Double critical behavior and micellar size effect in the multicomponent surfactant solution

Multicomponent surfactant system, sodium dodecyl sulfate and butanol in an aqueous NaCl solution, which exhibits a closed-loop type reentrant phase se paration, was investigated by light scattering experiments focusing on the double critical behaviors and the finite micellar size effect on the dynami c critical behavior. The loop size decreased with the decrease of NaCl conc entration, and the double critical point appeared. Approaching the double c ritical point, the critical exponents nu for the long-range correlation len gth xi and gamma for the isothermal osmotic compressibility chi(T) determin ed by use of the spinodal divergency theory showed a crossover behavior fro m Fisher's renormalized Ising model values to the doubling of them. xi and chi(T) formed master curves as a function of epsilon(UL) = \(T-U-T)(T-L-T)\ /TUTL, and had the effective critical exponents nu=0.73 and gamma=1.41 asce rtaining the validity of Fisher's renormalized Ising model. Here, T-U and T -L are the upper and lower critical solution temperatures, respectively. Dy namic critical behaviors were well explained in a unified manner over the h ydrodynamic and critical regions by the modified dynamical droplet model ta king the finite micellar size into account, which assumes that the local fl uctuations activated thermally behave as the physical clusters with a fract al dimension of d(f)=2.49 and a polydispersity exponent of tau=2.21. The ev aluated monomer unit sizes are in good agreement with the micellar sizes ob tained independently. (C) 2000 American Institute of Physics. [S0021-9606(0 0)52033-9].