- To evaluate the potential of micellar enhanced ultrafiltration (MEUF) for treating groundwater contaminated with organics in northern Negev, Israel, the partition coefficient, K TBNPA, of tribromo-neopentyl alcohol (TBNPA) between aqueous and micellar phases was determined for nonionic surfactants. Two methods were used for the determination: the MEUF method using centrifugal UF equipped with 10 kDa MWCO regenerated cellulose and the solid phase matrix extraction (SPME) method using gas chromatography (GC) analysis. The distribution coefficient of TBNPA for nonionic surfactant Brij 58 (C 16 EO 20 ) was ∼630 M -1 and fairly constant over a wide range of loadings (mole fraction of TBNPA in micelle from 0.07-0.45) when determined by centrifugal MEUF; but is was only ∼340-400 M -1 when determined by SPME. This difference is attributed to concentration polarization of the micelles in the UF measurements. The partition coefficient was reduced about 10% in the presence of 1% NaCl. The presence of toluene in samples containing TBNPA increases the solubility of TBNPA in the micellar phases of Brij 58 and Igepal CO-720, as determined by SPME. This phenomenon is explained by assuming that toluene (a nonpolar molecule) dissolves in the hydrocarbon core of the micelles resulting in expansion of the micelle and increase of the distance between polar groups in the palisade layer. As a result, the micelle can incorporate more of the polar TBNPA molecules in that layer. Cryogenic-temperature transmission electron microscopy observations of micelles loaded with TBNPA showed that TBNPA loading did not significantly change micelle size, consistent with TBNPA concentrating in the palisade layer.