Additive-specific modulation of non-classical nucleation pathways

Additives, comprising small quantities of organic and inorganic molecules, are crucial for controlling the crystallisation of various (bio)materials; however, their mechanisms remain poorly understood. By integrating in situ high-energy X-ray scattering with potentiometric titrations, we examine the impact of industrially relevant additives on the nucleation pathways of portlandite and gypsum. While both minerals undergo multistep nucleation, portlandite transitions gradually from a disordered to an ordered phase, whereas gypsum exhibits an abrupt transition. These distinct non-classical pathways and their pH conditions correlate with the mineral-specific effects of the additives. Additive effects extend beyond the classical models, such as cation-binding, exerting influence primarily during the prenucleation stage. Moreover, additives demonstrate a dual role by simultaneously delaying and accelerating different stages of nucleation, highlighting their multifaceted impact on the crystallisation process. These findings offer insights for designing tailored additives to optimise industrial crystallisation processes and advance the understanding of biomineralisation mechanisms. Organic additives frequently shape crystallisation in natural and industrial settings, yet their precise influence on nucleation remains poorly understood. Here, Baken et al. investigate how additives affect the crystallization of the industrially relevant minerals portlandite and gypsum. Using controlled titration coupled with in situ synchrotron monitoring, the team demonstrates that both minerals form via intermediate steps: portlandite gradually becomes more ordered as it develops, while gypsum switches abruptly from a disordered to an ordered state. The study reveals that additives influence these pathways prior to nucleation by altering the nature of prenucleation clusters. How strongly an additive interacts with these clusters depends on its chemical state, which is controlled by the pH conditions specific to each mineral. These findings offer a starting point for creating an industrial “toolbox” to help select more effective additives, and they advance our understanding of biomineralisation processes.