Interfacial tension spontaneous emulsification

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

In buffered surfactant-enhanced alkaline flooding, it was found that the minimum in interfacial tension and the region of spontaneous emulsification correspond to a particular pH range, so by buffering the aqueous pH against changes in alkali concentration, a low interfacial tension can be maintained when the amount of alkali decreases because of acids, rock consumption, and dispersion [1826]. 

A similar technique, the so-called spontaneous emulsification solvent diffusion method, is derived from the solvent injection method to prepare liposomes [161]. Kawashima et al. [162] used a mixed-solvent system of methylene chloride and acetone to prepare PLGA nanoparticles. The addition of the water-miscible solvent acetone results in nanoparticles in the submicrometer range this is not possible with only the water-immiscible organic solvent. The addition of acetone decreases the interfacial tension between the organic and the aqueous phase and, in addition, results in the perturbation of the droplet interface because of the rapid diffusion of acetone into the aqueous phase. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

When the minimum of y with respect to T is negative (Fig. 3), the interfacial tension is negative in the range of temperatures in which the curve y = y(T) intersects the abscissa. A negative y makes the water oil interface unstable to thermal and mechanical perturbations and a spontaneous emulsification with the formation of globules of oil in water and water in oil takes place. At low temperatures, because of coalescence, only a fraction of the globules of oil survive... 

A second, new class of processes is that of membrane and micro-channel emulsification. A to-be-dispersed phase is here pushed through pores of a membrane or through micro-engineered micron-scale channels. At the pore or channel mouth, droplets are formed. These droplets can spontaneously detach from the pore or channel mouth (interfacial tension driven snap-off), due to the distortion of the droplet shape when it is still attached to the mouth. At higher fluxes or with channel mouths not giving a strong shape distortion, droplets are sheared off by a cross-flowing continuous phase. 

Emulsion systems can be considered a subcategory of lyophobic colloids. Like solid-liquid dispersions, their preparation requires an energy input, such as ultrasonication, homogenization, or high-speed stirring. The droplets formed are spherical, provided that the interfacial tension is positive and sufficiently large. Spontaneous emulsification may occur if a surfactant or surfactant system is present at a sufficient concentration to lower the interfacial tension almost to zero. 

The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability in terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10- dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. 

The concept of transient interfacial tension has been further extended by Davis and Haydon (43). They described an experiment by Ilkovic (44) in which a negative potential was applied to a mercury drop in an aqueous solution of a quaternary ammonium compound. At -8 v/cm applied potential, the spontaneous emulsification of mercury occurred. The spontaneous emulsification was observed for surfactant concentrations which exhibited negative values for interfacial tensions upon extrapolation. These results indicate that for spontaneous emulsification, the dynamic interfacial tension may approach transient negative values. Moreover, this does not mean that at equilibrium, the dispersed droplets will have a negative interfacial tension. 

This method is particularly useful for the measurement of very low interfacial tensions (<10 mN m ) that are particularly important in applications such as spontaneous emulsification and the formation of microemulsions. Such low interfacial tensions may also be achieved with emulsions, particularly when mixed surfactant films are used. In this case, a drop of the less-dense liquid A is suspended in a tube containing the second liquid, B. On rotating the whole mass (see Figure 5.4) the drop of the liquid moves to the centre and, with an increasing speed of revolution, the drop elongates as the centrifugal force opposes the interfacial tension force that tends to maintain the spherical shape, which is that having a minimum surface area. 

There are three similarities between macroemulsions and foams (1) They both consist of a dispersion of an immiscible state of matter in a liquid phase. Foams are dispersions of a gas in a liquid emulsions are dispersions of a liquid in a second immiscible liquid. (2) The tension y7 at the relevant interface is always greater than zero, and since there is a marked increase in interfacial area AA during the process (of emulsification or foaming), the minimum work involved is the product of the interfacial tension and the increase in interfacial area (Vkmm = AA x y7. (3) The system will spontaneously revert to two bulk phases unless there is an interfacial... 

In this paper we report first the spontaneous emulsification mechanisms in the petroleum sulfonate and caustic systems. This is followed by the kinetics of coalescence in alkaline systems for both the Thums Long Beach (heavy) crude oil and the Huntington Beach (less viscous) crude oil. Measurements of interfacial viscosity, interfacial tension, interfacial charge and micellar aggregate distributions are presented. Interrelationships between these properties and coalescence rates have been established. 

Importantly, in practically all common techniques of formation of drops and bubbles, the liquids are deformed geometrically with the use of a force of choice, and then they spontaneously break into smaller bits by the action of the interfacial tension. As we will discuss it below, emulsification in microfluidic devices constitutes a very different route to emulsification. 

Several micellar-polymer flooding models as applied to the EOR are discussed in [237]. It is noted that the co-solvent ordinarily used in this process considerably influences not only the microemulsion stabilisation, but also the removal of impurities in the pores of the medium. The idea of using an alkali in micellar-polymer flooding is discussed in [238] in detail. The alkali effect on the main oil components was studied aromatic hydrocarbons, saturated and unsaturated compounds, light and heavy resin compounds and asphaltenes. It is demonstrated that at pH 12 surfactants formed from resins allow to achieve an interfacial tension value close to zero. For asphaltenes, such results are achieved at pH 14. In the system alkali solution (concentration between 1300 to 9000 ppm)/crude oil at 1 1 volume ratio a zone of spontaneous emulsification appears, which is only possible at ultra-low interfacial tensions. 

Chemical flooding involves the injection of a surfactant solution that can cause oil-aqueous interfacial tension to drop from about 30 mN m to near-zero values on the order of 10 -10 mN m , allowing spontaneous or nearly spontaneous emulsification of the oU, with an increase in the capillary number by several orders of magnitude and with greatly increased displacement and recovery of the oil [6, 10,19, 67, 75, 79-87]. The micelles present also help to solubilize the released oil droplets hence, this process is sometimes referred to as micellarflooding. Having mobilized the oil, these processes are even more efficient if the oil droplets are... 

The importance of the properties of the interfacial region for spontaneous emulsification was first demonstrated by Gad (4), who observed that when a solution of lauric acid in oil was carefully placed on an aqueous alkaline solution, an emulsion spontaneously formed at the interface. The reason for this spontaneous emulsification is the formation of a mixed film of lauric acid and sodium laurate (produced by partial neutralization of the acid by alkali) which produces an ultra-low interfacial tension. 

Several mechanisms may be proposed to explain the process of spontaneous emulsification, all of which are related to the properties of the interfacial film. The first mechanism is due to interfacial turbulence that may occur as a result of mass transfer or by non-uniform adsorption of the surfactant molecules at the OAV interface. The interface shows unsteady motions - streams of one phase are ejected and penetrate into the second phase. This is illustrated in Figure 4.1(a) which shows the localized reduction in interfacial tension caused by non-uniform adsorption of surfactants or mass transfer of surfactants across the interface (5-7). When the two phases are not in chemical equilibrium, convection currents may be formed which transfers the liquid rich in surfactants towards the areas deficient in surfactants. These convection currents may give rise to... 

Figure 4.1. Schematic representation of spontaneous emulsification (a) interfacial turbulence (b) diffusion and stranding (c) ultra-low interfacial tension...

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