Environ. Sci. Technol. 1997, 31, 3318-3324
Water Absorption by Organics: Survey of Laboratory Evidence and Evaluation of UNIFAC for Estimating Water Activity PRADEEP SAXENA* AND LYNN M. HILDEMANN Environmental and Water Studies Program, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020
The composition of organic material in atmospheric particles and the influence of these organics on aggregate particle properties have remained less well characterized than that of the inorganic ionic fraction. While laboratory and atmospheric studies strive to quantify the formation rates and concentrations of water-soluble and other organic compounds in atmospheric particles, concerted efforts are being devoted by many scientists to develop models for simulating the formation and gas-particle distribution of condensible organics in the atmosphere. Within this research milieu, as a first step toward developing a capability to simulate the thermodynamics of aqueous, organic-containing submicron droplets under atmospheric conditions, in this paper we (i) synthesize published laboratory data to evaluate the water absorption behavior of multifunctional oxygenated organic compounds and (ii) test the reliability of the UNIFAC group contribution method (1) for estimating water activities of aqueous organic solutions. The laboratory data show that multifunctional oxygenated compounds can absorb water over the entire range of relative humidities. For a wide variety of compounds (e.g., glycols, dicarboxylic acids, keto acids) and a wide range of solute concentrations (0 to >90% by wt), we find that, in most cases, water activities can be estimated to within approximately 15%.
Introduction Air quality models encompassing the formation and properties of atmospheric particles enable scientists and air quality managers to estimate quantitative relationships between emissions and ambient concentrations of particles as well as between emissions and hazes (e.g., refs 2-7). Such models could be used to forecast the changes in atmospheric aerosol concentrations and hazes that would result from changes in emissions (8, 9). Typically, important physical and chemical processes pertaining to particles (nucleation, condensation, coagulation, interfacial mass transfer, interfacial and chemical equilibrium, and condensed phase reactions) are represented in these models by various algorithms (aka modules). Here, we focus on the algorithms for calculating the equilibrium between gas and particle phases. For atmospheric inorganic aerosols, computer algorithms are available for estimating the concentration of each component in each phase from the given total concentration * Corresponding author present address: Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303-0813. Fax: 650-8551069; e-mail:
[email protected].
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over all phases, the temperature, and the relative humidity (RH) (10-14). Using the assumption that the vapor and particle phases are in equilibrium, the approach is to solve the interfacial and chemical relationships using laboratory data on equilibrium constants and activity coefficients. In addition to serving as building blocks for air quality models, these algorithms are useful by themselves for estimating properties that are difficult to measure: particle water content and pH, for instance (15, 16). Atmospheric aerosols contain condensed organics as well as inorganics (9, 17-22); thus far, organics have not been included in equilibrium algorithms. This is largely because organics are difficult to sample and analyze, and consequently, the composition of the organic particulate matter (PM) itself is not well known: typically, 10% or less of the total organic PM in the atmosphere is resolved into individual compounds (19, 23, 24). In response to these gaps, there has been an upsurge in research dealing with condensible atmospheric organics. Laboratory experiments and theoretical assessments are providing air-water equilibrium constants as well as data on the mass and composition of organic PM formed from gaseous precursors (25-30). Simulation methods have been developed to estimate the formation rates and/or gas-particle distribution of condensible organics in the atmosphere (3138). Atmospheric experiments are attempting to more fully characterize the composition of organic aerosols and amass the data needed to evaluate air quality models for PM (e.g., refs 24 and 39-43). Recent analyses of data have provided (a) empirical evidence for water absorption by presumably water-soluble particulate organics in the atmosphere (44, 45) and (b) tentative identities of individual organic compounds that are likely or unlikely to contribute to the water-soluble fraction in atmospheric particles (27, 28, 46). In particular, from a review of compound solubility and condensibility, Saxena and Hildemann (28) have concluded that C2-C7 multifunctional compounds that contain carboxylic, hydroxy, carbonyl, and amino functional groups are likely to contribute substantially to the water-soluble organic PM in the atmosphere. Armed with this new information on potential contributors to the water-soluble organics in atmospheric particles, in this paper we evaluate the water absorption behavior of a wide variety of oxygenated multifunctional organic compounds using published laboratory data. Next, we test the reliability of a method for estimating the water activities of these organic compounds of interest in aqueous solutions. By identifying a method suitable for estimating water activities and solute activity coefficients for aqueous organic solutions of interest, we hope to pave the way for assembling a first generation equilibrium algorithm for water-soluble organics.
