Amino acid surfactants amphoteric

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. 

TABLE 4 Amino Acid-Based Amphoteric Surfactants on the Market... 

Amino-acid-based amphoteric surfactants containing a hydroxyl group have been obtained by reacting an amino acid with a perfluoroalkylated epoxide [197] ... 

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. 

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 often called amphoterics to distinguish them from betaines. They are characterized by a hydrophilic group in which the state of ionization depends on the pH, a behaviour which is typical of amino acids. At high pH values, the carboxyl group is... 

Rosenblatt, W., Amino acid amphoterics, in Amphoteric Surfactants, 2nd Edn, Lomax E. G. (Ed.), Surfactant Science Series 59, Marcel Dekker, New York (1996), pp. 49-73. 

This group of amphoteric surfactants is derived from various amino acids, and its members do not possess the hydroxyethyl grouping. Alkylation of the primary amino groups of an amino acid leads to secondary and tertiary amines that, as a rule, are more basic than the original primary amine. In addition, some of the alkyl substituents may carry a second amino group that provides an additional basic center. 

Chloroacetic acid and the dichloroacetic acid present in the reagent used in small amounts are both unwanted by-products in betaines (as well as in other zwitterionic or amphoteric surfactants such as amphoglycinates) because of their toxicity. Chloroacetic acid is almost completely depleted during the carboxymethylation reaction, but it is almost inert under the typical reaction conditions. By submitting betaines to additional posttreatment steps, the residual chloroacetic acids can be reduced. This can be achieved either by reacting at alkaline pH or by additional treatment with ammonia or amino acids that reduces the amount of monochloroacetic acid [27]. 

Recently, Infante et al. [51] synthesized acidic and basic A -lauroyl arginine dipeptides, as methyl esters and free carboxylic acids, from pure amino acids. They found that amphoteric A -lauroyl-l-arginine dipeptide surfactants containing glutamic acid or lysine had no antimicrobial activities. However, the cationic versions of these peptides were antimicrobial, particularly the dipeptide derivatives containing lysine [51]. 

Structurally, amino acid-based surfactants may be depicted as shown nearby. The fundamental stmctures may be considered as (1) N-acylated amino acid, which is essentially an anionic surfactant, or modified as in (b), where the carboxylic group is converted to ester or amide, as seen in cationic surfactants. The structures in (c) and (d) represent those of amphoteric surfactants, in which both amino and carboxyl groups represent hydrophilic moieties. 

A -Acylamino acids with basic amino acids such as lysine and arginine have an amphoteric structure. However, the solubility of the salt of these molecules is too poor to utilize them as a practical surfactant. Several modifications are applied to improve their solubility. Sakamoto et al. [45] reacted Al -acyllysine with ethylene oxide to introduce the polyoxyethylene group, whereas Sagawa et al. introduced additional the Af-methyl group. Both methods led to lysine derivatives with surface activities that were pH dependent (Fig. 18 and 19) because of their amphoteric structures [54-57]. 

This amphoteric surfactant, 0-oleoyl-L-homoserine, could be expected to have some characteristic properties different from those of nonionic food emulsifiers, such as monoglycerides and sugar esters. It may be safe for humans after digestion by pancreatic lipase, for two reasons (1) O-acyl-L-homo-serine is synthesized by an enzymatic process and can be hydrolyzed into fatty acids and L-homoserine by the digestive organs (2) L-homoserine is a naturally occurring amino acid and is distributed in plants, especially in germinating pea seeds [97],... 

In order to produce the amphoteric protein-based surfactant, the incorporation of lipophilic amino acid ester was attempted using the one-step method of plastein reaction with papain at pH 9. In a system containing succinylated ttsi-casein as a protein substrate and luecine n-dodecyl ester as a lipophile, the peptide bond between Phe and Tyr of casein was first hydrolyzed, and this is followed by the incorporation of luecine n-dodecyl ester at the same position, forming a new C-terminus [34]. The structure of the macropeptide with respect to the distribution of hydrophilic amino acid residues is shown in Fig. 4 [29,34]. Amphiphilic structure consisting of hydrophilic protein portion and lipophilic luecine n-dodecyl ester was clearly demonstrated. 

Published results on the toxicological properties of different lipoaminoacids suggest that amino acid-based surfactants are excellent nontoxic surfactants for a great variety of potential applications [33-36]. Ocular irritation by Ci2ArgGly, CijArg pp, LAM, and lauryl dimethyl amino betaine (LDAB), a commercial amphoteric surfactant, has been evaluated in male albino rabbits by a double-blind method. The eyes were examined after 24 h of administration and then each day for a week. The ocular damage was evaluated in accordance with the criteria of Draize [37]. 

Protein-based surfactants are composed of two naturally occurring moieties, amino acid and fatty acid such as acylglutamate. Acylglutamates by themselves are not ultramild surfactants by any means. Their in vitro Zein values are higher than those of alkyl polyglycosides, alkylamphoacetate, amphoteric... 

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