# On the mutual solubility of water with H2/N2/CO2 gas mixtures under high pressure/temperature conditions relevant to hydrothermal vent chemistry: A co-localized infrared/Raman spectroscopic investigation

Grégoire Boé, Thierry Tassaing

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Grégoire Boé, Thierry Tassaing. On the mutual solubility of water with H2/N2/CO2 gas mixtures under high pressure/temperature conditions relevant to hydrothermal vent chemistry: A co-localized infrared/Raman spectroscopic investigation. Journal of Molecular Liquids, 2025, 425, pp.127204. 10.1016/j.molliq.2025.127204. hal-05031393

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# On the mutual solubility of water with H₂/N₂/CO₂ gas mixtures under high pressure/temperature conditions relevant to hydrothermal vent chemistry: A co-localized infrared/Raman spectroscopic investigation

Grégoire Boé,¹ Thierry Tassaing,¹*

¹ Institut des Sciences Moléculaires (ISM), Univ. Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, F-33400 Talence, France

Corresponding author: thierry.tassaing@u-bordeaux.fr

## Abstract

This article reports an experimental investigation of the phase equilibrium and the local order of water molecules in the multicomponent system H₂O, CO₂, N₂, H₂ at high temperatures and pressures using a co-localized Infrared/Raman spectroscopic approach. Thus, we have investigated the gas and the liquid phase of the binary, ternary and quaternary mixtures of water-CO₂-N₂-H₂ in order to characterize the mutual solubility of each component and the evolution of the local order in both phases as a function of temperature in the range 40-320°C at a constant pressure P=20 MPa. In particular, the effect of H₂ and N₂ on the thermophysical properties of the CO₂/water mixtures are presented. The simultaneous recording of IR and Raman spectra using a single high-pressure cell for the analysis of the gas phase allows several benefits by saving time and resources, and allowing a direct correlation between IR and Raman data, and thus allowing quantitative analysis from Raman data. Raman spectra of the liquid phase have allowed to determine the fraction of water and of the different gases and the influence of the different gases on the local order of water molecules. Finally, the influence of the presence of NaCl in water has been evaluated.

## Keywords

Water/gaz mixtures, in situ co-localized Infrared/Raman spectroscopy, phase equilibrium, local order, high pressure, high temperature.
# I. Introduction

Understanding the mutual solubility of water with multi-component gas mixture (CO₂, N₂, H₂) at high pressures and temperatures is of great interest in various fields such as gas storage in deep aquifers¹, hydrogenation processes for biomass valorization² and for the understanding of the chemistry in hydrothermal vents.³ Hydrothermal vents form when seawater seeps into the ocean crust through fissures and becomes superheated by underlying magma. As the water warms and get enriched in gases, it dissolves minerals from the surrounding rocks and rises back to the seafloor, where it emerges as a superheated complex fluid in cold sea water. In particular, it has been shown that various gases (H₂, N₂, CO, CH₄, ...) are present in the hydrothermal fluids that can interact and react with the surrounding metal rich porous mineral (olivine, pyrite, magnetite, ...) to produce various prebiotic molecules.⁴,⁵ Therefore, a precise knowledge of the thermodynamic of such multicomponent system at different temperature and pressure is essential for the description of the interactions and the reactivity that may exist between the hydrothermal fluid and the mineral. In addition, solubility data under high pressure-high temperature conditions of these multicomponent systems allow feeding some databases necessary for the validation of thermodynamic models that attempt to describe these complex gas-liquid equilibria at high temperature and pressure, in particular when the aqueous phase present various salinity levels. However, the experimental studies of these hydrothermal environments are generally limited as it is difficult to analyze and sample these systems under extreme conditions. Until now, the binary system water-CO₂ is one of the most investigated one in the literature.⁶⁻⁹ In particular, phase behavior,⁸ spectroscopic studies¹⁰,¹¹ and atomic scale insights¹² of CO₂-water binary mixtures have been performed for a deeper molecular level understanding of their thermophysical properties. Thus, it was found that the increase with temperature of the CO₂-water mutual solubility with a complete miscibility reached at temperature close to the critical temperature of water was related to a progressive breakdown in the hydrogen bond network of water leading to a predominant number of H₂O molecules existing as monomers at the highest temperatures. Some experimental studies have been led on N₂-water¹³⁻¹⁵ and H₂-water¹⁶,¹⁷ binary mixtures, and mostly consist of measures of density in one or both phases of the system. However, little to none is known about the phase behavior of more complex systems such as ternary and quaternary mixtures. Adding new compounds makes the system always more complex, and lead up to a four-phase hydrate-liquid-liquid-vapor equilibrium, depending of the range of temperature and pressure studied. CO₂-H₂-Water¹⁸,¹⁹ and CO₂-N₂-Water²⁰⁻²² systems have been studied at high pressure-low temperature for
example, to understand gas hydrates formation and the interfacial tension of the mixtures. For quaternary or higher-order mixtures, the available literature mostly focus on either hydrates formation<sup>23</sup>, at low temperature, or gas and oil separation<sup>24</sup>.

