Microemulsion phase, description

A.l. A Water-in-Oil Droplet Microemulsion. (a) Description of the Phase. The microemulsion system is composed of Ngo droplets of various sizes g (each consisting of surfactant, alcohol, oil and water molecules) and outside the droplets,Noo oil molecules, Also surfactant molecules, Nao alcohol molecules, and ATW0 water molecules. The subscript g for a droplet denotes the total number of molecules of different kinds present in it (i.e., g =gs +8a +go +gw). The A ao alcohol molecules outside the droplets are present both as singly dispersed molecules and as aggregates. The number of alcohol aggregates containing j alcohol molecules is denoted by Njao-... 

As a general description, a microemulsion is a homogeneous phase that contains a substantial fraction of both oil and water, their mixing being induced by the dissolved surfactant. In contrast with macroemulsions, microemulsion systems form spontaneously in solution. As with micelles, microemulsions are thermodynamically stable. As a result, in contrast to macroemulsions, microemulsion formation is reversible. For example, if changes in a system parameter (e.g., temperature) alter the microemulsion system, when the system parameter returns to its original state, so does the microemulsion system. In contrast to macroemulsions, a microemulsion forms virtually independent of the volume of the oil phase, but once formed, the microemulsion enhances the aqueous solubility of the oil phase. Most often, an excess oil phase persists in the presence of a microemulsion phase—the oil and water phases do not completely mix. 

The phase diagrams of two quaternary mixtures made of sodium dodecylsulfate (SDS)-water-dodecane and hexanol (system A) or pentanol (system B) have been investigated in detail [22,23]. In both cases, sections of the three-dimensional diagram with constant water/surfactant ratio have been examined. These cuts were chosen because they allow a good description of the oil region and also because the water/SDS ratio, termed X in the following, fixes the size of the droplets in the inverse microemulsion phase and the thickness of the bilayers in the oil-rich lamellar phase. In the description of the quaternary mixtures, we emphasize the details of the evolution of the phase equilibria as X is varied. We have focused our attention not only on the characterization and the location of the boundaries of the various phases but also on the equilibria between the phases. 

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))-... 

Recently an alternative approach for the description of the structure in systems with self-assembling molecules has been proposed in Ref. 68. In this approach no particular assumption about the nature of the internal interfaces or their bicontinuity is necessary. Therefore, within the same formahsm, localized, well-defined thin films and diffuse interfaces can be described both in the ordered phases and in the microemulsion. This method is based on the vector field describing the orientational ordering of surfactant, u, or rather on its curlless part s defined in Eq. (55). 

The potential x as the difference of electrical potential across the interface between the phase and gas, is not measurable. But its relative changes caused by the change of solution composition can be determined using the proper voltaic cells (see Section IV). The name surface potential is unfortunately also often used for the description the ionic double layer potential (i.e., the ionic part of the Galvani potential) at the interfaces of membranes, microemulsion droplets and micelles, measured usually by the acid-base indicator technique (Section V). 

In that case the self diffusion coefficient - concentration curve shows a behaviour distinctly different from the cosurfactant microemulsions. has a quite low value throughout the extension of the isotropic solution phase up to the highest water content. This implies that a model with closed droplets surrounded by surfactant emions in a hydrocarbon medium gives an adequate description of these solutions, found to be significantly higher them D, the conclusion that a non-negligible eimount of water must exist between the emulsion droplets. 

We start our description of microemulsions by asking what is the most likely radius of drops R for given volume fractions of continuous phase, dispersed phase (j>d, and surfactant 4>al This radius can be estimated by using two equations. First, the total volume of the dispersed phase is given by... 

This chapter describes novel inkjet inks based on a variety of vehicles, and demonstrates several optical applications utilized by inkjet inks. It aims to provide a general description of inks which are based on unique components and structures, mainly micellar systems, polyelectrolyte complexes, microemulsions, miniemulsions, emulsions, liquid crystals, and interesting phase... 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

The most common definition of a microemulsion characterises it as a thermodynamically stable, transparent, optically isotropic, freely flowing surfactant mixture, often containing co-surfactants (e.g. alcohol) and added salts [37]. We restrict the definition further to non-crystalline (disordered) aggregates, since crystalline isotropic phases are better considered as liquid crystalline mesophases. Indeed, the most succinct description of a microemulsion would involve its microstructure. However, this has proven to be a very equivocal issue. So much so that until very recently it was widely believed that microemulsions were devoid of microstructure hence the thermod)mamic definition. 

Bartelt [5]. Therefore, due to historic precedent the description of water-in-oil polymerizations proceeds through an analogy, either colloidal or kinetic, to a process with a continuous aqueous phase. The prefix inverse is generally accepted for water-in-oil emulsions in contrast to direct or conventional oil-in-water emulsions/microemulsions for which the prefix is implied but not often explicitly stated. 

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