Amphoteric and Zwitterionic Surfactants

An amphiphilic compound is named amphoteric surfactant if its functional group is capable of carrying both anionic and cationic charges. The term amphoteric surfactants or 

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It is important to note that cationic quaternary compounds (monoquats) are rarely used alone in two-in-one shampoos because their substantivity to hair is either greatly reduced in the presence of anionic detergents [102] or else they form undesired precipitates with anionic surfactants. Therefore, combinations of anionic, nonionic, amphoteric, and zwitterionic surfactants are often employed to minimize the formation of insoluble complexes. 

Amphoteric surfactants can function either as anionic or cationic surfactants, depending on the pH of the system. They contain both anionic and cationic functions in the same moiecuie. More costiy to produce than ionic surfactants, amphoteric surfactants represent oniy about 3% of surfactant voiume in Europe and less than 1% in the United States. They are less irritating than other materials and are largely used in personal care products. A distinction can be made between amphoteric and zwitterionic surfactants. This distinction does not affect their analysis. For the analyst, a more important distinction is between am-photerics with a secondary or tertiary amine group and those containing a quaternary amine function. The former only have cationic properties when protonated at low pH, while the quaternary amines have cationic properties even under alkaline conditions. 

Amphoteric (or zwitterionic) surfactants carry a positive and a negative charge so that the net charge is zero. Some lipids, such as phophatidylcholine, are zwitterionic. Since lipids are practically not soluble in water, in a strict sense they do not belong to the class... 

The shape, size, and structure of these dispersed droplets depend upon a multitude of variables including the surfactant type, ionic strength, the presence of cosurfactant(s), and the amount of Avater. Commonly used surfactants are of the five general categories anionic, cationic, nonionic, amphoteric and zwitterionic. The nature of amphoteric surfactants, i.e., whether or not they behave as anionic or cationic species, is dependent on the pH or ionic strength of the aqueous phase. 

Zwitterionic surfactants have positive and negative charges on the head group. Amphoteric surfactants have a head group with a pH-dependent charge. The amine oxide shown in Fig. 3 is zwitterionic at high pH, but becomes cationic as protonation occurs at low pH. Because amphoteric surfactants are generally zwitterionic at some pH. and zwitterionic surfactants are often amphoteric, in practice, the terms zwitterionic and amphoteric are used as synonyms, and the term ampholytic is used to describe both surfactant types. 

Extra mild surfactants (nonionic) polyethoxylated sorbitan ester Combination with amphoteric or zwitterionic surfactants cocamidopropyl betaine Surfactants/glycol soap agents Cationic surfactants Cationic polymers Silicones and derivatives... 

Some compounds can be analyzed in both positive-ion and negative-ion mode. Because of the occurrence of intense alkah-metal cationization, it is sometimes preferred to perform the analysis in negative-ion mode. This was done for the analysis of the amphoteric or zwitterionic surfactant cocamidopropylbetaine (CAPB), which in the negative-ion mode predominantly shows [M-H], whereas in the positive-ion mode a range of ions is observed, inclnding monomeric, dimeric, and trimeric protonated, sodiated and potassiated molecnles, mixed double-charge ions, and a fragment ion [121]. 

Up to this point we have covered three of the four general classes of surfactants defined so far anionic, nonionic, and cationic. To produce an example of the fourth class, an amphoteric or zwitterionic surfactant, it is only necessary to react the dodecylchloride prepared as described above with a difunctional material such as A, A -dimethyl-3-aminopropane-l-sulfonic acid (path 8) ... 

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. 

The hydrophilic portion of a surfactant may carry a negative or positive charge, both positive and negative charges or no charge at all. These are classified respectively as anionic, cationic, amphoteric (or zwitterionic ) or non-ionic surfactant. 

