Thermal stability of three solid-state TRIMEB inclusion complexes.

NATALIA M. ROUGIER; RITA H. DE ROSSI; ELBA I. BUJÁN; DYANNE L. CRUICKSHANK; MINO R. CAIRA; SUSAN A. BOURNE

Villa Carlos Paz

Simposio; XVIII Simposio Nacional de Química Orgánica; 2011

Sociedad Argentina de Investigación en Química Orgánica

The inclusion of agrochemicals in cyclodextrins (CD) can contribute significantly to the safety and optimum use of these compounds by reducing their volatility and/or degradation and hence diminishing environmental contamination. For commercial use of solid-state CD inclusion complexes, it is important to know its thermal stability and this information can be obtained from kinetic studies in the solid state. We are presenting here results on the thermal stability of solid-state complexes of fenitrothion 1, fenthion 2 and acetochlor 3 with TRIMEB using thermogravimetry (TG). Compounds 1 and 2 are organophosphorus insecticides while 3 is a chloroacetanilide herbicide. TRIMEB (heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin) is a β-CD derivative modified at the C2-OH, C3-OH and C6-OH hydroxyl positions. The solid-state inclusion complexes of 1, 2, and 3 with TRIMEB are isostructural, as demonstrated by PXRD. The TG traces for these complexes show a two-step mass loss; the first one (between ~120 and 270 °C) is due to guest expulsion from CD cavity occurring concomitantly with fusion of the complex (m.p. = 158, 152 and 133 °C for CD.1, CD.2 and CD.3, respectively) and the second one is due to host decomposition. The kinetics of the first process was studied by isothermal and non-isothermal methods. The data obtained from the isothermal studies were well fitted using a first order reaction model (F1) as well as a 3-D diffusion model (3D). Activation energies of 126, 129 and 130 kJ M–1 for CD.1, CD.2 and CD.3, respectively, were obtained by F1 model while the 3-D model gave 110, 109 and 86 kJ M–1 for CD.1, CD.2 and CD.3, respectively. A model-free isoconversional method was used to determine the activation energy in the non-isothermal experiments. Values of 158, 149 and 151 kJ M–1 were obtained for CD.1, CD.2 and CD.3, respectively. As the majority of the temperatures recorded for the various heating rates at the specific conversion level were lower than the melting points of the complexes, the activation energies determined using non-isothermal procedures provide a better representation of the energy required to remove only the guest molecule from the solid-state complex.