Microemulsion microemulsions phase-forming

Dynamic light scattering methods were used to study reverse micelle and microemulsion phases formed in liquid and supercritical alkane continuous phases. 

In this paper we use dynamic light scattering (DLS) methods to examine micelle size and clustering in (1) supercritical xenon, (2) near-critical and supercritical ethane, (3) near-critical propane as well as (4) the larger liquid alkanes. Reverse micelle or microemulsion phases formed in a continuous phase of nonatomic molecules (xenon) are particularly significant from a fundamental viewpoint since both theoretical and certain spectroscopic studies of such systems should be more readily tractable. Diffusion coefficients obtained by DLS for AOT microemulsions for alkanes from ethane up to decane are presented and discussed. It is shown that micelle phases exist in equilibrium with an aqueous-rich liquid phase, and that the apparent hydrodynamic size, in such systems is highly pressure dependent. 

The dynamic behavior of the PDM system in the salinity range between optimum salinity and the upper-phase microemulsion region was similar to that below optimum in that intermediate brine and microemulsion phases formed. Figure 13 shows these phases for the 1.5 gm/dl-salinity PDM system. The middle-phase type of microemulsion, being high in oil content at these salinities, grew more rapidly in the direction of the oil phase than it did at low... 

Although the contacting experiments were performed with surfactant systems typical of those used in enhanced oil recovery, application of the results to detergency processes may be possible. For example, the growth of oil-rich intermediate phases is sometimes a means for removing oily soils from fabrics. Diffusion path theory predicts that oil is consumed fastest in the oil-soluble end of the three-phase regime where an oil-rich intermediate microemulsion phase forms. 

Scientifically described for the first time in 1943 by Hoar and Schulman (2), the latter author coined the term microemulsion in 1959 to describe these optically isotropic transparent oil and water dispersions (3). Since this early work, many experimental and theoretical efforts have shown that these dispersions are actually solutions, namely thermodynamically stable equilibrium phases (4). Consequently, the most widely, but still not universally accepted definition of a microemulsion is that of a thermodynamically stable mixture of oil and water. Occasionally, the term microemulsion (5) or miniemulsion (6) is used to describe long-lived emulsions with ultra-small droplet sizes (30-100 nm). Sometimes, stable emulsions may be created by agitation of systems while passing through regions of the phase diagram where microemulsion phases form however, the final state is in the emulsion region (7, 8). In this present chapter, we use the most widely accepted definition of microemulsions, namely equilibrium phases of oil and water (9). 

The surfactant concentration at the head of the fish, Yo, is a measure of the critical microemulsion concentration (cpc). At this surfactant concentration, the middle microemulsion phase forms first. Below the head of the fish (below yo), Or systems are observed where the surfactant is dissolved as monomers in the oil and water phases, and no mixing of oil and water is found. At yo, the three-phase region first appears indicating the formation of a middle microemulsion phase with an internal interface of surfactant separating oil and water microdomains. At yo, the concentration of surfactant in the excess water phase is identical to the critical micelle concentration (/water = cmc) (28). Thus, the amount of surfactant in the excess oil phase (/oil) is the overall surfactant concentration minus that dissolved... 

Here scalar order parameter, has the interpretation of a normalized difference between the oil and water concentrations go is the strength of surfactant and /o is the parameter describing the stability of the microemulsion and is proportional to the chemical potential of the surfactant. The constant go is solely responsible for the creation of internal surfaces in the model. The microemulsion or the lamellar phase forms only when go is negative. The function/(<))) is the bulk free energy and describes the coexistence of the pure water phase (4> = —1), pure oil phase (4> = 1), and microemulsion (< ) = 0), provided that/o = 0 (in the mean-held approximation). One can easily calculate the correlation function (4>(r)(0)) — (4>(r) (4>(0)) in various bulk homogeneous phases. In the microemulsion this function oscillates, indicating local correlations between water-rich and oil-rich domains. In the pure water or oil phases it should decay monotonically to zero. This does occur, provided that g2 > 4 /TT/o — go- Because of the < ), —<(> (oil-water) symmetry of the model, the interface between the oil-rich and water-rich domains is given by... 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

The addition of a dispersed droplet phase (forming a microemulsion) provides a convenient means of solubilizing highly polar or ionic species into the low polarity environment of the SCF phase. Hence, the combination of supercritical solvents with microemulsion stractures provides a new type of solvent with some unusual and important properties of potential interest to a range of technologies. These droplets have high diffusion rates in SCF and the properties of the continuous phase can be readily controlled by manipulation of system pressure (Beckman et al., 1995). 

The review by Attwood and Florence (1998) is valuable because it provides a comprehensive survey of the earlier pharmaceutical literature. From this we can learn that microemulsions are formed spontaneously from fixed compositions of surfactant(s), oil, and water but these compositions change with temperature. As a stable system is diluted progressively with, for example, water, the microemulsion phase will spontaneously revert to other phases on the diagram, including unstable emulsions or a solution. This becomes relevant when considering what happens to the system when it is administered. No matter how it is administered the microemulsion phase will be diluted, most likely with water in some form or another, and revert to some other composition in the body. 

