Hydration and Dynamics in Reverse Micelles of the Triblock Copolymer EO13PO30EO13 in Water/o-Xylene Mixtures: A Spin Probe Study

The reverse micellar phase L 2 of the triblock copolymer poly(ethylene oxide)- b -poly(propylene oxide)- b -poly(ethylene oxide) EO 13 PO 30 EO 13 (commercial name Pluronic L64), where EO is ethylene oxide and PO is propylene oxide, was studied in the ternary mixture L64/water/ o -xylene and in the binary mixture L64/water, using ESR spectroscopy with nitroxide spin probes. The spin probes differed in size, structure, and polarity and belong to two main types: (a) cationic probes 4-( N, N -dimethyl- N -alkyl)ammonio)-2,2‘,6,6‘-tetramethylpiperidine-1-oxyl iodide (CAT n ) with n, the number of carbon atoms in the alkyl substituent, equal to 1, 4, 8, 11, and 16; (b) amphiphilic probes based on x -doxylstearic acid ( x DSA) with x, the carbon atom to which the doxyl group is attached, equal to 5, 7, 10, and 16. X-band ESR spectra reflect the intercalation of the probes in the self-assembled L64 system, but different probes choose different locations in the aggregates and report on the local polarity, hydration, and degree of order. The hydration gradient in the poly(ethylene oxide) (PEO) core of the reverse micelles was estimated on a scale of ≤48 Å from the analysis of a N, the isotropic hyperfine splitting of 14 N, in the CAT n series by comparison with a “calibration curve” based on CAT4 in aqueous solutions of PEO. The lower homologs in the series ( n = 1 and 4) are located near the center of the hydrated core, while the higher homologs ( n = 11 and 16) are close to the interface between the hydrated core and the o -xylene-swollen poly(propylene oxide) (PPO) blocks. The x DSA probes have their head group near the interface between the core and the PPO regions. The nitroxide group in 5DSA is at the core/PPO interface, but the nitroxide groups in the other doxyl probes are at different depths in the PPO regions, depending on the value of x . The dynamics of the CAT n probes was elucidated by comparing the experimental B and C parameters in the expression for the line width in motionally averaged ESR spectra of nitroxides Δ H ( m I ) = A + Bm I + Cm I 2, with the corresponding values calculated as a function of the direction of the axis of rotation and N, the degree of anisotropy of the rotational reorientation. The comparison suggests that the dominant motional mechanism for all probes is rotation around the N−O bond ( x axis), but the anisotropy of the lower spin probe homologs is significantly higher ( N = 10) compared to that of the higher homologs ( N = 2−3). These results are consistent with the existence of a hydration gradient in the core of the reverse micelles.