Adsorption of zwitterionic oligomer-grafted silica nanoparticles on rock surface in enhanced oil recovery studied by molecular dynamics simulations

Abstract

The application of surface-functionalized nanoparticles (NPs) in enhanced oil recovery (EOR) has been rapidly increasing. Adsorption of NPs on the rock surface is the key step. In this study, we systematically investigated the adsorption and desorption mechanisms of various brush-like zwitterionic oligomers functionalized silica NPs on the hydrophilic rock surface in an aqueous solution using molecular dynamics simulations. Our simulations show that the adsorption is primarily influenced by the oligomer chains, while the neutral silica NP core contributes very weakly only when it has direct contact with the rock surface at the grafting density of 5 or below. The interactions from SO3⁻ groups located at the tail of zwitterionic side chains account for more than 72% of the non-bonded interaction energy Enb between the NPs and the rock. The adsorption becomes stronger for NPs with shorter (4 monomer units) oligomer chains but slightly weaker for NPs with longer (9 monomer units) oligomer chains when increasing the grafting density, which is well explained by the change in the number of adsorbed SO3⁻ groups. Desorption simulations with the umbrella sampling method indicate that the free energy of desorption decreases with increasing the grafting density. The silica NPs grafted with longer oligomer chains (9 monomer units) require higher free energies of desorption than the ones with shorter oligomer chains. The adsorption of NPs on the rock surface enhances surface hydrophilicity, and the effect of longer oligomer chains is more profound than that of short oligomer chains. This research provides valuable insights into the adsorption and desorption mechanisms of surface-modified silica NPs on the rock surface at the molecular level and thus gives hints to the design of nanofluids.