Interaction between bile salt sodium glycodeoxycholate and PEO-PPO-PEO triblock copolymers in aqueous solution

The interactions of the anionic bile salt NaGDC with three triblock copolymers based on poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), denoted P65, P123 and F127, were investigated using high-sensitive differential scanning calorimetry (DSC), turbidimetry, dynamic and static light scattering and small angle X-ray scattering (SAXS). P65 and P123 had the same hydrophilic PEO block lengths, whereas F127 and P123 had the same hydrophobic PPO block length. In water, the block copolymers self-assembled and formed spherical micelles at a critical micelle temperature, which depended on both the PPO/PEO composition ratio and the molecular weight of the copolymer. The mixed systems were studied at a constant P65, P123 or F127 concentration (i.e., 1.0 wt% or 5.0 wt%) with varying n(NaGDC)/n(polymer) molar ratio (MR) from 0 to 12. The DSC measurements presented endothermic enthalpy values (correlated to the amount of PPO that dehydrates in the aggregation process) that were suppressed at high MR. At 50 degrees C, the NaGDC molecules associated to the PPO core - PEO corona interface of the copolymer micelle forming a negatively charged block copolymer micelle-NaGDC complex. The complexes began to disintegrate upon NaGDC addition. Their resistance to disruption followed the stability order as inferred from the CMT values. At 20 degrees C, the unassociated block copolymer chains interacted with the NaGDC micelles and formed small NaGDC-rich complexes with a radius of similar to 2 nm as determined by SAXS.