Solubility dependence of peracylated d-glucopyranoses in supercritical carbon dioxide on the structure of their acyl moieties

Identification of chemical groups that enhance solubility of hydrocarbon-based surfactants and polymers in CO₂ is of current interest in expanding applications of this 'green solvent'. In particular, the acetyl moiety is believed to solvate effectively in CO₂ through a Lewis acid-base interaction between the carbonyl oxygen and CO₂ carbon and this is further enhanced by specific weak H-bonding interaction of the α-hydrogens with the CO₂ oxygen. In this communication we investigate the influence of this weak secondary interaction further by measurement of CO₂ solubility of a range of β-d-glucopyranose pentaalkanoates containing as the acyl functionality acetyl, propionyl, butyryl, isobutyryl and trimethylacetyl (pivaloyl) groups. The results show that progressive replacement of the α-hydrogens of the acetyl by methyl results initially in a remarkable drop in solubility but surprisingly this is regained in the trimethylacetyl derivative that demonstrates a similarly high solubility to that of the acetyl compound.