The bis- 2-ethylhexyl sulfosuccinate system

As stated above, the cubic phase of the AOT system is located differently in the phase diagram than the aminoxide system, which is due to the fact that AOT as a double-chain surfactant has a tendency to form reverse phases. Again it was of great interest to investigate the relation between the isotropic L2 phase and the cubic phase. Furthermore, this cubic phase is interesting since here the surfactant concentration can be varied over a large range (30-76 wt%). 

The transition from the L2 phase into the cubic phase has been studied by a variety of methods [73]. This can be done easily on a line of constant AOT content (in our case mostly 35 wt%). Then increasing the octanol/H20 ratio one crosses from the cubic phase into the 

L2 phase. Interestingly, measurements of the electric conductivity showed no discontinuity of the equivalent conductivity (specific conductivity/AOT concentration) upon crossing the phase boundary. Instead one observes a continuous increase of A with increasing H2O content. The value in the cubic phase is about 40 % of that of the free Na ions, which should mainly be responsible for the ionic conductivity (since the surfactant counterion will be largely immobilized being fixed in the amphiphilic film), which indicates that the structure must be water continuous. 

SANS spectra of the cubic phase show the typical spikes on the isotropic diffraction ring (compare Fig. 11.22). From the position of the peaks and from the total scattering intensity, it can be concluded that the size of the structural units decreases with increasing AOT (water) content, at constant octanol/water (AOT/octanol) ratio. 

Measurements of the shear modulus G° on a series of samples with constant ratio of octanol/water showed that G° is proportional to c(AOT) i.e. again, as in the case of the aminoxide system, the elastic modulus increases with decreasing size of the structural units, since the SANS experiments show that the size of the structural units decreases with increasing AOT concentration. Correspondingly the particle density N of these units increases and, as stated above, G° should be proportional to N. This means that the determination of the micro structure already allows the prediction of the elastic properties, as in the case of amin-oxide. 

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