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Behaviors of VO complexes in atmospheric residues (ARs) from two Kuwait crude oils and their resins and asphaltenes were studied using the electron spin resonance (ESR) to examine the effects of the surrounding matrix, concentration in solvent, and temperature on VO rotational mobility. The results show that the surrounding molecules in the petroleum fractions constrain the VO rotational mobility significantly. The constraint on the VO complexes by the surrounding matrix in different environments increases in the order of resin < AR < asphaltene. Less constraint by the AR than by the asphaltene can be ascribed to the solvent role of the lighter components in the AR. The higher measurement temperature of 100 °C significantly decreases the constraint on VO complexes by the matrix, while a higher sample concentration in the solvent shows stronger constraints. However, the constraint on the VO complexes in the asphaltene is hardly moderated, even when dissolved in toluene at 100 °C. Additionally, Kuwait Export Crude atmospheric residue (KEC-AR) and its resin dissolved in toluene show weaker constraints on the VO complexes than Lower Fars atmospheric residue (LF-AR) and its resin, possibly as a result of lower aggregation of KEC-AR and its resin.

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