Hydrated Micellar Structure of Ethoxylated Nonyl Phenols from PGSE NMR

Solvent self-diffusion coefficients D*(w) and D-w are respectively determined by PGSE NMR in the bulk solvent and in suitably prepared aqueous micellar solutions of a series of commercial ethoxylated nonyl phenols (ENPs) at 298 and 317 K. The capability of ENP micelles for obstructing the free diffusion of solvent molecules is characterized by the obstruction ratio K = D-w/D*(w), which turns out to depend merely on the volume fraction of EO units present-regardless of significant changes in solute concentration, distribution of EO chain length, micellar aggregation number and temperature. The experiments are interpreted in terms of a renewed analytical model of ENP micellar structure, based on Tanford's concept of hydrodynamic particle. The results from model fitting return the characteristic sizes of ENP monomers, support the ellipsoid shape of the micelles, indicate a stable EO shell, follow the trend in the variation of hydration number vs. the chain length predicted by thermal analysis in PEO solutions.