Estolide Molecular Weight Distribution via Gel Permeation Chromatography

Using the universal calibration and the Mark-Houwink equation (MHE) (eta j=KMj alpha), three batches of oleic estolide acids and their corresponding 2-ethylhexyl esters were characterized using gel permeation chromatography (GPC). The MHE parameters in tetrahydrofuran (THF) at 40 degrees C were determined (for acids: alpha = 0.442 +/- 0.003 and log(10)K=2.505 +/- 0.007, for esters: alpha =0.531 +/- 0.006 and log(10)K =2.794 +/- 0.018). The fits of the GPC chromatograms yielded also the oligomeric composition of the estolides, which can be used to calculate the estolide number (EN) of an estolide mixture, and other molecular-weight distribution parameters, such as number-average molecular weight (M-n), weight-average molecular weight (M-w), and dispersity (<SIC>). Using the Deming line fit, we concluded that the GPC should be expected to be approximately three times more sensitive than the currently used methods for determination of EN values.