The influence of heat capacity assumptions on the estimation of solubility parameters from solubility data

Steven H. Neau, Gordon L. Flynn, Samuel H. Yalkowsky

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Regular solution theory indicates that solubility parameters of crystalline organic compounds can be estimated from solubilities in London solvents. The equation for this purpose is: IN X2 - ΔHf RTf (Tf-T) T + ΔCp R (Tf - T) T - ln Tf T - V2θ21 RT (δ1 - δ2)2 where X2 is the mole fraction solubility of a compound in a solvent with a solubility parameter of δ1. With the exception of ΔCp, all parameters in the equation necessary to estimate the solute parameter, δ2, can either be suitably approximated or readily determined experimentally. In order to use the equation, simplifying assumptions have been made concerning ΔCp, namely: ΔCp = 0 or ΔCp = ΔSf, the entropy of fusion. In the present work, we have considered the extent to which these assumptions influence the magnitude of solubility parameters estimated from solubilities in n-hexane, n-heptane, n-dodecane, cyclohexane, carbon tetrachloride, toluene and benzene. Using n-alkyl p-aminobenzoates as test compounds, it is shown that solubility-based solubility parameters are relatively insensitive to the form of the equation used to calculate δ2. Specifically, solubility parameter estimations based on the two simplifying assumptions differ by no more than 0.2 (cal/ml) 1 2, an increment of the order of the presumed inherent error of estimation.

Original languageEnglish (US)
Pages (from-to)223-229
Number of pages7
JournalInternational Journal of Pharmaceutics
Issue number3
StatePublished - Feb 1 1989


ASJC Scopus subject areas

  • Pharmaceutical Science

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