamics of fluids. ,− …”