THE EXPANDED HANSEN APPROACH TO SOLUBILITY PARAMETERS - PARACETAMOL AND CITRIC-ACID IN INDIVIDUAL SOLVENTS

In this study two solubility-parameter models have been compared using as dependent variables the logarithm of the mole fraction solubility, lnX(2)(e) and ln(alpha)/U (originally used in the extended Hansen met hod), where alpha is the activity coefficient and U is a function of t he molar volume of the solute and the volume fraction of the solvent. The results show for the first time the proton-donor and -acceptor hyd rogen-bonding capacities of paracetamol, as measured by the acidic and basic partial-solubility parameters. The influence of solvents on the differential scanning calorimetry (DSC) pattern of the solid phases w as also studied in relation to the solubility models tested. Citric ac id was chosen as a test substance because of its high acidity and its proton donor capacity to form hydrogen bonds with basic solvents. The partial acidic and basic solubility parameters obtained from multiple regression were consistent with this property, validating the model ch osen. The results show that the more direct lnX(2)(e) variable was mor e suitable for fitting both models, and the four-parameter model seeme d better for describing the interactions between solvent and solute.