Thus, in order to get quantitative information about the phase behavior of the multi-component system H₂O, CO₂, N₂, H₂, we have performed in-situ IR/Raman measurements of the gas and the liquid phase as a function of temperature in the range 40-320°C at a constant pressure P=20 MPa for the binary, ternary and quaternary mixtures. To analyze the gas phase, we have used a novel co-localized Infrared/Raman spectroscopic approach<sup>25</sup> that allows the simultaneous recording of IR and Raman spectra of the same sample in a single high-pressure cell to reach a direct correlation between IR and Raman data, and thus allowing quantitative analysis from Raman data. From the analysis of the Raman and infrared spectra, the influence of the nature of the gas on the local order of water molecule is also discussed. To analyze the liquid phase, we have used only Raman scattering as the strong absorption of water precludes the analysis of this phase using IR spectroscopy. However, a calibration method of the Raman spectra proposed in this paper allowed us to determine the molar fraction of water and of the different gases in the liquid phase and the influence of the different gases on the local order of water molecules. Finally, the influence of the presence of NaCl in water at the concentration found in seawater has been evaluated.

## II. Materials and methods

### 1. Materials

The neat gases used in this study were used as received with the following characteristics: CO₂ (Air Liquide, 99.95% purity), N₂ (Messer, 99.995% purity), and H₂ (Air Liquide, 99.9999%). Ultra-high purity water was obtained from a Milli-Q Synergy UV unit with a resistivity of 18.2 MΩ.cm at 25 °C. Sodium chloride (ACS reagent, >99.0%) was acquired from Sigma Aldrich.

### 2. In Situ High-Pressure Co-Localized IR-Raman Spectroscopy

The in-situ analysis of the water-gases mixture using a combination of IR and Raman spectroscopy was done in one high-pressure high-temperature (HP-HT) optical cell (Figure 1). The homemade stainless-steel cell is composed of four cylindrical windows (two silica windows for Raman scattering and two sapphire windows for IR absorption using a path length of 8.3 mm. Even if silicon windows would allow to investigate the low wavenumber range up to 1200 cm⁻¹ for IR absorption, an oxidation of the silicon window is starting at about 150°C when mixtures with water are investigated which leads to a damage of the window that
precludes any investigation at higher temperature up to 320°C as we did in this work. Windows were positioned on the flat surface of an Inconel plug with a graphite foil placed between the window and the plug-in to compensate for imperfections at the two surfaces (unsupported area principle). Flat graphite rings were used to ensure sealing between the plug and the cell body. The total volume of the cell is 9 ml. The heating was achieved using four cartridge heaters disposed in the body of the cell and a thermocouple located close to one cartridge was used to regulate the temperature with an accuracy of about $ΔT = ± 0.5$ °C. The cell was connected using a stainless capillary to a pressure gauge allowing it to measure the pressure with an accuracy of about $ΔP = ± 0.01$ MPa.

<figure><img src="image_2.png" /><figcaption>Figure 1 Detailed scheme of the experimental setup and front and cut view of the high-pressure/high-temperature cell designed for this study.</figcaption></figure>

Infrared absorption measurements were carried out using a Thermo Fisher 6700 Fourier transform infrared spectroscopy (FT-IR) spectrometer equipped with a Globar source, a potassium bromide-germanium (KBr-Ge) beamsplitter, and a deuterated L-alanine doped glycine sulfate-KBr thermal detector to study the spectral range in the mid-IR from 400 to 7500 cm⁻¹. Single-beam spectra with a resolution of 1cm⁻¹ were obtained by Fourier transformation of 30 cumulated interferograms in order to improve the signal-to-noise ratio.

Raman scattering measurements were carried out using a portable spectrometer designed in the laboratory equipped with a PGL-VI-532 laser diode (EDMUND) (532 nm, 40 mW) as laser source and an iHR-320 spectrometer (Horiba Jobin Yvon) for measurement (array: 1200 lines/mm, slit: 100 µm). This device makes it possible to reach the spectral range between 200 and 4250 cm⁻¹ with a spectral resolution between 8 and 4 cm⁻¹ (~6 cm⁻¹ at 1000 cm⁻¹).
### 3. Experimental Procedure

Prior to all experiments, the cell was cleaned with ultra-pure water, dried, and flushed with the gas used in the experiment to remove atmospheric gases and water. Then, the cell was filled with 5ml of water with a syringe, and heated to 40°C before injecting the gases to reach 20 MPa. The mixtures were made at 40°C and are considered as ideal gas mixtures. Their initial composition is shown in Table 1. The spectra were recorded for each mixture by increasing the temperature and decreasing the pressure step by step by opening the cell purge valve, allowing the pressure to be maintained at 20 MPa.

