Of individual surfactants and

An unknown commercial detergent may contain some combination of anionic, nonionic, cationic, and possibly amphoteric surfactants, inorganic builders and fillers as well as some minor additives. In general, the analytical scheme includes separation of nonsurfactant and inorganic components from the total mixture, classification of the surfactants, separation of individual surfactants, and quantitative determination (157). 

In otlier words, tire micelle surface is not densely packed witli headgroups, but also comprises intennediate and end of chain segments of tire tailgroups. Such segments reasonably interact witli water, consistent witli dynamical measurements. Given tliat tire lifetime of individual surfactants in micelles is of tire order of microseconds and tliat of micelles is of tire order of milliseconds, it is clear tliat tire dynamical equilibria associated witli micellar stmctures is one tliat brings most segments of surfactant into contact witli water. The core of nonnal micelles probably remains fairly dry , however. 

One of the most important characteristics of micelles is their ability to take up all kinds of substances. Binding of these compounds to micelles is generally driven by hydrophobic and electrostatic interactions. The dynamics of solubilisation into micelles are similar to those observed for entrance and exit of individual surfactant molecules. Their uptake into micelles is close to diffusion controlled, whereas the residence time depends on the sttucture of the molecule and the solubilisate, and is usually in the order of 10 to 10" seconds . Hence, these processes are fast on the NMR time scale. 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

The cost/performance factor of individual surfactants will always be considered in determining which surfactants are blended in a mixed active formulation. However, with the recent advent of compact powders and concentrated liquids, other factors, such as processing, density, powder flowability, water content, stabilization of additives, dispersibility in nonaqueous solvents, dispersion of builders, and liquid crystalline phase behavior, have become important in determining the selection of individual surfactants. 

Methods to determine and control the properties of individual surfactant molecules and to determine the conditions needed to produce well-defined molecular assemblies are just beginning to emerge. We are at the threshold of being able to produce dehberately stmctured supramolecular entities with properties tailored to meet special applications. Some additional examples of problems that will have significant impacts over the next one or two decades follow ... 

Public information about the specific chemical identity of the surfactants and stabilizers in use is scant(353-355) (Figure 11). Performance of foamed fluids is heavily dependent upon the size and distribution of the individual foam cells that are present, therefore the generator, testing apparatus, pressure and procedures employed are critical parts of the evaluation and the observed results. Contaminants (salts, acids, alkalies, etc) in the liquid phase also can cause drastic changes in foam performance. 

During the past few years, the determination of the interfacial properties of binary mixtures of surfactants has been an area in which there has been considerable activity on the part of a number of investigators, both in industry and in academia. The Interest in this area stems from the fact that mixtures of two different types of surfactants often have interfacial properties that are better than those of the individual surfactants by themselves. For example, mixtures of two different surface-active components sometimes reduce the interfacial tension at the hydrocarbon/water interface to values far lower than that obtained with the individual surfactants, and certain mixtures of surfactants are better foaming agents than the individual components. For the purpose of this discussion we define synergism as existing in a system when a given property of the mixture can reach a more desirable value than that attainable by either surface-active component of the mixture by itself. 

Surfactant Activity in Micellar Systems. The activities or concentrations of individual surfactant monomers in equilibrium with mixed micelles are the most important quantities predicted by micellar thermodynamic models. These variables often dictate practical performance of surfactant solutions. The monomer concentrations in mixed micellar systems have been measured by ultraf i Itration (I.), dialysis (2), a combination of conductivity and specific ion electrode measurements (3), a method using surface tension of mixtures at and above the CMC <4), gel filtration (5), conductivity (6), specific ion electrode measurements (7), NMR <8), chromatograph c separation of surfactants with a hydrophilic substrate (9> and by application of the Bibbs-Duhem equation to CMC data (iO). Surfactant specific electrodes have been used to measure anionic surfactant activities in single surfactant systems (11.12) and might be useful in mixed systems. ... 

What types of two-dimensional phases can occur in a monolayer when the surface concentration of the surfactant and the temperature are changed individually ... 

The diversity of the chemical structures possible in these compounds is enormous, thus making it difficult to arrive at precise structure-activity relationships. In many instances commercial preparations are mixtures of surfactants with the mean length or weight of any side group or chain being distributed around a Poisson distribution curve. Thus, there is tremendous variation possible within individual surfactants and mixtures of surfactants, often making it difficult to interpret results. 

The nature of the surfactant and its concentration is expected to play a role. To achieve a mechanically strong interfacial film, which can ensure the stability of the emulsion, the interfacial film of adsorbed surfactant molecules should be condensed in order to have strong lateral intermolecular interactions. A blend of two surfactants with different areas of head groups rather than an individual surfactant can more easily generate a close-packed and mechanically strong interfacial film. 

Figure 2. Schematic representation in cross-section of a unilamellar vesicle formed by surfactant molecules containing two long-chain n-alkyl substituents. Polar head groups of individual surfactants are indicated as circles and the two alkyl tails as jagged lines.

A micelle is a dynamic aggregation of any number of individual surfactant molecules, or monomers. Although the molecules are intertwined, they are in constant motion like those of a liquid. Thus, the interior of a micelle can be thought of as a separate phase and a micellar solution can be thought of as a microdispersion of that phase in water. If the micelle is considered to be a separate phase, it is then convenient to evaluate the solubilization capacity (k), in... 

The reversal of the direction of the electro-osmotic flow by the adsorption onto the capillary wall of alky-lammonium surfactants and polymeric ion-pair agents incorporated into the electrolyte solution is widely employed in capillary zone electrophoresis (CZE) of organic acids, amino acids, and metal ions. The dependence of the electro-osmotic mobility on the concentration of these additives has been interpreted on the basis of the model proposed by Fuerstenau [6] to explain the adsorption of alkylammonium salts on quartz. According to this model, the adsorption in the Stern layer as individual ions of surfactant molecules in dilute solution results from the electrostatic attraction between the head groups of the surfactant and the ionized silanol groups at the surface of the capillary wall. As the concentration of the surfactant in the solution is increased, the concentration of the adsorbed alkylammonium ions increases too and reaches a critical concentration at which the van der Waals attraction forces between the hydrocarbon chains of adsorbed and free-surfactant molecules in solution cause their association into hemimicelles (i.e., pairs of surfactant molecules with one cationic group directed toward the capillary wall and the other directed out into the solution). 

Strategems to overcome some of these basic problems include (i) the use of a high viscosity oil phase in w/o/w emulsions to prevent or decrease diffusion of individual surfactant molecules and water molecules, (ii) the polymerisation of interfacially adsorbed surfactant molecules, and (iii) the gelation of the oily or aqueous phases of the emulsions. 

This work reports the FAB spectra of the most representative surfactants used in the area. A library of all studied compotmds with their relevant peaks is provided for a rapid identification of individual compotmds in complex mixtures. The feasibility of this table was proved with the identification of individual surfactants from organic extracts of raw and drinking water of Barcelona. 

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