Binding of methyl orange and its homologs by polycations containing apolar pendant groups

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Abstract
The binding of methyl orange, ethyl orange, and propyl orange by polycations involving various apolar pendant groups such as methyl, ethyl, benzyl, or dodecylbenzyl groups has been examined quantitatively by an equilibrium dialysis method at 5, 15, 25, and 35°C. The first binding constants and the thermodynamic parameters in the course of the binding have been calculated. The favorable free energy of the binding is accompanied by an entropy gain and an exothermic enthalpy change. The shorter the alkyl chain of the dyes or the polymers, the more negative is the enthalpy change and hence the smaller is the entropy change. Furthermore, an increase in binding affinity can be created in the polycation upon introduction of hydrophobic groups. In particular, the binding ability of the polycation containing a dodecylbenzyl group for methyl orange is almost 300‐fold that of bovine serum albumin. Therefore it is clear that hydrophobic interactions, as well as electrostatic ones, are involved in the binding.