Disordered systems microemulsions

This chapter concentrates on the results of DS study of the structure, dynamics, and macroscopic behavior of complex materials. First, we present an introduction to the basic concepts of dielectric polarization in static and time-dependent fields, before the dielectric spectroscopy technique itself is reviewed for both frequency and time domains. This part has three sections, namely, broadband dielectric spectroscopy, time-domain dielectric spectroscopy, and a section where different aspects of data treatment and fitting routines are discussed in detail. Then, some examples of dielectric responses observed in various disordered materials are presented. Finally, we will consider the experimental evidence of non-Debye dielectric responses in several complex disordered systems such as microemulsions, porous glasses, porous silicon, H-bonding liquids, aqueous solutions of polymers, and composite materials. 

Anderson and Wennerstrom [33] calculated the geometrical obstruction factors of the self-diffusion of surfactant and solvent molecules in ordered bicontinuous microstructures, which serve as good approximations also for the disordered bicontinuous microemulsions and L3 (sponge) phases. The geometrical obstruction factor is defined as the relative diffusion coefficient DIDq, where D is the diffusion coefficient in the structured surfactant system and Z)q is the diffusion coefficient in the pure solvent. In a bicontinuous microemulsion the geometrical obstruction factor depends on the water/oil ratio. An expansion around the balanced (equal volumes of water and oil) state gives, to leading order. 

It is seen above that the areas for microemulsions stabilized by ionic surfactants are decisively dependent on the structure of the cosurfactant to cause the necessary disorder in the system. Microemulsions stabilized by polyethylene glycol adduct nonionic surfactants, on the other hand, are characterized by the fact that co surfactant is not used. Instead, the areas of stability now rely on temperature (Figure 1.11) although the relation with the liquid crystal structure is still the essential element [14]. 

Lattice models for bulk mixtures have mostly been designed to describe features which are characteristic of systems with low amphiphile content. In particular, models for ternary oil/water/amphiphile systems are challenged to reproduce the reduction of the interfacial tension between water and oil in the presence of amphiphiles, and the existence of a structured disordered phase (a microemulsion) which coexists with an oil-rich and a water-rich phase. We recall that a structured phase is one in which correlation functions show oscillating behavior. Ordered lamellar phases have also been studied, but they are much more influenced by lattice artefacts here than in the case of the chain models. 

For the system studied in [174], it turns out that the oil/water interface is not wetted by the microemulsion, even though the latter is weakly structured. Hence fluctuations do shift the wetting transition beyond the disorder... 

K. V. Schubert, R. Strey. Small-angle neutron scattering from microemulsion near the disorder Une in water/formamide-octane-C,E systems. J Chem Phys 95 8532-8545, 1991. 

Polymerization in microemulsions allows the synthesis of ultrafine latex particles in the size range of 5 to 50 nm with a narrow size distribution [33], The deposition of an ordered monolayer of such spheres is known to be increasingly difficult as the diameter of such particles decreases [34], Vigorous Brownian motion and capillary effects create a state of disorder in the system that is difficult... 

The increase in conductivity is due to increase in dissolved surfactant, and this increase continues until all the crystallites dissolve. The peak in specific conductance is attained when the microemulsion is formed and the specific conductance levels off. The plateau of Figure 6 is often referred to as a "percolation threshold" (] ) and is reached when there is a disordered interspersion capable of bicontinuous structures ( ). Further addition of methanol results in a lowering of conductivity explained by the solution eventually approaching the conductivity of methanol. This is the region of molecular dispersion. These conductivity curves are similar to those observed by Lagues and Santerey (13) on a system of water, cyclohexane, sodium dodecylsulfate and 1-pentanol. 

Small-angle neutron scattering (SANS) can be applied to food systems to obtain information on intra- and inter-particle structure, on a length scale of typically 10-1000 A. The systems studied are usually disordered, and so only a limited number of parameters can be determined. Some model systems (e.g., certain microemulsions) are characterized by only a limited number of parameters, and so SANS can describe them fully without complementary techniques. Food systems, however, are often disordered, polydisperse and complex. For these systems, SANS is rarely used alone. Instead, it is used to study systems that have already been well characterized by other methods, viz., light scattering, electron microscopy, NMR, fluorescence, etc. SANS data can then be used to test alternative models, or to derive quantitative parameters for an existing qualitative model. 

