The PEG-SDS interaction results in the formation of In aqueous solution, the interaction between sodium dodecyl small aggregates or clusters of SDS attached to the PEG sulfate (SDS) and poly(ethylene glycol) (PEG) results in the forpolymer chain. These SDS-PEG clusters are smaller than mation of small aggregates or clusters of SDS attached to the normal SDS micelles, are more highly dissociated, and ap-PEG polymer chain. Selectivity coefficients for exchange of two pear to have a somewhat more ''open'' structure (2-5, 7, monovalent (N-methyl-4-cyanopyridinium cation and Tl / ) and 8). The PEG polymer chains are reported to be located at two divalent (methylviologen cation and Cu 2/ ) counterions at the the surface of the SDS clusters, but estimates of the fraction surface of SDS-PEG clusters, determined employing photophysiof the PEG segments bound to the micelles vary rather cal techniques, are similar, but not identical, to those for exchange widely (4, 5). Although a number of theoretical models at the surface of SDS micelles in the absence of PEG. The principal factor affecting ion exchange selectivity in SDS-PEG clusters does have been proposed to explain the onset of the interaction not appear to be aggregate size or surface charge density but rather (cac), the saturation, and the aggregate size, none provides the presence of poly(oxyethylene) subunits at the aggregate sura completely satisfactory description of all of the aspects of face. ᭧ 1997 Academic Press the PEG-SDS interaction (3-5).