# Influence of NH4Cl on Hydrothermal Formation of α-CaSO4·0.5H2O Whiskers

Authors: Haoyuan Chen; Jing Wang; Sichao Hou; Lan Xiang
Journal: Journal of Nanomaterials
 (2015)

---

## Abstract

The influence of NH4Cl on hydrothermal formation of CaSO4·0.5H2O whiskers from CaSO4·2H2O precursor at 135°C was investigated in this paper. Compared with the blank experiment, the presence of 3 × 10−2 mol·L−1 NH4Cl led to the increase of the lengths of the whiskers from 50 to 160 μm to 150 to 300 μm and the decrease of the diameters from 1.0 to 1.5 μm to 0.2 to 0.5 μm. The dissolution of CaSO4·2H2O was accelerated by the complex interactions with NH4Cl and the soluble cations, which led to the decrease of the induction time for the occurrence of α-CaSO4·0.5H2O from 46 minutes to 34 minutes and the formation of CaSO4·0.5H2O whiskers with high aspect ratios. Furthermore the critical supersaturation for the formation of α-CaSO4·0.5H2O was investigated.


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## Body

## 1. Introduction

The formation of calcium sulfate (CaSO4) whiskers with high aspect ratios has drawn much attention in recent years owing to their nontoxic and perfect mechanical properties and the wide applications in the fabrication of intensified composites [1–4]. Calcium sulfate whiskers were usually prepared by the first formation of α-CaSO4·0.5H2O whiskers followed by calcination at 600 to 800°C since the anisotropic –Ca–SO4–Ca–SO4–Ca–chains in α-CaSO4·0.5H2O favored their growth along c-axis [5].α-CaSO4·0.5H2O whiskers can be prepared by wet processes, including hydrothermal method, microemulsion, or acidification methods, and the hydrothermal method has been widely used owing to its high efficiency and easy control of the formation process [6–9]. α-CaSO4·0.5H2O whiskers were usually produced by hydrothermal conversion of CaSO4·2H2O at 100 to 150°C, and the hydrothermal dissolution of CaSO4·2H2O and the precipitation of CaSO4·0.5H2O were connected with the process parameters such as the supersaturation, temperature, pH, and the organic/inorganic additives [10–15]. For example, it was reported that the presence of Sr2+ or PO
4
3
- accelerated the precipitation of CaSO4·2H2O while the addition of Cd2+, Cu2+, Fe3+, and Cr3+ inhibited the crystallization of CaSO4·2H2O. The hydrothermal conversion of CaSO4·2H2O to CaSO4·0.5H2O was accelerated by the presence of CTAB and inhibited by the presence of arginine, aspartic acid, serine, glycine and sodium dodecyl sulfate, and so forth [16–20].In this paper, CaSO4·0.5H2O whiskers with high aspect ratios were produced by hydrothermal treatment of CaSO4·2H2O precursor at 135°C in the presence of NH4Cl. The influences of NH4Cl on supersaturation, induction time, and morphology of the CaSO4·0.5H2O whiskers were investigated and the corresponding phenomena were discussed.


## 2. Experimental

### 2.1. Experimental Procedure

Commercial CaSO4·2H2O with analytical grade was sintered at 150°C for 6.0 h, then mixed with deionized water and NH4Cl at room temperature to get the slurries containing 4.0 (wt/v) % CaSO4·2H2O and 0 to 7.48 × 10−2 mol·L−1 NH4Cl. The slurries were then transferred to the Teflon-lined stainless autoclaves with an inner volume of 60 mL and kept under isothermal condition at 135°C for 0 to 2.0 h. After hydrothermal treatment, the products were cooled down to 90°C naturally, filtrated, washed with alcohol for three times, and dried in air at 55°C for 12.0 h until the weight reached a stable value. The precipitates and the filtrates were collected and used for characterizations.


