Bilayer emulsion/interface

The larger thermodynamic affinity for the aqueous medium for the case of normal complexes as compared to interface complexes (Figure 7.16a) correlates well with ( -potential values of oil droplets in mixed and bilayer emulsions (Table 7.3). 

Therefore, two contributory factors may provide an explanation for more effective electrostatic / steric stabilization of the so-called mixed emulsions in comparison with the sequentially assembled biopolymer interfaces of the bilayer emulsions firstly, a greater hydrophilicity of the adsorbed protein-polysaccharide complexes, caused by the larger net negative charge, and, secondly, a more bulky architecture of the normal complexes as compared to the interface complexes. 

OS 41a] [R 19] ]P 30] A study was undertaken to compare extended (1 h) processing in small vials (2 cm ) with short-time (100 s) continuous micro reactor and mini-batch (10 cm ) operation for 10 different substrates (C4-C8 alcohols) which were reacted with rhodium(I)-tris(m-sulfophenyl)phosphane [111]. The vials were either directly filled with the two phases yielding a bilayered fluid system with small specific interfaces or by interdigital micro mixer action yielding an emulsion with large specific interfaces. 

FIG. 14. Transmission electron micrograph of Voltaren Emulgel the interface hetween the continuous hydrogel and the dispersed emulsion droplets consists of multiple bilayers of hydrated surfactant molecules, bar 500 nm. From Miiller-Goymann, C., and Schutze, W., Mehrschichtige Phasengrenzen in Emulsionen, Dtsch. Apoth. Ztg., 130 561-562 (1990). 

Rgure 2.30. Two adhesive emulsion droplets. A flat hquid fllm stabilized by the surfactant layers is located between the droplets. This fllm being very thin, it can be usually considered as a surfactant bilayer. Yf is the tension of the fllm and y nt the tension of single isolated interface. 

Table 7.3 Relationship between molecular parameters (A2, p) of sodium caseinate (0.5 wt%) + dextran sulfate complexes at pH = 6.0 formed in the bulk and at the interface of a protein foam, and the corresponding properties (J43, Q of the bilayer and mixed emulsions (20 vol% oil, 0.5 wt% sodium caseinate) containing 0.1 or 1.0 wt% dextran sulfate (Jourdain et aL, 2008 Semenova et al., 2009).

As w ell as lateral heterogeneity in mixed protein layers, there is also the possibility of segregation of biopolymer components perpendicular to the interface, z.e., bilayer formation (Dickinson, 1995, 2009). Let us consider the case of an interface containing casein and whey protein in an emulsion system. The images in Figure 8.4 are confocal micrographs... 

Drug is soluble in neither oil nor water however, it can be retained at the interface of an emulsion. Thus, if a liposomal preparation can be made in which the drug resides in the lipid bilayer, or if it can be solubilized into micelles by an appropriate detergent, an emulsion can probably be made wherein the drug resides at the interface. 

The formation of polymersomes from water in- oil-in-water drops. Initially, a double emulsion consisting of single aqueous drops within drops of a volatile organic solvent ( oil ) is prepared using a microcapillary device. Amphiphilic diblock copolymers dissolved in the middle phase assemble into monolayers at the oil-water interfaces. Evaporation of the solvent then leads to the formation of polymer bilayers (polymersomes). 

Transfer of water molecules (in evaporation control), transport of solvent across monolayers at oil-water interfaces (in Ostwald-ripening of emulsions) and transfer of ions across such interfaces (as models for ion conduction in bilayers and membranes) can often be treated in terms of surface concentration fluctuations. Their magnitudes can be expressed in standard deviations (cr for the standard deviation in the surface concentration), which are measures of the probability that random holes are formed in the layer, allowing material transport. We have presented the formal treatment in sec. 1.3.7. From this section we can immediately obtain = IcTOr / 0 ), for a Gibbs monolayer, with... 

What is the evidence supporting the emulsion particle model for LDLs One feature that may be used to distinguish an emulsion particle from a closed bilayer vesicle is that, for an emulsion particle, all of the phospholipid should be exposed to the external medium, whereas for a bilayer vesicle, somewhat more than one-half should be exposed. Enzymatic hydrolysis of LDLs by phospholipase A2 converted all of the phosphatidylcholine and phosphatidylethanolamine to their corresponding lysophospholipids (Aggerbeck et ai, 1976), indicating that ail of the phospholipid was located at the aqueous interface at the LDL surface. In addition, P NMR studies of LDLs have demonstrated that all of the phosphate was accessible to small amounts of Pr " ", a rare earth probe that should not cross a bilayer because of its ionic nature (Yeagle et ai, 1978). These results agree with the emulsion particle model. 

In this chapter, we report the influence of surface-active compounds on the stability of crude oil emulsions using the apparatus designed for bilayer lipid membrane studies. The electrical method we employed to measure the film lifetime and thickness of model oils and crude oils seems to be a convenient technique for monitoring the coalescence processes in emulsions. The results obtained show that the natural surface-active substances in crude oil, such as petroleum acids and asphaltenes, have a great effect on the film strength. The ionized acids formed by the reaction between the petroleum acids and the alkali can decrease the interfacial tension and accelerate the thinning and breakdown of the thin liquid film. The asphaltenes can adsorb on to the interface and improve the stability of the film. The order of stability of the films between different oils and alkaline solutions is as follows crude oil with asphaltenes removed < crude oil < crude oil with both asphaltenes and petroleum-acids removed (iv) < crude oil with petroleum acids removed. In addition. 

