Influence of NH 4 Cl on Hydrothermal Formation of α-CaSO 4 ⋅ 0 . 5 H 2 O Whiskers

The influence of NH 4 Cl on hydrothermal formation of CaSO 4 ⋅0.5H 2 O whiskers from CaSO 4 ⋅2H 2 O precursor at 135C was investigated in this paper. Compared with the blank experiment, the presence of 3 × 10mol⋅L NH 4 Cl led to the increase of the lengths of the whiskers from 50 to 160μm to 150 to 300 μm and the decrease of the diameters from 1.0 to 1.5μm to 0.2 to 0.5 μm. The dissolution of CaSO 4 ⋅2H 2 O was accelerated by the complex interactions with NH 4 Cl and the soluble cations, which led to the decrease of the induction time for the occurrence of α-CaSO 4 ⋅0.5H 2 O from 46 minutes to 34 minutes and the formation of CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios. Furthermore the critical supersaturation for the formation of α-CaSO 4 ⋅0.5H 2 O was investigated.


Introduction
The formation of calcium sulfate (CaSO 4 ) 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][2][3][4].Calcium sulfate whiskers were usually prepared by the first formation of -CaSO 4 ⋅0.5H 2 O whiskers followed by calcination at 600 to 800 ∘ C since the anisotropic -Ca-SO 4 -Ca-SO 4 -Ca-chains in -CaSO 4 ⋅0.5H 2 O favored their growth along -axis [5].
In this paper, CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios were produced by hydrothermal treatment of CaSO 4 ⋅2H 2 O precursor at 135 ∘ C in the presence of NH 4 Cl.The influences of NH 4 Cl on supersaturation, induction time, and morphology of the CaSO 4 ⋅0.5H 2 O whiskers were investigated and the corresponding phenomena were discussed.

Experimental
2.1.Experimental Procedure.Commercial CaSO 4 ⋅2H 2 O with analytical grade was sintered at 150 ∘ C for 6.0 h, then mixed with deionized water and NH 4 Cl at room temperature to get the slurries containing 4.0 (wt/v) % CaSO 4 ⋅2H 2 O and 0 to 7.48 × 10 −2 mol⋅L −1 NH 4 Cl.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.

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 K radiation ( = 1.54178Å).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 CaSO 4 ⋅2H 2 O and -CaSO 4 ⋅0.5H 2 O 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 Ca 2+ and SO 4 2− were analyzed by EDTA titration and barium chromate spectrophotometry (Model 722, Xiaoguang, China), respectively.[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][22][23].Intensity (a.u.) Figure 3 shows the influence of NH 4 Cl on pH and the concentrations of the total soluble Ca 2+ and SO 2− ] T and the decrease of pH with the increase of NH 4 Cl should be attributed to the complex interactions of NH 4 Cl and the hydrolysis of NH 4 Cl, respectively.The possible reactions involved in the hydrothermal solutions at 135 ∘ C are listed in Table 1.The solubility products of (NH 4 ) 2 SO 4 and CaCl + in the hydrothermal condition were 10 0.18 and 10 0.86 , respectively, which indicated that (NH 4 ) 2 SO 4 and CaCl + could

Thermodynamic Equilibrium Analysis.
The equilibrium constants were calculated using reactions listed in Table 1 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 NH 4 Cl on the concentrations of free Ca 2+ , SO   ].The morphology of -CaSO 4 ⋅0.5H 2 O 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 CaSO 4 ⋅0.5H 2 O whiskers with thin diameters and high aspect ratios.As shown in Figures 1 and 5, CaSO 4 ⋅0.5H 2 O whiskers formed in the presence of 3 × 10 −2 mol⋅L −1 NH 4 Cl were of the smallest diameter (0.2 m) and the highest aspect ratio (550) owing to the comparatively high  compared with those in the presence of 0, 7.48 × 10 −3 , and 7.48 × 10 −2 mol⋅L −1 NH 4 Cl.

Conclusion
A facile NH 4 Cl-assisted hydrothermal method was developed to synthesize -CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios.The presence of 0 to 3 × 10 −2 mol⋅L −1 NH 4 Cl 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 CaSO 4 ⋅0.5H 2 O was 209.5, irrelevant to the presence of NH 4 Cl.In the cases of 0 to 3 × 10 −2 mol⋅L −1 NH 4 Cl, the complex interactions among NH 4 Cl and the soluble ions led to the increase of the supersaturation, which favored the quick occurrence of -CaSO 4 ⋅0.5H 2 O nuclei with small sizes