### 2.2. Characterization

The morphology of the samples were detected with the field emission scanning electron microscopy (SEM, JSM 7401F, JEOL, Japan). The average diameters and the lengths of the hydrothermal products were estimated by direct measuring about 200 particles from the typical FESEM images with magnifications of 250 to 5000. The structures of the samples were identified by powder X-ray diffractometer (XRD, D8 advanced, Brucker, Germany) using Cu Kα radiation (λ
=
1.54178 Å). The solution pH was measured by a pH meter (pH meter, Mettler Toledo FE20, China).The composition of the hydrothermal precipitates which were composed of CaSO4·2H2O and α-CaSO4·0.5H2O was detected by analyzing the contents of the crystalline water in the precipitates using differential thermal-thermogravimetric (DTA-TG) analysis (TGA/DSC1/1600HT, Mettler-Toledo, Switzerland). The soluble Ca2+ and SO
4
2
- were analyzed by EDTA titration and barium chromate spectrophotometry (Model 722, Xiaoguang, China), respectively.


## 2.1. Experimental Procedure

Commercial CaSO4·2H2O with analytical grade was sintered at 150°C for 6.0 h, then mixed with deionized water and NH4Cl at room temperature to get the slurries containing 4.0 (wt/v) % CaSO4·2H2O and 0 to 7.48 × 10−2 mol·L−1 NH4Cl. The slurries were then transferred to the Teflon-lined stainless autoclaves with an inner volume of 60 mL and kept under isothermal condition at 135°C for 0 to 2.0 h. After hydrothermal treatment, the products were cooled down to 90°C naturally, filtrated, washed with alcohol for three times, and dried in air at 55°C for 12.0 h until the weight reached a stable value. The precipitates and the filtrates were collected and used for characterizations.


## 2.2. Characterization

The morphology of the samples were detected with the field emission scanning electron microscopy (SEM, JSM 7401F, JEOL, Japan). The average diameters and the lengths of the hydrothermal products were estimated by direct measuring about 200 particles from the typical FESEM images with magnifications of 250 to 5000. The structures of the samples were identified by powder X-ray diffractometer (XRD, D8 advanced, Brucker, Germany) using Cu Kα radiation (λ
=
1.54178 Å). The solution pH was measured by a pH meter (pH meter, Mettler Toledo FE20, China).The composition of the hydrothermal precipitates which were composed of CaSO4·2H2O and α-CaSO4·0.5H2O was detected by analyzing the contents of the crystalline water in the precipitates using differential thermal-thermogravimetric (DTA-TG) analysis (TGA/DSC1/1600HT, Mettler-Toledo, Switzerland). The soluble Ca2+ and SO
4
2
- were analyzed by EDTA titration and barium chromate spectrophotometry (Model 722, Xiaoguang, China), respectively.


## 3. Result and Discussion

### 3.1. Influence of NH4Cl on Morphology of α-CaSO4·0.5H2O

Figure1 shows the effect of NH4Cl on the XRD patterns and morphology of the hydrothermal products. XRD analyses showed that all of the hydrothermal products were composed of α-CaSO4·0.5H2O and most of the XRD peaks as (200), (020), and (400) were attributed to the planes parallel to c-axis, indicating the possible preferential growth of α-CaSO4·0.5H2O along the c-axis. The whiskers with a length of 50 to 160 μm and a diameter of 1.0 to 1.5 μm were prepared in the absence of NH4Cl (Figure 1(a)); the increase of NH4Cl from 7.48 × 10−3 mol·L−1 to 3 × 10−2 mol·L−1 led to the increase of the lengths from 100 to 220 μm to 150 to 300 μm, the decrease of the diameters from 0.8 to 1.2 μm to 0.2 to 0.5 μm, and the increase of the average aspect ratios from 180 to 550 (Figures 1(b) and 1(c)); in the case of 7.48 × 10−2 mol·L−1 NH4Cl, the lengths and diameters of the whiskers were 160 to 330 μm and 2.2 to 4.0 μm, respectively. The presence of ethanol, potassium sodium tartrate, and sodium citrate led to the increase in the aspect ratios of CaSO4·0.5H2O whiskers from 1.7 to 4.8. Hou and Xiang prepared CaSO4·0.5H2O whiskers with average aspect ratio of 325 by hydrothermal treatment of the active CaSO4·2H2O precursor [10]. It is reported that the presence of ethanol and potassium sodium tartrate led to the increase in the aspect ratios of whiskers from 1.7 to 4.8 [21–23].Figure 1
Influence of NH4Cl on XRD patterns and morphology of hydrothermal products. NH4Cl (mol·L−1): a-0, b-7.48 × 10−3, c-3 × 10−2, d-7.48 × 10−2; ∙: α-CaSO4·0.5H2O.


