Stabilization by surfactants

Surfactants provide temporary emulsion droplet stabilization of monomer droplets in tire two-phase reaction mixture obtained in emulsion polymerization. A cartoon of tliis process is given in figure C2.3.11. There we see tliat a reservoir of polymerizable monomer exists in a relatively large droplet (of tire order of tire size of tire wavelengtli of light or larger) kinetically stabilized by surfactant. 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

The substitution of water-borne versions of these primers is increasing as environmental restrictions on the use of organic solvents become stricter. These are generally aqueous emulsions of epoxy novolac or phenolic based resins stabilized by surfactants [34]. Non-ionic surfactants are preferred, as they are non-hygroscopic in the dried primer films. Hygroscopic ionic surfactants could result in excessive water absorption by the primer film in service. 

These routes involve the formation of (usually) prereduced metal particles that are then adsorbed or deposited onto the support. They have the advantage that the particle size of the particles is predetermined by the chemistry of the colloids and that resulting catalysts have narrow particle size distributions. However, the colloidal particles often are surface stabilized by surfactant molecules, which can be difficult to remove once the particles are adsorbed onto the support. One further disadvantage is that the colloidal particles are prepared at high dilution (typically millimolar concentrations— for example, 0.2 g Ft 1 ), which is a disadvantage in terms of scale-up. 

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

A paper contributed by J. E. Desnoyers, R. Beaudoin, C. Roux, and C. Perron described the use of microemulsions as a possible tool for the extraction of oil from tar sands. Using a technique called flow microcalorimetry recently developed at the University of Sherbrooke, these researchers studied the structure and stability of organic microphases in aqueous media. These microphases can be stabilized by surfactants and can dissolve large quantities of oil. In a similar vein, D. F. Gerson, J. E. Zajic, and M. D. Ouchi (University of Western Ontario) described the extraction of bitumen from Athabasca tar sands by a combined solvent-aqueous-surfactant system. 

The largest portion of the monomer (>95%) is dispersed as monomer droplets whose size depends on the stirring rate. The monomer droplets are stabilized by surfactant molecules absorbed on their surfaces. Monomer droplets have diameters in the range 1-100 pm (103-105 nm). Thus, in a typical emulsion polymerization system, the monomer droplets are much larger than the monomer-containing micelles. Consequently, while the concentration of micelles is 1019-1021 the concentration of monomer droplets is at most 1012-1014 L 1. A further difference between micelles and monomer droplets is that the total surface area of the micelles is larger than that of the droplets by more than two orders of magnitude. The size, shape, and concentration of each of the various types of particles in the... 

Mun, S., Decker, E.A., McClements, D.J. (2006). Effect of molecular weight and degree of deacetylation of chitosan on the formation of oil-in-water emulsions stabilized by surfactant-chitosan membranes. Journal of Colloid and Interface Science, 296, 581-590. 

Even closer to cell membranes than monolayers and bilayers are organized surfactant structures called black lipid membranes (BLMs). Their formation is very much like that of an ordinary soap bubble, except that different phases are involved. In a bubble, a thin film of water — stabilized by surfactants — separates two air masses. In BLMs an organic solution of lipid forms a thin film between two portions of aqueous solution. As the film drains and thins, it first shows interference colors but eventually appears black when it reaches bilayer thickness. The actual thickness of the BLM can be monitored optically as a function of experimental conditions. Since these films are relatively unstable, they are generally small in area and may be formed by simply brushing the lipid solution across a pinhole in a partition separating two portions of aqueous solution. 

As shown above, the pulsed field gradient NMR technique was first described by Tanner and Stejskal [1,2]. In addition to their work on unrestricted diffusion they also performed theoretical analyses of restricted diffusion and tested their results on octanol-in-water emulsions stabilized by surfactants. 

Lonnqvist et al. [14] performed NMR experiments on emulsions stabilized by surfactants to obtain information about the droplet size and size distribution and whether a particular emulsion is of the O/W or the W/O type. Murday and Cotts equation [7] was used with different droplet radii, each radius being weighted by its normalized volume fiaction. 

Figure 12.14 Schematic of an oil-in-water droplet stabilized by surfactant.

