Formation of Microemulsions

Considering a colloidal system at constant temperature, volume, and composition, the change of Helmholtz free energy (dF) for any process undergoing the expansion of the oil-water interfacial area dA 0) can be expressed as 

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Different methods are used in microemulsion formation a low-energy emulsification method by dilution of an oil surfactant mixture with water and dilution of a water-surfactant mixture with oil and mixing all the components together in the final composition. These methods involve the spontaneous formation of microemulsions and the order of ingredient addition may determine the formation of the microemulsion. Such applications have been performed with lutein and lutein esters. ... 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

The effectiveness of the method is most probably based on the fact that alkyl hypochlorite is formed at the oil/water interface where the cosurfactant alcohol resides. The oxidation that follows takes place either inside or on the surface of oil droplet. The rate of the reaction can result from a large hydrocarbon/water contact area permitting interaction between oil-soluble sulfide with interfacial cosurfactant that served as an intermediary. An extension ofthis procedure to mustard deactivation has also been proposed [20b]. Such systems could be also applied to the degradation of several environmentally contaminating materials The formation of microemulsions, micelles and vesicles is promoted by unfavourable interactions at the end sections of simple bilayer membranes. There is no simple theory of solute-solvent interactions. However, the formation of... 

It was observed that the titration of a coarse emulsion by a coemulsifier (a macromonomer) leads in some cases to the formation of a transparent microemulsion. Transition from opaque emulsion to transparent solution is spontaneous and well defined. Zero or very low interfacial tension obtained during the redistribution of coemeulsifier plays a major role in the spontaneous formation of microemulsions. Microemulsion formation involves first a large increase in the interface (e.g., a droplet of radius 120 nm will disperse ca. 1800 microdroplets of radius 10 nm - a 12-fold increase in the interfacial area), and second the formation of a mixed emulsifier /coemulsifier film at the oil/water interface, which is responsible for a very low interfacial tension. 

The formation of microemulsions usually involves a combination of oil, water, surfactant, and cosurfactant. The tendency toward a w/o (oil-in-water) or an o/w (water-in-oil) microemulsion is... 

Several theories have been proposed to account for the thermodynamic stability of microemulsions. The most recent theories showed that the driving force for microemulsion formation is the ultralow interfacial tension (in the region of 10 4-10 2 mN m 1). This means that the energy required for formation of the interface (the large number of small droplets) A Ay is compensated by the entropy of dispersion —TAS, which means that the free energy of formation of microemulsions AG is zero or negative. 

In some cases, substrates and enzymes are not soluble in the same solvent. To achieve efficient substrate conversion, a large interface between the immiscible fluids has to be established, by the formation of microemulsions or multiple-phase flow that can be conveniently obtained in microfluidic devices. Until now only a couple of examples are published in which a two-phase flow is used for biocatalysis. Goto and coworkers [431] were first to study an enzymatic reaction in a two-phase flow in a microfluidic device, in which the oxidation ofp-chlorophenol by the enzyme laccase (lignin peroxidase) was analyzed (Scheme 4.106). The surface-active enzyme was solubilized in a succinic acid aqueous buffer and the substrate (p-chlorophenol) was dissolved in isooctane. The transformation ofp-chlorophenol occurred mainly at... 

Figure.1. Effect of the nature of the oil and mononer on the percentage of surfactant(s) necessary for the formation of microemulsions as a function of HLB (oil to aqueous phase...

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. 

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. 

Gerbacia and Rosano (46) have determined the interfacial tension at oil-water interface after alcohol injection into one of the phases. They observed that the interfacial tension could be temporarily lowered to zero due to the diffusion of alcohol through the interface. They concluded that the diffusion of surfactant molecules across the interface is an important requirement for reducing interfacial tension temporarily to zero as well as for the formation of microemulsions. They further claimed that the formation of microemulsions depend on the order in which components are added. 

It has also been shown from thermodynamic consideration (Equation 3), that if the interfacial tension is very low, the thermodynamically stable emulsions can be formed. Previous investigators (20,45,47,48) have calculated that for a situation likely to occur in microemulsion formation, the interfacial tensions would need to be in the order of 10 to 10 5 dynes/cm for thermodynamic stabilization and for spontaneous formation of microemulsions. 

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. 

In addition, Shinoda and Friberg (53) have summarized their extensive studies on the formation of microemulsions using nonionic surfactants. They proposed the following conditions to form microemulsions with minimum amount of surfactants ... 

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. 

A thermodynamic definition of microemulsions can be obtained from a consideration of the energy and entropy terms for formation of microemulsions. The process of formation of microemuision from a bulk oil phase (for a O/W microemuision) or from a bulk water phase (for a W/O microemuision) is shown schematically in Figure 15.2. 

According to the second law of thermodynamics, the free energy of formation of microemulsions AG is given by the following expression,... 

The formulation of microemulsions or micellar solutions, like that of conventional macroemulsions, is still an art. In spite of exact theories that have explained the formation of microemulsions and their thermodynamic stabihty, the science of microemulsion formulation has not advanced to a point where an accurate prediction can be made as to what might happen when the various components are mixed. The very much higher ratio of emulsifier to disperse phase which differentiates microemulsions from macroemulsions appears at a first sight that the appHcation of various techniques for formulation to be less critical. However, in the final stages of the formulation it can be realised immediately that the requirements are critical due to the greater number of parameters involved. 

Liquid soil is usually removed by roll-up, emulsification, direct solubilization, and possibly formation of microemulsion or liquid crystalline phases. The oil emulsification capability of the surfactant solution and the oil-water interfacial tension are relevant physicochemical parameters. 

Formulation is important because the properties of surfactant-oil-water systems in general and the formation of microemulsions in particular, are very sensitive to it and slight deviations from a proper formulation may result in drastic changes of the properties. Consequently, formulation has to be controlled accurately, which is quite challenging because of the high number of degrees of freedom in any practical case. This is why formulation is sometimes considered as magic business . 

Figure 8.6 Pseudoternary phase diagram of a system containing 20 wt.% emulsifier (Cs/io-APG, Q2/14-APG and GMO), 20 wt.% perfume oil, 0.6 wt.% oil (dicapryl ether, octyldodecanol) and 59.4 wt.% water at 25°C. The formation of microemulsions was studied as a function of the emulsifier s composition. The dotted lines separate the o/w- from the w/o-region. ME indicates a one-phase microemulsion. (From Ref. [39], reprinted with permission of Elsevier.)...

The in situ formation of microemulsions can occur in washing processes depending on the oil type and conditions. During the oil removal from hard surfaces or fabrics ternary systems occur where two or three phases coexist in equilibrium. These systems are also referred to as Windsor I or Windsor III microemulsions. The effects were studied in detail for alkyl polyglycol ethers [77]. Depending on temperature different phases exist, having a three-phase region between the temperature T and Tu (see Fig. 1.3, Chapter 1). When... 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilisation of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the PITs of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One was the lamellar liquid crystalline phase La (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilisation of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C using n-decane as the hydrocarbon. At this temperature, which is near the PIT of the water/decane/C Es system, the existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evident. These results can be utilised to optimise surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and co-surfactants. 

In contrast to the formation of microemulsions from aqueous surfactant systems and oily soils during the cleaning process, less basic research has been carried out on micro emulsions as a direct cleaning medium [ 80 ]. Some examples will be presented in the following sections. 

Nakamura, N., Yamaguchi, Y., Hakansson, B., Olsson, U., Tagawa, T. and Kunieda, H. (1999) Formation of microemulsion and liquid crystal in biocompatible sucrose alkanoate systems. J. Disp. Sci. Technol., 20, 535. 

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