Structure-Dependent Formation of HFC-125a/CO₂ Mixed Hydrates: Raman Evidence and Implications for Fire Suppression

Abstract

Mixed gas hydrates containing 1,1,1,2,2-pentafluoroethane (HFC-125a) and carbon dioxide (CO₂) have emerged as promising candidates for next-generation fire suppression due to their combined cooling and capabilities. In this study, the formation behavior, hydrate yield, thermodynamic characteristics, and molecular-level guest occupancy of HFC-125a + CO₂ (20–80 mol %) mixed hydrates were systematically investigated under isochoric conditions. Pressure–temperature measurements revealed distinct differences between pure and mixed systems, with the mixed hydrate exhibiting a two-stage nucleation behavior. Primary nucleation was associated with rapid CO₂-driven hydrate formation, subsequent structural reorganization, and gas incorporation, while secondary nucleation occurred at lower temperatures and pressures. Comparative hydrate yield analysis showed that although the mixed system exhibits a lower overall water-to-hydrate conversion than that of pure HFC-125a, it provides improved structural stability and controlled gas incorporation. Raman spectroscopic analysis confirmed the encapsulation of CO₂ predominantly in structure I (sI) hydrates and HFC-125a in structure II (sII) hydrates, supporting the coexistence of sI and sII phases in the mixed system. These findings demonstrate how guest-specific kinetics and cage affinity govern hydrate structure and formation pathways, providing critical insight into tailoring mixed hydrate systems toward efficient and controllable fire-suppression applications.

1. Introduction

Recent large-scale fire disasters and industrial accidents continue to expose critical limitations in existing fire-suppression technologies. Conventional extinguishing agents suffer from well-known drawbacks: water is ineffective or hazardous for electrical and hydrocarbon fires, CO₂ disperses rapidly and poses asphyxiation risks in confined spaces, and dry chemical powders leave corrosive residues that damage sensitive equipment. − These challenges have intensified the demand for advanced fire-suppression materials that combine rapid heat absorption, effective flame inhibition, operational safety, and environmental compatibility.

Gas clathrate hydrates have emerged as a promising platform for next-generation fire suppression. These crystalline solids consist of hydrogen-bonded water cages encapsulating guest gas molecules and exhibit strongly endothermic dissociation upon heating. This behavior enables simultaneous heat absorption and controlled gas release, offering advantages over those of conventional agents. Hydrates adopt specific crystal structures depending on the guest molecule: structure I contains two 5¹² (small) and six 5¹²6² (large) cages, structure II contains sixteen 5¹² (small) and eight 5¹²6⁴ (large) cages, while structure H (sH) comprises three 5¹², two 4³5⁶6³, and one 5¹²6⁸ cage. Hydrates have been extensively studied in applications such as gas separation, energy storage, and carbon capture and more recently have attracted attention for fire-suppression use due to their dual cooling and inerting mechanisms. ,, Previous studies have demonstrated the effectiveness of CO₂ hydrates as fire-suppression agents. Hatakeyama et al. reported that CO₂ hydrates could extinguish pool flames with lower water consumption than ordinary ice and reduced CO₂ release compared to dry ice, highlighting the combined cooling and inerting advantages of hydrate-based suppression systems. In their study, the hydrate conversion mass fraction was approximately 0.76, enabling substantial gas storage and release during dissociation. Among candidate systems, hydrates formed with HFC-125a are of particular interest because of the gas’s strong flame-inhibition capability. , However, pure HFC-125a hydrates crystallize in the sII lattice, in which only the large cages are occupied, leaving the small cages vacant. While this incomplete cage occupancy results in a less densely filled hydrate structure, it limits the total amount of fire-suppressant gas released upon dissociation, thereby reducing the overall extinguishing effectiveness despite potentially facile dissociation kinetics. In contrast, CO₂ can readily occupy the small cages of sII hydrates and exhibits strong fire-suppression performance through inerting and cooling. Combining CO₂ with HFC-125a, therefore, offers a pathway to increase overall cage occupancy and tailor hydrate stability and dissociation behavior. A mixed HFC-125a + CO₂ hydrate system may enable synergistic performance by coupling enhanced heat absorption with optimized gas release dynamics.

Despite this potential, the guest distribution, thermodynamic behavior, and dissociation characteristics of mixed HFC-125a + CO₂ hydrates remain poorly understood. In this study, we systematically investigate the formation behavior, gas consumption, and molecular occupancy of mixed hydrates using experimental measurements and Raman spectroscopy. The results provide new insight into dual-guest hydrate systems and support the development of more effective hydrate-based fire-suppression materials. Though the hydrate conversion fraction is lower, the experiments were designed to investigate hydrate formation and dissociation behavior rather than to achieve a maximum hydrate yield. While such a conversion fraction may appear comparable to ice from a purely mass-based perspective, hydrate dissociation fundamentally differs from ice melting due to the additional contribution of guest gas release. Even at lower conversion, mixed and single-component hydrates exhibit staged dissociation behavior and thermally triggered gas liberation, which can enhance the suppression efficiency beyond simple cooling.

