Interaction, solubilization and location of p-hydroxybenzoic acid and its sodium salt in micelles of moderately hydrophilic PEO-PPO-PEO triblock copolymers.

Micelles of ABA type triblock copolymers (where A is polyethylene oxide PEO and B is polypropylene oxide PPO) viz. Pluronic® P103, P104 and P105 (each containing almost the same PPO mol wt. ~ 3250 g/mol and 30, 40 and 50 wt.% of PEO, respectively) in the presence of p -hydroxybenzoic acid (PHBA) and its sodium salt (Na-PHBA) were examined by viscosity, dynamic light scattering (DLS), small angle neutron scattering (SANS) and NMR. Spherical polymeric micelles (apparent hydrodynamic diameter ~ 20 nm) in water at 30 °C grow in the presence of PHBA and transform into prolate-ellipsoidal shape with an increased aggregation number. The micellar transition was favored at higher PHBA concentration, temperature and for copolymers with more hydrophobicity. The PHBA salt, however, increased cloud point and showed only a marginal decrease in aggregation number even at much higher concentrations. The location of PHBA in micelle was elucidated by nuclear Overhauser enhancement spectroscopy (NOESY).