Amphiphilic compound definition

Amphiphilic compounds as surfactants, 21 Amphoteric surfactant, definition, 386 Analysis speed, importance, 125 Anionic surfactants definition, 387... 

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. 

Nonionic surfactants are amphiphilic compounds the lyophilic (in particular hydrophilic) part of which does not dissociate into ions and hence has no charge. However, there are nonionics, for example such as tertiary amine oxides, which are able to acquire a charge depending on the pH value. Even polyethers, such as polyethylene oxides, are protonated under acidic conditions and exist in cationic form. Long-chain carboxylic acids are nonionic under neutral and acidic conditions whereas they are anionics under basic conditions. So, the more accurate definition is as follows nonionics are surfactants that have no charge in the predominant working range of pH. The main part of nonionics can be classified into alcohols, polyethers, esters, or their combinations. 

At least two of the recognized mechanisms for the formation of electrical double layers (Hunter, elal. 1981 Russel etal., 1989) are relevant to LB film depositions (1) ionization of carboxylic acid group and amphoteric acid groups on solid surfaces, and (2) differences between the affinities of two phases for ions or ionizable species. The latter mechanism includes the uneven distribution of anions and cations between two immiscible phases, the differential adsorption of ions from an electrolyte solution to a solid surface, and the differential solution of one ion over the other from a crystal lattice. Since the solid-liquid and the film-liquid interfaces are flat, large surfaces and since both have a large, solid-like concentration, the analysis that follows applies to both interfaces. For an interface conformed by a thin film of an amphiphilic compound with the hydrophilic end of the molecule in contact with the water subphase, the equilibrium of charges is based on pH and subphase concentration. The effect of pH is highlighted by the definition of the of the carboxylic acid ... 

There is some disagreement within the surfactant literature as to the exact definition of solubilization, particularly as the ratio of surfactant to additive decreases, and one approaches the nebulous frontier between swollen micellar systems and the micro- and macroemulsion regions. For present purposes, solubilization will be defined as the preparation of a thermodynamically stable, isotropic solution of a substance (the additive ) normally insoluble or only slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. By the use of such a definition, a broad area can be covered that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation and hydrotropes. It does not, however, limit the phenomenon to any single mechanism of action. 

An interesting consequence of this definition is that it can be used to define univocally surface excess amounts. As shown in Figure 2.3, where the case of an amphiphilic compound adsorbed at a liquid/liquid interface is considered, it can be defined as the algebraic sum of the hatched zones. Classically this quantity is denoted by Pj" for a compound with reference to solvent 1, The unit for P is moles per square meter (mol m ). 

Hydrotropes are a class of compounds widely used in industrial applications for their outstanding ability to enhance the aqueous solubility of hydrophobic compounds and surfactants. Very different chemical families can lead to a hydrotropic behaviour and the hydrotropy phenomenon has not been clearly elucidated so far. No definite structure-properties relationships have been established either. The common feature is that hydrotropes exhibit amphiphilicity and can be regarded as weak surfactants, with some similarities in their behaviours in water. This chapter focuses on hydrotropes obtained from sugars and polyols, such as glycerol, sorbitol and isosorbide, which represent relatively new and promising alternatives to the currently used hydrotropes from petroleum origin. Emphasis is put on the preparation of such amphiphiles and on their specific physico-chemical properties. 

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