Wax crystallization and aggregation in a model crude oil

The high-molecular-weight paraffinic (‘wax’) fraction separates from crude oils at low temperatures, a process that can lead to a sol–gel transition when the mass of wax solids exceeds 1–2%. Attractive interactions between the micron-size wax solids suspended in the non-polar medium have been suggested to be responsible for gel formation. The present study reports an optically transparent model oil system, based on a mixture of linear and branched paraffins. Rheology and optical microscopy show that the model system reproduces essential features of crude oil gels. Small-angle light scattering studies show that phase separation and wax solid aggregation are rapid processes, leading to the formation of dynamically arrested structures well above the sol–gel transition determined rheologically. Clusters formed by the aggregated wax solids possess a fractal dimension of about 1.8, consistent with diffusion-limited cluster–cluster aggregation.