Studies involving the phase equilibria of liquid-liquid binary systems contribute to the knowledge powering industrial chemistry. Identification of coexistence temperatures as a function of composition is necessary to characterize partially miscible liquid-liquid system behavior. In several binary systems, pre-transition droplet formation (PTDF) has been observed, in which trace droplets separate from the mother phase ~0.1 K before the bulk phase transition. The premature separation occurs in near-critical samples on one side of the phase diagram. This suggests a third component in systems that are usually considered binary; the third component could possibly be naturally present isotopologues. In order to determine if deuterium substitution would result in PTDF, the 1,2-propanediol + benzene (PDB) and 1,2-propanediol + benzene-d6 (PDDB) liquid-liquid systems were characterized. Vacuum-pumped and flame-sealed 3-mL samples of PDB and PDDB were prepared using the synthetic method. Intensities of transmitted, 2°-, and 90°-scattered laser light were measured as a function of temperature to identify each sample’s coexistence temperature Tcx. Analysis of the Tcx data for the 21 samples of PDB yielded a critical temperature TC of 351.96 ± 0.03 K, and analysis of the 90°-scattered light gave a critical composition XProp,C of 0.3637 ± 0.0010. Analysis of the 17 samples of PDDB yielded a TC of 349.83 ± 0.04 K and XProp,C of 0.3621 ± 0.0008. No PTDF was observed in any of the samples, or in a single sample of 1,2-propanediol + benzene + benzene-d6. The two XProp,C values were not significantly different. As seen in the accompanying figure, the TC values differed by 2.13 ± 0.04 K, or 0.355 ± 0.007 K per deuterium atom. This change is consistent with other studies where TC values have been found to decrease by ~0.3 K per deuterium substitution of a hydrogen atom not engaged in hydrogen bonding.

Characterization of the 1,2-propanediol + benzene and 1,2-propanediol + benzene-d6 liquid-liquid phase equilibria

254th National Meeting (2017)

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