Figure 1
shows the effect of NH 4 Cl on the XRD patterns and morphology of the hydrothermal products.XRD analyses showed that all of the hydrothermal products were composed of -CaSO 4 ⋅0.5H 2 O and most of the XRD peaks as (200), (020), and (400) were attributed to the planes parallel to -axis, indicating the possible preferential growth of -CaSO 4 ⋅0.5H 2 O along the -axis.The whiskers with a length of 50 to 160 m and a diameter of 1.0 to 1.5 m were prepared in the absence of NH 4 Cl (Figure 1(a)); the increase of NH 4 Cl 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 m to 150 to 300 m, the decrease of the diameters from 0.8 to 1.2 m to 0.2 to 0.5 m, 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 NH 4 Cl, the lengths and diameters of the whiskers were 160 to 330 m and 2.2 to 4.0 m, respectively.The presence of ethanol, potassium sodium tartrate, and sodium citrate led to the increase in the aspect ratios of CaSO 4 ⋅0.5H 2 O whiskers from 1.7 to 4.8.Hou and Xiang prepared CaSO 4 ⋅0.5H 2 O whiskers with average aspect ratio of 325 by hydrothermal treatment of the active CaSO 4 ⋅2H 2 O precursor
(2−b)− ion pair"-directed mechanism[16].[Ca2+ ] T and [SO 42− ] 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 CaSO 4 ⋅2H 2 O was dominant in the initial stages and the precipitation of -CaSO 4 ⋅0.5H 2 O became gradually faster than the dissolution of CaSO 4 ⋅2H 2 O in the later stages.The above phenomena confirmed the possible dissolutionprecipitation mechanism for the hydrothermal formation of -CaSO 4 ⋅0.5H 2 O from CaSO 4 ⋅2H 2 O precursor.

2 −
SO 4 .[Ca 2+ ] increased with the increase of NH 4 Cl (Figure 4(a)), indicating that the complex interactions among NH 4 Cl and the soluble cations as Ca 2+ accelerated the dissolution of CaSO 4 ⋅2H 2 O. Compared with [Ca 2+ ] shown in Figure 4(a), [SO 4 2− ] was much lower and decreased with the increase of NH 4 Cl (Figure 4(b)), which should be related to the hydrolysis of NH 4 Cl.The hydrolysis of NH 4 Cl 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 NH 4 Cl, the free SO 4 showed an opposite trend and decreased.In the case of the blank experiment without NH 4 Cl (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 [NH 4 Cl] up to 7.48 × 10 −2 mol⋅L −1 .
Figure 5 shows the influence of NH 4 Cl on the supersaturation () for the formation of -CaSO 4 ⋅0.5H 2 O, where  = [[Ca 2+ ][SO 4 2− ]/ sp ] and  sp for -CaSO 4 ⋅0.5H 2 O at 135 ∘ C was 10 −5.344 . increased with the increase of the reaction time up to 60-70 minutes and then decreased with further prolongation of the reaction time. also increased with the increase of NH 4 Cl up to 3 × 10 −2 mol⋅L −1 , while  became quite low at 7.48 × 10 −2 mol⋅L −1 NH 4 Cl.The above work showed that a higher supersaturation favored the faster formation of -CaSO 4 ⋅0.5H 2 O.It was noticed that the  values at the end of the induction in solutions containing 0 to 7.48 × 10 −2 mol⋅L −1 NH 4 Cl 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 NH 4 Cl, respectively.The points of A, B, C, and D were arranged around a horizontal line, corresponding to  = 209.5.Therefore, it was concluded that the critical supersaturation for the formation of -CaSO 4 ⋅0.5H 2 O at 135 ∘ C was 209.5, which was irrelevant to the presence of NH 4 Cl.-CaSO 4 ⋅0.5H 2 O occurred if the solution supersaturation was higher than the critical supersaturation. increased with the increase of NH 4 Cl up to 3 × 10 −2 mol⋅L −1 , which led to the decrease of the induction time for -CaSO 4 ⋅0.5H 2 O.In the case of 7.48 × 10 −2 mol⋅L −1 NH 4 Cl, the low  led to the prolongation of the induction time for the formation of -CaSO 4 ⋅0.5H 2 O due to the hydrolysis of NH 4 Cl and the decrease of [SO 4 2−

4
Cl on Composition of Precipitates and Solutions.Figure 2 shows the influence of NH 4 Cl on the conversion of CaSO 4 ⋅2H 2 O to -CaSO 4 ⋅0.5H 2 O. -CaSO 4 ⋅0.5H 2 O occurred at about 46 minutes and converted to CaSO 4 ⋅0.5H 2 O completely at about 60 minutes in the absence of NH 4 Cl, while -CaSO 4 ⋅0.5H 2 O occurred at 42