Amphoteric surfactants general

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). 

An unknown commercial detergent may contain some combination of anionic, nonionic, cationic, and possibly amphoteric surfactants, inorganic builders and fillers as weU as some minor additives. In general, the analytical scheme iacludes separation of nonsurfactant and inorganic components from the total mixture, classification of the surfactants, separation of iadividual surfactants, and quantitative determination (131). 

Protease performance is strongly influenced by detergent pH and ionic strength. Surfactants influence both protease performance and stabiUty in the wash solution. In general, anionic surfactants are more aggressive than amphoteric surfactants, which again are more aggressive than nonionic surfactants. 

Surfactants can be divided into four general areas cationic surfactants, anionic surfactants, nonionic surfactants, and amphoteric surfactants. Major anionic surfactants are soaps, linear alcohol sulfates, linear alcohol ethoxy-sulfates, and linear alkylbenzenesulfonates. 

This group of surfactants is discussed first because they are among the oldest amphoterics in commerce with patents dating back to 1949 and because they best illustrate the amphoteric surfactants. The first products introduced to the market in this category were the Deriphat [1] products introduced by General Mills Chemicals. 

Most of the amphopropionate surfactants produced are of the amphodipropionate type, 2 mol of methyl acrylate or sodium acrylate added per mole of imidazoline. Depending on the reaction conditions, 1 mol of acrylate can add to the fatty R group at the alpha carbon. Upon hydrolysis of the imidazoline, the second reacts with the liberated secondary amine to produce the beta alanine derivative. If methyl acrylate is used, the methyl ester of the amphoteric surfactant is formed. An equimolar amount of sodium hydroxide is added to effect saponification to the sodium salt of the surfactant. Methanol is formed as a by-product and it is generally left in the final product as part of the solvent system. 

In addition, because they are generally based on aminoacid structures, they are among the most biodegradable surfactants available to the formulator. Products based on amphoteric surfactants are usually readily biodegradable, thus having a minimal impact on the environment. 

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. 

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). 

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). 

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. 

Anionics are the largest class of surfactants in terms of volume, and include the work-horse surfactants, linear alkylbenzene sulfonate (LAS), alcohol sulfate (AS) and alcohol ether (or ethoxy) sulfate (AES). Cationic surfactants generally include various quaternary salts, used predominantly as fabric conditioners ( fabric softeners ), anti-static agents and anti-microbial agents. Amphoteric surfactants represent the smallest class of surfactants, and generally are used when solubility, mildness and compatibility issues are important. 

Zwitterionic and amphoteric surfactants contribute to the overall generation of foam, foam stability and foam quantity. Regardless of the type of amphoteric, a generalization can be made concerning the hydrophobic R group The total number of carbon atoms in the R group has a profound effect upon performance. 

It is generally known that aqueous solutions of true amphoterics can be difficult to thicken. Viscosity control is best achieved by using either the amphoteric salts or by combining with anionic surfactants. The traditional thickening aids, the alkanolamides, are not particularly effective with amphoterics. Nonionic surfactants offer the best thickening support for amphoteric surfactants, especially those based on fatty acids or alcohols ethoxy-lated with 50-200 moles of ethylene oxide, but like all nonionics, they could exert a foam depressing effect if used at a higher level. When amphoteric surfactants are combined with anionic surfactants, the traditional alkanolamides are effective. The final pH adjustment can also make a difference to the viscosity of the product. 

Amphoteric surfactants are finding increased use in shampoo formulations because of several benefits that are consumer perceivable. Amphoteric surfactants in general are known to be poor foamers when used alone,and milder than traditional primary surfactants. Their use therefore is in many baby shampoos that often consist entirely of them, and also they are often incorporated in formulations to mitigate the effects of harsher primary surfactants. " Amphoteric surfactants also maintain their compatibility with all anionic, cationic, and nonionic surfactants over a wide pH range. Combination of these surfactants with other anionic surfactants provides decreased irritancy of a formulation while increasing the active content level of the product and therefore the quality of the lather produced. 

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