Phase diagrams and microstructure of mixtures of n-hentriacontane and saturated fatty acids

We determined the phase behavior and microstructure of binary mixtures of n-hentriacontane (C31) and one of three saturated fatty acids (SFA; myristic acid (MA), palmitic acid (PA), or stearic acid (SA)). Phase diagrams were developed through differential scanning calorimetry and X-ray diffraction. The C31/MA, C31/PA, and C31/ SA systems all showed eutectic phase behavior. Below the solidus line, these systems consisted of a mechanical mixture of C31 crystals and SFA crystals in the C polymorph, irrespective of the C31/SFA ratio. The C31/SFA eutectic composition increased in C31 content with SFA chain length: XC31 = 0.06, XC31 = 0.37, and XC31 = 0.54 for MA, PA, and SA, respectively. Within specific concentration ranges, small amounts of SFA were incorporated into the C31 crystal lattice. Smaller crystals of C31 and SFA were obtained for mixtures close to the eutectic mixture composition.