Kinetics and mechanism of the solid phase dehydration of 11-aminoundecanoic acid dihydrate

The dehydration of 11-aminoundecanoic acid dihydrate was studied on both powdered material and single crystals by means of thermogravimetric measurements, optical observations and diffractometric procedures. The reaction can be described empirically by the Avrami-Erofe'ev equation, with parameters K_A and n that change with water vapour pressure and temperature. No Smith-Topley effect is observed, and n ranges between 1 and 2.2, with maximum values at intermediate water vapour pressures. Optical observations show that the reaction starts through surface nucleation and proceeds inwards and laterally from exposed (100) and (010) faces, which correspond to water movement in planes perpendicular to the direction of the molecule chains. Diffractometric observations show that dehydration of single crystals gives rise topotactically to a coherent array of anhydrate microcrystals. These observations are rationalized through a qualitative model that involves surface nucleation, growth and coalescence of nuclei and ingestion of undeveloped nuclei; growth rate has to be described by two parameters representing growth normal to (100) or (010) planes and growth perpendicular to these faces. Under limiting conditions, such a model may give rise to boundary advance or even diffusional control of the dehydration process.