Quantitative Study of Gas-Liquid Interface Adsorption Based on Theoretical Modeling and Molecular Dynamics Simulation

The study of surfactant adsorption behavior at the interface is particularly important to guide the treatment of wastewater containing surfactants and the application of surfactants. However, there are still many challenges to fully understand their interfacial adsorption behavior. In this work, the limitations of Gibbs' formula in describing interfacial adsorption were summarized. A new isothermal adsorption formula based on the thermodynamic equilibrium of the system when adsorption reaches equilibrium was developed. Through this approach, a quantitative description of interface adsorption can be realized. Thus, the relationship between surface concentration and surface tension can be further elucidated. Taking anionic surfactant sodium dodecyl sulfate as an example, the numerical curve of surface concentration (Cs) was obtained, and the Cs was 1.486 mol/L at saturation. Molecular simulations were used to study the behavior of surfactant molecules near the interface and to study the properties of the solution surface phase. Based on the simulated data, the Cs at saturation was calculated to be 1.480 mol/L, which validates the model result. In addition, to address the abnormal expansion of the simulated interface caused by surfactant adsorption, an algorithm for calculating the surface area of the fluctuating interface is established, and this algorithm realizes the convenient and accurate calculation of the interfacial area.