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) EO13PO3EO13 (co mmercial name Pluronic L64), where EO is ethylene oxide and PO is prop ylene oxide, was studied in the ternary mixture L64/water/o-xylene and in the binary mixture L64/water, using ESR spectroscopy with nitroxid e spin probes. The spin probes differed in size, structure, and polari ty and belong to two main types: (a) cationic probes l-N-alkyl)ammonio )-2,2',6,6'-tetramethylpiperidine iodide (CATn) 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 (xDSA) with x, the carbon atom to which the doxyl group is attached, equal to 5, 7, 10, a nd 16. X-band ESR spectra reflect the intercalation of the probes in t he self-assembled L64 system, but different probes choose different lo cations in the aggregates and report on the local polarity, hydration, and degree of order. The hydration gradient in the poly(ethylene oxid e (PEG) core ofthe reverse micelles was estimated on a scale of less t han or equal to 48 Angstrom from the analysis of a(N), the isotropic h yperfine splitting of N-14, in the CATn series by comparison with a '' calibration curve'' based on CAT4 in aqueous solutions of PEG. The low er homologs in the series (n=1 and 4) are located near the center of t he 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 xDSA 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 CATn probes was eluci dated by comparing the experimental B and C parameters in the expressi on for the line width in motionally averaged ESR spectra of nitroxides Delta H(m(I))=A+Bm(I)+ Cm-I(2), with the corresponding values calcula ted as a function of the direction of the axis of rotation and N, the degree of anisotropy of the rotational reorientation. The comparison s uggests that the dominant motional mechanism for all probes is rotatio n around the N-O bond (x axis), but the anisotropy of the lower spin p robe homologs is significantly higher (N=10) compared to that of the h igher homologs (N=2-3). These results are consistent with the existenc e of a hydration gradient in the core of the reverse micelles.