<table><caption>Table 1 Composition of laboratory prepared mixtures with the pressure of each gas and their corresponding concentrations added in the high-pressure cell at 40°C.</caption><thead><tr><th rowspan="2"></th><th colspan="3">Partial Pressure (MPa)</th><th colspan="3">Concentration (mol.l<sup>-1</sup>) (NIST)<sup>26</sup></th></tr><tr><th>CO<sub>2</sub></th><th>H<sub>2</sub></th><th>N<sub>2</sub></th><th>CO<sub>2</sub></th><th>H<sub>2</sub></th><th>N<sub>2</sub></th></tr></thead><tbody><tr><td>H<sub>2</sub>O-CO<sub>2</sub></td><td>20</td><td>-</td><td>-</td><td>19.082</td><td>-</td><td>-</td></tr><tr><td>H<sub>2</sub>O-H<sub>2</sub></td><td>-</td><td>20</td><td>-</td><td>-</td><td>6.8682</td><td>-</td></tr><tr><td>H<sub>2</sub>O-N<sub>2</sub></td><td>-</td><td>-</td><td>20</td><td>-</td><td>-</td><td>7.2114</td></tr><tr><td>H<sub>2</sub>O-CO<sub>2</sub>-H<sub>2</sub></td><td>10</td><td>10</td><td>-</td><td>14.283</td><td>3.6300</td><td>-</td></tr><tr><td>H<sub>2</sub>O-CO<sub>2</sub>-N<sub>2</sub></td><td>10</td><td>-</td><td>10</td><td>14.283</td><td>-</td><td>3.7932</td></tr><tr><td>H<sub>2</sub>O-H<sub>2</sub>-N<sub>2</sub></td><td>-</td><td>10</td><td>10</td><td>-</td><td>3.6300</td><td>3.7932</td></tr><tr><td>H<sub>2</sub>O-CO<sub>2</sub>-H<sub>2</sub>-N<sub>2</sub></td><td>10</td><td>5</td><td>5</td><td>14.283</td><td>1.8668</td><td>1.9184</td></tr></tbody></table>

### 4. Data analysis

#### Gas phase:

The determination of the mutual solubility of each component in the gas phase is allowed by the coupling of the two spectroscopic methods, which are carried out simultaneously on the cell filled with one of the studied mixtures. The detailed procedure used to calibrate the Raman data using the infrared measurements in order to determine the mutual solubility of each component in the gas phase is given in ESI1. In order to validate our calibration method, we have compared some of our results with few literature data that have been obtained on the binary mixtures H₂O-CO₂, H₂O-H₂ and H₂O-N₂ in the pressure and temperature domain studied here. Most of the results have been obtained by ex situ density measurement of gas and liquid phases, after extraction from high-pressure vessel. The results reported present a really high precision on the mass measurement, but may exhibit a systematic error linked to the transfer of liquid or
gas necessary for the measurements. Our results obtained from in situ spectroscopic measurements on the gas phase are compared to the literature results obtained at the same pressure in Figure 2.

<figure><img src="image_3.png" /><figcaption>Figure 2 Comparison between measured mole fraction and literature data for the binary mixture's gas phase (20MPa from 40 to 320°C)</figcaption></figure>

The H₂O-CO₂ mixtures comparison validates the results obtained with really low differences in the mole fraction. However, a slightly bigger difference is observed for H₂O-H₂ and H₂O-N₂ mixtures, obtained from the co-localized calibrated Raman measurements. The results obtained by this method, as presented in more details previously, show a greater uncertainty (up to 10%) than the results obtained by direct infrared quantification.²⁵ The error linked to the experimental limits at the time of the results available in the literature could also partly explain the difference with the results of R. Wiebe et al.¹³ and A.W. Saddington et al.¹⁵ for each of the two mixtures. All in all, these results highlight the reliability of the experimental approach and the robustness of the IR/Raman co-localized measurements.

