Amphiphilic compounds surface activity

The solute may itself be amphiphilic like a surface-active lipid (emulsifier) or just a simple hydrophilic or hydrophobic compound. 

Terpenoids are structurally based on the isoprenoid (C5) unit and include monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids, steroids and carotenoids. These compounds can be further modified to generate greater structural complexity. Thus the saponins are surface active amphiphiles deriving from the glycosylation of steroid (C27) or triterpenoid (C30) entities. Plant triterpenoids with very specific biochemical effects include those that mimic the effects of mammalian steroid hormones or of insect developmental hormones. 

Saponin Saponin is a type of glycoside widely distributed in plants. It is an amphiphilic compound that has surface activity. Purified quillaja saponin has been reported to... 

A. A. Gorkovenko, E. L. Berman, S. Y. Zaitsev, and V. A. Ponomarenko, Synthesis and surface-active properties of carbohydrate amphiphilic compounds and polymers based on them, Bull. Acad. Sci. USSR, Chem. Sci., 36 (1987) 2135-2139. 

Surfactants are amphiphilic molecules widely used for different purposes in industrial processes, with a worldwide annual demand of about 10 billion (1,2). The most used surfactants are produced from petrochemical sources (3) however, compounds having surface activity characteristics may be synthetized by a wide variety of microorganisms (4,5). Such compounds, called biosurfactants, when compared with the syn-... 

The molecular structure of retinoic acid is typical for an amphiphilic compound that is concentrated at interfaces. Further, the carboxylic acid groups allow such compounds to adjust their amphiphilic character by the degree of their dissociation. Surface tension measurements were carried out in order to determine the surface activity of retinoic acid [179]. The surface tension with respect to the concentration at pH 5 decreases more strongly than at pH 9. This reflects the fact that the protonated form of retinoic acid is more efficient in its surface activity than the deprotonated form. The critical micelle concentrations are 3.7 0.5 mg/L (pH 5) and 19 2 mg/L (pH 9). The limiting surface tension values in both curves is about 35 mN/m. Due to the precipitation of retinoic acid, the highest concentration in the surface tension curve at a pH of 5 was 20 mg/L. By contrast the solubility at pH 9 is at least 1 g/L. In order to verify the results from the FTIR measurements, films of the complexes were immersed in a solution of 0.15 mol/L sodium... 

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]. 

Fatty acid salts and many polar derivatives of fatty acids are amphiphilic, possessing both hydrophobic and hydrophilic areas within the one molecule. These are surface-active compounds that form monolayers at water/air and water/surface interfaces and micelles in solution. Their surface-active properties are highly dependent on the nature of the polar head group and, to a lesser extent, on the length of the alkyl chain. Most oleochemical processes are modihcations of the carboxyl group to produce specihc surfactants. 

Certain compounds, because of their chemical structure, have a tendency to accumulate at the boundary between two phases. Such compounds are termed amphiphiles, surface-active agents, or surfactants. The adsorption at the various interfaces between solids, liquids and gases results in changes in the nature of the interface which are of considerable importance in pharmacy. For example, the lowering of the interfacial tension between oil and water phases facilitates emulsion formation the adsorption of surfactants on the insoluble particles enables these particles to be dispersed in the form of a suspension and the incorporation of insoluble compounds within micelles of the surfactant can lead to the production of clear solutions. 

Amphiphilic compounds with surface-active properties, such as surfactants or ten-sides, assemble in water to form spherical aggregates. The size and shape of these aggregates depends on the structure of the amphiphile. As a general rule amphi-philes with one polar head group and one nonpolar alkyl chain form micelles. 

However there is still a great discussion about the definition of their representative properties and even about the proper meaning of the word "microemulsion". Indeed such a name would indicate a dispersed system while microemulsions show the appearence of true solutions,i.e., of homogeneous systems. Since an essential requisite for the existence of a microemulsion is the presence of water (l), we think that a study of the fundamental properties of such systems should require the use of experimental approaches specifically apt to reveal "in primis" the behavior of water. Moreover,besides of course the hydrocarbon,being the other components necessary for the existence of a microemulsion amphiphilic compounds acting as surface active agents, also techniques suitable for the study of systems with a high surface-to-volume ratio, are requested. 

Amphiphiles (surface-active compounds or surfactants) are characterized by a molecular structure with both hydrophilic and hydrophobic domains. They tend to adsorb to surfaces and interfaces, with a concomitant lowering of surface tension. At the critical micellar concentration (CMC), the limit of surface tension reduction is reached and a spontaneous self-assembly takes place with the formation of aggregates (micelles). The size and structure of the micelles depend on the type and concentration of surfactant(s) present. 

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],... 

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

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. 

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,... 

Amphiphilic compounds are also surface-active their differently polarized regions cause them to accumulate at interfacial zones in the environment. For example, at the air-water interface, amphiphiles tend to orient themselves in surface microlayers or surface films (see Section l.B.2d), where the polar region of the molecule is associated with the water phase and the nonpolar region is forced out of solution and extends up into the air phase. Often these surface layers are visible by the damping effect they exert on wave action (Figure 1.8) they are apparent as smoother patches among the ripples on a lake or in the ocean. 

Surface-active or amphiphilic molecules, in contrast to insoluble compounds, interact with the water phase. There are essentially two types of film-forming, surface-active compounds dry surfactants, in which the major portion of the molecule extends out of the water (Figure 1.10. A), and wet surfactants, in which the bulk of the molecule is within the water phase and only small portions of the structure project into the air (MacIntyre, 1974 Figure I.IO.B). Synthetic detergents (20), long-chain aliphatic compounds with a polar benzenesulfonate tail, are typical dry surfactants, whereas proteins, with many hydrophilic regions and only a limited... 

Micelles are spherical supramolecular assemblies of amphiphilic compounds with surface-active properties (surfactants, tensides) in a colloidal dimension [1] (see also Section 2.3.1). Typical micelle-forming molecules carry a hydrophilic headgroup and a hydrophobic tail. This is shown schematically in Figure 1. 

Traditional models for calculation of adsorption isotherms are based on the assumption that surface-active compounds at the interface can substitute for adsorbed molecules of one solvent but cannot penetrate the second phase. Although these models are useful for metal-water interfaces, recent interest has focused on the surface chemistry of amphiphilic compounds that can penetrate both phases and replace adsorbed molecules of both solvents, for example, water and oil. Amphiphilic molecules consist of two moieties with opposing properties a hydrophilic polar head and a hydrophobic hydrocarbon tail. We present here, a theoretical analysis of the generalized Frumkin adsorption isotherm for amphiphilic compounds. 

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