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Thermodynamic Control of Amorphous Precursor Phases for Calcium Carbonate via Additive Ions Gwan Yeong Jung, Eunhye Shin, Ju Hyun Park, Byoung-Young Choi, Seung-Woo Lee, and Sang Kyu Kwak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02346 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Chemistry of Materials

Thermodynamic Control of Amorphous Precursor Phases for Calcium Carbonate via Additive Ions

Gwan Yeong Jung†,||, Eunhye Shin†,||, Ju Hyun Park†, Byoung-Young Choi‡, Seung-Woo Lee§, and Sang Kyu Kwak†,*

†Department

of Energy Engineering, School of Energy and Chemical Engineering, Ulsan

National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea ‡Center

for CO2 Geological Storage, Petroleum & Marine Division, Korea Institute of

Geoscience and Mineral Resources (KIGAM), 124 Gwahang-no, Yuseong-gu, Daejeon 34132, Republic of Korea §Center

for Carbon Mineralization, Mineral Resources Division, Korea Institute of

Geoscience and Mineral Resources (KIGAM), 124 Gwahang-no, Yuseong-gu, Daejeon 34132, Republic of Korea

*Corresponding

author. E-mail: [email protected]

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Calcium carbonate is an earth-abundant biomineral that exists in a variety of marine environments, including corals, shells of mollusks, sea urchins, etc. Particularly, amorphous calcium carbonate (ACC) phases have increasingly received scientific attentions because their local order in the short-range can affect the subsequent pathways for phase transition. In this regard, various types of additives have been employed to tailor the local structures and stability of ACC; however, their precise roles in controlling the phase transition pathways are still unclear. To address this ambiguity problem, the effects of additive ions on the structure and stability of amorphous precursor phases were theoretically traced using molecular dynamics simulation. Starting from the nucleation cluster in aqueous solution, the hydrated and anhydrous forms of ACC were systematically examined by varying the hydration levels and molar compositions of additive ions (e.g., Mg2+, Fe2+, Sr2+, and Ba2+). Our results revealed that each ion can exert promoting or inhibiting effect by tuning the local structures and stability of amorphous precursor phases depending on their hydrophilicity and ionic radii. Moreover, our findings suggested that the thermodynamic spontaneity of the overall phase transition process can be determined by the balance between two opposing factors – endothermic dehydration and exothermic crystallization.


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INTRODUCTION Biomineralization refers to a process that living organisms produce minerals, which are ubiquitously found in natural environments such as corals,1 shells of mollusks,2 sea urchins,3-4 teeth5 and bones.6 Among them, calcium carbonate is the most abundant biomineral, which has been regarded as a useful material for various applications in the fields of agriculture,7 medicine,8-9 and industry.10-11 The particular interest on this material concerns the disordered precursor phase called amorphous calcium carbonate (ACC),3-4, 1215

which exhibits a distinct local order in the short range (up to ~5 Å).16 ACC usually exists

in a hydrated form and can be transformed into final crystalline polymorphs (i.e., calcite crystals in many cases) via a stepwise phase transition process, including dehydration into anhydrous ACC and its subsequent solid-state transformation (i.e., crystallization).17-18 As a transient precursor phase, tailoring the local structure of ACC has been increasingly recognized as a crucial factor for controlling the phase transition pathways19-21; however, fundamental understanding about the structure-property relationship is still unclear. From this viewpoint, the introduction of additive ions can be a promising approach as it enables the tuning of local orders imprinted in ACC by altering the coordination geometries and hydrophilicity with constituent water molecules. Ions are usually soluble in aqueous environments and can control the phase transition pathways by affecting the lifetime of intermediate phases, or by inducing different morphologies.22-25 For example, the presence of Mg2+ and PO43- in ACC retards the phase transition process.26 Intriguingly, in the case of Fe2+ ions, some contradictory results have been reported for the phase transition process, although most studies concluded that they have an inhibitory effect.27 Meanwhile, in the presence of Ba2+, amorphous phases became more ordered at short and medium ranges (increasingly similar to their crystalline counterparts).28



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To date, the structural and energetic characteristics of ACC in the presence of additives have been extensively investigated using a variety of experimental techniques such as X-ray absorption spectroscopy combined with total distribution function (TDF) analysis,29 nuclear magnetic resonance (NMR),30-31 Raman spectra,32 thermogravimetric analysis (TGA),33-34 etc. However, these experimental approaches have some limitations in accurately describing the structural complexity and attaining the precise thermodynamic properties for ACC. In the course of resolving these issues, molecular dynamics (MD) simulation can be a suitable technique to complement experiments, as it provides reliable structural information with specific atom positions and energetic changes depending on the additive ions.35-41 In this work, we extensively conducted MD studies to theoretically determine the effect of additive ions on the structure and stability of amorphous intermediate phases of calcium carbonate. For this purpose, we examined the stepwise phase transition process, including dehydration into anhydrous ACC from its hydrated nucleation clusters, and the subsequent transformation into crystalline counterpart, for a wide range of water content (n) and molar compositions of additive ions (x) (i.e., Ca1-xMxCO3·nH2O, where 0

x

1 and 0

n

9).

