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New UNIFAC Parameterization C Water activity in polyol/water systems: new UNIFAC parameterization C. Marcolli, Th. Peter To cite this version: C. Marcolli, Th. Peter. Water activity in polyol/water systems: new UNIFAC parameterization. Atmospheric Chemistry and Physics, European Geosciences Union, 2005, 5 (6), pp.1545-1555. hal- 00295677 HAL Id: hal-00295677 https://hal.archives-ouvertes.fr/hal-00295677 Submitted on 17 Jun 2005 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Atmos. Chem. Phys., 5, 1545–1555, 2005 www.atmos-chem-phys.org/acp/5/1545/ Atmospheric SRef-ID: 1680-7324/acp/2005-5-1545 Chemistry European Geosciences Union and Physics Water activity in polyol/water systems: new UNIFAC parameterization C. Marcolli and Th. Peter Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland Received: 1 February 2005 – Published in Atmos. Chem. Phys. Discuss.: 14 March 2005 Revised: 11 May 2005 – Accepted: 25 May 2005 – Published: 17 June 2005 Abstract. Water activities of a series of polyol/water systems 90% in tropical forested areas (Yamasoe et al., 2000; Roberts were measured with an AquaLab dew point water activity et al., 2002). The amount of water absorbed by aerosol meter at 298 K. The investigated polyols with carbon num- particles can be significantly altered by the presence of or- bers from n=2–7 are all in liquid state at room temperature ganics (Saxena et al., 1995; Saxena and Hildemann, 1997). and miscible at any molar ratio with water. In aqueous solu- Conversely, gas/particle partitioning of semivolatile species tions with the same molar concentration, the diols with lower is influenced by the presence of water in particles (Griffin molecular weight lead to lower water activities than those et al., 2003). To tackle these interdependences, thermody- with higher molecular weights. For diols with four or more namic models are required that describe gas/particle parti- carbon atoms, the hydrophilicity shows considerable differ- tioning and water activity (Bowman and Melton, 2004). The ences between isomers: The 1,2-isomers – consisting of a hy- wide variety of organic species present in the ambient aerosol drophilic and a hydrophobic part – bind less strongly to wa- can only be handled by models that parameterize functional ter than isomers with a more balanced distribution of the hy- groups rather than individual compounds. The most common droxyl groups. The experimental water activities were com- group contribution method for organic substances is UNI- pared with the predictions of the group contribution method FAC (Fredenslund et al., 1975, 1977). Although it has been UNIFAC: the model predictions overestimate the water activ- shown that the performance of UNIFAC is not satisfactory ity of water/polyol systems of substances with two or more for predicting water activities of mixtures containing mul- hydroxyl groups and can not describe the decreased bind- tifunctional organic species (Saxena and Hildemann, 1997; ing to water of isomers with hydrophobic tails. To account Peng et al., 2001; Ming and Russell, 2002), it is commonly for the differences between isomers, a modified UNIFAC pa- used to describe gas/particle partitioning of semivolatile or- rameterization was developed, that allows to discriminate be- ganic species and water (Pun et al., 2002; Cai and Griffin, tween three types of alkyl groups depending on their position 2003; Griffin et al., 2003; Erdakos and Pankow, 2004). Ming in the molecule. These new group interaction parameters and Russell (2002, 2004) have therefore developed an im- were calculated using water activities of alcohol/water mix- proved parameterization to model the influence of organic tures. This leads to a distinctly improved agreement of model compounds in affecting droplet number densities in fog. predictions with experimental results while largely keeping This study investigates more closely how the performance the simplicity of the functional group approach. of UNIFAC depends on the number and the position of func- tional groups. This is done for a large variety of alcohols including monofunctional as well as polyfunctional ones. 1 Introduction Polyfunctional alcohols – so called polyols – together with polyethers have been identified by HNMR as a main class Organic species are emitted into the atmosphere by a va- of the water-soluble organic fraction of atmospheric aerosols riety of natural and anthropogenic sources. They account (Decesari et al., 2000, 2001). Various individual polyols and for up to 50% of the total fine aerosol mass at continental carbohydrates have been observed in biomass burning sam- mid-latitudes (Saxena and Hildemann, 1996) and for up to ples (Graham et al., 2002; Gao et al., 2003; Claeys et al., 2004, Simoneit et al., 2004). The hydroxyl group therefore Correspondence to: C. Marcolli can be considered as one of the most important functional (claudia.marcolli@env.ethz.ch) groups of organic aerosol constituents. A variety of mono- © 2005 Author(s). This work is licensed under a Creative Commons License. 