Critical fluctuations of the micellar triethylene glycol monoheptyl ether-water system

Abstract

Using the equal volume criterion and also the pseudospinodal conception the critical demixing point of the triethylene glycol monoheptyl ether/water system (C7E3/H2O) has been determined as Y-crit=0.1 and T-crit=296.46 K (Y, mass fraction of surfactant). From density measurements the critical micelle concentration (cmc) followed as Y-cmc=0.007 at 288.15 K and Y-cmc=0.0066 at 298.15 K. The (static) shear viscosity eta(s) and the mutual diffusion coefficient D of the C7E3/H2O mixture of critical composition have been evaluated to yield their singular and background parts. From a combined treatment of both quantities the relaxation rate Gamma of order parameter fluctuations has been derived. Gamma follows power law with universal critical exponent and amplitude Gamma(0)=3.1x10(9) s(-1). Broadband ultrasonic spectra of C7E3/H2O mixtures exhibit a noncritical relaxation, reflecting the monomer exchange between micelles and the suspending phase, and a critical term due to concentration fluctuations. The former is subject to a relaxation time distribution that broadens when approaching the critical temperature. The latter can be well represented with the aid of the dynamic scaling model by Bhattacharjee and Ferrell (BF) [Phys. Rev. A. 31, 1788 (1985)]. The half-attenuation frequency in the scaling function of the latter model is noticeably smaller (Omega(BF)(1/2)approximate to 1) than the theoretically predicted value Omega(BF)(1/2)=2.1. This result has been taken as an indication of a coupling between the fluctuations in the local concentration and the kinetics of micelle formation, in correspondence with the idea of a fluctuation controlled monomer exchange [T. Telgmann and U. Kaatze, Langmuir 18, 3068 (2002)]. (c) 2006 American Institute of Physics.