Phase Transition of Normal Higher Primary Bromides. III.

Abstract

The phase transition phenomena of four normal higher primary bromides containing 22, 24, 26 and 28 carbon atoms were investigated using a differential scanning calorimeter (Perkin Elmer DSC-1B), and irreversible transformation phenomena which take place on prolonged storage at constant temperature were also studied.The rotational phase transitions expected from the results of previous X-ray and dielectric studies of these bromides are observed as a first order phase transition accampanying latent heat.The recrystallized sample (β0 form) shows only one endothermic melting peak on heating DSC. This form is most stable one and reveals no phase transition phenomena. The molten sample shows two exothermic peaks on cooling. These peaks correspond to freezing and transition phenomena. The low temperature phase which first appears below the transition point (β1 form) shows phase transition phenomenon on heating. This β1 form is metastable one and gradually transforms to another form (β2 form) on storage spontaneously. The β2 form shows no transition phenomenon on heating and is more stable than the β1 form. However, this β2 form transforms to most stable one (β0 form) irreversibly, on prolonged storage at constant temperature The β0 form melts at higher temperature than the melting point of β2 form. These irreversible transformations occur a wide temperature range below the transition points.The complicated thermal behaviors of these bromides is interpreted using a free energy-temperature diagram.