Spontaneous emulsification system

In situ formation of oil-in-water emulsions adds the requirement that the emulsification proceed spontaneously or at least with very little energy input due to mixing. Most such systems are associated with the agent-in-oil procedure, and spontaneous emulsification to oil-in-water emulsions sometimes occurs when aqueous caustic is mixed with petroleum oils containing naphthenic acids. Some researchers propose that mass transfer of the naturally occurring surfactants across the interface is the mechanism that causes this phenomena... 

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. 

Formation of Hposomal vesicles under controlled conditions of emulsification of Hpids with phosphoHpids has achieved prominence in the development of dmgs and cosmetics (42). Such vesicles are formed not only by phosphoHpids but also by certain nonionic emulsifying agents. Formation is further enhanced by use of specialized agitation equipment known as microfluidizers. The almost spontaneous formation of Hposomal vesicles arises from the self-assembly concepts of surfactant molecules (43). Vesicles of this type are unusual sustained-release disperse systems that have been widely promoted in the dmg and cosmetic industries. 

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. 

K. J. Ruschak and C.A. Miller Spontaneous Emulsification in Ternary Systems with Mass Transfer. Ind. Eng. Chem. Fundam. 11, 534 (1972). 

These equations, referring to completely unstirred systems, are not usually valid in practice complications such as spontaneous interfacial turbulence and spontaneous emulsification often arise during transfer, while, if external stirring or agitation is applied to decrease Ri and R2, the hydrodynamics become complicated and each system must be considered separately. The testing of the above equations will be discussed below, after a consideration of overall coefficients and of interfacial turbulence. 

The concept of interfacial mesophases promoting spontaneous emulsification (21.22) can be applied to the Tagat TO - Miglyol 812 system, where stable liquid crystalline dispersion phases are adequate to promote the process of self-emulsification. The stability of the resulting emulsion systems can also be accounted for by liquid crystalline interface stabilisation (23.24). Phase separation of material as observed above 55f surfactant, in conjuction with the increased viscosities of such systems, will inhibit the dynamics of the self-emulsification process and hence the quality of self-emulsified systems declines when the surfactant concentration is increased above 55. ... 

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],... 

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. 

A novel nanoparticulate lipid-based carrier system was developed by Mumper et al. at the University of Kentucky. ° This carrier system is composed of a lipophilic-emulsifying wax such as cetyl alcohol/ polysorbate 60 and other surfactants such as Brij 72, Brij 78, and Tween 80. The nanoparticles were formed through a warm microemulsion technique where encapsulates have included paclitaxel and plasmid DNA. The emulsification process is spontaneous, and cooling of the emulsion causes solidification of the nanoparticle-containing drug. This novel carrier has shown high efficiency in drug delivery across the blood-brain barrier. 

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. 

As in the previous study (4), the experiments involve brine-alcohol-petroleum sulfonate mixtures brought into contact with oil. In these systems, which are the type used for enhanced oil recovery, the initial mixtures are, depending on the salinity, either stable dispersions of lamellar liquid crystal and brine or a single liquid crystalline phase. The latter is formed at higher salinities than the former, in accordance with the general pattern of phase behavior in such systems described elsewhere ( 5). Indeed, one aspect of our work which differs from studies made by others of spontaneous emulsification in enhanced oil recovery processes (6-7) is emphasis on the need to understand the role of liquid crystals in the overall nonequilibrium process. Related studies of spontaneous emulsification in other systems are reviewed elsewhere... 

Finally, we present an interpretation of our observations in terms of diffusion paths. Basically, the diffusion equations are solved for the case of two semi-infinite phases brought into contact under conditions where there is no convection and no interfacial resistance to mass transfer. Other simplifying assumptions such as uniform density and diffusion coefficients in each phase are usually made to simplify the mathematics. The analysis shows that the set of compositions in the system is independent of time although the location of a particular composition is time-dependent. The composition set can be plotted on the equilibrium phase diagram, thus showing the existence of intermediate phases and, as explained below, providing a method for predicting the occurrence of spontaneous emulsification. 

The diffusion path method has been used to interpret nonequilibrium phenomena in metallurgical and ceramic systems (10-11) and to explain diffusion-related spontaneous emulsification in simple ternary fluid systems having no surfactants (12). It has recently been applied to surfactant systems such as those studied here including the necessary extension to incorporate initial mixtures which are stable dispersions instead of single thermodynamic phases (13). The details of these calculations will be reported elsewhere. Here we simply present a series of phase diagrams to show that the observed number and type of intermediate phases formed and the occurrence of spontaneous emulsification in these systems can be predicted by the use of diffusion paths. 

