Aspects of asphaltene aggregation obtained from coarse- grained molecular modelling Julio F. Jover a,b , Erich A. Müller a , Andrew J. Haslam a,c , Amparo Galindo a , George Jackson a , Hervé Toulhoat b , Carlos Nieto-Draghi b ,* a Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. b IFP Energies Nouvelles, 1 et 4, Avenue de Bois-Préau, 92852 Rueil-Malmaison, France. c Qatar Carbonates and Carbon Storage Research Centre (QCCSRC), Imperial College London, London SW7 2AZ, U.K. KEYWORDS: molecular dynamics, crude oil, fouling Supporting Information : SI.m4v; movie of simulations of a Coarse grained asphaltene model in Heptane and Toluene ABSTRACT: We have performed a molecular-simulation-based study to explore the some of the underlying mecha- nisms of asphaltene aggregation. The daunting complexity of the crude-oil + asphaltene system precludes any type of meaningful molecular simulation unless some assumptions are made with respect to the key physical and chemical properties that must be explicitly described. In the present work we focus on molecular simulations of a coarse-grained (CG) model of asphaltene molecules in pure solvents, which are based on the assumption that the general size asym- metry and asphaltene morphology play a key role in the aggregation process. We use simple single isotropic Lennard- Jones sites to represent paraffinic and aromatic C6 segments, which are used as building blocks for the description of continental asphaltene models and solvent moieties. Parameters for the intermolecular models (ε and σ) for solute and solvent molecules are chosen to reproduce the experimental density of the liquid phase for different mixtures. An ex- plicit pure solvent has been considered and the relationship between the aggregation mechanism and the solvent nature has been investigated through direct simulation of the aggregation process. The results reproduce accurately expected trends observed for more-complex models as well as experiments, e.g., strong aggregation of asphaltene molecules in n- heptane and high solubility in toluene. Different asphaltene models based on different geometries reveal that even at this level of simplification the topology of the molecules (number and position of aliphatic branches) does affect the way molecules aggregate. 1. INTRODUCTION Asphaltenes have been studied for well over 50 years 1 , yet remain one of the least-understood components of crude oil. They are formally defined as a solubility class 2,3 ; specifically they are classified as soluble in toluene but insoluble in n-heptane (although in some cases they are defined instead with reference to insolu- bility in n-hexane or n-pentane). This definition, how- ever, does not suffice to provide any insight into the nature of the molecules themselves. The quantity and properties of asphaltenes ex- tracted from a single source vary according to the pre- cipitant, the temperature, the pressure and the length of time elapsed between mixing and precipitation 1 . Accordingly, early in the studies of asphaltenes the need for standardising the procedures used to engineer precipitation was identified. In the late seventies Dickie and Yen 4 introduced a hierarchical approach to the study of asphaltene precipitation. Two levels of aggre- gation were proposed: a first level in which asphaltene molecules adhere forming a “nano-aggregate”, and a second level wherein the nano-aggregates themselves aggregate forming clusters. Interestingly more empha- sis was put on the structures of the two levels of aggre- gates than on the actual molecular detail. A modern version of this model is still used as the reference for asphaltene deposition 5 .With the reduction of light-oil reserves around the world, crudes previously discarded as problematic are now considered to be economically viable. Consequently more effort and resources are being put into improving our understanding of why and how asphaltenes, once called the cholesterol of oil 6 , aggregate and precipitate, causing all kinds of serious extraction and production problems. Yet, in