Structural and Phase Behavior Studies of Pyridine and Alkyl Pyridine at the Interface of Oil/Water by Molecular Dynamics Simulation

Polar fractions of crude oil play an important role in the state and structure of the oil/water interface, and understanding their behavior is crucially important in oil recovery. In studying different properties of oil/water interface, there are issues to be resolved and understood on the basis of molecular structure. A synthetic oil/water system is usually used to model and to simulate molecular-based behavior of the interface involving a polar surfactant. This paper is devoted to modeling the behavior of pyridine and its alkyl derivative at the interface of synthetic oil/water by molecular dynamics simulation. Solutions of pyridine and 4-ethylpyridine in octane/water system were studied for the density distribution and the structure in bulk phases as well as their molecular orientation at the interface. The interface of the octane/water system was first studied by simulation of density profile and interfacial properties to validate the force field applied. The distribution of pyridine and 4-ethylpyridine in and at the interface was studied, and the orientation of these molecules was determined accordingly. The results of simulation indicate that pyridine dissolves in water and avoids the interface leaving no residue in the octane phase, while 4-ethylpyridine mainly occupies the interface and dissolves partly in water. The bivariat maps gives molecular orientations in terms of spherical polar angles and indicate that the pyridine ring plane takes a parallel position with respect to interfacial plane surface; the nitrogen atom in pyridine takes any direction toward water and octane equally, but in 4-ethylpyridine preferentially is oriented toward water.