Polymorphism and transformation energetics of saturated monoacid triglycerides from differential scanning calorimetry and theoretical modeling

Differential scanning calorimetry studies on saturated monoacid triglycerides were extended to include most odd and even chain lengths from tricaprylin (C₈) through tritriacontanoin (C₃₀). Two β’-forms were common with triglycerides C₁₅ through C₂₄: shorter odd chain length triglycerides (C₉-C₁₃) exhibited only one β’-form; short even chain length triglycerides (C₈-C₁₄) exhibited three. Odd chain length C₂₁ and C₂₃ triglycerides showed two β-forms. Triglycerides of even chain lengths greater than C₂₁ produced two α-forms. Apparent energies of phase excitation for α-form transformations (determined from scans at different heating rates) showed odd-even alternation for short chain lengths, but increased linearly with chain length above C₁₄, evidencing the importance of extended chain conformation and interactions as determinants of polymorph properties. Changes in melting point patterns, particularly for β’ and β-forms, at C₁₄ correlated with the change in apparent phase excitation energy. Comparisons of X-ray data with dimensions from space-filling models and agreement between observed entropies of fusion and values calculated for probabilistic models also emphasize the importance of extended chain conformation and suggest configurational differences, and possibly different polymorph conversion pathways for odd versus even chain length triglycerides.