Article pubs.acs.org/cm
Structure and Transformation of Amorphous Calcium Carbonate: A Solid-State 43Ca NMR and Computational Molecular Dynamics Investigation Jared Wesley Singer,† A. Ö zgür Yazaydin,‡,§ R. James Kirkpatrick,∥ and Geoffrey M. Bowers*,†,⊥ †
Department of Material Science and Engineering, New York State College of Ceramics at Alfred University, Kazuo Inamori School of Engineering, Alfred University, Alfred, New York, 14802 United States ‡ Department of Chemical Engineering, University of Surrey, Guildford, GU2 7XH, United Kingdom § Department of Chemistry, Michigan State University, East Lansing, Michigan, 48824 United States ∥ College of Natural Science, Michigan State University, East Lansing, Michigan, 48824 United States ⊥ Department of Chemistry, College of Liberal Arts & Sciences, Alfred University, Alfred, New York, 14802 United States S Supporting Information *
ABSTRACT: Amorphous calcium carbonate (ACC) is a metastable precursor to crystalline CaCO3 phases that precipitates by aggregation of ion pairs and prenucleation clusters.1,2 We use 43 Ca solid-state NMR spectroscopy to probe the local structure and transformation of ACC synthesized from seawater-like solutions with and without Mg2+ and computational molecular dynamics (MD) simulations to provide more detailed molecular-scale understanding of the ACC structure. The 43Ca NMR spectra of ACC collected immediately after synthesis consist of broad, featureless resonances with Gaussian line shapes (FWHH = 27.6 ± 1 ppm) that do not depend on Mg2+ or H2O content. A correlation between 43Ca isotropic chemical shifts and mean Ca−O bond distances for crystalline hydrous and anhydrous calcium carbonate phases indicates indistinguishable maximum mean Ca−O bond lengths of ∼2.45 Å for all our samples. This value is near the upper end of the published Ca−O bond distance range for biogenic and synthetic ACCs obtained by Ca-X-ray absorption spectroscopy.3−5 It is slightly smaller than the values from the structural model of Mgfree ACC by Goodwin et al.6 obtained from reverse Monte Carlo (RMC) modeling of X-ray scattering data and our own computational molecular dynamics (MD) simulation based on this model. An MD simulation starting with the atomic positions of the Goodwin et al.6 RMC model using the force field of Raiteri and Gale2 shows significant structural reorganization during the simulation and that the interconnected carbonate/water-rich channels in the Goodwin et al. model shrink in size over the 2 ns simulation time. The distribution of polyhedrally averaged Ca−O bond distances from the MD simulation is in good agreement with the 43Ca NMR peak shape, suggesting that local structural disorder dominates the experimental line width of ACC. KEYWORDS: amorphous calcium carbonate, ACC, biomineralization, 43Ca NMR, molecular dynamics
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spectra of ACCs with and without Mg2+ and on complementary computational molecular dynamics (MD) modeling. Previous experimental studies of ACC local structure using Ca X-ray Absorption Spectroscopy (XAS), 13C NMR spectroscopy, and various X-ray scattering methods have provided constraints on parameters such as mean Ca−O bond lengths and coordination numbers and have also shown that ACC samples with different levels of hydration have different local structures.1,2,6,9,10,16 For instance, Gebauer and colleagues have used Ca-XAS and 13C NMR results to suggest that synthetic, Mg-free ACC can have a “vaterite-like” or “calcite-like” structure depending on the pH of synthesis.3 In contrast, based on X-ray pair distribution function (PDF) analysis, Ca-
INTRODUCTION Amorphous calcium carbonates (ACCs) are commonly occurring precursors to inorganic and biogenic crystalline calcium carbonates7−9 and are at the heart of current discussions of nonclassical nucleation theory.1,2,10,11 Their aqueous precipitation, crystallization behavior, and natural occurrences have been widely studied.2,3,6−9,12−30 ACC can be readily synthesized in the laboratory and also occurs naturally as inclusions in sea urchin spicules31 and bivalve shells25 and in crustaceans,32 earthworms,33 and plant cystoliths.34 Several studies have shown that Mg2+,35 organicand biomolecules,28 silica encrustations,23 phosphate,34 and physical confinement7,17 can stabilize ACC such that its transformation to crystalline CaCO3 occurs over periods of days to weeks instead of minutes or less, as for synthetic, pure Ca-ACCs. This paper discusses the structure and transformation of ACC based on the first 43Ca solid-state NMR © 2012 American Chemical Society
Received: February 3, 2012 Revised: April 20, 2012 Published: April 23, 2012 1828
dx.doi.org/10.1021/cm300389q | Chem. Mater. 2012, 24, 1828−1836
Chemistry of Materials
Article
Table 1. Synthesis Conditions and Compositions of ACC Samples Used in This Studyb sample description
Mg/Casolution
T (°C)
Ca
Mg
Na
H2O
Cl
Si
low-Mg, RT Mg-free, low-T Mg-free, low-Ta Mg-free, low-Ta high-Mg, RT high-Mg, low-T
>0.05 0 0 0 5 5
20−25 2−5 2−5 2−5 20−25 2−5
0.93 NA 0.99 0.99 0.44 0.99
0.05 NA 0.01 0.00 0.51 0.00
0.03 NA 0.01 0.00 0.05 0.01
10% NA 15% 15% 40% 30%