There are an enormous variety of commercial emulsifiers that are employed in emulsion polymerization. Emulsifiers are generally categorized into four major classes anionic, cationic, nonionic and zwitterionic (amphoteric). The anionic and nonionic emulsifiers are the most widely used. In addition, mixtures of emulsifiers are also often used. Since the effects of the molecular structme and chemical and physical properties of an emulsifier on particle formation are still far from being well understood, numerous experimental investigations on particle formation have been carried out to date with various nonionic emulsifiers [99-102], mixed emulsifiers (ionic and nonionic emulsifiers) [18,103-106] and reactive surfactants [33, 107-110]. Recently, polymeric surfactants have become widely used and studied in emulsion polymerizations [111-116]. A general review of polymeric surfactants was published in 1992 by Piirma [117]. Recently, emulsion polymerization stabilized by nonionic and mixed (ionic and nonionic) emulsifiers was reviewed by Capek [118]. 

Zwitterionic Surfactant A surfactant molecule whose polar group contains both negatively and positively charged groups. Example lauramidopropylbetaine, CnH23CONH(CH2)3N (CH3)2CH2COO" at neutral and alkaline solution pH. See also Amphoteric surfactant. 

The change in charge with pH of amphoteric surfactants affects their properties, such as wetting, detergency, and foaming. Atthe i.e.p., the properties of amphoterics resemble those of nonionics very closely, but below and above the i.e.p. the properties shift towards those of cationic and anionic surfactants, respectively. Zwitterionic surfactants have excellent dermatological properties, and also exhibit low eye irritation consequently, they are frequently used in shampoos and other personal care products (e.g., cosmetics). 

Anionic, cationic, amphoteric, zwitterionic surfactants thickening agents benefit agents (silicones, fats and oils, vitamins, plant extracts, sunscreens, alkyl lactate, essential oils, etc.) small amount of soap... 

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. 

Zwitterionic surfactants, also known as amphoteric surfactants, have both cationic and anionic centers present in the same molecule. Generally, their properties are highly dependent on the pH of the solution. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. The anionic part can be more variable and include sulfonates, as in CHAPS (3-[(3-Cholamidopropyl)dimethylammonio]-l-propanesulfonate). Other anionic groups are sultaines illustrated by cocamidopropyl hydroxysultaine. Examples of zwitterionic surfactants include betaines (such as cocamidopropyl betaine, dodecyl betaine, lauramidopropyl betaine and cocoamido-2-hydroxypropyl sulfobetaine) and phosphates (such as lecithin). 

The betaines, especially the coconut-oil-derived cocamidopropyl betaines, are the most important zwitterionic surfactants. The name betaine refers to trimethylglycine, which was first isolated from sugar beet beta vulgaris). Cocoamphomonoacetate and cocoamphodiacetate are the most important types for the amphoteric surfactants. The structures and names for both product types are summarized in Table 15.1. 

Amphoteric surfactants have a special application profile which favours their use mainly in cosmetics. In recent years, they have also found increasing application in the development of dishwashing agents or household cleaners. Compared to the amounts produced world-wide of anionic and nonionic surfactants, the volume of amphoterics is still relatively small. For cosmetic products, their consumption in Europe in 1992 was about 15 000 t. The main carbon chain includes the Cg-Cig range. The zwitterionic character of amphoterics strongly influences their particular behaviour, i.e. both anionic and cationic... 

Note here that though the terms zwitterionic and amphoteric are often used as a single entity, amphoteric is the more general term, including zwitterionic surfactants. 

As mentioned in Chapter 1, there can be thousands of molecules with polar heads and nonpolar tails, usable as surfactants. The applications are also many. Thus, the commercial anionic surfactants, recording about 50% of all surfactant production, are literally used all over the place shampoos, dishwashing detergents and washing powders are some common examples. Cationic surfactants likewise are used in hair-conditioners, fabric softeners, asphalt coating, corrosion inhibitor formulations for metal surfaces etc. The major applications of non-ionic surfactants are in the areas of food and drinks, as also pharmaceuticals and cosmetics. Amphoteric /zwitterionic surfactants have only limited applications one area is cosmetics, especially skin care products. 

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