Nucleation, Growth, and Pseudo-Polymorphic Behavior of Citric Acid As Monitored in Situ by Attenuated Total Reflection Fourier Transform Infrared Spectroscopy

The crystallization, dissolution, and associated pseudo-polymorphic behavior of citric acid crystals from aqueous solution is investigated using temperature-programmed and isothermal batch experiments. Quantitative attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is used to measure in situ the solution concentration and hence the reactant supersaturation over a wide range of solution undercoolings within the metastable zone. Detailed mapping out of the solubility−supersolubility diagram reveals poor nucleation behavior as characterized by a very wide metastable zone width (typical value, 55 °C for a cooling rate of 0.05 K/min). Simultaneous ATR FTIR and optical turbidometric measurements are used to cross-correlate the supersaturation driving force to the nucleation behavior as followed prior to and during crystallization within the metastable zone. Both temperature-programmed and isothermal measurements reveal behavior consistent with spontaneous liquid-phase separation wi...