Effect of asphaltenes on equilibrium and rheological properties of waxy model systems

This experimental work evaluates the effect of the chemical nature of asphaltenes and waxes on crystallization and gelation of waxy model systems studied by means of calorimetric and rheological measurements. The molecular parameters of the asphaltenes used in this work were obtained by means of C-13 and H-1 nuclear magnetic resonance (NMR). The characterization of the asphaltenes allows us to get a better understanding of the mechanisms involved into the asphaltene-wax interactions and their effect on wax precipitation, melting point, flow, gelation and yielding behavior. Results showed that phase equilibrium properties (crystallization and melting) of the waxy model systems are just slightly affected by the asphaltenes whereas rheological behavior (pour point, gelation, flow curves and yielding) is significantly influenced by the concentration and chemical nature of asphaltenes and waxes. The results showed that asphaltenes with greater abundance of aliphatic moieties, promotes a better asphaltene-wax interaction resulting in an easier incorporation of this kind of asphaltenes into the wax crystal network where the normal crystal growth is just moderately affected. On the other hand, asphaltenes with higher aromaticity, which have very few aliphatic substitutions and a bigger aromatic core, inhibits and disturbs the normal wax crystal growth in a greater extent retarding the gel structuring and promoting the formation of a weak and less stable gel structure. (C) 2011 Elsevier Ltd. All rights reserved.