Microemulsions water-continuous systems

The effect of hydrated radii, valence, and concentration of counterions on oil-external and middle-phase microemulsions was investigated by Chou and Shah [28]. It was observed that I mol of CaCb was equivalent to 16-19 mol of NaCl for solubilization in middle-phase microemulsions, whereas for solubilization in oil-external microemulsions, 1 mol of CaCb was equivalent to only 4 mol of NaCl. For monovalent electrolytes, the values for optimal salinity of solubilization in oil-external and middle-phase microemulsions are in the order LiCl>NaCl>KCl>NH4Cl, which correlates with the Stokes radii of hydrated counterions. The optimal salinity for middle-phase microemulsions and critical electrolyte concentration varied in a similar fashion with Stokes radii of counterions, which was distinctly different for the solubilization in oil-external miroemulsions. Based on these findings, it was concluded that the middle-phase microemulsion behaved like a water-continuous system with respect to the effect of counterions [28]. 

The cosmetics and transdermal drug delivery fields are also expected to further benefit from the formulation of microemulsions from mild sugarbased surfactants. Lehmann et al. have studied the effect of such a microemulsion on dermal and corneal irritation, and hydrocortisone incorporation [105]. A microemulsion containing commercially available sucrose esters, isopropyl myristate, and propylene glycol and water was prepared as a water continuous system, and 16.5% hydrocortisone was loaded into the anhydrous base mixture. The formulation spread well on the skin due to the low surface tension of the system at 26 mN/m. While the microemulsion provided greater drug penetration, it also resulted in irritation and barrier compromise. The authors make the point that the formulation may be better suited to drugs that do not induce an irritation themselves. 

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],... 

Addition of salting-out type electrolytes to oil-water-surfactant (s) systems has also a strong influence on their phase equilibria and interfacial properties. This addition produces a dehydration of the surfactant and its progressive transfer to the oil phase (2). At low salinity, a water-continuous microemulsion is observed in equilibrium with an organic phase. At high salinity an oil-continuous microemulsion is in equilibrium with an aqueous phase. At intermediate salinity, a middle phase microemulsion with a bicontinuous structure coexists with pure aqueous and organic phases. These equilibria were referred by Vinsor as Types I,II and III (33). 

Figure (3) shows the solubilization parameters as functions of water concentration for SDS/2- entanol ratios of 0.25 and 0.40 at 25 C. The solubilization parameters are defined as Vo/Vs and Vw/Vs, where Vo, Vs and Vw are the volumes of organic phase, surfactant and aqueous phase in the microemulsions. The parameters are related to the drop size and also interfacial torsions f7.23). The bicontinuous phase is located around the composition range corresponding to equal values of solubilization parameters. The solubilization parameters are dependent on the initial surfactant and/or cosurfactant concentration. Similar dependence has been observed in other systems as a function of salinity and pH (7.231. Conductivity measurements performed as a function of water content indicate an S-shaped curve as shown in Figure (4). This is typical of microemulsions showing transition from oil-continuous to bicontinuous to water-continuous microstructure with increasing water content. 

Equilibrium with Aqueous Phases. The formation and properties of reverse micelle and microemulsion phases in equilibrium with a second predominantly water continuous phase is of practical interest for extraction processes. Figure 7 compares apparent hydrodynamic diameters observed in the ethane/AOT/water system at 37 C for values of 1, 3 and 16. In single phase systems at W - 1 (a) and 3 (b) the apparent hydrodynamic diameter decreases with increased pressure due to decreased micelle-micelle interactions as the solvent power increases. In contrast for a system with an overall W - 16 (c), where a second aqueous phase exists, hydrodynamic diameter increases continuously with pressure. 

Although water allows for greater uptake of acrylamide by the microemulsion, water alone ([acrylamide] 0) will not produce a one-phase system with the Brij 52/30 blend in an ethane/propane continuous phase. As postulated earlier, acrylamide is a cosurfactant with the B52/B30 blend, as evidenced by the results in Figure 5. The existence of the maximiun in the allowable water as a function of [AM] has been observed in other micelle systems where acrylamide behaves as a co-surfactant ). When more than the maximum allowable water level is added at a particular acrylamide content, the system becomes turbid, followed by the appearance of what appears to be a solid second phase. That acrylamide behaves as a co-surfactant is possibly due to its... 

