Effects of Cooling Rate, Saturation Temperature, and Solvent on the Metastable Zone Width of Triethanolamine Hydrochloride

The metastable zone widths (MZWs) of triethanolamine hydrochloride ([HTEA]Cl), defined as the maximum temperature difference Delta T(max) a solution can withstand before crystallization of a solid phase, in water, ethanol-water, and HCl-water mixed solvents have been determined at a saturation temperature T(0) range from 293.15 to 303.15 K with cooling rate R from 3 to 8 K.h(-1). Solubilities of [HTEA]Cl in ethanol water mixtures have also been obtained using a dynamic method. An equation of MZW with similar form to Sangwal's equation was derived from the fundamental driving force of crystallization based on chemical potential. With the newly obtained equation, the experimental values of MZW Delta T(max) for [HTEA]Cl-water system at 298.15 K was correlated with the cooling rate R by ln(Delta T(max)/T(0)) = -6.140 + 1.222 ln R. The enthalpy of mixing Delta H(mix) was calculated with the solubility data, and the result of differential scanning calorimetry (DSC). For [HTEA]Cl-water system, the value of -Delta H(mix) was determined to be 30.88 kJ.mol(-1), and the addition of both HCl and ethanol led to a decrease of -Delta H(mix). The experimental results also showed that MZW could be greatly enhanced when ethanol and HCl were added. It was found that the values of MZW decrease with -Delta H(mix) for both [HTEA]Cl-ethanol-water and [HTEA]Cl-HCl-water systems at the same saturation temperature and cooling rate.