Four kinds of divalent cations, Mg2+, Fe2+, Sr2+, and Ba2+ ions, were chosen as model additive ions for biomineralization. Note that Mg2+ is one of the most abundant ions in marine environments where biomineralization takes place,42 and the remaining three ions are also present in sea water with carbonate forms in some bacteria.43-45 Our results revealed that the local coordination environments of amorphous precursor phases distinctively changed depending on the intrinsic properties of constituting additive ions, which are the ionic radius and hydrophilicity. Strikingly, each additive ion had a promoting or inhibiting effect on the phase transition, which was determined by the balance between two opposing factors – endothermic dehydration and exothermic crystallization. Moreover, we captured several structural features for promoter or inhibitor additives by TDF analyses, leaving open


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a possibility for experimental observations. These findings provide a fundamental understanding for the precise role of additive ions in controlling the phase transition pathways of calcium carbonate, which can shed light on the biomineralization processes.

RESULTS AND DISCUSSION Model Systems and Overview of Dehydration Process. In general, the phase transition of calcium carbonate is known to occur via a stepwise process from hydrated ACC, to its anhydrous form (via dehydration), and to the final crystalline form (via crystallization).17-18 Assuming this mechanism to be a proof of concept, we constructed model systems to describe the dehydration process from hydrated nucleation clusters to anhydrous ACC for a wide range of water content (n) and molar compositions of additive ions (x) (i.e., Ca1-xMxCO3 nH2O, where 0

x

1 and 0

n

9) (see Figure 1 and Table

S1). The initial structure was modelled by randomly packing 200 pairs of divalent cations (i.e., Ca2+, Mg2+, Fe2+, Sr2+, and Ba2+ ions) and carbonate ions (CO32-) to satisfy a water-tocation ratio (n) of 9.0 (see “Simulation Details” in the Computational Methods section). The existing force field of the alkaline-earth carbonate-based system, which was previously reported by Raiteri et al.,46 was extended to include the Fe2+-water and Fe2+-carbonate interactions for our model systems (see “Incorporation of Fe2+ Interaction Parameters into Forcefield” in the Computational Methods section).



4H2O

Ca1-xMx CO3 9H2O

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4H2O

Ca1-xMx CO3 5H2O

Ca1-xMx CO3

Figure 1. Dehydration scheme of the Ca1-xMxCO3NnH2O system (i.e., x = 0–1, n = 0–9), where M = Mg, Fe, Sr, and Ba. The gray, red, green, yellow and cyan colors represent carbon, oxygen, calcium, additive ion, and water, respectively.

To briefly overview the dehydration process without additive ions, including the transformation from nucleation clusters to anhydrous ACC, which has been similarly examined in several computational studies,38-40 we conducted MD simulations for the CaCO3 nH2O systems (Figure 2a). From the initial stage of dehydration (i.e., n = 9), the majority of the calcium and carbonate ions associated with each other to form the nucleation clusters. During the dehydration process, the clusters started to aggregate, and at the end of dehydration (i.e., n = 0), all the Ca2+ and CO32- ions constituted the ionic framework, which corresponded to the anhydrous ACC structure. To examine the detailed structural information, we investigated the coordination number (CN) environments of Ca–Oc (oxygen in carbonate) pairs by varying the water content (n) (Figure 2b). At n = 9.0, the chain-like form (CN = 2) was mostly observed, followed by the terminal- (CN = 1) and branch-like (CN = 3) forms, indicating that the nucleation cluster had a polymeric chain form. The chains were held together by ionic interactions, and showed a dynamic nature by repeatedly breaking and reforming with each other; this was previously named as a dynamically ordered liquid-like oxyanion polymer (DOLLOP) by Gale and coworkers.37 Subsequently, as


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Furthermore, the self-diffusion coefficient (D) of water and the ions (i.e., Ca2+ and CO32-) were estimated using mean square displacement (MSD) analysis (Figure 2c). The detailed procedure and examples of fitting are given in the “Investigation of Diffusion Coefficients by MSD Analysis” in the Computational Methods section and Figure S1, respectively. The diffusivities of the Ca2+ and CO32- ions are provided as an average value for two species because they were found to be very similar in the system. Overall, the diffusivities of the ions were much lower than those of water molecules in the CaCO3 nH2O system. As previously reported by Bushuev et al.,38 the D for ions and water in the amorphous precursor phases (i.e., n = 0–9) were between those of liquid water (i.e., 2.3 (i.e., 0.8

10-9 m2/s) and calcite

10-23 m2/s).47-48 This implies that structural water, which has a lower mobility

than that of liquid water, was formed in the amorphous intermediate state,49 and that the ionic species of the amorphous phase have a higher mobility than those of the crystalline phase. Moreover, the D for water and ions in the amorphous phase gradually decreased, except for the anhydrous state (i.e., n = 0). From the abrupt changes of D for ions at the range of n = 0~1, the phase transition from solution to the solid phase was predicted to occur, which was more specifically found at n R 0.8–0.9 in the previous study.38

