Calorimetric Relaxation and the Glass-Liquid Temperature Range of Acetaminophen-Nifedipine Alloys

Glassy states of nine acetaminophen-nifedipine compositions have been made by slowly supercooling their melts, and calorimetric Tg and the nonexponential, nonlinear relaxation parameters β and x that are used in modeling the mobility of a pharmaceutical determined. The Tg-endotherm's shape varies with the alloy's composition, Tg increases approximately linearly with the  mol% of nifedipine, β and x increase, and the activation enthalpy Δh* decreases. At Tg, the relaxation time τcal of acetaminophen, nifedipine, and their alloys differs from 100 s to different extents. The distribution of relaxation times is lesser than that for polymers and other glasses. For a given composition, Δh*, β, x, and τcal anomalously depend upon the heating rate, indicating that variation of β with temperature would not yield better fits for modeling their stability. It is suggested that a pharmaceutical's relaxation is generally influenced by changes in intermolecular hydrogen bonds, chemical short-range order, vibrational frequency, isomerization, and impurity electrolyte dissociation, all of which contribute to the energy change with a distinctive kinetics.