Time-resolved dielectric investigation of relaxation kinetics in metastable caffeine

Using time-resolved dielectric relaxation spectroscopy, we have studied the kinetics of the first-order phase transformation of caffeine from its high temperature rotational solid form to the room temperature phase. The results indicate that the kinetics can be fitted by a highly stretched exponential law. The kinetics, thus, differ from that of a ‘typical’ nucleation and growth process. This unusual non-sigmoidal evolution can be understood as being dominated by slow nucleation in grains with a dispersion of their size ranging below the characteristic dimension of the nucleation and growth process. The relaxation times of polarization are characterized by an activation energy closely correlated with that of the nucleation process.