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

Small angle X-ray (SAXS) and neutron scattering techniques were combined to study mixed complexesformed between micelles of the nonionic amphiphilic PEO-PPO-PEO copolymer (P123) and the anionicbile salt (NaGDC) in aqueous solution. The purpose was to investigate the structural parameters of thecharged complexes, such as size and internal structure, as well as their interparticle interactions inaqueous solution. The overall aim of this work was to gain understanding of how thermoresponsivePEO-PPO-PEO block copolymers interact with bile salts in order to make predictions as to whether theycan 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 1.74 mM and bile salt concentrationswere varied up to a molar ratio nNaGDC/nP123(MR) = 5.7. It was found that the NaGDC molecules prefer-entially associated to the PEO corona of the P123 micelle and due to their amphiphilic nature, close tothe core/corona interface. Because of this association the micelles became charged causing their recip-rocal interparticle repulsions in solution to increase. In parallel, the association caused a decrease inthe 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, aninteraction 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 becauseof nonadsorbed bile salt in the surrounding solution. Meanwhile, a further decrease in total radial sizeof the P123 micelle-NaGDC complexes occurred due to a decrease in the aggregation number of P123as the bile salt finally disintegrated the complexes. However, the micelles were found to be more stableand less prone to disintegration in D2O. This investigation demonstrated the importance of using smallangle scattering techniques for studying intermolecular interactions in order to gain understanding ofhow natural surfactants influence the aggregation behavior of amphiphilic polymers