Extensive light scattering measurements, including the intensity, turbidity, and linewidth, on a three-component microemulsion system consisting of mixtures of water, decane, and a surfactant sodium di-2 ethylhexylsulfosuccinate (AOT) (WDA), have been made. The critical and several off-critical mixtures have been studied along constant microemulsion droplet volume fraction lines in the one-phase region over a very large temperature range. In the vicinity of the lower phase separation temperature Tp the intensity data are very well accounted for by the standard theory of critical binary fluids using a single value for the short range correlation length ξ0=(13.5±1.5) Å. By combining a mode-coupling theory including the background effects and a linear model equation of state applicable in the critical region, we have been able to fit the dynamic light scattering data using a Debye cutoff length q−1D which is equal to the constant average diameter of microemulsion droplets. Furthermore, we find clear evidence for a crossover from critical to single particle behavior in both static and dynamic light scattering data. A crossover temperature Tx has been identified at which qDξ(Tx)=1. Analyses of the dynamic light scattering data show that qD, which can only be measured far away from Tp, in fact plays a decisive role in controlling the critical dynamics in the whole temperature range.
An extensive static and dynamic light-scattering experiment on a water-in-oil microemulsion system (water/n-decane/AOT) showed that the critical dynamics of this three-component microemulsion is completely governed by a Debye cut-off wavenumber qD, which is equal to the inverse diameter of the microemulsion droplet. Furthermore, the authors found occurrence of a crossover phenomenon for the statics and dynamics, at a correlation length xi c such that qD xi c=1, the crossover temperature being at about 10 degrees C below the critical temperature.
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