Temperature dynamics of dimer formation in behenic acid: FT-IR spectroscopic study

The structural evolution of behenic acid molecule dimers has been investigated in a wide temperature range 10–365 K by FT-IR spectroscopy. In a crystal phase the two dimer conformations, cis and trans forms, coexist in the whole temperature region up to the melting point. The trans configuration prevails at low temperatures and amounts to 96% at T = 10 K. At heating these associates transform gradually into the cis form. The dynamic equilibrium of 75% cis and 25% trans dimer configurations is established at T = 250 K and retains up to the melting, where dimers start to dissociate to monomers. The calculated hydrogen bonding energy for both types of behenic acid dimers is ∼8 kcal/mol in average, being 0.3–0.8 kcal/mol (depending on temperature) higher for the trans form. At heating the energy of both dimer configurations drops gradually, i.e. the hydrogen bonding in a crystal weakens slowly with temperature. The results can be generalized and applied for other homologues of carboxylic fatty acids.