### Liquid phase:

The same IR/Raman co-localized measurements couldn't be performed for the liquid phase, as the strong absorption of water in the studied spectral domain precludes FTIR measurements in transmission mode using our HP-HT cell. However, semi-quantitative measurement can be done from the measured Raman intensities of each compound and allows to obtain molar fraction of each compound in the mixture. The detailed procedure used to extract from the spectra the molar fraction of each compound in the liquid phase is given in ESI1. The results obtained for the liquid phase are much more difficult to compare as almost no experimental data are available. The rare data we could find were determined by collecting water in the temperature and pressure conditions studied, and results were obtained by weighting the gas
phase of the sample once returned to normal pressure and temperature conditions. Those literature data for the H₂O-N₂ and H₂O-H₂ binary mixtures are compared in Figure 3 to the ones obtained in-situ according to the method described in ESI1. We emphasize that the data published in the literature for the binary mixture H₂O-CO₂ were used as reference for our calibration method.

<figure><img src="image_4.png" /><figcaption>Figure 3 Comparison between measured mole fraction and literature data for the liquid phase of the binary mixtures (20MPa from 40 to 320°C)</figcaption></figure>

For the reasons expressed above, we observe a bigger difference between our results and those obtained from the literature. For the binary H₂O-H₂ mixture, the limit of sensibility was reached and doesn't allow to measure any hydrogen in the liquid phase at 40°C. However, at higher temperature, our data gets closer to the ones obtained by R. Dohrn et al.²⁴ and validate the correction done on the SCS relative to water. For the nitrogen mixture, data at low temperatures match well with the ones of the literature, but show a bigger difference at higher temperature. In both mixtures, we clearly reach the limits of our method, linked to the sensitivity of the Raman measurements. However, these experimental results will still allow to determine relative quantities to characterize the mixture phase behavior.

### III. Results and discussion

In order to understand the equilibrium of the systems studied and the influence of the different compounds on the stability of both phases, we analyze individually each phase present in the high-pressure optical cell.

#### 1. Gas phase

The infrared spectra recorded for a binary mixture H₂O-CO₂ and a quaternary mixture H₂O-CO₂-H₂-N₂ at constant pressure of 20 MPa at different temperatures from 40 to 320°C are
presented in Figure 4. The spectral regions characteristic of H₂O and CO₂ and the assignment of the vibrational modes are also reported.

<figure><img src="image_5.png" /><figcaption>Figure 4 FTIR-spectra of the gas phase of H₂O-CO₂ and H₂O-CO₂-H₂-N₂ mixtures at 20MPa</figcaption></figure>

We observe qualitatively that upon an increase of the temperature, the intensity of the contribution of CO₂ decreases while H₂O absorption rapidly increases. Naturally, the signals attributed to CO₂ are weaker in the quaternary mixture since it is present in a lower proportion. At a first glance, there is no significant influence of the presence of N₂ and H₂ on the band shape of H₂O and CO₂ in comparison with the binary mixture H₂O-CO₂. However, we can notice the presence of the induced hydrogen infrared absorption band, centered around 4200cm⁻¹.²⁷

<figure><img src="image_6.png" /><figcaption>Figure 5 Raman-spectra of the H₂O-CO₂-H₂-N₂ mixtures at 20MPa</figcaption></figure>
The Raman spectra obtained simultaneously with the infrared spectra for the quaternary mixture H₂O-CO₂-H₂-N₂ are presented in Figure 5. In the low frequency range, several rotational transitions of H₂ are active in Raman (denoted S₀(0), S₀(1), S₀(2), and S₀(3)) and are observed respectively at 343, 580, 806 and 1031 cm⁻¹. At higher frequencies, hydrogen can be identified by the fundamental stretching band of H₂ (the vibron) with its rovibrational Q-branch, which can be observed between 4125 and 4160 cm⁻¹.²⁸ These transitions appear through two peaks in our spectra, as the resolution of our Raman spectrometer doesn't allow to identify them individually (Figure 5). The two bands observed at 1284 and 1387 cm⁻¹ are attributed to the Fermi dyad of CO₂.²⁹ Nitrogen displays a fundamental stretching band at 2228 cm⁻¹.³⁰ The band observed at about 3640 cm⁻¹ is associated with the symmetric O-H stretching mode v₁ of monomeric water (see discussion below). Finally, the peak observed at 2980 cm⁻¹ is an artefact due to the scattering of the He-Ne laser of the FTIR spectrometer.

From both spectroscopic techniques, we clearly observe the water signal increasing in the gas phase, while other compounds are getting lower with the increase of the temperature. To have a quantitative approach of this effect, the methodology explained in details in the ESI1 is applied and allows the complete quantification of the system. The results obtained are represented in Figure 6 which illustrates the composition of the gas phase of the pressurized mixtures as a function of the temperature. For every mixture studied, the solubility of water increases in the gas phase at higher temperature. CO₂ concentration decrease rapidly with the augmentation of the temperature (Figure 6 a.), which can be explained by the rapid diminution of its density as we move away from its critical temperature. CO₂, H₂ and N₂ are supercritical in the whole domain of the temperature studied at 20 MPa. However, H₂ and N₂ have no brutal density loss as the temperature increases (Figure 6 b.) since they behave closer to a low-density-gas phase due to their much lower critical temperature (respectively -239.96°C and -146.9 °C).

