A thermodynamic study of F108 and F127 block copolymer interactions with liposomes at physiological temperature

The interactions of egg yolk phosphatidylcholine liposomes with F108 and F127 triblock copolymers, in the monomer state, were analyzed by isothermal titration calorimetry (ITC) at 37 °C. According to the results, the critical micelle concentration was determined to be 0.4 and 0.04 wt.% for F108 and F127, respectively, by surface tension at 37 °C. According to the results, liposomes/poloxamers were not favoured energetically, since endothermic interactions were observed. However, positive changes in entropy promoted a spontaneous process. F127 had a greater partition coefficient (51.97 ± 1.77 × 10⁴), stronger affinity, than F108 (8.19 ± 0.37 × 10⁴) towards the vesicle lipid bilayer due to its larger hydrophobic block. After the ITC experiments, an increased vesicle size (within about 1–3 nm average) by dynamic light scattering and the formation of bilayer discs by electron microscopy (EM) was observed at low copolymer concentrations (0.57 mol% of F108 and 1.01 mol% of F127). The EM and ITC results confirmed the intimate association of the copolymers with the membrane instead of being simply absorbed onto the bilayer surface. Our results indicate that the temperature of the system (37 °C), the copolymer concentration and hydrophobic chain length are important factors for the interaction of poloxamers with lipid bilayers and the stability of liposomes.