Macroscopic Dehydration Control of ZnSO₄·7H₂O: Infrared and Raman Spectra of ZnSO₄ Hydrates

Abstract

With microscopic thermogravimetric analysis, it is difficult to prepare metastable intermediate phases with precise water contents during the dehydration process of hydrates, making it a challenge to acquire their related spectra. The gradual dehydration process of ZnSO₄·7H₂O proceeds through 7 → 6 → 4 → 1 → 0. Vibrational spectra of ZnSO₄ hydrates, especially ZnSO₄·6H₂O and ZnSO₄·4H₂O, have not been previously reported. By macroscopic thermogravimetric analysis of ZnSO₄·7H₂O, the dehydration process can be precisely controlled to produce a variety of ZnSO₄ hydrates with specific water contents. In this study, powder X-ray diffraction confirmed the purities of 7H₂O, 6H₂O, 4H₂O, 1H₂O and anhydrous ZnSO₄. IR and Raman spectra of ZnSO₄ hydrates were obtained and compared for the first time. Spectroscopic and crystallographic analysis revealed that structural similarity plays a key role in the 7 → 6 → 4 → 1 → 0 dehydration process. Macroscopic thermogravimetric analysis combined with powder X-ray diffraction is a valuable method for investigating the intermediate phases in the hydrate dehydration process.

1. Introduction

Inorganic salt hydrates can lose their crystallized water with increasing temperature or decreasing humidity [1,2,3]. The dehydration process can involve multiple steps before the hydrate reaches a thermodynamically stable state [4,5,6]. The dehydration and reverse processes of inorganic salt hydrates involve the absorption and release of a large amount of energy, which makes inorganic salt hydrates a potential candidate for thermal energy storage and release [7,8]. Understanding the multi-step dehydration process of inorganic salt hydrates is of great significance for understanding their fundamental properties and their applications as energy storage materials.

The main research methods for hydrate dehydration are X-ray diffraction (XRD) and microscopic thermal analysis (TA) techniques [9,10,11,12], such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). It is a challenge to combine powder XRD (PXRD) and microscopic TA, even in ex situ conditions, for the following reasons. (1) The atmosphere for PXRD testing is usually air at ambient humidity, and that for microscopic TA is usually dry air or N₂. These different atmospheres can make it difficult to discern changes in water content in ex situ conditions. (2) The sample mass for microscopic TA is milligram-scale, which is usually not large enough for PXRD, and is not easily transferred from the crucible. (3) Some inorganic salt hydrates do not have a clear dehydration step, and it is difficult to obtain intermediate phases with precise water contents in microscopic TA. For inorganic salt hydrates with clear dehydration steps, these problems are not critical. For example, the dehydration process of CuSO₄·5H₂O, a common standard material for calibrating DSC-TGA instruments [13], is 5 → 3 → 1 → 0, with clear dehydration steps [14]. The stability of CuSO₄·3H₂O, CuSO₄·H₂O and anhydrous CuSO₄ is sufficient in air. Thus, IR [15,16] and Raman [17] spectra of various CuSO₄ hydrates have been reported to align well with microscopic TGA. However, in the case of ZnSO₄·7H₂O (

Table 1. Some reported dehydration processes of ZnSO₄·7H₂O.

Year	Process	Method	Refs.
1953	7 → 6 → 1	DTA	[21]
1955	7 → 1 → amorphous	XRD (vacuum)	[22]
1969	7 → 6 → 1	TGA-DTA	[23]
1975	7 → 6 → 4 → 1	DSC, DTA, TGA	[18]
1982	7 → 6 → 1	Calorimetric containers of fused silica	[24]
1997	7 → 6 → 1	DTA, DTG, TG (5 K·min−1, 20–1000 °C)	[25]
2005	7 → 6 → 4 → 1	Neutron PXRD and single-crystal XRD	[19]
2006	7 → 6 → 1 → 0	DTA, TG (5 K·min−1)	[26]
2011	7 → 1	TGA-DTA (30 °C·min−1 in air)	[27]
2012	7 → 6 → 4 → 1	In situ diffraction	[20]
2013	7 → 1 → 0	TPD-MS-SMB	[28]
2014	7 → 6 → 1	TG-DTA, fluidized bed drier	[29]
2014	7 → 6 → 1 → X → 0	TG-DTA (5 K·min−1, 25–1100 °C, air)	[30]
2018	7 → 6 → 1	Water vapor sorption analyzer, microbalance	[31]
2020	7 → 5 → 1	DSC-TG (1 K·min−1, 25–150 °C, N2)	[32]
2022	7 → 6 → 1	All-fiber terahertz time-domain spectroscopy	[33]
2022	7 → 6 → 1	TGA (10 K·min−1, 25–1000 °C, N2)	[34]

