Characteristics of Boiling, Saturation Vapor Pressures, and Compositions of Melts of Alkali Metals with Allowance Made for the Existence of Small Self-Associates: Computer Experiment

Abstract

The validity of the hypothesis that melts of alkali metals (AMs) contain small self-associates (or clusters) [AM 2 ]‐[AM 5 ] was tested using the thermodynamic properties of volatile and condensed clusters of alkali metals [1] by computer experiments [2] in liquid alkali metal‐argon systems at a total pressure of 1 atm. The compositions of melts were described using a model of ideal solutions of interaction products [2]. The components of the solutions were atoms and various clusters; the components of the gas‐vapor phase were atoms, volatile clusters, alkali metal ions, and electron gas. The integral compositions of the initial systems were identical: 99 wt % of alkali metal and 1 wt % of argon. The components of the solutions and the gas phase for ten variants (variants 1‐10) of modeling are presented in Table 1. In this paper, we revealed model systems for which our calculated and published boiling points T b are in best agreement. For these systems, we analyzed the correspondence between the change in the enthalpy of boiling Δ H b and the saturation vapor pressure P s as functions of temperature according to the modeling and experimental data; we also calculated the compositions of model melts and the gas phase as functions of temperature. The calculated boiling points T b (AM) for variants 1‐10 and their comparison with published data [3‐7] are presented in Table 2 and Fig. 1. The calculated and published boiling points T b are seen to be in best ( ~0.5% ) agreement for variant 5 (the solution contains [AM 1 ], [AM 3 ], and [AM 5 ]; the gas phase contains AM 1 ‐AM 3 , AM 5 , alkali metal ions, and electron gas), and this is characteristic for all alkali metals. Table 3 shows that the calculated enthalpies of boiling Δ H b (AM) agree with published data [4, 5, 7]. Further, for alkali metals for variant 5, the saturation vapor pressures P s were calculated at various temperatures and the results obtained were compared with available data [3‐5]. An example of this is presented for a lithium system in Table 4. Similar comparisons were made for all alkali metals; the results are summarized in Table 5. Tables 4 and 5 show that the published P s values [3‐5] differ noticeably, the calculated P s values agree with the published values within the range of their difference between one another, and agreement is best between the calculated P s and P s according to Bystrov et al. [4]. Table 6 presents the coefficients of the expressions for P s (AM) as functions of temperature. Figure 2 demonstrates of the partial pressures of the i th components as functions of temperature by the example of the gas‐vapor phase over molten lithium. For all alkali metals, these data are given in Table 7. The compositions of model melts of alkali metals as functions of temperature are illustrated in Fig. 3 and Table 8. A detailed discussion of these data will be presented elsewhere. P i log