Structural Analysis based on CN Environment. The dehydration-induced structural transformations of calcium carbonate, from nucleation clusters to anhydrous ACC, were systematically examined by tracking the CN environment of constituting atom pairs. For this investigation, we representatively chose two atom pairs (i.e., M–Oc and M–Ow pairs), which were comprised of divalent cations (i.e., Mtotal, including Ca2+ and additive ions (M2+)) and oxygen atoms in carbonates (Oc) or in water molecules (Ow) (Figure 3). Note that the average CNs were analyzed based on the first minimum distance of the radial distribution function (RDF) for each pair, to count the pairs within the first coordination shell (Table ACS Paragon Plus Environment

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S2). Overall, regardless of the type of additive ions, the CNs of Mtotal–Oc pairs increased during the dehydration process, whereas those of Mtotal–Ow pairs decreased throughout (Figures 3a and 3b). Particularly, the average CNs for either Mtotal–Oc or Mtotal–Ow pairs changed exponentially (with positive or negative slope) with decreasing water content, indicating that aggregation occurred more intensely as dehydration progressed.

(a) 10

(b) 10 Ca0.5Mg0.5CO3·nH2O

Ca0.5Mg0.5CO3·nH2O

Ca0.5Fe 0.5CO3·nH2O

Ca0.5Sr 0.5CO3·nH2O Ca0.5Ba0.5CO3·nH2O

8

CaCO3·nH2O

CaCO3·nH2O

CN (Mtotal–Ow)

CN (Mtotal–Oc)

Ca0.5Fe 0.5CO3·nH2O

Ca0.5Sr 0.5CO3·nH2O Ca0.5Ba0.5CO3·nH2O

8

6

4

2

6

4

2

0

0

9

8

7

6

5

4

3

2

1

0

9

8

Water content (n)

7

6

5

4

3

2

1

0

Water content (n)

(c) 1.2

(d) 1.2 CaCO3·nH2O

Ca0.5Mg0.5CO3·nH2O Ca0.5Fe 0.5CO3·nH2O Ca0.5Sr 0.5CO3·nH2O Ca0.5Ba0.5CO3·nH2O

0.4 0.0

CaCO3·nH2O

Ca0.5Mg0.5CO3·nH2O Ca0.5Fe 0.5CO3·nH2O Ca0.5Sr 0.5CO3·nH2O Ca0.5Ba0.5CO3·nH2O

0.8

CN (Mtotal–Ow)

0.8

CN (Mtotal–Oc)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.0

-0.4

-0.4

-0.8

-0.8

-1.2

-1.2 9

8

7

6

5

4

3

2

1

0

9

Water content (n)

8

7

6

5

4

3

2

1

0

Water content (n)

Figure 3. CN environment for divalent cations (Mtotal, including Ca2+ and additive ions (M2+)) with oxygen atoms in the Ca1-xMxCO3·nH2O systems. Mole fractions (x) for additive ions were fixed to 0.5. (a, b) Average CN for (a) Mtotal–Oc (O in carbonate), (b) Mtotal–Ow (O in water) pairs. (c, d) Relative CN ( CN) from the system without additive ions for (c) Mtotal–Oc and (d) Mtotal–Ow pairs.



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To independently evaluate the effect of each additive ion, we compared the relative change of CNs from those in the absence of additive ions (i.e., xMxCO3·nH2O)

CN = CN(Ca1-

– CN(CaCO3·nH2O), Figures 3c and 3d). Obviously, the variation trends

depending on the additive ions were different for both Mtotal–Oc and Mtotal–Ow pairs. Mg2+ and Fe2+ ions had a declining effect on the CN of both atom pairs, while Sr2+ and Ba2+ ions had an increasing effect. These results were in consistent with the order of ionic radii of constituting ions (i.e., Mg2+ < Fe2+ < Ca2+ < Sr2+ < Ba2+ ions, Table S3), implying that the coordination capacity is proportional to the size of ions. Note that the variation trends of CNs for atom pairs, both in the increasing (i.e., Sr2+ and Ba2+) and decreasing directions (i.e., Mg2+ and Fe2+), appeared more distinctly with increasing mole fraction of the additive ions (Figures S2 and S3). Additionally, it is particularly notable that Mtotal–Oc and Mtotal–Ow pair exhibited the reverse tendency for Mg2+- and Fe2+-containing atom pairs at relatively high hydration levels (i.e., n = ~3–9, Figures 3c and 3d). Specifically, Fe2+ ions preferred the coordination with water ( CN = Fe–Ow > Mg–Ow), while Mg2+ ions favored the coordination with carbonate ions ( CN = Mg–Oc > Fe–Oc). These results could be attributed to the stronger hydrophilicity of Fe2+ ions (i.e., hydration free energy ( Ghyd) = –437.4 kcal/mol for Fe2+ ions, and –423.7 kcal/mol for Mg2+ ions, see Table S3). Therefore, the hydrophilicity as well as the ionic radii of constituting ions are predicted to play a viable role in controlling the local structure of ACC phases. We also examined the overall molar volume changes ( Vm) of the systems depending on the additive ion types and hydration levels (Figure S4). The Vm refers to the difference in molar volume between the amorphous precursor phases (Ca1-xMxCO3·nH2O) and phase-separated crystalline mixture and liquid water. As the dehydration progressed, the Vm gradually decreased in a quasi-linear manner, which was similar with the variation tendency for CNs (Figure 3b). Although the


Vm’s for Mg2+

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additives decreased somewhat at high hydration levels (i.e., n = ~3–9), the overall effects of additive ions on the system volume were relatively insignificant. This result also agreed well with recently reported experimental work by Zou et al.,50 which demonstrated the little effect on the particle size of ACC in the presence of Mg2+, Sr2+, and Ba2+ ions.