), the gradual dehydration process proceeds through 7 → 6 → 4 → 1 → 0, which has been confirmed by microscopic TA [18] and diffraction methods [19,20]. Dehydration from ZnSO₄·7H₂O to ZnSO₄·H₂O is only a continuous process in microscopic TGA, making it challenging to isolate intermediate products with precise compositions. The single-crystal data of ZnSO₄·7H₂O, 6H₂O, 4H₂O, 1H₂O and anhydrous ZnSO₄ have been reported (

Table 2. Unit cell parameters of ZnSO₄·7H₂O, ZnSO₄·6H₂O, ZnSO₄·4H₂O, ZnSO₄·H₂O and anhydrous ZnSO₄.

	7H2O	6H2O	4H2O	H2O	Anhydrous
Year	1959	1979	2001	1991	1978
ICSD No.	107287	41708	280878	71348	2456
System	Orthorhombic	Monoclinic	Monoclinic	Monoclinic	Cubic
Space Group	P212121	C2/c	P21/n	C2/c	F-43m
a (Å)	11.779	9.981	5.904	6.925	7.176
b (Å)	12.050	7.250	13.519	7.591	7.176
c (Å)	6.822	24.280	7.883	7.635	7.176
α (°)	90	90	90	90	90
β (°)	90	98.45	90.26	118.19	90
γ (°)	90	90	90	90	90

), but the IR and Raman spectra of ZnSO₄·6H₂O and ZnSO₄·4H₂O have not been reported.

Macroscopic TGA is widely used for determining the moisture content in food [35,36]. Different from TGA, DSC and DTA, macroscopic TGA is operated with samples of mass in the order of grams under air conditions. This method enables the dehydration process to be interrupted at any time and the product to be transferred. Using grams of sample can enhance the reproducibility of the experiment and the storage stability of the sample, which is conducive to the combination of this method with PXRD and spectroscopic analysis.

In this work, macroscopic TGA was successfully used for the preparation of various ZnSO₄ hydrates with precise water contents. PXRD confirmed that the prepared ZnSO₄·7H₂O, ZnSO₄·6H₂O, ZnSO₄·H₂O and anhydrous ZnSO₄ are pure and ZnSO₄·4H₂O has high purity with a small amount of ZnSO₄·6H₂O and ZnSO₄·H₂O. IR and Raman spectra of the various ZnSO₄ hydrates were obtained for the first time. Spectroscopic and crystallographic analysis indicate that the gradual dehydration of ZnSO₄·7H₂O through 7 → 6 → 4 → 1 → 0 is consistent with the principle of structural similarity. The combination of macroscopic TGA analysis and PXRD has advantages in preparing the intermediate phases with precise water contents, determining the polymorphs of the intermediate phases and obtaining the spectra of the intermediate phases in the dehydration process.

2. Materials and Methods

2.1. Materials

Zinc sulfate heptahydrate (ZnSO₄·7H₂O, 99.5%) was bought from Xilong Chemical Co. Raw ZnSO₄·7H₂O would dehydrate during storage, and the fresh ZnSO₄·7H₂O used in this work was prepared by the following method. A total of 5 g of ground raw ZnSO₄·7H₂O was totally dissolved in 10 mL of water, and then the solution was evaporated in air (26 °C, ~60% humidity) until most of the solution had crystallized. The crystals were collected by vacuum filtration, and the moisture on the surface of the crystals was absorbed with dry filter paper.

