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

Aqueous solutions containing insoluble p-cyclodextrin (beta-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 beta-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 beta-CD: linker reactant ratios 1:1, 1:2, and 1:3, respectively. The decolourization studies provided estimates of the 1: 1 binding constants (K-1) for the monomer beta-CD/phth inclusion complex. It was concluded that the Values of K-1 for copolymer/phth systems for highly accessible beta-CD inclusion sites in copolymer materials closely resembles the K, value for the 1:1 beta-CD/phth cornplex.The Surface accessibility of the beta-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 beta-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 (Delta G degrees) and site occupancy (0) of phth adsorbed to the copolymer materials was estimated independently using the Sips isotherm model. The Delta G degrees values ranged between -27 and -30 kJ mol(-1) and are in agreement with the Gibbs free energy for the 1: 1 beta-CD/phth complexes (similar to-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 beta-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 beta-CD based sorbent materials. (C) 2009 Elsevier Ltd. All Fights reserved.