Effect of glycerol on the order of the mesophase transitions of supercooled itraconazole

Abstract

Itraconazole, an antifungal drug, is a thermotropic liquid crystal that exhibits nematic (N) and smectic A (SmA) phases, when cooled from the melt. By means of high-resolution adiabatic scanning calorimetry (ASC), we have obtained the temperature dependence of the heat capacity as well as the enthalpy (including latent heats) of the nematic to smectic A (N-SmA) and the isotropic to nematic (I-N) phase transitions. The N-SmA transition is weakly first-order, with substantial pretransitional heat capacity increases. The critical exponent α, obtained from power law fits to the heat capacity data, is 0.50 ± 0.05, suggesting that the N-SmA transition must be very close to a tricritical point. Indeed, with this character, the small molecule dopant glycerol (in binary mixtures with ITZ), causes interesting changes to the mesomorphic phase sequence and to the order of the phase transitions. With increasing glycerol content, the temperature width of the nematic phase systematically reduces, until a critical concentration, at which the nematic phase disappears, leading to a direct isotropic-smectic A (I-SmA) transition. The I-SmA transitions of the ITZ-glycerol mixtures show stronger first-order character with substantial latent heats and wide two-phase regions, when compared to the (N-SmA and I-N) transitions of neat itraconazole. The ability of glycerol to drive the ITZ transitions to stronger first-order character indicates a possible coupling of the additive concentration to the smectic order parameter, which leads to the development of highly ordered, stable smectic structures.