Thermodynamic Study on Surface Adsorption and Micelle Formation of a Hybrid Anionic Surfactant in Water by Surface Tension (Drop Volume) Measurements

Abstract

To study the formation of micelles in bulk water and adsorbed film at the air/water interface, we have thermodynamically studied a novel type of anionic surfactant, the so-called “hybrid type”. We used sodium 1-oxo-1[4-(tridecafluorohexyl)phenyl]-2-hexane sulfonate (here, abbreviated as FC6-HC4), which involves a perfluorocarbon chain attached to the phenyl group as the primary hydrophobic group and a short hydrocarbon side chain attached to a sulfonate group.The surface tension (γ) of solutions at different temperatures was measured by means of a drop volume technique. The critical micellization concentrations (cmc's) were determined from the plot of γ vs logarithmic molality as a function of temperature ranging from 5 to 50 °C and the cmc−temperature curve was found to have a minimum of ca. 15 °C. The degree of counterion binding (β) was estimated at each temperature from the Corrin−Harkins plot. The β value itself shows a little temperature dependency ranging around 0.57, which is smaller than those of common surfactants (β = 0.7−0.8), reflecting the low charge density on the micellar surface formed by top-heavy FC6-HC4 molecules. The collected data on the cmc and β as a function of temperature and a literature value of the aggregation number allowed us to calculate the Gibbs energy change ( ) of micelle formation, and the Gibbs−Helmholtz plot of enabled us to estimate the enthalpy change and then the entropy change . Between and a compensation rule was found to hold as = /Tc + σ as has been observed for more than 18 species of different kinds of surfactants in water where Tc is the compensation temperature and σ, the entropy change at = 0. The effect of added salt on γ or the surface excess (the surface absorbed amount relative to H2O), Γ, was also discussed in detail. Compared with common surfactants, even a small addition of NaCl (below 5 mmol kg-1) strikingly enhanced the surface activity inducing a great depression not merely of surface tension but of cmc as well.