Journal of Physical Chemistry B, Vol.101, No.20, 3973-3979, 1997
In-Situ Optical Interferometric Studies of the Growth and Dissolution Behavior of Paracetamol (Acetaminophen) .1. Growth-Kinetics
Paracetamol (acetaminophen, 4-hydroxyacetanilide) is a typical representative of over-the-counter analgesic and antipyretic drugs which are commonly administered as tablets. For these materials, crystallization from solution is an important stage of pharmaceutical production which defines many physicochemical properties of the solid dosage forms. The approach developed in this work involves investigation of the surface growth kinetics of single paracetamol crystals using in situ laser interferometry. All the major crystal faces of paracetamol (e.g. {110}, {001} and <{20(1)over bar }>) have shown a dislocation growth mechanism. The structure and the effective Burgers vector of the dislocations have been obtained and the specific step free energy, alpha, and the step kinetic coefficient, beta, determined. The dissolution behavior of paracetamol crystals is affected by dislocation core instability and, to a lesser extent, by stress in the uppermost crystal layer for which the critical undersaturations have been determined. The step velocity is a nonlinear function of supersaturation. This nonlinearity is particularly important for the {110} crystal faces and results in the formation of large macrosteps, growth hysteresis, and overall kinetic instability. The crystal defects are mostly concentrated in the {110} growth sectors which therefore define the overall crystal quality. Analysis of the paracetamol crystal structure, surface energies, and the surface-solvent interactions suggests that specific hydrogen bonding plays a major role in defining the surface kinetics and, consequently, the crystal shape and crystal quality.