Quantitative comparison of surface coverages derived from surface tension and XPS measurements

The presence of surfactants has a strong influence on the properties of interfaces of aqueous solutions with air and is thus of importance for a better understanding of processes involving liquid-vapor interfaces in the environment, atmosphere, and in industrial applications. The most widely used method to quantify the density of surfactants at the interface is the measurement of surface tension. Even though this is an indirect way to determine the surface concentration of surfactant molecules, surface tension measurements have provided a wealth of information for more than a century. Over the past decade, X-ray photoelectron spectroscopy (XPS) has established itself as a direct method to investigate the properties of liquid-vapor interfaces, including the concentration of ions and surfactants in the interface region. Here we compare the results of surface tension and XPS measurements on the example of n-alkane sulfonates with aliphatic chain lengths from 0 to 9 carbons. Our results show that for monodisperse systems, as those here under study, there is good quantitative agreement between the surface coverages obtained from surface tension measurements and those determined using XPS.