### 3.2. Influence of NH4Cl on Composition of Precipitates and Solutions

Figure2 shows the influence of NH4Cl on the conversion of CaSO4·2H2O to α-CaSO4·0.5H2O. α-CaSO4·0.5H2O occurred at about 46 minutes and converted to CaSO4·0.5H2O completely at about 60 minutes in the absence of NH4Cl, while α-CaSO4·0.5H2O occurred at 42 minutes, 34 minutes, and 66 minutes in the presence of 7.48 × 10−3 mol·L−1, 3 × 10−2 mol·L−1, and 7.48 × 10−2 mol·L−1 NH4Cl, respectively. The above work showed that the formation of α-CaSO4·0.5H2O was accelerated with the increase of NH4Cl up to 3 × 10−2 mol·L−1 and then slowed down if NH4Cl ≥ 7.48 × 10−2 mol·L−1.Figure 2
Influence of NH4Cl on formation of CaSO4·0.5H2O. NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.Figure3 shows the influence of NH4Cl on pH and the concentrations of the total soluble Ca2+ and SO
4
2
- (abbreviated as Ca
2
+
T and SO
4
2
-
T, resp.) detected in the experiments. The increase of Ca
2
+
T and SO
4
2
-
T and the decrease of pH with the increase of NH4Cl should be attributed to the complex interactions of NH4Cl and the hydrolysis of NH4Cl, respectively. The possible reactions involved in the hydrothermal solutions at 135°C are listed in Table 1. The solubility products of (NH4)2SO4 and CaCl+ in the hydrothermal condition were 100.18 and 100.86, respectively, which indicated that (NH4)2SO4 and CaCl+ could exist stably in the hydrothermal solutions. These reactions would promote the dissolution of CaSO4·2H2O due to the shift of chemical equilibrium and the concentration of the total soluble Ca2+ (including Ca2+, CaCl+, CaCl2, Ca(OH)+, and Ca(OH)2) and the total soluble SO
4
2
- (including SO
4
2
-, HSO
4
-, (NH4)2SO4, NH4HSO4, and H2SO4) would increase. Similar phenomena have been reported. Jiang et al. found that the interaction between univalent NH
4
+ and SO
4
2
- could promote the dissolution of CaSO4·2H2O significantly, which was called “MSO
4
(
2
-
b
)
- ion pair”-directed mechanism [16]. Ca
2
+
T and SO
4
2
-
T increased with the increase of the reaction time within 60 minutes, reached up to the maximum values at 60 to 75 minutes, and then decreased with further prolongation of the reaction time, revealing that the dissolution of CaSO4·2H2O was dominant in the initial stages and the precipitation of α-CaSO4·0.5H2O became gradually faster than the dissolution of CaSO4·2H2O in the later stages. The above phenomena confirmed the possible dissolution-precipitation mechanism for the hydrothermal formation of α-CaSO4·0.5H2O from CaSO4·2H2O precursor.Table 1
Thermodynamic equilibrium reactions.
NH
4
+ + Cl− = NH4Cl (K
sp = 10−1.66)
CaCl+ + Cl− = CaCl2  (K
sp = 10−7.88)
NH
4
+ + OH− = NH3(a) + H2O (K
sp = 104.88)
Ca2+ + OH− = Ca(OH)+  (K
sp = 102.57)
2NH
4
+ + SO
4
2
- = (NH4)2SO4  (K
sp = 100.18)
Ca(OH)+ + OH− = Ca(OH)2  (K
sp = 104.46)
NH
4
+ + HSO
4
- = NH4HSO4  (K
sp = 10−8.66)
H+ + SO
4
2
- = HSO
4
-  (K
sp = 103.52)
Ca2+ + Cl− = CaCl+  (K
sp = 100.86)
H+ + HSO
4
- = H2SO4  (K
sp = 10−7.68)Figure 3
Influence of NH4Cl on Ca
2
+
T (a), SO
4
2
-
T (b), and pH (c). NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.
(a)
(b)
(c)


