In this paper, properties of binary mixture of polar portions of the crude oil are studied by classical molecular dynamics simulation. Particularly, the bulk and the interface of pyridine (and its alkyl derivatives) mixture with n-octane are studied, including density and structure in bulk as well as their molecular orientations at the liquid-vapor interface. Initial simulations of density profile and interfacial properties are to validate the force field applied. Additionally, the structure of solution of pyridine derivatives in bulk and at the interface is studied. As a result, the derivatives with long alky chain (of low surface tension) approaches to the surface while those with short chain (of high surface tension) aggregate in the solution and avoid approaching the surface. Finally, the molecular orientations of pyridine and alkyl derivatives are investigated in the bulk and at interface by the intrinsic bivariate orientational maps. (C) 2015 Elsevier B.V. All rights reserved.