Investigation of the Precipitation Behavior of Asphaltenes in the Presence of Naphthenic Acids Using Light Scattering and Molecular Modeling Techniques

A delay in the onset of flocculation is observed for asphaltenes in the presence of several naphthenic acids: methyl abietate, hydrogenated methyl abietate, 5β-cholanic acid, and 5β-cholanic acid-3-one. This flocculation behavior is monitored as a function of the added precipitant (<i>n</i>-heptane) to solutions of suspended asphaltenes and naphthenic acids in model solutions of toluene/<i>n</i>-heptane, using a combination of dynamic light scattering (DLS) and near-infrared (NIR) spectroscopic techniques. DLS and NIR show very good correlation in indentifying the onsets of flocculation, which varied among the series of naphthenic acids. Specific interaction energies and equilibrium intermolecular distances of asphaltenes and naphthenic acids are calculated using molecular mechanics. The results from molecular mechanics calculations support the experimental results of the titrations, and structure–property relationships are defined. Structure–property relationships are established for naphthenic acids, defining the relative contributions and importance of various functional groups: CC, CO, COOR, and COOH. The additive effects of naphthenic acids, defined by an increase in the precipitation onset, increase in the order of 5β-cholanic acid-3-one < hydrogenated methyl abietate < methyl abietate < 5β-cholanic acid, with experiments containing 5β-cholanic acid-3-one containing unexpected and interesting results

Noncovalent Interactions in Microsolvated Networks of Trimethylamine <i>N</i>‑Oxide

The effects of the formation of hydrogen-bonded networks on the important osmolyte trimethylamine N-oxide (TMAO) are explored in a joint Raman spectroscopic and electronic structure theory study. Spectral shifts in the experimental Raman spectra of TMAO and deuterated TMAO microsolvated with water, methanol, ethanol, and ethylene glycol are compared with the results of electronic structure calculations on explicit hydrogen-bonded molecular clusters. Very good agreement between experiment and theory suggests that it is the local hydrogen-bonded geometry at TMAO’s oxygen atom that dominates the structure of the extended hydrogen-bonded networks and that TMAO’s unique stabilizing abilities are a result of the “indirect effect” model. Natural bonding orbital (NBO) calculations further reveal that hyperconjugation results in vibrational blue shifts in TMAO’s C–H stretching region when solvated and a red shift in methanol’s C–H stretching region when hydrogen bonding with TMAO