From the results of self-diffusion, Lindman et al. (71) have proposed the structure of microemulsions as either the systems have a bicontinuous (e.g. both oil and water continuous) structure or the aggregates present have interfaces which are easily deformable and flexible and open up on a very short time scale. This group has become more inclined to believe that the latter proposed structure of microemulsion is more realistic and close to the correct description. However, no doubt much more experimental and theoretical investigations are needed to understand the dynamic structure of these systems. 

A general pattern of microemulsion phase behavior exists for systems containing comparable amounts of water and a pure hydrocarbon or hydrocarbon mixture together with a few percent surfactant. For somewhat hydrophilic conditions, the surfactant films tend to bend in such a way as to form a water-continuous phase, and an oil in water microemulsion coexists with excess oil. Drops in the microemulsion are spherical with diameters of order 10 nm. Both drop size and solubilization expressed as (VJVX the ratio of oil to surfactant volume in the microemulsion, increase as the system becomes less hydrophilic. At the same time interfacial tension between the microemulsion and oil phases decreases. Just the opposite occurs for somewhat lipophilic conditions. That is, a water in oil microemulsion coexists with excess water with drop size and solubilization of water (VJV,) increasing and interfacial tension decreasing as the system becomes less lipophilic. When the hydrophilic and lipophilic properties of the surfactant films are nearly balanced, a bicontinuous microemulsion phase coexists with both excess oil and excess water. For a balanced film (VJV,) and (VJV ) in the microemulsion are nearly equal, as are 7, 0 and... 

Figure 2.1 Comparison of SANS curves obtained for the system D20/n-octane-di8/C1oE4 on the (a) water-continuous (o/w-droplet microemulsion) and (b) the oil-continuous (w/o-droplet microemulsion) side, respectively. The solid lines in both plots are from factor curves according to Eq. (2.11). Usually, the polydispersity is slightly higher for w/o-droplet microemulsions. (Figures redrawn with data from Ref. [67].)...

After these first experiments it took 11 years until this problem was studied again exploiting the unique possibilities of NSE with respect to contrast variation and energy resolution [29]. The studied microemulsion was an o/w-droplet microemulsion in the system H2O/ -octane/C10E5. It turned out that the NSE data can be analysed using a double exponential fit according to Eq. (2.8), when the translational diffusion coefficient is already measured in advance using PCS. The same approach was also successfully applied to study another water-continuous microemulsion in the system H2 0/n-dodecane/Cio E-[49]. Since the approach works as well for oil-continuous systems an extended example for the approach will be discussed in the next subsection. 

Figure 8.15 Diagram showing cleaning of petroleum jelly versus microemulsion structure containing 50 wt.% of water. Adding electrolyte drives microemulsions from water-continuous towards solvent-continuous (low conductivity) and improves cleaning performance to a value similar to that of the water-free control system. (From Ref. [92], reprinted with permission of Wiley VCH.)...

The diffusion studies described in the above sections pertain to water-continuous and bicontinuous microemulsions. Chen and Georges [34] were the first to study diffusion in oil-continuous microemulsions using steady-state microelectrode voltammetry. Ferrocene was used to probe diffusion in an SDS-dodecane-1-heptanol-water system. The diffusion coefficient of the hydrophobic probe indicated the microviscosity of the oil rather than the bulk viscosity of the microemulsion. Owlia et al. [36] reported diffusion coefficient measurements of water droplets in an Aerosol OT [AOT, bis(2-ethylhexyl)sulfosuccinate] microemulsion using a microelectrode. Water-soluble cobalt(II) corrin complex (vitamin Bi2r) was used in an oil-continuous microemulsion containing 0.2 M AOT, 4 M water buffered at pH 3, and isooctane. The apparent diffusion coefficient decreased with the probe concentration in accordance with Eq. (13) as shown in Fig. 6 [36]. The water droplet size was... 

Microemulsions are composed of two mutually immiscible liquid phases, one spontaneously dispersed in the other with the assistance of one or more surfactants and cosurfactants. While microemulsions of two nonaqueous liquids are theoretically possible (e.g., fluorocarbon-hydrocarbon systems), almost all of the reported work is concerned with at least one aqueous phase. The systems may be water continuous (o/w) or oil continuous (w/o), the result being determined by the variables such as the surfactant systems employed, temperature, electrolyte levels, the chemical nature of the oil phase, and the relative ratios of the components. 

