EFFECT OF UREA ON MICELLAR PROPERTIES OF AQUEOUS-SOLUTIONS OF NONIONIC SURFACTANTS

We present results of a systematic experimental and theoretical investigation of the effect of urea on micellization, micellar growth, and phase separation of aqueous micellar solutions of the nonionic surfactant n-dodecyl hexaethylene oxide (C12E6). The experimental studies, which cover a wide range of urea concentrations from 0 to 6 M, involve determinations of the critical micellar concentration (cmc), micellar shape and size as a function of temperature, and liquid-liquid phase separation coexistence curves. We rind that the addition of urea significantly affects micellar solution properties. In particular, (i) the cmc increases, (ii) micellar size decreases, (iii) the "sphere-to-rod" shape transition temperature shifts to higher values, and (iv) the critical point, associated with the coexistence curve, shifts to a higher temperature and concentration. We also find that the measured coexistence curves, at 0, 2, 4, and 6 M urea concentrations, collapse onto a single universal curve when plotted in reduced coordinates. In addition, we find that plots of the mean micellar hydrodynamic radius, R(h), vs T(c) - T, where T(c) is the critical temperature and T is the actual temperature, overlap for 0, 2, 4, and 6 M urea concentrations, with the resulting universal curve exhibiting a pronounced break at a value of T(c) - T almost-equal-to 37-degrees-C.