Water Enhances the Aggregation of Model Asphaltenes in Solution via Hydrogen Bonding

Abstract

The self-association properties of two 2,2′-bipyridine derivatives, 4,4′-bis(2-pyren-1-yl-ethyl)-2,2′-bipyridine (PBP) and 4,4′-bis[2-(9-anthryl)ethyl]-2,2′-bipyridine (ABA), in water-saturated solvents were studied as models for petroleum asphaltenes. 1H NMR spectroscopy titration experiments in chloroform established that both compounds self-associate in solution. The addition of water (ranging from 0.43 to 0.86 g/L) increased the thermal stability and the aggregation behavior of PBP and ABA at low concentration (3.06 × 10−4 and 1.28 × 10−3 M, respectively). The chemical shift, line-width, and spin−spin (T2) relaxation time of water protons in these solutions showed that the O−H···N hydrogen bonds between the pyridyl nitrogens of PBP/ABA and water were responsible for their aggregation at low concentration, although FTIR spectroscopy suggested that the H-bond between PBP and water was relatively weak. The solubility of water in PBP solutions showed that the number of water molecules per PBP molecule varied between 1 and 2 in water-saturated CHCl3 and that the Gibbs free energy for the exchange of water between the aqueous phase and the solution of PBP was −16 kJ/mol at 22 °C. Water likely enhanced the stability of aggregates by reinforcing π−π interactions via water-bridged intermolecular H-bonding between the pyridyl nitrogen.