Effect of aspartic acid and glycine on calcite growth

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 (Asp_n, 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 Asp_n, 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 (Asp_n) = -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.