Formation of rodlike micelles of dimethyloleylamine oxide in aqueous solutions: effects of addition of HCl and NaCl on the micelle size and the intermicellar interaction

Angular dependence of light scattering has been measured for aqueous solutions of dimethyloleylamine oxide in the presence of HCl and NaCl. In micellar solutions more concentrated than 0.1×10^–2 g cm^–3, rodlike micelles are dominantly formed, and their properties are strongly reflected in the characteristics of the solutions. The aggregation number, radius of gyration and even flexibility of the rodlike micelles increase with the addition of HCl as well as NaCl. The increase of HCl concentration up to 10^–3 N makes the aggregation number of rodlike micelles as large as 58,000, when 0.01 M NaCl is present. The large micelle size would be stabilized by the dehydration of the amine oxide group and the hydrogen bonding between nonprotonated and protonated molecules in a micelle.In aqueous solutions without HCl and NaCl or in their presence at very low concentrations, the light scattering is subject to the effects of both external and internal interferences. The effect of external interference can be separated from the effect of internal interference by applying the Zernike-Prins equation to the observed angular dependence of light scattering. Then the second virial coefficient and the pair interaction potential of rodlike micelles can be derived by means of certain approximate methods. The addition of HCl to 10^–3 N makes both repulsive and attractive forces stronger and the resulting potential well deeper, but the addition of NaCl depresses such an effect of HCl considerably.