<figure><img src="image_7.png" /><figcaption>Figure 6 Evolution of the molar concentrations in the gas phase as a function of temperature for mixtures a. H₂O-CO₂ and b. H₂O-CO₂-H₂-N₂ at 20MPa</figcaption></figure>
Figure 7 shows the evolution of the water molar concentration and density of the binary mixtures and the quaternary mixture, as a function of temperature. The H₂O-CO₂ mixture shows a more pronounced increase in water concentration, reaching a value close to 7 mol.l⁻¹ at 320°C. The mixtures containing H₂O-H₂ and H₂O-N₂ show a lower water molar concentration over the entire temperature range studied. The water concentration of the quaternary mixture H₂O-CO₂-N₂-H₂ follows an intermediate trend to that of the binary mixtures presented.

The second part of the graph shows the density evolution of the gas phase of the different gas mixtures with increasing temperature. The H₂O-CO₂ and H₂O-CO₂-H₂-N₂ mixtures exhibit the highest densities over the entire temperature range which decrease with the temperature increase. In particular, mixtures containing CO₂ exhibit higher densities at low temperatures, due to the quasi-liquid behavior of CO₂ near its critical point. On the other hand, the H₂O-H₂ and H₂O-N₂ mixtures have the lowest densities over the entire temperature range which slightly increase with the temperature. Finally, as the amount of water increases with the temperature, the density of the different gas mixtures converges at the highest temperature roughly towards the density value of the gas phase of pure water that is 0.695 g.cm⁻³ at 320°C and 11.3 MPa (along the liquid-gas coexistence curve).

<figure><img src="image_8.png" /><figcaption>Figure 7 Evolution of a. the concentration of water in the gas phase and b. the density of the gas phase with the temperature, for the binary and quaternary mixtures under 20MPa.</figcaption></figure>

In order to evaluate the influence of the presence of H₂ or N₂ on the phase equilibrium of the H₂O-CO₂ binary mixture, we have also investigated the H₂O-CO₂-H₂ and H₂O-CO₂-N₂ ternary mixtures (Figure 8). As seen above, the H₂O-CO₂ mixture has a higher concentration of water
in the gas phase, while H₂O-H₂ and H₂O-N₂ mixtures display a lower concentration. The ternary mixtures H₂O-CO₂-H₂ and H₂O-CO₂-N₂ show an intermediate solubility of water between the ternary and the binary mixtures that is directly related to the lower amount of CO₂ in the ternary mixture (10MPa) than in the binary ones (20 MPa). Finally, the H₂O-N₂-H₂ ternary mixture was also investigated and shows that the solubility of water is equivalent to that of the H₂O-N₂ and H₂O-H₂ binary mixtures.

<figure><img src="image_9.png" /><figcaption>Figure 8 Evolution of the concentration of water in the gas phase for the binary and ternary mixtures under 20MPa.</figcaption></figure>

Therefore, Figure 8 highlights the significant influence of the nature of the gas mixture on the water concentration in the gas phase and put in evidence that mixtures containing CO₂ show a higher water concentration as found in Figure 7 that could be due to both molecular interactions and density-related effects.

<figure><img src="image_10.png" /><figcaption>Figure 9 FTIR spectra of the gas phase of the binary and quaternary mixture at 240°C and 20 MPa.</figcaption></figure>

These molecular interactions and/or density effects are highlighted by the observation of the FTIR and Raman spectra of water in the gas phase at 240°C and 20 MPa reported in Figure 9 and Figure 10 respectively. In infrared, for the H₂O-CO₂ mixture, the fine rotational P, Q, R
and P, R structures displayed for the band centered at 5350 and 7220 cm⁻¹ respectively are found to be broader compared to mixtures with non-polar gases such as H₂ and N₂. Such broadening, resulting from more frequent collision of water molecule with the surrounding molecules (CO₂, H₂ or N₂), could be due to the higher density of the CO₂ gas phase in comparison with both the H₂ and N₂ gas phase (see figure 7b). Alternatively, this broadening could result from the formation of weak bonds such as Lewis acid-base interactions between H₂O and CO₂ as reported in the literature³¹,³² that should lead to a stronger hindering of the rotational freedom of water molecules in CO₂ in comparison with H₂ and N₂. Indeed, mixtures containing H₂ and N₂ exhibit narrower and better resolved bands indicating an almost free rotation of water in these two gases due to expected weaker interactions of H₂ and N₂ with water.³³

