Studies on the partial specific volume of a poly(ethylene glycol) derivative in different solvent systems

The specific volume of charged supramolecular compounds dissolved in organic solvents varies considerably with the solvent system applied; in addition, it is influenced by the presence of salt. In this study we determined the specific volume of an uncharged molecule from the same molar mass range in order to find out whether it shows the same dependencies. To allow application of solvents of widely differing polarity, including water, a poly(ethylene glycol) derivative of molar mass 3,650 g/mol was used as a model system. The primary method applied for determining the specific volume was the buoyant density method, in which sedimentation equilibrium experiments using solvent mixtures of different density are performed and the specific volume is obtained as the reciprocal of that solvent density for which the compound is neutrally buoyant. A second method applied for determination of the specific volume was digital densimetry. We found that the strong influence of the solvent on the specific volume observed with charged compounds is also shown by the uncharged poly(ethylene glycol) derivative, the differences in the specific volume between different solvent systems amounting up to 15%; however, no significant dependence on the presence of salt was observed. We also found that, with the compound studied, a simple rule relating the specific volume and solvent polarity apparently does not exist.