Mitigating crystallization of saturated FAMES in biodiesel: 5. The unusual phase behavior of a structured triacylglycerol dimer and methyl palmitate binary system

Abstract

A model binary system made of methyl palmitate (MeP) and (E)-1-(1-(oleoyloxy)-3-(stearoyloxy)propan-2-yl) 18-(1-(oleoyloxy)-3-(stearoyloxy) propan-2-yl) octadec-9-enedioate (dimer, D), a structured triacylglycerol dimer, one of a series of fatty molecules that have been found to significantly reduce the crystallization temperature of biodiesel, was investigated with DSC (differential scanning calorimetry), XRD and PLM (polarized light microscopy). A complex and unusual phase behavior was uncovered for this system. The exclusive orthorhombic crystal packing that was detected by XRD was shown to be guided by the bulky D molecules. The findings are explained by the formation of dimer/methyl palmitate composite units in which the dimer associates with the MeP molecules at its fatty acid branches as well as bridges. This association was initiated in the liquid state where the mobility of MeP and free rotation of the fatty branches of D are favorable. The disruptive effect of D on the packing of the saturated FAME was effective only at low concentrations (<0.10) with the system presenting a eutectic at 5% of D, followed by three stepped transformation lines indicating three concentration regions of defined phases of increasing stability. The findings indicate that the dimer may be an effective cold flow improver of biodiesel but only at concentrations lower than 5%.