Thermodynamic Analysis for Dehydration and Crystallization Processes. To investigate the thermodynamics of phase transition for calcium carbonates, we calculated the enthalpy of formation ( Hf) from the amorphous precursor phases to the phase-separated crystalline mixture and liquid water. The Hf is defined as follows, Hf

(1 x) Ecalcite

xEMCO3 (cryst.)

nEwater

ECa1-x M x CO3 nH2O

(1)

where Ecalcite is the enthalpy of calcite; EMCO3 (cryst.) is the enthalpy of crystalline structures of metal carbonates (i.e., magnesite, siderite, strontianite, witherite); and ECa1-x M x CO3 nH2O is the enthalpy of each amorphous intermediate state of Ca1-xMxCO3 nH2O systems. Figure 4 shows the Hf of the Ca1-xMxCO3NnH2O systems for a wide range of mole fraction of additive ions (x) and water content (n). It is particularly notable that the Hf from the anhydrous ACC without additive ions to the calcite (i.e., –9.81 kcal/mol for x = 0, n = 0) was similar to an experimentally reported value (i.e., –9.77 kcal/mol),51 which supports the validity for our results.



Ca1-xMgxCO3·nH2O

Hf (kcal/mol) 15

Mole fraction (x)

1

10

0.75

5

0.50

0 -5

0.25

(b)

Ca1-xFexCO3·nH2O

Hf (kcal/mol) 45

1

Mole fraction (x)

(a)

35

0.75

25

0.50

15 5

0.25

-10

0 9

8

7

6

5

4

3

2

1

0

-5

0

-15

9

8

Water content (n)

7

6

5

4

3

2

1

0

Ca1-xSr xCO3·nH2O 1

Hf (kcal/mol) 15 10

0.75

5

0.50

0 -5

0.25

(d)

Ca1-xBaxCO3·nH2O

Hf (kcal/mol) 15

1

10

0.75

5

0.50

0 -5

0.25

-10

-10

0 9

8

7

6

5

4

3

2

1

Water content (n)

-15

Water content (n)

Mole fraction (x)

(c) Mole fraction (x)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

-15

0 9

8

7

6

5

4

3

2

1

0

-15

Water content (n)

Figure 4. Enthalpy of formation ( Hf) for Ca1-xMxCO3NnH2O systems in the range of n = 0–9, where M = (a) Mg, (b) Fe, (c) Sr, and (d) Ba. The black circles represent the actual simulation points used for constructing the contour plot. The black dotted lines represent the base line for the thermo-neutral state (i.e., Hf = 0). The gray dotted lines represent the line for guidance of the location at n = 3.

In general, the variation trends for Hf were distinctively changed depending on the hydration level (n), specifically at n = ~3 as a branch line. At high hydration levels (i.e., n = ~3–9), the Hf changed significantly along the y-axis direction of the graph, indicating that the dependence of Hf on the mole fraction of additive ions (x) was much stronger than that on the water content. Conversely, the Hf at low hydration levels (i.e., n = ~0–3) was more dependent on the water content (along x-axis direction in the graph). To further demarcate the different trends regarding with hydration level, we examined the Hf with two divided regions; at n = ~3–9 and n = ~0–3 (Figure S5 and Figure 5). At n = ~3–9, the Hf showed


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clearly different trends for the more hydrophilic additives (i.e., Mg2+, Fe2+) and the less hydrophilic ones (i.e., Sr2+, Ba2+) compared to the Ca2+ ions (Figure S5). In specific, with an increase in the mole fraction of Mg2+ or Fe2+ ions, the endothermicity of the Hf increased further, implying that the dehydration process in the amorphous intermediate states became more difficult to occur due to the increased hydration stability (Figures S5a and S5b). Particularly, the endothermicity of the Hf for Fe2+ ions greatly increased, induced by their stronger hydration. On the other hand, with an increase in the mole fraction of Sr2+ or Ba2+ ions, the endothermicity of the Hf significantly decreased, mainly due to the more facile dehydration (Figures S5c and S5d). With an extremely high content of additive ions (i.e., xSr > ~0.75–0.9, and xBa > ~0.55–0.75), some exothermic regions were observed by passing over the thermo-neutral line (represented by black dotted lines), where the phase transformation can occur without any energetic cost during the dehydration process. Ca1-xMgxCO3·nH2O

