Microemulsions ionic systems

We recall that comparatively sharp and even nonmonotonous crossover from Ising to mean-field behavior has been deduced from experiments for a diversity of ionic systems. We note that this unusually sharp crossover is a striking feature of some other complex systems as well we quote, for example, solutions of polymers in low-molecular-weight solvents [307], polymer blends [308-311], and microemulsion systems [312], Apart from the fact that application of the Ginzburg criterion to ionic fluids yields no particularly... 

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). 

Ionic surfactants with only one alkyl chain are generally extremely hydrophilic so that strongly curved and thus almost empty micelles are formed in ternary water-oil-ionic surfactant mixtures. The addition of an electrolyte to these mixtures results in a decrease of the mean curvature of the amphiphilic film. However, this electrolyte addition does not suffice to drive the system through the phase inversion. Thus, a rather hydrophobic cosurfactant has to be added to invert the structure from oil-in-water to water-in-oil [7, 66]. In order to study these complex quinary mixtures of water/electrolyte (brine)-oil-ionic surfactant-non-ionic co-surfactant, brine is considered as one component. As was the case for the quaternary sugar surfactant microemulsions (see Fig. 1.9(a)) the phase behaviour of the pseudo-quaternary ionic system can now be represented in a phase tetrahedron if one keeps temperature and pressure constant. 

Figure 1.11 Section through the phase tetrahedron of the pseudo-quaternary system E O/NaCI-n-decane-sodium dodecyl sulphate (SDS)-1-butanol (C4E0) at tf> = 0.58, 8 = 0.10 and T = 20°C [26]. Note that the pseudo-quaternary ionic system can be driven through phase inversion by adding C4E0 as was the case for the quaternary alkylpolyglucoside microemulsions. (From Ref. [26], reprinted with...

Microemulsions have been achieved with supercritical fluids, particularly light hydrocarbons that can be solubilized in ionic systems [62,63], while carbon dioxide could be solubilized in only fluorocarbon surfactant and other nonionic systems [64]. 

Since salt must be added to Aerosol OT surfactant in order to form balanced middle-phase microemulsions (ionic surfactants are salt-sensitive), and salt also induces a lipophilic shift in ethoxylated alcohol mixtures (nonionics are less salt-sensitive than ionics), the salt concentration (e) is an important tuning parameter in addition to 5. As an example, consider the system of C)2E5/AOT/decane/water/NaCl, where 8 is the amount of AOT in the C12E5 plus AOT mixture. 

Measurements of the optimal temperature for microemulsion formation (T) are plotted as a function of AOT surfactant concentration 0) and salinity ( ) in Figure 4.11 (46). Rather than T simply averaging upon mixing the two surfactants, due to the opposing temperature-dependence of the nonionic and ionic systems, the curves of constant salinity (e) diverge at a value of 8 70 wt%. A pole is found in the phase behaviour (a flip in the curvature of the lines of constant e) for e between 0.8 and 1.0 wt%. Thus, at <5 ... 

This is not the end of the story, however. A third type of effect can alter the selfassociation structure and is directly related to the surfactant and or alcohol inherent properties. For instance, straight-chain ionic surfactants would produce liquid crystals of the lamellar type unless the temperature is quite elevated. Thus, in most cases of ionic systems, a large amount of alcohol (as much as two or three times the surfactant amount on a mole fraction basis) is required to melt the liquid crystal into a microemulsion, particularly for middle-phase ones [33]. Note, however, that too much alcohol could be detrimental to a high-performance microemulsion because the alcohol molecules which are not playing a cosurfactant role at the interface would dissolve into the bulk of one or both excess phases, making them more compatible [i.e., the alcohol would make the water less polar and the oil more polar (depending on the alcohol, but most particularly, intermediate solubility ones such as secondary butanol or tertiary pentanol)]. This is, of course, a way to narrow the miscibility gap, but this time by favoring the formation of a cosolubilized random mixture of all molecules instead of a microemulsion structure [50,65]. 

Our experiments demonstrated that micelle formation is possible in ILs and that size and aggregation number of the micelles can be timed by changing the exact nature of the IL. Moreover, as observed in water the formation of lamellar structures of DPPC in different ILs was demonstrated. Lastly three micro-regions of the microemulsion - ionic hquid in oil, bicontinuous and oil in ionic liquid - were identified in the ternary system... 

Not surprisingly, nonionic microemulsion systems are much less sensitive to electrolytes than are ionic systems, although any effect will be in the same sense as that for ionic systems. 

The original work was on ionic reactions in normal micelles in water, but subsequently there has been extensive work on reactions in reverse micelles (O Connor et al., 1982, 1984 Kitahara, 1980 O. A. El Seoud et al., 1977 Robinson, et al., 1979). There also has been a great deal of work on photochemical and radiation induced reactions in a variety of colloidal systems, and microemulsions have been used as media for a variety of thermal, electrochemical and photochemical reactions (Mackay, 1981 Fendler, 1982 Thomas, 1984). 

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. 

Effect of Ionic Strength. Both yE systems were examined for ionic strength effects. Microemulsion compositions were prepared at 70% water, with a cyanide concentration of 0.032 M with respect to the water content. Potassium bromide was used to vary the ionic strength of the reaction mixtures. Ionic strength in the CTAB yE was varied from 0.04 to 0.34. Since the Brij yE tolerated a much higher salt concentration without phase separation, ionic strength in that system was varied between 0.04 and 1.80. As will be seen, the Brij system exhibits a salt effect, while the CTAB yE does not. Rate constants obtained for reaction (1) in the Brij yE were therefore corrected to take into account the effect of ionic strength in that system (vide infra). 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

Consequently, the SDS microemulsion system is the best model for indirect measurement of log Pow. However, this is valid only for neutral solutes. We reported that the relationship between MI and log Pow for ionic solutes is different from that for neutral solutes (49). This would be caused by the ionic interaction between ionic solutes and the ionic microemulsion as well as ionic surfactant monomer in the aqueous phase. Kibbey et al. used pH 10 buffer for neutral and weak basic compounds and pH 3 buffer for weak acidic compounds (53). Although their purpose was to avoid measuring electrophoretic mobility in the aqueous phase, this approach is also helpful for measuring log Pow indirectly. 

Separation of antibiotics and cephalosporins can be achieved successfully by CZE because most of them are ionic species. As an alternative to CZE, antibiotics and cephalosporins have been separated by MEEKC. The separation of cephalosporins in different systems (micelles, mixed micelles, and microemulsions) was investigated. The best separation was achieved in microemulsions (Fig. 4). Figure 4 shows that cephalosporins have better affinity to ME in the ME systems than in the MC systems. The affinity of cephalosporins in the ME systems decreases with decrease in the migration time. The MEEKC was also particularly suitable for neutral cephalosporins that could not be separated by CZE or MEKC (14) (see Fig. 5). The method provided good reproducibility and rapid separation with high efficiency. 

Dye-Doped Silica Nanoparticle Synthesis Using Ionic Surfactant-Based Microemulsion Systems... 

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