1546 C. Marcolli and Th. Peter: New UNIFAC parameterization 3 UNIFAC group contribution method Table 1. UNIFAC volume (Rk) and surface area (Qk) parameters for subgroup k (from Hansen et al., 1991). The UNIFAC group contribution method (Fredenslund et al., 1975, 1977) is a broadly used tool for the prediction of liquid- Main group Subgroup k R Q k k phase activity coefficients parameterized for a wide range CHn (n=0,1,2,3) CH3 0.9011 0.848 of structural groups (Hansen et al., 1991). In the UNIFAC CH2 0.6744 0.540 model, the activity coefficients of a molecular component i CH 0.4469 0.228 (γi) in a multicomponent mixture are expressed as the sum C 0.2195 0 of two contributions: a combinatorial part (C), accounting OH OH 1.0000 1.200 for size and shape of the molecule and a residual part (R), a H OH O 0.9200 1.400 2 2 result of inter-molecular interactions C R ln γi = ln γi + ln γi . (1) The water activity is calculated as and polyfunctional alcohols are commercially available cov- ering a large number of chain lengths and isomers. The focus aw = γwxw, (2) of this study is on polyols that are present as liquids at room where xw is the mole fraction of water and γw is the wa- temperature and miscible at any molar ratio with water. This ter activity coefficient accounting for the non-ideality of the allows the measurement of water activities of polyol/water mixture. For an ideal mixture (γw=1), the water activity is bulk samples over the whole composition range. The experi- simply the mole fraction of water. mental data is compared with UNIFAC predictions and used The combinatorial part of UNIFAC uses the pure compo- together with vapour-liquid equilibrium data of alcohols at nent properties such as volumes and surface areas to account the boiling temperature to develop a new improved UNIFAC for the excess entropic part of the activity coefficients parameterization. C 8i z 2i 8i X ln γi = ln + qi ln + `i − xj `j (3) xi 2 8i xi j 2 Experimental methods where r x q x = i i ; = i i The water activities, aw, were measured using an AquaLab 8i P 2i P (4) rj xj qj xj water activity meter (Model 3TE, Decagon devices, USA). j j This instrument applies the chilled mirror technology to de- termine the dewpoint temperature of air equilibrated with and z the sample. In addition, infrared thermometry pinpoints the `i = (ri − qi) − (ri − 1) (5) sample temperature. Therefore, accurate measurements are 2 not dependent on precise thermal equilibrium. An internal with temperature control allows to have a temperature-stable sam- X X r = v(i)R ; q = v(i)Q . (6) pling environment from 15–40◦C. For all measurements, the i k k i k k k k volatile sample block available as an accessory to the in- strument was used. With this sample block, the water ac- In these equations, xi is the mole fraction of component i, (i) tivity in the presence of other semivolatile components can vk is the number of groups of type k in molecule i, and z be determined. Experimental errors for the volatile sample is the lattice coordination number, a constant set equal to ten block are ±0.015 aw. To correct for instrument drifts and (Fredenslund, 1975). The group volume and surface area pa- offset, the performance of the sample block was frequently rameters Rk and Qk are based on the work of Bondi (1968). controlled and readjusted with reference samples. All mea- The residual part of the activity coefficient is given by surements were performed at 298 K. The substances were X h i γ R = ν(i) 0 − 0(i) , purchased from Sigma-Aldrich in the best available purity. ln i k ln k ln k (7) For glycerol, ethanediol, 1,2-, and 1,3-propanediol as well k as 1,3-, 1,4-, and 2,3-butanediols the purity was above 99%, where 0k is the group residual activity coefficient in the mix- (i) for 1,4-pentanediol it was 99%, for 1,2-butanediol and 2,4- ture and 0k the one in a reference liquid containing only pentanediol ≥98.0%, for 1,2- and 2,5-hexanediol ≥97.0%, molecules of type i. The residual activity coefficients are for 1,2- and 1,5-pentanediol 96%, and for 1,2,4-butanetriol calculated as and 1,7-heptanediol ≥95%.
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