Kinetics and Mechanism of Transitions Involving the Lamellar, Cubic, Inverted Hexagonal, and Fluid Isotropic Phases of Hydrated Monoacylglycerides Monitored by Time-Resolved X-ray Diffractiont

Abstract

A study of the dynamics and mechanism of the various thermotropic phase transitions undergone by the hydrated monoacylglycerides monoolein and monoelaidin, in the temperature range of 20–120 °C and from 0 to 5 M NaCl, has been undertaken. Measurements were made by using time-resolved X-ray diffraction at the Cornell High-Energy Synchrotron Source. The lamellar chain order/disorder, lamellar/cubic (body centered, space group No. 8), cubic (body centered, No. 8)/cubic (primitive, No. 4), cubic (body centered, No. 12)/cubic (primitive, No. 4), cubic (primitive, No. 4)/fluid isotropic, cubic (body centered, No. 12)/inverted hexagonal, cubic (primitive, No. 4)/inverted hexagonal, and hexagonal/fluid isotropic transitions were examined under active heating and passive cooling by using a jump in temperature to effect phase transformation. All of the transitions with the exception of the cubic (body centered, No. 8)/cubic (primitive, No. 4) and the cubic (body centered, No. 12)/cubic (primitive, No. 4) cooling transitions were found (1) to be repeatable, (2) to be reversible, and (3) to have an upper bound on the transit time (time required to complete the transition) of s. The shortest transit times recorded for the various phase changes in the heating direction were (lamellar chain melting), [lamellar liquid crystal/cubic (body centered, No. 8)], <0.5 [cubic (body centered, No. 8)/cubic (primitive, No. 4)], <0.9 [cubic (primitive, No. 4)/ hexagonal], <1.3 [cubic (body centered, No. 12)/cubic (primitive, No. 4) and cubic (body centered, No. 12)/hexagonal], and <0.6 s (hexagonal/fluid isotropic). For the exceptions noted above, the transitions were slow with transit times ranging from 0.5 to 30 min and displayed pronounced hysteresis and/or undercooling. Regardless of the direction of the transitions, all but one appear to be two state to within the sensitivity limits of the time-resolved method. In the case of the lamellar liquid crystal/cubic (body centered, No. 8) transition a stable intermediate of unknown identity was apparent. In addition to the time-resolved measurements, data were obtained on the stability of the various phases in the temperature range of 20–120 °C and from 0 to 5 M NaCl. In the case of fully hydrated monoolein, high salt strongly favors the hexagonal over the cubic (body centered, No. 8) phase and slightly elevates the hexagonal/fluid isotropic transition temperature. With fully hydrated monoelaidin, the hexagonal phase which is not observed in the absence of salt becomes the dominant phase at high salt concentration. In this case, the cubic (body centered, No. 8)/cubic (primitive, No. 4) and lamellar liquid crystal/cubic (body centered, No. 8) transition temperatures decrease while that of the lamellar chain order/disorder transition increases with NaCl concentration. Structural parameters and thermal expansivity of the long spacings for the various phases as a function of salt concentration are presented. © 1987, American Chemical Society. All rights reserved.