EFFECT OF N-BUTYLUREA ON THE CRITICAL MICELLE CONCENTRATION OF SODIUM LAURYL SULFATE IN WATER AT DIFFERENT TEMPERATURES

The critical micelle concentration (cmc) of sodium lauryl sulfate (NaLS) was determined in aqueous n-butylurea (BU) solutions at 15, 20, 25, 30, 35, and 45 °C. At each of these temperatures the cmc initially decreases, reaches a minimum, and then starts to increase with increasing BU concentration. Corresponding changes in the thermodynamic parameters of micellization show similar trends except at 45 °C where both ΔH°m and ΔS°m tend to increase only with increasing BU concentration. The degree of micelle ionization (α) was calculated from the slopes of the conductance curves and shows the characteristic behavior of increasing, reaching a maximum, and then decreasing as the concentration of additive is increased. This was explained as due to increasing BU solubilization in the micelle, reaching a maximum, and then decreasing as the BU concentration is increased. The results are discussed in terms of the effect of BU and temperature on the structure of the solvent and of micelle penetration. n-Butylurea is strongly adsorbed at water-air and water-dodecane interfaces and increases the dielectric constant of water, the molar increment being about 3 ± 1 at 20 °C. © 1979 American Chemical Society.