Microemulsions with ionic surfactants

In the preceding sections, the phase behaviour of rather simple ternary and quaternary non-ionic microemulsions have been discussed. However, the first microemulsion found by Schulman more than 50 years ago was made of water, benzene, hexanol and the ionic-surfactant potassium oleate [1, 3]. Winsor also used the ionic-surfactant sodium decylsulphate and the co-surfactant octanol to micro-emulsify water/sodium sulphate and petrol ether [2], In the last 30 years, in-depth studies on ionic microemulsions have been carried out [7, 8, 65, 66]. It toned out that nearly all ionic surfactants which contain one single hydrocarbon chain are too hydrophilic to build up microemulsions. Such systems can only be driven through the phase inversion if an electrolyte and a co-surfactant is added to the mixture (see below and Fig. 1.11). 

However, using double-chain ionic surfactants, e.g. sodium-bis-ethylhexylsulfo-succinate (AOT) [9, 67] and didodecyl dimethyl ammonium bromide (DDAB) [68], no co-surfactant is necessary to time the mean curvature of the amphiphilic film from positive to negative. In the following the quaternary (pseudo-ternary) system H20/NaCl (A)-n-decane (B)-AOT (D) will be discussed to show the main features of ionic microemulsions. Subsequently, the knowledge gained for alkylpolyglucoside micro emulsions (see Section 1.2.3) will be applied to understand the complex behaviour of five component ionic mixtures. 

Considering the phase behaviour of the system H20/NaCl (A)-n-decane (B)-AOT (D) as an example, the temperature-dependent phase behaviour of the system can be represented as a first approximation in an upright Gibbs phase prism, if the mixture of H20 and NaCl (often referred to as brine) is treated as a pseudo-component. It holds for the mass fraction of NaCl in the H20/NaCl mixture 

The phase behaviour of the pseudo-ternary ionic mixture can again be explained with the interplay of the three binary base systems. At low temperatures the ionic surfactant is 

The variation of the phase behaviour as a function of the salinity is shown in Fig. 1.10(b) in the form of an (y)-section through the phase tetrahedron of the quaternary H20/NaCl-n-decane-A0T system at a = 0.50 and a constant temperature of T = 40° C. In order to compare the variation of the phase behaviour with temperature and salinity a rectangular representation is used also for the (y)-section through the phase tetrahedron. As can be seen, the phase boundaries also resemble the shape of a fish in this isothermal (y)-section. However, with increasing mass fraction of salt the phase sequence 2, 3, 2 is found which is inverse to the 2, 3, 2 sequence observed with increasing temperature. 

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In many instances the SOW ideal ternary case is not sufficient to describe the behavior of an actual system. It has been noted that the addition of alcohol as a disordering agent is often required to avoid viscous or sol id-like mesophases. particularly to produce microemulsions with ionic surfactant systems (108). According to the corre)ations for optimum formulation, the alcohol effect can also be that of a cosurfactant that modifies the overall balance of affinity through the flA) and 0(i4) terms. The use of two surfactants i.s also often recommended to attain a better emulsion stability, a statement that. should not be taken for granted in all cases, although it could prove correct in some ca.ses. Hence, it is often... 

PREPAR LTION OF MICROEMULSIONS WITH IONIC SURFACTANTS... 

The effect of the addition of short chain alcohols on the chromatographic selectivity and peak efficiency was extensively exposed in previous chapters. The addition of such alcohols to a micellar solution forms mixed micelles. This is the first step toward the achievement of microemulsions with ionic surfactants. The oil in water microemulsion (LI structure, see Chapter 2) has a continuous aqueous phase containing oil swollen micelles or microdroplets of oil stabilized by an alcohol-surfactant interphase layer. The medium is transparent and stable, however it 1ms a dynamic structure. Then, it is interesting to see if LI microemulsion mobile phases could be useful in... 

Like water-oil-surfactant, IL-oil-surfactant can also form microemulsions. A special feature of IL-based microemulsions is the flexibility of the IL, which can be the polar or nonpolar as well as the surfactant component. When ILs are incorporated into microemulsions with ionic surfactants, electrostatic interactions between the components can play a crucial role to form a more stable palisade layer surrounding the droplets. By using polar ILs, water-free microemulsions can be formed. The tunable properties of two different ILs may allow spontaneous formation of a new class of IL-oil-lL microemulsions with special features opening new fields of application. 

With ionic surfactants for which V/1 <0.7, microemulsion formation needs the presence of a cosurfactant. The latter has the effect of increasing V without affecting 1 (if the chain length of the cosurfactant does not exceed that of the surfactant). These cosurfactant molecules act as "padding" separating the head groups. 

The extent of solubilization of the oily soil depends on the chemical structure of the surfactant, its concentration in the bath, and the temperature (Chapter 4, Section IB). At low bath concentrations only a relatively small amount of oily soil can be solubilized, whereas at high surfactant concentrations (10-100 times the CMC), solubilization is more similar to microemulsion formation (Chapter 8, Section II) and the high concentration of surfactant can accommodate a much larger amount of oily matter (Schwartz, 1972). With ionic surfactants, the use concentration is generally not much above the CMC consequently, solubilization is almost always insufficient to suspend all the oily soil. When insufficient surfactant is present to solubilize all the oily soil, the remainder is probably suspended in the bath by macroemulsification Antiredeposition agents, such as the POE terephthate polyesters mentioped in Section 1 above, help prevent redeposition of suspended oily soil particles. 