At approximately optimum salinity, spontaneous emulsification of brine drops in the oil phase began in both systems. This phenomenon resulted from local supersaturation of the oil phase, as explained in the discussion section below. The amount of emulsification tended to increase with increasing salinity. As a result, the cloud of emulsion drops began to obscure the interface between the microemulsion and oil, making interface position measurements difficult. These observations of spontaneous emulsification confirm the results of the earlier contacting experiments performed in the horizontal configuration ( 4). 

The transition from the three-phase to two-phase region in the PDM system was marked by a sudden increase of spontaneous emulsification in the oil phase. Because formation of an intermediate microemulsion ceased at this point, the emulsion drops remained near the brine interface rather than rapidly moving away to form a single-phase region above the brine. An example of this behavior is shown in Figure 15 for the 2.1 gm/dl-salinity PDM system. 

Calculated diffusion paths also successfully predicted the occurrence of spontaneous emulsification in the systems. Near optimum salinity where this phenomenon first appeared, brine drops spontaneously emulsified in the oil but were isolated from the bulk brine phase by a microemulsion. At high salinities, a more common type of spontaneous emulsification was seen with brine emulsifying in the oil directly above a brine layer. 

Complete information on phase behavior including tie-lines and on diffusion coefficients is rarely available for oil-water-surfactant systems. Nevertheless, Raney and Miller used plausible phase diagrams for an anionic surfactant-NaCl brine-hydrocarbon system as a function of salinity to calculate diffusion paths that exhibited intermediate phase formation and spontaneous emulsification in agreement with experimental observations made using the vertical cell technique. For example. Figure 9.12 shows a diffusion path for a surfactant-alcohol-brine mixture of composition D in contact with oil for a case when initial salinity is high. An intermediate brine phase is predicted as well as spontaneous emulsification in the oil phase, both of which were, in fact, observed. 

Mixtures containing 1 wt% of the pure nonionic surfactant C,2E5 in water were contacted with pure n-hexadecane and n-tetradecane at various temperatures between 25 and 60°C using the vertical cell technique. Similar experiments were performed with C,2E4 and n-hexadecane between about 15 and 40°C. In both cases the temperature ranged from well below to well above the phase inversion temperature (PIT) of the system, i.e., the temperature where hydrophilic and lipophilic properties of the surfactant are balanced and a middle phase microemulsion forms (analogous to the optimal salinity for ionic surfactants mentioned above). The different intermediate phases that were seen at different temperatures and the occurrence of spontaneous emulsification in some but not all of the experiments could be understood in terms of known aspects of the phase behavior, e.g., published phase diagrams for the C12E 5-water-n-tetradecane system, and diffusion path theory. That is, plausible diffusion paths could be found that showed the observed intermediate phases and/or spontaneous emulsification for each temperature. 

Similar experiments were carried out in which drops that were mixtures of /i-decane and various alcohols were injected into dilute solutions of a zwitterionic (amine oxide) surfactant. Here, too, the lamellar phase was the first intermediate phase observed when the system was initially above the PIT. However, with alcohols of intermediate chain length such as /i-heptanol, it formed more rapidly than with oleyl alcohol, and the many, small myelinic figures that developed broke up quickly into tiny droplets in a process resembling an explosion.The high speed of the inversion to hydrophilic conditions was caused by diffusion of n-heptanol into the aqueous phase, which is faster than diffusion of surfactant into the drop. The alcohol also made the lamellar phase more fluid and thereby promoted the rapid breakup of myelinic figures into droplets. Further loss of alcohol caused both the lamellar phase and the remaining microemulsion to become supersaturated in oil, which produced spontaneous emulsification of oil. 

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... 

The study of the age of the interface, as illustrated in Figure 4, clearly demonstrates that Distillate 3 undergoes spontaneous emulsification. England and Berg (18) attributed this to the transfer of surfactant across the interface with a large desorption barrier when tension dropped continuously. In the present case, the rapid increase in tension accompanied by the single drop instability may also suggest a rapid coalescence process for emulsification. As pointed out by Schechter and Wade (19), the systems which emulsify and rapidly coalesce... 

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. 

---