Foamed emulsions

Slight variations of this method using reflux temperatures29-31 and a foam-emulsion system30 have been used industrially since the oxidant is cheap and readily available and yields are usually above 70%. 

Liquid Liquid Liquid-gas Liquid-liquid Liquid-solid Solution Foam Emulsions Slurry Suspension Metal plating effluent spent acids wash-waters Detergent foam Oil-in-water (e.g. suds) water-in-oil Aqueous effluent from fume scrubbing... 

As is evident from Eq. (3-20) or (3-21), the Bingham plastic exhibits a shear thinning viscosity i.e., the larger the shear stress or shear rate, the lower the viscosity. This behavior is typical of many concentrated slurries and suspensions such as muds, paints, foams, emulsions (e.g., mayonnaise), ketchup, or blood. 

It is also important that the phases be separated after the reactor, and mists, foams, emulsions, and dust in separation units can cause major problems in design of these multiphase reactors. Separation of phases after the reactor is easier for gas-Hquid and gas-Hquid systems than for Hquid-Hquid and liquid-solid systems because of the larger density differences for the former. Reactor orientation plays an obvious role in ary multiphase reaction processes. 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

There are a number o-f processes which have not been discussed (because o-f space) in which mixture o-f sur-f actants are important. Among these are foaming, emulsion formation, liquid crystal formation, microemulsion formation, adsorption as 1iquid—1iquid interfaces, and phase partitioning of surfactants between immiscible liquid phases. These areas will also see increased interest in the use of surfactant mixtures. 

The structure and stability of foamed emulsions, such as whipped cream, ice cream or whipped toppings, strongly depend on the interparticle interactions and on the orientation of drops/particles at the foam films. Further development of the surface force balance and... 

A foam in which the liquid consists of two phases in the form of an emulsion. Also termed foam emulsion . Example whipped cream consists of air bubbles dispersed in cream, which is an emulsion. See also Foam. See Foaming Agent. 

Dependence of the lifetime of foam bilayers on the concentration of dissolved surfactant. The stability of foam, emulsion and membrane bilayers can be characterised by their mean lifetime r which is the time elapsing form the moment of formation of a bilayer with a given radius until the moment of its rupture. Obviously, this is a kinetic characteristic of the bilayer stability and can only be applied to thermodynamically metastable bilayers. 

Similar steep x(C) dependences, which are an immediate indication for nucleation-mediated rupture, have been obtained for all investigated cases of foam, emulsion and membrane bilayers [300, 421], In some cases, for example, for bilayers of phospholipids, the rtO dependence is so steep that it is hard to obtain short-lived bilayers. 

In conclusion, let us outline some more important aspects of the hole-nucleation theory for stability of amphiphile bilayers of Kashchiev-Exerowa and its experimental support. The outlined theoretical and experimental investigations of the stability and permeability of foam, emulsion and membrane bilayers represent a new approach towards... 

Linear energy ft. of holes in foam, emulsion and membrane bilayers of various amphiphiles... 

Foamed Emulsions with Large Volume Fraction of the Disperse Phase... 

Foamed emulsions are disperse systems with two disperse phases (gas and liquid) in the disperse medium (surfactant solution). Water foamed emulsions are formed when foams or aqueous surfactant solutions are used to clean up oil deteriorated surfaces, in the process of oil flotation of waste waters, in firefighting when the foam contacts various organic liquids and in the processes of chemical defoaming (foam destruction by antifoams). Individual foamed emulsions can have practical importance e.g. a foamed emulsion of bitumen is used in road coating foamed emulsions from liquid fuels are used as explosives. 

Since the densities of gas and oil are quite different, foamed emulsions can separate into concentrated emulsions and a polyhedral foam in which no oil (or very little) is present. The ability to control such instability proves to be the determining factor in the practical application of such systems. In that respect the only study known to the authors is [126]. 

Fig. 7.24. Kinetics of destruction of foamed emulsion from 1% NaDoS containing 0.5% PVA...

Fig. 7.24 depicts the results from the kinetics of destruction of a foamed emulsion produced from 1% NaDoS solution and 0.05% PVA, its drainage and separation of the emulsion from it. The determined period of half-life of the foamed emulsion the time of drainage of one half of the volume of the aqueous solution (tl) and the time for separation of... 

