INTERACTION OF POLY(ETHYLENE OXIDE) WITH THE SODIUM DODECYL-SULFATE MICELLE INTERFACE STUDIED WITH NITROXIDE SPIN PROBES

Electron spin resonance (ESR) line widths of 5-, 7-, 12-, and 16-doxyl stearic acid (x-DSA) and tempo nitroxides versus the concentration of poly(ethylene oxide) (PEG) in sodium dodecyl sulfate (SDS) micelles sh ow different trends. The ESR line widths of 5-, 7-, and 16-DSA increas e with increasing concentration of PEG, which is interpreted as due to increasing viscosity in the environment of the nitroxide spin probe. The tempo and 12-DSA line widths were independent of the concentration of PEG. The line width showed the highest value for 5-DSA and the low est value for tempo. The line width of x-DSA decreases from 5-DSA to a minimum value for 2-DSA and then increases somewhat for 16-DSA. This is interpreted as bending of the alkyl chain to provide different loca tions for the nitroxide moiety relative to the micelle interface. The relative distances of the nitroxide moiety of x-DSA from deuterated wa ter at the SDS micelle interface was measured by deuterium electron sp in echo modulation. The distances increased from 5-DSA to 12-DSA and t hen decreased for 16-DSA. The interpretation df the ESR line width tre nd is supported by the deuterium modulation depth trend. The deuterium modulation depths of x-DSA also increase with increasing concentratio n of PEO. They are largest for 5-DSA, less for 12-DSA, and then larger again for 16-DSA. These changes of the ESR line widths and deuterium modulation depths for x-DSA indicate alkyl chain bending of x-DSA. The alkyl chain of x-DSA is bent near the 12-carbon position which corres ponds to the deepest nitroxide location into the hydrocarbon region of SDS micelles. The data also show that the interaction site of PEO wit h SDS micelles is near the interface region.