Influence of Alkyl Trimethyl Ammonium Bromides on Hydrothermal Formation of α-CaSO₄·0.5H₂O Whiskers with High Aspect Ratios

Abstract

In this paper, the influence of alkyl trimethyl ammonium bromides (C_nH_2n+1(CH₃)₃NBr, n = 10, 12, 14, 16, 18, abbreviated as ATAB) on the formation of alpha calcium sulfate hemihydrate (α-CaSO₄·0.5H₂O) whiskers under a hydrothermal condition (135 °C, 3.0 h) was analyzed. Specifically, it focuses on cetyl trimethyl ammonium bromide (C₁₆H₃₃(CH₃)₃NBr, abbreviated as CTAB). The rising CTAB concentration from 0 to 9.2 × 10⁻⁴ mol·L⁻¹ led to the increase of the average aspect ratio of α-CaSO₄·0.5H₂O whiskers from 80 to 430, since the selective adsorption of CTAB on the negatively-charged side facets of the whiskers inhibited the growth of the whiskers along the direction normal to the lateral facets. The further increase of CTAB concentration above the critical micelle concentration (abbreviated as CMC) showed little effect on the morphology of α-CaSO₄·0.5H₂O whiskers, considering that CTAB tended to form micelles instead of being adsorbed on the whisker surfaces. Similar phenomena were observed in other ATABs (n = 10, 12, 14, 18).

1. Introduction

Calcium sulfate whiskers with high aspect ratios are promising reinforcing materials for composites as rubbers, plastics, plaster and friction materials, etc. [1,2,3,4], showing excellent thermal stability and mechanical strength [5,6,7]. Calcium sulfate whiskers can be produced by preparing α-CaSO₄·0.5H₂O whiskers through hydrothermal, acidification, reverse micro-emulsion, and microwave-assisted approaches [8,9,10,11,12,13] followed by calcination of the α-CaSO₄·0.5H₂O whiskers above 600 °C [14,15]. Compared with the other methods, the hydrothermal route was widely used thanks to its moderate conditions and easily controlled parameters and whisker properties [16].

Ca²⁺ ions and SO₄²⁻ tetrahedrons are stacked along the [001] direction and attached −Ca−SO₄−Ca−SO₄−Ca−SO₄− chains in α-CaSO₄·0.5H₂O lattice. The chains form a framework parallel to the [001] direction with continuous channels, where the water molecules are combined and interact with −Ca−SO₄−Ca−SO₄−Ca−SO₄− chains via hydrogen bonds. This crystal structure is favorable to the one-dimensional growth of α-CaSO₄·0.5H₂O whiskers along the c-axis [17,18,19,20]. The distribution of SO₄²⁻ ions on the side facets as (200), (400), and (020) is denser than that of Ca²⁺ ions, while the distribution of Ca²⁺ ions on the top facet as (001) is denser than that of SO₄²⁻ ions. Therefore, the side facets and top facet of the α-CaSO₄·0.5H₂O crystal are negatively and positively charged, respectively [18].

Many studies have been conducted in the effort to synthesize α-CaSO₄·0.5H₂O whiskers with high aspect ratios or varying morphologies, since the elasticity modulus of one-dimensional materials with homogeneous structures increases with increasing aspect ratios [21]. For example, Zhao et al. produced α-CaSO₄·0.5H₂O whiskers with an aspect ratio of 240 at 140 °C in the presence of AlCl₃, because the adsorption of Al³⁺ on the negatively charged side facets of the whiskers inhibited the growth of the whiskers along the direction normal to the lateral facets [22]. Hou et al. produced α-CaSO₄·0.5H₂O whiskers with an aspect ratio of up to 370 in the presence of 1.97 × 10⁻³ mol·L⁻¹ MgCl₂ due to the adsorption and doping effects of Mg²⁺ on the side facets of the whiskers [23]. Based on the adsorption and inhibition mechanism, previous work focused on studying the influence of inorganic ions on the growth of α-CaSO₄·0.5H₂O whiskers, aimed at preparing the whiskers with high aspect ratios. Alkyl trimethyl ammonium bromides (C_nH_2n+1(CH₃)₃NBr, n = 10, 12, 14, 16, 18, abbreviated as ATAB) are commonly-used cation surfactants that may inhibit the growth of the whiskers along the direction normal to the lateral facets more effectively. They have long alkyl chains, which possibly provide a strong steric hindrance and reduce the surface energy of the side facets of the whiskers. Currently, the influence of ATABs on the hydrothermal formation of α-CaSO₄·0.5H₂O whiskers has yet to be reported.

