Probing the Interaction Mechanism between Alkanes and Hydrophobic Substrate Using Atomic Force Microscopy and Molecular Dynamics Simulation

Abundant organic-rich nano-/micropores in unconventional oil reservoirs result in relative hydrophobic pore surface and extreme difficulty to displace the oil stored in the matrix. Hence, it is imperative to reveal the nanomechanical features between crude oil and hydrophobic rock surfaces. In this work, the effects of hydrophobicity of pore surface on oil/solid surface interactions and oil recovery were investigated using atomic force microscope (AFM), molecular dynamics (MD) simulation, and core displacement experiments, at molecular, nano-, and macroscales, respectively. The core displacement experiments revealed that the recovery of the hydrophobic core (contact angle 123.0 degrees) was 9.78% lower than that of the hydrophilic core (contact angle 18.4 degrees) with the same porosity and permeability. By combining AFM force measurements with theoretical force analysis, it was found that the alkanes/hydrophilic surface interaction could be well described by the classical Deriaguin-Landau-Verwey-Overbeek (DLVO) theory model. However, alkanes/hydrophobic surface interaction was much stronger than the theoretical value derived by the DLVO theory model. Hydrophobic interaction was conformed and measured, and the decay length D-0 was found to be 1.65 nm. Furthermore, the contribution of hydrophobic interaction accounted for more than 90% of the resultant force in the range from 0.68 to 9.38 nun. The attractive depletion force and migration force, induced by density depleted region and the migration of water molecules, are probably the underlying mechanism of the origin of hydrophobic interaction. Owing to higher hydration number and larger hydration radius, the divalent ions like Ca2+ possess a stronger shielding ability to hydrophobic effect than the monovalent ions like Na+. Our results provide a novel insight into hydrophobic interactions and offer consequential guidance not only for unconventional reservoir exploitation but also for other industrial processes involving hydrophobic surface, such as protein folding, oriented gas transport, and mineral flotation.