Standard Partial Molar Volumes of Aqueous 2- and 3-Hydroxypropionic Acid from 100 to 325 °C: Functional Group Additivity in Isomers with Closely Spaced Polar Groups.

Abstract   Apparent molar volumes have been determined using a high-pressure vibrating-tube densimeter for aqueous solutions of 2- and 3-hydroxypropionic acid, at temperatures from 100 �C to 325 �C and pressures as high as 15 MPa. The results were corrected for acid ionization and extrapolated to infinite dilution to obtain the standard partial molar volumes, V 2o. The standard partial molar volumes of both isomers increase with temperature towards a positive discontinuity at the critical point, which is typical for almost all non-electrolytes. The temperature dependence of V 2o for the sodium salts of the acids is consistent with a negative discontinuity at the critical point, as displayed by all other aqueous electrolytes. Values of the apparent molar volumes of 2-hydroxypropionic acid are more positive than 3-hydroxypropionic acid by ∼2 cm3⋅mol−1, both in the neutral form and as the sodium carboxylate salt. This is the first demonstration at such high temperatures that functional group additivity in alkyl organic solutes with closely spaced polar groups is preserved to within such small differences. The onset of thermal decomposition prevented measurements at temperatures above 325 �C. [ABSTRACT FROM AUTHOR]