Available online at www.sciencedirect.com J. of Supercritical Fluids 46 (2008) 245–251 Applying UNIFAC-based models to predict the solubility of solids in subcritical water Tiziana Fornari a,∗ , Roumiana P. Stateva b , F. Javier Se˜ norans a , Guillermo Reglero a , Elena Iba ˜ nez c a Secci´ on Departamental Ciencias de la Alimentaci ´ on, Facultad de Ciencias, Universidad Aut´ onoma de Madrid, 28049 Madrid, Spain b Institute of Chemical Engineering, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria c Instituto de Fermentaciones Industriales, CSIC, c/Juan de la Cierva, 3, 28006 Madrid, Spain Received 10 October 2007; received in revised form 6 November 2007; accepted 13 November 2007 Abstract This work explores the capabilities of UNIFAC-based models to predict the solubility of different solid solutes in subcritical water as a function of temperature. The original UNIFAC, its modified (Dortmund) version and the A-UNIFAC model, which explicitly includes association effects between groups, are applied to calculate the solubility of solid compounds in subcritical water in the temperature range 298–500 K. The comparison between the three models is carried out using polycyclic aromatic hydrocarbons (PAHs) as test substances and for which reliable physical properties and experimental solubility data are available in the literature. The results obtained indicate that modified UNIFAC (Dortmund) provides the best representation of the aromatic compounds’ solubility as a function of temperature. In addition, the application of the A-UNIFAC model confirms the hypothesis that a decrease in the level of association between the subcritical water molecules (in accordance with the dielectric constant decrease with temperature), greatly improves the solubility of hydrophobic organic compounds. © 2007 Elsevier B.V. All rights reserved. Keywords: UNIFAC-based models; Subcritical water; Hot pressurized water; Polycyclic aromatic compounds; Solid solubility 1. Introduction Hot pressurized water (HPW), also called subcritical water, is increasingly used as a green extraction solvent. It has been demonstrated that, depending on temperature, it can be very effective to selectively extract a variety of polar or non-polar organic compounds from many different matrices. Some recent applications involve the extraction of alkyl benzenes from industrial soil and petroleum waste sludge [1], polychlorinated biphenyls from soil and river sediments [2], therapeutic sub- stances from different plants matrices [3], natural antioxidants from aromatic plants [4], etc. The main characteristic of the extraction of hydrophobic organic compounds using water is the decrease of water polarity (measured by its dielectric constant) with increasing tempera- ture. Thus, raising temperature with enough pressure to maintain ∗ Corresponding author. Tel.: +34 914972380; fax: +34 914978255. E-mail address: tiziana.fornari@uam.es (T. Fornari). water in the liquid state has a dramatic effect on the solu- bility of non-polar compounds. Proper representation of the solute + subcritical water phase behaviour is particularly impor- tant in order to select the optimum extraction temperature. Nevertheless, there is a lack of studies that focus on the devel- opment of methods to estimate solute solubilities in HPW. Several approaches have been developed [5–8] to correlate the solubility of solutes, mainly polycyclic aromatic hydrocar- bons (PAHs), in pressurized hot water. First attempts by Miller et al. [5] present a simple equation to correlate the effect of increasing temperature on the solubility of PAHs in liquid water at high temperatures. A recent contribution by del Valle et al. [7] demonstrated the strong effect of other factors, related with the solute itself. The semi-empirical relationship described in [7] allowed for an excellent correlation of the solubility for up to 34 different compounds including PAHs, pesticides, flavonoid-type compounds and some essential oil components in HPW. Another semi-empirical relationship (Kar´ asek et al. [8]) uses the tem- perature dependency of pure water physical properties (internal pressure, cohesive energy density and dielectric constant) and 0896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2007.11.017