Approach We envision that the results of this work are likely to be used, at least initially, in conjunction with observations of aerosols in the lower atmosphere (near earth’s surface); therefore, we have reviewed the literature to seek laboratory observations of activity coefficients (or related properties like osmotic coefficients) in binary and multicomponent aqueous organic solutions for T = 25 °C and P = 1 atm. The interest here is in oxygenated water-soluble compounds known or expected to be present in atmospheric particles. In particular, we have sought data for the candidate compounds identified by Saxena and Hildemann (28): these are aliphatic C2-C7 compounds containing two or more of the following functional groups: COOH, COH, CdO, and CNH2. Recognizing that straightchain dicarboxylic acids and straight-chain and cyclic poly-
S0013-936X(97)00363-5 CCC: $14.00
1997 American Chemical Society
hydroxy alkanes are potentially the two most important compound classes (47-50), we have sought observations for each homologue in these series. We have found that few if any observations are readily available for many compounds (e.g., glutaric acid); therefore, we have included here compounds which, though unlikely in the atmosphere, are structurally similar to compounds of interest and are therefore relevant for characterizing water absorption. For instance, although not a known constituent of atmospheric particles, sucrose is included here since it is relevant for characterizing water absorption by the hydroxy- and polyhydroxy carbonyls (e.g., glyceraldehyde) that we do expect to find in the atmosphere (51-55). Turning to activity coefficient estimation, since atmospheric droplets may contain numerous (tens and perhaps hundreds of) compounds, treating each compound as a molecule would not only be cumbersome but would also require at minimum binary aqueous data for each and every compound; for many compounds of interest such data are unlikely to be available. Fortunately, the problem of characterizing activity coefficients in multi-organic solutions has previously been faced in many chemical engineering applications (process design, pharmaceutical, biotechnology, wastewater treatment): as a result of past research motivated by these applications, we have at our disposal structured and matured approaches known as group contribution methods or GCMs (56-58). The rationale behind the GCM approach is that a solution containing numerous organic compounds can be treated as a solution of fewer and thus a more manageable number of functional groups. GCMs are attractive because parameters are defined for groups and group pairs; therefore, binary activity coefficient data for specific molecule pairs are not required for estimating the parameters and thus the activity coefficients. From a suite of available GCMs, we have opted to test the original UNIFAC (59-61), which is perhaps the most widely used method for estimating activity coefficients of organic solutes (UNIFAC stands for the UNIQUAC Functional Group Activity Coefficients Model where UNIQUAC stands for Universal Quasi Chemical). In UNIFAC expressions (59, 60), the activity coefficient of component i is based upon (a) Raoult’s law (RL) aka LewisRandall standard state and (b) the mole fraction concentration scale (for definitions, see refs 28 and 62). Often it is necessary to convert activity coefficients based upon the RL standard state to that based upon the Henry’s law (HL) standard state. For instance, the numerical values of air-water and other equilibrium constants are typically derived from observations by assuming that the activity coefficients in dilute solutions are unity; a choice of HL standard state is implicit in this assumption (28). Therefore, while using these numerical values in solving vapor-liquid and chemical equilibria equations, we must use activity coefficients based upon the HL standard state. The activity coefficients based upon the two standard states are inter-convertible as follows:
γi(HL) γi(RL)
)
1 γi∞
where γi(RL) and γi(HL) are the activity coefficients based upon RL and HL standard states, respectively, at solute concentration xi, and γi∞ is the activity coefficient at infinite dilution (based upon the RL standard state). The value of γi∞ can be derived by solving the UNIFAC equations for a solute concentration of zero. In applying UNIFAC, one needs the following parameters: the volume (R) and surface area (Q) for each group and a pair of group interaction parameters (amn and anm) for each group pair. UNIFAC authors derived each volume and surface area from atomic and molecular structures by taking into consideration bond angles and bond distances. The pub-
