Surface-Active Substances Amphiphiles

Surfactants exhibit surface activity at different kinds of interfaces  

FIGURE 1.23 Orientation of soap (SAS) at the surface of water (alkyl group , polar group ). 

The orientation of surface molecule at the interface will be dependent on the system. This is shown as follows  

Air-water. Polar part toward water and hydrocarbon part toward air Oil-water. Polar part toward water and hydrocarbon part toward oil Solid-water. Polar part toward water and hydrocarbon part toward solid (in general) 

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Surface-active substances — are electroactive or elec-troinactive substances capable to concentrate at the interfacial region between two phases. Surface-active substances accumulate at the electrode-electrolyte - interface due to -> adsorption on the electrode surface (see -> electrode surface area) or due to other sorts of chemical interactions with the electrode material (see - chemisorption) [i]. Surface-active substances capable to accumulate at the interface between two immiscible electrolyte solutions are frequently termed surfactants. Their surface activity derives from the amphiphilic structure (see amphiphilic compounds) of their molecules possessing hydrophilic and lipophilic moieties [ii]. 

Surface active substances or surfactants are amphiphilic compounds having a lyophilic, in particular hydrophilic, part (polar group) and a lyophobic, in particular hydrophobic, part (often hydrocarbon chain). The amphiphilic structure of surfactants is responsible for their tendency to concentrate at interfaces and to aggregate in solutions into various supramolecular structures, such as micelles and bilayers. According to the nature of the polar group, surfactants can be classified into nonionics and ionic, which may be of anionic, cationic, and amphoteric or zwitterionic nature. 

Microemulsions [191, 192] are transparent, optically isotropic and thermodynamically stable liquids. They contain dispersion of polar and nonpolar solvent, usually water or aqueous solutions and oils. Adding surfactants stabilizes droplets of 1-100 nm in size. Due to amphiphilic properties of the surface active substances containing lipophilic groups and one or two lyophobic C-H chains mainly collected at the interface of two liquid phases, they cannot be mixed under normal conditions. Unlike traditional macroemulsion, which is kinetically stabilized only by the external mechanical energy supply, nano-domains in the microemulsions are formed spontaneously. Their size depends on the microemulsion composition, temperature and elastic properties of the separating film of surfactant. In particular, in the case of water-oil microemulsions with spherical nanosized micelles of water dispersed in oil, water droplets can be used as nanoreactors and templates for the solid nanoparticles fabrication. Since the reaction is initiated by the spatially restricted water and micelle, heterogeneous nucleation and crystal growth can be controlled. 

They are sold in bioshops as green detergents to the eco-lifestyle consumer community. Biosurfactants refer to surface active substances synthesized by living cells and as microbial amphiphilic compounds with surface active properties. They are in the focus of white biotechnology in order to promote the use of bio-based surfactants as possible alternative to chemical surfactants due to environmental and health concerns, stringent regulations and volatile petroleum, and other raw material price. 

Therefore, the use of surfactants for the modification of interfaces is very versatile, both with respect to the nature of the interfaces (between solid and liquid, polar and nonpolar), as well with respect to the assortment of the available surfactants. Up to this point, we have been talking about amphiphilic synthetic organic surfactants. However, the adsorption phenomenon is universal in nature and industry and takes place at all interfaces without any exceptions. It is worth emphasizing one more time that the general reason for the accumulation of surface-active substances at interfaces is the lowering of free energy as a result of the partial compensation of the disrupted bonds between interfacial atoms. 

The past two decades have seen the introduction of a new class of surface-active substance, so-called polymeric surfactants or surface active polymers, which result from the association of one or several macromolecular structures exhibiting hydrophilic and lipophilic characters, either as separated blocks or as grafts. They are now very commonly used in formulating products as diverse as cosmetics, paints, foodstuffs and petroleum production additives. A macromolecule can obviously exhibit an amphiphilic structure. 

For instance, surfactants dissolve in water and give rise to low surface tension even at very low concentrations (a few grams per liter or 1-100 mmol/L) of the solution therefore, these substances are also called surface-active molecules (surface-active agents or substances). On the other hand, most inorganic salts increase the surface tension of water. All surfactant molecules are amphiphilic, which means that these molecules exhibit hydrophilic and hydrophobic properties. Ethanol reduces the surface tension of water, but one will need over a few moles per liter to obtain the same reduction as when using a few millimoles of surface-active agents. 

