Self-Aggregation of Sodium Ibuprofenate in Water. II. Equilibrium Modeling and Structural Characterization from Simulation and SAXS Experiments

A complete characterization of the self-aggregation process of sodium ibuprofenate (NaIbu) in water is presented based on both scattering experiments and computer simulation techniques. The micellization process was rationalized in terms of the classical mass-action model. Microscopic details about the aggregation number and the degree of ionization of the aggregates, which are necessary to achieve the aforementioned description, were obtained from molecular dynamics simulations and small-angle X-ray scattering (SAXS) experiments. Molecular dynamics simulations showed the existence of ellipsoidal stable micelles with characteristic lengths on the order of ∼20 Å, an average aggregation number of <i>n</i>_Ibu = 45 ibuprofenate molecules, and a degree of micelle ionization α = 0.63. SAXS experiments revealed similar structural characteristics and low polydispersity across a wide NaIbu concentration range above the critical micellar concentration, reinforcing the results obtained from the simulations. By combining data from molecular dynamics simulations and time-resolved experiments within the framework of the Burchfield and Woolley model for the activities of all the species involved, values of In(<i>K</i>_Agg) = 41, = 45, and α = 0.64 were determined, along with a corresponding value for the critical micellar concentration of 0.185 m for the equilibrium aggregation model. Findings regarding the modulation of the dynamical modes of ibuprofenate imposed by the restricted micellar environment and its potential influence on the photochemical response of the probe were also reported and discussed in the context of the photophysical experimental data previously measured by our group.