Amphiphilic substance

Carboxylate groups are hydrophilic ( water loving ) and tend to confer water sol ubility on species that contain them Long hydrocarbon chains are lipophilic ( fat loving ) and tend to associate with other hydrocarbon chains Sodium stearate is an example of an amphiphilic substance both hydrophilic and lipophilic groups occur within the same molecule... 

Immiscible solvents like water and oil can be transformed by addition of solubilizers to single-phase solutions. Amphiphilic substances are known as effective solubilizers. Solubilization depends on the HLB of the components that ought to form a single phase and on the kind of solubilizer used. Phosphorus-containing surfactants with their variety of possible molecular structures are solubilizers that can be tailored to the task demanded. 

The main peculiarity of solutions of reversed micelles is their ability to solubilize a wide class of ionic, polar, apolar, and amphiphilic substances. This is because in these systems a multiplicity of domains coexist apolar bulk solvent, the oriented alkyl chains of the surfactant, and the hydrophilic head group region of the reversed micelles. Ionic and polar substances are hosted in the micellar core, apolar substances are solubilized in the bulk apolar solvent, whereas amphiphilic substances are partitioned between the bulk apolar solvent and the domain comprising the alkyl chains and the surfactant polar heads, i.e., the so-called palisade layer [24],... 

Phospholipids are amphiphilic substances i.e. their molecules contain both hydrophilic and hydrophobic groups. Above a certain concentration level, amphiphilic substances with one ionized or polar and one strongly hydrophobic group (e.g. the dodecylsulphate or cetyltrimethylammonium ions) form micelles in solution these are, as a rule, spherical structures with hydrophilic groups on the surface and the inside filled with the hydrophobic parts of the molecules (usually long alkyl chains directed radially into the centre of the sphere). Amphiphilic substances with two hydrophobic groups have a tendency to form bilayer films under suitable conditions, with hydrophobic chains facing one another. Various methods of preparation of these bilayer lipid membranes (BLMs) are demonstrated in Fig. 6.10. 

Attempts have recently been made to link the RNA world with the lipid world. Two groups involved in RNA and ribozyme research joined up with an expert on membrane biophysics (Szostak et al., 2001). They developed a model for the formation of the first protocells which takes into account both the most recent experimental results on replication systems and the self-organisation processes of amphiphilic substances to give supramolecular structures. 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

The interactions of drugs with various amphiphilic substances, which could be used as excipients, are discussed in the next chapter. 

Liquid crystals can broadly be classified into lyotropic and thermotropic. Lyotropic liquid crystals are formed by amphiphilic substances in the presence of a solvent. The... 

These statements lead to the conclusion that the limiting proportion of 1 gram of Na cholate associated to 1 gram of lecithin is simply imposed by the size of a certain form of mixed micelle which can remain in equilibrium with an excess of Na cholate in micellar solution. Thus, it clearly appears that association is governed by the necessity of securing the proper hydrophilic-lipophilic balance of the mixture of two components. Here, as in the case of other amphiphilic substances, by the progressive increase in proportion of the more hydrophilic amphiphile. the association can reach complete micellar dispersion in water. 

The aggregates discussed above are all anisodimensional, which is the reason for the anisotropic character of the mesophases. In some systems it has been possible to prove the existence of isotropic highly viscous phases of similar structure but which clearly consist of almost isodimensional aggregates. The exact structure of these phases is still the subject of discussion, as is also the case with the complex mesophases. The relation between the isotropic phases and globular proteins and plastic crystals of non-amphiphilic substances has been discussed by Gray and Winsor (5). 

T he phase equilibria of ionic surfactants combined with water and an A amphiphilic substance such as a long chain alcohol, carboxylic acid, or ester have been investigated in detail for a long time (1, 2). The nonionic surfactants have not attracted as much interest despite the fact that they are suitable models for illustrating the association conditions which are responsible for the structure and function of biomembranes they also present interesting problems in their temperature dependent interaction with water and hydrocarbons. 