### 3.3. Thermodynamic Equilibrium Analysis

The equilibrium constants were calculated using reactions listed in Table1 and the basic data in HSC chemistry 7.0 [24].The equilibrium concentrations of the soluble species can be calculated based on the above equilibrium equations,Ca
2
+
T, SO
4
2
-
T, and pH detected in the experiments (Figure 3). Figure 4 shows the influence of NH4Cl on the concentrations of free Ca2+, SO
4
2
-, and HSO
4
- (abbreviated as [Ca2+], [SO
4
2
-], and [HSO
4
-], resp.). Free Ca2+ and SO
4
2
- are the simple ion of Ca2+ and SO
4
2
-, not including CaCl+, CaCl2, Ca(OH)+, and Ca(OH)2 or HSO
4
-, (NH4)2SO4, NH4HSO4, and H2SO4. [Ca2+] increased with the increase of NH4Cl (Figure 4(a)), indicating that the complex interactions among NH4Cl and the soluble cations as Ca2+ accelerated the dissolution of CaSO4·2H2O. Compared with [Ca2+] shown in Figure 4(a), [SO
4
2
-] was much lower and decreased with the increase of NH4Cl (Figure 4(b)), which should be related to the hydrolysis of NH4Cl. The hydrolysis of NH4Cl led to the decrease of pH or the increase of [H+], which promoted the conversion of SO
4
2
- to HSO
4
- owing to the strong complex between H+ and SO
4
2
-. As a result, although the total soluble SO
4
2
- increased with the increase of NH4Cl, the free SO
4
2
- showed an opposite trend and decreased. In the case of the blank experiment without NH4Cl (Figure 4(c)), [HSO
4
-] was lower than 2 × 10−4 mol·L−1, which was much lower than [SO
4
2
-]. [HSO
4
-] increased up to 5.3 × 10−2 mol·L−1 as the increase of [NH4Cl] up to 7.48 × 10−2 mol·L−1.Figure 4
Influence of NH4Cl on [Ca2+], [SO
4
2
-] and [HSO
4
-]. NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.
(a)
(b)
(c)


### 3.4. Effect of Supersaturation on Induction Time

Figure5 shows the influence of NH4Cl on the supersaturation (S) for the formation of α-CaSO4·0.5H2O, where S
=
[
[
Ca
2
+
]
[
SO
4
2
-
]
/
K
sp
] and K
sp for α-CaSO4·0.5H2O at 135°C was 10−5.344. S increased with the increase of the reaction time up to 60–70 minutes and then decreased with further prolongation of the reaction time. S also increased with the increase of NH4Cl up to 3 × 10−2 mol·L−1, while S became quite low at 7.48 × 10−2 mol·L−1 NH4Cl. The above work showed that a higher supersaturation favored the faster formation of α-CaSO4·0.5H2O.Figure 5
Influence of NH4Cl on supersaturation. NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.It was noticed that theS values at the end of the induction in solutions containing 0 to 7.48 × 10−2 mol·L−1 NH4Cl were quite similar, being 209.5 (Point C), 210.2 (Point B), 208.4 (Point A), and 209.2 (Point D) at 0, 7.48 × 10−3, 3 × 10−2, and 7.48 × 10−2 mol·L−1 NH4Cl, respectively. The points of A, B, C, and D were arranged around a horizontal line, corresponding to S
=
209.5. Therefore, it was concluded that the critical supersaturation for the formation of α-CaSO4·0.5H2O at 135°C was 209.5, which was irrelevant to the presence of NH4Cl. α-CaSO4·0.5H2O occurred if the solution supersaturation was higher than the critical supersaturation. S increased with the increase of NH4Cl up to 3 × 10−2 mol·L−1, which led to the decrease of the induction time for α-CaSO4·0.5H2O. In the case of 7.48 × 10−2 mol·L−1 NH4Cl, the low S led to the prolongation of the induction time for the formation of α-CaSO4·0.5H2O due to the hydrolysis of NH4Cl and the decrease of [SO
4
2
-].The morphology ofα-CaSO4·0.5H2O was connected with the solution supersaturation. According to the traditional crystal theories, nuclei with small sizes prefer to be produced in solutions with high supersaturations, which favored the formation of CaSO4·0.5H2O whiskers with thin diameters and high aspect ratios. As shown in Figures 1 and 5, CaSO4·0.5H2O whiskers formed in the presence of 3 × 10−2 mol·L−1 NH4Cl were of the smallest diameter (0.2 μm) and the highest aspect ratio (550) owing to the comparatively high S compared with those in the presence of 0, 7.48 × 10−3, and 7.48 × 10−2 mol·L−1 NH4Cl.


