The aminoxide system

For the cubic phase the SANS experiment shows stUl the same type of aggregates hut more concentrated. However, here the packing exceeds the critical volume fraction of 53 vol%, which is typical for a hard sphere crystallization [63, 64]. In the cubic phase the spherical aggregates are packed similar to metal atoms in a cubic lattice. From SANS investigations of samples in the cubic phase it seems that the packing is not primitive cubic but either face-centred cubic (fee) or body-centred cubic (bcc). Between these two possibilities an experimental distinction was not feasible [65]. 

A similar series of samples as in the SANS experiments was studied in cooperation with the group of Prof. Wokaun by NMR self-diffusion experiments. Tbe pulsed field gradient spin echo (PGSE) method [67, 68] allows the determination of the self-diffusion coefficient of each of the individual constituent components in particular water, surfactant, and hydrocarbon. Here, in order to obtain simpler NMR spectra the hydrocarbon was cyclohexane. The molar ratio of C14DMAO cyclohexane was chosen to be 1 1.2, with three samples in the Lj phase and three samples in the cubic phase. 

The obtained self-diffusion coefficient of water shows a continuous, approximately linear decrease with increasing volume fraction of the micellar aggregates and with no dis- 

A much different picture is obtained for the diffusion of the surfactant and the hydrocarbon (Fig. 11.24). Within the L phase both diffuse with the same coefficient. This is in good agreement with the diffusion coefficient of the micellar aggregates calculated from their size, determined by scattering methods. Again some decrease of the diffusion coeffi- 

In summary, this experiment clearly demonstrates that in the L phase and in the cubic phase the same water continuous structure is present. But while in the L phase the micellar aggregates are freely mobile their translational mobility is blocked in the cubic phase, i.e. the microemulsion droplets are condensed into a glass-like liquid crystalline state of high elasticity. 

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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%). 

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. 

PEO/PPO/PEO triblock copolymers exhibit properties similar to typical surfactants, i.e. they reduce surface and interfacial tension of aqueous solutions and form micellar aggregates above a critical micellar concentration [74, 78]. For some compounds of this type, like P104 (EOigPOsgEOis), P123 (E02oP07oE02o)> 3nd F127 (EO106PO70EO106), a similarly located cubic phase like the one of the aminoxide systems has been found in tbe binary aqueous system [74, 79]. 

These systems are insofar similar as they both are next to an isotropic phase. Moreover a binary system containing triblock copolymers of the polyethyleneoxide/polypropy-leneoxide/polyethyleneoxide (PEO/PPO/PEO) type has been studied for which we also have foimd a formation of a cubic phase with location in the phase diagram similar to the aminoxide case. 

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