Physisorption of Three Calix[4]arenes on Alkyl-Functionalized Mesoporous Silica for Biomimetic Ligand Development

Abstract

Calixarene physisorption onto alkyl-functionalized silica support is presented as an alternative to tethering to reduce synthetic complexity. Three versions of calix[4]arene were synthesized with differing upper rim functional groups: aromatic amine, tert-butyl, and unfunctionalized. Calixarene molecules physisorbed throughout the alkyl layer on the silica following a linear isotherm with a maximum observed loading of 300 μmol g–1. Large exothermic adsorption enthalpies (−60 to −260 kJ mol–1), attributed to van der Waals interactions, were measured for the calixarene physisorption using flow microcalorimetry; large negative entropies were calculated (−140 to −780 J mol–1 K–1). Amine-calix[4]arene was physisorbed onto C18-silica (NCS) through solvent evaporation. The myoglobin uptake on NCS (47 μmol g–1) was twice that on C18-silica; additionally, a greater heat of adsorption (−16.9 ± 2.9 kJ mol–1) was observed on NCS than on C18-support (−12.7 ± 2.2 kJ mol–1).