Enthalpy-entropy compensation phenomenon observed for different surfactants in aqueous solution

Based on previously reported thermodynamic data such as changes of the Gibb s energy (Delta G(m)degrees), the enthalpy (Delta H degrees(m)), and the en tropy (Delta S(m)degrees) on micelle formation of more than 15 species of s urfactants (including nonionic, anionic, and cationic surfactants), plots o f Delta H(m)degrees vs Delta S(m)degrees (not of Delta S(m)degrees vs Delta H(m)degrees, as is usually done) were made. For each surfactant, a linear relation having almost the same slope (1/307 K-1) within a small error (+/- 2.3%) but a different intercept (sigma) depending on the surfactant species was obtained, i.e., Delta S(m)degrees = (1/307)Delta H(m)degrees + sigma, where 1/307 (K-1) means that the so-called compensation temperature (T-C) i s 307 K, Strictly speaking, T-C ranges from 299 to 315 K, depending on the species. The intercept corresponds to the entropy change at a specific temp erature giving Delta H(m)degrees = 0, at which the driving force of micelle formation comes only from the entropy term; this temperature is characteri stic of the surfactant species. On the other hand, the compensation tempera ture has no significant meaning other than a mean temperature studied. (C) 1999 Academic Press.