Vapor Pressure and Enthalpy of Vaporization of Guanidinium Methanesulfonate as a Phase Change Material for Thermal Energy Storage

Abstract

This paper reports the vapor pressure and enthalpy of vaporization for a promising phase change material (PCM) guanidinium methanesulfonate ([Gdm][OMs]), which is a typical guanidinium organomonosulfonate that displays a lamellar crystalline architecture. [Gdm][OMs] was purified by recrystallization. The elemental analysis and infrared spectrum of [Gdm][OMs] confirmed the purity and composition. Differential scanning calorimetry (DSC) also confirmed its high purity and showed a sharp and symmetrical endothermic melting peak with a melting point (T_m) of 207.6 °C and a specific latent heat of fusion of 183.0 J g⁻¹. Thermogravimetric analysis (TGA) reveals its thermal stability over a wide temperature range, and yet three thermal events at higher temperatures of 351 °C, 447 °C, and 649 °C were associated with vaporization or decomposition. The vapor pressure was measured using the isothermogravimetric method from 220 °C to 300 °C. The Antoine equation was used to describe the temperature dependence of its vapor pressure, and the substance-dependent Antoine constants were obtained by non-linear regression. The enthalpy of vaporization (Δ_vapH) was derived from the linear regression of the slopes associated with the linear temperature dependence of the rate of weight loss per unit area of vaporization. Hence, the temperature dependence of vapor pressures ln P_vap (Pa) = 10.99 − 344.58/(T (K) − 493.64) over the temperature range from 493.15 K to 573.15 K and the enthalpy of vaporization Δ_vapH = 157.10 ± 20.10 kJ mol⁻¹ at the arithmetic mean temperature of 240 °C were obtained from isothermogravimetric measurements using the Antoine equation and the Clausius–Clapeyron equation, respectively. The flammability test indicates that [Gdm][OMs] is non-flammable. Hence, [Gdm][OMs] enjoys very low volatility, high enthalpy of vaporization, and non-flammability in addition to its known advantages. This work thus offers data support, methodologies, and insights for the application of [Gdm][OMs] and other organic salts as PCMs in thermal energy storage and beyond.

1. Introduction

Thermal energy storage technologies employing phase change materials (PCMs) offer a promising solution for the intermittency of solar energy and industrial waste heat recovery and utilization [1,2,3,4,5]. PCMs can be divided simply into two categories: inorganic PCMs and organic PCMs [5].

Table 1. Some PCMs with melting points in the intermediate temperature range (100–230 °C).

PCMs	Tm (°C)	ΔfusH (J g−1)	ΔT (°C) [b]
MgCl2·6H2O [6]	123	117	41
Indium [7]	157	29	/
Selenium [7]	220	86	/
Erythritol [8,9]	118	340	86
D-dulcitol [11]	186	334	/
C3(mim)2(Br)2 [12]	173	116	46
[Pzy][OMs] [13]	168	160	/
[Pzy][OTf] [13]	147	17 ss, 24 ss, 27	/
[Pzy][C6H5SO3] [13]	137	105	/
[PyH][OMs] [14]	180	21 ss, 77	16
[PyH][OTf] [14]	226	24 ss, 38	41
[pyH][C6H5SO3] [14]	135	123	80
[Gdm][OMs] [15,16]	208	190	/
[Gdm][OTf] [15,16]	160	6 ss [a], 130	/
[Gdm][C6H5SO3] [15,16]	210	138	/
C2(mim)2(Br)2 [17]	188	116	24
C2(mim)2(PF6)2 [17]	191	109	/

^[a] ss: solid–solid phase transition. ^[b] ΔT: degree of supercooling, i.e., difference between T_m and crystallization temperature.

