IN-SITU OPTICAL INTERFEROMETRIC STUDIES OF THE GROWTH AND DISSOLUTIONBEHAVIOR OF PARACETAMOL (ACETAMINOPHEN) CRYSTALS .3. INFLUENCE OF GROWTH IN THE PRESENCE OF P-ACETOXYACETANILIDE

The influence of p-acetoxyacetanilide (PAA) as an additive in the surr ounding paracetamol solution was investigated during growth and dissol ution of paracetamol crystals. The different crystal faces of paraceta mol, {110}, {20 (1) over bar}, and {001}, exhibited two different mech anisms of interaction with the additive. According to the first mechan ism, PAA molecules are adsorbed onto the crystal surfaces, i.e. {110} faces, and decrease both the step free energy and the step velocity fu nctions. The second mechanism, which is characteristic for the {001} c rystal faces, consists of a selective adsorption of PAA onto the activ e dislocation sites and suppression of more powerful growth centers in favor of elemental dislocations. The character of adsorption is defin ed by the affinity between the molecular structure of the crystal surf ace and the additive molecules, which permits incorporation into the { 110} growth sectors. Lattice strain produced by incorporation of PAA e xhibits a maximum followed by a relaxation process. This relaxation is caused by formation of cleavage defects and corresponds to an increas e of the enthalpy of fusion and decrease of the crystal density as fun ctions of PAA concentration.