Correlation of Jet Fuel Interfacial Tension with Water Separation and Effects of Surfactant Species

It is critical to remove surfactants from fuels before passing through fuel–water separator systems because surfactants are known to inhibit separation. Micro-separometer (MSEP) analysis is a field-portable method to measure the water separation characteristics of fuels using coalescence followed by a turbidity measurement. The objective of this effort is to explore the correlation of interfacial tension (IFT) and surface tension (ST) to MSEP data. These properties were analyzed in conjunction with the fuel composition data (bulk hydrocarbon and trace polars) to explore the underlying factors that affected MSEP measurements. It was found that IFT has a moderate positive correlation with MSEP for neat conventional jet fuels (F-24, Jet A, and Jet A-1), while no correlation between ST and MSEP was observed. Because neither IFT nor MSEP correlate with bulk hydrocarbon classes, their correlation may be due to a shared correlation with trace polar species, namely, esters and phthalates. IFT and ST were measured for jet fuel samples containing corrosion inhibitor/lubricity improver (CI/LI, dimer of linoleic acid and related derivatives) and aqueous film-forming foam (AFFF, fluorinated C8 and shorter sulfonic and carboxylic acids and their conjugate bases), in addition to F-76 diesel samples with lubricity improver additive (LIA, exact structure unknown, though, typically, they are organic acids). IFT data fit an exponential decay function with high R2 values for all three additives studied. A lower IFT limit of 37.8 mN/m is proposed for conventional jet fuels based on the IFT versus MSEP correlation and the existing ASTM D1655 minimum specification for MSEP. It was found that the IFT for jet fuel with CI/LI does not drop below the IFT limit at the expected dosage, while the IFT for jet fuel containing AFFF drops below the IFT limit before the expected dosage.