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

Finally there are dynamic methods to measure the surface tension. For example, a liquid jet is pushed out from a nozzle, which has an elliptic cross-section. The relaxation to a circular cross-section is observed. An advantage of this method is that we can measure changes of the surface tension, which might be caused by diffusion of amphiphilic substances to the surface. 

Plots of surface tension versus concentration for n-pentanol [49], LiCl (based on Ref. [50]), and SDS in an aqueous medium at room temperature are shown in Fig. 3.7. The three curves are typical for three different types of adsorption. The SDS adsorption isotherm is typical for amphiphilic substances. In many cases, above a certain critical concentration defined aggregates called micelles are formed (see Section 12.1). This concentration is called the critical micellar concentration (CMC). In the case of SDS at 25°C this is at 8.9 mM. Above the CMC the surface tension does not change significantly any further because any added substance goes into micelles not to the liquid-gas interface. 

The adsorption isotherm for pentanol is typical for lyophobic substances, i.e., substances which do not like to stay in solution, and for weakly amphiphilic substances. They become enriched in the interface and decrease the surface tension. If water is the solvent, most organic substances show such a behaviour. The LiCl adsorption isotherm is characteristic for lyophilic substances. Most ions in water show such behaviour. 

The cation and anion exchangers are amphiphilic substances that are adsorbed at the interface. The latter is then rigid and independent of the droplet diameter, since friction forces are shielded. This is similar to physical extraction systems, in which an analogous behavior is caused by surfactants in the aqueous feed accumulated at the interface. 

This chapter reviews the wide range of colloidal systems amenable to investigation by FT - IR spectroscopy. Molecular level information about die interactions of amphiphilic substances in aggregates such as micelles, bilayers, and gels can be obtained and related to the appearance and stability of the various phases exhibited. The interactions of polymers, surfactants and proteins with interfaces, which substantially modify the solid - liquid or liquid - air interface in many important industrial and natural processes, can also be monitored using FT - IR. 

Dimitrov, K., Gancel, F., Montastruc, L. and Nikov, I. (2008) Liquid membrane extraction of bio-active amphiphilic substances Recovery of surfactin. Biochemical Engineering Journal, 42, 248. 

The dilute solution ( 10-2 M) of the amphiphilic substance, most commonly a long-chain fatty acid, should be prepared in a volatile organic solvent immiscible with the subphase, such as chloroform. The required amount of solution should be carefully transferred dropwise onto the air-subphase interface. It is important not to allow the drops to sink into the subphase, since on one hand it will result in contaminating the subphase, and on the other hand the drowned amphiphile molecules will not become part of the film, thus introducing error into the calculations of area per molecule. The drops of solution should be placed on the surface at large distances from each other, in order to allow the liquid to spread unobstructed. 

Solutions of amphiphilic substances (e.g., surfactants), which have one polar part and one large nonpolar part. These solutions are called association colloids and they can also dissolve other molecules in their interior. They are thermodynamically stable. 

The inverse micellar solubility areas in systems of water, surfactants and a third amphiphilic substance frequently are of a shape according to Fig. l. iZ/ Such shapes are also found in W/0 microemulsions —when water solubility is plotted against cosurfactant/surfactant fraction. 

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. 

Langmuir-Blodgett technique (LB technique) is one of the most promising methods for the preparation of ultra thin ordered multilayer films with uniform thickness.(2) Typical monomeric multilayer films of amphiphilic substances such as long alkyl carboxylic acids are thermally and mechanically unstable. Improvements in the stability have been achieved using polymeric multilayer films which have been... 

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. 

G. Kretzschmar and R. Miller. Dynamic Properties of Adsorption Layers of Amphiphilic Substances at Fluid Interfaces, Adv. Colloid Interface Set 36 (1991) 65. See also R. Miller, G. Kretzschmar, Ibid 37 (1991) 97. 

As the concentration of aqueous solutions of many amphiphilic substances increases, there... 

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