Solubilisation and microemulsion

Optically transparent micellar solutions are often able to dissolve considerable quantities of a substance molecular insoluble in the pure solvent. The transparency of such a solution is not affected by this. The solubilisation is controlled by the shape and size of the micelles which determine the amotmt of components incorporated into the interior of the micelles (Schulman etal. 1959). 

In Fig. 1.17a an example of the solubilisation of oil in water and of water in oil as a function of temperature is illustrated. The system consists of 5 wt% polyoxyethylene-8,6-nonylphenyl ether, 47.5 wt% water and 47.5 wt% cyclohexene (Shinoda Friberg 1975). 

With rising temperature the volume of the aqueous phase grows, the micelles swell until suddenly, at the so-called phase inversion temperature, the oil phase has the larger volume. This effect is explained by the polyoxyethylene chains dehydrating as the temperature rises. The hydrophilic/hydrophobic balance of the molecule is thereby altered and the solubility in the oil phase grows. When concentration is great enough, micelles are formed in the oil phase and water is solubilised. If the two phases do not exist as stratified layers but as emulsion a 

A similar phenomenon was observed by Muller Kretzschmar (1982) with ethylene oxide adducts at significantly increased concentration of the background electrolytes connected with clouding of the emulsion. 

The solubilisation of oil or water in a micellar solution of non-ionic surfactant, a) two-phase diagram (O - oil, W - water, 0, - oil in micellar solution, - water in inverse micellar solution, D - phase separation temperature region), b) interfacial tension as a function of T, according to Shinoda Friberg 1975 

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Mukerjee P, Cardinal JR, Desai NR. In Mittal KL, editor. Micellisation, solubilisation and microemulsions. New York Plenum 1977. p. 241. 

K. L. Mittal, ed., Micellicyation, Solubilisation, and Microemulsions, Vols. 1 and 2, Plenum Press, New York, 1977. 

The choice of suitable surfactants and additional chemicals for the decontamination of source zones largely depends on the type of pollutant and the structure of the soil (mainly on adsorption behaviour and hydraulic conductivity). Adsorbed and solid pollutants or very viscous liquid phases cannot be mobilised. Preformed microemulsions, co-solvents or co-surfactants can be favourably used for such contaminations in order to enhance the solubilisation capacity of surfactants. NAPL with low viscosity can easily be mobilised and also effectively solubilised by microemulsion-forming surfactant systems. Mobilisation is usually much more efficient. It is achieved by reducing the interfacial tension between NAPL and water. Droplets of organic liquids, which are trapped in the pore bodies, can more easily be transported through the pore necks at lower interfacial tension (see Fig. 10.2). The onset of mobilisation is determined by the trapping number, which is dependent on... 

At that time, some large-scale field tests on surfactant and microemulsion technologies had alreadybeen performed [50-55]. In most cases, the applied surfactants or microemul-sion components were selected in laboratory experiments by determining phase behaviour, interfacial tension, solubilisation capacity, viscosity and extracting power in soil columns. 

Efficient degreasing was found to be closely connected to the three-phase state and hence to the ultra-low interfacial tension between water and oil [170]. The so far unidentified mechanism of degreasing of animal skins could be understood and explained. Correlation of results obtained from phase behaviour measurements and degreasing experiments revealed that Eusapon OD shows the best degreasing performance and lead to the clarification of the four-step process of degreasing as shown in Fig. 10.13 [171]. The first step is the penetration of the surfactant into the skin. In a second step the natural fat is solubilised. A microemulsion phase coexists with a fat- and a water-excess phase and the interfacial tension between water and oil is ultra-low. On the surface of the skin dilution of the microemulsion with pure water, i.e. reduction of the salt concentration in the float, leads to the formation of a stable emulsion via shearing. The stable emulsion prevents the deposition of the fat on the skin and enables the transport of the natural fat away from the skin. 

The use of microemulsions in the dying process [186] is relatively new and little practiced. The major part of patents and other references are dedicated to the solubilisation of various dyes in surfactant micelles, and microemulsion compositions are given, which consist of a classical triad surfactant, co-surfactant, solubilisable substance (dye). In some cases, o/w emulsions were added, in which xylene or hexadecane were used as oil phase. Only few examples are... 

The packing ratio also explains the nature of microemulsion formed by using nonionic surfactants. If v/a 1 increases with increase of temperature (as a result of reduction of a ), one would expect the solubilisation of hydrocarbons in nonionic surfactact to increase with temperature as observed, until v/a l reaches the value of 1 where phase inversion would be expected. At higher temperatures, va l > 1 and water in oil microemulsions would be expected and the solubilisation of water would decrease as the temperature rises again as expected. 

Thus it can be concluded that the structure of microemulsions depends on the structure of surfactant and cosurfactant. Moreover, this structure also determines the amount of solubilisation of oil and or water in microemulsions. 

