Thermodynamic study of the adsorption of octylsulfinylethanol at the micellar solution/air interface

The surface tension of the aqueous solution of octylsulfinylethanol (OSE) was measured as a function of temperature at concentrations around the critical micelle concentration (CMC). The derivative of surface tension with respect to the temperature vs concentration curve was found to change abruptly at the CMC like that of dodecylammonium chloride (DAC) reported previously [1]. By comparing the experimental results with the thermodynamic relations derived for the nonionic surfactant, it has been proved in the case of the OSE solution that the micelle formation can be treated thermodynamically as the appearance of a macroscopic bulk phase in the solution. Further, it was found that the entropy change associated with the transfer of surfactant from the micelle to the water/air interface has a positive value for OSE whereas for DAC it had a negative one. This has been accounted for by the difference between OSE and DAC in contributions of the hydrocarbon chain and polar group parts of the surfactant molecule to its mean partial molar entropy.