Control of crystallization by polymer additives

Abstract

Understanding how crystallization processes are controlled by polymer additives is relevant for a vastly diverse number of fields, for example in biomineralization, where the morphology of minerals is controlled by proteins or in scale inhibition strategies in industrial plants, in which macromolecules are added to prevent the precipitation of minerals. While numerous investigations have focused on elucidating additive-controlled crystallization, a detailed understanding of the underlying mechanisms is still desired. Herein, based on calcium carbonate as a mineral system, a comprehensive study on the manifold effects of selected polycarboxylate additives on the distinct species along the crystallization pathway is presented. Poly(glutamic acid) and poly(aspartic acid) are chosen as additives, as these polyaminoacids resemble biomineralization-associated peptides, as well as poly(acrylic acid), a commercially used scale inhibitor. Using potentiometric titrations, it is shown that even the basic interaction of polycarboxylates with calcium ions is more complex than commonly assumed. Quantitative determination of the Langmuir parameters of the binding process reveals that higher order effects and contributions arising from the whole polymer chain play a significant role, while the chemistry of the monomer unit constituting the polymer plays a subordinate role. The results put a question mark on whether the binding processes can be accurately described using solely the Langmuir binding model that assumes non-interacting binding sites. The investigation of the following stages of the crystallization pathway reveals that the polymer additives show the largest effect in the stabilization of liquid-like mineral precursors. A key step of this inhibition is the additive-driven binding of bicarbonates. Quantitative evaluation of ion association in the prenucleation regime reveals that more than 20% of bicarbonate species are bound in mineral precursors at pH 9.8, which can also be detected in isolated solid amorphous intermediates. Surprisingly, the protons introduced by bicarbonates are highly mobile, causing the formation of amorphous mineral ion conductors, which opens up possibilities for novel applications of mineral materials. The importance of liquid-like precursors for the mineral formation pathway is still highly debated in the community, and they are often ignored in the explanation of crystal formation. Using a refined gas diffusion method, it is demonstrated that liquid-like precursors show sufficient kinetic stability to be detected, both in presence of polymers and in additive-free systems. Observing the time dependent formation and transformation of the precursors shows that they play an important role in the early stages of crystallization and must be generally considered for the interpretation of gas diffusion experiments. Regarding the technological application of liquid-like minerals, a new and easily scalable synthesis method is presented, which solves existing limitations of the available synthesis methods. The “scalable controlled synthesis and utilization of liquid-like precursors for technological applications” (SCULPT) method effectively allows the isolation of the precursor on a gram scale and to access to the full potential of these mineral precursors for material synthesis. Implementing the gained insights into the current picture of nonclassical mineral formation, which was subject of many advancements in recent years, an updated view on additive-controlled mineralization is presented. The discoveries presented in this work are beneficial for the scientific and industrially-related communities far beyond the field of nucleation and crystallization mechanisms, such as materials chemistry, and improve the understanding of the mechanisms underlying biomineralization and mineral formation in general.