2.2. Methods

Dehydration Sample Preparations. ZnSO₄·6H₂O was obtained after 1 g of fresh ZnSO₄·7H₂O lost its 6.3% moisture content on a thermobalance (MB120ZH, OHAUS, Changzhou, China) set at 50 °C. The dehydration sample, mainly composed of ZnSO₄·4H₂O (ZnSO₄·4H₂O*), was obtained by heating 1 g of fresh ZnSO₄·7H₂O with 18.9% water lost on a thermobalance set at 70 °C. ZnSO₄·H₂O was obtained while 1 g of fresh ZnSO₄·7H₂O lost its 37.8% moisture content on a thermobalance set at 150 °C. Anhydrous ZnSO₄ was obtained after dehydration treatment for two hours at 350 °C on a hot stage (HP380-Pro, DLAB SCIENTIFIC, Beijing, China) with 1 g of raw ZnSO₄·7H₂O.

Spray Drying of ZnSO₄ Aqueous Solution. A total of 25 mg·mL⁻¹ ZnSO₄ aqueous solution was spray dried (Shanghai Pilotech YC-015, feed rate 1.5–2 L·h⁻¹, inlet temperature 200 °C, outlet temperature ~110 °C, atomization air pressure 24 kg·cm⁻², drying air flow rate 30 m³·h⁻¹).

Conventional Characterization. ZnSO₄ samples were characterized with differential scanning calorimetry–thermogravimetric analysis (DSC-TGA, NETZSCH STA 449F3, NETZSCH, Germany, dry N₂, 10 K·min⁻¹), powder X-ray diffraction (PXRD, Philips X’Pert Pro, PANalytical, Netherlands, Cu Kα, 40 kV, 30 mA, 10–30°, 4°·min⁻¹ and Rigaku SmartLab, Cu Kα, 40 kV, 50 mA, 10–30°, 5°·min⁻¹), Fourier transform infrared spectroscopy (IR, Shimadzu IRAffinity-1S, Shimadzu, Japan, absorbance, 400–4000 cm⁻¹) and confocal Raman spectroscopy (Thermo Fisher Scientific, USA, DXR3xi, 532 nm, 40 mW, 0.002–0.02 s, 1000 scans of 50–3400 cm⁻¹, 50× objective lens).

Difference Spectra Analysis. The IR spectrum of ZnSO₄·4H₂O* is a composite spectrum of the main ZnSO₄·4H₂O and a small amount of ZnSO₄·6H₂O and ZnSO₄·H₂O. The Raman spectrum of ZnSO₄·4H₂O* is equal to that of ZnSO₄·4H₂O because the Raman spectroscopy was obtained from a micro area. The Raman spectrum of ZnSO₄·4H₂O in the ZnSO₄·4H₂O* sample can be easily identified, which is different from that of ZnSO₄·6H₂O and ZnSO₄·H₂O. All the obtained spectra were normalized for difference spectra analysis. Briefly, IR bands at 1084 cm⁻¹ of ZnSO₄ saturated aqueous solution, 1053 cm⁻¹ of ZnSO₄·7H₂O and 6H₂O, 1063 cm⁻¹ of ZnSO₄·4H₂O*, 1082 cm⁻¹ of ZnSO₄·H₂O, and 1050 cm⁻¹ of anhydrous ZnSO₄ were used for normalization, and Raman bands at 982 cm⁻¹ of aqueous solution and ZnSO₄·6H₂O, 983 cm⁻¹ of ZnSO₄·7H₂O, 991 cm⁻¹ of ZnSO₄·4H₂O, 1020 cm⁻¹ of ZnSO₄·H₂O, and 1040 cm⁻¹ of anhydrous ZnSO₄ were used for normalization. Difference spectra were obtained by the subtraction of two related spectra with further baseline deduction based on a polynomial fitting method. Absolute deviation of the values in the difference spectrum (a.d.) was used to compare the similarity.

$$a.d.={\displaystyle \frac{\sum \left|x\right|}{n}}$$

Each sample was characterized three times individually, and the corresponding nine difference spectra can be obtained. Further, the error of the a.d. can be calculated.

