In characterising ACC it is important to understand the material and to compare with crystalline polymorphs. As well as features on the order of Angstroms, such as those provided in section 7.3.1, analysis of the supramolecular structure of ACC can shed light on a material which can be difficult to study by experiment because of its often transient nature. Figure 7.2 shows snapshots of typical configurations
Figure 7.2: Snapshots of a 4 ˚A slice through the centre of the simulation z axis of equilibrated configurations of n= 0.55, 1 and 2 (from left to right respectively) for CaCO3·nH2O. Calcium and carbon atoms are connected by yellow bonds if rCa−C <3.825 ˚A. Water is shown as a blue surface, where the probe radius is 1σ of oxygen atoms. Hydrogen and carbonate oxygens have been omitted for clarity. for systems containing n = 0.55, 1 and 2 water molecules per CaCO3 unit. A heterogeneous distribution of water can be seen, with water clusters tending to occupy channels and cavities within an ionic framework formed by calcium and carbonate. The distribution of ions locally was equivalent for all hydration levels, but there was a clear difference between the local concentration of water with respect to ions. At low levels of hydration (i.e. n = 0.55), water tended to form small clusters which were isolated in cavities, but increased water content led to structures with very few isolated water in cavities and large interconnected water clusters that formed within channels in the ionic framework. The clusters extended throughout the framework, and the hydration level taken from different regions of time averaged configurations was consistent with the average stoichiometry for the whole system, as shown in Figure F.2 for ACC for the highest water content.
Goodwinet al. have used Reverse Monte Carlo (RMC) simulations to gen- erate a proposed structure of hydrated ACC based on experimental measurements [Goodwin et al., 2010]. They showed a similar heterogeneity within ACC, identi- fying water-rich regions that had excess carbonate, within a porous, calcium-rich ionic framework. This is further support for heterogeneous water distributions in ACC; however, it is unlikely that cation- and anion-rich regions will form in ACC as this would lead to a high energy state, as was shown by Singeret al. [Singer et al., 2012].
The distribution of water in mesoporous ACC was not consistent throughout the simulation cells. Two systems were simulated, with overall average stoichiome- tries of CaCO·H2O and CaCO3·3 H2O. However, heat treatment led to partial phase separation, and introduced a concentration gradient to both systems. While
the local concentrations of ions was again consistent, as for bulk systems, there was a depletion of water around the centre of the simulation cell, leading to an increase in the amount of water close to the simulation cell boundaries, exemplified by Figure F.3 (a). This led to a mesoporous system where repeating periodic units of calcium carbonate were encapsulated within channels of water.
Analysis of the water content in mesoporous systems is provided in Figure F.3 (b). For both low and high hydration, a plateau region is found in H2O/Ca2+ up to around 23 ˚A (from the simulation cell origin), after which there is a sharp increase in water content to the edge of the simulation cell. This cut-off signifies a partition in the systems from a core with low hydration, to increasingly hydrated calcium carbonate, whereupon the concentration of ions gradually decreases. The surface of ACC is ill-defined and is both rough and fluctuating, in agreement with other studies [Raiteri and Gale, 2010]. The chemical composition in the core of overall low and high hydration mesoporous ACC was measured asn= 0.16±0.01 and n = 0.64±0.01, respectively, where n is the number of water molecules per calcium ion. Within the core, water was mainly isolated in cavities within the ionic framework, with larger clusters observed in channels at the periphery of the ionic surface as the hydration level increased. In the mesoporous system with total average composition of CaCO3·3 H2O, no ions were found at the outermost regions of the simulation cell, and so this water can be considered to be contained in mesoporous channels.
In this work, it has been helpful to define three types of water molecule which can be found in ACC: occluded, channel and mesoporous channel water. The dis- tance between water molecules was defined by the relative positions of oxygen atoms, and the average maximum distance between nearest neighbour water molecules (as measured from the minimum following the first peak ingO−O(r) for CaCO3·H2O)
was∼3.8 ˚A. Water molecules which maintainedrO−O>3.8 ˚A over the course of the trajectory were deemed to be “occluded” in the ionic framework and were labelled H2OOcc. Water molecules for which rO−O < 3.8 ˚A at any stage in the trajectory were described as “clustered” water. For clustered water, it was further helpful to differentiate between water molecules found within nanometre sized channels in the ionic framework of ACC, and those which were contained within mesoporous channels (where a relatively low concentration of ions was found). Water in chan- nels within the ACC framework and in water-rich regions in mesoporous ACC were labelled H2OChan and H2OM es, respectively. Water clusters can be occluded in the ACC framework, and so the definition of occluded water is a conservative one. A snapshot showing H2OOcc and H2OChan in mesoporous ACC is provided in Figure
F.4.
In the case of bulk ACC, H2OOcc and H2OChan were present, and in meso- porous systems, additional H2OM es was found. On analysis of bulk systems, 4.76 (0.25) % of water molecules were measured to be H2OOcc in CaCO·H2O. The
probability of finding H2OOcc in all types of hydrated ACC, measured as a function of hydration, was observed to decrease exponentially as shown in Figure 7.3, with almost all water in CaCO3·3 H2O present in clusters, while 80.76 (2.01) % of water in CaCO3·(0.16)H2O was H2OOcc. The results suggest that the size of water-rich channels in ACC decreases as the water content in the system is reduced, leading to a reduction in the amount of H2OChanfor bulk ACC and mesoporous systems.
Figure 7.3: Probabilities of finding occluded water, H2OOcc, in hydrated ACC. Water types were analysed from bulk ACC and the core of mesoporous ACC systems. H2OOcc was defined as water molecules which maintained a distance of 3.8 ˚A to all other water throughout simulation trajectories.