Langmuir, Vol.10, No.12, 4468-4474, 1994
Intermicellar Migration of Surfactant Molecules in Entangled Micellar Solutions
Surfactant self-diffusion coefficients (D) have been measured on semidilute solutions of nonionic surfactants (C(16)E(7), C(14)E(7), and C(14)E(6)) at different temperatures by using the pulsed-gradient spin echo method. The self-diffusion coefficient of these surfactants first decreases with increasing concentration (C) and then increases. Above about 10% by weight, the log D-log c plot gives a straight Line whose slope is in good agreement with the theoretical prediction (2/3) of the previously reported model which takes into account intermicellar migration of surfactant molecules. In order to simulate the observed diffusion coefficient in the whole concentration range, this model has been modified by taking into account the micellar diffusion. The activation energy for the molecular diffusion processes in the higher concentration range has been obtained from the simulation results at different temperatures. The observed activation energy is anomalously large (110-160 kJ.mol(-1)) and increases as the hydrophobicity of surfactant molecules is increased, which can also be explained by the above model. Concentration dependence of the viscosity has been measured on C(16)E(7), C(14)E(6), and C(12)E(5) systems. The slope of the double logarithmic plot decreases with increasing temperature. The plot of the slope vs T - T-c (T-c, the lower critical solution temperature) for different systems falls into a smooth Line. Transition from an entangled network to a multiconnected network is discussed.