Surfactant systems, quantitative

The main advantage to the Winsor system is its heuristic feature of treating all cohesive interactions in a two-phase surfactant system. However, to date only the simple form of Equation 22 has been exploited quantitatively (21, 23) as... 

Normal aqueous micellar media can also be employed to extract and purify components from solid matrices. Proteins have been extracted from wheat kernals using aqueous NaLS (399). This same surfactant system has been employed in an improved method for the extraction of filth from cheese (417). In another application, aqueous solutions of Brij-35 micelles have been employed to extract components (i.e. vanillin and ethylvanillin) from smoking tobacco (106). In a similar manner, various phenolic compounds have been extracted from herbal/plant leaves using nonionic Triton X-100, Brij-35, or octyl glucoside (0G) (393). In both of these latter examples, the indicated compounds could be identified and quantitated by reversed phase HPLC using as mobile phase the same micellar solutions (refer... 

QUANTITATIVE REPRESENTATION OF PHASE BEHAVIOR OF AN ALKALINE-SURFACTANT SYSTEM... 

Quantitative Representation of Phase Behavior of an Alkaline-Surfactant System 475... 

Experimental set-ups as well as the corresponding theoretical models for these and other capillary pressure methods have been described in detail in a chapter by Liggieri and Ravera in the book on drops and bubble [197], These authors also discuss very detailed the problems coimected with the various experimental procedures used in the investigations with drop pressure methods. Most of all the theoretical basis of this group of experiments is well described and offers a good chance to quantitatively understand surfactant systems. 

In principle, we can distinguish (for surfactant self-assemblies in general) between a microstructure in which either oil or water forms discrete domains (droplets, micelles) and one in which both form domains that extend over macroscopic distances (Fig. 7a). It appears that there are few techniques that can distinguish between the two principal cases uni- and bicontinuous. The first technique to prove bicontinuity was self-diffusion studies in which oil and water diffusion were monitored over macroscopic distances [35]. It appears that for most surfactant systems, microemulsions can be found where both oil and water diffusion are uninhibited and are only moderately reduced compared to the neat liquids. Quantitative agreement between experimental self-diffusion behavior and Scriven s suggestion of zero mean curvature surfactant monolayers has been demonstrated [36]. Independent experimental proof of bicontinuity has been obtained by cryo-electron microscopy, and neutron diffraction by contrast variation has demonstrated a low mean curvature surfactant film under balanced conditions. The bicontinuous microemulsion structure (Fig. 7b) has attracted considerable interest and has stimulated theoretical work strongly. 

Theoretical models have reached a state that allows a quantitative description of the equilibrium state by thermodynamic models, the adsorption kinetics of surfactants at fluid interfaces, the transfer across interfaces and the response to transient or harmonic perturbations. As result adsorption mechanisms, exchange of matter mechanisms and the dilational rheology are obtained. For some selected surfactant systems, the characteristic parameters obtained on the various levels coincide very well so that a comprehensive understanding was reached. 

Clearly, the process of selecting the best surfactant or surfactants for the preparation of an emulsion has been greatly simplified by the development of the more or less empirical but theoretically based approaches exemplified by the HLB, solubility parameter, and PIT methods. Unfortunately, each method has its significant limitations and cannot eliminate the need for some amount of trial-and-error experimentation. As our fundamental understanding of the complex phenomena occurring at oil-water interfaces, and of the effects of additives and environmental factors on those phenomena, improves it may become possible for a single, comprehensive theory of emulsion formation and stabilization to lead to a single, quantitative scheme for the selection of the proper surfactant system. 

In contrast, at later stages [t > t) quantitatively different trends for water and surfactant systems were observed, even in the normalized presentation. Hole coalescence, droplet formation, and ripening proceeded much faster when y was lower as for aqueous L77 surfactant solution as a bounding medium. In Fig. 4.9, we indicate by multiplying the normalized time by the factor 10 that for the low values of y of the L77 surfactant solution the process... 

Despite all these shortcomings, the theory seems to be in quantitative agreement with experiment for at least one system studied in detail. Additional experiments with other surfactant systems are quite straightforward to conduct, because the data on the bending moduli, spontaneous curvature and interfacial tensions of several surfactant systems have become available recently. 

While infrared spectrophotometry is most useful for the qualitative analysis of surfactants, various quantitative methods have been developed for well-characterized systems. For example, an attenuated total reflectance cell with a ZnSe crystal is useful for direct analysis of aqueous anionic surfactant solutions by FTIR, while avoiding the deleterious effects of water on the usual transmission cells. In this case, the sulfonate absorbance at 1175 cm" , or the sulfate absorbance at 1206-1215 cm , is used for quantification (10,26). In another application, the weak absorption bands in the 1429-1333 cm" region are used to measure the relative amounts of linear and branched chain alkylbenzene sulfonates extracted from environmental waters (27). This is the one advantage of the infrared technique over those that have supplanted it for wastewater analysis its ability to differentiate the straight and branched chain compounds (28). No procedure will be given here, since the cleanup prior to IR analysis can be handled adequately by the method for LAS analysis by desulfona-tion/gas chromatography, described in Chapter 8. 

Because the PIT approach to surfactant evaluation is newer than the HLB method, the effects of variables on the relationship between PITs, surfactant structures, and emulsion stability have not been as clearly defined in a quantitative way. It has been found, however, that there is an almost linear correlation between the HLB of a surfactant under a given set of conditions and its PIT under the same circumstances. In essence, the higher the HLB of the surfactant system, the higher will be its PIT. 

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