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Hydrophilic-lipophilic balance, solubility parameter, and oil-water partition coefficient as universal parameters of
1School of Pharmacy, Temple University, Philadelphia, PA 19140, USA.
Abstract:
The following three parameters describing the balance between the hydrophilicity of the polar moieties and the lipophilicity of the hydrocarbon moieties of nonionic surfactants are tabulated and examined: hydrophilic-lipophilic balance (HLB), oil-water partition coefficient (KWO), and a solubility parameter (delta O) modified to take into account hydrogen bonding between water and the ether and hydroxyl groups of the surfactants. The purpose of the study is to ascertain whether these parameters are universal properties applicable to different categories of nonionic surfactants. Included were the following five nonionic surfactant categories or homologous series, comprising a total of 51 surfactants for which KWO values are available: octoxynols, nonoxynols, polyoxyethylated linear alcohols and dodecylamines, and sorbitan monoesters. The HLB of each homologous surfactant series gave a nearly linear correlation with log KWO. However, the five lines were spread far apart rather than falling on a single master curve. Therefore, the HLB is not a universal property, because it is based exclusively on the weight-percent of polyoxyethylene or polyol in the surfactant molecule while disregarding its molecular weight, the chemical nature of its hydrophilic and lipophilic moieties, and the structural features of the latter. These characteristics are taken into account when computing delta O. The correlation between the 51 experimental log KWO values and their calculated delta O is linear, with a regression coefficient of 0.902. Moreover, it is possible to estimate the KWO value of a nonionic surfactant (which is difficult to determine experimentally) from its delta O value (which is readily calculated) within a 95% confidence limit. Our conclusion, subject to continuing reappraisal as additional KWO values become available, is that delta O is a universal property applicable to all categories of nonionic surfactants.