Ionic Surfactant-Cosurfactant Systems

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]. 

Kunieda, H. (1988) Triaitical phenomena in a brine/ionic surfactant/ cosurfactant/oil system. J. Colloid Interface Sci., 122, 138-142. 

Li and L2 can be described by considering the phase diagram of a ternary system of water-surfactant-cosurfactant system, as shown in Figure 13.20 for water-ionic sulphate-long-chain alcohol system. 

Rather than fix the water/salt ratio as a brine pseudocomponent, another useful possibility is to fix the alcohol/ionic surfactant ratio as the pseudo-component (fix 8) and vary the salt concentration. An increase of the lyotropic salt concentration in ionic surfacant plus alcohol cosurfactant systems has the same effect as increasing temperature or salt in nonionic surfactant mixtures - a lipophilic shift is observed, and the phase behaviour progresses from 2 to 3 to 2 (15). If salt is placed in the position occupied previously by the cosurfactant in Figure 4.8, and the fixed ratio of alcohol/ionic surfactant placed as a combined pseudocomponent (fixed 5) at the surfactant position, at equal amounts of oil and water (a = 0.5) a plot of salt concentration (e) versus overall cosurfactant/surfactant concentration (y) also yields a fish -shaped phase diagram (45). Therefore, in either the case of fixed salt concentration (fixed ) or the case of fixed alcohol/ionic surfactant ratio (fixed 8), the optimally formulated microemulsions for the chosen fixed ratio of 8 or s, are found at X, where the tail and body of the fish meet (see Figure 4.8). Consequently, the phase behaviour of simple monodisperse ethoxylated alcohol surfactants in oil and water qualitatively mimics that of much more complicated mixtures containing ionic surfactants, cosurfactants and salt. Alcohol... 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

Most challenging are the systems that have ionic surfactants and are free of electrolyte. In this case it is possible to obtain a swollen phase, by the addition of a mixture of cosurfactant and oil.2,28 24 The thickness of the water lamellae, evaluated from the composition of the liquid crystal and the repeat distance, is about 17 A.2 If... 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

In this article we evaluate interactions in a system stabilized with an ionic surfactant and with a carboxylic acid as the cosurfactant. Such a system is distinguished from the common soap/alcohol stabilizer combinations by the fact that the soap/acid system does not require a minimum water concentration to dissolve the soap. 

As a brief conclusion it can be noted that many nonaqueous microemulsions reported do not seem to contain an organized structure, being simply molecular solutions. Since the degree of organization already in many aqueous microemulsion is low, in particular for quaternary systems containing ionic surfactant and cosurfactant, this is not really surprising. 

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 9 gives a generic picture, but the parameter controlling the spontaneous curvature is different for different surfactant systems. For nonionic surfactants, temperature controls the spontaneous curvature, changing it from positive (surfactant film curved toward oil) a low temperatures to negative at higher tempemtures. For other systems, salinity (for ionics), surfactant composition in a mixture, and cosurfactant and cosolvent concentrations are controlling parameters. The generic structural change as a function of the spontaneous curvature is demonstrated in Fig. 9. At markedly positive spontaneous curvatures we have...

Experimentally, the bending elastic modulus K as measured by ellipsometry is indeed found to be on the order of (0.1-1)A 7 for quite a wide range of microemulsion systems see, e.g.. Ref. 45 for a system with a single-chain ionic surfactant plus cosurfactant. Ref. 46 for a system with a double-chain ionic surfactant in which the chain length of the oil (linear alkane) is varied, and Ref 47 for a system with a nonionic surfactant in which the chain length of the surfactant is varied. 

By the way, ionic surfactants generally do not lead to microemulsions at ambient temperature, but to liquid crystals, which are organized systems. To produce a microemulsion, it is often required to introduce disorder either by increasing temperature (up to 50°Q or by adding a cosurfactant, generally an alcohol [9,10]. 

Systems of a single-chain ionic surfactant and a cosurfactant, where the spontaneous curvature is controlled by controlling the salinity [44] (Fig. 10). 

An important aspect in all drug delivery is the toxicity of the drug as well as that of the drug carrier. Therefore, toxicity has to be assessed also for microemulsion formulations. In microemulsion systems, the main concern regarding toxicity has to do with the cosurfactants used. For example, the majority of the work on the pharmaceutical application of microemulsions has involved the use of short- or medium-chain alcohols, e.g., butanol. In a range of studies it has been shown that these cause toxic side effects. For example, inhalation studies of the toxicity of 1-butanol, 2-butanol, and / -butanol in rats showed a dose-dependent reduction in fetal weight [56]. Furthermore, aqueous solutions of ethanol, propanol, and butanol were shown to result in elongated mitochondria in hepatocytes after 1 month of exposure [57]. (In addition to the toxicity aspects of these alcohols, microemulsions formed in their presence are often destabilized on dilution of the continuous phase.) Furthermore, many studies so far have involved aliphatic or aromatic oils, such as hexane or benzene, which obviously are unsuitable for pharmaceutical use. Moreover, ionic surfactants could in themselves be toxic and irritant [58]. 

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... 

Worm-like micelles are formed in solutions of both ionic and nonionic surfactants. For aqueous solutions of ionic surfactants, inorganic or organic salt or cosurfactants are necessary except for special system like dimeric surfactants. On the other hand, nonionic surfactants can form worm-like micelles in water without anyadditionalcomponents. Surfactants ofpolyoxyethylene type C H2 +i(OC2H4) OH... 

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. 

The monomer cosurfactant effect alone is not enough to explain the very high values of HLBopt- Electrolytic effects of certain monomers must also be taken into account. For example, adding sodium acrylate or MADQUAT to water/surfactant/oil systems drastically reduces solubility of non-ionic surfactants in water. This is called salting out. Reducing solubility in this way shifts HLBopt to higher values. 

The conductive and dielectric behavior of water-in-oil, (w/o), type transparent isotropic systems using either non-ionic surfactants or a combination of an ionic surfactant with a medium chain-length alcohol used as the cosurfactant was investigated between 400 Hz and 2 GHz or so. 

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