Prediction of Sodium Substitution Sites in Octacalcium Phosphate: The Relationships of Ionic Pair Ratios in Reacting Solutions

Abstract

Octacalcium phosphate (OCP) is widely used in biomaterial fabrication by virtue of its unique crystal structure and low environmental loading. Although various ion and molecule substitution methods into the OCP unit lattice have been introduced, it remains unclear which factors and mechanisms dominate the substitution process. Experimental studies have indicated that Na alkali metal ions are substituted at the P3 PO₄ conjugated site in acidic to weakly acidic conditions and the P5 PO₄ conjugated site in neutral to weak basic conditions. Ionic species calculation methods have indicated that the pair ratios of Na and HPO₄²⁻ (NaHPO₄⁻) are small in acidic reacting solutions but large under weakly basic conditions. Consequently, the roles played by NaHPO₄⁻ and ionic pair formation processes are thought to dominate ion and molecule substitution into the OCP unit lattice. Such ionic pair formation strongly inhibits dicarboxylic acid substitution into the OCP unit lattice due to the replacement of the Ca ion, which conjugates P5 PO4 as an anchor of dicarboxylic acid.

1. Introduction

Calcium phosphates constitute a diverse group of minerals that play important roles in geology and biology [1,2,3]. Among these minerals, some are constituent of bones and teeth and are thus closely related to hard tissue health and diseases [4,5]. They are also essential components in fertilizer, food additives, and other engineering aspects while constituting an important part of the phosphate cycle at the Earth’s surface [1,6]. Octacalcium phosphate (OCP: Ca₈H₂(PO₄)₆·5H₂O), a major inorganic component of immature bone, has received much attention for its excellent biocompatibility and the ability to substitute various ions and molecules whose properties could be applied to green and low-toxicity drug delivery systems (DDS) and combination medical products [7,8,9]. In addition, the substitute ions and molecules in the OCP unit lattice could be controlled by radiation processes [10]. Therefore, OCP has been attracting attention from radiation medicine as novel DDS devices.

OCP are mainly synthesized via hydrolysis in solution [11,12,13,14]. During this process, co-existing ions and molecules present in the ambient fluid are substituted into the OCP unit lattice. Previous studies have indicated that metal ions [7,15,16,17], cationic molecules [18], ammonium ions [19] and dicarboxylic molecules [20,21,22,23] could substitute within the interlayer of the OCP unit lattice. Although the sizes of substituted ions and molecules have been extensively investigated, it remains unclear which factors, such as pH and co-existing ions, control their substitution.

For OCP synthesis, three processes have been identified based on the initial pH conditions: an acidic solution with a pH increasing process, a weak acidic solution with a titrate pH process, and a weak basic solution with a pH decreasing process [12,14,15,16].

Although it has been suggested that ionic species in reacting solutions preferentially form OCP with molecular substitution, the relationships between substitution and solution ionic species are not known. In this study, we focus on the specific substitution modes of Na into OCP, evaluating the relationships between Na ionic complexes in reacting solutions and the corresponding substitution modes into the OCP unit lattice.

2. Materials and Methods

2.1. Fabrication Methods of Na-Substituted OCP

All reagents were purchased from FUJI Film Wako Pure Chemical Inc., Japan. H₃PO₄, NaCl, and Na₂HPO₄ were diluted to 2 mol/L using distilled water. Thus, experimental solutions consisted of a 1.0 mol/L PO₄ and a 2.0 mol/L Na mixture of either Na₂HPO₄ or H₃PO₄-NaCl.

In an acidic solution with increasing pH, 5.0 g of CaCO₃ was immersed into 20 mL of H₃PO₄-NaCl solution. Following the elimination of initially formed CO₂, the samples were packed and incubated at 60 °C for 1 day.

In weak basic solutions with decreasing pH, 2.39 g of dicalcium hydrogen phosphate hydrate (DCPD: CaHPO₄·2H₂O) was immersed into 20 mL of Na₂HPO₄ solution and incubated at 60 °C for 1 day.

The initial and final pH values of the solutions were measured using a pH electrode (LAQUA ToupH 9615S-10D) connected to a pH meter (Horiba Co. D-72, Kyoto, Japan). These treated samples were washed several times using distilled water and 99.5% ethanol to eliminate the residual immersion solution and were subsequently dried in an oven at 40 °C overnight.

2.2. Characterization

Crystallographic characterization of the samples was obtained via X-ray diffraction (XRD, MiniFlex600, Rigaku Co., Tokyo, Japan) at an accelerating voltage of 40 kV and an amplitude of 15 mA. The diffraction angle was continuously scanned over the 3−70° 2θ range at a scanning rate of 2°/min for characterization and over the 2−12° 2θ range at a scanning rate of 0.8°/min for crystallographic parameter analysis.

The chemical bonding structures of the samples were determined using Fourier-transform infrared spectroscopy (FT-IR, Nicolet NEXUS670, Thermo Fisher Scientific Co., Waltham, MA, USA) with a triglycine sulfate detector using the KBr method (32 scans, 2 cm⁻¹ resolution) and using a ZnSe attenuate total reflection (ATR) prism (64 scans, 2 cm⁻¹ resolution). Measurements were obtained in the air.