Although HFC-125a is an effective flame-retardant agent, its practical use in fire suppression requires careful evaluation due to the potential formation of harmful oxidation byproducts under high-temperature conditions and its regulatory restrictions as a fluorinated gas. Consequently, the present study shows the fire extinguishing ability of these gases and also demonstrates the fundamental advantages of hydrate-based suppression systems, including controlled encapsulation, staged gas release, and combined cooling and inerting effects. Importantly, the hydrate-based framework developed here can be transferred to environmentally benign and regulatory-compliant agents, which will be the focus of future investigations.

2. Experimental Section

2.1. Materials

High-purity HFC-125a and CO₂ gases (99.9%) were supplied by the Jang Yoo Integrated Gas Company. Deionized water (Milli-Q grade, 99.9%) was produced using a Direct-Q3 purification system and used for all experiments.

2.2. Experimental Setup

Experiments were conducted in a stainless-steel (SS-316) high-pressure batch reactor with an internal volume of 365 mL and a maximum operating pressure of 15 MPa. Gas delivery and pressure regulation were achieved by using an auxiliary gas reservoir (1800 mL). Reactor temperature was controlled by a recirculating chiller (RW3–2035, JEIO Tech, Korea). Pressure and temperature were continuously monitored using a calibrated pressure transducer (±0.25%) and an RTD sensor (±0.1 K), with data recorded via a computer-based acquisition system.

2.3. Experimental Procedure

The reactor was cleaned, assembled, and charged with 100 mL of Milli-Q water under ambient conditions. After it was sealed, the system was purged multiple times with the selected gas to remove residual air. The reactor was then pressurized, cooled to the target temperature, and agitated at 400 rpm. Hydrate formation experiments were performed under isochoric conditions. Pressure and temperature data were logged at 10 s intervals. A schematic of the setup is shown in Figure .

1.

Schematic of the experimental reactor and hydrate formation and dissociation.

2.4. Gas Uptake and Hydrate Conversion

Eq gives the gas consumption during hydrate formation and was determined from the decrease in gas-phase moles, calculated using real-time pressure and temperature measurements and corrected with the compressibility factor obtained from Pitzer’s correlation. ,

$$\mathrm{\Delta }{n}_{H,\downarrow }=n_{H,0}-n_{H,t}=(\mathit{PV}/zRT{)}_{G,0}-(\mathit{PV}/\mathit{zRT}{)}_{G,t}$$ 1

Here, Δn _H, ↓ represents the total moles of gas consumed for hydrate formation by the end of the experiment. n _H,0 and n _H , _t correspond to the number of moles present in the gas phase at the start of the experiment and at any time t, respectively. The subscripts G,0, and G,t indicate gas-phase conditions at the initial moment and at time t. V is the volume of the gas phase inside the reactor, and any slight change in this volume during hydrate formation was neglected. R is the universal gas constant, while P (kPa) and T (K) denote the measured pressure and temperature inside the reactor, respectively.

Eq provides the hydrate conversion and was calculated based on the ratio of consumed gas to the initial water content, incorporating the appropriate hydration numbers. For pure hydrates, hydration numbers of 17 for HFC-125a, ,, and 6.8 for CO₂ were used. For mixed hydrates, a value of 5.67 was assumed, considering HFC-125a occupancy in large cages and CO₂ in small cages. −

$$\mathrm{Hydrate}\mathrm{conversion}(\%)=\frac{\mathrm{\Delta }{n}_{\mathrm{gas}}}{n_{\mathrm{water}}}\times H_N\times 100$$ 2

Δn _gas denotes the amount of gas consumed during the hydrate formation, and n _water denotes the initial amount of water in the reactor. H _N is the hydration number.

2.5. Raman Spectroscopy

Raman spectroscopy was employed to investigate the hydrate formation and guest inclusion at the molecular level. Measurements were conducted by using a Dimension-P1 Raman spectrometer (Lambda Solutions, USA) equipped with fiber-optic probes integrated into the reactor, enabling in situ analysis of both gas and liquid phases. A 532 nm excitation laser with a maximum output power of 100 mW was used, and spectra were collected over the range of 120–3460 cm^–1 with an integration time of 60 s per spectrum. Before analysis, all Raman spectra were subjected to baseline correction to remove the fluorescence background and instrumental drift. Spectral fitting was performed using Voigt line-shape functions, which account for the combined effects of Gaussian instrumental broadening and Lorentzian vibrational line widths. Peak deconvolution was carried out using consistent fitting constraints for the peak position and bandwidth to ensure a reliable comparison of band intensities and assignments across different experimental conditions. The processed spectra were used to identify the hydrate structure and confirm guest molecule enclathration at selected stages of hydrate formation.