3. Results

3.1. Microscopic DSC-TGA and Macroscopic TGA of ZnSO₄·7H₂O

Fresh ZnSO₄·7H₂O was prepared via slowly evaporating the aqueous solution of ZnSO₄. The microscopic TGA curve of fresh ZnSO₄·7H₂O with a heating rate of 10 K·min⁻¹ reveals a two-stage dehydration process within the temperature range of 25–400 °C (Figure 1a). The first dehydration step takes place below 200 °C with a weight loss of 37.1%, confirmed as the 7 → 1 dehydration process with theoretical weight loss of 37.6%. The second dehydration step occurs in the temperature range of 200–400 °C with a weight loss of 7.6%, similar to the 1 → 0 dehydration process with theoretical weight loss of 6.3%. The microscopic TGA curve does not provide additional insights about the 7 → 1 dehydration process even with a slow heating rate of 1 K·min⁻¹ (Figure S1). The corresponding DSC curve exhibits four distinct endothermic peaks at 56, 95, 146, and 315 °C, indicating a four-step dehydration process denoted as 7 → X₁ → X₂ → 1 → 0. X₁ and X₂ are two unknown intermediate phases. The X₁ phase can be obtained because the endothermic peak corresponding to the transition from 7H₂O to X₁ is isolated. The synthesis of the X₂ phase with good purity seems to be rather challenging because the endothermic peaks of X₁ → X₂ and X₂ → 1H₂O transformation processes are overlapping even with a slow heating rate of 1 K·min⁻¹. The X₂ phase might be unstable and easy to transform into ZnSO₄·H₂O. Macroscopic TGA curves of ZnSO₄·7H₂O at different temperatures only show one continuous step in a 7 → 1 process (Figure 1b), similar to the microscopic TGA curves. The weight loss of ZnSO₄·7H₂O is 36.0% at 50 °C and 100 °C (ZnSO₄ + 1.2H₂O) and 37.8% (ZnSO₄ + 1.0H₂O) at 150 °C. The macroscopic TGA curves reach equilibrium within 10 min at 100 and 150 °C, which is too rapid to capture intermediate phases. The macroscopic TGA curve obtained at 50 °C shows a gradual and nearly linear weight loss, which is conducive to obtaining intermediate products with precise compositions (e.g., ZnSO₄ + 6.0H₂O, ZnSO₄ + 4.0H₂O).

3.2. Macroscopic Dehydration Control of ZnSO₄·7H₂O

Four phases, including ZnSO₄·7H₂O, ZnSO₄·6H₂O, ZnSO₄·H₂O and anhydrous ZnSO₄, can be prepared with good purity and robust repeatability via macroscopic TGA, confirmed by the simulated PXRD patterns of the pure phases. The characteristic 2θ peaks of ZnSO₄ hydrates are compared and identified (Table S1). The fresh ZnSO₄·7H₂O is confirmed again from the PXRD pattern (Figure 2a). The sample of ZnSO₄ + 6.0H₂O is the product of fresh ZnSO₄·7H₂O with 6.3% weight loss in the macroscopic TGA carried out at 50 °C, whose PXRD pattern aligns with the simulated PXRD pattern of ZnSO₄·6H₂O (Figure 2b). ZnSO₄ + 1.0H₂O is the equilibrium product of ZnSO₄·7H₂O in the macroscopic TGA carried out at 150 °C, and its PXRD pattern agrees with the simulated PXRD pattern of ZnSO₄·H₂O (Figure 2c). The equilibrium product of ZnSO₄·7H₂O in the macroscopic TGA at 50 °C is ZnSO₄ + 1.2H₂O, and its PXRD pattern is also consistent with that of ZnSO₄·H₂O. The water content of ZnSO₄·H₂O can be variable. Anhydrous ZnSO₄ is the equilibrium product of the macroscopic TGA at 350 °C confirmed by the PXRD pattern (Figure 2d). After being stored in a sealed environment for one week, the PXRD patterns of these four phases remained unchanged, indicating good storage stability. Through macroscopic TGA, the exact water contents of ZnSO₄·6H₂O and ZnSO₄·H₂O can be precisely determined and controlled, which is an advantage that other methods do not possess.

