Amphoteric surfactant molecule

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

As mentioned in Table 8.1, amphoteric surfactants contain both an anionic and a cationic group. In acidic media they tend to behave as cationic agents and in alkaline media as anionic agents. Somewhere between these extremes lies what is known as the isoelectric point (not necessarily, or even commonly, at pH 7), at which the anionic and cationic properties are counterbalanced. At this point the molecule is said to be zwitterionic and its surfactant properties and solubility tend to be at their lowest. These products have acquired a degree of importance as auxiliaries in certain ways [20-25], particularly as levelling agents in the application of reactive dyes to wool. 

A surfactant can be grouped in one of the four classes - anionic, nonionic, cationic and amphoteric surfactants, depending on what charge is present in the chain-carrying hydrophilic portion of the molecule after dissociation in aqueous solution. Tab. 4.1 shows examples of surfactants most commonly used for detergents. 

The two distinctive affinities in the surfactant molecule mentioned above serve as the basis for the commonly accepted definition of surfactant groups. According to the charge of their hydrophilic moiety, surfactants can be classified into four categories anionic, non-ionic, cationic and amphoteric. 

An L-B film is formed by the dispersion of amphoteric molecules at an air-water surface (Figure 8.20). These molecules have a polar group at one end, something like a carboxy substituent (in this respect they resemble the surfactant molecules which make micelles), and a long non-polar aliphatic chain. The polar group stays in the polar water phase, and the aliphatic chain stays in the non-polar air environment. The L-B film at the water surface is then made by the controlled compression of these molecules by means of a floating barrier. The molecules then line up to form a mono-molecular layer on the water surface. 

Amphoteric surfactants contain both an acidic and basic hydrophilic group. Ether or hydroxyl groups may also be present to enhance the hydrophilicity of the surfactant molecule. Examples of amphoteric surfactants include amino acids and their derivatives in which the nitrogen atom tends to become protonated with decreasing pH of the solution. Amino acid salts, under these conditions, contain both a positive and a negative charge on the same molecule. 

A surfactant molecule that contains negatively and positively charged groups. Example lauramidopropylbetaine, C11H23CONH(CH2)3N+(CH3)2CH2COO-, at neutral and alkaline solution pH. See Amphoteric Surfactant. 

Amphoteric surfactants carry both a positive and a negative charge in the organic part of the molecule. They still have a long hydrocarbon chain as the hydrophobic tail and behave as anionic surfactants or cationic surfactants, depending on the pH. 

The main characteristics of amphoteric surfactants is their dependence on the pH of the solution in which they are dissolved. In acid solutions, the molecule acquires a positive charge and it behaves like a cationic, whereas in alkaline solutions they become negatively charged and behave like an anionic. A specific pH can be defined at which both ionic groups show equal ionization (the isoelectric point of the molecule). 

Adsorption can be measured by direct or indirect methods. Direct methods include surface microtome method [46], foam generation method [47] and radio-labelled surfactant adsorption method [48]. These direct methods have several disadvantages. Hence, the amount of surfactant adsorbed per unit area of interface (T) at surface saturation is mostly determined by indirect methods namely surface and interfacial tension measurements along with the application of Gibbs adsorption equations (see Section 2.2.3 and Figure 2.1). Surfactant structure, presence of electrolyte, nature of non-polar liquid and temperature significantly affect the T value. The T values and the area occupied per surfactant molecule at water-air and water-hydrocarbon interfaces for several anionic, cationic, non-ionic and amphoteric surfactants can be found in Chapter 2 of [2]. 

N-alkyl (3-alanine derivatives, the monopropionates , have distinct isoelectric points while most other amphoteric surfactants do not. The isoelectric point is the pH at which the molecule is internally neutralized, existing as a zwitterion. Aqueous solubility and the propensity to foam are lowest at the isoelectric point. 

These surfactants contain both cationic and/or anionic groups on the same molecule. Amphoteric surfactants are relatively expensive due to the raw material and processing costs. Typical applications include liquid... 

Amphoteric surfactants. They include both acidic and basic groups in the same molecule. 

Amphoteric surfactants. These compounds have the characteristics of both anionic detergents and cationic fabric softeners. They tend to work best at neutral pH, and are found in shampoos, skin cleaners, and carpet shampoos. They are very stable in strong acidic conditions and are favorably used with hydrofluoric acid. For example, compounds of alkyl-betain or alkylsulfobetaine type possess both anionic and cationic groups in the same molecule even in aqueous solution. These surfactants are rarely employed in laundry detergents because of their high costs [3, 4]. 

Amphoteric Surfactants. Amphoteric surfactants in aqueous solution contain both positive and negative charges in the same molecule. Thus, a hydrophobic fatty chain is attached to a hydrophilic group that contains both positive and negative charges. Its behavior depends on the condition of the medium or its pH value. Examples of this type are the alkyl betaines. 

Amphoteric Surfactant A surfactant molecule for which the ionic character of the polar group depends on solution pH. For example, lauramidopropylbetaine, CnH23CONH(CH2)3N (CH3)2CH2COO", is positively charged at low pH but is electrically neutral, having both positive and negative charges at intermediate pH. Other combinations are possible, and... 

These compounds are permanently anionic and are moderately polar (surfactants are organic molecules that are surface active). This means that they concentrate on the surface of a liquid in which they are dissolved. Generally, these types of analytes contain both a hydrophobic and a hydrophilic segment. There are anionic, cationic, neutral, and amphoteric surfactants. They may be readily sorbed from water by reversed-phase SPE. Elution requires methanol or acetonitrile rather than ethyl acetate because of their polar, ionic functional groups, which are typically sulfate esters or sulfonic acids (Fig. 7.18). 

Amphoteric surfactants are sometimes referred to as 2witterionic molecules. They are soluble in water, but their solubility shows a minimum at the i.e.p. Amphoterics show excellent compatibility with other surfactants, forming muKd micelles they are also chemically stable both in acids and alkalis. The surface activity of amphoterics varies widely, and depends on the distance between the charged groups. Amphoterics display a maximum in surface activity at the i.e.p. 

The tested amphoteric surfactants (betaine and sulfobetaine) are highly salt-tolerant and are excellent foamers. Adsorption of these surfactants is less dependent on salinity than adsorption of anionic surfactants, and the trends are not monotonic (Figure 13b). Adsorption of amphoteric surfactants may proceed by a complex interplay of mechanisms involving electrostatic and complexation mechanisms of both the cationic and the anionic group in the surfactant molecule (12, 87, 88). The trends in ad-... 

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