Diffusion permeability of hybrid chitosan/polyhedral oligomeric silsesquioxanes (POSS™) membranes to amino acids

Hybrid membranes of chitosan (Chi), hydrophilic polyhedral oligomeric silsesquioxanes (POSS™, 2.5 mol%) and ethylene glycol diglycidyl ether (EGDGE, 75 mol%) as a crosslinker were prepared by casting the blends onto nonwoven microporous polymer support, drying at 70 °C and treating with 0.5% sodium hydroxide solution. The POSS differed by the tether used: {[3-(2,3-Dihydroxypropoxy)propyl]dimethylsilyloxy}in DL-POSS, 3-aminopropyl in OA-POSS and (tetramethylammonio)oxy in TMA-POSS. The diffusion permeability of the hybrid membranes both to hydrophobic (alanine, tryptophane) and hydrophilic (glutamic acid, lysine) amino acids was investigated to elucidate the effect of POSS on the membrane transport properties. The experiments were carried out at pH 3 and 11, i.e., with the chitosan amino groups fully protonated or deprotonated. Diffusion transport of amino acids was affected by their pI, hydrophilic indexes and molecular sizes. Membrane permeability was indirectly affected by the POSS tethers due to changes in the morphology of the hybrid membranes on nanoscale. The longer the tether used, the higher the packing density of chitosan chains in the membrane and the lower its diffusion permeability to amino acids. The TMA-POSS, undergoing hydrolytic-condensation in acidic chitosan solution during membrane preparation, formed large silica segments, which caused the largest morphological disorder in Chi/TMA-POSS membranes and, as result, their highest permeability to all amino acids studied. The contribution of incorporated POSS to transport of amino acids is discussed.