Micellar-enhanced ultrafiltration can be used to remove multivalent anions or cations from aqueous streams. In the removal of chromate ions (CrOz-), the cationic surfactant hexadecylpyridinium chloride is added to the solution, and the chromate ions preferentially adsorb at the surface of the highly charged surfactant micelles. The solution is processed by ultrafiltration, using a membrane with pore sizes small enough to block the passage of the micelles and adsorbed ions. The permeate solution has a chromate concentration less than 0.1% that in the original stream. A new equilibrium model, combining the simple twophase polyelectrolyte theory of Oosawa with thermodynamic activity, material-balance, and charge-balance equations, successfully correlates ultrafiltration and equilibrium dialysis results for chromate solutions.
As shown in Figure 8 and suggested by Guinier,12 differences between intraparticle form factors and the Guinier exponential expression given in eq 6 are minimal for axial ratios, e s 0.4. This allows us to extend the use of eq 6 for our samples to values several times those appropriate for spheres with acceptable error.
Precise vapor pressure-solubility measurements have been made for aqueous solutions of benzene with -, ß-, and 7-cyclodextrins (CD) at temperatures in the range 15-45 °C. From the inferred values of benzene fugacity and solubility at known concentrations of the CD's, equilibrium constants, enthalpy changes, and heat capacity changes have been determined for formation of 1:1 complexes of each of the three CD's. Thermodynamic results have also been obtained for higher-order complexes (2:1 and 1:2) of benzene and -CD and for the complex of 2 benzenes with 1 /3-CD. The thermodynamic data indicate that hydrophobic effects become increasingly important in stabilizing the benzene-CD complexes as the cavity size of the CD's increases. The 1:1 complex of /3-CD is considerably more stable than the 1:1 complexes of a-and -CD. Heat capacity changes are quite negative for formation of all the 1:1 benzene-CD complexes.Registry No. -Cyclodextrin benzene complex (1:1), 30915-12-9; acyclodextrin benzene complex (2:1), 88945-47-5; a-cyclodextrin benzene complex (1:2), 88932-63-2; /3-cyclodextrin benzene complex (1:1), 88932-64-3; /3-cyclodextrin benzene complex (2:1), 88932-65-4; -cyclodextrin benzene complex (1:1), 88932-66-5.
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