Metastable crystalline lamella of cetylpyridinium chloride in the Krafft transition

The metastable crystalline lamella was found in the Krafft transition of aqueous cetylpyridinium chloride (CPC) solutions. Temperature-dependent profiles of small-angle X-ray scattering (SAXS) for the CPC solution incubated for 10 min at 5 degrees C exhibited the metastable lamella structure with a lattice spacing of d(L) = 3.19 nm at temperatures below 12 degrees C and the stable lamella structure with a lattice spacing of d(L) = 2.85 nm at temperatures between 12 and 19 degrees C. The former lamella structure, however, was not observed in the temperature scanning SAXS profiles of the CPC solution incubated for 24 h at 5 degrees C. The latter lamella structure observed in the SAXS profile mentioned above started melting at 18 degrees C. The electric conductance change of the CPC solution with a time elapsed after dropping the temperature showed the existence of the temperature-dependent induction period in the Krafft transition, indicating high activation energies for the transition. In the differential scanning calorimetry measurements over temperatures ranging from 5 to 30 degrees C, a single endothermic enthalpy peak at 19 degrees C observed for the CPC solution incubated at 5 degrees C for a longer period than 6 h was split into double peaks at 14 and 19 degrees C when the same solution was incubated at 5 degrees C for a shorter period than 6 h. The observed calorimetric behavior is explained by the existence of the metastable crystalline state that grows faster and melts at a lower melting temperature than the stable crystalline state.