Amphoteric surfactants characterization

FLUORAD Surfactant FC-100 is a general purpose amphoteric surfactant characterized by outstanding surface activity and solubility in aqueous solutions over a wide pH range. It is highly effective in solutions having a neutral pH, particularly those with a high electrolyte content. 

As mentioned above, most commercial products are based on either a lauric (mainly C-12) or a whole coconut distribution (C-8 to C-18, with approximately 50% C-12) since these alkyl distributions give the best detergency. Early on, the imidazoline derived amphoterics were characterized as exceptionally mild to the skin and eyes relative to most surfactants available at the time. This made them excellent candidates for use in baby shampoos, geriatric cleansing products, hand wash for medical facilities and so on. 

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... 

With regard to bioconcentration, it is important that surfactants are characterized by combining a lipophilic and a hydrophilic moiety in the same molecule. This is true for all four classes, namely anionic, nonionic, cationic and amphoteric surfactants. Although these classes possess quite different hydrophilic groups, the lipophilic part usually consists of an alkyl chain or alkyl chains of different lengths. There is some evidence that the lipophilic groups of surfactants are metabolized after uptake by aquatic invertebrate species (Daphnia and Chironomus) and fish. 

Amphoteric surfactants are characterized by a molecular structure containing two different fnnc-tional groups, with anionic and cationic characters, respectively [19]. Most amphoteric surfactants are able to behave like cationic snrfactants in acidic medinm, and like anionic surfactants in alkaline medium. However, betaines are different in that they cannot be forced to assume anionic active behavior through an increase in the pH valne [20,21]. Fignre 12.7 shows strnctnres of the most widely nsed amphoteric surfactants, as prodnced, in dependence of the pH valne. 

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). 

Potentiometric titration with HCl gives a characteristic value proportional to the content of amphoteric surfactant (1,10,11). In some cases, solvent systems have been optimized so that acid-base titration is suitable for assay of the product, as described below for characterization of alkylbetaines (12). 

Although amphoteric surfactants are generally free of Al-nitrosamines, those used in cosmetics may occasionally be analyzed by the method for total nitrosamines described under characterization of nonionic surfactants (Chapter 2, Section II). 

Konig and Waldorf discuss analysis of hair and body shampoos (32). The shampoo is first evaporated to dryness and extracted with isopropanol or 95% ethanol to separate the surfactant, which is qualitatively identified by IR. The alcohol-insoluble residue is also examined by IR in case a less-soluble surfactant remains there. The alcohol extract is then separated into anionic, cationic, nonionic, and amphoteric surfactant fractions by ion exchange. Anionics can be separated into sulfonates and carboxylates by use of strongly basic anion exchange resins in the Cl" and OH" form, respectively. Anionics are further characterized by TLC. Nonionics are likewise characterized by TLC. Once the components are identified, quantitative analysis is by the usual methods, described elsewhere in this volume. 

Levine, L.H. Garland, J.L. Johnson, J.V. HPLC/ESI-Quadrupole Ion Trap-MS for Characterization and Direct Quantification of Amphoteric and Nonionic Surfactants in Aqueous Samples. Anal. Chem. 2002, 74, 2064-2071. 

L.H. Levine, J.L. Garland, J.V. Johnson, LC-ESl-quadrupole ion trap MS for characterization and direct quantification of amphoteric and nonionic surfactants in aqueous samples. Anal. Chem., 74 (2002) 2064. 

Henrich developed a comprehensive TLC method for identification of surfactants in formulations (4). She specified two reversed-phase and four normal phase systems, with detection by fluorescence quenching, pinacryptol yellow and rhodamine B, and iodine. Prior to visualization, one plate was scanned with a densitometer at 254 nm, and UV reflectance spectra were recorded for each spot detected. Tables were prepared showing the Rf values of 150 standard surfactants in each of the six systems, along with the reflectance spectra and response to the visualizers. This system allows for systematic identification of compounds of a number of surfactant types (LAS, alcohol sulfates and ether sulfates, alkane sulfonates, sufosuccinate esters, phosphate compounds, AE, APE, ethoxylated sorbi-tan esters, mono- and dialkanolamides, EO/PO copolymers, amine oxides, quaternary amines, amphoterics and miscellaneous compounds). Supplementary analysis by normal phase HPLC aided in exactly characterizing ethoxylated compounds. For confirmation, the separated spots may be scraped from one of the silica gel plates and the surfactant extracted from the silica with methanol and identified by IR spectroscopy. 

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