(T1330-09-56) Sodium Sulfide Nonahydrate (Na₂S·9H₂O), a Hydrogen Sulfide (H₂S) Donor and Its Solid-State Characterization

Purpose: Sodium sulfide nonahydrate (Na₂S·9H₂O) is widely employed in thermochemical heat storage systems due to its high enthalpy of hydration and reversible dehydration behavior, making it effective for energy storage applications [1–3]. Interestingly, beyond its role in energy systems, sodium sulfide has also gained attention in the biomedical field as an inorganic hydrogen sulfide (H₂S) donor. Upon contact with water, it rapidly dissociates to release H₂S, a biologically active gas known to play key roles in physiological processes such as neuroprotection, cardiovascular modulation, and in glaucoma treatment [4,5]. However, due to its hygroscopicity and thermal instability, it is crucial to understand its solid-state properties (e.g. phase transitions, dehydration patterns, and overall stability) critically needed for developing a reliable delivery system for providing a low but sustained level of H₂S.

Methods: The thermal behavior of Na₂S·9H₂O (98% extra pure) was investigated using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Hot Stage Microscopy, and Variable Temperature X-ray Powder Diffraction (VTXRD). For DSC measurements, approximately 3–5 mg of the sample was sealed in an aluminum pan with a hermetic lid and heated from 25°C to 280°C at a rate of 5°C/min under nitrogen purge. TGA was performed using 18 mg of the sample placed in an open platinum pan, heated under identical conditions (5°C/min up to 280°C under nitrogen purge). Hot Stage Microscopy was used to observe any physical transformations such as melting, crystallization, and dehydration. The sample was placed on a glass slide with silicone oil immersion, covered with a cover slip, and heated to 280°C at a rate of 5°C/min while being visually observed. VTXRD [(Rigaku SmartLab, Japan) equipped with cross-beam optics, HyPix-3000 detector, and SmartLab Studio II software] was utilized to identify crystalline phase transitions occurring during thermal treatment. The sample was heated up to 280°C at a rate of 5°C/min, and diffraction patterns were recorded at selected temperature.

Results: The DSC thermogram of Na₂S·9H₂O (Fig.1.A) showed an endothermic peak at 36°C, which can be attributed to the onset of dehydration of the surface bound water. This was followed by an exothermic peak at around 56°C, corresponding to simultaneous recrystallization into lower hydrates because of partial dehydration during melting. Beyond this temperature, a broad exothermic transition was observed between 76°C and 200°C, suggesting a stepwise conversion of nonahydrate to lower hydrates. However, the precise interpretation of this broad peak remains inconclusive due to the complex interplay of overlapping endothermic and exothermic processes occurring simultaneously. The TGA curve (Fig.1.B) displayed a gradual stepwise weight losses, with major weight loss at 84°C and 180°C, corresponding to the progressive release of water of crystallization. The final weight loss near 180°C confirms the formation of anhydrous Na₂S. However, the DSC curve of the triturated Na₂S·9H₂O sample (Fig.1.C) showed multiple overlapping endothermic events within the broad exothermic peak up to 200°C, indicating complex and irregular thermal events arising from reduced crystallinity due to trituration. When Na₂S·9H₂O crystals were placed in a desiccator for 12 hours, all the thermal events were shifted to a higher temperature (Fig.1.D). This shift is likely due to the loss of adsorbed moisture from the hygroscopic Na₂S·9H₂O crystals. Hot-stage microscopy revealed melting of nonahydrate crystals followed by recrystallization into small tetragonal crystals, indicating conversion to lower hydrates around 51–55°C. Water bubble evolution began around 140°C and continued beyond 200°C, supporting the broad exothermic peak associated with multiple thermal events (exothermic and endothermic) occurring simultaneously (Fig.2). VTXRD analysis revealed a continuous change in diffraction patterns starting at 35°C, indicating progressive dehydration and recrystallization of Na₂S·9H₂O to lower hydrates up to 200°C (Fig.3).

Conclusion: Sodium sulfide nonahydrate (Na₂S·9H₂O) exhibits a complex thermal behavior involving melting, recrystallization, and stepwise dehydration. DSC, TGA, VTXRD, and hot-stage microscopy collectively revealed transition of nonahydrate to lower hydrates and eventual formation of anhydrous Na₂S around 180–200°C. Trituration and desiccation influenced thermal transitions by altering crystallinity and adsorbed moisture content, respectively.

References: [1] de Boer R, Haije W G and Veldhuis J B J Determination of structural, thermodynamic and phase properties in the Na2S–H2O system for application in a chemical heat pump
[2] Roelands M, Cuypers R, Kruit K D, Oversloot H, De Jong A-J, Duvalois W, Van Vliet L and Hoegaerts C 2015 Preparation & Characterization of Sodium Sulfide Hydrates for Application in Thermochemical Storage Systems Energy Procedia 70 257–66
[3] Aandersson J Y, De Pablo J and Azoulay M 1985 Kinetics of the rehydration of sodium sulphide dehydrated in situ, under formation of its pentahydrate Thermochim. Acta 91 223–34
[4] Zheng Y, Yu B, De La Cruz L K, Roy Choudhury M, Anifowose A and Wang B 2018 Toward Hydrogen Sulfide Based Therapeutics: Critical Drug Delivery and Developability Issues Med. Res. Rev. 38 57–100
[5] Mhatre S, Opere C A and Singh S 2022 Unmet needs in glaucoma therapy: The potential role of hydrogen sulfide and its delivery strategies J. Controlled Release 347 256–69

Acknowledgements: Financial support from School of Pharmacy and Health Professions, Creighton University. Erik Moore for the VTXRD studies at the Drug Development and Characterization Core facility. Conflict of Interest Statements: We confirm that there are no conflicts of interest associated with this work and there has been no significant financial support for this work that could have influenced its outcome.

Figure 1: A: DSC Thermogram of untreated Sodium sulfide nonahydrate Crystals ; B:TGA Curve of untreated Sodium sulfide nonahydrate Crystals; C: DSC Thermogram of triturated Sodium sulfide nonahydrate Crystals; D: DSC Thermogram of Sodium sulfide nonahydrate Crystals placed in desiccator for 12 hours

Figure 2: Hot Stage Microscopy images of untreated Sodium sulfide nonahydrate Crystals in silicon oil

Figure 3: VTXRD Diffraction patterns of Sodium sulfide nonahydrate Crystals