ZnSO₄ samples with precise water contents of 5.0H₂O, 4.0H₂O, 3.0H₂O and 2.0H₂O can be easily prepared by macroscopic TGA at 50 °C. After a careful comparison, we found that the PXRD patterns of these fresh samples only show the characteristic 2θ peaks of ZnSO₄·6H₂O and ZnSO₄·H₂O (Figure 3a). The characteristic 2θ peaks of ZnSO₄·4H₂O at 13.0, 22.5, 24.8, 27.4° are all not observed. The diffraction intensity ratio of the peak at 20.3° of ZnSO₄·6H₂O and peak at 26.2° of ZnSO₄·H₂O shows a good linear relationship with the mass ratio of ZnSO₄·6H₂O and ZnSO₄·H₂O (Figure S2), indirectly confirming that these samples only contain ZnSO₄·6H₂O and ZnSO₄·H₂O. ZnSO₄·4H₂O is a distinct kinetic intermediate phase, which can only be obtained under a suitable dehydration rate. The ZnSO₄ + 4.0H₂O sample can also be easily prepared by conducting macroscopic TGA at a higher temperature (Figure 3b). When the operation temperature was 60 °C, a small amount of ZnSO₄·4H₂O appeared. ZnSO₄·4H₂O became the main product when macroscopic TGA was carried out at 70–80 °C. The proportion of ZnSO₄·4H₂O declined at the operation temperature of 90 °C. The ZnSO₄ + 4.0H₂O obtained at 70 °C (ZnSO₄·4H₂O*) was selected to study the physical and chemical properties of ZnSO₄·4H₂O due to its high content of ZnSO₄·4H₂O and the low operation temperature. By carefully comparing the PXRD patterns, it can be concluded that ZnSO₄·4H₂O* is a mixture of primary ZnSO₄·4H₂O, ZnSO₄·6H₂O and ZnSO₄·H₂O. The composition of ZnSO₄ + 5.0H₂O obtained at 70 °C is mainly ZnSO₄·6H₂O with a small amount of ZnSO₄·4H₂O (Figure 3c). The ZnSO₄ + 4.0H₂O obtained from ZnSO₄·6H₂O at 70 °C is also mainly composed of ZnSO₄·4H₂O, similar to ZnSO₄·4H₂O*. The direct transformation from ZnSO₄·7H₂O to ZnSO₄·4H₂O was not observed. ZnSO₄·4H₂O was mainly formed by the dehydration of ZnSO₄·6H₂O. Thus, we consider that for the 7 → X₁ → X₂ → 1 → 0 dehydration process indicated by microscopic DSC-TGA mentioned above, X₁ is ZnSO₄·6H₂O and X₂ is ZnSO₄·4H₂O. The gradual dehydration process of ZnSO₄·7H₂O can be 7 → 6 → 4 → 1 and 7 → 6 → 1 in this work. ZnSO₄·4H₂O* was not stable and would transform into a product composed of ZnSO₄·6H₂O (main) and ZnSO₄·H₂O even after only one day of storage (Figure 3d). ZnSO₄·4H₂O was hardly detectable in the PXRD pattern after storage for 5 days. ZnSO₄·4H₂O* should be characterized as soon as possible to avoid its polymorphism transition.

3.3. Vibrational Spectroscopic Analysis

The spectroscopic method has been widely used in the field of hydrates [37]. Spectroscopic techniques are usually non-destructive, and can provide information at the molecular levels, which is helpful for understanding the dehydration process. The limitation of the spectroscopic method lies in the fact that without the PXRD results or standard spectra, the crystal form cannot be determined. The combination of macroscopic TGA and PXRD makes it possible to study the dehydration of ZnSO₄·7H₂O by infrared (IR) and Raman spectroscopy.

Mid-frequency IR spectra (in the range of 400–1800 cm⁻¹, Figure 4a,

Table 3. Mid-frequency IR bands of ZnSO₄ samples.

Solution	7H2O	6H2O	4H2O*	1H2O	Anhydrous	Assignment [38]
					470
	561	557	536	534
601	602	597	600	601	599	δas-${\mathrm{SO}}_4^{2-}$
				623
			665	665	689
			861	855
980	982	982	984		992	ν1-${\mathrm{SO}}_4^{2-}$
			1017	1017
	1053	1053	1063		1050
1084				1082		ν3-${\mathrm{SO}}_4^{2-}$
	1142				1129
			1504	1504
1643	1661	1651	1632			δ-OH

