Disordered Micellar Solutions

First we consider surfactant phases containing aggregates without positional or orientational order. 

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It is quite noticeable that these figures are very close to those known for melted paraffins of similar chain lengths. Then the development of NMR studies of deuterated moleoules and of NMR permitted to show that the amplitude of the deformations are about the same in all structures, ordered liquid crystals as well as in disordered micellar solutions. Thus there does not appear any definite correlation between one structure and the molecular behavior in it. The chains are about equally disordered in all structures, in spite of important changes of interfacial curvature and mean area per molecule at the interface. 

Fig. 4.13 (a) Phase diagram for aqueous solutions of Pluronic 25R8 (PPOI5PE01WPPO 5) determined using SANS, SLS, DLS and rheometry (Morlensen 1997 Mortensen el al. 1994). Phases 1 and V are disordered micellar networks, V with excess water. Phases II and III are cubic micellar phases. Phase IV is a coexistence regime of micelles and crystalline layered PEO. (b) Schematic of the micellar network. 

As explained above, the hexagonal micellar mould is only present for the micellar solutions that contain between 30 and 65 wt.% of sur ctant. The addition of TMOS probably disturbs this array, leading to a disordered fiamework. However, in the case of less concentrated micellar solutions, where only individual micellar rods exist, it appears that the addition of the silica source plays the key role in the obtention of very regular hexagonally arranged channels. Such a co-operative mechanism could be analogous to the LCT pathway initially proposed by Mobil researchers [2]. 

In a bicontinuous CTAC microemulsion, ferrocene had a k° twice as large as 1 and 2 [33], but values for 2-Fc and 5-Fc were similar. These relatively small differences in electron transfer rates in the microemulsion cannot be explained by head down-tad up orientation of 1 and 2 at the time of electron transfer as proposed for micellar solutions. The results suggest an increased disorder and mobd-ity in the electrode-fluid interface in the CTAC microemulsion compared to micellar CTAB solutions. 

Gast and co-workers (Gast 1996 McConnell et al. 1993,1995) have used SAXS to probe ordered micellar structures in PS-PI block copolymers dissolved in decane, which is a preferential solvent for PI. They determined the form factor of a range of diblocks by performing SAXS in dilute solution (McConnell et al. 1993). The same diblocks in more concentrated solutions were found to form cubic micellar structures, and the gelation (micellar disorder-order) transition on increasing concentration was determined. Remarkably, both BCC and FCC struc-... 

In subsequent work, ordering in solutions of the same matched diblock and triblock spanning a broader range of volume fractions, 0.1 < < 0.4, was explored (Hamley et al. 1997). For liquid-like and SAXS showed that there was no inter-micellar order in the liquid. Above a crossover concentration 0.2, ordering of micelles was shown by the presence of a structure factor peak. The ordered micellar structure, identified as hexagonal-packed cylinders for more concentrated solutions, persisted up to an order-disorder transition located from a discontinuity in the... 

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