Computational Simulations of Adsorption Behavior of Anionic Surfactants at the Portlandite-Water Interface under Sulfate and Calcium Ions.

The adsorption behaviors of two kinds of anionic surfactants (called HSO4 and HPO4, respectively) with different negatively charged hydrophilic head groups (sulfate and phosphate groups) under different concentrations of sulfate and calcium ions at the portlandite-water interface are investigated by molecular dynamics simulations. Although the adsorption strength of HPO4 is much greater than that of HSO4, the desorption energy of HSO4 is slightly greater at an early stage of desorption due to a more perpendicular orientation and denser packing of hydrophobic tail chains. After adding ions, the sulfate ion has a significant weakening effect due to competitive adsorption, and the negative influence of the calcium ion is weaker, and it even slightly promotes the adsorption at low concentration. Due to the stronger electrostatic interaction of phosphate head groups with the portlandite surface, adsorption strength and adsorption stability for HPO4 are always greater than that of HSO4 under the interference of sulfate ions. The competitive adsorption of the sulfate ion significantly weakens the interaction of hydrophilic head groups with portlandite and the dense packing of two surfactants. The calcium ion with low concentration approaches the portlandite surface and acts as an ion bridge to slightly enhance the adsorption of the surfactant. The ion bridging effect is stronger in the HPO4 system than in the HSO4 system.