Mechanistic study of the effect of hydrocarbon unsaturation on the distribution state of water molecules at the oil-water interface by oblique incident reflectance difference technique

The molecular distribution of the oil–water interface has an important effect on the efficiency of water–oil displacement. However, challenges persist with interfacial thickness and dielectric properties and their interactions at the nanoscale. In this study, the oil–water interface with different compositions is detected by the oblique incident reflectance difference (OIRD) technique to analyze the influence of oil composition on the molecular distribution of the interface. When the oil layer is completely spread out, the electron cloud distribution of water molecules at the interface is affected by the electronegativity of oil layer molecules. When the unsaturation of oil layer molecules gradually increases, the density of water molecular electron cloud at the interface decreases due to the electronegativity of CC, which results in the decrease of hydrogen bond density. Therefore, smaller clusters of molecules can be formed, resulting in a thicker transition layer at the interface. The ability of the OIRD technique to qualitatively and quantitatively analyze the oil-water interface under actual drive conditions was demonstrated by the determination of the brine/surfactant solution-oil formation interface. In addition, the OIRD signal can characterize the effect of reservoir medium composition on the interface molecular distribution state, which provides a new idea for evaluating the effect of displacement fluid.