We employed heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy to obtain a molecular-level understanding of the interaction between the anionic surfactant sodium dodecyl ammonium sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). We observed that these surfactants show a strong cooperative effect on their adsorption to the water-air interface. Even at bulk concentrations 1000 times lower than the critical micelle concentrations of SDS and DTAB, a nearly complete surface surfactant layer is observed when both surfactants are present. This strong enhancement of the surface concentrations of DS- and DTA+ can be quantitatively explained from the favorable Coulomb interaction of the oppositely charged headgroups of DS- and DTA+ and the electrostatic interactions with their counterions. The HD-VSFG results are complemented by a modified Langmuir adsorption model in which we include the free energy associated with the electrostatic interactions of the surfactant ions and their counterions.

The Netherlands Organisation for Scientific Research (NWO)
J. Phys. Chem. Lett.
Ultrafast Spectroscopy

Vilangottunjalil, A., Versluis, J., & Bakker, H. (2024). Observation of Electrostatically Driven Surface Adsorption in Mixed Surfactant Systems. J. Phys. Chem. Lett., 15(6), 1596–1602. doi:10.1021/acs.jpclett.3c03377