Measurement and correlation of physicochemical properties of phosphonium-based deep eutectic solvents at several temperatures (293.15 K–343.15 K) for CO 2 capture Hosein Ghaedi, Muhammad Ayoub ⇑ , Suriati Sufian, Bhajan Lal, Azmi Mohd Shariff Department of Chemical Engineering, Universiti Teknologi Petronas, 32610 – Bandar Seri Iskandar, Perak, Malaysia article info Article history: Received 17 February 2017 Received in revised form 6 May 2017 Accepted 13 May 2017 Available online 18 May 2017 Keywords: DES Salt HBD Density Refractive index Temperature abstract Recently, deep eutectic solvents (DESs) as the new solvents have received considerable amount of atten- tion between researchers in different research fields and are under investigation so find out their poten- tial to become a versatile alternative to ionic liquids (ILs) and traditional solvents. DESs are derived from two or more salts as the hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs). Six DESs were synthesized namely allyltriphenylphosphonium bromide- diethylene glycol (ATPPB-DEG) and allyl- triphenylphosphonium bromide- triethylene glycol (ATPPB-TEG) using three mole ratios of 1:4, 1:10 and 1:16 salt to HBDs. In this work, we report physicochemical properties of these DESs, which include den- sity, molar volume, isobaric thermal expansion, refractive index, specific refraction, molar refraction, free molar volume, electronic polarization and internal pressure at several temperatures from 293.15 K to 343.15 K. Most of these properties are fitted to a linear equation by the method of least-squares using the Levenberg-Marquardt algorithm to derive the corresponding parameters and estimate the root mean square error (RMSE) and least squared correlation coefficient (R 2 ). Ó 2017 Published by Elsevier Ltd. 1. Introduction Deep eutectic solvents (DESs) are derived from two or more salts as the hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) including amides, amines, alcohols, and carboxylic acids. In 2003, Abbot et al. [1] introduced DESs by synthesis of urea and a range of quaternary ammonium salts. In recent times, deep eutectic solvents (DESs) have gained pop- ularity between researchers for many purposes, especially in CO 2 capture [2–12] in comparison with ionic liquids (ILs) which have a low melting point, high solvency power for both polar and non-polar solvents, high ionic conductivity, high thermal and chemical stability, tuneable physicochemical character, low flammability and volatility [13–22]. However, in spite of unique properties of ILs, they have some drawbacks such as the high price for their industrial applications in the large scale, the complex reaction steps and purification procedures for synthesizing, high viscosity, potential toxicity, limited biodegradability and low CO 2 loading capacity (in comparison with traditional alkanolamines) [23,24]. That is why researchers are trying to find new solvents as an alternative to ILs. Similar to ILs, DESs have many unique properties, for instance, nonreactive with water, non-volatile, ther- mally stable, highly conductive and also composed of low-cost, nontoxic, natural, easy preparation and synthesizing way, lower melting point than the constituents of the mixture, a wide liquid range, high solvation capacity, biodegradable constituents which are important for the environmental and economical perspective [2,25–30]. Therefore, DESs can be regarded as the alternative to ILs for many purposes. It has been reported that the cost of synthesizing a DES was only about 20% less than that of ionic liquids (ILs) [31]. Recently, Kar- eem et al. [26,32], Luo et al. [33], Shahbaz et al. [34], Ghareh Bagh et al. [35] and Hayyan et al. [36] have measured the physical prop- erties of DESs based on phosphonium and demonstrated their potential use in industrial applications. Aissaoui et al. [37] con- ducted the FT-IR investigation, and investigated the functional groups of some DESs based on phosphonium. There is scarce infor- mation regarding toxicity of DESs. Radoševic ´ et al. [38] evaluated the toxicity of choline chloride based deep eutectic solvents using fish and human cell line. Their results showed that choline chloride-glucose and choline chloride-glycerol have low cytotoxic- ity, while choline chloride-oxalic acid has modest cytotoxicity. Hayyan et al. [39] investigated the toxicity of several DESs base on phosphonium using the Gram positive and negative bacteria. http://dx.doi.org/10.1016/j.jct.2017.05.020 0021-9614/Ó 2017 Published by Elsevier Ltd. ⇑ Corresponding author. E-mail address: muhammad.ayoub@utp.edu.my (M. Ayoub). J. Chem. Thermodynamics 113 (2017) 41–51 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct t