Structural characterization, physicochemical properties, suspension stability, and adsorption properties of four solid forms of amitraz

This paper reports the identification and preparation of three crystalline (A-C) and one metastable form (D) of amitraz. These were identified by their crystal morphology, crystal structures, aqueous solubility, and thermal properties. Form C was the least soluble (7 mug/mL) and had the highest melting point (115 degrees C). The differences in melting point (82 vs 72 degrees C) and solubility (20 vs 23 mug/mL) of forms A and B were not significant. The metastable noncrystalline form D (Tg = 38 degrees C, transition temperature = 62 degrees C, and melting point = 78 degrees C) was obtained by deposition on the surface of activated carbon from acetonitrile solutions. When the anionic surfactant sodium lauryl sulfate was added to the solubility medium, the solubilities of forms A and B were increased 10-11-fold and that of form C was increased 28-fold. Changing the crystal form had an effect on the stability of amitraz suspensions. Form C was the most stable with a t(1/2) of 136 days, and form B was the least stable with a t(1/2) of 28 days. The stability correlated with solubility differences. The addition of sodium lauryl sulfate to the suspensions increased the rate of hydrolysis (mean t(1/2) = 17 h), and because of increased solubility, there was no difference in the stability of the crystal forms in the anionic surfactant solution.