The Ca, P(PO₄), and Na ion concentrations of the samples were measured using inductively coupled plasma-atomic emission spectroscopy (ICP-ES, 5110VDV, Agilent Technology Co., Tokyo, Japan) following the dissolution of the samples in 2% HNO₃.

2.3. Calculation of Ionic Species in Solutions

The ionic complexes present in the reaction solutions were simulated as data inputs for speciation modeling using the PhreeqC software [24,25]. The input thermodynamic data for each ionic species and the reaction code are listed in

Table 1. Selected thermodynamic data for reactions in this system and their corresponding complexes. All data refer to a reference temperature of 298.15 K and standard state, i.e., a pressure of 0.1 MPa and, for aqueous species, infinite dilution [26,27].

H2O = O2 + 4H+ + 4e	-log_k	−86.08
2H+ + 2e− = H2	-log_k	−3.15
PO43− + H+ = HPO42−	-log_k	12.346
PO43− + 2H+ = H2PO4−	-log_k	19.553
PO43− + 3H+ = H3PO4	-log_k	21.721
Ca2+ + H2O = CaOH+ + H+	-log_k	−12.78
Ca2+ + PO43− = CaPO4−	-log_k	6.459
Ca2+ + HPO42− = CaHPO4	-log_k	2.739
Ca2+ + H2PO4− = CaH2PO4+	-log_k	1.408
Na+ + HPO42− = NaHPO4−	-log_k	0.29
K+ + HPO42− = KHPO4−	-log_k	0.29

. Briefly, each ionic species in solution was calculated at 333 K (60 °C), and 0.1 MPa with O₂ contents assumed to be in equilibrium with the atmosphere. The phase diagrams of each ionic species were drawn using Microsoft Excel (Microsoft Office 365 MSO, Microsoft Co., Redmond, WA, USA).

3. Results

As described in Scheme 1, the OCP unit lattice used in these experiments consisted of eight Ca ions, four PO₄ ions, two HPO₄ ions, and five H₂O molecules [11,28,29]. Some Ca, and PO₄ ions exhibited conjugated relationships. Detecting ionic replacement of Ca ions used the vibration alternations in conjugated PO₄ and HPO₄ ions afforded by spectroscopic methods [28,29].

We then evaluated whether synthesis methods dominated Na substitution mode and controlled the site of the OCP unit lattice into which it substituted. We chose two methods for OCP synthesis: an acidic solution with an increasing pH and a basic solution with a decreasing pH. Both solutions contained 1 mol/L PO₄ and 2 mol/L Na. Figure 1 shows the XRD patterns of the treated materials. In both methods of synthesis, the treated materials were monophasic OCP.

Table 2. The initial and final pH values of reacting solutions for each synthesis method.

Sample	Initial	Final
OCP-pure	<1.00	6.26
OCP(Na)	<1.00	5.85
OCP-Na	9.00	6.87

shows the initial and final pH values of the reacting solutions. For convenience, we denote the OCP synthesized acidic solution with increasing pH as OCP(Na) and that OCP basic solution with decreasing pH as OCP-Na. The Na contents of OCP(Na) and OCP-Na were 6.68 ± 0.21 at % and 6.83 ± 0.74 at %, respectively. There was no significance between Na contents of OCP(Na) and of OCP-Na (p = 0.39).

Spectroscopic analysis indicated Na-substituted sites as features exhibiting alternation of vibration in conjugated sites of PO₄ and/or HPO₄. Figure 2 and Figure S1 showed the transmitting FT-IR (KBr method) spectra, and ATR-FT-IR spectra of each OCP studied with conventional OCP provided for comparison. In OCP-Na, the P3 PO₄ band of PO₄ revealed v_3′ PO₄ mode rather than v_3′ HPO₄. It was indicated that the cations substituted in the conjugated site of P3 PO₄ were likely to conjugate PO₄ rather than HPO₄. Considering the charge of cations, Ca²⁺ is likely to be placed into the conjugated site of P3 PO₄ rather than Na⁺. On the other hand, in OCP(Na), the v_3′ PO₄ mode of the P3 PO₄ band was significantly attenuated. Then, in OCP-Na, the P5 PO₄ band was significantly altered relative to conventional OCP and OCP(Na). Furthermore, in ATR-FT-IR observation, unlike in the OCP and OCP(Na) cases, clearly γ_OH HPO₄ band corresponded to hydrous layer structure around 633 cm⁻¹ was observed. This strongly suggests that Na could be substituted into the conjugated site of P5 PO₄ for OCP-Na, and into P3 PO₄ for OCP(Na).

