(174x) Entropic Contributions to Supramolecular Assembly of Liquid Hydrocarbons

Hydrocarbon molecules can spontaneously associate in the liquid phase to yield local organized structures. These fluid structures might be responsible for modulating the self-assembly of polyaromatic hydrocarbons (e.g., discotic liquid crystals, proteins, asphaltenes) in the dispersed phase when the hydrocarbons are used as solvents. In this talk, we will discuss an integrated simulation and scattering experimental approach to investigate the self-aggregation mechanism of polyaromatic hydrocarbons in solutions. Structural investigation of a petroleum derivative (i.e., asphaltene) in 1-methylnaphthalene (1-MN) suggested an increase in the stacking of aromatic cores with temperature. To further investigate this behavior, we conducted wide-angle neutron scattering experiments on three different isotopologues of 1-MN (H, D, and HD) as a function of temperature (20 to 160 ˚C) to determine the solvent fluid structure. Representative 3-dimensional structures of the liquid system were obtained using Empirical Potential Structural Refinement (EPSR). With increasing temperature, 1-MN molecules showed an apparent increase in parallel (and antiparallel) stacking resulted from the ordered alignment of aromatic cores. The observed “increased stacking” feature of liquid 1-MN with temperature appears to contradict the conventional concept of entropy, wherein temperature increases are expected only to induce more disorder. The asymmetric molecular structure of 1-MN might induce directional entropic forces on its surface to direct parallel or antiparallel stacking. This behavior is well-known in colloidal systems; however, this effect has not been reported in simple liquid hydrocarbon systems. The results shed light on the role of molecular shape in understanding hydrocarbon liquid structure and its effect on the assembly of polyaromatic hydrocarbons in solutions.