Complexes of PEO-PPO-PEO triblock copolymer P123 and bile salt sodium glycodeoxycholate in aqueous solution: A small angle X-ray and neutron scattering investigation

Abstract

Small angle X-ray (SAXS) and neutron scattering techniques were combined to study mixed complexes formed between micelles of the nonionic amphiphilic PEO-PPO-PEO copolymer (P123) and the anionic bile salt (NaGDC) in aqueous solution. The purpose was to investigate the structural parameters of the charged complexes, such as size and internal structure, as well as their interparticle interactions in aqueous solution. The overall aim of this work was to gain understanding of how thermoresponsive PEO-PPO-PEO block copolymers interact with bile salts in order to make predictions as to whether they can be put forward as a new class of bile salt sequestrants in the treatment of bile-salt related diseases. The system was investigated at a constant P123 concentration of 1.74mM and bile salt concentrations were varied up to a molar ratio nNaGDC/nP123 (MR)=5.7. It was found that the NaGDC molecules preferentially associated to the PEO corona of the P123 micelle and due to their amphiphilic nature, close to the core/corona interface. Because of this association the micelles became charged causing their reciprocal interparticle repulsions in solution to increase. In parallel, the association caused a decrease in the core radius accompanied by dehydration, which in turn led to a decrease in total radius of the “P123 micelle-NaGDC” complexes. To elucidate the effect of the interactions on their diffusive motion, an interaction model based on a spherical particle with a hard-core interaction shell was employed using the fitted SAXS data. At higher molar ratios, the interparticle interaction was increasingly screened because of nonadsorbed bile salt in the surrounding solution. Meanwhile, a further decrease in total radial size of the P123 micelle-NaGDC complexes occurred due to a decrease in the aggregation number of P123 as the bile salt finally disintegrated the complexes. However, the micelles were found to be more stable and less prone to disintegration in D2O. This investigation demonstrated the importance of using small angle scattering techniques for studying intermolecular interactions in order to gain understanding of how natural surfactants influence the aggregation behavior of amphiphilic polymers.