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Organic molecules control calcite growth and crystal morphology, influence biomineralization processes, and offer clues for optimizing antiscalants for industry. Here we quantified the effect of amino acid monomers, aspartic acid (Asp1), and glycine (Gly1), and their polymers (Aspn, Asp5, and Gly5), on calcite growth rate, in a constant composition setup. Asp1 and its polymers inhibit growth, with rate decreasing as amino acid chain length increases. For 2 mM Asp1, fractional inhibition (FI, where 1 represents complete inhibition) was 0.54; for 0.0012 mM Aspn, FI = 0.94. Gly1 and Gly5 only marginally affect growth (−0.1 < FI < 0.1); indeed, they slightly promote growth at most tested concentrations. Fitting of adsorption isotherms (Langmuir, Langmuir–Freundlich, Flory–Huggins) confirmed that Asp polymers adsorb strongly, explaining their strong control on calcite growth, but Gly1 and Asp1 adsorb less due to competition with carbonate ions. ΔG ads (Aspn) = −39 kJ/mol; ΔG ads (Asp5) = −50 kJ/mol; ΔG ads (Asp1) = −21 kJ/mol; and ΔG ads (Gly1) = −22 kJ/mol. The morphology was equally affected. Crystal edges became rougher, and corners, more rounded. Overall, the number of carboxyl groups and length of the carbon chain correlated with the lowest growth rate.

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Polysaccharides play a key role in the biomineralization of the elaborate and species specific calcite platelets, known as coccoliths. Coccoliths cover some species of algae and the polysaccharides serve as controlling agents, directing crystal growth. We conducted experiments of calcite growth in the presence of five well described polysaccharides to test the influence of composition and structural properties. Alginate and polygalacturonate contain a carboxylate group for every glycosyl unit, whereas the unbranched amylose and the progressively branched polysaccharides, ß-limit dextrin and amylopectin, contain only glucosyl units. Calcite growth was monitored by the consumption of Ca 2+ and CO 3 2− in a constant composition setup. Langmuir isotherms effectively describe the uptake data from experiments where polysaccharide concentration was varied. Our results demonstrate that polysaccharides containing acidic glycosyl units strongly inhibit calcite growth, compared to neutral polysaccharides, but fine structure, i.e., the anomeric configuration and/or the position of functional groups in the polymer, also plays a role, as illustrated by the difference in inhibitory properties of the acidic polysaccharides, alginate, and polygalacturonate. Of the neutral polysaccharides, the branched molecules were stronger inhibitors than the linear molecule and the longer the branches, the stronger the inhibition. Thus, amylopectin was 2 orders of magnitude more effective than ß-limit dextrin and amylose was inactive.

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