A CORRELATION OF MOLECULAR FLEXIBILITY WITH VOLUME AND HEAT OF MIXING OF ORGANIC SOLUTES WITH WATER AND GLYCOL-WATER MIXTURE

The presence of unsaturation, small rings, and especially aromatic rings causes the volume of mixing of organic compounds to tend toward zero. Evidence is presented to show that the space requirement of methylene groups is approximately 2 ml/mol greater when they are conformationally free than when they are restricted, and it is hypothesized that the net mixing volume for open-chain compounds results from an interference with their rotational freedom by the cohesive forces of water. The volume of mixing appears to be unrelated to icelike structure formation because it is independent of temperature over a range of 80° and is nearly the same for the glycol-water mixture in which the solutes have normal heat capacities. Mixing is more exothermic for flexible molecules, and the magnitude of the variation is comparable with the PΔV work done by the internal pressure.