Amphoteric Dyes

In 1862 Nicholson sulfonated basic dyes and thus created the first amphoteric and acid leather dyes. Triaiylmethane (triarylmethine) dyes (Table 5.3) and their different uses are typical examples of this development. 

Aniline blue/blue T base methine Solvent Blue 23 solvent 

Alkaline blue carbinol base, one sulfo group Acid Blue 119 silk 

Sea blue/ reflex blue B two sulfo groups Acid Blue 48 wool, paper 

Ink blue three sulfo groups Acid Blue 93 ink, leather 

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Dyes are usually divided into water-soluble and water-insoluble groups. The water soluble group is further divided into anionic (acid) and cationic (basic) dyes. Gun 3 provides a comprehensive chart of the synthetic organic dyes. However, he does not address the resonant amphoteric dyes. The specific placement of the Rhodonines within his chart requires additional analysis and probably laboratory measurements to determine their relative level of amphoterism. The chart does not specifically delineate resonant dyes such as the Rhodonines. 

FIGURE 17.7 Computer-simulated distributions of the 140 carrier components and the three dyes for the pH 5-8.5 gradient system after 10, 1000, and 10,000 min of constant voltage application. The numbers refer to the pi values of the dyes and the arrowheads mark their locations. Successive graphs are presented with a y-axis offset of 30 mM. The insets a and b depict the concentration profiles of the pi 6.6 and 7.4 amphoteric dyes and the pH profiles, respectively, at the indicated time points. Simulations were performed with Ax = 50 xm and having column ends that are impermeable to any sample and carrier compounds. (Modified from Mosher, R.A. and Thormann, W., Electrophoresis, 23, 1803, 2002. With permission.)... 

Analytical flotation, i.e. shaking out with organic liquids, may be used to advantage when colored or colorless precipitations are conducted in a test tube. Minute amounts of a colored precipitate produced in a colored solution can often be made visible if the test is made as a drop reaction on filter paper or, in case this is not feasible because of the need of warming, etc., a drop of the suspension (after dilution if necessary) can be placed on filter paper. The precipitate will be retained in the center of the fleck while the colored liquor diffuses away through the capillaries of the paper and can be effectively separated from the precipitate by repeated treatments with one to three drops of water. However, it must be remembered that not all colored solutes may be washed away in this manner, because certain soluble colored substances (usually with colored anions) are irreversibly adsorbed on filter paper. Examples include the red solutions of complex ferrous salts of a,a -dipyridyl and phenanthroline, and likewise solutions of acid or amphoteric dyes (for instance rhodamine dyes). 

Aminophenols and their derivatives are of commercial importance, both in their own right and as intermediates in the photographic, pharmaceutical, and chemical dye industries. They are amphoteric and can behave either as weak acids or weak bases, but the basic character usually predominates. 3-Aminophenol (2) is fairly stable in air unlike 2-aminophenol (1) and 4-aminophenol (3) which easily undergo oxidation to colored products. The former are generally converted to their acid salts, whereas 4-amiaophenol is usually formulated with low concentrations of antioxidants which act as inhibitors against undesired oxidation. 

The incorporation of metal salts of amphoteric surface active agents (Mostat Series) as internal antistatic agents in polypropylene fibers has been reported (95). Metal salts of alanine, amidoamine, and imida2oiine-type amphoteric surface-active agents show excellent performance as internal antistatic agents and also improve the dyeing abiUty of the fibers with acid dyes. 

ACID DYES Commercial acid dyes contain one or more sulfonate groups, thereby providing solubility in aqueous media. These dyes are apphed in the presence of organic or mineral acids (pH 2—6). Such acids protonate any available cationic sites on the fiber, thereby making possible bonding between the fiber and the anionic dye molecule. Wool, an animal fiber, is an amphoteric coUoid, possessing both basic and acidic properties because of the amino and carboxylic groups of the protein stmcture. In order to dye such a system, coulombic interactions between the dye molecule and the fiber must take place ie, H2N" -wool-COO + H2N" -wool-COOH. The term acid dye is appHed to those that are capable of such interactions. Acid dyes... 

Acid dyes used for coloring animal fibers via acidified solution (containing sulfuric acid, acetic acid, sodium sulfate, and surfactants) in combination with amphoteric protein. 

Any hydrophobe can yield each of the main (i.e. anionic, cationic, nonionic or amphoteric) types of surfactant in much the same way as the same chromogenic system can be used in anionic, basic or disperse dyes. This will be demonstrated in the following sections, dealing with each class of surfactant, using the cetyl-containing (C16H33) hydrophobe. 

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. 

Wool and silk are protein-like substances and hence are amphoteric. Accordingly, they can combine with acids as well as with bases. For this reason wool and silk can be dyed directly by dyes in virtue of their auxochromic groups. 

During the appropriate tests phthalic anhydride was smoothly esterified with 2-ethylhexanol in the mole ratio 1 4. The reaction temperature rose to 200°C. after about 15 minutes, then rose slowly up to 210°C. till the end of the esterification. Table I shows the esterification time in hours after 99.9% conversion, while continuously removing the reaction water, according to the acid number relative to monooctyl phthalic acid. Furthermore, it indicates the ester colors according to the iodine scale in milligrams iodine/100 ml., whereby 1 mg. iodine/100 ml. can be more or less compared with a dye number of 120 APHA. Since, the values are obtained with the same starting materials, it is possible to compare the color numbers. They show that with the same times, the amphoteric catalysis achieves better colors than in the reaction course containing catalytically effective acids. 

In the afterchroming method with chrome-developing dyes, the dyeing liquor is prepared with formic acid (pH 3.5-3.8), sodium sulfate, and an amphoteric leveling agent as a wool protectant. The process is started at 40 °C, and dissolved dye is added. The system is heated, and dyeing is performed at 90°C for 30-... 

To improve leveling a nonionic or amphoteric auxiliary agent is used. In the case of fiber-affinitive agents, the sum of the dye anions and auxiliary anions should not exceed the binding capacity of the fiber to avoid blocking effects [105], The use of computerized dosing pumps for dyes and acid permits reliable control of dye sorption [105a]. 

The adsorption of dyes to proteins has been studied extensively in the textile industry241. A critically important fact is that acid dyes do not dye cotton242. They can dye wool, silk and other natural proteins. Rattee Breuer have provided considerable information concerning the mechanism of dye interaction with proteins, including amphoteric proteins. Their Chapter 5 discusses the molecular nature of proteins. They have also addressed the subject of hydrogen bonding between dyes and proteins. 

H. Oda, Photostabilisation of photochromic materials contribution of amphoteric counterions on the photostability of spiropyrans and related compounds, Dyes Pigm., 23, 1-12 (1993). 

Compatibilizer for simultaneous dyeing of anionic and cationic dye systems—amphoteric. 

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