Date Published: February 14, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Marie Albéric, Luca Bertinetti, Zhaoyong Zou, Peter Fratzl, Wouter Habraken, Yael Politi.
Many organisms use amorphous calcium carbonate (ACC) and control its stability by various additives and water; however, the underlying mechanisms are yet unclear. Here, the effect of water and inorganic additives commonly found in biology on the dynamics of the structure of ACC during crystallization and on the energetics of this process is studied. Total X‐ray scattering and pair distribution function analysis show that the short‐ and medium‐range order of all studied ACC samples are similar; however, the use of in situ methodologies allow the observation of small structural modifications that are otherwise easily overlooked. Isothermal calorimetric coupled with microgravimetric measurements show that the presence of Mg2+ and of PO43− in ACC retards the crystallization whereas increased water content accelerates the transformation. The enthalpy of ACC with respect to calcite appears, however, independent of the additive concentration but decreases with water content. Surprisingly, the enthalpic contribution of water is compensated for by an equal and opposite entropic term leading to a net independence of ACC thermodynamic stability on its hydration level. Together, these results point toward a kinetic stabilization effect of inorganic additives and water, and may contribute to the understanding of the biological control of mineral stability.
Since the discovery of the central role of amorphous calcium carbonate (ACC) in the crystallization pathways of biogenic calcium carbonate in sea urchin larva,1 the use of amorphous precursors has been recognized as a common strategy employed by many organisms across various phyla to build biominerals with superior properties.2, 3, 4, 5 This strategy has also inspired the development of new synthetic routes for controlling morphology, phase, and physical properties of various materials.6, 7, 8
Our results show that ACC stabilization in the studied systems is dominated by kinetic control and depends on several factors: particle size, the presence of additives and the water content. Using in situ methodology and novel analyses, we describe the crystallization dynamics of ACC, with and without additives induced by heating or high relative humidity. Despite similar initial short‐ and medium‐range order of all studied ACC samples studied, additives increase the thermal stability of the amorphous phase and, at high humidity, delay the onset of crystallization as well as the time required for its completion. Nonetheless, neither additive has a large effect on the crystallization enthalpy at room temperature. Most importantly, water content was found to correlate linearly with the molar enthalpy difference to calcite. Due to a balancing effect of the molar entropy change between hydrated and anhydrous ACC the free energy of ACC relative to calcite is independent of its hydration level from 0 to 1.3 H2O. Water, however, has a strong effect on the ion mobility and is the key to structural rearrangement as is reflected in the structural changes occurring during water loss at moderate temperatures. We anticipate that the developed methodology will aid further study, for example to determine to what extent the role of additives is synergistic or antagonistic in the stabilization of ACC. In addition, the present study contributes to the understanding of biological mineralization and may guide materials synthesis for various applications.
ACC Synthesis: ACC was synthesized in the presence (or absence) of inorganic additives (Mg2+, PO43− ions) by mixing of CaCl2 (or CaCl2/MgCl2, ratio: 9/1) and Na2CO3 (or Na2CO3/NaH2PO4, ratio: 9/1) solutions. The mixing was either performed using pipettes on site at the European Synchrotron Research Facility (ESRF) beamline in order to have fresh samples or using a controlled titration set up as in.32 The latter was used to control the particle size of ACC. The titration set up allows measuring the pH, which changes from 11.4 to 10.6 from beginning to the end of the reaction. The ACC synthesis was followed by a fast filtration and drying procedure using cold ethanol (4 °C, 100%). The samples were stored for further use in a vacuum desiccator. By varying the concentration of the initial solutions (CaCl2 and Na2CO3) pure ACCs (with no additives) with different particle sizes were also synthetized. Pure ACC with water content of n = 0.01 used in isothermal experiment was obtained by heat treatment at 200 °C for 2 h. Pure ACCs with water content from n = 0.77 to 1.3 were obtained by storing the ACC under vacuum for several days.
The authors declare no conflict of interest.