Kinetic studies on hydrated solid transform of cetylpyridinium chloride in aqueous solution

Kinetic studies were performed on the transitions of hydrated solid formation and melt in an aqueous cetylpyridinium chloride (CPC) solution. The electric conductivity change of the CPC solution exhibited a long induction period, t(ind), for the formation of a hydrated solid after dropping the temperature. The solubility was obtained as a function of temperature from the conductivity of the supernatant over the precipitated solid. The t(ind) for the solid formation in the solution at a degree of supersaturation, S, below 16 was about 1 000 000 s. The t(ind) in the stirring solution, however, was less than 1/10 of that in the standing solution without stirring. The approximated relation of t(ind) = A(k)(ln S)(-3) was found for the formation of stable solid in the stirred solution. The A(k) values obtained for the micelle solutions were 2400 times greater than those obtained for the monomer solutions. The transition fraction from the metastable solid to the stable solid, r(stable), was obtained from the melting enthalpy ratio of the stable solid to the total melting enthalpy given by differential scanning calorimetry of the metastable solid suspension incubated at 3 degrees C for a given time. The changing rate of r(Stable) observed in applying the shear to the metastable solid suspension was about 20 times the rate in the shear-free case. The present experiments demonstrate that the shear in the solution accelerates the formation of a stable solid of CPC.