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The factors determining the micellar structure of non-ionic detergents in aqueous solutions have been investigated by means of light scattering, specific viscosity, sedimentation velocity, and partial specific volume measurements. The nonionic detergents used, viz., ethylene oxide condensates of nonylphenol, n-dodecanol, and «-octadecanol, have been characterized by analytical as well as surface chemical methods. The critical micelle concentration of non-ionic detergents is much lower than that of ionic detergents with comparable hydrophobic groups; consequently, the aggregate molecular weights are much larger than those of ionic detergents. Variations in aggregation number from 19 to 370 have been observed. These aggregation numbers depend on the length of both the hydrophobic and the hydrophilic groups. They increase with increasing length of the hydrophobic group, but decrease with increasing length of the hydrophilic group. Addition of electrolyte increases the aggregation number of the nonylphenol + 50 EO micelles. This increase is proportional to the electrolyte concentration, but is inversely proportional to the lyotropic number of the ionsv The effect of variations in the lyotropic number of the anions is more pronounced than that of the cations. _ Whereas with ionic detergents micelle formation is ascribed to a balance between hydrocarbon-chain attractions and ionic repulsions, with non-ionic detergents the hydrocarbon-chain attractions are opposed by the hydration and space requirement of the ethylene oxide chains. For the aggregate molecular weight region of 45,000 to 100,000, spheres appear to be the most probable shape, and for larger micelles, disks or rods.

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