Marine oil dispersants are typically composed of a surfactant blend dissolved in solvents and, when introduced to the crude oil-seawater interface (OWI), they reduce the oil-water interfacial tension (IFT). This IFT reduction, coupled with wave action, facilitates the breakup of crude oil into droplets that can be dispersed into the water column and eventually broken down by bacteria. Recently, questions about the environmental persistence and toxicity of commercial dispersant formulations have led to the development of 'greener' dispersant blends composed of the food grade surfactants L-α-phosphatidylcholine (lecithin) and polyoxyethylenated sorbitan monooleate (Tween 80). Individually, neither surfactant is effective at dispersing crude oil, but when combined at specific weight ratios, lecithin-Tween 80 (LT) blends work synergistically for effective dispersion-- the reasons for this synergy remain unexplained. More broadly, an unresolved challenge is the ability to <i>predict</i> whether a given surfactant (or surfactant blend) can act as an effective dispersant. Herein, we investigate whether LT dispersant effectiveness can be correlated with <i>thermodynamic phase behavior</i> in model dispersant-oil-water ternary systems. Specifically, we have studied systems comprised of LT dispersant (D) + hexadecane (O) + synthetic seawater (W) with the D formulation being systematically varied (across 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0 L:T weight ratios), and we hypothesize that the thermodynamic phase equilibria in DOW systems of a given LT dispersant correlates with that blend’s dispersant effectiveness. To test this, we have mapped microemulsion phase behavior over the course of 8 weeks in DOW systems across the 6 different L:T weight ratios noted above. From this study, we have determined that there is an empirical correlation between LT dispersant effectiveness and the microemulsion phase behavior in these DOW ternary systems and believe that the observed fundamental thermodynamic principles can be used as a predictive model for effective dispersant blends. Further, this presentation will outline a protocol that allows the phase behavior to be observed on industrially relevant timescales and discuss a set of general guidelines that can be used to interpret the results of these tests. These findings expand on the fundamental understanding of dispersant interfacial chemistry and provide a tool to guide the efficient formulation of future marine oil dispersants.

Using microemulsion phase behavior as a predictive model for lecithin-tween 80 marine oil dispersant effectiveness

ACS Fall 2021

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