3. Results and Discussion

Proven their ability and high impact on the fire suppression, HFC-125a hydrates show a good thermodynamic promotion and stability. , Pure HFC-125a hydrates leave all small cages vacant, resulting in underutilization of the hydrate lattice and limiting gas-storage capacity. Mixed hydrate formation effectively unlocks this untapped potential by filling small cages with CO₂, thereby increasing the total gas loading per unit of water. This enhanced storage efficiency is particularly important for fire-suppression applications, where maximizing the amount of the active agent stored in the hydrate form directly translates to improved cooling capacity, longer suppression duration, and reduced material requirements.

To understand these mixed hydrates, phase equilibrium lines serve as fundamental design tools for hydrate-based technologies. They define the precise pressure–temperature conditions required for the formation, stability, and dissociation. The hydrate equilibrium line shows the thermodynamic boundary above which gas hydrates are stable and below which they dissociate into free gas and water (or ice). Knowledge of these boundaries enables the selection of optimal operating conditions for hydrate synthesis, storage, and controlled gas release. In mixed hydrate systems, equilibrium data are especially critical because guest–guest interactions can shift stability regions relative to pure hydrates. Accurate phase equilibrium characterization, therefore, underpins rational hydrate design, allowing the performance to be tuned for specific applications such as fire suppression and gas storage.

Figure presents the pressure–temperature (P–T) phase equilibrium behavior of the CO₂ and HFC-125a hydrates. The dashed lines represent the vapor–liquid equilibrium (VLE) boundaries, while the solid circles denote the hydrate equilibrium lines for each gas. The CO₂ hydrate phase equilibrium was generated using CSMGem software, and vapor–liquid equilibrium data were adapted from Bhavya et al., while the HFC-125a hydrate equilibrium data were adapted from Hashimoto et al. The vapor–liquid equilibrium data for HFC-125a were taken from Richard F. Kayser. From the figure, it is evident that the CO₂ hydrate exhibits relatively high equilibrium pressures over the investigated temperature range. At a given temperature, CO₂ requires substantially higher pressure to stabilize the hydrate phase compared to that of HFC-125a. This behavior arises from the smaller molecular size of CO₂ and its weaker van der Waals interactions with the hydrate cages, which necessitate higher pressures to maintain lattice stability. , The steep slope of the CO₂ hydrate equilibrium line indicates a strong temperature sensitivity. Small increases in temperature lead to pronounced increases in dissociation pressure, implying rapid gas release upon heating. This characteristic is advantageous for fire-suppression applications, where fast endothermic dissociation and inert gas release are desirable. In contrast, HFC-125a hydrate forms at much lower pressures, reflecting the strong stabilization of the hydrate lattice by the larger HFC-125a molecules occupying the large cages of the sII hydrate. The hydrate equilibrium line lies well below that of CO₂, indicating superior thermodynamic stability under mild conditions. However, the flatter slope of the HFC-125a hydrate equilibrium line suggests slower dissociation kinetics and a reduced sensitivity to temperature changes. While this stability is beneficial for hydrate formation and storage, it can limit rapid gas release, which is critical for effective fire suppression. For both gases, the hydrate stability shifts to higher pressures with increasing temperature, reflecting the endothermic nature of hydrate dissociation. However, the absolute pressure levels and slopes of the equilibrium lines differ significantly between CO₂ and HFC-125a, highlighting fundamental differences in their hydrate-forming characteristics.

2.

Pressure–temperature phase equilibrium of CO₂ hydrate (black circles), generated from GSMGem ref , and of HFC-125a (green circles), adapted with permission from ref , Copyright 2010 American Chemical Society. Dashed lines indicate vapor–liquid equilibrium (VLE) of CO₂ (gray dashed line), adapted with permission from ref Copyright 2021 American Chemical Society, and of HFC-125a (purple dashed line), adapted from ref .

