Molecular dynamics simulation of the effect of calcium ions on the foamability of anionic surfactants

The presence of calcium ions in hard water significantly affects the foaming properties of surfactants, thereby limiting their effectiveness in industrial and everyday applications. This study aims to investigate how varying concentrations of calcium ions influence the foaming abilities of several surfactants. A combination of experimental analysis and molecular dynamics (MD) simulations was employed to examine three typical anionic surfactants: sodium laureth sulfate (AES), sodium alkane sulfonate (SAS-60), and sodium dodecylbenzene sulfonate (SDBS). Experimental results indicated that calcium ions exert inhibitory effects on the foaming characteristics of these anionic surfactants to varying degrees. MD simulations further revealed that calcium ions hinder the interactions between surfactant head groups and water molecules, displaying a strong tendency to bind with the head groups. The presence of calcium ions diminishes the electrostatic interactions among surfactants and alters their molecular arrangement. In addition, calcium ions infiltrate the interface between the head groups and water, restricting the mobility of water molecules. These findings suggest that interactions between counterions, such as sodium and calcium ions, and surfactants are critical factors contributing to the reduction in foaming ability. However, the ethylene oxide (EO) groups in AES can partially mitigate the binding of calcium ions to the head groups, thereby maintaining their foaming ability in hard water.