) show the main signals of ZnSO₄ samples, while high-frequency IR spectra do not provide much information (Figure S3). The IR spectrum of ZnSO₄ saturated aqueous solution shows two strong bands at 1084 cm⁻¹ (ν₃-${\mathrm{SO}}_4^{2-}$, asymmetric stretching vibration band) and 1643 cm⁻¹ (δ-OH), and two weak bands at 601 cm⁻¹ (δ_as-${\mathrm{SO}}_4^{2-}$, asymmetrical angular vibration band) and 980 cm⁻¹ (ν₁-${\mathrm{SO}}_4^{2-}$, symmetric stretching vibration band) [17,38]. ν₃ at 1053 cm⁻¹ is observed in ZnSO₄·7H₂O and ZnSO₄·6H₂O, and shifts to 1063 cm⁻¹ for ZnSO₄·4H₂O* and 1082 cm⁻¹ for ZnSO₄·H₂O. δ-OH exists in ZnSO₄·7H₂O, 6H₂O and 4H₂O*, and disappears in ZnSO₄·H₂O and anhydrous ZnSO₄. The IR spectrum of anhydrous ZnSO₄ is quite different from those of the other samples, indicating a different molecular packing pattern. IR difference spectra analysis was used to establish the similarity orders among ZnSO₄ samples, as reported in our previous work [39]. The absolute deviations of values in the IR difference spectra show that the similarity can be easily divided into three regions (Figure 4b). The most similar region is 7–6. The second similar region includes aq-7, aq-6, aq-4,7-4, 6-4, and 4-1. The other comparisons show poor similarities.

Low-frequency Raman spectroscopy (LFRS, below 300 cm⁻¹) is associated with lattice vibrations, which is often used to distinguish polymorphs [40,41]. The LFRS of ZnSO₄ aqueous solution does not show obvious bands, confirming the features of the solution. The LFRS values of ZnSO₄·7H₂O (59, 69, 107, 127, 193, 220, 238 cm⁻¹), ZnSO₄·6H₂O (66, 96, 111, 133, 221, 236 cm⁻¹), ZnSO₄·4H₂O (52, 78, 102, 126, 149, 178, 221, 277 cm⁻¹), ZnSO₄·H₂O (112, 171, 218, 278 cm⁻¹) and anhydrous ZnSO₄ (95, 139, 184 cm⁻¹ with a poor signal-to-noise ratio) show different characteristic Raman bands (Figure 5).

Mid-frequency Raman spectra (MFRS, range of 300–1800 cm⁻¹, Figure 6a,

Table 4. Mid-frequency Raman bands of ZnSO₄ samples.

Solution	7H2O	6H2O	4H2O	1H2O	Anhydrous	Assignment [43,44]
448	442		421	420	442	ν2-${\mathrm{SO}}_4^{2-}$
		466	483		468
				503	496
613	612	619	624	623	600	ν4-${\mathrm{SO}}_4^{2-}$
					691
982	983	982	991		993	ν1-${\mathrm{SO}}_4^{2-}$
		1020	1022	1020	1040
	1058		1073	1071
1104	1094	1080		1082	1098	ν3-${\mathrm{SO}}_4^{2-}$
	1140	1144	1151
			1188	1188	1186
					1218

) showing the molecular vibration signals with a good signal-to-noise ratio are commonly used for compound identification [42], whereas high-frequency Raman spectra do not provide useful information (Figure S4). The MFRS of ZnSO₄ solution shows one strong band at 982 cm⁻¹ (ν₁-${\mathrm{SO}}_4^{2-}$), and three weak bands at 448, 613, and 1104 cm⁻¹, corresponding to ν₂-${\mathrm{SO}}_4^{2-}$ (depolarized deformation modes), ν₄-${\mathrm{SO}}_4^{2-}$ (depolarized deformation modes), and ν₃-${\mathrm{SO}}_4^{2-}$ [43,44]. The MFRS values of ZnSO₄ solution, ZnSO₄·7H₂O, 6H₂O, 4H₂O and 1H₂O are similar in visual. The strong ν₁ of ZnSO₄ solution, ZnSO₄·7H₂O and 6H₂O is at 982 cm⁻¹, and shifts to 991 cm⁻¹ for 4H₂O and 1020 cm⁻¹ for 1H₂O. The MFRS of anhydrous ZnSO₄ is different from those of the other ZnSO₄ samples. Mid-frequency Raman difference spectra analysis was also used to establish the similarity orders [39] among ZnSO₄ samples. The absolute deviations of the values in Raman difference spectra show that the similarity can be divided into four regions (Figure 6b). The most similar region is aq-7, aq-6, and 7-6. The second similar region includes aq-4, 7-4, and 6-4. The third similar region includes aq-1, 7-1, and 6-1. The comparison of 4-1 is between the second region and the third region. The other comparisons show poor similarities.