Micelles are colloidal dispersions that form spontaneously, under certain concentrations, from amphiphilic or surface-active agents (surfactants), molecules of which consist of two distinct regions with opposite afL nities toward a given solvent such as water (Torchilin, 2007). Micelles form when the concentration of these amphiphiles is above the critical micelle concentration (CMC). They consist of an inner core of assembled hydrophobic segments and an outer hydrophilic shell serving as a stabilizing interface between the hydrophobic core and the external aqueous environment. Micelles solubilize molecules of poorly soluble nonpolar pharmaceuticals within the micelle core, while polar molecules could be adsorbed on the micelle surface, and substances with intermediate polarity distributed along surfactant molecules in intermediate positions. 

Throughout the discussion, the terms surface active agent, surfactant, and detergent are used interchangeably to refer to amphiphilic substances which form association colloids or micelles in solution. Amphiphilic substances or amphiphiles are molecules possessing distinct regions of hydrophobic and hydrophilic character. 

The dissolution of surface-active components in the spreading liquid greatly affects its spreading behaviour. For Instance, an amphiphilic substance (e.g. oleic acid) will adsorb strongly at the water-benzene and benzene-air Interface, thereby reducing and considerably. As a result, a solution of oleic acid in benzene spreads readily over the water surface. 

Solubilization [20, 358-360] was defined by Elworthy, Florence and Macfar-lane as the preparation of thermodynamically stable isotropic solutions of substances normally insoluble or slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components [358], Like surface activity, the ability to solubilize water-insoluble compounds represents a key property in the performance of surfactants [21], Therefore, solubilization by polysoaps has raised interest from the very beginning [46-51],... 

Surface-active materials consist of molecules containing both polar and nonpolar portions, i.e., amphiphilic molecules. The proteins are typically amphiphilic, polymeric substances made of amino acid residues combined in definite sequences by peptide bonds (primary structure). In many cases polypeptide chains are present in helical or /3-sheet configuration (secondary structure) which are stabilized by intramolecular (S-S and hydrogen) bonding. The next structural level, the tertiary structure, is determined by the folding of the polypeptide chains to more or less compact globules, maintained by hy-... 

The aggregates of many surface-active compounds are capable of enhancing the solubility of nonpolar organic compounds by bringing them into association with the hydrophobic portion of the amphiphilic cluster (Kile and Chiou, 1989). Humic aggregates and mieelles, in particular, may incorporate nonpolar materials such as hydrocarbons, a property that gives them the ability to act as soaps and detergents and transport hydrophobic substances into the aqueous phase (Carter and Suffer,... 

The fundamental property of a surface active agent, as mentioned before, is that it contains both polar and nonpolar moieties in its structure. This property is termed amphiphilicity or amphipathicity, and the substances that possess it are called amphiphiles. An amphiphile can be anionic or cationic, depending on whether its hydrophobic moiety is an anion or a cation. A zwitterion is an ion that possesses both anionic and cationic groups on the hydrophobic moiety and can behave either as an anionic, cationic or neutral species. An example of an ionic type is sodium dodecyl sulfate a cationic dodecyltrimethylammonium bromide a zwitterionic A-dodecyl-3-aminopropionic acid and a nonionic N, AT-dimethyldodecylamine oxide. 

With respect to the properties of polar groups, surfactants can be subdivided into ionic (cation- and anion-active, ampholytic, and zwitterionic) and nonionic surfactants. If the effect produced by the polar group of the surfactant molecule is more significant than that of the lipophilic group, this substance is soluble in water. It is less surface active as compared to any substance characterized by an optimum balance between the activities of hydrophilic and lipophilic groups. Similar conclusions can be drawn also with respect to the solubility in oil here, the role of the lipophilic group is determining. Clearly, the efficiency of a surfactant is not determined solely by the amphiphilicity, but depends on the hydrophilic/lipophilic balance (HLB) characteristic for this compound. Therefore, this balance is an important characteristic of both the surfactant and the interface. 

We emphasize that a micelle may for many purposes be considered as a microscopic droplet of oil. This explains the large solubilization capacity towards a broad range of non-polar and weakly polar substances. We note, however, that the locus of solubilization will be very different for different solubilizates. While a saturated hydrocarbon will be rather uniformly distributed over the micelle core, an aromatic compound, being slightly surface-active, will be concentrated to the interfacial region. An amphiphilic solubilizate, like a long-chain alcohol, tends to orient in the same way as the surfactant itself. 

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