(114b) In Situ Asphaltenic Film Characterization of W/O Emulsions by Sans: the Role of Film Thickness and Composition in Emulsion Stability

We describe here the use of small angle neutron scattering (SANS) for the characterization of interfacial films directly related to the stabilization of water-in-oil emulsions frequently encountered in the petroleum industry. These emulsions are often detrimental in crude oil transport as they are typically of high viscosity and they tend to transport so-called poisons (usually inorganic salts) to downstream catalytic processes. Asphaltenes, the heaviest component in crude oil, have been established as a likely culprit in the formation of the elastic interfacial films that protect these emulsion systems from coalescence. The physicochemical properties of these films are expected to correlate with mechanical properties and, ultimately, emulsion stability.

Presented here are the results of SANS experiments aimed at ascertaining the thickness and composition of the stabilizing films formed on the surface of aqueous droplets dispersed in asphaltene-rich organic solvents. Three liquids of increasing solvency ? 1-methylnaphthylene, toluene, and decalin ? were used as the organic phase along with 1 wt% Hondo whole (HOW) asphaltenes. In this work we describe two avenues for handling aggregate scattering that occurs in these systems: 1) subtraction of the bulk phase scattering from the total emulsion scattering and 2) the physical removal of the bulk aggregates by sequential replacement with fresh organic solvent.

Using a polydisperse core/shell model, we have determined the average thickness and scattering length density (SLD) of each emulsion system tested. The film thickness values were 9-11 nm, and did not appear to directly correlate with emulsion stability ? determined by centrifugation @ 14000g for 1 hr ? but do trend with solvent strength/aggregate RG. However, from the fitted film SLD values, and known SLDs for the solvents and asphaltenes, we have been able to determine the average film composition in each system. In general, the interfacial films tended to be quite solvent-rich, with only 15-45% of the film thickness and SLD accounted for by asphaltene. Surprisingly, the strongest emulsions, those made with decalin, had the thinnest films (9.3 nm), but exhibited the highest surface concentration (4.4 mg/m2), more than twice that of toluene (1.9 mg/m2) or 1-methylnaphthalene (2.0 mg/m2). These results demonstrate the power of SANS as a tool for probing physicochemical properties of asphaltenic W/O emulsions in situ, enhancing our understanding of how asphaltenic films lead to emulsion stability.