Thermodynamic study on surface adsorption and micelle formation of a hybrid anionic surfactant in water by surface tension (drop volume) measurements

To study the formation of micelles in bulk water and adsorbed film at the a ir/water interface, we have thermodynamically studied a novel type of anion ic surfactant, the so-called "hybrid type". We used sodium 1-oxo-1[4-(tride cafluorohexyl)phenyl]-2-hexane sulfonate (here, abbreviated as FC6-HC4), wh ich involves a perfluorocarbon chain attached to the phenyl group as the pr imary hydrophobic group and a short hydrocarbon side chain attached to a su lfonate group. The surface tension (gamma) of solutions at different temper atures was measured by means of a drop volume technique. The critical micel lization concentrations (cmc's) were determined from the plot of gamma vs l ogarithmic molality as a function of temperature ranging from 5 to 50 degre es C and the cmc-temperature curve was found to have a minimum of ca. 15 de grees C. The degree of counterion binding (beta) was estimated at each temp erature from the Corrin-Harkins plot. The beta value itself shows a little temperature dependency ranging around 0.57, which is smaller than those of common surfactants (beta = 0.7-0.8), reflecting the low charge density on t he micellar surface formed by top-heavy FC6-HC4 molecules. The collected da ta on the cmc and beta as a function of temperature and a literature value of the aggregation number allowed us to calculate the Gibbs energy change ( Delta G(m)degrees) of micelle formation, and the Gibbs-Helmholtz plot of De lta G(m)degrees enabled us to estimate the enthalpy change Delta H(m)degree s and then the entropy change Delta S(m)degrees. Between Delta H(m)degrees and Delta S(m)degrees a compensation rule was found to hold as Delta Sm deg rees = Delta H(m)degrees/T-c + sigma as has been observed for more than 18 species of different kinds of surfactants in water where T-c is the compens ation temperature and sigma, the entropy change at Delta H(m)degrees = 0. T he effect of added salt on gamma or the surface excess (the surface absorbe d amount relative to H2O), Gamma, was also discussed in detail. Compared wi th common surfactants, even a small addition of NaCl(below 5 mmol kg(-1)) s trikingly enhanced the surface activity inducing a great depression not mer ely of surface tension but of cme as well.