The CO₂ gas forms a sI hydrate, , and HFC-125a gas forms sII hydrate, , but when CO₂ and HFC-125a are mixed, their complementary molecular sizes enable cooperative occupancy of the sII hydrate lattice. , HFC-125a preferentially occupies the large cages, while CO₂ can fill the previously unoccupied small cages. This dual-guest configuration increases overall cage occupancy and gas-storage density compared with pure HFC-125a hydrate. Thermodynamically, the mixed hydrate is expected to exhibit an equilibrium behavior intermediate between those of the pure systems. The presence of CO₂ can slightly shift the hydrate equilibrium line to higher pressures while simultaneously enhancing dissociation responsiveness due to CO₂’s higher temperature sensitivity. As a result, mixed hydrates can combine the low-pressure stability of HFC-125a with the rapid dissociation characteristics of CO₂.

Figure a illustrates the temporal evolution of the temperature and pressure during CO₂ hydrate formation. Initially, the system undergoes controlled cooling, resulting in a gradual decrease in the temperature accompanied by a corresponding pressure decline due to gas contraction and dissolution of CO₂ in water. After the cooling stage, the temperature stabilizes near ∼273.84 K, indicating that the system has reached conditions favorable for hydrate nucleation. At approximately 140 min, a sudden pressure drop is observed, accompanied by a minor temperature perturbation. This behavior is attributed to the onset of CO₂ hydrate formation, during which the CO₂ gas is rapidly consumed and incorporated into the hydrate lattice. The sharp temperature spike suggests the exothermic nature of hydrate nucleation and effective heat dissipation under the experimental conditions. The extended induction period reflects the fresh hydrate formation. Figure b presents the temperature and pressure profiles during the HFC-125a hydrate formation. In contrast to that of CO₂, HFC-125a exhibits hydrate nucleation at a higher temperature (∼273.15 K) and significantly lower pressure. Following the cooling phase, a distinct temperature rise is observed at around 45–50 min, which is characteristic of exothermic hydrate nucleation. Simultaneously, a sharp pressure decrease occurs, indicating rapid gas uptake into the hydrate phase. The pronounced thermal response and shorter induction time demonstrate faster hydrate formation for HFC-125a. This behavior can be attributed to the stronger guest–host interactions and the stabilization of structure II hydrates, which lowers the thermodynamic barrier for nucleation compared to CO₂. Comparison of Figure a,b clearly demonstrates that HFC-125a hydrates form more readily than CO₂ hydrates, requiring lower pressures and shorter induction times and exhibiting stronger exothermic signatures. These results highlight the intrinsic differences in hydrate stability and formation kinetics between the two gases and provide experimental evidence supporting the potential role of HFC-125a as a promoter in mixed gas hydrate systems.

3.

Temporal variation of temperature and pressure during hydrate formation: (a) CO₂ hydrate formation and (b) HFC-125a hydrate formation. The highlighted region shows nucleation and pressure inflection.

Figure presents the time-dependent evolution of temperature and pressure during hydrate formation from an HFC-125a + CO₂ gas mixture with a composition of 20–80 mol % under isochoric conditions. The 20–80 mol % HFC-125a-CO₂ composition was deliberately selected to achieve an optimal balance between the CO₂ uptake efficiency and stable sII hydrate formation. At high CO₂ fractions (80 mol %), CO₂ effectively promotes early hydrate nucleation by preferentially occupying the small 5¹² cages, thereby enhancing overall gas consumption and CO₂ intake. , Simultaneously, the presence of 20 mol % HFC-125a, a large guest molecule with a strong affinity for the 5¹²6⁴ large cages, is sufficient to stabilize the hydrate lattice into the sII structure. This composition avoids the formation of unstable or incomplete hydrates that may occur in CO₂-rich systems alone while also preventing excessive kinetic inhibition that can arise at higher HFC-125a concentrations.

4.

Temporal evolution of temperature (red symbols, left y-axis) and pressure (black symbols, right y-axis) during hydrate formation from a binary gas mixture of HFC-125a and CO₂ (20–80 mol %) under isochoric conditions.

At the beginning of the experiment, both temperature and pressure decrease smoothly as the system is externally cooled, and the injected gases dissolve into the aqueous phase. During this initial stage, no hydrate formation occurs; the observed pressure reduction is primarily associated with gas dissolution and thermal contraction, indicating that the system is progressively moving toward a supersaturated and thermodynamically favorable region for hydrate nucleation. As cooling continues, the first distinct hydrate formation event, termed primary nucleation, is observed at approximately 284.59 K and 2462 kPa. This stage is characterized by a noticeable change in the temperature and pressure–time slope, signifying the onset of gas consumption due to hydrate nucleation. At this temperature, it is ascribed that the CO₂, owing to its smaller molecular size and higher diffusivity, preferentially occupies the 5¹² small cages, facilitating the initial formation of hydrate nuclei. ,, However, hydrate growth remains limited and relatively slow, suggesting that the initially formed hydrate phase is either metastable or structurally incomplete, with HFC-125a participation still kinetically constrained. Further, the system undergoes a prolonged growth might be due to the reorganization phase, during which pressure continues to decrease gradually while temperature declines steadily without a pronounced exothermic signal. , The slow hydrate growth indicates a local lattice rearrangement where the hydrate structure evolves as HFC-125a and gas molecules progressively begin to incorporate into the hydrate framework due to their large molecular size and preference for large cages. ,,

