Cluster structure of interfacial molecules studied by oblique incidence reflectance difference: Influences of salt ion on oil-water interfacial properties

Tuning the concentration of the ions is beneficial for improving oil recovery by water flooding. Despite the widely recognized distribution of salt ions at the water interface, their effects on the structure of interfacial water, such as hydrogen(H) bonds, are unclear. In this study, using oblique incident reflectance difference (OIRD) technique and interfacial rheometer to analyze the alkanes-ion solution interface, we show that ions have a significant effect on the perturbation of hydrogen bonds at the alkanes-water interface. The change in the water layer structure follows the gradual increase in the concentration of Na₂SO₄/Na₂CO₃ and the decrease in the interfacial tension, and dielectric constant at the alkane-solution interface. Specifically, structure-breaking anions such as SO₄²⁻ and CO₃²⁻ decrease the average H-bonding of water at the alkane/water interface, thus damaging the molecular cluster structure at the interface. Although Cl⁻ will form hydration ions with water molecules, it will not break the hydrogen bond structure between water molecules at the interface. These results indicate the mechanism of anion effects on the alkane/water interface, and for samples with high saturated alkane content, a repellent solution containing SO₄²⁻ can be preferentially selected for repelling, providing a new idea for the study of the molecular boundary of the oil-water interface.