1

Hf (kcal/mol) 10 5

0.75

0

0.50

-5 -10

0.25

(b) Mole fraction (x)

Mole fraction (x)

(a)

Ca1-xFexCO3·nH2O

1

Hf (kcal/mol) 30

0.75

20 10

0.50

0

0.25 -10

-15

0 3

2

1

0

0

-20

3

Water content (n)

(c)

2

1

0

(d)

Ca1-xSr xCO3·nH2O

0

0.50

-5 -10

0.25

Mole fraction (x)

5

0.75

Ca1-xBaxCO3·nH2O Hf (kcal/mol) 10

1

Hf (kcal/mol) 10

5

0.75

0

0.50

-5 -10

0.25

-15

-15

0 3

2

1

0

Water content (n)

-20

Water content (n)

1

Mole fraction (x)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-20

0 3

2

1

Water content (n)


0

-20


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Figure 5. Enthalpy of formation ( Hf) for Ca1-xMxCO3NnH2O systems in the range of n = ~0–3, where M = (a) Mg, (b), Fe, (c) Sr, and (d) Ba. The black circles represent the actual simulation points used for constructing the contour plot. The black dotted lines represent the base line for the thermo-neutral state (i.e., Hf = 0).

Figure 5 shows the Hf of the Ca1-xMxCO3NnH2O system at low hydration levels (i.e., n = ~0–3), including the transition from solution to solid phase, as identified by abrupt change of D at n R 1 (Figure 2c). The remarkable change of the Hf at n = ~0–3 is the increasing trend of exothermicity with decreasing water content (along the x-axis direction). Assuming that the dehydration process of hydrated nucleation clusters in the amorphous precursor phases is obviously related to the endothermic behavior, the subsequent transformation into crystalline structures (i.e., crystallization) can be speculated as the origin of exothermicity. To further delineate these processes from the thermodynamic viewpoint, we split the Hf into each enthalpic contribution for dehydration ( Hdehyd) and crystallization ( Hcryst) of the Ca1-xMxCO3NnH2O systems as follows, Hf

where

H dehyd

(2)

H cryst

H dehyd is the enthalpy change of dehydration (from hydrated form of amorphous

precursor phase into anhydrous ACC and liquid water), and

H cryst is the enthalpy change

of crystallization (from anhydrous ACC into separated crystalline phases of metal carbonates). The each enthalpic contribution was defined as follows, H dehyd H cryst

Eanhyd

(1 x) Ecalcite

nEwater

Ehyd

xEMCO3 (cryst.)

ECa1-x M x CO3

(3) (4)

where Ehyd is the enthalpy of hydrated forms of amorphous precursor phases of Ca1xMxCO3·nH2O

systems; and Eanhyd is the enthalpy of anhydrous ACC (i.e., n = 0),

respectively.


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The results for decomposed enthalpic contributions including Hdehyd and Hcryst of the Ca1-xMxCO3·nH2O systems are presented in the Figures S6 and S7. Basically, the Hdehyd is in charge of the endothermic part, whereas Hcryst is responsible for the exothermic part of the Hf. Depending on the balance between the two opposing factors, the thermodynamic spontaneity of the Hf can be determined; it was endothermic in the solution phase (i.e., n = ~3), and exothermic in the solid phase (i.e., n = 0). Each enthalpic contribution varied as a function of the type and mole fraction (x) of additive ions (M2+), and the water content (n). Among these determining factors, we initially focused on the variation trends of

Hf

according to the n. As the dehydration proceeded (from n = ~3 to 0), the Hdehyd was gradually decreased, while the Hcryst was constant because it is naturally independent on the water content by assuming the solid-state transformation (i.e., n = 0). Accordingly, the Hf changed from endothermic to exothermic by crossing over the thermo-neutral line (i.e., Hf = 0), implying that dehydration effect was more dominant in the solution phase (i.e., n = ~3), while the crystallization effect gradually won over the opposite as it got closer to the solid phase. Note that these trends (from endothermic to exothermic) with progressive dehydration are equally found in the Figure 5 (from left to the right along the x-axis direction in the graph). Next, we moved our focus to the variation trends of Hf according to the x. With an increase in the content of Mg2+ or Fe2+ uptake (from bottom to top in the Figures 5a and 5b), the endothermicity slightly (for Mg2+) or largely (for Fe2+) increased at the hydration level of n = ~3, but the tendency reversed in the anhydrous phase (i.e., n = 0) by propagating in the exothermic direction. These different trends for Hf with increasing amount of Mg2+ and Fe2+ uptake were further examined from each enthalpic contribution (Figure S6). The increased Hdehyd at n = ~3 was predicted to be originated from the stronger hydrophilic effect of Mg2+ and Fe2+ additives, as similarly stated above at high hydration levels (n = ~3–