lists some PCMs with melting points in the intermediate temperature range between 100 °C and 230 °C. Among them, inorganic salt hydrates (e.g., MgCl₂·6H₂O) have poor cycling stability due to water separation and high supercooling degree [6]. Metals such as indium and selenium are unsuitable for use as large-scale PCMs due to their low latent heat of fusion (Δ_fusH) [7]. Organic PCMs such as erythritol and D-dulcitol have high values of Δ_fusH, but their significant supercooling is not conducive to heat release [8,9,10,11]. As a new kind of PCM, ionic liquids (or organic salts) have unique physicochemical properties, including non-flammability, low volatility, excellent thermal stability, tunable melting points, and high heat storage density, offering potential applications in thermal energy storage [12,13]. Currently, protic ionic liquids derived from pyridines [14], guanidines [15,16], pyrazoles [13], and imidazoles [12,17] have been explored as potential PCMs. Among them, guanidinium methanesulfonate ([Gdm][OMs], i.e., guanidinium mesylate, Figure 1) exhibits the highest Δ_fusH to date (T_m = 208 °C, Δ_fusH = 190 J g⁻¹, Table 1) with total volumetric energy storage measured as 622 MJ m⁻³ (173 kWh m⁻³) and an excellent cyclic stability after 420 cycles between 150 °C and 215 °C, making it the state-of-the-art PCM based on protic organic salts [15,16]. Overall, [Gdm][OMs] has been identified as a very promising PCM for inexpensive renewable energy storage applications at intermediate temperatures [7,15,16,18]. Moreover, [Gdm][OMs] is a representative guanidinium organomonosulfonate (GMS), which displays a series of lamellar crystalline architectures and a two-dimensional hydrogen-bonding network of complementary guanidinium ions (G) and sulfonate moieties (S), the so-called GS sheet [19]. Previous studies on [Gdm][OMs] as a PCM in terms of thermal energy storage include melting temperature, latent heat of fusion, heat capacity, thermal conductivity, volume change, advanced thermal stability, long-term cycling, and economic analysis [15,16]. However, so far, there has been no report on the vapor pressure and vaporization enthalpy of [Gdm][OMs] to the best of our knowledge.

The determination of the temperature-dependent vapor pressure of ionic liquids (ILs) is useful for their industrial application. A higher vapor pressure of the storage medium can lead to thicker walls and the potential loss of PCMs, increasing cost [5], and safety concerns for thermal energy storage containers. Hence, vapor pressure (P_vap) and enthalpy of vaporization (Δ_vapH) are not only important basic thermal properties but also key parameters for designing thermal energy storage systems, which are critical for the long-term safety of PCMs and thermal energy storage systems. In addition, Δ_vapH reflects molecular interactions in the liquid state and can serve as a foundation for calibrating and validating force fields in molecular dynamics simulations and as an anchoring parameter in P−V−T equations for neat ILs [20,21]. For example, Liu et al. [21] calculated the values of Δ_vapH of tetrabutylammonium bis(trifluoromethanesulfonyl)imide ([N₄₄₄₄][NTf₂]) and investigated its cohesive energy, enthalpy, and entropy. The Gibbs free energy of the transition from condensed state to gas was successfully obtained using Born–Fajans–Haber cycles.

In this work, the protic organic salt [Gdm][OMs] was prepared and purified via recrystallization (Figure 1). Its composition and purity were verified by elemental analysis and Fourier transform infrared spectroscopy (FT-IR). The melting temperature (T_m), latent heat of fusion (Δ_fusH), and thermal decomposition temperature (T_d) were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). P_vap and Δ_vapH of [Gdm][OMs] were determined using the isothermogravimetric method. The functional relationship between vapor pressure and temperature was fitted using the Antoine equation. [Gdm][OMs] exhibits very low vapor pressure, high enthalpy of vaporization, and non-flammability in addition to its reported merits like high thermal and cyclic stability, offering data support for its application as a PCM in thermal energy storage.

2. Experimental

2.1. Materials

The materials used in this study were as follows: guanidinium carbonate ((CH₅N₃)₂·H₂CO₃, 99%, Acros Organics, Geel, Belgium), methanesulfonic acid (CH₃SO₃H, 99%, Sigma-Aldrich, Steinheim, Germany), ethanol (200 proof, anhydrous, ≥99.5%, Sigma-Aldrich, St. Louis, MO, USA), and glycerol (HOCH₂CHOHCH₂OH, ≥98%, Beijing Solarbio Science & Technology Co., Ltd., Beijing, China). They were all utilized without further purification.

2.2. Synthesis of the Organic Salt

The synthesis of guanidinium methanesulfonate ([Gdm][OMs], i.e., guanidinium mesylate) involves the slow addition of 22.61 mL of methanesulfonic acid to 15.69 g of guanidinium carbonate in a round bottom flask at room temperature. It should be noted that the adopted ratio of these two raw materials (CH₃SO₃H:(CH₅N₃)₂·H₂CO₃) slightly exceeds the stoichiometric ratio of 2:1, ensuring excess methanesulfonic acid and thus the complete reaction of the guanidinium carbonate. Then, the resulting wet slurry was heated to 210 °C in an oil bath to form a homogenous melt, followed by stirring at 210 °C via a magnetic stirrer for 5 h to promote the complete formation of the expected salt. The reaction was accompanied by the production of large quantities of gas and water. Most of the water was evaporated off during the reaction while the excess methanesulfonic acid was removed under vacuum at 250 °C.

