The effect of water vapor pressure on desolvation kinetics of caffeine 4/5-hydrate

Abstract

The kinetics of dehydration of caffeine 4/5-hydrate at different relative humidities (RH) was studied at 25 °C for two crystal preparations with different crystal sizes. The weight change was evaluated over phosphorus pentoxide and relative humidities of 9, 13, 24, 36 and 43%, using special hygrostats and a below-weight balance. Additionally the samples were stored for 8 months at RH between 0 and 98%. A mechanistic interpretation of the reaction was derived by analyzing the data with numerous kinetic models as well as by microscopic investigations and various thermoanalytical techniques (thermomicroscopy, thermogravimetry, differential scanning calorimetry). The mechanism of the dehydration reaction was found to be very complex, depending on the crystal size and water vapor pressure. Diffusion control as well as nucleation and growth reaction are considered as the fundamental principles of the process. Due to the occurrence of a Smith-Topley effect, a constant loss of water rate of a coarse crystalline batch between 0 and 13% relative humidity takes place. The dehydration rate of small crystals is much more affected by changes in water vapor pressure. Over desiccants, complete transformation to the anhydrate requires about 4 h, a 30th of the time required for the coarse crystals. Caffeine hydrate loses the water of crystallization even at 61% RH (25 °C), which should be considered during processing and storing conditions. Beyond that, this paper suggests that a variety of analytical techniques are necessary for sufficient information about the dehydration characteristics of a crystalline hydrate and to avoid misinterpretations of phase transformations in the solid state.