<figure><img src="image_11.png" /><figcaption>Figure 10 Raman spectra of the gas phase of the binary and quaternary mixtures at 240°C and 20 MPa.</figcaption></figure>

In Raman, the evolution as a function of the nature of the gas of the peak position of the symmetric O-H stretching band (ν₁) of water molecules located around 3650 cm⁻¹ is reported in Figure 10. A shift to lower frequencies of about 4 cm⁻¹ and 5 cm⁻¹ of the (ν₁) vibrational mode in the case of the H₂O-CO₂ mixture is observed in comparison with the peak position measured with the H₂O-N₂ and H₂O-H₂ mixtures, respectively. Consistently with the infrared results (Figure 10), this observation can be explained by the stronger interaction of CO₂ with H₂O in comparison with N₂ and H₂ as well as the higher density of the CO₂ gas phase in comparison with both the H₂ and N₂ gas phase (see figure 7b). In addition, even if the difference is less pronounced, a weak shift of about 1 cm⁻¹ to lower frequencies is observed for the H₂O-N₂ mixture in comparison with the H₂O-H₂ mixture which is in line with the higher density of the N₂ gas phase in comparison with the H₂ gas phase (see figure 7b). Finally, we observe for the quaternary mixture that the peak associated with the (ν₁) vibrational mode appears to be
very close to that observed for the H₂O-CO₂ mixture that might be explained by the higher concentration of CO₂ in comparison with the other gases in the quaternary mixture.

## 2. Aqueous phase

The Raman spectra of the water phase recorded for a quaternary mixture H₂O-CO₂-H₂-N₂ at constant pressure of 20 MPa at different temperatures from 40 to 320°C are presented in Figure 11. The signals obtained for H₂ and N₂ are particularly weak, given their low solubility in water. As the temperature increases, the signal-to-noise degrades with increasing scattering in the observed solution, causing the baseline to increase (not observable in Figure 11 since the spectra are normalized).

<figure><img src="image_12.png" /><figcaption>Figure 11 Raman-spectra of the liquid phase of the H₂O-CO₂-H₂-N₂ mixture at 20MPa</figcaption></figure>

Raman scattering band of all compounds are integrated and treated as explained in the ESI1. This method doesn't allow an absolute determination of the concentration of the mixture, but allows us to determine the evolution of the mole fraction of every compound of the mixture in the liquid phase. We can, thanks to those data, evaluate the solubility of the different gases in the liquid aqueous phase, and how it varies with the temperature and the addition of other compounds.
<figure><img src="image_13.png" /><figcaption>Figure 12 Evolution of the mole fractions in the liquid phase of a quaternary mixture (H₂O-CO₂-N₂-H₂) as a function of temperature at 20 MPa.</figcaption></figure>

We have reported in Figure 12 the evolution as a function of temperature at a constant pressure of 20 MPa of the mole fraction of each component in the liquid phase of the quaternary mixture (H₂O-CO₂-N₂-H₂). Water makes up the majority of the liquid mixture, with a very high mole fraction, always above 98%, across the entire temperature range. Its mole fraction shows slight variations linked to the variation in CO₂ solubility. CO₂ is the second most important component in the liquid phase, with a mole fraction varying around 1% to 1.5%. The solubility of CO₂ in water starts to decrease from room temperature up to 100°C as usually expected for the solubility of CO₂ in various liquids and polymer at constant pressure. But in the case of water, due to the peculiar hydrogen bond network, increasing the temperature will also allow some hydrogen bonds breaking and will allow to increase again the solubility of CO₂ from 100°C. Therefore, the hydrogen bond network weakening might explain the minimum observed at 100°C. A maximum of solubility is then observed at 240°C, before decreasing again when the conditions approach the critical point of water. The mole fraction of N₂ and H₂ are about one order of magnitude lower than that of CO₂ (0.13% and 0.09 % for N₂ and H₂ respectively at 40°C). Although a slight decrease of their mole fraction is observed with temperature to reach a minimum around 100°C (0.06% and 0.05% for N₂ and H₂ respectively at 120°C), a further increase of the temperature from 100°C to 320°C leads to a significant increase of their mole fraction in particular for N₂ (0.34% and 0.15 % for N₂ and H₂ respectively at 320°C).