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9). These results are in line with previous experimental observations regarding with the extended lifetime of amorphous phases in the presence of Mg2+ or Fe2+ additives,26,27 which can be interpreted as the increased endothermicity of dehydration ( Hdehyd). Meanwhile, the increased Hcryst in the exothermic direction was mainly due to the decreased stability of the anhydrous Ca1-xMxCO3 system. It is noteworthy that similar effect of the structural instability due to Mg2+ substitution was previously reported for Mg-calcite systems, which was verified using surface energy calculations.52 Conversely, with an increase in the Sr2+ or Ba2+ uptake (Figures 5c, 5d and Figure S7), the endothermicity tended to decrease due to the more facile dehydration at all hydration levels, but the degree of variation decreased as the dehydration progressed. Finally, in the anhydrous phase (i.e., n = 0), the thermodynamic stability was nearly preserved regardless of the additive content, without an increase in the exothermicity. These different trends depending on the additive ions were further identified from each contribution for electrostatic and van der Waals interactions (Figures S8 and S9). Note that the Hf can be decomposed into two enthalpic contributions including electrostatic ( Hf(ES)) and van der Waals ( Hf(vdW)) interactions as follows. Hf

H f(ES)

H f(vdW)

(5)

In the presence of Mg2+ or Fe2+ ions, the variation tendency of Hf was mainly responsible for the electrostatic interactions, which increased the endothermicity in the solution phase (i.e., n = ~3–9) while increased exothermicity in the solid phase (i.e., n = ~0) (Figure S8). Meanwhile, in the cases of Sr2+ or Ba2+ ions, the van der Waals interactions were mostly attributed to the variation tendency of

Hf, where the exothermicity increased for both

solution and solid phases (Figure S9). These results implied that the hydrophilicity is closely related to the electrostatic interactions, but the van der Waals effect can play a major role for larger size ions.


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To sum up with the results, we schematically redesigned the results in the Figure 5, which represents the Hf as a function of water content (n) and mole fraction of additive ions (x) at low hydration levels, by signifying the relationship between two opposing factors – dehydration and crystallization (Figures 6a and 6b). Two contrasting factors were exerted on the Hf either in the same or opposite directions; the former refers to a synergistic relationship (i.e., for Sr2+ and Ba2+ cases) and the latter to a rivalry relationship (i.e., for Mg2+ and Fe2+ cases). Based on these concepts, we constructed a schematic diagram to describe the phase transition from hydrated forms of amorphous precursor phases (i.e., Ca1xMxCO3·nH2O)

into phase-separated crystalline structures and liquid water (i.e. (1-x)CaCO3

+ xMCO3 + nH2O), including dehydration and crystallization processes (Figure 6c). In the cases of Mg2+ and Fe2+, which had stronger hydration strengths than Ca2+, the endothermicity increased at the dehydration stage. However, at the crystallization stage, the exothermicity increased due to an increase in the structural instability, owing to additive substitution. Accordingly, the inhibiting effect in the solution phase and the promoting effect in the solid phase can simultaneously coexist in the Mg- (or Fe-) substituted carbonate systems. Notably, Fe2+ had a higher effect in the dehydration stage than Mg2+, but a lower effect in the crystallization stage, thus exhibiting the strongest inhibitory effect among the additives of our interest (Figure 5b). On the contrary, in the cases of Sr2+ and Ba2+, the promoting effects were enhanced due to the propagation in the exothermic direction, both in the dehydration (solution) and crystallization (solid) stages. Finally, Ba2+ had the strongest promoting effect among the additives of our consideration (Figure 5d).


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TDF Analysis for ACC Structure Involving Additive Ions. The total distribution function (TDF, D(r)) is a commonly used method for the examination of the atomic structure of ACC, which can be determined from X-ray scattering spectroscopy,53 and is thus directly comparable with computational models. The TDF is defined as the weighted sum of the partial distribution functions (PDFs) for all the atom pairs, as shown by the following equation: n

D(r )

4

0r

ci c j i, j 1

zi z j n

2

gij (r ) 1

(6)

c zk

k 1 k

where

0

is the total number density in system; ci and cj are the concentrations of atoms i

and j, respectively; zi and zj are the atomic numbers of atoms i and j, respectively; and gij(r) is the PDF between atoms i and j. First, the TDF of the CaCO3NnH2O system with n = 1, which corresponds to the hydrated ACC, was compared with available experimental data56 (Figure S10). The simulation results showed a good agreement with the experimental models for the whole range (up to ~13 Å), implying that our ACC model structures would be actually accessible in a real situation. Next, we investigated the TDFs for hydrated ACCs (i.e., Ca1-xMxCO3NnH2O, where n = 1) in a wide range of molar compositions for additive ions (i.e., x = 0.25, 0.5, 0.75, and 1) (Figure 7). In common, all the systems with additive ions exhibited very sharp peaks at 1.3 and 2.2 Å, which corresponded to the C–Oc and Oc–Oc pair for the internal atom pairs of the carbonate ion. We particularly note that the shoulder peaks associated with M–Ow pairs slightly appeared in the short-range only for both the Mg- and Fe-ACC systems, at 2.0 Å (i.e., Mg–Ow pair) and 1.9 Å (i.e., Fe–Ow pair) in the Figures 7a and 7b, respectively. For detailed examination of these structural differences, we separated the PDF for M–Ow pairs from each TDF. It was found that M–Ow contribution decreased in the order of Fe2+, Mg2+, Sr2+, and Ba2+ (Figure S11), and this tendency was originated from the relative



