Engineering the viscosity and melting behaviour of triacylglycerol biolubricants via interesterification

The effects of blending, chemical and enzymatic esterification on the viscosity, melting and crystallization behaviour of blends of high oleic algal oil (HOAO), tricaproin (trihexanoin) and tricaprylin (trioctanoin) were determined. Blending HOAO with lower molecular weight tricaproin and tricaprylin reduced viscosity in a predictable fashion related to the average molecular weight of the molecular mixtures. For example, the HOAO kinematic viscosity at 40 °C decreased from 39.6 cSt to 33.3 cSt upon addition of 30% (mol mol−1) tricaprylin. However, molecular randomization by interesterification did not affect the kinematic and dynamic viscosities of the blends; the viscosity of the 30:70 HOAO:tricaprylin blend decreased non-significantly to 31.4 cSt. On the other hand, addition of tricaprylin and tricaproin combined with chemical and enzymatic interesterification did significantly reduce the crystallization and melting temperatures of the mixtures. The melting temperatures of HOAO decreased from −3.7 °C to −4.5 °C upon addition of 30% (mol mol−1) tricaprylin, while the crystallization temperature decreased from −34.8 °C to −37.8 °C. Surprisingly, enzymatic interesterification of this mixture drastically decreased the crystallization and melting temperatures of the blends, to −58.3 °C and −21.4 °C, respectively. A statistical thermodynamic model was developed to explain the decrease in melting temperature as a function of molecular randomization of the mixtures, which predicted the observed decrease quantitatively. This decrease in melting temperature was governed by an enthalpic effect. Thus, viscosity reduction in triacylglycerol mixtures is solely affected by the average molecular weight of the constituent triacylglycerol molecules, but their melting points can be significantly reduced by randomizing chemical structure. Blending and interesterification are both required to reduce viscosity and decrease the melting point of triacylglycerol biolubricants.