In this work, a facile method was employed to synthesize α-CaSO₄·0.5H₂O whiskers with high aspect ratios by hydrothermal treatment of CaSO₄·2H₂O precursor at 135 °C in the presence of trace amount of CTAB (cetyl trimethyl ammonium bromide, C₁₆H₃₃(CH₃)₃NBr). For the first time, α-CaSO₄·0.5H₂O whiskers with an average aspect ratio of up to 430 were obtained. The influence of CTAB on the aspect ratios of α-CaSO₄·0.5H₂O whiskers was investigated, along with the corresponding mechanism. In addition, the effects of ATABs with varying alkyl chain lengths on the formation of α-CaSO₄·0.5H₂O whiskers was analyzed.

2. Results and Discussion

2.1. Influence of CTAB on the Formation of α-CaSO₄·0.5H₂O Whiskers

Figure 1 shows the XRD pattern and morphology of the CaSO₄·2H₂O raw material after calcination and hydration treatment. The XRD peaks were indexed to CaSO₄·2H₂O (JCPDS 33-0311). The activated CaSO₄·2H₂O was composed of irregular rectangle planes with a length of 2.0–8.0 μm and a width of 0.5–5.0 μm.

Figure 2 and Figure 3 show the morphology and aspect ratio distributions of the α-CaSO₄·0.5H₂O whiskers formed in the presence of 0–1.5 × 10⁻³ mol·L⁻¹ CTAB. Whiskers with an average aspect ratio of 80 were prepared in the absence of CTAB (Figure 2a and Figure 3a). The increase of the CTAB concentration from 1.0 × 10⁻⁴ to 9.2 × 10⁻⁴ mol·L⁻¹ led to the gradual increase of the average aspect ratio of the whiskers from 240 to 430 (Figure 2b,c and Figure 3b,c). Little change in the aspect ratios was observed when further increasing the CTAB concentration (Figure 2d and Figure 3d). TEM and high-resolution TEM (HRTEM) images (Figure 2e,f) reveal that the inter-planar spacing parallel to the axial direction of the whiskers was 0.598 nm (corresponding to the spacing of (002) plane (0.599 nm) of α-CaSO₄·0.5H₂O), indicating the intrinsic preferential growth of the whiskers along the c axis. The diffraction spots in the SAED pattern (Figure 2f) could be indexed to the [$1\overline{1}0$] zone axis of α-CaSO₄·0.5H₂O, reconfirming the preferential growth of the whiskers along the c axis.

2.2. Adsorption of CTAB on α-CaSO₄·0.5H₂O Whiskers

The previous work showed that the side facets of α-CaSO₄·0.5H₂O whiskers are negatively charged, while the top facets of the whiskers are positively charged [18,24]. As a cationic surfactant, CTAB could possibly be adsorbed on the side facets of α-CaSO₄·0.5H₂O whiskers by electrostatic interactions. Figure 4 presents the variations of Fourier transform infrared (FT-IR) spectra and zeta potentials of the α-CaSO₄·0.5H₂O whiskers with CTAB concentration. As shown in Figure 4a, the peak at 660 cm⁻¹ corresponded to the bending (ν₄) mode of SO₄²⁻, while the peaks at 1620 cm⁻¹ and 1685 cm⁻¹ derived from the stretching (ν₂) mode of water in α-CaSO₄·0.5H₂O. The peaks at 2850 cm⁻¹ and 2920 cm⁻¹ were ascribed to the stretching of C–H bonds in CTAB, the intensities of which increased with the CTAB concentration in the range of 0 to 9.2 × 10⁻⁴ mol·L⁻¹ and remained unchanged when the CTAB concentration rose to 1.5 × 10⁻³ mol·L⁻¹. This indicated the adsorption of CTAB on the negatively-charged side facets of the whiskers. The zeta potential of the whiskers formed in the absence of CTAB declined from −8.05 to −20.92 mV as the pH increased from 2.52 to 10.44 (curve 1), revealing that the surfaces of the whiskers were negatively charged. The increase of zeta potentials with the CTAB concentration in the range of 0–9.2 × 10⁻⁴ mol·L⁻¹ should be attributed to the enhanced adsorption of cationic CTAB on the negatively-charged side facets ((200), (400), and (020)) of the whiskers by electrostatic interactions, which provided a strong steric hindrance and inhibited the growth of the whiskers along the direction normal to the lateral facets.

The surface energy of the whiskers was influenced by the adsorption of CTAB. It was calculated by measuring the contact angles of water and CH₂I₂ on the whisker film. As shown in

Table 1. Surface energy and polar and dispersive components of α-CaSO₄·0.5H₂O whiskers.

