Cosurfactant molecules

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. 

Negative interfacial tension [58,61-66] Due to adsorption of surfactants or cosurfactant molecules, the interfacial tension can become extremely low (less than 1 mN/m) and eventually transiently negative. Therefore, the interface can increase and any fluctuation can break it. 

Structural entitles In water-ln-oll (W/0) mlcroemul-slon at low water content are reviewed. These structures Include monomers of surfactant associated with a few water and cosurfactant molecules. These small aggregates are stable In a non-polar environment In spite of their polar character and provide an Interesting case of unusual Interaction between polymers and mlcroemulslon structures. Examples are provided of cases when this Interaction Is Important for stability of mlcroemulslons with added organic or Inorganic polymers. 

The simplest representation of the structure of microemulsions is the droplet model in which microemulsion droplets are surrounded by an interfacial film consisting of both surfactant and cosurfactant molecules, as illustrated in Fig. 7.18. The orientation of the amphiphiles at the interface will, of course, differ in o/w and w/o microemulsions. As... 

The film (which may consist of surfactant and cosurfactant molecules) is considered as a liquid two-dimensional third phase in equilibrium with both oil and water... 

Contributions to are considered to be due to crowding of the surfactant and cosurfactant molecules and penetration of the oil phase into the hydrocarbon chains of the interface. According to Equation (15.3), if. >(yo/w)a> Yt becomes... 

The two surfactant molecules should adsorb simultaneously and they should not interact with each other, otherwise they lower their respective activities. Thus, the surfactant and cosurfactant molecules should vary in nature, one predominantly water-soluble (e.g., an anionic surfactant) and the other predominantly oil-soluble (e.g., a medium-chain alcohol). 

If it is assumed that all surfactant and cosurfactant molecules are adsorbed at the interface, it is possible to calculate the total interfacial area of the microemulsion from a knowledge of the area occupied by the surfactant and cosurfactant molecules. 

Total interfacial area = Total number of surfactant molecules (n ) x area per surfactant molecule (A ) -H total number of cosurfactant molecules x area per cosurfactant molecule... 

FIGURE 13.1 Surfactant and cosurfactant molecules oriented at the surface of the oil microdroplet. 

Micelles are dynamic aggregates of surfactant molecules [1,2]. Microemulsions are isotropic, thermodynamically stable dispersions of two immiscible liquids, generally oil and water that are interfacially stabilized by the surfactant and cosurfactant molecules [3-5]. These systems have important industrial [5-8] and biomedical [9-11] applications. 

These results and those using other methods (Shah, 1971 Clausse, 1957 Ache, 1977) agree on the following interpretation. At low water concentration the surfactant molecules associate a few water and cosurfactant molecules around its polar part forming an aggregate such as the one in Fig. 5, A. With increasing water concentration a stepwise association to inverse micelles. Fig. 6, B, takes place (Eicke and Christen, 1974). ... 

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. 

Exchange rates of cosurfactant molecules between interface and bulk solution seem to vary with molecular size (mobility) and environment, but a typical rate of 10 s was found by Lang et al. [104] for butan-l-ol in water/toluene/SDS microemulsions. They also found that surfactant exchange could be on a similar timescale, but may be greatly affected by many other parameters [105]. It also seems that the exchange times found in different studies are dependent on the method of measurement, so the picture is not yet complete. 

The film, which may consist of surfactant and cosurfactant molecules, is considered as a liquid two-dimensional third phase in equilibrium with both oil and water. Such a monolayer could be a duplex film, i.e. giving different properties on the water side and oil side. The initial flat duplex film (Figure 10.3) has different tensions at the oil and water sides. This is due to the different packing of the hydrophobic and hydrophilic groups (these groups have different sizes and cross sectional areas). 

Contributions to re are considered to be the crowding of surfactant and cosurfactant molecules and penetration of the oil phases into the hydrocarbon part of the molecules. If re > (yo/w)a hen yj becomes negative, leading to the expansion of the interface until yj becomes zero or a small positive value. Since (yi-)a is of the order of 15-20 mN m surface pressures of that order have to be reached for y-j-to reach an ultralow value that is required for microemulsion formation. This is best achieved by the use of two surfactant molecules, as discussed above. 

The P values did vary fairly linearly with temperature for the above-referred studied systems. But both the intercepts and slopes have shown exponential dependence on the concentration of surfactants. The authors have suggested that the dependence of P with temperature is a measure of the relative adjustment between the surfactant and cosurfactant molecules at the interface and in the continuous oil phase for the sake of stability [35]. 

The w/o microemulsion droplets are assumed spherical and mono disperse with a surface monolayer (called the interphase) comprising closely arranged surfactant and cosurfactant molecules determined by the composition, temperature, and the system type. The total droplet volume and their surface area Aj are then... 

The number of surfactant (N ) and cosurfactant molecules (NJ at the droplet interface is consequently related by the equations... 

FIGURE 16.1. Simplified representation of microemulsions showing (a) fluctuating domains of oil and water and (b) surfactant and cosurfactant molecules adsorbed at the oil and water interface. 

The continuous exchange of the surfactant molecules (as well as cosurfactant molecules in case of mixed micelles) constitutes a major dynamic process in micellar systems. The situation in microemulsions, although more complex, directly derives from that in simple micelles. For this reason, we will briefly recall here the main conclusions that have been estab-hshed concerning the micellar dynamics, reviewed in detail in Chapter 3. 

It is more difficult to obtain information concerning local motions in cosurfactant molecules (usually short-chain alcohols) due to their very fast exchange rate between the different pseudo-phases. 

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