Then, in order to evaluate the influence of the presence of each compound in this multi-component quaternary mixture on the respective solubility of each compound, we have investigated the binary (H₂O-CO₂, H₂O-N₂, H₂O-H₂) and ternary (H₂O-CO₂-N₂, H₂O-CO₂-H₂, H₂O-N₂-H₂) mixtures and determine the molar fraction of the different components as a function of temperature from 40°C to 320°C at 20 MPa (Figure 13).
<figure><img src="image_14.png" /><figcaption>Figure 13 Evolution of the molar fraction of the different components in the liquid phase of binary, ternary and quaternary mixtures, as a function of temperature at 20MPa. Each subgraph corresponds to the variations of molar fraction for each component: CO₂, H₂, H₂O, and N₂.</figcaption></figure>

First of all, the variation in the mole fraction of CO₂ appears to be primarily governed by the partial pressure of CO₂ in the system. Indeed, in the binary mixture (H₂O and 20 MPa of CO₂), the liquid phase exhibits a significantly higher mole fraction of CO₂, nearly twice that of the other mixtures where the initial partial pressure of CO₂ is 10 MPa (Table 1). The presence of N₂ or H₂ does not seem to have a notable impact on the solubility of CO₂ in water.

The evolution of the mole fraction of H₂ is more complex to assess. CO₂ appears to enhance the solubility of H₂ at lower temperatures, while N₂ seems to restrict H₂ solubility at higher temperatures. At 320°C, we observe that increasing the partial pressure of H₂ leads to an increase of its mole fraction in the liquid phase: 5 MPa for the quaternary mixture results in a mole fraction of about 0.15%, 10 MPa for the ternary mixtures results in a mole fraction of about 0.25% and 20 MPa for the binary mixture results in a mole fraction of about 0.4%.

Regarding the case of N₂, we observe that the molar fraction of N₂ in the ternary and quaternary mixtures are quite similar over the temperature range investigated and lower than that measured for the binary mixture. Although this is consistent with the higher partial pressure of N₂ in the binary mixture, it seems that the addition of any other component (whether CO₂ or H₂) in the ternary and quaternary mixture substantially limits its solubility in the aqueous phase in particular at high temperature.
<figure><img src="image_15.png" /><figcaption>Figure 14 Evolution of the Raman band of water in the liquid phase for the three binary mixtures at different temperatures</figcaption></figure>

In order to discuss the influence of the nature of the gas on the structure of water, we have reported in Figure 14 the Raman spectra of water in the OH-stretching region of the binary mixtures H₂O-CO₂, H₂O-N₂ and H₂O-H₂ for three different temperatures. At the lowest temperature, the broad profiles that are characteristic of the hydrogen bonded network of water in these binary mixtures are superimposed thus showing that under these temperature and pressure conditions, the nature of the gas does not have a peculiar influence on the structure of water. However, upon an increase of the temperature up to 240°C, we observe a shift to higher frequencies and a narrowing of the three profiles that is due to a progressive weakening of the hydrogen bond network and the formation of oligomers that could further dissociate into dimers and monomers as the temperature increases up to 320°C.¹⁰ Interestingly, a noticeable shift toward higher frequencies is observed in the presence of CO₂, compared to other binary mixtures, a shift that is even more pronounced at the highest temperature of 320°C. This shift indicates the increased breaking of hydrogen bonds between water molecules in the presence of CO₂. In other words, CO₂ reduces structuring of nearby water molecules and acts as a “structure breaker” in water, particularly at elevated temperatures, facilitating the dissociation of water into smaller aggregates.³⁴,³⁵ By promoting the breaking of these bonds, CO₂ “shifts” the phase equilibrium of water, which likely explains the observed increase in water concentration in the gas phase, as discussed in the earlier section.

### 3. Saline water

In order to assess to which extent the presence of NaCl in the liquid phase could modify the phase equilibrium of the binary and quaternary mixture H₂O-CO₂-N₂-H₂, an additional study
was conducted using saline water. The salt concentration was selected to match the average salinity of oceans, approximately 35 g.L⁻¹.³⁶ Data are available in ESI 3, Table S5.

As reported in Figure 15 for the binary mixtures H₂O-CO₂ and H₂O-H₂ with and without NaCl, we observe that the presence of NaCl reduces the water concentration in the gas phase for both binary mixtures.

<figure><img src="image_16.png" /><figcaption>Figure 15 Comparison of the concentration of water as a function of the temperature in the gas phase of H₂O-CO₂ and H₂O-H₂ binary mixtures and their salted equivalent under 20MPa</figcaption></figure>

Considering the H₂O-CO₂ mixture in the presence of NaCl, our result is consistent with the previous investigation by Oparin et al.³⁷ putting in evidence that the transfer of water molecules from the water rich phase to the CO₂ rich phase is limited as NaCl is dissolved in the water rich phase. This phenomenon has been interpreted in terms of electrostriction effect, which is related to an increase in the density of water around ions compared to bulk water. This electrostriction effect results from a locally enhanced solvation of ions by water molecules hence limiting the concentration of water in the CO₂ phase as observed in Figure 15. In our study, the salt concentration is relatively low, and thus, the effect is correspondingly moderate in comparison with the study by Oparin et al.³⁷ showing that at higher concentrations of NaCl, the solvation of the salt mobilizes a larger proportion of water molecules, amplifying the electrostriction effect. Considering the H₂O-H₂ mixture, we observe a similar electrostriction effect leading to a lower concentration of water in the H₂ phase in the presence of NaCl in water.
<figure><img src="image_17.png" /><figcaption>Figure 16 Evolution of the concentration of water in the gas phase of the salted mixtures, as a function of the temperature and under 20 MPa</figcaption></figure>

