Estolide molecular weight distribution via gel permeation chromatography

Authors: Bantchev, Grigor; Cermak, Steven - Steve; Durham, Amber; Price, Neil

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2018
Publication Date: 3/5/2019
Publication URL: https://handle.nal.usda.gov/10113/6896870
Citation: Bantchev, G.B., Cermak, S.C., Durham, A.L., Price, N.P. 2019. Estolide molecular weight distribution via gel permeation chromatography. Journal of the American Oil Chemists' Society. 96(4):365-380. https://doi.org/10.1002/aocs.12165.
DOI: https://doi.org/10.1002/aocs.12165

Interpretive Summary: Estolides are novel bio-based products that have been successfully used to formulate engine oils. The resulting engine oils are high-performing and highly biodegradable. An important parameter to characterize estolides is their estolide number (EN), which indicate the average degree of oligomerization of an estolide. ARS scientists in Peoria, IL, evaluated the applicability of gel permeation chromatography (GPC) as a method to evaluate estolide samples. They demonstrated that GPC is ~3 times more sensitive than the traditional method of EN determination. Additionally, in comparison to the traditional method, GPC yields additional information about the estolide samples, such as polydispersity index and others. The scientists conclude that the GPC method is better than the traditional method for EN determination.

Technical Abstract: Using the universal calibration and the Mark-Houwink equation (MHE) ([n]j = K(Mj)^a), 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 °C were determined (for acids: a = 0.442 ± 0.003 and log10K = 2.505 ± 0.007, for esters: a = 0.531 ± 0.006 and log10K = 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 (Mn), weight-average molecular weight (Mw), and dispersity (Ð). 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.