Nucleation, growth, and pseudo-polymorphic behavior of citric acid as monitored in situ by attenuated total reflection Fourier transform infrared spectroscopy
H. Groen et Kj. Roberts, Nucleation, growth, and pseudo-polymorphic behavior of citric acid as monitored in situ by attenuated total reflection Fourier transform infrared spectroscopy, J PHYS CH B, 105(43), 2001, pp. 10723-10730
The crystallization, dissolution, and associated pseudo-polymorphic behavio
r of citric acid crystals from aqueous solution is investigated using tempe
rature-programmed and isothermal batch experiments. Quantitative attenuated
total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is u
sed to measure in situ the solution concentration and hence the reactant su
persaturation over a wide range of solution undercoolings within the metast
able zone. Detailed mapping out of the solubility-supersolubility diagram r
eveals poor nucleation behavior as characterized by a very wide metastable
zone width (typical value, 55 degreesC for a cooling rate of 0.05 K/min). S
imultaneous ATR FTIR and optical turbidometric measurements are used to cro
ss-correlate the supersaturation driving force to the nucleation behavior a
s followed prior to and during crystallization within the metastable zone.
Both temperature-programmed and isothermal measurements reveal behavior con
sistent with spontaneous liquid-phase separation within the highly supersat
urated mother liquor prior to crystallization, the occurrence of which is k
nown as oiling-out, a phenomenon poorly understood in industrial crystalliz
ation reactions. Parallel examination of the phase of the product crystals,
using in situ and ex situ powder X-ray diffraction (XRD) and differential
scanning calorimetry (DSC), reveals the formation of the anhydrous form of
citric acid via temperature-programmed experiments and the monohydrate phas
e being crystallized via isothermal experiments. These results, which corre
late with the solubility-supersolubility phase diagram, are rationalized in
terms of the respective crystal chemistry of the anhydrate and monohydrate
structures of citric acid, which is consistent with a solvent-mediated pha
se transformation mechanism effecting the change from the anhydrate to the
monohydrate form.