Abstract
Abstract. We have used the COSMOtherm program to estimate activity coefficients and solubilities of mono- and α,ω-dicarboxylic acids and water in binary acid–water systems. The deviation from ideality was found to be larger in the systems containing larger acids than in the systems containing smaller acids. COnductor-like Screening MOdel for Real Solvents (COSMO-RS) underestimates experimental monocarboxylic acid activity coefficients by less than a factor of 2, but experimental water activity coefficients are underestimated more especially at high acid mole fractions. We found a better agreement between COSMOtherm-estimated and experimental activity coefficients of monocarboxylic acids when the water clustering with a carboxylic acid and itself was taken into account using the dimerization, aggregation, and reaction extension (COSMO-RS-DARE) of COSMOtherm. COSMO-RS-DARE is not fully predictive, but fit parameters found here for water–water and acid–water clustering interactions can be used to estimate thermodynamic properties of monocarboxylic acids in other aqueous solvents, such as salt solutions. For the dicarboxylic acids, COSMO-RS is sufficient for predicting aqueous solubility and activity coefficients, and no fitting to experimental values is needed. This is highly beneficial for applications to atmospheric systems, as these data are typically not available for a wide range of mixing states realized in the atmosphere, due to a lack of either feasibility of the experiments or sample availability. Based on effective equilibrium constants of different clustering reactions in the binary solutions, acid dimer formation is more dominant in systems containing larger dicarboxylic acids (C5–C8), while for monocarboxylic acids (C1–C6) and smaller dicarboxylic acids (C2–C4), hydrate formation is more favorable, especially in dilute solutions.
Highlights
Note that the COSMO-RS-DARE method is not used in the effective equilibrium constant calculations, because the clusters are already included in the calculation as products
We see that when using COSMO-RS-DARE, COSMOtherm is able to predict the miscibility of the smaller monocarboxylic acids (n = 1–4), but the experimental solubilities of pentanoic (n = 5) and hexanoic (n = 6) acids are overestimated to a greater degree than when using COSMO-RS
We compared COSMOtherm-estimated activity coefficients and aqueous solubilities of simple carboxylic acids with experimental values and a commonly used UNIFAC model, and we generally found a good agreement between experiments and COSMO-RS estimates
Summary
Mono- and dicarboxylic acids (which are CH3(CH2)n−2COOH and COOH(CH2)m−2COOH, respectively) are common atmospheric compounds that have been detected in both the gas (Kawamura et al, 2000; Fisseha et al, 2006) and aerosol phases (Fisseha et al, 2006; Verma et al, 2017; Guo et al, 2015; Hyder et al, 2012). Succinic, and glutaric acids (m = 3, 4, and 5) have been measured by Davies and Thomas (1956) and Soonsin et al (2010) in bulk and particle experiments, respectively Group contribution methods, such as UNIFAC (Fredenslund et al, 1975) and AIOMFAC (Zuend et al, 2008), are often used to estimate activity coefficients of atmospherically relevant compounds. Michailoudi et al (2020) estimated the activity coefficients of monocarboxylic acids with even numbers of carbon atoms (n = 2, 4, 6, 8, 10, 12) at infinite dilution They estimated the solubility of the same acids in pure water and different aqueous electrolyte solutions. We estimate aqueous solubilities and effective equilibrium constants of cluster formation of the acids
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