CTAB (mol·L−1)	θ (Water) (°)	θ (CH2I2) (°)	Polar Component (mN·m−1)	Dispersive Component (mN·m−1)	Surface Energy (mN·m−1)
0	14.700	18.013	39.279	33.085	72.364
1.0 × 10−4	25.468	16.438	34.359	34.452	68.811
9.2 × 10−4	32.586	15.533	30.165	35.594	65.759
1.5 × 10−3	33.336	15.239	29.649	35.776	65.425

, the increase of CTAB led to the decline of the polar component of the surface energy and the decrease of the total surface energy (even if the dispersive component increased a little). Considering that CTAB was adsorbed on the side facets of the whiskers, it can be deduced that the decrease of the whisker surface energy was caused by the decrease of the surface energy of the side facets. The lower surface energy implied the increased stability of the side facets, which reduced the growth tendency of the whiskers along the direction normal to the lateral facets. In summary, the adsorption of CTAB on the side facets of the whiskers not only provided a strong steric hindrance preventing the addition of Ca²⁺ and SO₄²⁻, but also reduced the surface energy of the side facets. Both aspects inhibited the growth of the whiskers along the direction normal to the lateral facets and led to the formation of α-CaSO₄·0.5H₂O whiskers with high aspect ratios.

As observed, the saturated adsorption seemed to occur when the CTAB concentration exceeded 9.2 × 10⁻⁴ mol·L⁻¹, which was shown by the unchanged zeta potential and the aspect ratios. The adsorption quantity of CTAB by α-CaSO₄·0.5H₂O whiskers was quantitatively analyzed by determining the dissociative CTAB in the reaction filtrate by an ultraviolet spectrophotometer. Figure 5 shows the variation of the adsorption quantity of CTAB on the whiskers with the CTAB concentration. The adsorption quantity of CTAB increased almost linearly with the increase of the CTAB concentration in the range of 0–9.2 × 10⁻⁴ mol·L⁻¹, and the dissociative CTAB in the solution was negligible compared with the adsorbed CTAB. The adsorption reached saturation at 9.2 × 10⁻⁴ mol·L⁻¹, which is quite close to the critical micelle concentration (CMC) of CTAB. Based on the above results, the effects of CTAB were summarized and schematically illustrated in Figure 6. CTAB may be adsorbed mostly on the negatively charged side facets of the whiskers as free cationic ions when its concentration was less than the CMC, thus promoting the anisotropic growth of the whiskers along the c-axis; while the extra CTAB tended to form micelles in the solution instead of being adsorbed on the whisker surfaces when its concentration was greater than the CMC; this was why the whisker morphology and the adsorption quantity of CTAB remained unchanged when the CTAB concentration was greater than 9.2 × 10⁻⁴ mol·L⁻¹.

2.3. Influence of ATABs on α-CaSO₄·0.5H₂O Whiskers

The effect of CTAB on the formation of α-CaSO₄·0.5H₂O whiskers could also be extended to the other ATABs with varying alkyl chain lengths (C_nH_2n+1(CH₃)₃NBr, n = 10, 12, 14, 18). Figure 7 shows the variations of the aspect ratios of the whiskers with the ATAB concentration and alkyl chain length. The aspect ratios of the whiskers increased with increasing ATAB concentration, achieving its maximum at the CMCs of ATABs (indicated by the dashed line). The aspect ratios of the whiskers obtained at the CMCs increased approximately linearly with the alkyl chain length of ATABs. This may be attributed to the enhanced steric hindrance in ATABs with longer alkyl chains.

3. Materials and Methods

Commercial chemicals of analytical grade and deionized water with a resistivity >18 MΩ·cm⁻¹ were used in the experiments. Calcium sulfate dihydrate (CaSO₄·2H₂O, 99%) was purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Decyl trimethyl ammonium bromide (C₁₃H₃₀BrN, 99%), dodecyl trimethyl ammonium bromide (C₁₅H₃₄BrN, 99%), tetradecyl trimethyl ammonium bromide (C₁₇H₃₈BrN, 99%), cetyl trimethyl ammonium bromide (C₁₉H₄₂BrN, 99%), and octadecyl trimethyl ammonium bromide (C₂₁H₄₆BrN, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Ethanol (CH₃CH₂OH, 99.7%) was purchased from Beijing Tong Guang Fine Chemicals Company (Beijing, China). All of the chemicals were used without any further purification.

