group^.^-^ We could not obtain evidence for such secondary aggregation in CHAPS.Thus, GFC can provide very accurate C, values for CHAPS, which make possible the determination of aggregation numbers and stepwise aggregation constants. These results will be helpful J . Phys. Chem. 1991Chem. , 95, 1850Chem. -1858 for applications of CHAPS in membrane biochemistry. Furthermore, the present approach &n be used for other compounds, such as bile salts, drugs, nucleosides, and dye^.^-'^ The odd-even alternation observed in the aggregation number dependence of stepwise aggregation constants is a novel phenomenon, which has been revealed by the determination of very accurate V, values.The properties of aqueous solutions of low molecular weight triblock copolymers of PEO/PPO/PEO have been investigated, mainly by dynamic and static light scattering. At low concentrations (C < 10%) and temperature (<25 O C ) the relaxation time distributions from dynamic light scattering show the coexistence of the monomer (RH = 18 A), micelles (RH = 80 A), and micellar aggregates in relative proportions which depend critically on temperature and concentration. Micelles are formed at about C = 5% at 25 O C . At 40 "C and above micelles are present at all concentrations used (C > 0.3%). At infinite dilution the hydrodynamic radius of the micelles is approximately constant over the temperature range 15-50 OC. At finite concentrations the apparent micellar radius increases with increasing temperature. The growth into asymmetric particles with increasing concentration is stronger as demonstrated by ultracentrifugation and combining static and dynamic light scattering data. At higher concentrations, a solidlike gel is formed at a well-defined temperature as shown by oscillatory shear measurements. It is characterized by a dynamic correlation length which decreases monotonically with increasing concentration to about 20 A.

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Micellar solutions of a highly hydrophilic ethylene oxide−propylene oxide triblock copolymer, pluronic F88 (EO10 3PO39EO10 3), in aqueous solution are examined by small-angle neutron scattering (SANS) at different concentrations and temperatures and in the presence of different salts. At temperature less than 30 °C, F88 solution in water (5 wt %) showed unimers which are fully dissolved Gaussian chains. The unimer-to-micelle transition takes place when the temperature or concentration is increased. Added neutral salt favors micellization of the copolymer at lower concentration/temperature. At temperature close to ambient, block copolymer forms micelles that consist of a central core, presumably dominated by the propylene oxide blocks, surrounded by a corona of highly hydrated ethylene oxide subchains. SANS analysis shows that the size of hydrophobic core increases as a part of PEO adjacent to the PPO core loses water with increasing temperature or salt (KCl) concentration. The micellar volume fraction increases with increasing concentration of block copolymer. The salting out effect of different electrolytes on structure of micelles is in the order KCI > KBr > KI. The effect of temperature on micellar solutions in the presence of KCl is also examined. It has been observed that the effect of added salt on the structural phase behavior of block copolymer is analogous to that of temperature. In all the above study, micelles are found to be spherical.