Surface active compounds

Fig. 5. Effect of surfactant type on surface resistivity, (a) Concentration of surface-active compound in low density polyethylene (LDPE) requked to achieve 10 Q/sq surface resistivity and (b) effect on surface resistivity of an acrylic polymer. Concentration of surface-active compound is 0.3%.

Surface-active compounds, especially the anionic surfactants, are derived from fossil raw materials as well as from recent raw materials. The portion of the biomass on the production of anionic surfactants is about 75% if the soap, the quantitatively most important anionic surfactant, is included. Considering only the synthetic surfactants, the syndets, the portion of fossil raw materials in the production of these surfactants, is about 75%. Without the lignosulfonates (and the petroleum sulfonates) this portion is about 90%. Due to strong efforts... 

It has also been found that surface-active compounds are obtained by reaction of higher molecular weight hydroxy compounds containing nitrogen with... 

The yielded product can be converted to a surface-active compound if at least one ester group has been transformed to the free acid or an alkali metal salt thereof [160]. There are also many compounds from phosphinic acid derivatives claimed to be useful as sequestrants and builders to improve detergency, especially bisphosphonylmethylphosphinic acids and polyphosphinic acids [structures (9) and (10)], respectively ... 

The possession of surface activity per se may be an important faetor in the antibacterial action of a group of drugs, for example the eationie detergents. The addition of low concentrations of surface-active compounds may potentiate the biological effect of an antibacterial agent. Thus, phenols are often more aetive in the presence of soaps. 

Certain surface-active compounds [499], when dissolved in water under conditions of saturation, form self-associated aggregates [39,486-488] or micelles [39,485], which can interfere with the determination of the true aqueous solubility and the pKa of the compound. When the compounds are very sparingly soluble in water, additives can be used to enhance the rate of dissolution [494,495], One can consider DMSO used in this sense. However, the presence of these solvents can in some cases interfere with the determination of the true aqueous solubility. If measurements are done in the presence of simple surfactants [500], bile salts [501], complexing agents such as cyclodextrins [489 191,493], or ion-pair-forming counterions [492], extensive considerations need to be applied in attempting to extract the true aqueous solubility from the data. Such corrective measures are described below. 

Other surface-active compounds self-assemble into bilayer structures (schematically illustrated in Fig. 10b), which normally spherilize into structures termed vesicles. When vesicles are formed from phospholipids, the term liposome is used to identify the structures, which also provide useful drug delivery systems [71]. Solutes may be dispersed into the lipid bilayer or into the aqueous interior, to be subsequently delivered through a variety of mechanisms. Liposomes have shown particular promise in their ability to act as modifiers for sustained or controlled release. 

The long-chain, fatty acid esters of sucrose are non-ionic, nontoxic, and biodegradable, and compare well in overall performance with other surface-active compounds in detergency, emulsification, and... 

Neu, T. R. (1996). Significance of bacterial surface-active compounds in interaction of bacteria with interfaces, Microbiol. Rev., 60, 151-166. 

High polarity is one of the reasons why both the ionic and amphoteric surfactants, and especially their metabolites, are difficult to detect. This property, however, is important for the application tasks of surface-active compounds, but is also the reason for their high water solubility. Due to this fact, their extraction and concentration from the water phase, which can be carried out in a number of very different ways, is not always straightforward. Furthermore, they are often not volatile without decomposition, which thus prevents application of gas chromatographic (GC) separation techniques combined with appropriate detection. This very effective separation method in environmental analysis is thus applicable only for short-chain surfactants and their metabolites following derivatisation of the various polar groups in order to improve their volatility. 

Surfactants are surface-active compounds, which are used in industrial processes as well as in trade and household products. They have one of the highest production rates of all organic chemicals. Commercial mixtures of surfactants consist of several tens to hundreds of homologues, oligomers and isomers of anionic, non-ionic, cationic and amphoteric compounds. Therefore, their identification and quantification in the environment is complicated and cumbersome. Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, still poses the analyst considerable problems. 

Figure 1.35 On-demand protection of an electrochemical sensor. Trace analysis of metal (M) analyte in the presence of surface active compounds (S) using the active and passive states. (Reprinted with permission from Ref. [172]. 2006 American Chemical Society.)...

Fig. 9.6 Tailored SAMs for surface engineering provides the control of the surface physical properties, chemical reactivity and heterogeneity on the molecular level, a) Self-assembly of one kind of surface active compound results in homogeneous monolayers, b) Adsorption of two components give ride to mixed SAMs, combining the physical and...

