Complex formation between n-alkyltrimethylammonium bromides and water-soluble calix[n]arenes

Abstract Complex formation between n-alkyltrimethylammonium bromides and water-soluble calix[n]arenes (n = 4, 6, 8) (CALXSn) was studied by potentiometric titration and molecular dynamic simulation. The binding isotherms were found to be composed of two phases; the first phase is strong specific binding to a few sites and the second phase is cooperative binding to the residual sites. Binding constant for the first specific binding was found to increase with an increase in the number of ring members of CALXSn. Binding constants for CALXS4 and CALXS6 systems did not change significantly with the length of the surfactant's alkyl chain and with the pH change from 7.0 to 12.5, while that for CALXS8 systems tended to increase with an increase in the alkyl-chain length. The specific binding disappeared with the pH increase. The thermodynamic parameters showed an enthalpy-entropy compensation relation with a compensation temperature of 289 K. The second phase was observed mainly for n-dodecyltrimethylammonium-CALXSn systems. The cooperativity parameter was found to increase with an increase in the number of ring member of CALXSn and is attributed to the contribution of increasing conformational flexibility of CALXSn as the number of ring members increases. MD simulation on CALXSn with and without n-alkyltrimethylammonium ion provided structural interpretation for the features of binding data.