3.4. Crystallographic Analysis

The crystal structure of ZnSO₄·7H₂O (Figure 7a) reveals the presence of two distinct zinc ions in the unit cell. The coordination environments of the two Zn²⁺ are identical, coordinated with six water molecules in the form of [Zn(H₂O)₆]²⁺, but the orientations of the two [Zn(H₂O)₆]²⁺ are different. ${\mathrm{SO}}_4^{2-}$ does not coordinate with Zn²⁺. ZnSO₄·7H₂O can be represented as [Zn(H₂O)₆]SO₄·H₂O. The two O atoms in the sulfate group are connected to the water molecule of two [Zn(H₂O)₆]²⁺ molecules with hydrogen bonding, forming a zigzag one-dimensional chain structure denoted as [Zn(H₂O)₆SO₄]_n. The additional water molecule is linked to [Zn(H₂O)₆]²⁺ and ${\mathrm{SO}}_4^{2-}$ with hydrogen bonding. Although the crystal structure of ZnSO₄·6H₂O (Figure 7b) is different from that of ZnSO₄·7H₂O, the coordination environments of Zn²⁺ and ${\mathrm{SO}}_4^{2-}$ in ZnSO₄·6H₂O are almost the same as those in ZnSO₄·7H₂O, which can be represented as [Zn(H₂O)₆]SO₄. The unit cell of ZnSO₄·6H₂O contains two [Zn(H₂O)₆]²⁺ molecules with different orientations. The crystal structure of ZnSO₄·4H₂O (Figure 7c) reveals that the coordination environment of Zn²⁺ is [Zn(H₂O)₄(1/2SO₄)₂] with two orientations. The two O atoms of ${\mathrm{SO}}_4^{2-}$ are coordinated with a Zn²⁺ ion. The crystal structure of ZnSO₄·H₂O (Figure 7d) reveals that the coordination environment of Zn²⁺ changes [Zn(1/2H₂O)₂(1/4SO₄)₄], the O atom of water is coordinated with two Zn²⁺ ions and each O atom of ${\mathrm{SO}}_4^{2-}$ is coordinated with one Zn²⁺ ion. The crystal structure of anhydrous ZnSO₄ (Figure 7e) has a high degree of symmetry compared to ZnSO₄ hydrates. The coordination environment of Zn²⁺ can be written as [Zn(1/4SO₄)₄] which is different from the hexacoordinated Zn²⁺, and the coordination environment of ${\mathrm{SO}}_4^{2-}$ is similar to that in ZnSO₄·H₂O. The significant differences of the Zn²⁺ coordination environment between ZnSO₄·H₂O and anhydrous ZnSO₄ make the direct transformation between them difficult, which can explain the appearance of a liquid intermediate phase [45].

3.5. Similarity Analysis

Similarity analysis based on different methods may yield different results. It is necessary to adopt various methods to perform the structural similarity analysis. Herein, vibrational spectroscopy and crystal structure are used to determine the similarity order. IR difference spectra indicate that 7H₂O and 6H₂O are most similar, suggesting a kinetic dehydration process of 7 → 6. 4H₂O is the most similar to 6H₂O in the dehydration process of 6H₂O, suggesting a kinetic process of 6 → 4. The kinetic process of 4 → 1 is proposed with similar analysis. The IR spectra analysis suggests that the gradual dehydration process of 7H₂O is 7 → 6 → 4 → 1 → 0 (Figure 8a). Raman difference spectra analysis shows similar results to IR difference spectra analysis, except in the dehydration process of 6H₂O; the kinetic process can be 6 → 4 or 6 → 1. The Raman spectra analysis suggests that the gradual dehydration process of 7H₂O is 7 → 6 → 4 → 1 → 0 and 7 → 6 → 1 → 0 (Figure 8b). The simplified coordination environments of Zn²⁺ in various crystal structures are schematized and compared (Figure 8c). The simplified Zn²⁺ environments are [Zn(H₂O)₆]²⁺ in 7H₂O and 6H₂O, [Zn(H₂O)₄(1/2SO₄)₂] in 4H₂O, [Zn(1/2H₂O)₂(1/4SO₄)₄] in 1H₂O, and [Zn(1/4SO₄)₄] in anhydrous ZnSO₄. The coordination environment analysis of Zn²⁺ suggests that the gradual dehydration process of 7H₂O is 7 → 6 → 4 → 1 → 0. The simplified coordination environment of ${\mathrm{SO}}_4^{2-}$ suggests that the gradual dehydration process of 7H₂O is 7 → 6 → 4 → 1 → 0 and 7 → 6 → 4 → 0 (Figure 8d). Both the similarity analysis based on vibrational spectroscopy and crystal structure suggest that the kinetic gradual dehydration process of ZnSO₄·7H₂O is 7 → 6 → 4 → 1 → 0.