lished group interaction parameters were derived using a substantial corpus (over 14 000 data sets) of vapor-liquid equilibria (VLE) observations in the Dortmund Data Bank (63). The published UNIFAC parameters are dynamic compilations: new observations are being continually added to the Data Bank and group interaction parameters are being continually added or revised (61, 64-68). Interaction parameters are available for most groups of interest to us (1). Before proceeding to test UNIFAC for aqueous organic solutions, we ask ourselves: if UNIFAC parameters are derived by fitting an equation to observations, then would we be (i) repeating the tests already performed and published by UNIFAC authors and (ii) conducting a cyclical exercise by comparing measurements with estimates based upon UNIFAC parameters that were derived in the first place to achieve best agreement with the experimental data? The answer to both questions is “no” for the following reasons: (a) Published UNIFAC performance results depict the deviations between the estimated and the observed values for vapor-phase composition (e.g., ref 66) as well as, in some cases, for activity coefficients (e.g., ref 59). To characterize overall performance, UNIFAC authors have also published the deviations averaged over all data points for all solutions (1, 69, 70). To our knowledge, the results we present here of UNIFAC performance in estimating water activities for specific organic compounds have not been published before. (b) In deriving the interaction parameters for a particular group pair, the fitting procedure used by UNIFAC authors is to minimize the deviation (either in activity coefficient or in vapor-phase mole fraction and pressure) summed over all observations for all relevant binary and multicomponent solutions (64, 68). For instance, the interaction parameters for the methyl-carboxyl pair were derived by considering all available data for solutions with alkanes and organic acids. Since the parameters for the methyl-carboxyl pair are needed for deriving the parameters for the water-carboxyl pair from VLE data for aqueous organic acid solutions, instead of estimating the parameters sequentially, the authors estimated them simultaneously by applying a minimization procedure to the combination of the two data sets (61). Therefore, the published UNIFAC parameters we have used here were derived wherever possible from observations for more than one specific binary molecule pair. In fact, in estimating the group interaction parameters, UNIFAC authors did not use any observations for many of the multifunctional compounds we consider in this paper (e.g., malic acid). (c) UNIFAC is not recommended for compounds in which two strongly polar groups are separated by less than three or four carbon atoms because the interaction (hydrogen bonding) between these polar groups is important but not accounted for in UNIFAC (1). While more fundamental solutions (e.g., application of ab-initio quantum mechanics) are presently being sought for treating the interactions between proximate polar groups in a molecule (71-73), in the interim UNIFAC authors and others have resorted to ad hoc empiricism to deal with this information gap: for instance, 1,2-ethanediol is defined as a special group (glycol) because large deviations between UNIFAC estimates and observations result if this compound is described by two alcohol groups (1). Since we are interested in applying UNIFAC to multifunctional oxygenated compoundssprecisely those compounds for which the method is known to have shortcomingssit is necessary to specifically test and document the method performance. (d) UNIFAC authors estimated the group interaction parameters using VLE measurements for a wide range of temperatures and pressures; observations for temperatures up to 152 °C were used (69, 70). For aqueous solutions of relatively less volatile organic compounds (e.g., multifunctional compounds like polyols), the majority of observations in the Dortmund Data Bank (74-76) are either for relatively
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FIGURE 1. Laboratory evidence for water absorption by organics in equilibrium with the vapor phase (liquid phase water activity ) relative humidity). We (i) assume that the vapor phase is an ideal gas and (ii) use Raoult’s law standard state for the solvent. These data apply to bulk solutions and particles for which the Kelvin effect is negligible. high temperature (T) and/or for relatively low pressure (P); for instance, for 1,2-ethanediol, most observations are for temperatures between 50 and 197 °C. Therefore, our tests will be valuable in delineating the magnitude of errors that result when water activities at T = 25 °C and P = 1 atm are estimated using UNIFAC parameters derived from VLE observations for a different range of temperatures and pressures.
Results Laboratory measurements show that at equilibrium a wide variety of multifunctional organics in the condensed phase absorb water from the vapor phase (Figure 1). Finitely soluble compounds deliquesce at the water activity of their saturated solutions: this is the case for sucrose, mannitol, succinic acid, malic acid, citric acid, R-keto glutaric acid, tartaric acid, glycine, and urea. We would expect similar, deliquescent behavior for other finitely soluble compounds (e.g., malonic, glutaric, and maleic acids) observed in atmospheric particles (e.g., ref 79) for which water activity data are not readily available. For the compounds considered here, the relative humidity of deliquescence (RHD), i.e., the relative humidity at which that compound deliquesces, is in the range of 75% (urea) to >99% (succinic acid). In contrast, compounds completely miscible with water absorb water at all relative humidities. This is the case for 1,2-ethanediol, 1,2-propanediol, diethylene glycol, and 1,2,3propanetriol (glycerol); note that although observations are not available for water activities
