Kinetic Analysis of the Thermal Decomposition of Amitriptyline Hydrochloride by Nonisothermal Thermogravimetry

Abstract

The nonisothermal thermogravimetric decomposition of amitriptyline hydrochloride was investigated under pure nitrogen atmosphere at four different heating rates (β =5, 10, 15, 20 o C min-1). Friedman, Flynn-wall-Ozawa, and Tang isoconversional methods were used to calculate the activation energy E a. The results obtained was compared with that obtained by the distributed activation energy model, Miura procedure. Activation energy, E a, results showed that the isoconversional kinetic methods used (Friedman, Flynn-Wall-Ozawa and Tang methods) were in good agreement with each other, together with the distributed activation energy model. It was found that the activation energy E a was not really changed and was almost independent with respect to the level of conversion (α). This result suggests that the nonisothermal decomposition process of amitriptyline hydrochloride follows a single-step reaction. The master-plots were used to obtain the reaction kinetic model, which was confirmed by the integral composite I method. The kinetic triplet determined was, activation energy, E a = 74.68 kJmol-1 , pre-exponential factor, A= 3.71×10 3 min-1 and the reaction kinetic model follows the Avrami-Erofeev model, (nucleation and growth) , A4, f(α)= 4(1-α)[-ln(1-α)] 3/4 , g(α) =[-ln(1-α)] 1/4. 1-INTRODUCTION Amitriptyline hydrochloride (AMT), with trade names, Vanatrip, Elavil, Endep, is an antidepressant drug under tricyclic antidepressant group. AMT has been widely used in the treatment of chronic pain, regardless of the presence of depression (Burke et al.,2015). It is used to treat many psychological disorder including major depressive disorder, anxiety, attention-deficit hyperactivity disorder, and bipolar disorder (Dipalma and Katzung,1983). The thermal decomposition study, has received considerable attention all along the modern history, especially in the pharmaceutical field. Thermogravimetry, derivative thermogravimetry (TG/DTG) and differential thermal analysis (DTA), are analytic tools of high importance that helps in identifying the polymorphic forms, phase transitions, active-excipients