Selection of solvents based on Hansen solubility parameters for calix[4]arene acetic-acid derivatives as extractants of metal ions

Abstract

Abstract Calixarene derivatives exhibit excellent extractability and selectivity for metal ions based on their chelating and size-discriminating effects; however, their poor solubility in aliphatic hydrocarbons, which are used as solvents in industrial processes, hinders their practical use. In this study, the relationship between solubility of calix[4]arene acetic-acid derivatives in various organic solvents and Hansen solubility parameters of the solvents was investigated. The center coordinates of the Hansen solubility sphere of the p-tert-octylcalix[4]arene acetic-acid derivative (tOct[4]CH2COOH), as determined from its solubility in 50 different solvents, are δD = 17.24, δP = 4.73, δH = 4.45, which indicate that this extractant is less polar than p-tert-butylcalix[4]arene acetic-acid derivative. The solubilities of tOct[4]CH2COOH in diethylene glycol dibutyl ether (DBC) and 2-nonanone (2-NON) were 92.6 mM and 91.1 mM, respectively. DBC and 2-NON are relatively preferable as solvents from the viewpoint of physical properties (low viscosity, relatively low water solubility, and high flash point). Extractability of Pb(II) using tOct[4]CH2COOH dissolved in various solvents was compared. Extraction in DBC proceeded at much higher pH than that in other solvents. Extraction of various metals in 2-NON proceeded at higher pH than that in chloroform. The order of metal selectivity of tOct[4]CH2COOH in 2-NON was similar to that in chloroform. The extraction reaction for Pb(II) using tOct[4]CH2COOH was determined by slope analysis. These results suggest that 2-NON can be an alternative to chlorinated solvents for use of tOct[4]CH2COOH as a metal extractant.