Fluorescence depolarization measurements are used to determine the size of asphaltene molecules and of model compounds for comparison. Mean molecular weights of roughly 750 amu with a range of roughly 500-1000 amu are found for petroleum asphaltenes. A strong correlation is established between the size of an individual fused ring system in an asphaltene molecule and the overall size of this corresponding molecule, showing that asphaltene molecules have one or perhaps two fused ring systems per molecule. Subtle differences in molecular size are found for different virgin crude oil asphaltenes and for a vacuum resid asphaltene. Coal asphaltene molecules are found to be much smaller than petroleum asphaltenes. The molecular sizes of resins and asphaltenes are found to form a continuous distribution.

The rotational correlation times of individual asphaltene molecules have been determined using fluorescence depolarization techniques, addressing an active, long-standing controversy. Using simple theoretical models and using model-independent comparisons with known chromophores, a range of asphaltene molecular diameters is obtained of 10−20 Å. Comparison with corresponding data of known chromophores indicates a molecular mass for asphaltene molecules of 500−1000 amu. Furthermore, we have performed the first direct measurement correlating molecular size with constituent chromophore size; we establish that the bulk of asphaltene molecules possess 1 or 2 (aromatic) chromophores per molecule. Similar results are found for the largest aromatic molecules of the de-asphaltened crude oil.

Asphaltenes are defined in terms of their solubility classification. This operational definition combined with the previous controversy over asphaltene molecular weight have obscured the governing chemical and structural parameters that define the asphaltene fraction. Here, asphaltenes are investigated by several techniques to elucidate relations between structure and properties. In particular, the asphaltene molecular size is compared to the ratio of aromatic to saturated carbon. The conclusion is obtained that asphaltene molecular structure is governed by the balance between the propensity of fused aromatic ring systems to stack via π-bonding, reducing solubility, vs the steric disruption of stacking due alkane groups, increasing solubility.

Sum frequency generation (SFG) vibrational spectroscopy is applied to study the surface characteristics of a film composed of nanoscale TiO2 particles, in which methanol is employed as a molecular probe. The SFG signal from methanol adsorbed on the TiO2 film surface in the CH region consists of four primary peaks located at 2828, 2855, 2935, and 2968 cm-1. The peaks at 2855 and 2968 cm-1 arise from molecular methanol; the other two are attributed to methoxy produced by dissociative chemisorption of methanol on TiO2. Changes in the SFG features with methanol pressure and with surface temperature provide the first direct proof of methoxy generation on nanoparticulate films. In comparison with molecular methanol, methoxy sits more perpendicularly on the surface. The chemisorption is barrierless, and a Langmuir constant of (2.13 ± 0.14) × 103 is derived from the adsorption isotherm. Moreover, UV pretreatment of the TiO2 film substantially increases the methoxy signal. Changes in SFG intensity are in good agreement with the current models for surface active sites and face specificities on TiO2.