Surface viscosity emulsions

Monolayers can be used to retard evaporation (water reservoirs), a related effect is the use of an oil film to dampen waves (surface viscosity), interfacial films are often the stabilizing agent in emulsions (lower y and increase rjs). 

In conclusion we will note that the main difference between aqueous emulsion films and foam films involves the dependences of the various parameters of these films (potential of the diffuse double electric layer, surfactant adsorption, surface viscosity, etc.) on the polarity of the organic phase, the distribution of the emulsifier between water and organic phase and the relatively low, as compared to the water/air interface, interfacial tension. 

A generalization of Equation 5.288 to the more comphcated case of two nondeformed (spherical) emulsion droplets which account for the influence of surface viscosity has been published in Reference 440. 

Concentrated compositions that increase in viscosity when diluted are described in U.S. Patent 6,150,320. A Bostwick consistometer is used for all viscosity measurements and equivalence is offered to Brookfield measurements using Spindle no. 1, at 60 r/min, and the Zahn viscometer, no. 1 cup. A low-viscosity hard surface cleaning emulsion, approximately 12 mPa s, is described in U.S. Patent 5,934,375 that increases in viscosity upon dilution with water to 800 to 1200 mPas. 

Table I lists values of the steady-state tensions for the various individual proteins at the n-hexadecane—water interface. There is no obvious relationship between interfacial tension and apparent surface viscosity. Gelatin and o<-lactalbumin have similar viscosities but very different tensions p-casein and x-casein have similar tensions but very different viscosities. It is interesting to note that, although at short times (. 15 min) the surface activity of sodium caseinate lies intermediate between that for

Monolayers can be used to retard evaporation (water reservoirs), a related effect is the use of an oil film to dampen waves (surface viscosity), and monolayer interfacial films are often the stabilizing agent in emulsions (lower y and increase i/ ). The first scientific studies of spread monolayers appear to have included those by Benjamin Franklin in 1774 and Agnes Pockels in 1891. Experimental methods for determining the retardation of evaporation by monolayers have been reviewed by Barnes [52]. 

With respect to the rheological parameters fliey come to the conclusion that surface elasticity effects are superior to surface viscosity effects. This, however, apphes to pure surfactant layers and may be different for pure protein or mixed surfactant/protein adsorption layers. It has been stressed also by Langevin (26), in her review on foams and emulsions, fliat studies on the dynamics of adsorption and dilational rheology studies for mixed systems, in particular surfactant-polymer systems, are desirable in order to understand these most common stabilizing systems. 

A detailed analysis of the shape-recovery experiments can be found in Ref 29, which provides, we believe, the first measurement of interfacial transport parameters on die micrometer scale. It is noted that, although different droplets in an emulsion have uniform IFT values, their rates of shape recovery - and hence surface viscosities - appear more varied. Further study on this variability will be required. 

Depending on the type of crude oil, the adsorbed film at the interface can be either fluid or very viscoelastic and able to form a skin. Depending on the properties of the crude oil (e.g. API gravity (2), sulfur, salt and metals content, viscosity, pour point, etc.), the structure of the film can vary significantly. Therefore, the molecular packing, surface viscosity, surface elasticity and surface charge of the adsorbed film are very important parameters that determine various phenomena such as coalescence of emulsion droplets, as well as oil drop migration in porous media. 

Let us first consider the case of System II (surfactant inside the drops. Fig. 20) in which case the two drops approach each other like drops from pure liquid phases (if only the surface viscosity effect is negligible). Therefore, to estimate the velocity of approach of such two aqueous droplets one can use equation (38). On the other hand, the velocity V of droplet approach in System I can be expressed by means of Eq. (37). Note that the Taylor velocities for System I and System II are different because of differences in viscosity and droplet-droplet interaction. Then the following criterion for formation of emulsion of type I and II is obtained (Ivanov and Kralchevsky 1997 and Danov et al. 2001) ... 

It is the increase in surface viscosity produced by adsorbed films (insoluble and Gibbs monolayers, adsorbed polymers, etc.) that leads to the production of persistent foams, helps stabilize emulsions, and explains the role of spread monolayers in dampening surface waves, among other important interfacial phenomena. 

Further experimental and theoretical investigations are expected to give us a better understanding of the existence of low interfacial tensions and of the thermodynamic stability of the phases. Indeed, interfacial tensions can be related theoretically to micellar interactions in bulk phases (3). The role of surface viscosity is less clear. But it probably influences the phase stability as it happens for ordinary emulsions. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [27], For example, in some cases, the addition of a small amount of nonionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [21, 126], To the extent that viscosity and surface viscosity influence emulsion and foam stability one would predict that stability would vary according to the effect of temperature on the viscosity. Thus, some petroleum industry processes exhibit serious foaming problems at low process temperatures, which disappear at higher temperatures [21],... 

It was pointed out earlier that surfactant adsorption at liquid interfaces can influence emulsion stability by lowering interfacial tension, increasing surface elasticity, increasing electric double layer repulsion (ionic surfactants), lowering the effeetive Hamaker constant, and possibly increasing surface viscosity. Surfactant can determine the arrangement of the phases in an emulsion, that is, which phase will form the dispersed versus continuous phase. We will briefly summarize several rules of thumb. A very qualitative rule of emulsion type, Bancroft s rule, states that if a... 

The factor in Eq. (36) accounts for the effect of finite surface viscosity (j/s) and has been computed by Desai and Kumar [61] as a function of the inverse of the dimensionless surface viscosity (y = 0.4387/i /5jiIt must be emphasized that their results for the calculation of Cy are valid only for foams because they neglected the viscosity of the dispersed phase. Equation (36) can be used for liquid liquid concentrated emulsions only when the surfaces are immobile (i.e., when Cy = 1). The pressure gradient (dp/dz) can be computed as follows. 

A generalization of Equation 4.301 to the more complicated case of two nondeformed (spherical) emulsion droplets with account for the influence of surface viscosity and the solubility of surfactants in both phases has been published in Ref. [783]. The terms related to the surface viscosities K and E, and the surface elasticity N, are scaled with the drop radius, Rj, as follows [783] ... 

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