Characterization of surfactant adsorption at a liquid-liquid interface by drop volume tensiometry

Surfactant characterization is important for many practical applications, such as the removal of hydrophobic contaminants from soils through surfactant-enhanced washing or flushing processes. Dynamic interfacial tension measurements can be useful in describing the adsorption behavior of surfactants at liquid-liquid interfaces. In this work, selected surfactants are tested at the mineral oil-water interface using a drop volume tensiometer. A model is developed to account for the neck formation in suspended drops under certain experimental conditions. The model relates apparent values of interfacial tension calculated from drops possessing necks to actual values. Dynamic interfacial tension measurements are then used to describe surfactant adsorption. In many cases, surfactant adsorption can be described by either kinetic or diffusion-controlled models. A kinetic-controlled model is found to fit the dynamic interfacial tension data obtained for sodium dodecyl sulfate. Span 80 and Triton X-100 are found to fit diffusion-controlled models. For Span 80 dissolved in the mineral oil phase, the short diffusion time approximation is valid. For Triton X-100 dissolved in the aqueous phase, the long diffusion time approximation is valid. A dimensionless parameter relating adsorption time, $\rm t\sb{a},$ to a characteristic diffusion time, $\rm\tau\sb{D},$ is introduced in order to determine the general applicability of the diffusion approximations. The ratio $\rm t\sb{a}/\tau\sb{D}$ is found to be useful in characterizing diffusion-controlled adsorption