Charge-modulated calcite surface for anionic surfactant adsorption from molecular dynamics simulations

Since surfactant adsorption on reservoir rock means the loss of a valuable component, it makes surfactant flooding economically less feasible. In this work, adsorption behaviors of anionic surfactant on calcite surface were investigated with different charge states for the first time using molecular dynamic (MD) simulations. To reveal the underlying adsorption mechanism, adsorption processes were systematically analyzed with the adsorption energy, mean square displacement (MSD), relative concentration distribution (RCD), and radial distribution function (RDF). The results showed that sodium dodecyl sulfate had the strongest adsorption on the surface of calcite. When calcite surface was modelled with a charge state up to 80 e (8.24 × 1014 e/cm2), sodium dodecyl benzene sulfonate (SDBS) was found to be most stable on 10 e (1.03 × 1014 e/cm2) positively charge surface, with the maximum adsorption energy and the minimum diffusion coefficient. Increasing the carbon chain length of surfactant tail, the adsorption energy of SDBS became weaker, which can be attributed to the van der Waals interactions (long-range dispersion interactions) among surfactants. A monolayer or bilayer of surfactants can be derived based on the RCD. RDF curves revealed that hydrogen bonds played an important role in interactions among calcite surface, water molecules, and head groups of surfactants. The obtained results offer important insights into charge-modulated anionic surfactant adsorption on calcite surface at molecular scale and a guide to optimal flooding conditions with reduced surfactant loss.