Commonly, the reaction is conducted in an emulsion stabilized by surfactants and containing the substrate (PC). In a microporous membrane reactor, PLD stability could be increased sevenfold by the addition of ethers to the chamber side. Additionally, no surfactants were required as the product could be separated in simple fashion 20% product was formed, compared with 4% in the simple emulsion system. 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

A thin film of water in air that is stabilized by surfactant. The term is used even though the film is not a film of soap and even where the surfactant is not a soap. See also Fluid Film. 

Pertraction (PT) can be realized through a liquid membrane, but also through a nonporous polymeric membrane that was applied also industrially [10-12]. Apart from various types of SLM and BLM emulsion liquid membranes (ELM) were also widely studied just at the beginning of liquid membrane research. For example, an emulsion of stripping solution in organic phase, stabilized by surfactant, is dispersed in the aqueous feed. The continuous phase of emulsion forms ELM. Emulsion and feed are usually contacted in mixed column or mixer-settlers as in extraction. EML were applied industrially in zinc recovery from waste solution and in several pilot-plant trials [13,14], but the complexity of the process reduced interest in this system. More information on ELM and related processes can be found in refs. [8, 13-16]. 

It is well-known that free films of water stabilized by surfactants can exist as somewhat thicker primary films, or common black films, and thinner secondary films, or Newton black films. The thickness of the former decreases sharply upon addition of electrolyte, and for this reason its stability was attributed to the balance between the electrostatic double-layer repulsion and the van der Waals attraction. A decrease in its stability leads either to film rupture or to an abrupt thinning to a Newton black film, which consists of two surfactant monolayers separated by a very thin layer ofwater. The thickness of the Newton black film is almost independent of the concentration of electrolyte this suggests that another repulsive force than the double layer is involved in its stability. This repulsion is the result of the structuring of water in the vicinity of the surface. Extensive experimental measurements of the separation distance between neutral lipid bilayers in water as a function of applied pressure1 indicated that the hydration force has an exponential behavior, with a decay length between 1.5 and 3 A, and a preexponential factor that varies in a rather large range. 

Nanosuspensions Drug nanocrystals dispersing in aqueous media commonly stabilized by surfactants Suitable for insoluble drugs to obtain good bioavailability and targeting 19... 

In froth flow the gas and the liquid are intimately mixed as a froth, with the liquid forming very thin films surrounding the gas bubbles. This flow is created by passing the gas through a glass frit. The froth flow is usually unstable but can be stabilized by surfactants. 

The better understanding of the mechanisms of stability incomplex dermatological emulsions stabilized by surfactants and amphiphiles has enabled the development of a rapid microscopic method for evaluation of potential emulsifiers. The method is based on the observation that good emulsifier blends that stabilize emulsions by the formation of multilayers of stable gel phase also swell spontaneously in water at ambient temperature and this process can be observed microscopically. Mixtures that do not form gel phase or form metastable gels only after a heating and cooling cycle cannot be observed to swell spontaneously at ambient temperature. ... 

These are transparent isotropic structured fluids composed of two immiscible phases that are stabilized by surfactants. Often a co-surfactant and a co-solvent are present in the formulation. Microemulsions form spontaneously and are thermodynamically stable. Their transparency is due to the small droplet size (<100 nm) in microemulsions (Flanagan and Singh, 2006 Garti and Aserin, 2007). 

Particles may be formed at the inorganic-C02 interface. Inorganic suspensions of metals and metal oxides have been formed by reaction or by mixing particles stabilized by surfactants or ligands (25). Semiconductor nanoparticles have been formed through chemical reduction in water-in-C02 microemulsions. The vast number of potential interfaces formed by combination of hydrophilic, lipophilic, and C02-philic phases will continue to offer... 

The most interesting property of Weimarn sols is their stabilization by surfactants as well as by certain biopolymers. Most effective seems to be gelatine of which only 0.025 /tg are needed to prevent the precipitation of 2 ml sol by aqueous NaCl (10%) within 5 min. The same result is obtained by 3 yUg of saponin, by 5 /tg each of egg albumin, gum arabic or sodium stearate, by 25 /tg of dextrin, or by 500 /tg of sodium cholate [13-15]. It can be assumed that the biopolymer molecules are adsorbed by their hydropho-... 

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