CaSO₄·2H₂O was calcined at 150 °C for 5.0 h followed by hydration in deionized water at 25 °C for 1.0 h. Then, 0.4 g of the calcination–hydration-treated CaSO₄·2H₂O was mixed at room temperature with 40.0 g of deionized water and a minor amount of ATABs with an alkyl chain length of 10 to 18 to get suspensions containing 1.0 wt % CaSO₄·2H₂O and 0–1.5 × 10⁻³ mol·L⁻¹ ATAB. Then, the suspensions were transferred to the Teflon-lined stainless steel autoclaves with a volume of 80 mL and stirred with magnetic stirrers at a speed of 200 r/min for 15 min. Then, the autoclaves were put into a homogeneous reactor and kept under hydrothermal conditions (135 °C) for 3.0 h. The products were filtered, washed three times with ethanol, and dried at 105 °C for 4.0 h.

The morphology and microstructure of the samples were characterized with a field emission scanning electron microscope (FESEM, JSM 7401F, JEOL, Hitachi, Tokyo, Japan) and a high-resolution transmission electron microscope (HRTEM, JEM-2010, JEOL, Hitachi, Tokyo, Japan) equipped with selected area electron diffraction (SAED). The average diameters and lengths of the whiskers were estimated by directly measuring about 200 whiskers from the typical FESEM images with the magnifications of 250–5000. The functional groups of the samples were examined using a Fourier transform infrared spectrometer (FT-IR, Nexus, Nicolet, Madison, MI, USA). The surface electric potentials of the samples were measured with a zeta potential analyzer (ZETAPALS, Brookhaven Instruments Corporation, Brookhaven, MS, USA) in the pH range of 2.0–13.0. The amount of the dissociative CTAB in the filtrate was determined with an ultraviolet spectrophotometer (U-3010, Hitachi, Tokyo, Japan).

The contact angles (θ) of deionized water and diiodomethane (CH₂I₂) on the surfaces of the whiskers were measured by a dynamic contact angle tensiometer (OCA20, Dataphysics, Filderstadt, Germany). The surface energy (${\mathsf{\gamma }}_s^T$) of the whiskers and its dispersive component (${\mathsf{\gamma }}_s^d$) and polar component (${\mathsf{\gamma }}_s^p$) were figured out by the following equation [24]: ${\mathsf{\gamma }}_l^T\left(1+cos\theta \right)=2{\left({\mathsf{\gamma }}_l^d{\mathsf{\gamma }}_s^d\right)}^{0.5}+2{\left({\mathsf{\gamma }}_l^p{\mathsf{\gamma }}_s^p\right)}^{0.5}$, ${\mathsf{\gamma }}_l^T={\mathsf{\gamma }}_l^d+{ \mathsf{\gamma }}_l^p$, ${\mathsf{\gamma }}_s^T={\mathsf{\gamma }}_s^d+{ \mathsf{\gamma }}_s^p$, where ${\mathsf{\gamma }}_l^T$, ${\mathsf{\gamma }}_l^d$, ${\mathsf{\gamma }}_l^p$ are the surface energy of the immersion liquid, its dispersive component, and its polar component, respectively. For deionized water, ${\mathsf{\gamma }}_l^T$ = 72.8 mN·m⁻¹, ${\mathsf{\gamma }}_l^d$ = 21.8 mN·m⁻¹, and ${\mathsf{\gamma }}_l^p$ = 51.0 mN·m⁻¹. For diiodomethane, ${\mathsf{\gamma }}_l^T$ = 50.8 mN·m⁻¹, ${\mathsf{\gamma }}_l^d$ = 48.5 mN·m⁻¹, and ${\mathsf{\gamma }}_l^p$ = 2.3 mN·m⁻¹. Each measurement was repeated five times, and the average results were adopted.

4. Conclusions

α-CaSO₄·0.5H₂O whiskers with high aspect ratios were synthesized in the presence of CTAB. The increase of the CTAB concentration from 0 to 9.2 × 10⁻⁴ mol·L⁻¹ (CMC) led to the increase of the average aspect ratio of the whiskers from 80 to 430. The adsorption of the positively charged cationic CTAB on the negatively charged side facets of the whiskers provided a strong steric hindrance and reduced the surface energy of the side facets, which favored the one-dimensional growth of the whiskers along the c-axis, promoting the formation of whiskers with high aspect ratios. When the CTAB concentration was greater than the CMC (9.2 × 10⁻⁴ mol·L⁻¹), the whisker morphology and the adsorption quantity of CTAB remained unchanged with the increase of the CTAB concentration, since the extra CTAB tended to form micelles instead of being adsorbed on the whisker surfaces. Similar phenomena were also observed in other ATABs with varying alkyl chain lengths (C_nH_2n+1(CH₃)₃NBr, n = 10, 12, 14, 18).