Molecular Insights into the Eutectic Tripalmitin/Tristearin Binary System

A molecular interpretation of the eutectic behavior of a binary mixture of tristearin (SSS) and tripalmitin (PPP) triglycerides was formulated using computer simulations and experimental techniques (calorimetry and X-ray scattering). A eutectic composition was identified using both experimental and computer simulation techniques at a composition of 70% PPP and 30% SSS, in agreement with previous findings in the literature. The decrease in the melting temperature at the eutectic composition can be ascribed to an interplay between enthalpic and entropic effects. In particular, a lower global melting enthalpy at the eutectic composition was detected here, caused by a less efficient packing of the triglycerides in the crystal. On the other hand, a higher crystalline disorder is reflected in a lower change in the entropy of melting. The simultaneous decrease in global enthalpy and entropy has a contrasting effect on the melting temperature, with a slight melting point depression found in both experiment and simulations, resulting from a combination of enthalpic and entropic factors. Computer simulations showed, in fact, that the eutectic effect can be ascribed to the reduction of crystalline order when SSS molecules are incorporated into the PPP crystal structure. This decrease of the crystalline order is due to the protrusion of SSS end-chains (last three carbons of each alkyl chain) into the interlamellar space between adjacent lamella. These end-chains disturb the orderly stacking of the lamella, as evidenced by low-density regions in the interlamellar space. Thus, the greater disorder of the last atoms of the SSS alkyl chains is consequently due to the greater conformational freedom. At molecular level, in fact, the conformational freedom of terminal atoms of SSS surrounded by shorter PPP molecules is larger than the conformational freedom of longer SSS in the neighborhood of short PPP.