Diffusion-adsorption kinetics of anionic surfactants at a liquid–liquid interface: An analytical and experimental study

We present a new dynamic interfacial adsorption model for estimating the diffusion and adsorption kinetics of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) at an n-decane–water interface. The migration of surfactant molecules to the interface progressively reduces the dynamic interfacial tension. From these data, equilibrium fitting parameters are estimated using adsorption models, including the Langmuir, Frumkin, van der Waals, and Volmer models. The experimental data are further analyzed using the proposed analytical model, which calculates the surface concentration as a function of diffusivity and adsorption rate. This time-dependent surface concentration is then related to interfacial tension by the Frumkin equation of state. Consequently, the apparent diffusion coefficient and adsorption rate constant are estimated from the best fit of the model to the experimentally measured data. The results show that SDS exhibits higher initial diffusivity and faster adsorption rates. In contrast, SDBS achieves a lower interfacial tension at equilibrium despite its lower diffusivity and slower adsorption rates. The comparative observations span concentrations from dilute to near the critical micelle concentration at 25 °C.