FUGACITY IN GAS MIXTURES DETERMINED FROM RAMAN SPECTROSCOPY

Fugacity is a fundamental thermodynamic quantity that is conceptually nebulous because it is generally obtained from thermodynamic calculations rather than by direct analysis. Specifically, fugacities of gases are sometimes described as analogues of partial pressures, adjusted to account for the non-ideality of real gases. Non-ideality, in turn, represents the effects of molecular-scale interactions (attraction and repulsion) between gas particles. These same molecular interactions are also reflected in spectroscopic analyses whereby Raman scattering yields information about the internal vibrational modes of covalently-bonded molecules, which are sensitive to the local chemical environment of the molecules. The Raman band positions of pure gases are strongly dependent on pressure and temperature, but the pressure-temperature-composition dependence of Raman bands in gas mixtures is poorly understood.

In this study, we performed spectroscopic analyses of gas mixtures of known composition at variable and known pressures using a high-pressure optical cell, and coupled these results with thermodynamic calculations of gas fugacities. The gases studied are mixtures of N₂, CH₄ and CO₂. We have analyzed mixtures of these gases at pressures from 1 to 50 MPa, at ambient temperature of 23 °C. Our results indicate that Raman waveshifts in gas mixtures are directly correlated with the fugacities of the component gas species, thus providing experimental evidence of a fundamental relationship between fugacities and Raman spectroscopic properties of gas mixtures. Thus, our results indicate that Raman spectroscopy can be used to estimate fugacities of gases in situ at elevated pressure. By this approach, the thermodynamic quantities fugacity and activity are linked directly with underlying molecular interactions (attraction, repulsion), according to the vibrational properties of gas species. This study represents a tractable method to efficiently obtain large datasets on thermodynamic properties of gas mixtures, and with some adjustments, may lead to a viable method for developing non-ideal mixing rules for other substances, such as crystalline solid solutions, aqueous ions and electrolytes.