Room temperature sphere-to-rod growth of Pluronic® P85 micelles induced by salicylic acid

Abstract

The influence of salicylic acid (SA) on the aggregation characteristics of Pluronic® P85 has been studied in the aqueous medium by dynamic light scattering (DLS), small angle neutron scattering (SANS), steady-state fluorescence and rheological measurements. DLS studies suggest that SA, which is used in the treatment of various skin diseases, induces a room temperature growth of the P85 micelles. Steady-state fluorescence studies and cloud point measurements attribute this to the ability of SA to stay in micellar corona and consequently dehydrate the copolymer micelles by displacing water from the corona region. A large increase in the viscosity of the P85 solutions accompanying the observed micellar growth, and SANS and rheological studies reveal that the P85 micelles undergo a sphere-to-rod shape transition in the presence of SA. Pluronics® being biomedically important surfactants, the observed effects suggest that P85 could be considered as a suitable agent as a carrier of SA, as well as a viscosity modifier of the formulations of SA. The studies also show that the effectiveness of SA in causing the observed micellar dehydration increases with a decrease in pH, which suggests that the observed effects of SA on the P85 micelles arise due to the presence of the unionized form of the SA in the micellar corona region. Rheological studies show that unlike other Pluronic® worm like micellar systems, the viscoelastic behavior of the present system shows close conformity with the Maxwell law. Such difference in the behavior has been attributed to faster restructuring and breaking processes of the P85 micelles due to the high hydrophilic PEO group content (50%) of P85 molecules.