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Yabalak, E.; Akay, S.; Kayan, B.; Gizir, A.M.; Yang, Y. Solubility of Organic Compounds in Subcritical Water. Encyclopedia. Available online: (accessed on 15 April 2024).
Yabalak E, Akay S, Kayan B, Gizir AM, Yang Y. Solubility of Organic Compounds in Subcritical Water. Encyclopedia. Available at: Accessed April 15, 2024.
Yabalak, Erdal, Sema Akay, Berkant Kayan, A. Murat Gizir, Yu Yang. "Solubility of Organic Compounds in Subcritical Water" Encyclopedia, (accessed April 15, 2024).
Yabalak, E., Akay, S., Kayan, B., Gizir, A.M., & Yang, Y. (2023, March 07). Solubility of Organic Compounds in Subcritical Water. In Encyclopedia.
Yabalak, Erdal, et al. "Solubility of Organic Compounds in Subcritical Water." Encyclopedia. Web. 07 March, 2023.
Solubility of Organic Compounds in Subcritical Water

Data on the solubility and decomposition of organic compounds in subcritical water, a green solvent, are needed in environmental remediation, chemistry, chemical engineering, medicine, polymer, food, agriculture, and many other fields. The solubility of organics is significantly enhanced with increasing water temperature. Likewise, the percentage of organic decomposition also increases with higher temperature.

subcritical water (SBCW) solubility decomposition

1. Introduction

The use of subcritical water as a green solvent for extraction or reaction media has gained importance with advanced scientific studies in the last 20 years. Subcritical water has variable physical properties compared to water at ambient conditions such as the dielectric constant, which is typically used for measuring polarity and can easily be tuned by changing temperature and pressure. As the temperature rises above 373, 473 and 505 K, the dielectric constant of water reaches the normal values of dimethyl sulfoxide (DMSO) (46.68), acetonitrile (37.5), and methanol (32.7), respectively [1][2][3]. Therefore, recent studies have demonstrated that subcritical water is successfully used as the sole medium in both extraction and chromatography, thus completely removing organic solvents in these processes.

2. Solubility in Subcritical Water

A simple and reliable system for the determination of solubility and partitioning behavior of fuel components in subcritical water up to 523 K was developed by Yang et al. [4]. The solubility of toluene increased ~23 times by increasing the temperature from ambient temperature to 473 °C, but the pressure change (from 1 to 50 bar) did not affect the solubility values in the solubility studies performed at ambient temperature. The increases in the separation of benzene, toluene, ethylbenzene, xylenes, and naphthalene from gasoline to liquid water when the temperature is increased from ambient temperature to 473 °C range from 10 times for benzene to 60 times for naphthalene. Similarly, increases in the partitioning of polycyclic aromatic hydrocarbons from diesel fuel to liquid water when the temperature was increased from ambient temperature to 523 °C ranged from 130-fold for naphthalene to 470-fold for methylnaphthalene.
After this study in 1998, Miller et al. [5] studied that the solubility of anthracene, pyrene, chrysene, perylene, and carbazole were determined at temperatures ranging from 298 to 498 K and at pressures from 30 to 60 bar in subcritical water. They estimated the solubility equation based on simplifying assumptions and empirical correlations based on data presented in this work and previous reports. The calculation of solubility at desired temperature needs only knowledge of ambient temperature solubility. Equation (1) is given below:
ln x 2 ( T ) = ( T 0 T ) ln [ x 2 ( T 0 ) ] + 15 ( T T 0 1 ) 3
where x2(T0) refers to the solubility of organic compounds at ambient temperature, and x2(T) refers to the solubility of organic compounds at a calculated temperature.
The solubilities of benzene, toluene, m-xylene, p-cymene, octane, 2,2,4-trimethylpentane (isooctane), tetrachloroethylene, 1,2-dichlorobenzene, and tetraethyltin were investigated at temperatures ranging from 298 to 473 K. Increasing the temperature by 175 K increased the solubilities by a factor of 10–250 [6].

2.1. Solubilities of Polycyclic Aromatic Hydrocarbons and Derivatives in Subcritical Water

The solubility of PAHs is important for many industrial plants. Furthermore, their aqueous solubility determines both their uptake by the roots of plants and their transfer to other parts of the plant and their mobility in the soil. The solubilities of three PAHs, namely acenaphthene, anthracene, and pyrene, in water were measured in temperature and pressure ranges of 323–573 K and 50–100 bar, respectively, by Andersson et al. [7]. The solubility values of the employed compounds below their melting point were determined to be consistent with literature values, and the solubility of pyrene and anthracene exponentially varies with temperature. The solubilities of acenaphthene, anthracene, and pyrene were calculated as mole fraction solubilities (x2) and were determined as 1.25 × 10−3 at 300 K and 100 bar, within 1.02 × 10−7–3.78 × 10−3 at a temperature range of 373–573 K and pressure of 50 bar and 6.87 × 10−8–1.41 × 10−3 at a temperature range of 323–573 and pressure range of 50–100 bar, respectively.
Karásek et al. developed a semiempirical relationship to correlate the solubility of PAHs (naphthalene, anthracene, pyrene, chrysene, 1,2-benzanthracene, triphenylene, perylene, p-terphenyl) in pressurized hot water within the temperature range of 313–498 K, a pressure of 1–77 bars and equilibrium mole fraction (x2) of 10−11–10−3. They used only pure-component properties such as cohesive energy density, internal pressure and dielectric constant of water and enthalpy of fusion, triple-point temperature, the molar volume of the solid compound and the molar volume of the subcooled liquid of PAHs [8]. The x2 data were experimental values of the previously reported research studies. γ2 (Raoult’s law activity coefficient of the solute) values of each PAH mentioned above were calculated using Equation (3), where fs02(the fugacity of the pure solid solute) and f102(the pure subcooled liquid solute) values were calculated by Equations (2) and (3). 
x 2 = f 2 s 0 γ 2 f 2 10
x 2 = f 2 s 0 γ 2 f 2 10
The aqueous solubilities (x2) of solid heterocyclic analogues of anthracene, phenanthrene and fluorene at a specific temperature range (313 K–the melting point of each compound) under 50 bar of pressure were reported by Karásek et al. [9]. They collected the solubility data of each compound via the dynamic saturation method based on pressurized hot water extraction. x2 values for the employed compounds were found to be within the 3.17 × 10−9–8.27 × 10−4 range and were widely changeable based on the applied temperature. It was also indicated that no appreciable degradation was observed for any compound in the temperature range studied based on the GC/MS results. Obtained solubilities were converted to activity coefficients of individual solutes in saturated aqueous solutions, and the relationship between temperature and type or number of heteroatoms was evaluated (Equation (4)). 
ln x 2 = b 1 + b 2 ( T 0 T ) + b 3 ln ( T T 0 )
where T0 and γ2 refer to 298.15 K and Raoult’s law activity coefficient of the related compound, respectively. b1, b2, and b3 denote the least-squares estimates of the coefficients, and T is the absolute temperature at the experimental conditions. Hence, the increase in the aqueous solubilities of solid heterocyclic analogues of anthracene, phenanthrene and fluorine was reported to strongly depend on the increasing temperature and variance with the heteroatoms.
The solubility values of PAHs in subcritical water were calculated using Equations (5) and (6) with the above-mentioned three UNIFAC-based thermodynamic models. 


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Update Date: 09 Mar 2023