4. Discussion

4.1. Macroscopic TGA Combined with PXRD

Macroscopic TGA and PXRD are very common methods in the laboratory and industry. The combination of macroscopic TGA and PXRD is a feasible method that enables the acquisition of intermediate products with precise water content and accurate polymorph during the dehydration process of hydrates. Macroscopic TGA ensures the precise determination of compositions, while PXRD guarantees the accurate identification of crystalline forms. Macroscopic TGA conducted under different conditions may induce different dehydration processes and result in different products. Products with precise compositions and accurate polymorphs are conducive to obtaining the fundamental spectra, which is difficult to achieve through other methods. Acquisition of these fundamental data facilitates the proposal of dehydration mechanisms and designs of the thermal energy storage and release systems based on ZnSO₄·7H₂O [46]. The method combining macroscopic TGA with PXRD is also suitable for capturing the intermediate phases during the desolvation process of solvates and the solvent evaporation crystallization process.

4.2. Preparations of ZnSO₄·4H₂O

ZnSO₄·4H₂O single crystal can be prepared via vapor phase diffusion of diethyl ether into a reflux acetonitrile solution of hydrated zinc tetrafluoroborate and 2,6-bis(2-mercapto-1-methylimidazole)pyridine after a period of seven months [47], showing that it is difficult to prepare ZnSO₄·4H₂O. Based on the results of similarity analysis, it is possible to obtain ZnSO₄·4H₂O at room temperature with a suitable humidity through the 7 → 6 → 4 dehydration process. The PXRD patterns indicate that under atmospheric conditions (26 °C, 60% humidity), ZnSO₄·7H₂O mainly transforms into ZnSO₄·6H₂O after 3 days and into ZnSO₄·4H₂O with a small amount of ZnSO₄·H₂O after 5 days (Figure 9a). The sample mainly composed of ZnSO₄·4H₂O remained stable even after being exposed to the air for one month. IR and Raman spectroscopic analyses also indicate that it is possible to crystalize ZnSO₄·4H₂O from ZnSO₄ aqueous solution. The spray drying product of ZnSO₄ aqueous solution is a mixture of ZnSO₄·4H₂O and ZnSO₄·H₂O (Figure 9b). Similarity principle can guide the preparation methods of metastable intermediate phases. However, neither of these two methods can prepare pure phase of ZnSO₄·4H₂O with accurate composition. The macroscopic dehydration of ZnSO₄·7H₂O at 70 °C to ZnSO₄ + 4.0H₂O is a quick and feasible method for the preparation of ZnSO₄·4H₂O.

5. Conclusions

In the dehydration process of ZnSO₄·7H₂O, intermediate hydrates with precise water contents, including ZnSO₄·6H₂O, ZnSO₄·4H₂O and ZnSO₄·H₂O, were prepared by macroscopic TGA and confirmed by PXRD. IR and Raman spectra of the various ZnSO₄ hydrates were obtained and compared for the first time. IR, Raman spectroscopic analysis and crystallographic analysis explain the gradual dehydration process of 7 → 6 → 4 → 1 → 0 based on the principle of similarity. Macroscopic TGA combined with PXRD is a feasible method for preparing intermediate hydrates with precise water contents and accurate polymorphs, and obtaining the related spectra in the dehydration process of hydrates.