The influence of the various compounds on the relative solubility of water in the gas phase is consistent with that observed under non-saline conditions (Figure 16). CO₂ promotes the solubility of water in the gas phase, while hydrogen has a comparatively lower effect. Nitrogen appears to have an intermediate influence. The quaternary mixture exhibits a water concentration that is intermediate between the three binary mixtures. Therefore, the electrostriction effect decreases the concentration of water in the gases phase for all the gases investigated and does not depend on the nature of the gas as the solubility ‘order’ of water is the same than that measured in neat water (C<sub>H2O</sub> in CO₂ > C<sub>H2O</sub> in N₂ > C<sub>H2O</sub> in H₂).

By mobilizing water molecules to interact with the ions formed during salt dissolution, this electrostriction effect will result into a salting-out effect that corresponds to the reduction of the solubility of gases in the aqueous phase in the presence of a salt.³⁸

In other words, gases are expected to be less soluble in the aqueous phase because water molecules preferentially interact with the dissolved ions and less “free” water molecules are available to stabilize dissolved gases. In order to put in evidence this salting-out effect, Figure 17 shows the evolution as a function of temperature of the mole fraction of H₂O and of the gaseous components (H₂, CO₂, and N₂) in the aqueous phase of two quaternary mixtures H₂O-CO₂-N₂-H₂: one containing pure water and the other containing saline water (3.5% NaCl).
<figure><img src="image_18.png" /><figcaption>Figure 17 Evolution of the mole fraction of the different components in the liquid phase of the quaternary mixture with pure and saline water, as a function of temperature at 20MPa. Each subgraph corresponds to the variations of molar fraction for each component: CO₂, H₂, H₂O, and N₂.</figcaption></figure>

The salting-out effect induced by NaCl clearly reduces the solubility of all the gases studied (H₂, CO₂, and N₂). This observation is consistent with the lower water concentration observed in the gas phase in the presence of salt. Furthermore, similar trends are observed in the evolution with temperature of gas mole fractions for the pure and saline water containing mixtures.

The results obtained for both the gas and aqueous phases consistently demonstrate the electrostriction and salting-out effect induced by NaCl. In the aqueous phase, the presence of NaCl significantly reduced the solubility of all gases studied (CO₂, H₂, and N₂), as water molecules preferentially interact with dissolved ions, limiting the availability of free water to stabilize gases. This results in lower water concentration in the gas phase. The convergence of the data from both phases clearly demonstrates the role of NaCl on the gas-phase equilibrium that leads to a shift of the region of existence of the two fluid phases to higher temperature.

#### IV. Conclusion

This study provides a detailed thermodynamic analysis of the phase equilibria and molecular interactions in multicomponent systems containing H₂O, CO₂, N₂, and H₂. The results highlight the influence of temperature, pressure, and the presence of dissolved NaCl, on the mutual solubility and phase behavior of these aqueous mixtures. The simultaneous use of IR and Raman spectroscopy allowed detailed insights into the molecular structures and interactions governing these systems. CO₂ acts as a key modulator of the phase equilibrium due to its relatively higher solubility and ability to engage in molecular interactions with water, while H₂ and N₂ demonstrate reduced solubilities due to their non-polar nature. The thermodynamic
equilibria between the gas and aqueous phases of mixtures of these compounds reflect the balance of these molecular interactions.

These findings not only have industrial implications, particularly for processes such as gas storage, carbon sequestration, and biomass conversion, but also offer significant insights into natural geochemical environments, such as hydrothermal vents. Hydrothermal systems, characterized by high pressures, temperatures, and salinities, represent ideal settings where the thermodynamic principles observed in this study could play a pivotal role in driving the formation of prebiotic organic molecules. The thermodynamic constraints within these environments, including the solubility limits and phase separation, may create conditions favorable for the synthesis of life-essential compounds. These results suggest that further thermodynamic modeling and experimental investigations into hydrothermal conditions could unravel new pathways for the abiotic synthesis of organic molecules, offering deeper insights into the origins of life on early Earth.

## Acknowledgments

This work was supported by the French "Agence Nationale de la Recherche" (ANR) [ANR-21-CE01-0018]; ANR is gratefully acknowledged.
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