Estimation of the surface accessible inclusion sites of β-cyclodextrin based copolymer materials

Aqueous solutions containing insoluble β-cyclodextrin (β-CD) based urethane copolymers were studied in aqueous solutions by measuring the absorbance changes (decolourization) of phenolphthalein (phth) at pH 10.5. The various copolymers were comprised of β-CD and five diisocyanate linkers (1,6-hexamethylene diisocyanate (HDI), 4,4′-dicyclohexyl diisocyanate (CDI), 4,4′-diphenylmethane diisocyanate (MDI), 1,4-phenylene diisocyanate (PDI), and 1,5-naphthalene diisocyanate (NDI)). The copolymers studied were prepared at the β-CD: linker reactant ratios 1:1, 1:2, and 1:3, respectively. The decolourization studies provided estimates of the 1:1 binding constants (K1) for the monomer β-CD/phth inclusion complex. It was concluded that the values of K1 for copolymer/phth systems for highly accessible β-CD inclusion sites in copolymer materials closely resembles the K1 value for the 1:1 β-CD/phth complex. The surface accessibility of the β-CD inclusion binding sites for the copolymers ranged from 1-100%. The observed variability was attributed to steric effects in the annular hydroxyl region of β-CD and the relative accessibility of the micropore sites within the polymer framework as a consequence of the variable cross linking. The Gibbs free energy of complex formation (ΔG°) and site occupancy (θ) of phth adsorbed to the copolymer materials was estimated independently using the Sips isotherm model. The ΔG° values ranged between −27 and −30 kJ mol−1 and are in agreement with the Gibbs free energy for the 1:1 β-CD/phth complexes (∼−27 kJ mol−1). The phth decolourization technique provides a simple, low cost and versatile method for the estimation of the surface accessible inclusion sites of β-CD in CD based urethane copolymer materials. This method is anticipated to have extensive analytical applications in materials research and for the design of functional β-CD based sorbent materials.