Mixtures of Pluronic (F-127 or L-61) and phospholipid were investigated for a wide range of Pluronic concentrations (0-15 wt %) using dynamic light scattering, differential scanning calorimetry, and fluorescence microscopy. The present study is aimed at better understanding how the amphiphilic triblock copolymers affect the lipid vesicles, particularly in the high-concentration regime. Our results show that L-61 interacts more strongly with phospholipid vesicles than F-127 when the copolymer is at the unimer state in the solution. For high concentrations, F-127 forms mixed micelles with solubilized lipid molecules in the form of bilayer patches. This novel behavior was observed for the first time. In contrast, more hydrophobic L-61 tends to precipitate with the solubilized lipids as large crew-cut mixed aggregates.

Interactions between phospholipid vesicles and poly(acrylic acid) (PAA) are of fundamental importance for pH-sensitive delivery systems in pharmaceutical applications. In this study, we investigated the behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at varied concentrations of PAA with low and high molecular weights. Differential scanning calorimetry and fluorescence microscopy experiments suggested that hydrogen bonding between the carboxyl group of PAA and the phosphodiester group of the lipid is the main driving force for association under acidic conditions. Complexation between DPPC and PAA depends on the PAA molecular weight and concentration. At high enough concentrations of the large molecular weight PAA, the polymer can completely disrupt the bilayers of DPPC. For the small molecular weight PAA, in contrast, the complexation leads to vesicle aggregation without destroying the bilayers. The difference in complex structure was detected by SEM and a UV-visible spectrophotometer. At alkaline pH, complexation did not occur because of strong dissociation of PAA.

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