Microemulsions fluctuations

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... 

For diffuse and delocahzed interfaces one can still define a mathematical surface which in some way describes the film, for example by 0(r) = 0. A problem arises if one wants to compare the structure of microemulsion and of ordered phases within one formalism. The problem is caused by the topological fluctuations. As was shown, the Euler characteristic averaged over the surfaces, (x(0(r) = 0)), is different from the Euler characteristics of the average surface, x((0(r)) = 0), in the ordered phases. This difference is large in the lamellar phase, especially close to the transition to the microemulsion. x((0(r)) =0) is a natural quantity for the description of the structure of the ordered phases. For microemulsion, however, (0(r)) = 0 everywhere, and the only meaningful quantity is (x(0(r) = 0))-... 

Figure 7. Topological fluctuations of the lamellar phase at different points of the phase diagram, (a) Single fusion between the lamellae by a passage (this configuration is close to the topological disorder line), (b) Configuration close to the transition to the disordered microemulsion phase the Euler characteristic is large and negative.

Michael W. Deem, A Statistical Mechanical Approach to Combinatorial Chemistry Venkat Ganesan and Glenn H. Fredrickson, Fluctuation Effects in Microemulsion Reaction Media... 

The preceding condition of thermodynamic equilibrium implies that the curvature of the dispersed phase becomes zero at the transition from two to three phases. In the middle-phase microemulsion, the pressures p and p2 fluctuate in time and space because of the instability of the interface between the two media (see below), and intuition suggests that Eqs. (37) and (40) be replaced with their average, hence that the condition of zero curvature be replaced with the condition of mean (with respect to time) average curvature. 

Venkat Ganesan and Glenn H. Fredrickson, Fluctuation Effects in Microemulsion Reaction Media David B. Graves and Cameron F. Abrams, Molecular Dynamics Simulations of Ion-Surface Interactions with Applications to Plasma Processing Christian M. Lastoskie and Keith E. Gubbins, Characterization of Porous Materials Using Molecular Theory and Simulation... 

Oil and water do not mix, but on addition of a suitable surfaetant a microemulsion can be formed depending on the relative concentrations of the three components. Microemulsions (i.e. surfactant/water/oil mixtures) can also be used as reaction media see references [859-862] for reviews. Microemulsions are isotropic and optically clear, thermodynamically stable, macroscopically homogeneous, but microscopically heterogeneous dispersions of oil-in-water (O/W) or water-in-oil (W/O), where oil is usually a hydrocarbon. The name microemulsion, introduced by Schulman et al. in 1959 [863], derives from the fact that oil droplets in O/W systems or water droplets in W/O systems are very small (ca. 10... 100 nm nanodroplets). Unlike conventional emulsions, microemulsion domains fluctuate in size and shape with spontaneous coalescence and breakup. The oil/water interface is covered with surfactant molecules and this area can amount to as much as 10 m per litre ( ) of microemulsion. 

This potential force occurs in microstructured fluids like microemulsions, in cubic phases, in vesicle suspensions and in lamellar phases, anywhere where an elastic or fluid boundary exists. Real spontaneous fluctuations in curvature exist, and in liposomes they can be visualised in video-enhtuiced microscopy [59]. Such membrane fluctuations have been invoked as a mechanism to account for the existence of oil- or water-swollen lamellar phases. Depending on the natural mean curvature of the monolayers boimding an oil region - set by a mixture of surfactant and alcohol at zero -these swollen periodic phases can have oil regions up to 5000A thick With large fluctuations the monolayers are supposed to be stabilised by steric hindrance. Such fluctuations and consequent steric hindrance play some role in these systems and in a complete theory of microemulsion formation. 

The ultrasonic absorption in relation to the transitions and critical phenomena in microemulsions has been studied by Lang et al. (72). The ultrasonic absorption is very sensitive to the concentration fluctuations which occur near the critical temperature or composition in binary liquids. Similar absorption maxima were also expected as the composition of the systems was varied in the vicinity of composition where water-in-oil microemulsions convert into the oil-in-water microemulsions. However, the most puzzling feature of these data is probably the very continuous change of the relaxation parameters with composition even in the range where W/0 microemulsions turn into 0/W microemulsions. 

Usually geometrical connectivity and concentration fluctuations are not related. However, in our case, the electrical percolation is not a simple geometrical connectivity. This fact can explain that electrical percolation and critical points seem to be associated and that (jp % [Pg.80]

The Kerr constant around cf> v0.1 becomes very large for ATB microemulsions and the decay curve becomes exponential.The measured decay time t is in fairly good agreement with the lifetime tg of a critical density fluctuation of size . is the correlation length of the density fluctuations previously measured in ATB microemulsion by light scattering and 3... 

The concentration fluctuations in our system can possibly further be subdivided into a solute (surfactant aggregates) concentration fluctuation and a solvent concentration fluctuation. The solute concentration fluctuation is similar to that of critical-like behavior observed in many W/0 microemulsion systems (28), while the solvent... 

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