When surfactant molecules are dissolved in organic solvents, the head groups cluster together and the hydrocarbon chains point into the bulk phase, forming reverse micelles. If water is trapped in the area of the head groups, the system is termed a microemulsion. Sometimes, a short chain alcohol (e.g. hexanol) is added to aid dispersion. 

The nano-sized particles were obtained at various ratios of the phases forming the microemulsion. They were about the same size, which confirmed the advantage of the method selected. 

With 7ow close to zero, microemulsions will form spontaneously and are thermodynamically stable. The droplets of microemulsions tend to be monodispersed. A microemulsion may form as a separate phase in equilibrium with excess oil (O/W) or water (W/O) (i.e. it is saturated with respect to droplets). Microemulsions are usually of low viscosity. 

There are other variations of this approach that involve the phase inversion temperature (PIT) (see Section 3.6.1). In one method an emulsion is formed at a temperature a few degrees lower than the PIT, where the interfacial tension is quite low and small droplets can be formed. The emulsion can then be quickly cooled. Another method uses a controlled temperature change to cause an emulsion to suddenly change from a coarse oil-in-water (O/W) emulsion, through a microemulsion phase, and into a fine water-in-oil (W/O) emulsion [432]. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

The partitioning of alcohol into the oil, water, and interfacial layer domains of a microemulsion controls whether a two-phase or a three-phase microemulsion system is formed, as well as the microscopic characteristics of the microemulsion phases. F or the typical alcohols used, the amount of alcohol present in the oil domain can be large and comparable to the amount present in the interfacial layer. This is in contrast to the behavior of the surfactant, most of which remains at the interfacial layer and only a negligibly small amount of which are partitioned into the oil and the water domains. Therefore, the accurate accounting of the partitioning of alcohol into the oil domain is a necessary part of any quantitative theory of microemulsions. Such a theory must account for the facts that the alcohol is present in the oil phase as both monomers and aggregates and that the self-association of alcohol in the oil is responsible for its appreciable presence in the oil domain. 

Several microemulsion inkjet inks have been described in the literature. An inkjet phase transition ink in the form of a microemulsion consists of an organic vehicle phase having a colorant dispersed therein, where the vehicle phase is preferably liquid while jetting at temperatures above 70°C and solid upon keeping the substrate at room temperature (22-25°C). This formulation undergoes a phase transition from a microemulsion phase to a lamellar phase upon heating, which allows build up of several layers of inks on the surface of the paper. In a similar concept for phase transition, an ink comprised of an aqueous phase, an oil phase. 

This is a result in the dehydration of the ethylene oxide part of the copolymer resulting in smaller volumes of water to be solubilised in the microemulsion droplets. The effect of temperature on the stability of the other regions of the phase diagram is complex to summarise though, it seems that increasing the temperature reduces the variety of phases formed. 

In the nonionic system observed under EVM, the initial microemulsion showed no tendency of gelation until it reached 60 C. After reaching 60<>C, the system gels and starts to polymerize after 10-12 hours. As polymerization proceeds, the water separates out. After about 20-24 hours, the gel starts to become a solid with an excess emulsion phase formed at the bottom. The polymerization is essentially complete after 36 hours. Due to different modes of polymerization in the anionic and nonionic surfactant systems, the mechanical properties of the solid are different. The polymers obtsuned from anionic microemulsions are brittle, while those obtmned from nonionic microemulsions are ductile. 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

The identity of the intermediate phase formed at these conditions can be deduced from the relative movement of the interfaces. Because the phase grew quickly in the direction of the aqueous surfactant solution, it contained predominantly brine. Although small in quantity, some oil did diffuse into it. From this information and from its isotropic appearance, one can conclude that the intermediate phase was an oil-in-water microemulsion. Additional support for this conclusion is that this type of microemulsion is an equilibrium phase at low salinities. 

At approximately S = 0.2 (this value is somewhat dependent on other phase behavior parameters), passage of the calculated diffusion path through the brine phase becomes possible. This change corresponds closely to the point at which brine began forming by diffusion in the contacting experiments and, as a result, indicates that formation of two intermediate phases is preferred over the formation of a single microemulsion phase. A... 

Phase Behavior on Equilibration with Oil. Microemulsions are formed when the aqueous surfactant-cosurfactant solutions are mixed with oil, and allowed to equilibrate. Figure 5(a) shows the phase behavior when 5 ml aqueous surfactant solutions (without any polymer) were equilibrated with equal volumes of n-dodecane. The salinity was varied from 0.8 to 2.2 gm/dl NaCl in 0.2 gm/dl increments. At low salinities a lower phase microemulsion exists in equilibrium with excess oil. The middle phase microemulsion appears at about... 

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