Ionic species calculation methods were used to determine the key factor for different Na substitution modes in the OCP unit lattice. The K_sp values of ions and each ionic pair are summarized in Table 1. Herein, we focused on the pH dependence of the ionic pair consisting of Na and HPO₄²⁻ as NaHPO₄⁻. NaHPO₄⁻ in the OCP unit lattice is described as having the form of Na substitution in a conjugated P5 PO₄ site. Figure 3 shows the calculation results of the amount and ratio of NaHPO₄⁻ versus total Na ions as a function of pH and Na concentration. Note that, in this calculation, we used NaCl instead of Na⁺ due to the inherent limitations of the calculation. In addition, the pH values of each pH series were slightly altered by, at most, 0.3 between Na-0 mol/L and Na-2 mol/L (Figure S2). The concentration of NaHPO₄⁻ in solutions increased with increasing Na concentration between pH 5 and pH 10. On the other hand, below pH 5 and above pH 10, limited NaHPO₄⁻ was formed, irrespective of the Na concentration used. Other ionic species in this system exhibited the same tendency as NaHPO₄⁻ (Figure S3).

During OCP synthesis, the Ca ion is initially released into reacting solution as dissolving Ca salts. Therefore, the Ca ion effect toward NaHPO₄⁻ ratios in solution should be evaluated. Figure 4 shows the concentration and ratio of NaHPO₄⁻ as a function of Ca concentration and pH. The concentrations and ratios of NaHPO₄⁻ and other ionic pairs were almost constant with pH regardless of Ca ion concentration (Figures S4 and S5).

The pH dependence of the ratio of NaHPO₄⁻ in the Na–Ca system suggested that this ionic pair was significantly affected by the mode of Na substitution into the OCP unit lattice. Our previous studies also indicated that the ionic radii of substituted cations were an important parameter in OCP substitution. Excluding ionic radii, we evaluated how ionic pair formation was influenced by OCP unit lattice substitution. In that study, we also considered K ion, i.e., an alkali metal ion with a larger ionic radius (1.38 Å) than Na⁺ (1.02 Å) and Ca²⁺ (1.00 Å) [31]. Figure 5 shows the calculated results of KHPO₄⁻ concentration and ratio as a function of pH and K concentration in solution. Both the concentration and ratio of KHPO₄⁻ were the same as those of NaHPO₄⁻. For alkali metal substitution into the OCP P5 PO₄ conjugated site, the ionic radius of the cation was found to be a more important factor than the mode of ionic pair formation.

These experimental and simulation results clearly indicate that the formation ratio of ionic pairs of Na⁺ and HPO₄²⁻ is influenced by the site of Na substitution into the OCP unit lattice. When NaHPO₄⁻ forms in solution, Na is likely to substitute into the conjugated site of P5 PO_4, whereas bare Na⁺ was likely to substitute into the conjugated site of P3 PO₄. These results support previous studies concerning the ionic potential and molecular orbital analysis of OCP crystals [32] and other trivalent cation substitution processes into OCP unit lattice [33,34]. Considering the unit lattice of OCP and FT-IR results, the cations were substituted at the conjugated site of P5 PO₄. Nevertheless, pH and related ionic pair formation are the key factors influencing molecule and ion substitution into the OCP unit lattice.

Based on the analytical results presented, we considered how such cations strongly inhibited dicarboxylic intercalation into the OCP unit lattice under neutral to weak basic conditions [35]. When dicarboxylic acid was intercalated into the OCP interlayer termed the hydrous layer, the a-axis of the OCP unit lattice expanded; this phenomenon was detected via a peak shift in OCP d(100) from 4.7° to ~4.2° [19,20,21,22,23,35,36]. As shown in Figure 6 and Table S1, when OCP fabricated in dicarboxylic ions (thiomalate: HOOC(SH)(CH₂)₂COOH) and Na-containing weak basic solutions, a small number of dicarboxylic ions are intercalated into the OCP unit lattice, whereas significant intercalation is observed under acidic to weak acidic solutions. The Na⁺ substitution process at conjugated sites of P5 PO₄ inhibited the Ca fulfilling process, which is the process as an anchor for such dicarboxylic molecules.

For calcium phosphate, including OCP so on formation process, Ca and PO₄ ionic pair formation has mostly been discussed. During forming crystalline calcium phosphates from amorphous calcium phosphate, Ca–PO₄ ionic pairs and clusters (such as Posner’s cluster) play essential roles [37,38,39,40,41,42,43]. Although the cluster structures of Ca–PO₄ have been extensively investigated, the formation of Ca–PO₄ with third and fourth molecules remains unclear, although molecular dynamics studies suggest that Posner-like clusters of Ca–Na-PO₄ are important [44]. For in vivo and physiological conditions, residual ions and molecules with Ca and PO₄ have not previously been considered. In this study, we demonstrate that Na also plays an important role in calcium phosphate formation processes.

4. Conclusions

In conclusion, using Na as a model cation, we investigated the role played by the cation substitution site in the OCP unit lattice on synthesis processes under acidic conditions with increasing pH and weak basic conditions with decreasing pH. Ionic species calculations indicated that the concentration of ionic pairs of Na⁺ and HPO₄²⁻ were the key factors determining the Na substitution site. Under NaHPO₄-poor conditions (i.e., pH < 5), Na was more likely to substitute in the conjugated site of P3 PO₄. On the other hand, under NaHPO₄-rich conditions (pH 5 to 10), Na was more likely to substitute in the conjugated site of P5 PO₄.