Role of Impurities in the Kinetic Persistence of Amorphous Calcium Carbonate: A Nanoscopic Dynamics View

Abstract

Amorphous calcium carbonate is found in the early stages of biomineral formation. Its moldability and its kinetic persistence against crystallization make it of potential interest to the development of biomimetic materials. The presence of water and of inorganic impurities such as Mg 2+ has been reported to increase its kinetic stability. Yet, the precise mechanisms of stabilization as well as the link between structural dynamics and crystallization kinetics are still missing. Here, we report the vibrational and diffusive dynamics of water and other atomic constituents in a series of Mg 2+-doped solid amorphous calcium carbonates revealed using coherent X-ray and incoherent neutron scattering. This unique combination of techniques presents a novel view on the nanoscopic dynamic behavior of these amorphous materials: the presence of Mg 2+ results in a higher frequency of structural rearrangements within the material. However, this increased dynamic behavior does not translate into a higher crystallization kinetics due to a concomitant stiffening of the water hydrogen bond network.

Water: How Does It Influence the CaCO3 Formation?

“…In addition, they affect the structure of the resulting crystals.W hen the Mg 2+ /Ca 2+ ratio in the aqueous solution is high, Mg-ACC preferentially transforms into aragonite or monohydrocalcite, as shown in Figure 8B. [148] As ar esult of these stronger interactions,t he activation energy for the dehydration of Mg-ACC is higher than that of pure ACCs uch that the temperature at which Mg-ACC starts to crystallize is higher. [139,141] Remarkably,t he mole fraction of Mg 2+ contained in Mg-calcite crystals is always identical to that in Mg-ACCp recursors.…”

“…[66,143,144,151] Additives such as Mg 2+ influence the crystallization kinetics of ACCi nt he presence of bulk water. [139,142,143,158,159] Thei ncorporation of Mg 2+ ions into ACCa lso delays its solid-state transformation:T he strong interactions between Mg 2+ and water molecules reduce the mobility of water, [148,154,160] thereby increasing the fraction of rigid water contained in Mg-ACC compared to that contained in ACC. [139,141] By contrast, when the Mg 2+ /Ca 2+ ratio is low,M g-ACC preferentially transforms into Mgcalcite.…”

Wasser: Wie beeinflusst es die CaCO3‐Bildung?

“…[145][146][147][148][149][150] The formation of Mg-ACC is thermodynamically favored over that of pure ACC [151] because the charge density of Mg 2+ is higher than that of Ca 2+ , [152,153] such that the Mg-ACC phase more strongly interacts with water. [154] This strong interaction increases the amount of water contained in the amorphous phase [135,155] such that the free energy of Mg-ACC is lower compared to pure ACC. However, the kinetics of the nucleation and growth of Mg-ACC is not significantly affected by the incorporation of Mg 2+ [156] such that the size of Mg-ACC particles is similar to that of pure ACC ones.…”

“…The amount of incorporated Mg 2+ influences the properties of Mg-calcite crystals and hence, is an important parameter that needs to be closely controlled if materials with well-defined properties want to be synthesized. [145,148,149,163,164] The incorporation of magnesium ions into ACC also delays its solid-state transformation: The strong interactions between Mg 2+ and water molecules reduce the mobility of water, [155,160,165] thereby increasing the fraction of rigid water contained in Mg-ACC compared to that contained in ACC. [155] As a result of this stronger interactions, the activation energy for the dehydration of Mg-ACC is higher than that of pure ACC such that the temperature where Mg-ACC starts to crystallize is higher.…”

“…[145,148,149,163,164] The incorporation of magnesium ions into ACC also delays its solid-state transformation: The strong interactions between Mg 2+ and water molecules reduce the mobility of water, [155,160,165] thereby increasing the fraction of rigid water contained in Mg-ACC compared to that contained in ACC. [155] As a result of this stronger interactions, the activation energy for the dehydration of Mg-ACC is higher than that of pure ACC such that the temperature where Mg-ACC starts to crystallize is higher. [74,135] For example, ACC containing 2.4 mol% Mg 2+ starts to crystallize around 190 °C, while pure ACC crystallizes around 160 °C, as shown in Figures 9 E and F. [74] Despite of the different crystallization temperatures, the structure of the resulting crystals is usually the same: calcite.…”