Isoelectric point, amphoteric surfactants

The term amphoteric (from amphos, meaning both) was first used for surfactants in the 1940s by H. Mannheimer (5). Amphoterics are substances which can have - like an amino acid - anionic, cationic or zwitterionic properties. These three forms exist in an equilibrium, depending on the pH range (Figure 15.1). At an acidic pH, the molecules will be protonated to form cations, while at an alkaline pH they will be depro-tonated to form anionic species. Only in a mid-pH range can they exist as neutral molecules and demonstrate their zwitterionic character. This pH is called the isoelectric point. Amphoteric behaviour requires the presence of a secondary or tertiary amine group, which can be protonated easily. 

Amphoteric surfactants are sometimes referred to as zwitterionic molecules. They are soluble in water, but the solubility shows a rninimum at the isoelectric point. Amphoterics show excellent compatibility with other surfactants, forming mixed micelles. They are chemically stable both in adds and alkahs. The surface activity of amphoterics varies widely and depends on the distance between the charged groups, showing maximum activity at the isoelectric point. 

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. 

Even if this class covers the smallest market segment, amphoteric surfactants still remain useful because of their unique properties, which justifies their comparably high manufacturing costs. Since they have partial anionic and cationic character, they can be compatible, under specific conditions, with both anionic and cationic surfactants. They can function in acid or basic pH systems and, at their isoelectric point, they exhibit special behaviour. Many amphoteric surfactants demonstrate exceptional foaming and detergency properties combined with antistatic effects. 

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). 

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. 

Betaine-type surfactants do not show anionic character in alkaline solutions or reduced water solubility close to the isoelectric point as other amphoterics do. These surface-active betaines generally exhibit excellent solubility in water and compatibility with anionics over a wide pH range. So, the betaines are often specified as zwitter-ionic surfactants. 

Amphoteric surfactants are surfactants that, depending on the pH, can have either anionic or cationic properties. They also have an isoelectric point at which they possess zwitterionic character. These surfactants are typically compatible with anionic, nonionic, or cationic surfactants. The structure and properties of two amphoteric surfactants used in hand dishwashing formulations are shown in Figure 3.3. 

Amphoteric surfactants are divided into two groups based on their response to pH. One class of compounds that contain carboxylated imidazoles and A-alkyl betaines are zwitterionic at pHs at, and above, their isoelectric points and cationic at lower pHs. Sulfobetaines and phosphobetaines, on the other hand, exhibit zwitterionic characteristics as the anionic portion is dissociated at all pHs. Although the former are the major ingredients in many baby shampoos or those products that provide mildness, they also function as foam and viscosity enhancers when utilized as secondary surfactants, having good water solubility over a wide pH range. ° ... 

Surfactancy. Amphoteric snrfactants have the ability to reduce the surface tension of water, as do all other snrfactants. The reduction of surface tension depends on the pH of the solution. At pH 6, close to the isoelectric point of most amphoteric surfactants, the net charge of the molecule is zero, which allows for closer packing of surfactant molecules at the interface and prodnces a great reduction in surface tension. 

Hidaka et al. [77] reported that amphoteric Af-(2-hydroxyethyl)- -(2-hydroxyalkyl)-P-alanines s cmc values greatly depend on the nature of the electrolytes added to its nearly neutral aqueous solution, and that the cmc value decreased in the following order NaCl > CaClj > Na2S04. Also, their calcium stability is superior to that of A-dodecyl-P-alanines. The effect of pH on this amphoteric amino acid surfactant was smdied in the presence of 0.1 M NaCl [78] and the results showed that the cmc value increased on the acidic side below the isoelectric point pi = 6.8 and remained almost unchanged on the alkaline side. Examination of the configuration with molecular models indicates that the cationic ionization of the amino group on the acidic side probably takes place within the micelle, whereas under alkaline circumstances the anionic ionization of the carbonyl group occurs on the micellar surface. This makes the electrostatic potential for ionization different on the acidic and alkaline sides. 

The change in charge with pH of amphoteric surfactants affects their properties, such as wetting, detergency, foaming, etc. At the isoelectric point (i.e.p.), the properties of amphoterics resemble those of non-ionics very closely. Below and above the i.e.p. the properties shift towards those of cationic and anionic surfactants, respectively. Zwitterionic surfactants have excellent dermatological properties. They also exhibit low eye irritation and are frequently used in shampoos and other personal care products (cosmetics). 

Amphoteric surfactants are characterized by the fact that these surfactants can carry a positive charge on a cationic site and a negative charge on an anionic site. The use of amphoteric terminology is still restrictive The charge of the molecule must change with pH, showing a zwitterionic form at an intermediate pH (i.e., around the isoelectric point). 

These surfactants show amphoteric properties and the zwitterionic form appears around neutral pH the water solubility is minimal at the isoelectric point. 

Amphoteric surfactants by definition are chimeric, exhibiting anionic character in alkaline solution, nonionic character near their isoelectric point, and cationic character in acidic solution [73]. As a result of their complex charge characteristics, their interactions with interfaces must be examined iudividually and as a function of pH. For example, the adsorption of alkyl betaines firom solution onto wool keratin is much greater at acidic than alkalide pH values. Although amphoteric surfactants are used extensively to improve the cosmetic attributes of many consmner products [74], their interactions with skin have received little attention. 

Amphoteric surfactants— have at least one anionic and one cationic group at their isoelectric point. 

Amphoteric fluorinated surfactants have at least one anionic and at least one cationic group at about their isoelectric point [181]. The cationic group is usually a quaternary amine or a protonated tertiary or secondary amine. The anionic functionality is a carboxylate, sulfonate, sulfate, or phosphate group. Most widely used fluorinated surfactants are carboxybetaine-type amphoteric surfactants which have, like betaines, a cationic amine functionality and an anionic carboxylate group. Instead of a carboxylate group, the anionic component of sulfatobetaines is a sulfate group and that of sulfobetaines is a sulfonate group. 

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