Prediction of activity coefficients in liquid aerosol particles containing organic compounds, dissolved inorganic salts, and water—Part 3: Organic compounds, water, and ionic constituents by consideration of short-, mid-, and long-range effects using X-UNIFAC.3

Abstract X-UNIFAC.3, a group contribution method for estimating activity coefficients of neutral and ionic components in liquid mixtures of organic compounds, inorganic salts, and water, is presented here. It is an extended UNIFAC method, in that traditional UNIFAC terms for short-range energetic interaction effects are extended to include ions as mixture components, and are combined with a Debye–Huckel long-range effect term and a second virial coefficient-type mid-range effect term. The method is formulated for application in modeling the formation of liquid aerosol particles consisting of general organic+inorganic salt+water solutions in which phase separation is likely to occur. Existing extended UNIFAC activity coefficient estimation methods can be problematic in modeling phase separation, since they require independent reference state corrections that may introduce significant errors. In X-UNIFAC.3, this problem is avoided by selecting appropriate reference states for all solution components, and imposing additional constraints on method parameters, when necessary, by inclusion of reference state correction terms within the activity coefficient expressions. Interaction parameters in the X-UNIFAC.3 equations are optimized for 12 different chemical groups (CH3–, –CH2–, - C | H - , - C | | - , −OH, −COOH, H2O, NH4+, Na+, Cl−, NO 3 - , and SO 4 2 - ) using available data for systems containing multi-functional oxygenated organic compounds and/or inorganic salts that are relevant to atmospheric aerosol applications. Estimations of water activities and mean ionic activity coefficients using X-UNIFAC.3 are compared with those of other extended UNIFAC methods. To demonstrate the use of X-UNIFAC.3 in predicting phase separation, the method is also applied to the butanoic acid+NaCl+water system, for which experimental liquid–liquid equilibrium data is available. The method performs well for aqueous salt solutions with salt concentrations within 30 mol kg−1 and for organic+inorganic salt+water solutions with salt concentrations less than or equal to 10 mol kg−1. Suggestions are proposed for improving the predictive capabilities of the method in future work.

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