Peculiar temperature dependence on the binding of methyl orange and its homologs by 2‐hydroxyethyl methacrylate‐N‐vinyl‐2‐pyrrolidone copolymers

Abstract

Abstract2‐Hydroxyethyl methacrylate (HEMA)‐N‐vinyl‐2‐pyrrolidone (VPy) copolymers of various compositions have been prepared. The copolymers obtained were examined for their ability to bind a homologous series of methyl orange derivatives, methyl orange, ethyl orange, propyl orange, and butyl orange, at 5, 15, 25, and 35°C, respectively, in an aqueous solution. The first binding constants and the thermodynamic parameters that accompanied the binding were evaluated. The binding ability of the copolymer for the small cosolute was enhanced with an increase of the HEMA content in the copolymer. Moreover, a bell‐shaped curve appeared in the binding of butyl orange by the copolymers having higher HEMA residues when the first binding constant was plotted as a function of temperature, whereas no such phenomenon was detected for the copolymers with less HEMA content or for the less hydrophobic dye, methyl orange, ethyl orange, or propyl orange. This peculiar temperature dependence of the first binding constant shows that the enthalpy of the binding varies from a positive (unfavorable) value below ca. 15°C to a negative (favorable) one above this temperature. This behavior can be accounted for in terms of more hydrophobic effects involved in the binding process.