Concentration-Dependent Interfacial Behavior of Calcium Ions and Sodium Dodecyl Sulfate Molecules at the Gas–Liquid Interface

The molecular structure and dynamic process of the gas-liquid interface are key factors determining the stability of foam, emulsion, and the gas absorption system. In this study, from the perspective of the concentration effect, the gas-liquid interface structure characteristics under the coupling influence of different SDS interface concentrations and calcium chloride concentrations were studied based on molecular dynamics theory. The results reveal that enhanced interfacial free energy and confined water diffusion constitute a dual-path mechanism that fundamentally strengthens foam film stability. Owing to its high charge density and strong headgroup affinity, Ca²⁺ dominates the competitive adsorption process and reconstructs the interfacial electric double layer (EDL), redefining the IHP-OHP structure through synergistic effects of charge repulsion, solvation, and electrostatic screening. At high SDS coverage (NSDS ≥ 49), a ternary interaction among counterions, water molecules, and headgroups forms a quasi-network structure supported by ion and water bridges. This network provides additional adsorption sites, reinforces lateral constraints, and stabilizes the interfacial morphology, allowing Na⁺ to partially regain its position in the IHP region. The three-dimensional synergistic structure effectively absorbs external perturbations, maintaining the thickness and integrity of the EDL and imparting enhanced mechanical and electrochemical resilience to the foam film.