The adsorption of surfactant molecules at oil-water interfaces has attracted much interest, in relation to the oil recovery techniques (1). The systems containing oil, water and emulsifier molecules form generally two phases an aqueous phase containing sometimes solubilized oil in the form of small droplets surrounded by emulsifier molecules and an oil phase which also can contain solubilized water. When the amount of emulsifier is large enough, the system can form only one phase, i.e. all the water (or oil) can be solubilized in the oil (or water). The system is again a dispersion of very small droplets of water (or oil), surrounded by emulsifier molecules, in a continuous medium containing the oil (or water). Such dispersions are currently called microemulsions. The droplet size is usually of the order of lOOA (2). 

FIG. 17 (a) Comparison between the thermal endotherms of water-dodecane (continuous line) and water-hexadecane (dotted line) microemulsion samples. C, = 0.195, Cj = 0.197. The two spectra differ only by the thermal event associated with the melting of the oil. (From Ref. 13.) (b) Similar comparison for three-component W/O microemulsions. Curve 1 Water-isooctane system. Curve 2 Water-decane system. The surfactant, Na-AOT, is the same in both samples. Compositions are given in Tables 2 and 3. 

Microemulsions are ternary systems containing oil, water, and surfactant. The terms oil and water in a microemulsion system normally refer to oil phase (oil and oil soluble components such as cyclosporine) and aqueous phase (water and water soluble components such as sodium chloride), respectively. The phase behavior of water-oil-surfactant mixtures was extensively studied by Winsor (1948). Based on his experimental observations, Winsor classified equilibrium mixtures of water-oil-surfactant into four systems (1) type I (Winsor I) system where water continuous or oil-in-water (0/W) type microemulsion coexists with the oil phase. In these systems, the aqueous phase is surfactant-rich (2) type II (Winsor II) system where oil continuous or water-in-oil (W/0) type microemulsion coexists with the aqueous phase. In these systems, the oil phase is surfactant-rich (3) type III (Winsor III) system where bicontinuous type microemulsion (also referred to as surfactant-rich middle-phase) coexists with excess oil at the top and excess water at the bottom and (4) type IV (Winsor IV) system where only a single-phase (microemulsion) exists. The surfactant concentration in type IV microemulsion is generally greater than 30 wt%. Type IV microemulsion could be water continuous, bicontinuous, or oil continuous depending on the chemical composition. The phase behavior of microemulsions is often described as a fish diagram shown in Figure lO.I (Komesvarakul et al. 2006). 

As shown in Figure 10.1, four types of microemulsion systems can be formulated. In type 1 to type III microemulsion systems, two or more phases are present in equilibrium with each other. Only in the case of type IV (Winsor IV) microemulsion, a single phase is present. However, type IV microemulsion could be either water continuous, bicontinuous, or oil continuous. Several techniques could be used to identify different types of microemulsions. For example, the Tyndall effect can be observed in the lower phase of type I, middle phase of type III, and upper phase of type II microemulsion by simply pointing a laser pointer toward the sample as shown in Figure 10.3. 

The purpose of a phase scan is to determine the temperature (in case of nonionic surfactants) or salinity (in case of ionic surfactants) that can produce a microemulsion of desired type (O/W, W/O, or bicontinuous) and properties (such as solubilization). Figure 10.5 shows a typical phase scan (Rosen 2004). Phase scan is normally run from type 1 (Winsor 1) system to type 11 (Winsor 11) system by increasing the temperature (in case of nonionic surfactant) or salinity (in case of ionic surfactant). As shown in Figiue 10.5, oil solubilization increases from sample 1 to 2 with the increase in temperature or salinity. With the increase in temperature/salinity, the surfactant becomes more lipophilic due to the increased dehydration. Consequently, the interaction between oil and water phase (Yai,) increases and the interfacial tension (yow) decreases. As surfactant becomes continuously more lipophilic with the increase in temperature/salinity, a phase (bicontinuous or middle phase) begins to separate from phase (water continuous O/W microemulsion phase). At the start of the separation (sample 3), the interfacial tension between oil and phase (Yob) is still high and the interfacial tension between B and water phase (ybw ) is zero (fi and water are miscible). 

---