A second, more pronounced hydrate formation event, termed secondary nucleation, occurs at approximately 269.63 K and 1435 kPa. This event is characterized by a small yet distinct temperature increase, indicative of the exothermic nature of accelerated hydrate growth accompanied by a renewed and steeper decline in system pressure. The secondary nucleation stage represents the incorporation of residual gas species that did not participate in the primary enclathration process. This residual gas is ascribed to be CO₂, since the HFC-125a is initially encapsulated due to its large size affinity. The CO₂ remains in the vapor phase and is observed to form the sI hydrate due to a change in the gas fugacity and favored by the equilibrium pressure conditions. After the secondary nucleation event, both temperature and pressure profiles gradually level off, indicating that the system approaches a quasi-equilibrium state. At this stage, most of the available gas has been consumed in hydrate formation, and further growth is minimal. From these observations, it is evident that hydrate formation in the HFC-125a + CO₂ system follows a two-step mechanism, where CO₂ promotes early nucleation, while HFC-125a governs late-stage growth and structural stabilization. This behavior provides important insight into mixed-guest hydrate kinetics and supports the coexistence of transient metastable hydrates followed by stable structure II hydrate formation, which is highly relevant for hydrate-based fire suppression and gas-storage applications.

Figure a presents the dissociation behavior of the mixed HFC-125a + CO₂ hydrate system (20–80 mol %) under isochoric conditions, showing the temporal evolution of temperature and pressure during controlled heating. The dissociation process clearly proceeds in two distinct stages, reflecting heterogeneous gas release mechanisms within the mixed hydrate structure. In the initial stage (0–600 min), a relatively abrupt increase in pressure with a rise rate of 1.01 kPa min^–1 is observed over a short time interval, accompanied by a rapid rise in the temperature of 0.013 K min^–1. This behavior is attributed to the destabilization and dissociation of less stable hydrate regions, likely associated with CO₂-rich domains and small-cage occupancy, which respond readily to thermal input. The rapid release of guest gas during this stage results in a pronounced pressure increase under constant-volume conditions.

5.

(a) Time evolution of temperature and pressure during dissociation of the HFC-125a + CO₂ (20–80 mol %) mixed hydrate, highlighting the pressure inflection around 600 min. (b) Pressure–temperature dissociation path of the mixed hydrate, compared with the CO₂ hydrate equilibrium line.

Following this initial event, the system transitions into a second, slower dissociation stage (600–2100 min), characterized by a more gradual and sustained increase in both the temperature (0.006 K min^–1) and pressure (0.54 kPa min^–1). This stage is indicative of progressive dissociation of the more stable hydrate domains, plausibly dominated by HFC-125a occupancy in the large cages. The reduced dissociation rate reflects stronger guest–host interactions and higher thermal resistance of these hydrate regions. At longer times, both pressure and temperature approach quasi-steady values, suggesting near-complete dissociation of the hydrate phase and establishment of gas–liquid equilibrium within the closed system. Figure b illustrates the temperature–pressure dissociation profile of the mixed HFC-125a + CO₂ hydrate system. The initial dissociation stage closely follows the phase equilibrium trend of pure CO₂ hydrate, indicating a rapid gas release associated with the decomposition of thermodynamically less stable CO₂-rich hydrate domains. This stage is characterized by a high gas release rate. As dissociation progresses, a distinct pressure inflection is observed, after which the system transitions to a dissociation pathway governed by the mixed hydrate phase behavior. The subsequent dissociation stage proceeds more gradually, reflecting the decomposition of the mixed hydrate structure and the controlled release of the remaining guest gases. The observed two-stage dissociation behavior highlights the controlled and nonexplosive nature of gas release from the mixed hydrate system, which is particularly relevant for fire-suppression applications where staged, thermally triggered gas discharge is desirable.

Figure presents a comparison of the hydrate yield obtained from pure CO₂ (3000 kPa), pure HFC-125a (1000 kPa), and the mixed HFC-125a + CO₂ (3000 kPa) hydrate systems. The initial operating pressure for the pure HFC-125a system was restricted to 1000 kPa to prevent liquefaction of the refrigerant gas. The hydrate conversion, expressed as a percentage, represents the fraction of water converted to the hydrate phase during the experiment, as defined in eq . Among the three systems, pure HFC-125a hydrate exhibits the highest yield, followed by pure CO₂, whereas the mixed hydrate system shows a comparatively lower overall yield. The associated error bars indicate good experimental reproducibility, with only minor variations observed between repeated measurements.