2.3. Recrystallization

The [Gdm][OMs] sample was purified by recrystallization before further use as follows: Dissolve the as-prepared [Gdm][OMs] in anhydrous ethanol in a sealed glass vial, and sonicate the solution in a water bath at 50 °C for 30 min until it is completely clear. The mass ratio of [Gdm][OMs] vs. anhydrous ethanol is about 1:15.0~15.4. Then, cool the solution and maintain it at 0 °C for 12 h. Afterward, the obtained crystals were filtered off and dried at 80 °C in an oven for 1 h.

2.4. Elemental Analysis

The composition of the [Gdm][OMs] sample was characterized using an elemental analyzer (UNICUBE, Elementar, Langenselbold, Germany) with a detection limit of less than 50 ppm using a thermally conductive detector.

2.5. Fourier Transform Infrared (FT-IR) Analysis

The FT-IR spectrum of the [Gdm][OMs] powders was recorded at room temperature in the range of 4000–400 cm⁻¹ on a spectrometer (INVENIO-R, Bruker, Ettlingen, Germany) with a universal ATR accessory and was accumulated for 16 scans at a resolution of 4 cm⁻¹.

2.6. Differential Scanning Calorimetry (DSC)

DSC measurements were conducted between 25 °C and 240 °C on a thermal analysis system (STAR^e DSC 3, Mettler-Toledo, Greifensee, Switzerland) at a heating and cooling rate of 5 °C min⁻¹ to identify the phase transition temperatures and latent heat of fusion (Δ_fusH). The values are reported using the second run of the DSC tests to eliminate possible thermal history. The melting point (T_m) was fixed as the peak temperature and Δ_fusH as the integrated area of the melting peak. Measurements were carried out using sealed aluminum pans (40 μL) under a nitrogen atmosphere (60 mL min⁻¹) with a sample weight of 8.34 mg.

2.7. Thermogravimetric Analysis (TGA)

All thermogravimetric tests were carried out on the same instrument (TGA 2, Mettler Toledo, Columbus, OH, USA), including variable temperature thermogravimetric analysis and isothermogravimetric analysis (IGA). Concerning the ramped TGA measurement, the sample was tested from 30 °C to 850 °C under N₂ (50 mL min⁻¹) and at a heating rate of 5 °C min⁻¹ with a sample weight of 11.07 mg using a covered Al₂O₃ pan. The decomposition temperature (T_d) is determined by the onset temperature.

The IGA experiments of [Gdm][OMs] and glycerol were carried out on the same instrument using the same batch of uncovered Al₂O₃ crucible (cross-sectional area (a): 0.204 cm²) under the same nitrogen atmosphere (50 mL min⁻¹). For [Gdm][OMs], it was conducted at 220 °C, 240 °C, 260 °C, 280 °C, and 300 °C successively. It was held for 30 min at each measurement temperature. The heating rate for each measurement temperature was 20 °C min⁻¹. Similarly, the IGA experiments for glycerol were conducted at 100 °C, 120 °C, 140 °C, and 160 °C successively, and at each temperature it was held for 30 min. The masses of [Gdm][OMs] and glycerol were 10.40 mg and 42.36 mg, respectively.

2.8. Flammability Test

The flammability of [Gdm][OMs] was tested by direct ignition in a battery case for 5 s.

3. Theoretical Basis

3.1. Vapor Pressure

The vapor pressures of [Gdm][OMs] can be derived based on the Langmuir equation using the thermogravimetric data [22,23]:

$$\left(-\mathrm{d}m/\mathrm{d}t\right)/S=P_{\mathrm{vap}}{\alpha }_{\mathrm{vap}}{\left(M_{\mathrm{w}}/2\mathsf{\pi }RT\right)}^{0.5}$$

(1)

where (−dm/dt)/S is the rate of mass loss of the sample per unit area of vaporization (g min⁻¹ cm⁻²), P_vap is the vapor pressure (Pa), α_vap is the vaporization coefficient, M_w is the molecular weight of the effusing vapor (g mol⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature (K). α_vap is considered to be dependent on the apparatus and experimental atmosphere rather than on the substance and temperature. Furthermore, it can be determined by using pure substances with reported precise vapor pressures [24,25]. Equation (1) is simplified as Equation (2):

$$P_{\mathrm{vap}}=k_{\mathrm{vap}}v$$

(2)

where k_vap is a constant (Pa min cm² g^−0.5 mol^−0.5 K^−0.5), v is dependent on the material (g^0.5 mol^0.5 K^0.5 min⁻¹ cm⁻²), k_vap = (2πR)^0.5/α_vap, and v = [(−dm/dt)/S](T/M_w)^0.5.

