Molecular physics in ion-bridging effect for wettability alteration of rock surfaces

Highlights: • Ion-bridging plays a crucial effect on the wettability alteration of rock surfaces via ion exchange mechanism. • Valence of ions in water film greatly affects the bridging effect and the wettability of substrate. • Ratio of monovalent and divalent ions can be varied to change the strength of ion-bridging. • Mechanism is revealed through the molecular and ionic distributions, system energy and others. We give a direct evidence of ion-bridging effect between polar molecules and substrates on the wettability alteration of rock surface by using molecular dynamics simulations. The contact angle of water in a water–oil-silicon dioxide system varies with the ratio of monovalent and divalent ions in hydrated water film. The contact angle gradually increases with increasing the proportion of Ca²⁺ ions owing to the enhanced strength of ion-bridging effect related to the valences of ions. The distinctive ion-bridging effects are confirmed from the distributions of polar molecules and ions, the theory of electrical double layers and the variation of system energy.