Microemulsions mesophases

Hentze H-P, Co CC, McKelvey CA, Kaler EW (2003) Templating Vesicles, Microemulsions and Lyotropic Mesophases by Organic Polymerization Processes. 226 197-223 Hergenrother PJ, Martin SF (2000) Phosphatidylcholine-Preferring Phospholipase C from B. 

Amphiphilic lipopeptides with a hydrophobic paraffinic chain containing from 12 to 18 carbon atoms and a hydrophilic peptidic chain exhibit lyotropic meso-phases and good emulsifying properties. The X-ray diffraction study of the mesophases and of dry lipopeptides showed the existence of three types of mesomorphic structures lamellar, cylindrical hexagonal and body-centred cubic. Two types of polymorphism were also identified one as a function of the length of the peptidic chain and the other as a function of the water content of the mesophases. The emulsifying properties of the lipopeptides in numerous pairs of immiscible liquids such as water/ hydrocarbons and water/base products of the cosmetic industry showed that small amounts of lipopeptides easily give three types of emulsions simple emulsions, miniemulsions and microemulsions. 

Many surfactants have been used to formulate microemulsions (1). They were of three types anionic surfactants such as petroleum sulfonates, sodium octyl benzene sulfonate, sodium dodecyl sulfate, alkaline soaps cationic surfactants such as dodecyl ammonium and hexadecyl eimmonium chlorides or bromides and nonionic surfactants such as polyoxyethylene glycols. Furthermore, many exhibit liquid-crystalline properties (2) and in some cases the structure of the mesophases has been established (3). Nevertheless, nearly nothing is known about their compatibility with blood and tissues, and, from our own experience, some exhibit a high lytic power for red cells... 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

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. 

Animal phospholipids, especially egg phospholipids or egg yolk, are the key precursors for a patent application, which claims the reaction with a sulphur-containing compound like cysteine to produce meaty flavours ]102], Phospholipids and monoglycerides can play a fundamental role in the physical structure of a process flavour. Vauthey and co-workers ]103] use these emulsifiers in water to generate nanostmctured mesophases or microemulsions at the reaction temperature. Better aroma yields and different aroma compounds were obtained in these nanostmctured reaction media compared to aqueous solutions. 

Figure 1.4 T(7)-sections through the phase prism of the systems H20-n-octane-C6E2, C8E3, Q0E4 and C12E5 at an oil/(water + oil) volume fraction of = 0.5. In order to determine the respective X-point the phase boundaries are measured only for surfactant mass fractions 7 > 7. An increase of both the hydrophobic chain length / and the size of the hydrophilic head group j shifts the X-point to lower values of 7, i.e. the efficiency increases. Simultaneously the stability range of the bicontinuous one phase microemulsion shrinks dramatically due to the increased extension of the lamellar mesophase (La). (From Ref. [26], reprinted with permission of Elsevier.)...

Formulation essentially relates to the content of the systems and generally not to the way it is attained if thermodynamically stable systems are considered. The simplest microemulsion system would contain an organic oil phase (O), an aqueous phase generally referred to as water (W), and a surfactant (S) at a given temperature (T) and pressure (p). This means that at least five variables are required to describe the system. In practice, the situation is much more complicated. Water always contains electrolytes. Moreover, oils as well as nearly all commercial surfactants are mixtures. In most cases, particularly with ionic surfactant systems, a co-surfactant (e.g. an alcohol (A)) is added, among other functions, to reduce the rigidity of the surfactant layer and thus to prevent the formation of gel-like mesophases. 

The ideas of the relevance of phase diagrams and thermodynamic stability as well as the bicontinuous structure were certainly not accepted immediately and many publications until well into the 1990s caused confusion as some authors still took droplet structures for granted. A title for a paper [31] in Nature as late as 1986 entitled Occurrence of liquid-crystalline mesophases in microemulsion dispersions illustrates both the slow acceptance and the ignorance of previous work on phase diagrams. 

Tabony, J. (1986) Occurrence of liquid-crystalline mesophases in microemulsion dispersions. Nature, 320, 339-341. 

Templating Vesicles, Microemulsions, and Lyotropic Mesophases by Organic Polymerization Processes... 

The most general definition of a template is as a structure-directing agent. In surfactant solutions the final templated polymers can be either discrete nanoparticles or mesostructured bulk materials as a consequence of polymerization, respectively, in the non-continuous or continuous domains of the template. Thermodynamically stable media, such as microemulsions, equilibrium vesicles, or lyotropic mesophases are especially useful as templates because of their structural definition and reproducible morphologies. The mesostructure of a thermodynamically stable template is defined by composition and temperature, but this same feature makes the structure unstable to changes in temperature, pH, or concentration. The aim of template synthesis is to transfer the self-organized template structure into a mechanically and chemically stable, durable, and processable material. 

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