Water activity in polyol/water systems: new UNIFAC parameterization

[摘要] Water activities of a series of polyol/water systems were measured with anAquaLab dew point water activity meter at 298K. The investigated polyolswith carbon numbers from n=2-7 are all in liquid state at roomtemperature and miscible at any molar ratio with water. In aqueous solutionswith the same molar concentration, the diols with lower molecular weightlead to lower water activities than those with higher molecular weights. Fordiols with four or more carbon atoms, the hydrophilicity shows considerabledifferences between isomers: The 1,2-isomers - consisting of a hydrophilicand a hydrophobic part - bind less strongly to water than isomers with amore balanced distribution of the hydroxyl groups. The experimental wateractivities were compared with the predictions of the group contributionmethod UNIFAC: the model predictions overestimate the water activity ofwater/polyol systems of substances with two or more hydroxyl groups andcan not describe the decreased binding to water of isomers with hydrophobictails. To account for the differences between isomers, a modified UNIFACparameterization was developed, that allows to discriminate between threetypes of alkyl groups depending on their position in the molecule. These newgroup interaction parameters were calculated using water activities ofalcohol/water mixtures. This leads to a distinctly improved agreement ofmodel predictions with experimental results while largely keeping thesimplicity of the functional group approach.

[学科分类] 大气科学