VIBRATIONAL SPECTROSCOPIC STUDY ON THE OCCURRENCE OF STEARIC-ACID B AND E FORMS - HETEROGENEOUS NUCLEATION OF THE B FORM ON THE SURFACE OF E CRYSTALS AND THE TOPOTACTIC PHASE-TRANSITION FROM E TO B

The mechanism of the occurrence and the crystal growth of the B form of stearic acid (n-C18 acid) has been investigated by means of IR spectroscopy. It has been clarified that on the crystallization process the B form occurs not directly from solutions but through a heterogeneous nucleation on the (001) face of the single crystals of E generated at the first stage and a subsequent solid-state phase transition from E to B. The E --> B transition achieves through the reorientation of dimerized carboxyl groups accompanied with the conformational change from trans to gauche at the C2-C3 bond, while most of acyl chain maintains its original orientation. Since the direction of the crystallographic axes as well as the crystal habit is kept unaltered during the transition, the E --> B phase transition is regarded as a topotactic transition.