Based on experimental data at several temperatures, vapor pressures in a given temperature range can be predicted by Antoine equation (Equation (3)) [26], which is an empirical equation that describes the P−T relationship of pure liquid substances and offers various applications in engineering:

$$\ln P_{\mathrm{vap}}=A-\frac{B}{T+C}$$

(3)

where A, B, and C are the substance-dependent Antoine constants (also known as Antoine coefficients) and P_vap is the vapor pressure (Pa) at temperature T (K).

3.2. Enthalpy of Vaporization

Enthalpy of vaporization of substances can be calculated by Equation (4), which is a regression equation derived from the Clausius–Clapeyron equation and was successfully used by Luo et al. [27] to calculate the enthalpy of vaporization of imidazolium-based ionic liquids:

$$\ln \left[(-\mathrm{d}m/\mathrm{d}t)T^{0.5}/S\right]=A-\frac{{∆}_{\mathrm{v}\mathrm{a}\mathrm{p}}H}{RT}$$

(4)

where (−dm/dt)/S is the rate of mass loss of the sample per unit area of vaporization (g min⁻¹ cm⁻²), T is the absolute temperature (K), R is the gas constant (8.314 J mol⁻¹ K⁻¹), Δ_vapH is the molar enthalpy of vaporization at the arithmetic mean temperature of all tested temperature points, and A is an empirical parameter. This method has recently been widely used to calculate Δ_vapH of various ionic liquids [28,29,30].

4. Results and Discussions

4.1. Basic Characterizations

The as-synthesized [Gdm][OMs] was examined by elemental analysis. The experimental values of the elemental contents of C, H, N, S, and O were as follows (the calculated values based on the molecular formula are given in parentheses): C-16.06% (15.48%), H-5.98% (5.81%), N-27.73% (27.10%), S-20.07% (20.65%), and O-30.16% (30.96%). It can be seen that the experimental values are in line with the calculated values for all the five elements. The chemical composition of the tested sample can be inferred to be C₂H₉N₃O₃S, indicating the high purity of the as-prepared sample.

The structure of the [Gdm][OMs] is also confirmed by FT-IR spectrum (Figure 2a). The appearance of new peaks that are different from the characteristic peaks of both guanidine carbonate and methanesulfonic acid, as well as the disappearance of some peaks, proved the formation of the proton transfer organic salt [31,32]. The as-prepared [Gdm][OMs] sample has a very low water content, indicated by the absence of the O−H stretching bands at 3400−3800 cm⁻¹ and the H−O−H bending band at around 1650 cm⁻¹ in the infrared spectrum [33,34,35]. Based on our previous work [32], the formation of CH₃SO₃⁻ anions in protic organic salts is confirmed by the bands at 1416, 1337, 1185, 1153, 1044, 773, 739, and 723 cm⁻¹, while the existence of guanidinium cations in [Gdm][OMs] is evidenced by the peaks at 3328, 3261, 3185, 1678, and 1582 cm⁻¹ (Figure 2a) [31,36]. The sharp absorption bands at around 1185 and 1153 cm⁻¹ are assigned to the SO³⁻ asymmetric stretching vibration, while the peak at 1044 cm⁻¹ relates to the SO³⁻ symmetric stretching vibration [37]. This indicates a transformation of the −SO₂OH group into the −SO³⁻ anionic group, implying the occurrence of a proton transfer process from the acidic site to the basic site. This spectral feature is typical of CH₃SO₃⁻ anions strongly involved in hydrogen bonding [38]. The peaks at 3328, 3261, and 3185 cm⁻¹ correspond to the asymmetric and symmetric NH₂ stretching vibrations, respectively [36,39]. In addition, the peaks at 1678 cm⁻¹ and 1582 cm⁻¹ are due to the asymmetric degenerated stretching of CN₃ and the degenerated scissoring vibration of NH₂, respectively [36,39]. Particularly, the peaks at 1351 cm⁻¹ and 908 cm⁻¹ associated with free CH₃SO₃H are not observed in [Gdm][OMs], implying no detectable excess acid in the product [40,41]. Moreover, the broadening of the vibrational bands between 3000 cm⁻¹ and 3500 cm⁻¹ implies a hydrogen bonding network [42,43]. The absorption bands at around 2825, 2755, and 2236 cm⁻¹ may be attributed to the N−H or N⁺−H stretching vibration, thus also proving the salt formation [44].