Dependence of the destruction of foamed emulsions on the hydrocarbon chain length of aliphatic alcohols... 

Composition of foamed emulsions surfactant solution/kerosene (1 1) in the nominator - 1% NaDoS solution in the denominator - 1% Volgonate solution ... 

The drainage studies of foamed emulsions produced from emulsions containing 50 vol.% oil indicates that the kinetics of drainage is described well by Eq. (5.50) used for the drainage of low expansion ratio foams. Most stable proves to be the foamed emulsions formed from xylol. 

Dependence of the destruction of concentrated foamed emulsions on the nature of the stabiliser and on the oil concentration in the liquid phase... 

The behaviour of foamed emulsions changes considerably when the oil volume reaches about 70%. The properties of a foamed emulsion at such a volumetric ratio are similar to those of a concentrated emulsion. Table 7.7 presents the results from the determination of the stability of foamed emulsions obtained from 10% Volgonate and OP-10 at equal initial gas content and different volumetric content of the organic phase (80-91%). 

The data indicate that all foamed emulsions have T/ xe and xp of the order of hours and increase with the increase in the surfactant concentration. The best stabilising ability exhibits OP-10 at high oil content. This is valid also for its mixture with Volgonate. 

The dependence of the stability on the gas content for concentrated foamed emulsions produced from 10% surfactants solutions at constant oil content (85.7%) are presented in Table 7.8. 

Time h Content of 10% surfactant solution Volume of the foam from which the foam emulsion is formed, cm ... 

In this case the stability of the foamed emulsion with respect to the separation of the emulsion and with respect to its destruction as a whole, falls significantly when the initial foam volume changes from 10 cm3 to 20 cm3, i.e. when the gas content augments. 

The data in Tables 7.7 and 7.8 reveal that most significant raise in the stability of the foamed emulsions is observed when the volume of the organic phase increases to a value that provides a concentrated foamed emulsion with droplets of polyhedral shape. In such an emulsion the gas remains longer due to the strong delay of its diffusion and to the high stability of the droplets towards coalescence. 

Information about O/W foamed emulsions are not found in literature. 

Foam films are usually used as a model in the study of various physicochemical processes, such as thinning, expansion and contraction of films, formation of black spots, film rupture, molecular interactions in films. Thus, it is possible to model not only the properties of a foam but also the processes undergoing in it. These studies allow to clarify the mechanism of these processes and to derive quantitative dependences for foams, O/W type emulsions and foamed emulsions, which in fact are closely related by properties to foams. Furthermore, a number of theoretical and practical problems of colloid chemistry, molecular physics, biophysics and biochemistry can also be solved. Several physico-technical parameters, such as pressure drop, volumetric flow rate (foam rotameter) and rate of gas diffusion through the film, are based on the measurement of some of the foam film parameters. For instance, Dewar [1] has used foam films in acoustic measurements. The study of the shape and tension of foam bubble films, in particular of bubbles floating at a liquid surface, provides information that is used in designing pneumatic constructions [2], Given bellow are the most important foam properties that determine their practical application. The processes of foam flotation of suspensions, ion flotation, foam accumulation and foam separation of soluble surfactants as well as the treatment of waste waters polluted by various substances (soluble and insoluble), are based on the difference in the compositions of the initial foaming solution and the liquid phase in the foam. Due ro this difference it is possible to accelerate some reactions (foam catalysis) and to shift the chemical equilibrium of some reactions in the foam. The low heat... 

Various techniques can be employed to determine the lowest (threshold) concentration (cmm) corresponding to the formation of a stable foam with > 1 [85-88], For example, it is possible to form a relatively small amount of a foam (or emulsion) in a large volume of a dispersion medium. This technique has been used in the study of the concentration dependence of the stability of foams [89,90] and of W/O emulsions [91,92], Another technique is based on the function foam (emulsion) volume versus concentration, presented by... 

An interface in contact with a mixture shows all sorts of interesting things. The layer next to the interface can have a composition very different from that of the bulk fluid this lowers the interfacial tension and can greatly enhance the stability of foams, emulsions and suspensions. 

---