## 3.1. Influence of NH4Cl on Morphology of α-CaSO4·0.5H2O

Figure1 shows the effect of NH4Cl on the XRD patterns and morphology of the hydrothermal products. XRD analyses showed that all of the hydrothermal products were composed of α-CaSO4·0.5H2O and most of the XRD peaks as (200), (020), and (400) were attributed to the planes parallel to c-axis, indicating the possible preferential growth of α-CaSO4·0.5H2O along the c-axis. The whiskers with a length of 50 to 160 μm and a diameter of 1.0 to 1.5 μm were prepared in the absence of NH4Cl (Figure 1(a)); the increase of NH4Cl from 7.48 × 10−3 mol·L−1 to 3 × 10−2 mol·L−1 led to the increase of the lengths from 100 to 220 μm to 150 to 300 μm, the decrease of the diameters from 0.8 to 1.2 μm to 0.2 to 0.5 μm, and the increase of the average aspect ratios from 180 to 550 (Figures 1(b) and 1(c)); in the case of 7.48 × 10−2 mol·L−1 NH4Cl, the lengths and diameters of the whiskers were 160 to 330 μm and 2.2 to 4.0 μm, respectively. The presence of ethanol, potassium sodium tartrate, and sodium citrate led to the increase in the aspect ratios of CaSO4·0.5H2O whiskers from 1.7 to 4.8. Hou and Xiang prepared CaSO4·0.5H2O whiskers with average aspect ratio of 325 by hydrothermal treatment of the active CaSO4·2H2O precursor [10]. It is reported that the presence of ethanol and potassium sodium tartrate led to the increase in the aspect ratios of whiskers from 1.7 to 4.8 [21–23].Figure 1
Influence of NH4Cl on XRD patterns and morphology of hydrothermal products. NH4Cl (mol·L−1): a-0, b-7.48 × 10−3, c-3 × 10−2, d-7.48 × 10−2; ∙: α-CaSO4·0.5H2O.