6.

Comparison of hydrate conversion obtained from pure CO₂, pure HFC-125a, and mixed HFC-125a + CO₂ (20–80 mol %) systems.

The enhanced hydrate conversion observed for the pure HFC-125a system can be attributed to its strong thermodynamic driving force for hydrate formation and its effective stabilization of structure II hydrates through preferential occupation of the 5¹²6⁴ large cages. Owing to its larger molecular size and strong host–guest interactions, HFC-125a promotes rapid hydrate growth and robust lattice stabilization, resulting in a higher degree of water-to-hydrate conversion under the investigated conditions. This behavior is consistent with the established role of large guest molecules as efficient sII hydrate stabilizers. , In contrast, the pure CO₂ system exhibits a moderate hydrate yield, which is primarily governed by the formation of structure I hydrates. CO₂ preferentially occupies the 5¹² small cages and partially fills the 5¹²6² large cages, leading to a lower overall water conversion compared to HFC-125a hydrates. Although CO₂ hydrates nucleate readily due to favorable kinetic characteristics, the absence of a strong, large-cage stabilizer limits extensive hydrate growth and, consequently, the total hydrate yield. The mixed HFC-125a + CO₂ hydrate system exhibits a reduced overall hydrate yield relative to that of the single-component systems, despite showing enhanced nucleation behavior and improved structural stability. This reduction is primarily associated with compositional effects and competitive cage occupancy in the mixed-guest system. While CO₂ facilitates early nucleation through preferential occupation of small cages, the relatively lower concentration of HFC-125a restricts complete occupation of the large cages, resulting in partial lattice filling and reduced net water conversion. Moreover, structural rearrangements and multistage nucleation processes in the mixed system may further impede continuous hydrate growth. It is important to note that the exact nature of competitive cage occupancy in mixed-guest hydrates remains difficult to quantify without the direct determination of guest–host affinities. A rigorous assessment of realistic cage occupancy would require evaluation of Langmuir adsorption constants for each guest species in both small and large cages. In the present study, it was assumed that HFC-125a and CO₂ possess comparable Langmuir affinities for the large cages under the investigated conditions and that relative cage occupancy is primarily governed by guest fugacity, such that large-cage occupancy is approximately proportional to the partial pressure of each component. Considering this, the hydration number was assumed to be 5.67 for yield estimation. Within these assumptions, the observed trends highlight a fundamental trade-off between hydrate yields and functional performance in mixed-guest systems. Although the mixed HFC-125a + CO₂ hydrate exhibits a lower overall yield, it offers advantages in terms of structural stability, controlled gas incorporation, and synergistic guest behavior, which are critical for applications such as gas storage, CO₂ capture, and hydrate-based fire suppression technologies.

To gain deeper insight into the hydrate formation mechanism, the mixed HFC-125a + CO₂ (20–80 mol %) hydrate system was subjected to Raman spectroscopic analysis after completion of hydrate formation. The gas-phase Raman spectra were analyzed to distinguish vibrational features associated with enclathrated guest molecules from those of the free gas phase. The Raman spectra presented in Figure a–d provide direct molecular evidence of how CO₂ and HFC-125a behave both in the gas phase and when enclathrated inside hydrate cages. In the gas-phase CO₂ spectrum (Figure a), the well-defined Fermi dyad at 1286 and 1388 cm^–1 reflects the free rotational and vibrational motion of unconfined CO₂ molecules, with minor accompanying bands at 1266 and 1409 cm^–1 arising from weak hot-band contributions. ,, The cumulative fitted peak (red line) overlays closely with the experimental points, indicating that the Voigt profile accurately captures the gas-phase spectral shape. When CO₂ becomes incorporated into the hydrate lattice (Figure b), these peaks shift to 1275.5 and 1381.2 cm^–1, a clear red shift that indicates the influence of the water cages on the vibrational modes of the trapped molecules. The confinement within the hydrate framework reduces the vibrational energy levels and restricts rotational freedom, producing narrower and more symmetric peaks typical of CO₂ in structure I hydrates. ,,, A similar trend is evident for HFC-125a. In the vapor state (Figure c), HFC-125a exhibits a sharp C–H stretching band at 3003 cm^–1, characteristic of an unconfined, freely rotating gas-phase molecule. , Upon hydrate formation (Figure d), this peak shifts to 2996.2 cm^–1 and broadens noticeably, demonstrating that the molecule experiences a different local environment when stabilized inside the larger cavities of a structure II hydrate. The magnitude and direction of this shift are consistent with strong guest–host interactions and the reduced vibrational freedom expected for bulky refrigerant molecules encaged within sII water lattices. Collectively, the spectral changes observed for both gases confirm their successful enclathration and provide clear vibrational signatures that distinguish gas-phase species from hydrate-bound guests. These Raman fingerprints not only validate the structural assignment of CO₂ as an sI hydrate former and HFC-125a as an sII former but also support the interpretation of mixed occupancy and dual-structure effects observed in the thermodynamic and kinetic analyses. ,,

7.

