Effect of polymer addition on the contact line crystallisation of paracetamol

In an evaporating aqueous paracetamol solution, crystallisation of the metastable orthorhombic polymorph can occur at the solution-substrate contact line due to a favourable meniscus geometry and enhanced evaporation rates at the edges. Upon the addition of polymers or other common excipients to these systems, it was found that crystallisation of this form was suppressed. This was attributed to the polymer blocking the nucleation sites at the edge of the meniscus, altering the evaporation rate or disrupting the flow of solution to the edges. These results are in contrast to previous conclusions by other groups suggesting that the addition of certain polymers may aid in the selective crystallisation of the orthorhombic polymorph from solution via the formation of specific interactions.