hydrophilicity of the additive ions. On the other hand, the Sr- and Ba-ACC systems exhibited the pronounced peaks for the M-Oc pair at 2.6 and 2.8 Å, corresponding to Sr–Oc and Ba– Oc, respectively (Figures 7c and 7d). These developed peaks indicated the higher coordination capacities for Sr2+ and Ba2+ ions due to their larger ionic size, as previously observed in the amorphous precursor phases for Sr2+- or Ba2+-containing systems at all hydration levels (Figures 3a and 3c). In addition, the Sr- and Ba-ACC systems showed an increase in the peak intensity for the M–M pairs at three regions including the new peaks in the mid-range, which corresponded to 4.3~4.4, 6.4~6.8, and 8.6~9.1 Å, respectively. The PDF for M–M pairs further clearly showed that the contributions for M–M pairs dominantly pronounced in the Ba- and Sr-ACC systems, compared to the Mg- and Fe-ACC systems (Figure S12). Note that similar observations for Ba-ACC systems were recently reported by Whittaker et al.,28 which stated that next-nearest-neighbor cation ordering at the mid-range increased with increasing barium content. Consequently, there were evidently different correlation patterns depending on the additive species; Ba- and Sr-ACC showed a more ordered structure than that of Mg- and Fe-ACC, and these structural features were mainly attributable to the developed M–M pairs in the mid-range. These developed mid-range order may contribute to the rapid formation of crystalline structures by lowering the barrier for subsequent transformation.


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CONCLUSIONS In this study, the effects of additive ions on the local structure and stability of amorphous precursor phases for calcium carbonate were theoretically elucidated using MD simulations. Starting from the nucleation clusters to the anhydrous ACC, a variety of forms in the amorphous precursor phases were systematically characterized for a wide range of water content and additive ions (i.e., Ca1-xMxCO3NnH2O, where x = 0–1, n = 0–9). Overall, the structural and thermodynamic characteristics of amorphous intermediate states were strongly related to the intrinsic properties of additive ions such as hydrophilicity and ionic size. Specifically, each additive ion was identified as inhibitors (e.g. Mg2+ and Fe2+ ions) or promoters (e.g. Sr2+ and Ba2+ ions) on the phase transition of amorphous precursor phases. Furthermore, we found that two opposing factors, which are the endothermic dehydration and exothermic crystallization processes, can work for the Hf either in a rivalry (e.g., for Mg2+, Fe2+ additives) or synergistic relationship (e.g., for Sr2+, Ba2+ additives) depending on the additive ions. TDF analysis captured the structural features for different trends of Mg2+ or Fe2+ additives by enhanced M–Ow contributions in the short-range, and Sr2+ or Ba2+ ions by enhanced M–M pairs up to the mid-range (~10 Å), which might be observable by experimental technique such as EXAFS in the future works. These findings will contribute to a fundamental understanding for controlling the phase transition pathways of aqueous calcium carbonate-based systems by additives. We note that a deeper understanding of how the local structures and thermodynamics of amorphous precursor phases vary as a function of temperature, pressure, pH, and other organic and inorganic additives is needed for practical applications into the biomineralization process, which are worthy of future investigations.


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COMPUTATIONAL METHODS Simulation Details. We investigated the dehydration and crystallization processes for amorphous precursor phases of Ca1-xMxCO3NnH2O systems using MD simulations. We started from the condition that the amount of water to Ca2+ was 9.0 (i.e., 200 Ca2+, 200 CO32-, and 1800 water molecules), assuming a locally high concentration right before aggregation. For the initial systems, 50 to 200 additive ions were replaced by Ca2+ ions (e.g., x = 0.25, 0.5, 0.75, and 1.0). Note that these high fractions of additive ions (i.e., x

0.25) have

been reported to be experimentally feasible in the previous literature29,54-55. Next, according to the previously reported dehydration scheme by Wallace et al.,56 we removed water molecules until the water-to-cation ratio (n) became zero. It was achieved through a total of 18 sequential NPT MD simulations; each simulation contained 200 (i.e., H2O/Ca2+ ratio = ~2–9) or 40 (i.e., H2O/Ca2+ ratio = ~0–2) fewer water molecules than the previous step. Note that the water molecules chosen to be removed were those farthest from the Ca2+ and CO32ions, to avoid unphysical structural distortion. Also, the crystal structures of calcium- or other metal- carbonates (e.g., calcite, magnesite, siderite, strontianite, and witherite) were separately modeled by creating the 5×5×3 supercell structures. For all the systems, each NPT MD simulation was performed for ~1–4 ns until reaching the equilibration, and the last 1 ns was analyzed for the results. The Nosé–Hoover–Langevin (NHL) thermostat and Berendsen barostat were used to adjust the temperature and pressure condition, respectively.57-58 The Ewald scheme59 and atom-based cutoff method (i.e., a radius of 12.5 Å) were applied to treat the long-range electrostatic and van der Waals interactions, respectively.