Raman spectroscopic signatures of (a) CO₂ in the gas phase, (b) CO₂ enclathrated in the hydrate phase, (c) HFC-125a in the gas phase, and (d) HFC-125a enclathrated in the hydrate phase, illustrating distinct vibrational shifts associated with gas-phase and hydrate-encaged states.

The experimental observations from the pure and mixed hydrate systems reveal distinct formation behaviors that provide new insight into mixed-guest hydrate formation mechanisms. In pure gas systems, hydrate formation is characterized by a single nucleation event followed by a monotonic pressure decrease, corresponding to continuous hydrate growth. In contrast, the mixed HFC-125a + CO₂ (20–80 mol %) system exhibits a two-stage nucleation behavior, consisting of an initial primary nucleation with a moderate pressure drop followed by a secondary nucleation event associated with slower, diffusion-limited hydrate growth. This multistage behavior highlights the complex interplay between guest composition, kinetics, and hydrate structures in mixed systems.

Direct identification of the guest species occupying hydrate cages during the primary and secondary nucleation stages is challenging solely on the basis of pressure–temperature data, as gas-phase discrimination is not possible without in situ gas chromatography or Raman spectroscopic measurements. Nevertheless, a careful examination of the experimental trends provides important mechanistic clues. The rapid pressure decrement observed at the onset of primary nucleation (Figure ), indicated by a lower density of pressure data points during this period, suggests that nucleation is triggered by CO₂. This interpretation is supported by the higher solubility and diffusivity of CO₂ in water compared to HFC-125a. Furthermore, the initial postnucleation kinetics at 15 to 18 min in Figure closely resemble the hydrate formation kinetics observed for pure CO₂ (rapid uptake, see Figure a), reinforcing the conclusion that CO₂ plays a dominant role in initiating hydrate nucleation. Following initial nucleation, the hydrate lattice likely stabilizes initially as a structure I framework dominated by CO₂ enclathration (15–18 min). As hydrate growth progresses, HFC-125a, owing to its strong affinity for large cages, can progressively reorganize the hydrate lattice toward structure II. ,, This structural transformation is consistent with the kinetic behavior observed between 19 and 26 min in Figure , which closely matches the hydrate formation kinetics of pure HFC-125a (Figure b). These observations suggest that HFC-125a contributes primarily during the lattice rearrangement and stabilization stages rather than during initial nucleation. The proposed mechanism is strongly supported by prior literature on mixed-guest hydrate systems. Subramanian et al. demonstrated, using Raman and ¹³C NMR spectroscopy, that methane–ethane mixtures, despite both gases forming sI hydrates individually, produce sII hydrates over a narrow compositional window. Their work established that mixed-guest competition can drive the hydrate structure selection toward the topology that best accommodates the larger guest molecule. In subsequent studies, they further showed that hydrate structure selection depends strongly on vapor composition and that the structure formed at incipient nucleation conditions may differ from that stabilized at later stages, emphasizing the role of guest-cage size matching and compositional evolution. Similar behavior has been reported by Mok et al., where they observed that CHClF₂ (R22), typically an sI former, can nucleate as sII when mixed with suitable coguests such as N₂ or CH₄. Their findings underscore that the presence and partial pressure of a secondary gas can force a hydrate lattice to adopt a structure different from that favored by the pure guest. Importantly, while structure switching was observed, stable coexistence within a single lattice was not reported, indicating that structure selection remains sensitive to the local composition and kinetics. Further evidence of structure-dependent behavior is provided by Lim et al., who showed that hydrate-based separation of R22 from R22 + N₂ mixtures is strongly governed by the hydrate structure. Using pressure–composition analysis combined with in situ Raman spectroscopy and PXRD, they demonstrated a transition from sI to mixed sI/sII and finally to sII hydrates as R22 concentration decreased, with a concomitant decrease in separation efficiency. Their work highlights the functional consequences of hydrate structural transitions in mixed systems.

