No abstract

Structural features of w/o microemulsions have been conductometrically evaluated. The radius of the water pool, the effective radius of the droplets, their number per unit volume, the effective surface area of the water pool, and the aggregation number of the surfactant and cosurfactant molecules per droplet in the oil/water interphase have been estimated. Three oils («-heptane, «-decane, and xylene), three surfactants (CTAB, SDS, and AOT) and two cosurfactants (1-butanol and n-hexylamine) have been used; their effects on the structural properties of microemulsions have been studied at two different temperatures, 293 and 303 K, at two weight ratios of surfactant/cosurfactant, 0.33 and 0.50. The droplet radius has been found to be oil dependent and follows the order «-heptane < «-decane < xylene. The values at 293 K are greater than those at 303 K. The dependence of the radius on the surfactant is in the order SDS > AOT > CTAB. 1-Butanol yields greater radius than «-hexylamine with SDS and AOT, and the order is reversed with CTAB. The aggregation number of the surfactant and cosurfactant per droplet increases and decreases, respectively, with elevation of temperature. The Rc//?w (ratio of the effective radius of the droplet and the radius of its water core) remains fairly constant under all situations, the average value being 1.15 ± 0.10.

The diameter of these close-packed clusters is estimated to be about 6 A, which well agrees with the value obtained by the method of Greegor and Lytle,j5 described above.The coordination number of Cu atoms in the metal clusters, on the other hand, remains unchanged through the CO-oxygen redox cycle, as recognized in Table I. It might be considered, therefore, that the number of Cu atoms in the species X is substantially retained during the interconversion between Cu metal and CuO clusters at the low temperature examined: Le., the CuO clusters also consist of I O Cu atoms.From the points of view described so far, the species X is plausibly small CuO clusters in the zeolites. Upon reaction with CO, these clusters are directly reduced into metal clusters at a low temperature, while the original Cu2+ species in the zeolite are reduced to Cu+ at a higher temperature. The CuO clusters, furthermore, function as the catalytic centers in CO oxidation at low temperatures, via the reversible redox mechanism. Acknowledgment.For hydrated dispersions, the appropriate extent of water binding is required to test the validity of the effective medium theory (EMT) equation of Bruggeman. The extents of hydration of five poly(ethy1ene g1ycol)s (PEG 200, 300, 400, 600, and 1000) have been determined from viscosity, and, through the measurement of conductance of a 0.01 mol dm-3 NaCl in PEG solution, the validity of the Bruggeman equation for them has been confirmed. The Bruggeman equation has been applied to evaluate the extents of hydration of 16 o/w microemulsion systems, derived from both aliphatic and aromatic oils and stabilized by nonionic surfactants (TX 100 an Tween 20) and cosurfactnts (I-butanol, I-hexanol, and n-hexylamine).Except Tween 20 + hexylamine stabilized xylene and toluene systems, the hydration of microemulsions has been found to be lower than that of TX 100 and Tween 20 micelles. It depends on the type of surfactant and the surfactant/cosurfactant ratio as well as the type of oil. Xylene-and toluene-derived microemulsions exhibited greater hydration than those derived from hexane, heptane, and decane. IntroductionMicroemulsions are likely to exhibit special conductance behavior. Water-in-oil type systems often show percolation tendency, where, after a threshold concentration of water,'-I0 the conductance increases very sharply. At lower water level and reasonable water/surfactant mole ratio, there can be also a percolation transition with respect to t e m p e r a t~r e .~. ' . '~~~ Percolation studies of microemulsions have been gaining importance in recent years. This can bring out potential information about the internal structures of the water solubilized in the continuous oil phase. Very recently, the effective medium theory (EMT) of conductance

The pseudo-first-order rates of the alkaline fading of crystal violet have been studied in detail in aqueous and ethylene glycol media as well as in water/Aerosol OT (AOTVdecane, water/AOT/isooctane, and ethylene glycol/AOT/isooctane microemulsion media. Both ethylene glycol and its microemulsion have shown reactionaccelerating effects compared with other environments. The rate of reaction in the microemulsion medium is inversely proportional to the water/AOT or the ethylene glycol/AOT mole ratio; significant increases in rates occur at ratios less than 8 in the former and less than 4 in the latter. The energetics of the reaction have offered distinct features in different environments; the AH* and AS* values have shown isokinetic effects. Normal salt effects have been observed in water-containing media; ethylene glycol alone and in a microemulsion has shown equivalent but abnormal effects.

[reaction: see text] The formation of nitrone (a water exclusion reaction) in aqueous media using surfactant and subsequent cycloaddition in the same pot, a new example of green chemistry, is reported. The control of regioselectivity favors the formation of trans-5-substituted isoxazolidine. This work not only may lead to an environmentally benign system but also will provide a new aspect of reactions in water.