Calculations of the small-angle x-ray scattering expected from a disordered array of reverse micelles (whose dimensions can be accurately determined for this system since the interfacial area and volume fractions are well known) differ markedly from measured scattering spectra, except in the most water-rich microemulsion mixtures. Only at the highest water contents which form microemulsions alone, are conductivity and X-ray spectra consistent with water-filled reverse micelles embedded within an oil continuum. 

Schubert, K.V. and Strey, R. (1991) Small-angle neutron-scattering from microemulsions near the disorder line in water formamide octane QEj systems. /. Chem. Phys., 95, 8532-8545. 

However, even without structural studies, Friberg et al. [32], Shinoda [33], and others noted that the broad existence range with respect to the water/oil ratio could not be consistent with a micellar-only picture. Also, the rich polymorphism in general in surfactant systems made such a simplified picture unreasonable. It was natural to try to visualize microemulsions as disordered versions of the ordered liquid crystalline phases occurring under similar conditions, and the rods of hexagonal phases, the layered structure of lamellar phases, and the minimal surface structure of bicontinuous cubic phases formed a starting point. We now know that the minimal surfaces of zero or low mean curvature, as introduced in the field by Scriven [34], offer an excellent description of balanced microemulsions, i.e., microemulsions containing similar volumes of oil and water. 

By the way, ionic surfactants generally do not lead to microemulsions at ambient temperature, but to liquid crystals, which are organized systems. To produce a microemulsion, it is often required to introduce disorder either by increasing temperature (up to 50°Q or by adding a cosurfactant, generally an alcohol [9,10]. 

The electrochemical oxidation of ferrocene and amphiphilic ferrocenes, 2- and 5-(ferrocenylcarboxy)dodecyltrimethylammonium nitrates (2-Fc and 5-Fc, respectively), were investigated in a bicontinuous microemulsion containing CTAC, -tetradecane, pentanol, and water [83]. The electron transfer rates for ferrocene, 2-Fc, and 5-Fc in microemulsions were an order of magnitude slower than in acetonitrile, possibly due to partial inhibition by microemulsion components adsorbed on the electrode [5,6]. The rates of electron transfer of 2-Fc and 5-Fc were only two-fold smaller than that of ferrocene, in contrast to 10-fold (2-Fc) and 100-fold (5-Fc) smaller in CTAB micelles [84]. These results indicate an increased disorder and mobility at the electrode/fluid interface in the CTAC microemulsion compared to CTAB micellar system [83]. 

In many instances the SOW ideal ternary case is not sufficient to describe the behavior of an actual system. It has been noted that the addition of alcohol as a disordering agent is often required to avoid viscous or sol id-like mesophases. particularly to produce microemulsions with ionic surfactant systems (108). According to the corre)ations for optimum formulation, the alcohol effect can also be that of a cosurfactant that modifies the overall balance of affinity through the flA) and 0(i4) terms. The use of two surfactants i.s also often recommended to attain a better emulsion stability, a statement that. should not be taken for granted in all cases, although it could prove correct in some ca.ses. Hence, it is often... 

As discussed in Sect. 4.1, one prominent example of a situation where the SCF theory fails on a qualitative level is the microemulsion channel in ternary mixtures of A and B homopolymers and AB diblock copolymers. Figure 5 shows an example of a mean-field phase diagram for such a system. Four different phases are found A disordered phase, an ordered (lamellar) phase (see Fig. 4, right snapshot), an A-rich and a B-rich phase. The SCF theory predicts the existence of a point where all three phases meet and the distance of the lamellar sheets approaches infinity, an isotropic Lifshitz point [100,101]. 

The simulations allow to analyze the phase transition and the microemulsion phase in more detail. Figure 11 compares different characteristic length scales of the system at the order/disorder transition. One length scale, Lo, is obtained from the maximum of Fo(q) (Eq. 128) and gives the typical distance between layers. The other is the mean absolute curvature radius Dc,... 

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