ABSTRACT
Pluronics® are considered as potential materials for the removal of contaminants like phenol from polluted water sources because of their superior solubilizing capacity of aromatic compounds. Systematic studies on the influence of solubilization of phenol on room temperature aggregation characteristics of Pluronics® in water are, however, conspicuous by their absence. In this manuscript, we thus report DLS, SANS and rheological studies on the influence of phenol on the aggregation characteristics of four Pluronics® viz. F127, P123, P104 and P103. The aim of this study has been to understand the role played by the composition of the Pluronics® in determining growth and interaction of the micelles induced by solubilization of phenol. The study shows that in the case of F127 and P123, phenol solubilization leads to a large increase in light scattering intensity due to an onset of attractive intermicellar interactions and consequent formation of micellar clusters. P123 being smaller than F127 shows a subsequent time dependent micellar growth, leading to a sphere-to-rod shape transitions in micelles. The copolymers P103 and P104, which are smaller and less hydrophobic than P123, respectively, exhibit a large increase in solution viscosity in the presence of phenol owing to a rapid sphere-to-rod micellar growth. The observation of such a fine interplay between the growth and interaction of the pluronic micelles in the presence of a hydrophobic solvent is first of its kind and highlights the role of composition of pluronic in determining the kinetics of the micellar restructuring process.
ABSTRACT
Aqueous micellar solutions of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) were examined using steady and dynamic rheology, small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM). Upon addition of NaNO(3), the CTAB spherical micelles transform into long, flexible wormlike micelles, conveying viscoelastic properties to the solutions. The zero-shear viscosity (eta(0)) versus NaNO(3) concentration curve exhibits a well-defined maximum. Likewise, upon increase in temperature, the viscosity decreases. Dynamic rheological data of the entangled micellar solutions can be well described by the Maxwell model. Changes in the structural parameters of the micelles with addition of NaNO(3) were inferred from SANS measurements. The intensity of scattered neutrons at the low q region was found to increase with increasing NaNO(3) concentration. This suggests an increase in size of the micelles and/or decrease of intermicellar interactions with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicates that addition of NaNO(3) leads to a decrease in the surface charge of the ellipsoidal micelles and consequently an increase in their length. The structural transition from spherical to entangled threadlike micelles, induced by the addition of NaNO(3) to CTAB micelles is further confirmed by cryo-TEM.
