Journal of Thermodynamics & Catalysis
Open Access

Regulation of Low-Molecular-Weight Organic Acids on CaCO₃ Crystallization

Yu Chenyu and Li Meng^{* *}Correspondence: Li Meng, Guangxi Key Laboratory of Natural Polymer Chemistry and Physics, Nanning Normal University, Nanning, P.R. China, Email:

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Description

In contrast to polymeric additives, monomeric organic acids possess relatively low molecular weights of less than a few hundred with few carboxyl groups and exhibit only a limited number of potential binding sites [1]. Low-Molecular-Weight Organic Acids (LMWOAs) are a favorable option for CaCO₃ due to the structural resemblance between the carboxyl groups present in the additive and carbonate groups found in the mineral [2]. Furthermore, LMWOAs exhibit high selectivity towards crystal faces [3], thereby enhancing our comprehension of the specific interaction between functional groups and crystal face. The carboxyl and hydroxyl anions of organic acids can interact with the CaCO₃ surface through geometrical matching, hydrogen bond, electrostatic interaction, or even incorporation into the crystal lattice. It is widely acknowledged that the interfacial interaction between organic acid and CaCO₃ can be influenced by variations in organic acid concentration, functional group composition and stereochemical structure.

Citric acid, with three carboxyl groups, predominantly exists as a metal-ligand complex in solution, which exhibits a strong affinity towards the partially positively charged surface of CaCO₃. It has been demonstrated that citrate can effectively regulate the crystallization process of CaCO₃ by inhibiting growth rates at relatively low concentration regime, resulting in the formation of both vaterite and calcite. However, at higher concentrations, citrate promotes nucleation leading solely to elongated calcite formation [4]. Besides, the concentration of citrate can significantly impact the morphology of CaCO₃ crystals, causing them to transform from dumbbell-like shapes to rod-like or flower-like structures [3].

The dicarboxylic acid of malic acid and succinic acid have been observed to exert an inhibitory effect on the calcite growth. The adsorption of malic acid and succinic acid on calcite predominantly occurs through the coordination of their deprotonated carboxyl groups to the same surface Ca²⁺ site, resulting in the formation of a seven-membered chelate ring for succinic acid. In contrast, for malic acid, its hydroxyl group and one of its two carboxylic groups bind to the CO₃ 2- and Ca²⁺ sites, respectively. Malic acid exerts a notable influence on CaCO₃ morphology by adsorbing onto specific crystal faces during calcite dissolution/crystallization processes. The sorption of malic acid onto the (011) faces of calcite leads to the elongated morphology along the c-axis [3]. Succinic acid is known for its ability to hinder calcite dissolution by competitive adsorbing succinate ions and carbonate ions on the surface of calcite. It shows a mild impact on calcite dissolution due to its adsorption on rate-controlling surface sites [5].

The regulatory effects of functional groups of LMWOAs on CaCO₃ crystallization are also influenced by molecular chain length and stereochemical isomerism. The adsorption experiments reveal that the dicarboxylic acids demonstrate significantly affinity for the aragonite surface with oxalic acid exhibiting the highest affinity followed by malonic acid and glutaric acid [6]. Chiral organic acids are enantiomeric forms of a chiral molecule, differing solely in the spatial orientation of their functional groups within the overall molecular structure. The calcite exhibits right-handed spirals when deposited in the presence of L-malic acid, while displaying an achiral microstructure in a racemic mixture of d-and l-malic acid [7].

Conclusion

Organic acids may exert regulatory effects on the crystallization of CaCO₃ through similar mechanisms, yet their stereochemical variances can result in disparities in binding energy between identical functional groups and the surface of CaCO₃. Although, the affinity of carboxylic acids towards CaCO₃ surfaces exhibits a stronger manifestation compared to alcohols. The adsorption behaviors are contingent upon the side groups present in organic molecules. The intriguing role of LMWOAs in controlling CaCO₃ crystallization serves as an inspiring lesson derived from nature, which can be further developed into an advanced strategy for precise material fabrication control.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (42207386), Middle-aged and Young Teachers’ Basic Ability Promotion Project of Guangxi (2021KY0390).

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