Phase transformations in solid cyclohexanol: Growth rates and morphology from visual observations

Abstract

Cyclohexanol exists in at least four solid forms: form I is a rotational form in which the molecules exhibit a hindered rotation; the other, non-rotational forms are II, III, and pre-II. The morphology and growth rates have been determined by optical methods as a function of temperature for the transformations I → II, I → III, I → pre-II → II, and II → III. The I → II transformation follows the theory of continuous growth with an activation energy of 7.3 ± 0.3 kcal/mol and a pre-exponential factor of 3 X 10 6 cm/hr. For very small supercooling there is some deviation from this behavior. The I → III transformation seems to proceed by a mechanism of lateral growth for supercoolings up to about 10° but then to change over at larger supercoolings to a mechanism of continuous growth, in close agreement with the predictions of the Cahn theory. In the region of continuous growth the activation energy is 9.9 ± 0.5 kcal/mol and the pre- exponential factor is 1 X 10 9 cm/hr. The II → III transformation gives evidence of being a transformation from a superheated rather than a supercooled phase. Very rough estimates give a transition temperature of –64°C, an activation energy of 17 kcal/mol, and a pre-exponential factor of 8 X 10 15 cm/hr.