Evidence of surfactant sub-monolayer adsorption at the air/water interface provided by laser scattering measurements of ultrafine gas bubbles

Surfactants are critical to many areas thanks to their characteristic adsorption property and surface activities; therefore, the understanding of their adsorption behaviors at interfaces is highly desirable. Typically, the adsorption of surfactants at the air/water interface has been interpreted based on monolayer models. However, recent studies have controversially proposed that there exists sub-monolayer adsorption underneath the air/water interface at surfactant concentrations well below its critical micelle concentration. In this report, we provide several pieces of experimental evidence to validate the presence of such sub-monolayer adsorption through laser scattering experiments on sodium dodecyl sulfate (SDS) adsorption at the air/water interfaces of ultrafine gas bubbles. In addition, by employing the calculation of the total surface area and Mie cross section, the Gibbs model was demonstrated to be inappropriate for SDS adsorption in bubble enriched solutions. Also, sum-frequency generation vibrational spectroscopy further reinforced the existence of the sub-adsorption layer instead of an adsorption monolayer. We believe that this work provides strong evidence supporting the existence of sub-monolayer adsorption which can be combined with other intrinsic surface-sensitive techniques to elucidate this phenomenon at the air/water interface. As the SDS concentration increases in bubble rich solutions, the surfactant layer alters its size and refractive index. The scattered light enhancement and SFG signal cancellation prove that sub-monolayer adsorption exists at the air/water interface.