Emulsions interfacial phenomena

An excellent discussion of the phenomenon of spontaneous emulsion has been included in a recent book by Davies and Rideal (D2). Interfacial turbulence has been advanced (D2, 01) as a possible cause but has been eliminated in at least one ease (D2). Diffusion and stranding seems... 

The time scale of fat crystallization is much shorter for topping powders than for ice cream mix as presented in Figure 2. This is due to the much higher emulsifier content in topping powder. The induction of fat crystallization in whippable emulsion systems is due to interfacial protein desorption from the fat globules of the emulsion mediated by the emulsifiers. This phenomenon is described in section 3.1. 

The presence of surfactants, either natural or added, promotes emulsion stability by the reduction of interfacial tension and the formation of highly rigid films on the surface of the droplets. This reduction of interfacial tension can increase the maximum, M, in Figure 4 significantly through charge stabilization or steric stabilization (J5). Because the nature and shape of the interaction energy curve determine the stability of OAV (and other types) of emulsions, any process, parameter, or phenomenon that affects the shape of this curve will ultimately control emulsion stability. 

Re-entrainment of liquid droplets that are captured can also occur as a result of squeezing when the local pressure drop is increased to overcome the capillary resistance force. The shape of the liquid droplets depends on the wettability of the rock. On the basis of this physical picture, Soo and Radke 12) proposed a model to describe the flow of dilute, stable emulsion flow in a porous medium. The flow redistribution phenomenon and permeability reduction are included in the model. Both low and high interfacial tension were considered. 

The interfacial tension values increase from A.l dynes/cm for SLS/ decanol to 8.3 dynes/cm for SLS/octadecanol. Conductometric titration results have indicated that all of these mixed emulsifier systems, except the one with decanol, should give a relatively stable emulsion (22,23). Interestingly, the SLS/decanol mixed emulsifier solution was the only case in which the presence of the fatty alcohol reduced the interfacial tension with styrene to below the value measured for SLS alone. Studies are in progress to investigate this phenomenon and to determine the effect of alcohol chain length on miniemulsion stability. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

Partial Coalescence. This is a complicated phenomenon. It can occur in O-W emulsions if part of the oil in the droplets has crystallized. The ultimate driving force is, again, a decrease in interfacial free energy, but the relations given by Hamaker, Laplace, and Young (Section 10.6.1) all are involved. 

One of the principal distinction between the liquid-liquid and liquid-gas interface is the possibility to achieve very low interfacial tensions down close to zero. This possibility is realised provided the surfactant is soluble both in the aqueous and hydrocarbon phase, as well as using binary mixtures of water-soluble and oil-soluble surfactants. This phenomenon is of special importance for the formation of emulsions and microemulsions, in the removal of dirt and in the enhanced oil recovery. 

PTC incorporated with other methods usually greatly enhances the reaction rate. Mass transfer of the catalyst or the complex between different phases is an important effect that influences the reaction rate. If the mass transfer resistance cannot be neglected, an improvement in the mass transfer rate will benefit the overall reaction rate. The application of ultrasound to these types of reactions can be very effective. Entezari and Keshavarzi [12] presented the utilization of ultrasound to cause efficient mixing of the liquid-liquid phases for the saponification of castor oil. They used cetyltrimethylammo-nium bromide (CTAB), benzyltriethylammonium chloride (BTEAC), and tetrabutylammonium bromide (TBAB) as the catalysts in aqueous alkaline solution. The more suitable PT catalyst CTAB can accumulate more at the liquid-liquid interface and produces an emulsion with smaller droplet size this phenomenon makes the system have a high interfacial surface area, but the degradation of CTAB is more severe than that of BTEAC or TBAB because of more accumulation at the interface of the cavity under ultrasound. 

Petrov etal. (1980) analyzed the causes for the entrapment of water between the solid substrate and the monolayer in Z-type depositions. This phenomenon has many common features with film thinning processes found during foam and emulsion breakdown and it is dependent on interfacial properties and on molecular interactions between the solid substrate and the monolayer. Petrov etal. (1980) measured the maximum speed of removal of the solid substrate before entrainment of a water layer and found it to be dependent on pH and ionic strength. There is no record in the publication of the measurement of dynamic contact angles. 

As indieated in Fig. 7, the emulsion viscosity passes through a minimum at optimum formulation (164). The value of this minimum is quite low, unusually low as it is known that the ulfralow interfacial tension tends to produce small droplets. Actually, this is not necessarily true as will be diseussed later, beeause the droplets formed can coalesce at once. It seems that the low emulsion viscosity is due to the ease of deformation of the droplets along the streaming lines, a phenomenon similar to the drag reduction by polymers (165). 

The main problem is the lack of knowledge about the interface between phases (1) and (2) where the exchange phenomenon occurs. Therefore, the interfacial area, the diffusion and mass transfer between the gas and the emulsion phase are unknown. 

As is the case in most discussions of interfacial systems and their applications, definitions and nomenclature can play a significant role in the way the material is presented. The definition of an emulsion to be followed here is that they are heterogeneous mixtures of at least one immiscible liquid dispersed in another in the form of droplets, the diameters of which are, in general, greater than 0.1 (.m. Such systems possess a minimal stability, generally defined rather arbitrarily by the application of some relevant reference system such as time to phase separation or some related phenomenon. Stability may be, and usually is, enhanced by the inclusion of additives such as surfactants, finely divided solids, and polymers. Such a definition excludes foams and sols from classification as emulsions, although it is possible that systems prepared as emulsions may, at some subsequent time, become dispersions of solid particles or foams. 

Besides coalescence, there is another mechanism by which emulsions degrade (or coarsen) into fewer, larger-sized droplets diffusional degradation. Monomer from smaller droplets diffuses to larger ones as the result of the process of interfacial free energy minimisation. This phenomenon is called Ostwald ripening (224). 

In Chapter 3, the solution and surface properties of a relatively new class of material, namely, polymeric surfactants, are illustrated in some detail using Flory-Huggins theory and current polymer-adsorption theory. This is followed by a discussion of the phenomenon of steric stabilization of suspended particles and how it is affected by the detailed structure of the stabilizing polymeric species. It concludes with a discussion of the stabilization of emulsions by interfacial and bulk theological effects, and presents closing comments on multiple emulsions. 

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