Influence of temperature, pressure, and pH on the group-inclusion equilibrium in water soluble calixarenes

Previously, we demonstrated, for the first time, that guest probes having more than one bulky group could produce a pair of bidirectional inclusion complexes with water soluble calixarene (Chem. Lett. 2006, 35, 772). The use of unique free radical probes and electron spin resonance has made this finding possible. In this report, we investigated thermodynamic and pH-dependent properties of the group-inclusion complex formation, i.e., the effects of temperature, pressure and the medium pH on group inclusion by p-sulfonatocalixarenes were determined. Temperature-dependence study indicated that enthalpy portions in ΔG play dominant roles in the group-in complexation. Pressure dependence experiments allowed us to calculate the reaction volumes, i.e., intrinsic volume changes related to inclusion into the calixarene cavity, which ranged from -5.1 to -16.3 cm³ mol ^-1. Data on pH dependence clarified the role of oxyanions in the entrant group of calixarenes. It should be emphasized that pressure-dependent study provided unique information on the structure of the group-inclusion complex.