Simultaneous determination of the heat and the quantity of vapor sorption using a novel microcalorimetric method

Purpose. In this study, instrumentation for measuring vapor sorption enthal pies and sorption uptakes simultaneously with an isothermal microcalorimete r is introduced. Various pharmaceutical model substances undergoing phase t ransitions when exposed to humid conditions (25 degrees C), were employed t o evaluate the usefulness and sensitivity of the constructed experimental m ethod. Methods. The sample is placed in the sample vessel of a RH cell and the moi sture content of the air flow is controlled. From the RH cell the air flow is conducted into a subsequent perfusion cell in which a saturated salt sol ution has been loaded. The RK cell and perfusion cells are positioned in th e sample sides of two twin calorimetric units. Depending on the moisture co ntent in the outlet flow leaving the preceding RH cell, the heat flow signa l from the subsequent perfusion cell will vary. By means of blank measureme nt with identical settings, the rate of water sorption can be calculated an d, by integration, the amount of sorbed water is obtained. Results. Amorphous lactose and cefadroxil undergo recrystallization when th e moisture level in the surroundings exceeds the threshold values specific to each compound. During the sorption phase, heat is evolved fairly linearl y as a function of consumed moisture, and also after the recrystallization, the heats indicate linear behavior. The heat values for the desorption pha se of amorphous lactose and the adsorption of crystalline lactose coincide. With the different anhydrous forms of theophylline, the hydration takes pl ace more rapidly in the metastable form I, and generally, the process is mo re energetic in form I. In all cases, the gravimetric results agree with th e water sorption uptakes calculated from the calorimetric data. Conclusions. The technique introduced offers a rapid and sensitive method t o gain new insights into the transitions in which vapors are involved. In a ddition, different kinds of surfaces with various energetics can now be stu died more closely.