Molecular dynamics simulations of adsorption behavior of mixtures of surfactant and polymer at C3A/water and Ettringite/water interfaces

Abstract

In cementitious system, surfactant is used as air-entraining agents, which is often incompatible with other chemical admixtures, such as superplasticizers. The adsorption of surfactants at solid–liquid interface is critical for the bubble stability. The measurement of adsorption properties of surfactant in fresh cement or concrete is very difficult. In this study, classical molecular dynamics simulations is used to analysis the influence of two polymers (call as HPMC and PCA, respectively) on the adsorption behaviors of the two anionic surfactants (call as HSO4 and HPO4, respectively) at the C3A/water and ettringite/water interfaces. At solid/water interface, both of HSO4 and HPO4 molecules form a monolayer with hydrophilic head groups adsorbed on the solid surfaces. After adding polymers, the adsorption conformations are changed to spherical micelle or multilayer. The electrostatic interaction between oxygen atoms of hydrophilic head groups and calcium atoms of solid surface plays a decisive role. Therefore, the value of adsorption energy of HPO4 with more negatively charged hydrophilic head groups is greater than that of HSO4. The adsorption energies of the two surfactants are weakened by the two polymers, but the adsorption energy value of HPO4 is always greater. The influence mechanism of HPMC and PCA is different. The negatively charged PCA has a strongly competitive adsorption with the surfactants. The neutral HPMC has a weak adsorption capacity but it carries many hydroxyl groups to interact with the hydrophilic head groups of surfactants.