(475c) The Role Of Asphaltene Acidity/basicity In Supramolecular Assembly At The Oil-Water Interface

Water-in-crude oil emulsions are notorious for their strong resistance to coalescence, often requiring specialty chemicals or equipment to facilitate oil/water separation. The high strength of these emulsions has been attributed to elastic third-phase films that form at the oil water interface. Asphaltenes are generally targeted in the investigation of these films, with a predominant focus on oil-side characteristics, including asphaltene chemistry, additive chemistry, as well as aggregate shape and size in different solvent conditions. This leaves much to investigate on the role of water-side chemistry in the formation of asphaltenic films. Under certain conditions, asphaltenes, and their aggregates, can be amphoteric, i.e. they exhibit both positive and negative charges. These charges will impose their influence at the interface, at which acidic and basic moieties of asphaltenes come into contact with the water. The propensity of these acidic and basic groups to become charged is known to be a strong function of the aqueous phase pH, and the associative range of these electrostatic charges depends heavily on the presence of electrolyte.

In this work, we ask fundamental questions about the important role of charged asphaltenic species in the formation of highly elastic interfacial films, including those about the functional properties of asphaltene sub-fractions required to form such films. Here we will describe a comprehensive look at the effect of charged species in asphaltenic film formation using a host of complementary interrogative experimental techniques, including interfacial shear rheometry, dilatational rheology, centrifugation, and the critical electric field (CEF) technique. Through inspection of film formation kinetics during extensive aging, we are able to discern an electrostatic effect on film formation in amphoteric asphaltenes at the oil/water interface. Specifically, asphaltenes obtained from Hondo crude oil are amphoteric and exhibit both positive and negative charges at the interface between 40:60 n-heptane:toluene and pH 7 deionized water. In interfacial shear rheology experiments we observed that the interfacial aging time required to form a film capable of withstanding an applied shear stress of ~0.4 mN/m was inversely proportional to the Debye length, k^-1. For charged colloids, k^-1 describes the range of influence for electrostatic interactions, which increases with decreasing ionic strength. For the same model oil system, we observe a similar delay in film formation kinetics using an oscillating drop tensiometer. We postulate that for amphoteric asphaltenic species at the interface, electrostatic attraction between positive and negative aggregate surface charges facilitate surface migration that allows aggregates to position themselves and facilitate the p-p overlaps that act as crosslinks in the interfacial network. We test the plausibility of this hypothesis with additional experiments performed at pH extremes and with asphaltenic species that are predominantly acidic or basic.