Solid state deracemisation through growth, dissolution and solution-phase racemisation

Abstract

Ripening is a collective name for a number of processes which are caused by the size-dependence of the solubility. In all these processes, ageing takes place at low supersaturation, where nucleation does not occur. In Ostwald ripening, large crystals grow at the expense of small crystals, leading to a change in the particle size distribution over time. Viedma ripening, or attrition-enhanced deracemisation, is a crystallisation process which converts a racemic, or nearly racemic, solid phase to an enantiomerically pure solid phase. In this work, we consider the case of Ostwald ripening acting on populations of crystals in a solution containing a racemising agent. We show that this leads to deracemisation but that there are two ways in which this can occur. In fast chiral ripening, the initial conditions are such that a whole population of crystals is dissolved, while the other remains. In slow chiral ripening, the deracemisation occurs only at the steady state limit. The simulations we have carried out show that fast chiral ripening can take place under special circumstances: a fast racemisation reaction, low particle concentration and a sufficient difference between the initial particle populations are required. These requirements and the dynamics of the process make this mechanism different to Viedma ripening, which occurs only when agglomeration takes place.