## 3.2. Influence of NH4Cl on Composition of Precipitates and Solutions

Figure2 shows the influence of NH4Cl on the conversion of CaSO4·2H2O to α-CaSO4·0.5H2O. α-CaSO4·0.5H2O occurred at about 46 minutes and converted to CaSO4·0.5H2O completely at about 60 minutes in the absence of NH4Cl, while α-CaSO4·0.5H2O occurred at 42 minutes, 34 minutes, and 66 minutes in the presence of 7.48 × 10−3 mol·L−1, 3 × 10−2 mol·L−1, and 7.48 × 10−2 mol·L−1 NH4Cl, respectively. The above work showed that the formation of α-CaSO4·0.5H2O was accelerated with the increase of NH4Cl up to 3 × 10−2 mol·L−1 and then slowed down if NH4Cl ≥ 7.48 × 10−2 mol·L−1.Figure 2
Influence of NH4Cl on formation of CaSO4·0.5H2O. NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.Figure3 shows the influence of NH4Cl on pH and the concentrations of the total soluble Ca2+ and SO
4
2
- (abbreviated as Ca
2
+
T and SO
4
2
-
T, resp.) detected in the experiments. The increase of Ca
2
+
T and SO
4
2
-
T and the decrease of pH with the increase of NH4Cl should be attributed to the complex interactions of NH4Cl and the hydrolysis of NH4Cl, respectively. The possible reactions involved in the hydrothermal solutions at 135°C are listed in Table 1. The solubility products of (NH4)2SO4 and CaCl+ in the hydrothermal condition were 100.18 and 100.86, respectively, which indicated that (NH4)2SO4 and CaCl+ could exist stably in the hydrothermal solutions. These reactions would promote the dissolution of CaSO4·2H2O due to the shift of chemical equilibrium and the concentration of the total soluble Ca2+ (including Ca2+, CaCl+, CaCl2, Ca(OH)+, and Ca(OH)2) and the total soluble SO
4
2
- (including SO
4
2
-, HSO
4
-, (NH4)2SO4, NH4HSO4, and H2SO4) would increase. Similar phenomena have been reported. Jiang et al. found that the interaction between univalent NH
4
+ and SO
4
2
- could promote the dissolution of CaSO4·2H2O significantly, which was called “MSO
4
(
2
-
b
)
- ion pair”-directed mechanism [16]. Ca
2
+
T and SO
4
2
-
T increased with the increase of the reaction time within 60 minutes, reached up to the maximum values at 60 to 75 minutes, and then decreased with further prolongation of the reaction time, revealing that the dissolution of CaSO4·2H2O was dominant in the initial stages and the precipitation of α-CaSO4·0.5H2O became gradually faster than the dissolution of CaSO4·2H2O in the later stages. The above phenomena confirmed the possible dissolution-precipitation mechanism for the hydrothermal formation of α-CaSO4·0.5H2O from CaSO4·2H2O precursor.Table 1
Thermodynamic equilibrium reactions.
NH
4
+ + Cl− = NH4Cl (K
sp = 10−1.66)
CaCl+ + Cl− = CaCl2  (K
sp = 10−7.88)
NH
4
+ + OH− = NH3(a) + H2O (K
sp = 104.88)
Ca2+ + OH− = Ca(OH)+  (K
sp = 102.57)
2NH
4
+ + SO
4
2
- = (NH4)2SO4  (K
sp = 100.18)
Ca(OH)+ + OH− = Ca(OH)2  (K
sp = 104.46)
NH
4
+ + HSO
4
- = NH4HSO4  (K
sp = 10−8.66)
H+ + SO
4
2
- = HSO
4
-  (K
sp = 103.52)
Ca2+ + Cl− = CaCl+  (K
sp = 100.86)
H+ + HSO
4
- = H2SO4  (K
sp = 10−7.68)Figure 3
Influence of NH4Cl on Ca
2
+
T (a), SO
4
2
-
T (b), and pH (c). NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.
(a)
(b)
(c)


## 3.3. Thermodynamic Equilibrium Analysis

The equilibrium constants were calculated using reactions listed in Table1 and the basic data in HSC chemistry 7.0 [24].The equilibrium concentrations of the soluble species can be calculated based on the above equilibrium equations,Ca
2
+
T, SO
4
2
-
T, and pH detected in the experiments (Figure 3). Figure 4 shows the influence of NH4Cl on the concentrations of free Ca2+, SO
4
2
-, and HSO
4
- (abbreviated as [Ca2+], [SO
4
2
-], and [HSO
4
-], resp.). Free Ca2+ and SO
4
2
- are the simple ion of Ca2+ and SO
4
2
-, not including CaCl+, CaCl2, Ca(OH)+, and Ca(OH)2 or HSO
4
-, (NH4)2SO4, NH4HSO4, and H2SO4. [Ca2+] increased with the increase of NH4Cl (Figure 4(a)), indicating that the complex interactions among NH4Cl and the soluble cations as Ca2+ accelerated the dissolution of CaSO4·2H2O. Compared with [Ca2+] shown in Figure 4(a), [SO
4
2
-] was much lower and decreased with the increase of NH4Cl (Figure 4(b)), which should be related to the hydrolysis of NH4Cl. The hydrolysis of NH4Cl led to the decrease of pH or the increase of [H+], which promoted the conversion of SO
4
2
- to HSO
4
- owing to the strong complex between H+ and SO
4
2
-. As a result, although the total soluble SO
4
2
- increased with the increase of NH4Cl, the free SO
4
2
- showed an opposite trend and decreased. In the case of the blank experiment without NH4Cl (Figure 4(c)), [HSO
4
-] was lower than 2 × 10−4 mol·L−1, which was much lower than [SO
4
2
-]. [HSO
4
-] increased up to 5.3 × 10−2 mol·L−1 as the increase of [NH4Cl] up to 7.48 × 10−2 mol·L−1.Figure 4
Influence of NH4Cl on [Ca2+], [SO
4
2
-] and [HSO
4
-]. NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.
(a)
(b)
(c)


