The density of Li2CO3-Na2CO3-K2CO3-Rb2CO3-Cs2CO3-CaCO3-SrCO3-BaCO3 liquids: New measurements, ideal mixing, and systematic trends with composition. (1st March 2019) - Hurt, Sean M.
Abstract: The densities of 15 liquids in the Li2 CO3 -Na2 CO3 -K2 CO3 -Rb2 CO3 -Cs2 CO3 -CaCO3 -SrCO3 -BaCO3 system were measured at 1 bar with the Pt double-bob Archimedean method between 758 and 1455 K. Melt compositions contain ≤50 mol% CaCO3, SrCO3 and BaCO3, and ≤100% Rb2 CO3 and Cs2 CO3 . These results were combined with double-bob density measurements from the literature on nine liquids in the Li2 CO3 -Na2 CO3 -K2 CO3 -CaCO3 system to calibrate a new linear volume equation for multicomponent carbonate liquids. The regression led to the following fitted partial molar volumes (±1σ cm 3 /mol) at 1100 K for Li2 CO3 (41.22 ± 0.09), Na2 CO3 (53.27 ± 0.11), K2 CO3 (71.59 ± 0.13), Rb2 CO3 (80.78 ± 0.11), Cs2 CO3 (94.00 ± 0.09), CaCO3 (40.18 ± 0.16), SrCO3 (44.33 ± 0.22) and BaCO3 (50.99 ± 0.19). At 1100 K, the thermal expansion coefficients of all alkali carbonate liquid components are indistinguishable within 1-sigma error (22.07 ± 1.66 × 10 −5 K −1 ), but differ from the thermal expansion coefficients of all alkaline-earth carbonate liquid components, which are also indistinguishable within 1-sigma error (16.40 ± 2.85 × 10 −5 K −1 ). The linear volume equation recovers the measurements within analytical error (±0.3%). The partial molar volumes of all eight carbonate components increase linearly along two different trends, one for the alkali carbonates (R 2 = 0.999) and another for the alkaline earth carbonates (R 2 = 0.999) as a function of cation volumes, where metal cation-oxygen coordination numbers are obtained from molecular dynamic simulations in the literature (ranging from 4- to 6-fold among the alkali metals and 7- to 8-fold among the alkaline-earth metals). The linear fits lead to two different partial molar volumes (∼38 and ∼31 cm 3 /mole) for the carbonate ion (CO3 2− ) at 1100 K, which reflects a more open topological arrangement of the carbonate ions when in 4-fold coordination with the alkali metals vs. 6-fold coordination with alkaline earth metals. The results permit the partial molar volume of MgCO3 and FeCO3 in multicomponent carbonate liquids to be calculated if the oxygen and carbonate ion coordination with Mg 2+ and Fe 2+ are known. For example, in the case where Mg 2+ and Fe 2+ are in 6-fold coordination with both oxygen and carbonate, the estimated partial molar volumes at 1100 K are 34.4 (±0.6), and 35.1 (±0.6) cm 3 /mol, respectively, with a thermal expansion coefficient of 16.40 (±2.85) 10 −5 K −1 . In contrast, if Mg 2+ and Fe 2+ are in 4-fold coordination with both oxygen and carbonate, the estimated partial molar volumes at 1100 K are 40.0 (±0.6), and 40.4 (±0.6) cm 3 /mol, respectively, with a thermal expansion coefficient of 22.07 (±1.66) 10 −5 K −1 . To resolve these different estimates of molar volume, molecular dynamic simulations are needed to determine the respective coordination environments for Mg 2+ and Fe 2+ in carbonate liquids.