The most often investigated enzymes in micro emulsions are lipases, because these enzymes are very stable and active in this medium [ 14]. Until now, most of the relevant interactions between the biomolecules and the reaction medium have been investigated. Many enzymes which are well investigated in aqueous reaction media can be solubilised in w/o-microemulsion, retaining their activity and stability, as shown in Table 1. 

An auspicious new strategy, in order to perform biocatalysis with hydrophobic substrates in w/o-microemulsion, is the usage of whole cells instead of purified enzymes [3,124,141]. There exist only a few surfactant-oil systems, in which whole cells are stable and suitable for a segmentation. Mainly the biodegradable surfactant based on sorbitan (Tween and Span) seems to be well suited for the solubilisation of whole cells in organic reaction media [142,143]. 

Microemulsions, with droplet diameters of 0.01-0.1 /un, can also be prepared and these are currently the subject of much fundamental investigation and new applications (e.g. oil recovery from porous rocks). Whether microemulsions should be regarded as true emulsions or as swollen micelles (see section on solubilisation, page 89) is a matter of controversy. 

C. Malcolmson, C. Satra, S. Kantaria, A. Sidhu, and M. J. Lawrence, Effect of oil on the level of solubilisation of testosterone propionate into non-ionic oil-in-water microemulsions, J. Pharm. Sci. 87 109-116 (1988). 

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]. 

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

Shinoda, K., and Kuineda, H. (1973), Condition to produce so-called microemulsions Factors to increase mutual solubility of oil and water solubilisers, J. Colloid Interface Sci, 42,381-387. 

Polymerized Microemulsion Systems. A microemulsion of styrene and divinylbenzene with CTAB + hexanol may readily be made, and subsequently polymerized to form a polymerized microemulsion (5,6,7). This system exhibits two sites of solubilisation for photosystems such as pyrene, one in the surfactant skin layer, and the other in the polymerized styrene-divinylbenzene core. Photochemical reactions induced in the surfactant skin are very similar to those observed in micelles and are not immediately of concern to us at this stage. However, photochemical reactions induced in the rigid polymerized core are of interest, as they essentially confine reactants to a small region of space where movement is restricted as compared to a fluid non-polymerised microemulsion or a micelle. Thus, diffusion is minimised, and it may be possible to investigate reactions which occur over a distance rather than reactions which occur by diffusion. In order to eliminate reactions in the surfactant skin a microemulsion can be constructed which contains cetyl pyridinium chloride in place of CTAB. The pyrene that resides in the surfactant skin layer is immediately quenched by the pyridinium group following excitation. 

A convenient way to describe microemulsions is to compare them with micelles. The latter, which are thermodynamically stable, may consist of spherical units with a radius that is usually less than 5 nm. Two types of micelles may be considered (i) normal micelles in which the hydrocarbon tails form the core and the polar head groups are in contact with the aqueous medium and (ii) reverse micelles (formed in nonpolar media) in which the water core contains the polar head groups and the hydrocarbon tails are now in contact with the oil. Normal micelles can solubiHse oil in the hydrocarbon core to form O/W microemulsions, whereas reverse micelles can solubilise water to form a W/O microemulsion. A schematic representation of these systems is shown in Figure 15.1. 

The research on microemulsions currently concentrates on even more complex mixtures. By adding amphiphilic macromolecules the properties of microemulsions can be influenced quite significantly (see Chapter 4). If only small amounts of amphiphilic block copolymers are added to a bicontinuous microemulsion a dramatic enhancement of the solubilisation efficiency is found [27,28]. On the other hand, the addition of hydrophobically modified (HM) polymers to droplet microemulsions leads to a bridging of swollen micelles and an increase of the low shear viscosity by several orders of magnitude [29]. 

The primary aim of microemulsion research is to find the conditions under which the surfactant solubilises the maximum amounts of water and oil, i.e. the phase behaviour has to be studied. As the effect of pressure on the phase behaviour is (in general) rather weak [30 ], it is sufficient to consider the effect of the temperature. Furthermore, it hasbeen shown that simple ternary systems consisting of water, oil and non-ionic n-alkyl polyglycol ethers (QEj) exhibit all properties of complex and technically relevant systems [6]. Therefore, we will first describe the phase behaviour of ternary non-ionic microemulsions. 

Considering now the variation of the phase behaviour with increasing mass fraction y of surfactant one can see that the volume of the respective microemulsion phase increases (see test tubes in Fig. 1.3(b)) until the excess phases vanish and a one-phase microemulsion is found. The optimal state of the system is the so-called X-point where the three-phase body meets the one-phase region. It defines both the minimum mass fraction y of surfactant needed to solubilise water and oil, i.e. the efficiency of the surfactant, as well as the corresponding temperature f, which is a measure of the PIT. 

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