Our observations are in strong agreement with the existing literature and support a mechanism in which CO₂ initiates structure I hydrate formation, followed by lattice transformation to structure II, driven by HFC-125a. If the sI-to-sII transformation occurs during primary nucleation, then HFC-125a is expected to be preferentially incorporated into the hydrate cages, leaving CO₂ largely in the vapor phase. Under such conditions, the secondary nucleation event observed in this study can be reasonably attributed to subsequent CO₂ hydrate formation. A comparable two-step crystallization behavior was reported by Uchida et al. for methane–propane mixed gas systems (propane < 10 vol %) at 274 K. Their gas chromatographic measurements showed a progressive decrease in propane concentration in the vapor phase during hydrate growth, with two distinct pressure drops corresponding to two hydrate formation stages. Structural analyses using X-ray diffraction and Raman spectroscopy confirmed that structure II methane–propane mixed hydrates formed during the first stage, followed by structure I methane hydrates in the second stage. Importantly, the second hydrate formation event occurred only when the methane partial pressure remained above its hydrate equilibrium pressure, underscoring the critical role of gas composition and hydrate structure in governing multistage crystallization behavior.

The Raman spectroscopic results in this study provide direct evidence for the enclathration of both CO₂ and HFC-125a. Strong Raman peaks at 1275.5 and 1381.2 cm^–1 confirm CO₂ enclathration characteristic of sI hydrates, while the peak at 2996.2 cm^–1 indicates strong HFC-125a enclathration consistent with sII hydrate formation. , Notably, the characteristic CO₂ peak shift associated with occupancy of sII small cages (∼1273.7 cm^–1) is not observed, suggesting that CO₂ does not significantly occupy sII small cages under the present conditions. , This observation supports the coexistence of sI and sII hydrates rather than a single fully converted sII structure. The coexistence of sI and sII hydrates observed here is consistent with reports by Menezes et al., who demonstrated using high-pressure micro-DSC and isochoric PVT measurements that methane–propane systems can exhibit simultaneous sI and sII formation depending on experimental protocol and kinetic conditions. They emphasized that hydrate structure selection can be kinetically controlled with sI appearing as a metastable phase even when sII is thermodynamically favored. Complementary insights were provided by Cai et al., who directly visualized spatially distinct sI and sII domains using in situ confocal Raman microimaging, proposing that sII nucleates at gas–water interfaces while sI forms from dissolved gas in the bulk liquid.

Based on the combined experimental observations and supporting literature, a reasonable formation mechanism can be proposed in which CO₂ first triggers the nucleation of structure I hydrates. As hydrate growth proceeds, HFC-125a is progressively incorporated into the larger cages, driving a reorganization of the hydrate lattice toward structure II. Changes in gas fugacity and diffusion following this transformation may then promote a secondary nucleation event dominated by CO_2, resulting in the two-stage hydrate formation behavior observed in this study. While this interpretation is well supported by the present results and earlier reports, direct confirmation will require dedicated in situ measurements capable of resolving guest composition and hydrate structure during formation. Such investigations are planned as part of the future work. Overall, the findings of this study provide clear insight into the formation dynamics and structural evolution of mixed hydrate systems. The observed two-stage nucleation and structure transformation behavior are particularly important for the design and optimization of mixed hydrates for fire-suppression applications. By understanding how individual gases influence nucleation, structural stabilization, and gas release characteristics, we can more effectively tailor hydrate composition and operating conditions to achieve stable, rapid, and controlled fire-suppression performance.

4. Conclusions

This study provides a detailed experimental assessment of hydrate formation, structural evolution, and guest distribution in HFC-125a + CO₂ mixed hydrate systems with direct relevance to fire-suppression applications. Pure gas systems exhibited single-step hydrate nucleation and continuous growth, whereas the mixed system showed a distinct two-stage nucleation behavior. Analysis of pressure–temperature trends indicates that CO₂ initiates primary hydrate nucleation due to its higher solubility and diffusivity, forming predominantly structure I hydrates. Subsequent lattice reorganization, driven by the preferential incorporation of HFC-125a into large cages, promotes structure II formation and stabilizes the hydrate framework. Raman spectroscopic evidence confirms the enclathration of CO₂ in sI hydrates and HFC-125a in sII hydrates, supporting the coexistence of both structures rather than complete structural conversion. Although the mixed hydrate system exhibits a lower overall hydrate yield compared to pure HFC-125a, it offers functional advantages through enhanced cage utilization, controlled gas incorporation, and tunable dissociation behavior. These characteristics directly address the objective of developing hydrate-based fire suppression materials that combine rapid heat absorption with the controlled release of inert and flame-inhibiting gases. The mechanistic understanding established in this work provides a foundation for the rational design of mixed hydrate compositions and operating conditions, enabling improved performance and reliability in hydrate-based fire suppression technologies.

The authors declare no competing financial interest.