Investigation of drug self-association in aqueous solution using calorimetry, conductivity, and osmometry

Based on heat of dilution, osmotic coefficient and conductivity data, it is evident that the compound of interest in this study undergoes self-association in aqueous solution. The average aggregation number for the association product is small, not greater than 4. The likely association products are cation-cation dimer, cation-cation-anion or all cation trimer, and/or cation-cation-cation-anion tetramer. The observed self-association in aqueous solution is purely enthalpy driven, being entropy disfavored, indicating that the driving force for the self-association is likely to be either hydrogen bonding or pi-pi interaction or both, but not hydrophobic interaction or pure electrostatic interaction.