## 3.4. Effect of Supersaturation on Induction Time

Figure5 shows the influence of NH4Cl on the supersaturation (S) for the formation of α-CaSO4·0.5H2O, where S
=
[
[
Ca
2
+
]
[
SO
4
2
-
]
/
K
sp
] and K
sp for α-CaSO4·0.5H2O at 135°C was 10−5.344. S increased with the increase of the reaction time up to 60–70 minutes and then decreased with further prolongation of the reaction time. S also increased with the increase of NH4Cl up to 3 × 10−2 mol·L−1, while S became quite low at 7.48 × 10−2 mol·L−1 NH4Cl. The above work showed that a higher supersaturation favored the faster formation of α-CaSO4·0.5H2O.Figure 5
Influence of NH4Cl on supersaturation. NH4Cl (mol·L−1): 1-0, 2-7.48 × 10−3, 3-3 × 10−2, 4-7.48 × 10−2.It was noticed that theS values at the end of the induction in solutions containing 0 to 7.48 × 10−2 mol·L−1 NH4Cl were quite similar, being 209.5 (Point C), 210.2 (Point B), 208.4 (Point A), and 209.2 (Point D) at 0, 7.48 × 10−3, 3 × 10−2, and 7.48 × 10−2 mol·L−1 NH4Cl, respectively. The points of A, B, C, and D were arranged around a horizontal line, corresponding to S
=
209.5. Therefore, it was concluded that the critical supersaturation for the formation of α-CaSO4·0.5H2O at 135°C was 209.5, which was irrelevant to the presence of NH4Cl. α-CaSO4·0.5H2O occurred if the solution supersaturation was higher than the critical supersaturation. S increased with the increase of NH4Cl up to 3 × 10−2 mol·L−1, which led to the decrease of the induction time for α-CaSO4·0.5H2O. In the case of 7.48 × 10−2 mol·L−1 NH4Cl, the low S led to the prolongation of the induction time for the formation of α-CaSO4·0.5H2O due to the hydrolysis of NH4Cl and the decrease of [SO
4
2
-].The morphology ofα-CaSO4·0.5H2O was connected with the solution supersaturation. According to the traditional crystal theories, nuclei with small sizes prefer to be produced in solutions with high supersaturations, which favored the formation of CaSO4·0.5H2O whiskers with thin diameters and high aspect ratios. As shown in Figures 1 and 5, CaSO4·0.5H2O whiskers formed in the presence of 3 × 10−2 mol·L−1 NH4Cl were of the smallest diameter (0.2 μm) and the highest aspect ratio (550) owing to the comparatively high S compared with those in the presence of 0, 7.48 × 10−3, and 7.48 × 10−2 mol·L−1 NH4Cl.


## 4. Conclusion

A facile NH4Cl-assisted hydrothermal method was developed to synthesize α-CaSO4·0.5H2O whiskers with high aspect ratios. The presence of 0 to 3 × 10−2 mol·L−1 NH4Cl led to the decrease of the induction period from 46 minutes to 34 minutes and the increase of the aspect ratios of the whiskers from 110 to 550. The critical supersaturation for the formation of CaSO4·0.5H2O was 209.5, irrelevant to the presence of NH4Cl. In the cases of 0 to 3 × 10−2 mol·L−1 NH4Cl, the complex interactions among NH4Cl and the soluble ions led to the increase of the supersaturation, which favored the quick occurrence of α-CaSO4·0.5H2O nuclei with small sizes and promoted the formation of α-CaSO4·0.5H2O whiskers with high aspect ratios. In the case of 7.48 × 10−2 mol·L−1 NH4Cl, the hydrolysis of NH4Cl led to the decrease of [SO
4
2
-], which reduced the solution supersaturation and prolonged the induction period of α-CaSO4·0.5H2O, producing α-CaSO4·0.5H2O with comparatively large diameters and low aspect ratios.



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*Source: 670872-2015-03-15.xml*