Micellar solubilization of tributylphosphate in aqueous solutions of Pluronic block copolymers: Part II. Structural characterization inferred by 1H NMR

Abstract

The solubilization of tributylphosphate (TBP), a polar oil, in various micellar solutions of Pluronic has been investigated by 1H NMR spectroscopy. Partial phase diagrams of the three components systems (Pluronic–TBP–water) have shown two characteristic temperatures, called CPT and SMT, which control the phase behavior (see Part I); Both temperature depend on the copolymer structure and, interestingly, are directly related to the TBP concentration in the medium. Monophasic microemulsions are observed only when the temperature ranges between the SMT and the CPT. Moreover, the evolution of the CPT with the TBP content clearly indicated the occurrence of a structural change of the microemulsions which allows higher quantities of TBP to be solubilized. In this second part, 1H NMR studies of TPB/micellar systems have essentially focused on elucidating the nature of the interactions between TBP and micelle, or on the location of the solubilized species, mainly from the dependence of chemical shifts or linewidths on TBP concentration. Especially, the NMR spectra of the microemulsions before and after the structural change have been compared with those obtained for pure solution of Pluronic in D 2O at different temperatures and in CDCl 3. The analysis of the 1H NMR chemical shifts suggests a structural transformation of the TBP–Pluronic micelles in the sense of an hydrophobic TBP–PPO core becoming more and more dense as the TBP concentration increases. Especially, 1H NMR data evidence an evolution of the hydration state of the hydrophobic core following addition of TBP in the micellar solutions. During the addition of TBP, the microemulsion structure turns from spherical swelled micelles to nanodroplets of pure TBP stabilized by the Pluronic (pure nanophase of TBP stabilized by the copolymer). It is shown that the structural change strongly depends on the temperatures (CPT and SMT, see Part I) and on the copolymer structure.