Small-angle neutron scattering study of crude oil emulsions: structure of the oil-water interfaces.

We have used SANS techniques to study in situ interfaces between crude oil and water in emulsions. These emulsions were stabilized by asphaltenes, which are natural surface-active molecules in viscous crude oils. By combining SANS and UV-vis spectrometry, we measured both the interfacial thickness and the adsorbed amount as well as the size of the asphaltene aggregates in the oil phase. We found that this size is comparable to the interfacial thickness, suggesting that the interface is covered by adsorbed aggregates. The thickness is a minimum at the pH at which the charge of asphaltenes in contact with water is zero (IEP). This suggests that asphaltene layers in contact with water are swollen and stretched at both low and high pH. The effect of salt addition on the interfacial characteristics is minor at the IEP, but a clear swelling is seen at high pH for 0.1 M salt, an effect that remains to be understood. Emulsion stability was found to correlate well with large interfacial thicknesses.