Unit Cell Volume and Liquid Phase Immiscibility in Oleate-Stearate Lipid Mixtures

Authors: Schmidt, Walter; Mookherji, Swati; CRAWFORD, MICHAEL - UNIV NORTH LONDON, UK

Submitted to: Chemistry and Physics of Lipids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2008
Publication Date: 1/7/2009
Citation: Schmidt, W.F., Mookherji, S., Crawford, M.A. 2009. Unit Cell Volume and Liquid Phase Immiscibility in Oleate-Stearate Lipid Mixtures. Chemistry and Physics of Lipids. 158:10-15.

Interpretive Summary: Biodiesel converts triglycerides into fatty acid methyl esters (FAME). The resulting mixture of FAME always has a variable lipid composition. Under specific conditions, FAME can become cloudy. Cloudiness is a highly undesirable trait in fuel; fuel line filters are clogged by the cloudy component(s) which precludes even the remaining liquid portion of the fuel from being combusted. MeOA often is the most abundant single component of biofuel. Mixtures of lipids SA, OA and MeOA were used as a simple model for cloudiness formation. Cloudiness can be induced simply by increasing the weight fraction of SA. However, the chemical composition of the cloudiness was still primarily MeOA, not SA. Adding more SA did not increase the ratio of SA to OA in the cloudy phase, it simply created a larger quantity of cloudiness and decreased the quantity of the clear liquid first phase. The end result of adding excess SA to OA is never 100% SA in the cloudy phase, it is 0% of OA in the clear layer. Thus on adding SA, SA inherently moves with OA forming the second phase. The same process occurs with MeOA as with OA. Adding saturated lipids causes cloudiness in lipid mixtures, but filtering out cloudiness removes more unsaturated lipid than saturated lipid. Thus, filtering lipids cannot preclude cloudiness formation. The solubility of lipids in either layer can be explained by the molecular size and shape of the component lipids and on how efficiently they pack. Mixtures of lipids may routinely pack more efficiently than chemically pure lipids.

Technical Abstract: The limited miscibility of stearic acid (SA) in two unsaturated lipids at room temperature was observed macroscopically. A 5% SA mixture in octadec-9-enoic acid (OA) is cloudy; a 5% SA solution in the methyl ester of OA (MeOA) is fully miscible and clear. At 10% SA in MeOA, an immiscible phase formed within a week. The composition of the immiscible phase however, was not 10% SA, but 25% SA. The stoichiometry of each mixture was confirmed by NMR spectroscopy. Raman mapping verified the chemical composition within each phase remained homogeneous despite the microscopically observed heterogeneity in the immiscible phase. The 3D basis for this stoichiometry was a cubic unit cell volume with the 1D length of a straight SA molecule. Molecular modeling supports that discrete ratios of mixtures of saturated and unsaturated lipids pack efficiently and stoichiometrically within the cubic unit cell volumes. Packing order of lipids within the unit volume explains the discrete lipid composition ratio forming each of the observed miscible and immiscible phases.