To realize surface-bonded initiating sites or their precursors, a variety of methods are applicable. Either organic (polymer) surfaces are irradiated or plasma treated to yield suitable functional groups [187, 195] or inorganic supports are covered with an interlayer of functional polymers bearing the desired groups. However, to gain control over the quantity of surface reaction sites and define the surface chemistry, interlayers of low molar mass a,co-functionalized surface active compounds are suit-... 

In the extraction of biologically active compounds, care must be taken to avoid the loss of activity that often occurs by contact with organic diluents. Thus a series of systems have been developed specifically with these compounds in mind. The first of these uses mixtures of aqueous solutions containing polymers and inorganic salts that will separate into two phases that are predominately water. A second system uses supercritical conditions in which the original two-phase system is transformed into one phase under special temperature-pressure conditions. Also the active organic compound can be shielded from the organic diluent by encapsulation within the aqueous center of a micelle of surface active compounds. AU these systems are currently an active area for research as is discussed in Chapter 15. 

An attempt to combine electrochemical and micellar-catalytic methods is interesting from the point of view of the mechanism of anode nitration of 1,4-dimethoxybenzene with sodinm nitrite (Laurent et al. 1984). The reaction was performed in a mixture of water in the presence of 2% surface-active compounds of cationic, anionic, or neutral nature. It was established that 1,4-dimethoxy-2-nitrobenzene (the product) was formed only in the region of potentials corresponding to simultaneous electrooxidation of the substrate to the cation-radical and the nitrite ion to the nitrogen dioxide radical (1.5 V versus saturated calomel electrode). At potentials of oxidation of the sole nitrite ion (0.8 V), no nitration was observed. Consequently, radical substitution in the neutral substrate does not take place. Two feasible mechanisms remain for addition to the cation-radical form, as follows ... 

Micellar catalytic methods were used to operate a choice between these two mechanisms. When an ion-radical has a charge opposite to that of the micelle surface, it is trapped by the micelle (Okamoto et al. 2001). In the presence of a surface-active compound, the aromatic substrate is nitrated in the very depth of a micelle, and the reaction rate depends on the local concentration of the nitrating agent on phase boundaries between the micelle and solution. A positively charged... 

As regards absorption through the skin, it is well known that surface-active compounds and skin irritants can enhance the absorption of other chemicals. 

Emulsions are one of the most important application areas of surface-active compounds, and are generally described as belonging to three different kinds ... 

In MEKC, mainly anionic surface-active compounds, in particular SDS, are used. SDS and all other anionic surfactants have a net negative charge over a wide range of pH values, and therefore the micelles have a corresponding electrophoretic mobility toward the anode (opposite the direction of electro-osmotic flow). Anionic species do not interact with the negatively charged surface of the capillary, which is favorable in common CZE but especially in ACE. Therefore, SDS is the best-studied tenside in MEKC. Long-chain cationic ammonium species have also been employed for mainly anionic and neutral solutes (16). Bile salts as representatives of anionic surfactants have been used for the analysis of ionic and nonionic compounds and also for the separation of optical isomers (17-19). 

For systems, which cannot be marked easily, the displacement method is an alternative, in particular for low concentration ranges (36). This is a modification of isotachophoresis, because current flow is not constant with time and field strength is a function of the position along the capillary. Instead of analyte peaks, profiles are obtained (Fig. 6a) as injection and separation are carried out in one step. Although this method is not suitable for all micellar systems, one outstanding advantage is the higher UV sensitivity (which is important for most surface-active compounds). Because of the reliance of the method on displacement, the micellar phase is not diluted. 

Biosurfactants are commercially available compounds for the in sim or ex situ treatment of hydrocarbons and non-aqueous-phase hquids (NAPLs) in soil and groundwater. Surfactants are highly surface-active compounds that solubilize and/or mobilize contaminants in the subsurface. 

Surfactants used in practical applications essentially always consist of a mixture of surface-active compounds. Isomerically pure surfactants are often expensive to produce and generally have only a small potential advantage in performance over the less expensive surfactant mixtures. In many applications, mixtures of dissimilar surfactants can have superior properties to those of the individual surfactant components involved. These synergistic properties of surfactant mixtures have provided impetus for much of the research on interactions between surfactants. 

---