Formulation essentially relates to the content of the systems and generally not to the way it is attained if thermodynamically stable systems are considered. The simplest microemulsion system would contain an organic oil phase (O), an aqueous phase generally referred to as water (W), and a surfactant (S) at a given temperature (T) and pressure (p). This means that at least five variables are required to describe the system. In practice, the situation is much more complicated. Water always contains electrolytes. Moreover, oils as well as nearly all commercial surfactants are mixtures. In most cases, particularly with ionic surfactant systems, a co-surfactant (e.g. an alcohol (A)) is added, among other functions, to reduce the rigidity of the surfactant layer and thus to prevent the formation of gel-like mesophases. 

Whenever a system has a composition that lies in the polyphasic region, it will generally separate (at equilibrium) into two phases, the representative points of which are located at the two extremes of the tie-line (see Fig. 3). In most cases the tie-hne is inchned i.e., one of the phases is rich in surfactant because it is located relatively near A or far from the OW side. If it is also located far from the AW and AO sides, then it contains both W and O in sizable amounts and fits the definition of a microemulsion (shaded region). Near the upper end of the tie-line in Fig. 3, it is an O/W type microemulsion. The other extreme of the tie-line is located near the OW side and near one of the component vertices (O in Rg. 3) and thus contains essentially one of the components. It is called an excess phase, in this case an oil excess phase. In most cases, particularly with ionic surfactant, the excess phase does contain a very small concentration of amphiphile, about the critical micelle concentration (cmc). In other words, the excess phase does not contain micelles, and as a consequence no micellar solubilization of the other phase can occur in the excess phase, an important feature when the mass balance is to be discussed. 

Table I summarizes the qualitative changes in the phase behavior of microemulsions containing ionic surfactants. Some details of the effects of different variables are available in Ref. 13 and various chapters in this book. The phase transitions are generally understood in terms of relative strengths of hydrophilic and hydrophobic properties of the surfactant film in the microemulsion. The phase behavior depends strongly on the type and structure of the surfactant. For example, microemulsions containing nonionic surfactants are less sensitive to salinity but are more sensitive to temperature than those with ionic surfactants. The partitioning of cosolvents such as alcohols between the surfactant film, the organic phase, and the aqueous phase also affects the phase behavior. Microemulsions can be tailored for specific applications by adjusting an appropriate variable. For example, as indicated in Table 1, the effect of salinity on the phase behavior can be counterbalanced by an increase in the pH of an appropriate microemulsion [18,19].

The relations between micellar solutions and microemulsions has been reviewed for microemulsion systems with ionic surfactants. The W/0 microemulsions are a direct continuation of the cosurfactant inverse micellar solution. At low water content no surfactant association takes place the surfactant molecules form small aggregates with a few water and cosurfactant molecules. The W/0 microemulsions are thermodynamically stable. 

One particular advantage of using mixtures of nonionic and ionic surfactants as microemulsifiers is the formation of temperature-insensitive microemulsions (17). Recall that the temperature-dependence of the phase behaviour of balanced microemulsion mixtures with ionic surfactants such as Aerosol OT (see Figure 4.10) is opposite to that found for ethoxylated alcohols (see Figure 4.5). Upon raising the temperature of Aerosol OT mixtures, a hydrophilic shift occurs (2-3-2), although with hoxylated alcohols, a lipophilic shift occurs (2-3-2). Intuitively, upon mixing ionic and nonionic surfactants, the temperature dependence should cancel at a particular ratio (5) of the two surfactants. 

Beside the implementation of protic ILs in microemulsions with nonionic surfactants, pseudo-ternary systems with ionic surfactants have been reported as well. We compared microemulsions composed of [Ci6mim][Cl]+decanol/ RTIL/dodecane with EAN and [bmim][BF4] as polar phase, respectively at ambient temperature (Zech et al., 2009 b). A significant difference with respect to phase behavior and microemulsions structure has been found. The area of the one phase region was considerably larger in the case of EAN than for microemulsions with [bmim][BF4]. For the microemulsions with EAN a typical percolation behavior for the EAN/o region with increasing EAN content has been found. DLS... 

The influence of surfactant structure on the nature of the microemulsion formed can also be predicted from the thermodynamic theory by Overbeek (17,18). According to this theory, the most stable microemulsion would be that in which the phase with the smaller volume fraction forms the droplets, since the osmotic term increases with increasing i. For w/o microemulsion prepared using an ionic surfactant, the hard sphere volume is only slightly larger than the water volume, since the hydrocarbon tails of the surfactant may interpenetrate to a certain extent, when two droplets come close together. For an oil in water microemulsion, on the other hand, the double layer may extend to a considerable extent, depending on the electrolyte concentration... 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

Markowitz et al. developed a different approach, again in an attempt to overcome some of the inherent difficulties that arise when imprinted bulk materials are used as catalysts [82], Here, the authors used a template-directed method to imprint an a-chymotrypsin TSA at the surface of silica nanoparticles, prepared with a number of organically modified silanes as functional monomers. Silica particle formation was performed in a microemulsion, where a mixture of a non-ionic surfactant and... 

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]. 

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