Liquid-phase characterization of molecular interactions in polyunsaturated and n-fatty acid methyl esters by 1H low-field nuclear magnetic resonance

Abstract

BackgroundTo identify and develop the best renewable and low carbon footprint biodiesel substitutes for petroleum diesel, the properties of different biodiesel candidates should be studied and characterized with respect to molecular structures versus biodiesel liquid property relationships. In our previous paper, 1H low-field nuclear magnetic resonance (LF-NMR) relaxometry was investigated as a tool for studying the liquid-phase molecular packing interactions and morphology of fatty acid methyl esters (FAMEs). The technological potential was demonstrated with oleic acid and methyl oleate standards having similar alkyl chains but different head groups. In the present work, molecular organization versus segmental and translational movements of FAMEs in their pure liquid phase, with different alkyl chain lengths (10–20 carbons) and degrees of unsaturation (0–3 double bonds), were studied with 1H LF-NMR relaxometry and X-ray, 1H LF-NMR diffusiometry, and 13C high-field NMR.ResultsBased on density values and X-ray measurements, it was proposed that FAMEs possess a liquid crystal-like order above their melting point, consisting of random liquid crystal aggregates with void spaces between them, whose morphological properties depend on chain length and degree of unsaturation. FAMEs were also found to exhibit different degrees of rotational and translational motions, which were rationalized by chain organization within the clusters, and the degree and type of molecular interactions and temperature effects. At equivalent fixed temperature differences from melting point, saturated FAME molecules were found to have similar translational motion regardless of chain length, expressed by viscosity, self-diffusion coefficients, and spin-spin (T2) 1H LF-NMR. T2 distributions suggest increased alkyl chain rigidity, and reduced temperature response of the peaks’ relative contribution with increasing unsaturation is a direct result of the alkyl chain’s morphological packing and molecular interactions.ConclusionsBoth the peaks’ assignments for T2 distributions of FAMEs and the model for their liquid crystal-like morphology in the liquid phase were confirmed. The study of morphological structures within liquids and their response to temperature changes by 1H LF-NMR has a high value in the field of biodiesel and other research and applied disciplines in numerous physicochemical- and organizational-based properties, processes, and mechanisms of alkyl chains, molecular interactions, and morphologies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0280-5) contains supplementary material, which is available to authorized users.

1H LF‐NMR Energy Relaxation Time Characterization of the Chemical and Morphological Structure of PUFA‐Rich Linseed Oil During Oxidation With and Without Antioxidants

“…Assignment of each peak was determined by 2D 1 H LF‐NMR energy relaxation time analysis of fatty acids and linseed oil standards and based on previous collected data of high resolution NMR regarding to linseed oil segmental motion rigidity analysis and oxidation products …”

“…To reinforce and to propose rationalization of the postulated assignment of the peaks determined by 2D 1 H LF‐NMR relaxation time analysis, standards of fatty acid and oxidation products were analyzed based on previous data of high resolution NMR regarding segmental motion and rigidity analysis . In addition, gas chromatography (GC) analysis of the treated samples oflinseed oil with and without α tocopherol for time periods up to 96 h of thermal heating at 80 °C (Figure ).…”

“…Autoxidation experimental design was based on previous studies . The samples were autoxidized by heating on an a 80 °C hot plate, 100 mL samples in a 250 mL beaker.…”

“…Autoxidation experimental design was based on previous studies. [17,21] The samples were autoxidized by heating on an a 80 C hot plate, 100 mL samples in a 250 mL beaker. Air was pumped into the beaker with maximum magnetic stirring for up to 96 h. Two different samples were treated in parallel.…”

Molecular Properties of Fatty Acid Mixtures Estimated by Online Time-Domain NMR

“…Impurities (10%) present in the oleic acid used may have also been responsible for the faster decaying component. In the previous works [30,31], the presence of two components in the transverse magnetization decays of oleic acid and the methyl ester of oleic acid was connected to the different mobilities of protons on the carbon chain. However, in our earlier paper [24], magnetization decays measured for mixtures of different fatty acids showed only one peak in the Laplace transformation.…”