A novel approach to the characterization of polar liquids. Part 1: pure liquids.

Liquid dosage forms, generally based on aqueous solutions, take an important role in drug administration. The approaches to a theoretical description of solvent and solubility properties have not yet proved completely satisfying. In this work, the Debye equation, which describes well polar and nonpolar molecules in an ideal gas, is extended to liquids. For this purpose, the Debye equation was modified and the term (E(i)/E) was introduced (E(i)=internal electric field, E=applied external electric field). Pure polar and nonpolar solvents were measured between 290.7 and 343.2 K. The values of (E(i)/E) were compared with the correlation factor g of the Kirkwood-Fröhlich equation, a measure for molecular pair correlations. For polar solvents, the relationship E(i)/E=m(1/T)+b as a function of temperature T was found. Associating compounds showed negative values of (E(i)/E) with a strong temperature dependency; the latter can be expressed by the slope m. A correlation between the absolute value of m and the corresponding Hildebrand solubility parameter delta could be established. This new approach allows to describe polar hydrogen-bonding liquids and provides a tool for a more rational design of liquid dosage forms.