Heat capacity and enthalpy of indapamide

Abstract

The heat capacities and total enthalpy of anhydrous indapamide (IND, CAS no.: 26807-65-8) are reported from temperature 1.84 to 523.15 K measured by a Quantum Design Physical Property Measurement System® (PPMS) and differential scanning calorimetry (DSC). The low-temperature experimental heat capacities of crystalline IND from 1.84 to 302.65 K were measured by PPMS and fit to a theoretical model in the low temperature range (T < 7.68 K), orthogonal polynomials in the middle temperature range (7.68 < T < 54.15 K), and a combination of Debye and Einstein functions in the high temperature range (T > 54.15 K). The calculated heat capacities were extended to a higher temperature to be used as a reference line of solid heat capacity, cp(solid). The experimental heat capacity of liquid, cp(liquid) for amorphous indapamide above the glass transition (Tg(midpoint on cooling) = 369.7 ± 0.6 K) was approximated by linear regression and was expressed as cp(liquid) = (0.954 + 0.0023T) J·K–1 g–1. The solid and liquid heat capacities of IND were applied as reference lines for advanced thermal analysis of the experimental, apparent heat capacity data measured by DSC. The change in heat capacity (Δcp) of 0.46 J·K–1 g–1 at Tg for fully amorphous IND and heat of fusion Δfush = 77.5 ± 0.7 J·g–1 at melting temperature (Tm = 439.7 ± 0.4 K) of 100% crystalline anhydrous IND were determined. Knowing equilibrium parameters of phase transitions of amorphous and crystalline forms of IND and heat capacities of solid and liquid IND, total enthalpies for both phases were calculated. Data for unaged and physically aged at 358.15 K for 32 h amorphous IND were presented in the context of absolute heat capacity and integral equilibrium enthalpy of solid and liquid IND phases.