Study of liquid-phase molecular packing interactions and morphology of fatty acid methyl esters (biodiesel)

Abstract

Background1H low field nuclear magnetic resonance (LF-NMR) relaxometry has been suggested as a tool to distinguish between different molecular ensembles in complex systems with differential segmental or whole molecular motion and/or different morphologies. In biodiesel applications the molecular structure versus liquid-phase packing morphologies of fatty acid methyl esters (FAMEs) influences physico-chemical characteristics of the fuel, including flow properties, operability during cold weather, blending, and more. Still, their liquid morphological structures have scarcely been studied. It was therefore the objective of this work to explore the potential of this technology for characterizing the molecular organization of FAMEs in the liquid phase. This was accomplished by using a combination of supporting advanced technologies.ResultsWe show that pure oleic acid (OA) and methyl oleate (MO) standards exhibited both similarities and differences in the 1H LF-NMR relaxation times (T2s) and peak areas, for a range of temperatures. Based on X-ray measurements, both molecules were found to possess a liquid crystal-like order, although a larger fluidity was found for MO, because as the temperature is increased, MO molecules separate both longitudinally and transversely from one another. In addition, both molecules exhibited a preferred direction of diffusion based on the apparent hydrodynamic radius. The close molecular packing arrangement and interactions were found to affect the translational and segmental motions of the molecules, as a result of dimerization of the head group in OA as opposed to weaker polar interactions in MO.ConclusionsA comprehensive model for the liquid crystal-like arrangement of FAMEs in the liquid phase is suggested. The differences in translational and segmental motions of the molecules were rationalized by the differences in the 1H LF-NMR T2 distributions of OA and MO, which was further supported by 13C high field (HF)-NMR spectra and 1H HF-NMR relaxation. The proposed assignment allows for material characterization based on parameters that contribute to properties in applications such as biodiesel fuels.

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

“…To characterize antioxidant efficacy by 1 H LF‐NMR, it is important to understand the mechanism of oxidation and the chemical morphological characterization of the oils at different stages of oxidation. In our previous paper on the autoxidation of different fatty acid oils, we described in detail the correlation between the molecular structure of linseed oil and its liquid phase morphological chemical domains characterized by 1 H LF‐NMR Linseed oil is primarily PUFA linolenic acid (53.21%), oleic acid (18.51%), and linoleic acid (17.25%), that are self‐associated as hydrogen‐bonded dimers in their liquid phase, that further aggregate to form clusters possessing the structure of a quasi‐smectic liquid crystal, of an interdigitated alternating head to tail arrangement . The clusters aggregate formation is responsible for the fatty acid liquid properties, such as fluidity and viscosity …”

“…Therefore, very similar energy relaxation time curves and/or a relatively low noise level in the measurements can result in very different reconstructed spectra and/or signal to noise ratio (SNR). We demonstrated an improved reconstruction program if a l 2 regularization term is added to the objective function, through , of a common regularization, as seen in Equation : where R is the residuals matrix and is the regularization parameter and a positive solution is forced into the system of Equation , it is possible to achieve a highly accurate and stable reconstruction for the 1D T 2 problem. This approach was generalized to the 2D T 2 versus T 1 spectra .…”

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.…”

Non-Invasive Detection of Adulterated Olive Oil in Full Bottles Using Time-Domain NMR Relaxometry

“…The viscosity of fatty acids comprising the same number of carbons decreased with an increase in fatty acid unsaturation. 15 Inverse Laplace transform (ILT) of the T 2 relaxation curves, using regularization parameter α = 1, of OO, 1:1 OO/SO blend, and SO samples are shown in Figure 3. The relaxation spectra showed two strong peaks for each sample, indicating the presence of two proton components in the fatty acid chains.…”