The DSC thermogram of [Gdm][OMs] is shown in Figure 2b. The experimental conditions and results of DSC measurements in comparison with the literature [15,16] for [Gdm][OMs] are summarized in

Table 2. Experimental conditions and results of DSC measurements for [Gdm][OMs].

Experimental Conditions and Results	This Work	Literature [15,16]
Sample mass	8.34 mg	3~8 mg
Scanning rate	5 °C min−1	10 °C min−1
Atmosphere and flow rate	N2, 60 mL min−1	N2 (flow rate not mentioned)
Measurement temperature range	25~240 °C	−40~220 °C
DSC instrument	STARe DSC 3, Mettler-Toledo (with 120 thermocouples)	DSC TA Q200 calorimeter (TA Instruments)
Sample condition	powders	NOT MENTIONED
Crucible	40 μL aluminum pan (sealed)	NOT MENTIONED
Data selection	the second run of the DSC cycle	the second run of the DSC cycle
Tm (peak maximum)	207.6 °C	208 ± 1 °C
ΔfusH	183.0 J g−1	190 ± 9.5 J g−1

. [Gdm][OMs] exhibits distinct melting and solidification peaks during the heating and cooling cycles. It shows a very sharp and symmetrical endothermic melting peak with an accurate melting point (T_m) of 207.6 °C and a specific latent heat of fusion of 183.0 J g⁻¹ (Figure 2b), indicating the high purity of as-prepared [Gdm][OMs]. The values of T_m and Δ_fusH agree well with the values reported by MacFarlane et al. (T_m = 208 ± 1 °C; Δ_fusH = 190 ± 9.5 J g⁻¹) [15,16].

4.2. Vapor Pressure and Enthalpy of Vaporization

Glycerol is used as the calibrator for the isothermogravimetric method to determine the constant k_vap (Equation (2)). Based on the IGA traces of glycerol at different temperatures (Figure 3a), the distinct mass loss rates of glycerol, the corresponding coefficient of determination R² (all of them exceeds 0.98) for linear regression, and the values of v were obtained and are listed in

Table 3. Vaporization rates, the values of v, and the reported vapor pressures of glycerol.

T/°C	T/K	(−dm/dt)/S/g min−1 cm−2	R2	v/g0.5 mol0.5 K0.5 min−1 cm−2	Pvap/Pa [45,46]
100	373.15	3.32 × 10−3	0.980	6.69 × 10−3	30.0
120	393.15	7.42 × 10−3	0.999	1.53 × 10−2	90.0
140	413.15	2.58 × 10−2	0.999	5.47 × 10−2	320
160	433.15	8.36 × 10−2	0.999	1.81 × 10−1	980

. Using the reported vapor pressure data of glycerol [45,46], the curve of P_vap vs. v is plotted in Figure 3b. Obviously, there is a linear relationship between P_vap and v, and the constant k_vap was evaluated from the slope as 5397 Pa min cm² g^−0.5 mol^−0.5 K^−0.5, with an excellent coefficient of determination (R²) of 0.999.

The TGA trace of [Gdm][OMs] at 5 K min⁻¹ in nitrogen atmosphere (Figure 4a) indicates that the onset temperature of weight loss of [Gdm][OMs] is as high as 332 °C (605.15 K), revealing its thermal stability over a wide temperature range. It also confirms the ultra-low water content as no loss of water was observed. Furthermore, derivative thermogravimetry (DTG), in which the rate of weight changes of [Gdm][OMs] upon heating is plotted against temperature, demonstrates three thermal events associated with vaporization or decomposition at higher temperatures of 351 °C, 447 °C, and 649 °C successively. Figure 4b shows the IGA traces of [Gdm][OMs] at 220 °C, 240 °C, 260 °C, 280 °C, and 300 °C, respectively. Clearly, the weight loss rate of [Gdm][OMs] increases as the temperature increases, demonstrating that the higher the temperature is, the easier it is for [Gdm][OMs] to evaporate.

Table 4. Vaporization rates, the values of v, and derived vapor pressures of [Gdm][OMs].

