Adsorption of ionic surfactants at microscopic air-water interfaces using the micropipette interfacial area-expansion method: Measurement of the diffusion coefficient and renormalization of the mean ionic activity for SDS

The dynamic adsorption of ionic surfactants at air-water interfaces have been less-well studied than that of the simpler non-ionics since experimental limitations on dynamic surface tension (DST) measurements create inconsistencies in their kinetic analysis. Using our newly designed “Micropipette interfacial area-expansion method”, we have measured and evaluated both equilibrium and dynamic adsorption of a well-known anionic surfactant, sodium dodecyl sulphate (SDS), in the absence or presence of 100 mM NaCl. Our focus was to determine if and to what extent the inclusion of a new correction parameter for the “ideal ionic activity”, A_±i, can renormalize both equilibrium and dynamic surface tension measurements and provide better estimates of the diffusion coefficient of ionic surfactants in aqueous media obtained from electroneutral models, namely extended Frumkin isotherm and Ward-Tordai adsorption models. We found that the estimated value of the new parameter, A_±i = 0.29, is key to obtain the diffusion coefficient D = 5.3 ± 0.3 × 10⁻⁶ cm²/s for SDS, in excellent agreement with the literature. These new technique and analyses can now be applied to study the interfacial adsorption of a range of both ionic and non-ionic surface-active molecules, including the potentially slower-diffusing polymers and biological materials like lipids, peptides, and proteins.