Surfactants system definition

Surfactants by definition self-organise in water giving rise to micelles of varying size and shape. The core of micelles is non-polar and can solubilise reactants that are insoluble in water. Thus, a simple surfactant-water system at a surfactant concentration well above the critical micelle concentration can be used to overcome the problem of reactant incompatibility the polar reagent will be situated in the bulk aqueous domain, the non-polar reagent will be present in the micelles, and the reaction will occur at the micelle boundary. Organic reactions in micellar systems have been reported more than 40 years ago [1,2]. 

Our own involvement in microemulsion research was very much influenced by the contacts with the Swedish masters in the field of phase behaviour, Ekwall and Friberg, and at a later stage Shinoda, as well as by our previous experience of studying molecular interactions and association phenomena for other types of surfactant systems. Regarding the stability issue, we found it useful to suggest a definition [32] of a microemulsion as a system of water, oil and amphiphile which is a single isotropic and thermodynamically stable liquid solution . While this definition certainly provided nothing new, we felt it contributed to eliminate some confusion. 

Pressure effects on surfactant systems containing conventional liquid alkanes have not often been studied because of the very low compressibility of liquids. Conflicting results have been reported [38-40]. It is likely that the changes in cohesive energy density (solubility parameter) of the phases over the pressure ranges used were too low to produce definitive trends in phase behavior. The solubility parameter of compressed liquid propane, however, is moderately adjustable with pressure, and therefore a propane-brine-AOT system could be expected to show pressure-driven phase transitions [20,22,41]. 

The phase behavior observed in surfactant systems can be viewed from several theoretical perspectives. One is the qualitative Winsor R theory. In its simplest form, the Winsor R parameter is the ratio of the forces acting on the oil side of the surfactant interface to those on the water side. This definition has been extended to include other interactions that tend to oppose surfactant aggregation [34]. When R>, the forces on the oil side of the interface are the strongest, so the interface curves about water, resulting in a 2 system. When i < 1, the water-side forces are dominant, which causes the formation of a 2 system. When R = U the forces at the interface are balanced, which leads to a bicontinuous microemulsion... 

For ionic surfactant systems the definition of SAD as a function of flic formulation variables is... 

Nonionics can function as cosurfactants in microemulsions. This cosurfactant role, which can also be performed by short-chain alcohols or glycol ethers, can make it possible to form microemulsions or other phases from anionic surfactant systems that would otherwise be unreachable in a pure anionic system. By one definition, a microemulsion system is one that achieves a zero or close-to-zero curvature [108]. This is easily achievable in nonionic systems, but very difficult in pure anionic surfactant systems due to the head group repulsions. Mixing the nonionic with the anionic makes these types of systems, which sometimes have greater cleaning ability, easily accomplished. 

Reference has already been made to the interesting finding by Laurent and Scott (65) that precipitation of various polyanion/cationic surfactant systems can be totally inhibited by the addition of a sufficient amount of simple salt. This work allowed the definition of a critical electrolyte concentration (c.e.c.), which was found to vary from system to system. Clearly, electrostatic screening effects are again involved. This phenomenon has been confirmed and examined in some detail by Lindman and co-workers (see next section). Less work has been carried out in this respect on polycation/anionic surfactant systems and, at least in some systems involving cationic cellulosic polymer/SDS combinations, resolubilization by salt addition was found not to be facile (59,103). 

If a micelle is regarded as a chemical species and no melting of the surfactant solids is assumed, two degrees of freedom still remain along the line of Po — P — P (because there are four components, three phases, and one equilibrium equation). Temperature therefore specifies the binary surfactant system at a definite pressure as long as two surfactant solid phases coexist in the system, which is consistent with much experimental evidence. 

Microemulsions are a convenient medium for preparing microgels in high yields and rather uniform size distribution. The name for these special emulsions was introduced by Schulman et al. [48] for transparent systems containing oil, water and surfactants, although no precise and commonly accepted definitions exist. In general a microemulsion may be considered as a thermodynamically stable colloidal solution in which the disperse phase has diameters between about 5 to lOOnm. 

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. 

When chloro-octadecane was found to give the same result as a so-called cosurfactant, an argument arose in terms of the real role of this highly hydrophobic compound because it is not surface active and has no cooperation with surfactant. Taking account of these systems, the definition of miniemulsion polymerization will be revised to the polymerization in which a water-insoluble compound in the dispersed phase retards or inhibits diffusion degradation of the emulsion. ... 

Nature utilizes surfactants for a variety of additional roles. A natural surfactant, using a strict definition, is a surfactant taken directly from a natural source (isolated by a separation procedure from either a plant or an animal origin).42 Lecithin, obtained either from soybean or from egg yolk, is probably the best example of a truly natural surfactant. Other natural originated surfactants are the various soap-like surfactants for the removal of fatty/oily substances. These compounds produce a rich lather when dispersed into water and are found in various natural systems (such as chestnuts, in leaves and seeds of Saponaria Officinalis (soapwort), in the bark of the South American soaptree Quillaja saponaria Molina and in the fruits of Acacia Auriculiformis (Figure 7.2) 43,44... 

As far as I am aware, independent experimental evidence for the values of the surface potential and salt fractionation factor have not been obtained for any system other than the n-butylammonium vermiculite gels. For this isolated system, the predicted values of 5 from the Donnan equilibrium and the new equilibrium based on the coulombic attraction theory, namely 4.0 and 2.8, respectively, are definitely distinguished by the experimental results. It would be highly desirable to obtain further tests of our prediction for 5 in systems of interacting plate macroions, both in clay science and lamellar surfactant phases. 

The possibility to use Cm as a parameter characterising foam inhibition has been demonstrated for the first time in [60]. It was shown that the increase in the concentration of silicon oil Caf (antifoam) led to increase in Cm- That is why it was proposed to used the ratio Caf/Cm as a quantitative measure of the defoaming ability. However, it should be noted that the silicon oil concentrations at which inhibition of black spot formation was observed, were very low (10 5-10 9 %). For that reason it is difficult to conclude definitely whether the system was a real solution or represented a diluted emulsion of the antifoam in the surfactant solution. 

---