T/°C	T/K	(−dm/dt)/S/g min−1 cm−2	R2	v/ g0.5 mol0.5 K0.5 min−1 cm−2	Pvap/Pa
220	493.15	9.01 × 10−5	0.939	1.61 × 10−4	0.87
240	513.15	5.39 × 10−4	0.998	9.79 × 10−4	5.29
260	533.15	1.52 × 10−3	0.999	2.82 × 10−3	15.23
280	553.15	1.10 × 10−3	0.980	2.08 × 10−2	112.26
300	573.15	8.68 × 10−2	0.996	1.67 × 10−1	899.98

shows the linear fitting results of the IGA traces with R² in the range of 0.939–0.999.

The measured vaporization rates and the calculated values of v (v = [(−dm/dt)/S](T/M_w)^0.5) are listed in Table 4. The vapor pressures of [Gdm][Oms] at 220 °C, 240 °C, 260 °C, 280 °C, and 300 °C are then calculated via Equation (2) (P_vap = k_vapv) thanks to the calibrated value of the constant k_vap (5397 Pa min cm² g^−0.5 mol^−0.5 K^−0.5) using glycerol. Note that k_vap is dependent on the apparatus and experimental atmosphere. The derived vapor pressure of [Gdm][OMs] at 533.15 K (260 °C) is 15.23 Pa (Table 4), which is much lower than that of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄mim][NTf₂]) (P_vap = 4660 Pa at 517.45 K), indicating that this protic organic salt has very low volatility even at high temperatures [47]. The ultra-low vapor pressure means less loss and better durability of [Gdm][OMs] as a PCM and also implies a simpler design, a lower cost, and higher safety for the containers in terms of pressure vessels.

The three Antoine constants are obtained by fitting P_vap vs. T using the Antoine equation (Figure 4d, R² = 0.999), where A, B, and C are 10.99, 344.58, and −493.64, respectively. The temperature dependence of vapor pressures ln P_vap (Pa) = 10.99 − 344.58/(T (K) − 493.64) in the temperature range 493.15–573.15 K is thus acquired. The approximated semi-empirical equation indicates the exponential relationship between the vapor pressure and the temperature of [Gdm][OMs]. Moreover, the vapor pressures of [Gdm][OMs] between 220 °C and 300 °C can thus be interpolated.

Figure 5 shows a linear fit based on the linear weight loss rates at 220 °C, 240 °C, and 260 °C calculated from Figure 4c (also shown in Table 4) according to Equation (4). The calculated Δ_vapH at the average temperature of 513.15 K (240 °C) for [Gdm][OMs] is 157.10 ± 20.10 kJ mol⁻¹. The estimated value of Δ_vapH is higher than those of [C₄mim][NTf₂] (Δ_vapH = 118.5 ± 0.4 kJ mol⁻¹ at 496 K) and guanidinium nonaflate ([Gdm][NfO], Δ_vapH = 120.9 kJ mol⁻¹ at 523 K), which again proves that [Gdm][OMs] has essentially low volatility [27,31].

Finally, the flammability test (Figure 6) indicates that [Gdm][OMs] cannot be ignited after 5 s of combustion in air and will not explode when exposed to open flames, either. This demonstrates that [Gdm][OMs] is non-flammable and has excellent thermal safety.

5. Conclusions

Pure guanidinium methanesulfonate ([Gdm][OMs]) was prepared by a slight deviation from the stoichiometric ratio of methanesulfonic acid to guanidinium carbonate (i.e., the acid is slightly excess), followed by heating, stirring, vacuum drying, and then recrystallization. The elemental analysis, infrared spectrum measurement, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) all confirmed its high purity. TGA confirms its thermal stability over a wide temperature range while it also indicates three thermal events due to vaporization or decomposition between 350 °C and 650 °C. Using isothermogravimetric analysis, the Antoine equation was successfully employed to describe its vapor pressure as a function of temperature between 493.15 K and 573.15 K: ln P_vap (Pa) = 10.99 – 344.58/(T (K) – 493.64), indicating the vapor pressures ranging from 0.87 Pa at 220 °C to 899.98 Pa at 300 °C. Furthermore, its enthalpy of vaporization is derived as Δ_vapH = 157.10 ± 20.10 kJ mol⁻¹ at the average temperature of 240 °C via the Clausius–Clapeyron equation. In summary, [Gdm][OMs] exhibits a series of additional advantages, including essentially low volatility, high enthalpy of vaporization, and non-flammability. This work is expected to offer data support, methodologies, and insights for the application of organic salts as a new type of phase change materials in thermal energy storage and other industrial fields.