Molecular Interactions between Orinoco Belt Resins Olga Castellano,* ,† Raquel Gimon, ‡ Carlos Canelon, † Yosslen Aray, § and Humberto Soscun* ,§,∥ † Gerencia Departamental de Investigació n Estrate ́ gica RIIE, ‡ Gerencia Departamental de Refinació n e Industrializació n, Gerencia General de Refinació n, Petró leos de Venezuela (PDVSA)-Intevep, Los Teques, Estado Miranda, Apartado 76343, Caracas 1070-A, Venezuela § Centro de Quimica, Instituto Venezolano de Investigaciones Científicas (IVIC), Altos de Pipe, Caracas 1020-A, Venezuela ∥ Laboratorio de Química Inorga ́ nica Teó rica (LQIT), Departamento de Química, Facultad Experimental de Ciencias, La Universidad del Zulia, Apartado 526, Maracaibo 4001-A, Venezuela ABSTRACT: This paper describes the characterization of structural, energetic, and electric properties of the molecular complexes between Orinoco belt resins through the application of computational molecular mechanics (MM), semi-empirical parametrization (PM6), and density functional theory (DFT) (PW91 and HCTH) in conjunction with the double numeric and polarized (DNP) basis set. The resin sources for the studied molecules are Orinoco belt vacuum residues (Carabobo, Hamaca, Merey, Merey-Mesa, and Zuata). Molecular structures of these compounds were proposed from analysis of experimental characterization. The study of molecular interactions has shown that these resins are able to form stable van der Waals complexes, where their computed stability has a large dependence upon the applied theory level. A qualitative description of the formation of these complexes could be obtained with these methodologies. In particular, MM overestimates resin interaction energies when compared to PM6 and DFT (PW91 and HCTH) results. However, the HCTH/DNP approach leads to interaction energy values for resin−resin complexes that lie in the range from −2.39 to −7.09 kcal/mol, in better agreement with literature reports and chemical expectations than PW91/DNP interaction energy values. The stability of these complexes and the strength of the resin self-association can be rationalized considering their chemical nature and the induced electric properties (dipole moment and polarizabilities) by molecular interactions. Additionally, inclusion of dispersion in the DFT calculations of the resin molecular complexes improves the energetic pattern of the studied molecules significantly. 1. INTRODUCTION Petroleum is a complex mixture of organic and inorganic materials, in which the chemical composition depends upon the source, and their components are operationally classified in terms of solubility through the saturates, aromatics, resins, and asphaltenes (SARA) separation method. 1 The petroleum stabi- lity in only one fluid phase is governed by a delicate com- bination of molecular interactions between their polar and nonpolar components, leading to a dynamic physical chemistry equilibrium. 2 In general, saturate (S) and aromatic (A) com- ponents are nonpolar compounds, while the polarity in petroleum is introduced by the presence of low- and high- polar compounds, such as resins (R) and asphaltenes (A), respec- tively. Because of the great variety of forms, sizes, and chemical complexities of petroleum components, a detailed description of their structure is a very difficult experimental and theoretical challenge. Asphaltenes are the heaviest and most complex components of petroleum, which can be separated by using paraffinic solvents of low molecular weight, such as n-pentane and n-heptane compounds, which are often referred as A(n-C5) and A(n-C7) asphaltenes, respectively. The structure of these compounds, soluble in toluene and other organic solvents, is characterized by the presence of aromatic fused rings with terminal aliphatic chains of different lengths and the presence of heteroatoms (N, S, and O) in different coordination modes. 3 These heteroatoms can be chemically coordinated into either the aromatic rings or the alkyl groups. With regard to the structural shape and molecular weight (MW) of asphaltenes, there has been a large controversy in the literature. Recently, the employment of high-definition experimental techniques, such as diffusion and mass spectroscopy, has allowed for the determination that asphaltene structures are characterized by island-shape models with molecular weights of about 750 Da, 4 which are independent of the petroleum origin source. Maltenes, referred to as the fraction of oil that remains in solution after the asphaltene separation, contain the A, S and R (insoluble in propane) components of oil. In particular, the shape and chemical composition of resins resemble those of asphaltenes but with smaller molecular structures and relatively longer aliphatic side chains, where these groups are respon- sible for their higher solubility in aliphatic solvents. 5 It has recently been shown that different source resins can be chemically analyzed using electrospray ionization−mass spec- trometry (ESI−MS), where the corresponding MWs are in the range of 367−423 Da. 6 Resins are sticky, dark brown materials, where their prop- erties lie between those of asphaltenes and the rest of the oil components. Structurally, the H/C resin ratio is in the 1.4−1.5 range, suggesting less aromatic structures and more alkyl and cycloalkyl groups than asphaltenes. Polarity is a chemical property that characterizes resin and asphaltene compounds, where resin polarity is intermediate Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 26, 2011 Revised: January 30, 2012 Published: March 2, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 2711 dx.doi.org/10.1021/ef2014452 | Energy Fuels 2012, 26, 2711−2720