Author
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Schmidt, Walter |
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Mookherji, Swati |
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Mitchell, Alva |
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CRAWFORD, M - Imperial College |
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Submitted to: Book Chapter
Publication Type: Book / Chapter Publication Acceptance Date: 9/2/2011 Publication Date: N/A Citation: N/A Interpretive Summary: Lipid or fats have counter-intuitive physical properties. Some compounds are water soluble, others are fat soluble. From this rule, clearly fats will be soluble in fats. Place too much sugar in hot water and allow it to cool: pure sugar will precipitate out. Add an excess of one lipid (e.g., stearic acid, a solid fat at room temperature) to another fat (e.g., pig fat, seal oil): no solid lipid (e.g., stearic acid) forms. Instead two forms of lipids, both liquid, both immiscible form and each has exactly the same lipids in them. The ratio of lipids in each form however is discretely different. This manuscript explains why this happens. The same physical chemistry applies to problems of agricultural interest. Poultry fat has been proposed as a source for production of biodiesel. Poultry fat, clear and free-flowing, from a poultry processing facility, routinely separates into two viscous intermingled lipid phases, neither of which has any discretely different chemical or physical properties. If they are so similar, how come they separate? A sudden and delayed increase in macroscopic heterogeneity and viscosity of lipids is a highly undesirable property that can show up in biodiesel fuels. Excess saturated fat in human’s diet can lead to a sudden and delayed increase in macroscopic heterogeneity and viscosity of lipids which deposit inside arteries and can result in stroke and heart attacks. In each case, the same physical chemistry is involved. This manuscript explains the reason this occurs. At the molecular level, unsaturated fats can absorb a finite amount of saturated fats into unit cell volumes. Above this threshold level, saturated fats repack the existing unsaturated fats into a more tightly packed volume. The two sized volumes cannot pack uniformly so therefore aggregate, resulting in increased solids-like properties [this is measured using a technique called DSC]. The mixture of the two sized volumes each with a different density results in the macroscopically viscous heterogeneous lipid. The two lipid forms are very difficult to separate physically because at the molecular level, the process of heating and/or cooling itself, both alters the size of the unit cell volume [and therefore the number of which lipids that can fit inside it], disrupts the aggregates of unit cells, and enables remixing among unit cells of different unit cell volumes. Technical Abstract: Although natural lipids often are present in one single liquid phase, a small specific change in composition of the same lipids can trigger the formation of two lipid phases which are immiscible with each other. Stearic acid (SA) as an added secondary lipid component had a discrete solubility in methyl oleate (MeOA), docosahexaenoic acid (DHA), pig fat and seal oil. In each case, adding SA in excess of its solubility resulted not in the precipitation of insoluble SA, but in the lipid itself re-ordering into a second phase [immiscible with the first]. An ordered second phase was confirmed by differential scanning calorimetry (DSC) and modulated DSC. The presence of the same lipids in both phases was verified using Raman microscopy. Each layer maintained its own discrete stoichiometry, and the fats other than SA, determine phase transition temperature of the immiscible phase. Structurally diverse lipids may have a finite porosity to structurally large molecules like SA. SA added in excess to that finite porosity can only “fit” into the lipid by enhancing the lipid packing and/or packing efficiency, i.e., forming the immiscible phase. A unit cell is the smallest regularly shaped volume within which the mole ratio of lipids corresponds to the macroscopic mole ratio of lipids. Computational chemistry calculations enable modeling of a cube about 25 A (2.5 nm) per side as a volume in which discrete lipid compositions can effectively pack. Despite entropy, unit cells of similar size and shape will slowly pack uniformly, resulting in macroscopic lipid order. Translational motion of excess SA lipid molecules from an immiscible phase to a miscible phase unit cell volume unpacks the former, and repacks the latter: the unit cell appears to diffuse to a different site and only a fraction of the lipids had exchanged places. Interestingly, a localized increase in packing efficiency itself creates the temporary space through which other large molecules such as SA can diffuse. Slow release of latent heat in lipid mixtures detected by modulated DSC can be due to localized non-uniformity in lipid chemical composition and/or slowness in packing efficiently. |
