Ionic Vapor Composition in Critical and Supercritical States of Strongly Interacting Ionic Compounds Vitaly V. Chaban † and Oleg V. Prezhdo* ,‡ † Instituto de Ciê ncia e Tecnologia, Universidade Federal de Sã o Paulo, 12231-280, Sã o Jose ́ dos Campos, SP, Brazil ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States 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. ■ INTRODUCTION Ionic compounds (ICs) are omnipresent in chemistry. They are generally known for lower volatility and flammability and a larger liquid-state temperature range, as compared to molecular compounds. 1-19 ICs can be inorganic, organic, and mixed. For instance, many room-temperature ionic liquids (RTILs) 20-28 comprise an organic cation and inorganic anion, whereas inorganic crystals are essential for the Earth crust. Most ICs exhibit hydrophilic properties, to a larger or smaller extent. Furthermore, ICs are significantly miscible with each other, due to electrostatic forces. The latter feature permits tunability of the physicochemical properties, and, thus, opens up a wide avenue of possible applications. Purely inorganic ICs have high melting, boiling, and critical temperatures, 2,3,6,14 and are used predominantly as solid materials. In turn, mixed and organic ICs can be low melting salts due to unfavorable ionic packing for steric and symmetry reasons. They serve as versatile solvents and tunable reaction media. Because of their organic nature, these ICs thermally decompose well below their critical and even normal boiling points. An accurate knowledge of the critical points (CPs), triple points, and all their parameters constitutes a starting point of many investigations aiming to predict thermophysical proper- ties of ICs, most important of which are liquid state density and surface tension. The thermodynamic behavior of the majority of simple fluids is quite universal, provided that the corresponding state variables are expressed in the reduced form. This reduction is achieved by scaling all parameters by their respective values at the CP. Specific intermolecular and interionic interactions, such as hydrogen bonding, dipole- dipole association, and π-electron interactions between relatively large species, complicate correlations of the thermophysical properties and give rise to deviations that are difficult to predict, although major trends still persist. Roman and co-workers 29 published an encouraging theoreti- cal advance, demonstrating a universal behavior of the thermodynamic properties of a wide class of pure fluids along the entire vapor-liquid coexistence curve. The CP and triple point parameters are used to convert surface tension (ST), enthalpy of vaporization (EV), and coexistence densities difference (CEDD) into the corresponding reduced values. Once reduced, the experimental data can be described by a relatively simple expression containing only two empirical parameters, which are specific for every thermodynamic property: the critical-point exponent and the slope at the triple point. Noteworthy, saturation pressure (SVP) does not obey the established universal behavior. SVP is conventionally fitted by the three-parameter Antoine equation, whereby the parameters are different below and above the normal boiling point. The analyzed fluids include noble gases, short-chain alkanes, molecular nitrogen, carbon dioxide, and aprotic polar fluids, which may be expected to exhibit more sophisticated trends. Hydrogen-bonded fluids, such as water, hydrofluoric acid, methanol, carboxylic acids, etc., were omitted from the analysis, together with other fluids exhibiting various types of specific site-site intermolecular interactions. Recently, Weiss 30 published an analytic solution of the reformulated problem for selected molecular and ionic Received: March 7, 2016 Revised: April 19, 2016 Published: April 21, 2016 Article pubs.acs.org/JPCB © 2016 American Chemical Society 4302 DOI: 10.1021/acs.jpcb.6b02405 J. Phys. Chem. 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