Incorporation of Fe2+ Interaction Parameters into Forcefield. Existing interaction parameters for alkaline-earth metal carbonate (i.e., Mg2+, Ba2+, and Sr2+) were extended to include the Fe2+ ions (Table S4). The interaction parameters for Ca2+, CO32- ions and water



Page 24 of 34

molecules were taken from Demichelis et al.’s work.37 The interaction parameters for Mg2+, Ba2+ and Sr2+ ions were taken from Raiteri et al.’s work.46 For Fe2+ ion species, the Fe2+water interaction and the Fe2+-carbonate interaction were separately parameterized, by following the methods in the Raiteri et al.’s work.46 The Fe2+-water interaction parameter was adjusted by comparing the experimental values for Ghyd, ion-water distance (rOw-Fe), and CNs. The LJ 12-6 potential was used to describe the intermolecular interaction between Fe2+ ions and water molecules, E

D0

R0 R

12

R 2 0 R

6

(7)

where D0 and R0 indicate the energy well depth and equilibrium distance. The Ghyd was calculated by using thermodynamic integration method.60 For this calculation, a single Fe2+ ion was packed with 2000 flexible SPC water model61 in the cubic box (i.e., 38.8 × 38.8 × 38.8 Å3). Next, each van der Waals and Coulombic interaction between Fe2+ ion and water molecules was progressively turned on through 20 steps, where each step consisted of equilibrium run of 200 ps and production run of 500 ps. Note that finite size correction62 (i.e., 0.5q 2 ò 1 ò

Ew

2 R 2 3L3 R 52.1 kcal/mol) was added into Ghyd values in

order to complement the size effect of system. The rOw-Fe and CN was obtained by last production run of simulation. Additionally, Ghyd of Ca2+, Mg2+, Sr2+ and Ba2+ ions were calculated to rank the hydrophilicity, where the values were well agreed with experimentally reported ones63-64 (Table S3). The interaction parameter of Fe2+-carbonate was described by Buckingham potential,

E

A exp

R

C R6

(8)

where A and refer to the Buckingham parameters, which are related to repulsion force, and C parameter is related to attraction force. Note that C parameter was neglected in this study.


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Buckingham parameter was adjusted to experimental physical properties, including lattice parameter65 and bulk moduli66 of siderite, by using the GULP relaxed fit method.67 Lattice parameters and volume thermal expansion coefficient (V) of siderite were analyzed by Forcite program in Materials Studio 2019,68 and linear thermal expansion coefficients ( a, c)

were analyzed by LAMMPS program.69 Note that

a

(in K-1) and

c

(in K-1) of siderite

structure were obtained from NPT MD simulation at different temperature (i.e., 200, 300, 400, and 500 K) and atmospheric pressure conditions for 1 ns, while keeping the cell shape. The V (in K-1) was calculated from NPT MD simulation at different temperature (i.e., 300 and 400 K) and 1 atm conditions. Considering that thermal expansion coefficients was not included in the training sets, it showed well matched results with experiment, supporting the validity of our parameterization.

Investigation of Diffusion Coefficients by MSD Analysis. To obtain the self-diffusion coefficient (D) of water and ions (i.e., Ca2+ and CO32-) in the CaCO3 nH2O systems, we analyzed the mean square displacement (MSD) over time using the Stokes-Einstein relation as follows,

D

2 1 ( r) t 6

(9)

where r is the displacement change within the time interval, t. The factor 6 arises from three dimensions (i.e., x, y, and z) and two directions to move (i.e., forward and backward). The MSD values were calculated over last 1 ns trajectory from the NPT MD simulation for 5 ~ 10 ns to secure the statistical accuracy. The slope of MSD was calculated to estimate the D from equation (9). Note that the first 10% of the MSD curve (i.e., about 100 ps MD run) was excluded for the fitting since it has not yet entered the linear diffusive regime.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional data, including Figure S1 – S12, Table S1 – S4. Mean square displacement analysis for water and ions; CN environment for divalent cations around oxygen atoms in the Ca1-xMxCO3·nH2O systems; Molar volume changes during the dehydration; Enthalpy of formation for Ca1-xMxCO3·nH2O systems in the range of n = ~3–9; Enthalpy of dehydration and crystallization for Ca1xMxCO3·nH2O

systems in the range of n = 0–3; Decomposed enthalpic contributions

of Hf including electrostatic and van der Waals interactions; TDF for hydrated ACC; PDF for M–Ow and M–M pairs; Summary of model systems for MD simulations; First minimum distance of RDF for M–Oc and M–Ow pairs; Ionic radius and Ghyd of divalent cations; Interaction parameters of Fe2+–water and Fe2+–carbonate including comparison of physical properties.

AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected] (S.K.K.)

Author Contributions ||G.

Y. Jung and E. Shin contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This work was supported by the Korea Institute of Geoscience and Mineral Resources (KIGAM), and computational resources from KISTI-HPC (KSC-2018-CHA-0041) and UNIST-HPC.

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