Artificial lipid bilayer membranes can be made [22,23] either by coating an orifice separating two compartments with a thin layer of dissolved lipid (which afterwards drains to form a bilayered structure—the so-called black film ) or by merely shaking a suspension of phospholipid in water until an emulsion of submicroscopic particles is obtained—the so-called liposome . Treatment of such an emulsion by sonication can convert it from a collection of concentric multilayers to single-walled bilayers. Bilayers may also be blown at the end of a capillary tube. Such bilayer preparations have been very heavily studied as models for cell membranes. They have the advantage that their composition can be controlled and the effect of various phospholipid components and of cholesterol on membrane properties can be examined. Such preparations focus attention on the lipid components of the membrane for investigation, without the complication of protein carriers or pore-forming molecules. Finally, the solutions at the two membrane interfaces can readily be manipulated. Many, but not all, of the studies on artificial membranes support the view developed in the previous sections of this chapter that membranes behave in terms of their permeability properties as fairly structured and by no means extremely non-polar sheets of barrier molecules. 

The catalyst was a sulfonated derivative of Wilkinson s catalyst which did not appear to affect the structure of bilayers with respect to their permeability barrier properties [2]. The catalyst was found to hydrogenate oil-in-water emulsions and two-phase oil-water systems without the need for organic co-solvents [3]. The reaction rate could be increased significantly by screening the electrostatic charge on the sulfonate groups with inorganic cations added to the aqueous phase. This allowed the catalyst to penetrate into the substrate at the interface partition of the catalyst from the aqueous into the lipid phase could not be detected. 

The next chapter, by Kvamme and Kuznetsova, presents a theoretical approach to molecular-level processes taking place at the W/0 interface. The chapter comprises state-of-the-art concepts, experimental results, and atomic-level computer simulations of processes de-terming the stability of the dispersions. Parallels are drawn to lipid bilayers. A strategy suitable for molecular dynamics simulation of water-in-crude-oil emulsions is presented, with most of its constituents elements proved by computer simulations of less complex systems. 

Qualitatively, the preceding discussion of surface interactions tells us that free surfaces are inherently unstable and will usually experience a net attraction for similar surfaces in the vicinity. The practical repercussion is that if only the van der Waals forces were involved, systems involving the formation and maintenance of expanded interfaces would all be unstable and spontaneously revert to the condition of minimum interfadal area, thereby making impossible the preparation of paints, inks, cosmetics, many pharmaceuticals, many food products, emulsions of all kinds, foams, bilayer membranes, etc. It would be a decidedly different world we lived in. In fact, life as we know it (or can conceive of it) would not exist Obviously, something is or can be involved at interfaces that alters the simple situation described above and makes things work. In the following chapters we will introduce other actors that allow nature (and humankind, when we re lucky) to manipulate surfaces and interfaces to suit our purposes. 

Flow also induces the formation of micro-emulsions, as it can detach the pinch-offs from the interface. As shown by Kim et al. [76] for a bilayer of reactive PS-end-carboxylic and PMMA-ran-epoxy, micro-emulsions could be observed in a shear fiow under a shear rate of 100 s after less than 50 min, while for static conditions they would form after 15 h [57[. In the case of melt blends prepared in internal mixers, micro-emulsions could be detected after less than 5 min of mixing, thus demonstrating again the importance of fiow [68[. 

Stable oil-in-water emulsions can also be obtained by dispersing polar lipids such as phospholipids into triglycerides and then emulsifying the oil in water. The presence of charged phosphatidylcholine components of phosphohpids improves the stabilization of the emulsions. In most of these systems, the polar phospholipids form a separate phase at the interface where they form lamellar bilayers and a monolayer separated by triglyceride oil, between the outer water phase and the iimer triglyceride oil phase (Figure 10.3). 

As depicted in Figure 4.5, the surfactant bilayer created at the solid/aqueous solution interface provides hydrophobic loci for the solubilization of monomer or radicals these can polymerize further on the modified inorganic surface and effectively coat the sohd particles according to an emulsion-like polymerization reaction. Three steps are involved in the coating reaction. In the first step, the emulsifier adsorbs onto the mineral surface, forming micelle-like aggregates. In a second step, the monomer is solubihzed in the adsorbed micelles. Finally, the polymerization takes place as a conventional emulsion polymerization in the monomer-sweUed admiceUes (Fig. 4.6). 

Table 1 Elastic modules G, yield stress xys su d viscosity of emulsion bilayer films and infiafacial adsorption layers formed at the interface with heptane from aqueous gelatin/lecithin mixed solutions...

Liquid interfaces with monolayers play an important role in many diverse industrial processes, creating colloidal dispersions such as emulsions, micelles or liposomes. When two monolayers come into contact, bilayers can be synthesized. Phospholipid bilayers form liposomes, which are used as vehicles for drug delivery, in cosmetics and in gene therapy [1]. 

Nanoparticles phase transfer behaviors at the oil—water interface have many in common with fipid bilayer crossing behavior and the Pickering emulsion formation. The phase transfer behavior and interfacial behavior are intuitive indicators for the application potential of nanoparticle materials. Polymer brush modification enables nanoparticles to behave differendy in hydrophilic solvent, hydrophobic solvent, and their interface region. 

The transition between the lamellar liquid crystalline phase emd the gel pheise can be utilized to stabilize the emulsion, provided the actual gel phase is stable. If an aqueous dispersion of the emulsiher is hrst formed, and the emulsihcation is then performed under cooling, an emulsion is formed with the gel phase forming the 0/W interface. Such an emulsion has a much higher stability when compened with that produced with the lamellar phase at the 0/W interface. This is probably due to the higher mechanical stability of the crystalline lipid bilayers compared to bilayers with liquid chain conformation. 

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