Using caffeine and maleic acid as example compounds, the researchers achieved 100 per cent crystallisation in just one minute using very little solvent, making the process faster and more environmentally friendly than conventional methods.
Professor Paradkar, Director of the University of Bradford’s Centre for Pharmaceutical Engineering, explains: “We chose caffeine and maleic acid as these compounds have different levels of solubility which is a common problem in pharmaceutical manufacturing. Conventional methods of co-crystallisation aren’t effective when you’re working with two compounds that dissolve differently, but we found that by using microwave radiation we could still get excellent results.”
The team worked with five different solvents, of which water and methanol produced the best results. These two solvents were able to transmit the heat from microwave radiation more effectively than the others tested: acetone, ethyl acetate and toluene.
The experiments have only, to date, been carried out at laboratory scale, but Professor Paradkar believes that, if the process can be scaled up, it could offer significant benefits to the pharmaceutical industry:
“Other methods of co-crystallisation, such as solvent crystallisation, grinding or ultrasound, all have associated problems, for example increased use of solvents or lack of purity in the resulting co-crystals,” he says. “Our research shows that microwave radiation is a very promising alternative for producing pure co-crystals quickly, with limited use of solvents.”