Interfacial films, salt effects

To mitigate the effects of corrosion resulting from the presence of salts, it is advantageous to reduce the salt concentration to the range of 3 to 5 ppm. Typically, brine droplets in crude oil are stabilized by a mixture of surface-active components such as waxes, asphaltenes, resins, and naphthenic acids that are electrostatically bound to the droplets surface. Such components provide an interfacial film over the brine droplet, resulting in a diminished droplet coalescence. Adding water to the crude oil can decrease the concentration of the surface-active components on the surface of each droplet, because the number of droplets is increased without increasing component concentration. 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

Salt Effects in Interfacial Films Formed from Nonionic Surfactants... 

All these results should convince us that the monomer plays a crucial role in the phase diagrams of these systems. In fact, this role, more or less ignored by formulators until recently, is two-fold. As a cosurfactant, the monomer increases flexibility of the interfacial film. It can therefore deform more easily to produce a sponge structure. And as an electrolyte, the monomer reduces aqueous solubility of ethoxylated surfactants. It thereby favours their gradual transfer into the organic phase and the formation of a bicontinuous structure. This effect of salt in the formation of bicontinuous microemulsions is well known to the users of such systems. It is significant in this respect that the same systems without monomer or electrolyte (if the monomer is neutral) do not lead to bicontinuous structures. 

Specific ion effects are important because formation water that can be associated with oil field emulsions may vary greatly with respect to salt content. The most pronounced effects observed, particularly with North Sea crudes, is the condensing effect of Ca on the interfacial film rendering these more incompressible, resulting in more stable emulsions (1). 

Pons et al. have studied the effects of temperature, volume fraction, oil-to-surfactant ratio and salt concentration of the aqueous phase of w/o HIPEs on a number of rheological properties. The yield stress [10] was found to increase with increasing NaCl concentration, at room temperature. This was attributed to an increase in rigidity of films between adjacent droplets. For salt-free emulsions, the yield stress increases with increasing temperature, due to the increase in interfacial tension. However, for emulsions containing salt, the yield stress more or less reaches a plateau at higher temperatures, after addition of only 1.5% NaCl. 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

The NS-100 membrane is capable of giving salt rejections in excess of 99% in tests on salt solutions simulating seawater (18 gfd, 3.5% synthetic seawater, 1,500 psi, 25°C). If the polyurea interfacial reaction step is omitted, and the polyethylenimine-coated polysulfone film is heat-cured as usual, a crosslinked polyethylenimine semipermeable barrier film is generated. This membrane gives 70% salt rejection and 55 gfd water flux under the same test conditions as above. Also, if the fully formed NS-100 membrane is dried at 75°C, which is too low a temperature to effectively crosslink the amine layer, the resulting film will exhibit a salt rejection of 96% or less. 

In the following discussions the published experimental findings are presented interrelatedly first in terms of internal oil chemistry at the interface and instabilities based on its composition, secondly in terms of effects of water chemistry, and thirdly in terms of demulsifier interaction. We include the activity of interfacial components involved in the structure of the protective skin, the behavior(s) of this structure with changes to water chemistry or solvency, or the effects of changes in film stmeture itself due to modification of relative proportions of interfacially active components. In some examples, developments in interfacial rheology, which is both a tool for understanding stable films and a means of rationalizing the effects of demulsifiers in demulsification, are discussed interrelatedly. Films may be sensitive to crude oil type, gas content, aqueous pH, salt content, temperature, age, and the presence of demulsifiers. Demulsifier performance is also influenced by many of these variables. 

Strassner (127) examined a third eomponent, waxes . He showed that resin and waxes do not oil-wet siliea. Waxes had no significant effect. He eoncluded that the waxes only contribute to increased viscosity of the oil phase. At low salt concentrations asphaltenes plus resins will oil-wet silica at acidic pH, but will water-wet siliea at basie pH. He examined Venezuelan crude oil and distilled water, and found the most breakout of water occurred at pH 10. In this case there was a transition to a mobile weak film, and interfacial tension was still high. However, when the water was changed to a bicarbonate solution, this transition occurred at pH 6, where maximum water breakout was observed at the high interfacial tension. The role of interfacial tension is discussed later in this text. 

Wasan et al. (163) used a deep-channel viseometer in studying the interfaeial shear viseosity of Salem crudes/water with and without the addition of petroleum sulfonate and salts, as well as Illinois erude/brine with pen-tadecyl benzenesulfonate. They showed a decreased coalescence time with decreased shear viseosity. A viscous traction shear viscometer was used for fraetionated crude oil/brine by Pasquarelli and Wasan (164), who showed that increased interfacial shear viseosity is eorrelated with decreased coalescence. Later, Wasan eorrelated interfacial shear viscosity with film-drainage time to determine effective demulsifiers (178). 

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