Complexing Agents

In document 23550908 Laundry Detergents (Page 74-79)

3. Detergent Ingredients

3.2.2. Complexing Agents

Soda ash induces precipitation of calcium and magnesium salts from wash water.

This can lead to formation of lime deposits on both laundry and washing machine parts. By contrast, sequestering agents form stable, water-soluble complexes with alkaline-earth ions, as well as with traces of heavy-metal present in water. Often the resulting complexes are chelates (Fig. 45). Sodium triphosphate forms a stable water-soluble complex with calcium when their stoichiometric ratio is 1 : 1 (Fig. 45). However, when a substoichiometric quantity of sodium triphosphate is present, the water-insol-uble dicalcium triphosphate is formed, which precipitates on textile fibers and washing machine parts. Its precipitation can be impeded by adding small amounts of hydroxy-ethanediphosphonate and/or special polycarboxylates to the detergent formulation [78].

Temperature and complexing agent concentration are generally the decisive factors in successful elimination of polyvalent metal ions. Table 13 shows the calcium binding capacity of various sequestering agents as a function of temperature. Calcium binding

Figure 44. Comparison of soil removal (given as remission R in %) on permanent-press cotton at 0 d and 90 C for sodium triphosphate- and so-dium carbonate-containing detergents, formulated on the basis of

a) 40 % Sodium triphosphate; b) 20 % Sodium triphosphate + 20 % sodium sulfate; c) 40 % So-dium carbonate

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capacity is here to be viewed as a quantitative measure of the stoichiometry of the resulting complexes. For most sequestering agents, this capacity decreases markedly with increasing temperature.

Regarding stability, the data presented show only that the stability constants (which are a function of the method of study employed, e.g., the dissolving power for freshly precipitated calcium carbonate) exceed a specific value that is dependent on the solu-bility of the calcium carbonate. Extremely high stasolu-bility constants or stabilities, which would result in very low calcium ion concentrations, are not required; indeed, they are normally undesirable. It is important that those salts present to the greatest extent, such as calcium carbonate or others with even higher solubility products, be prevented from precipitating during the washing process. Less soluble calcium salts normally play a minor role.

Relative to other polyvalent ions, sequestration of alkaline earth ions is of primary concern because these ions are likely present in high concentration in tap water.

Nevertheless, heavy-metal ions must also be eliminated because even in trace amounts their presence can have a negative effect on the washing process. For this reason, low concentrations of selective complexing agents are generally added as well, usually specific phosphonic acids, e.g., sodium hydroxyethanediphosphonate or sodium diethy-lenetriaminepentakis(methylenephosphonate).

If a sequestering agent is present in less than stoichiometric amounts relative to polyvalent metal ions, precipitation of carbonates and insoluble salts of the sequester-ing agent with the ions caussequester-ing the water hardness usually results. Even with adequate amounts of sequestering agent, this effect is important because dilution during the rinse cycle can cause sequestrant concentration to drop below the necessary value, thereby permitting undesirable precipitates to form. These can build up on both fabric and washing machine components, leading eventually to serious accumulations. The prob-lem can be particularly severe if conditions permit large crystals to form as a result of seed crystals located on fabric or machine components.

Figure 45. Metal complexes

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Table 13. Calcium binding capacity of selected sequestering agents [4]

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There are a number of compounds that even in substoichiometric amounts are capable of retarding, hindering, or otherwise interfering with precipitation of insoluble salts. In some cases their action induces salts to precipitate in amorphous form, thereby strongly reducing the tendency toward formation of crystals such as calcite, whose sharp edges can be damaging to fabrics. Sodium polycarboxylates and sodium hy-droxyethanediphosphonate are frequently used additives in modern detergents, which strongly exhibit the latter property even at low concentrations; this is known as a threshold effect [78].

Despite the many desirable properties shown by sodium triphosphate in the washing process, its continued use has been the subject of an international debate between industry, governments, and water authorities in Europe, the USA, Japan, and other regions for many years. The problem has been that sodium triphosphate is a contrib-utor to eutrophication of standing or slowly flowing surface waters; that is, it may lead to overfertilization, which in turn encourages extreme algal growth and adversely affects marine organisms. Recognition of the problem led to an intense worldwide search for suitable replacements in the 1960s and 1970s (see also Chap. 10). Developments have been concentrated not only on sequestering agents, but also on ion exchangers, since these are capable of binding polyvalent metal ions [66] – [68]. Most of the promising substitutes examined so far were organic compounds, primarily those derived from raw materials produced by the petrochemical industry. However, few of these substances were available in large quantity. The cost for commercial production on the necessary scale of > 106t/a was in many cases prohibitive.

Apart from eliminating cations and achieving good soil and stain removal, other important factors in the washing process are dispersion of soil and prevention of soil redeposition (cf. Chap. 2). Because of the presence of distinct localized charges, seques-tering agents are readily adsorbed onto particulate soil. As a result, these compounds often act as effective dispersing agents for such soils.

Figures 46 and 47 show the relationship between dispersing agent concentration and sediment volume for two structurally different soil pigments where the sediment volume is understood as an indicator of the effectiveness of the dispersion process.

Figure 46. Sediment volume Vsof kaolin clay as a function of active ingredient concentration for water of hardness 16 d (285 ppm CaCO3) [4]

a) Sodium sulfate; b) C16–18n-Alkyl decaglycol ethers; c) Hydroxyethyliminodiacetic acid, sodium salt; d) Nitrilotriacetic acid, sodium salt; e) So-dium triphosphate; f ) SoSo-dium n-dodecyl sulfate

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Voluminous sediments are formed in the presence of poor dispersing agents; adsorption is insufficient to produce an adequate negative surface charge, and the result is coagu-lation. Figure 46 shows that the extent of dispersion, in this case of kaolin clay, depends on the specificity with which a sequestering agent is adsorbed. Virtually no effect is produced by sodium sulfate (a) and a nonionic surfactant (b). Anionic surfactants begin to show stabilization only at high concentrations that are rarely used in practice (f ).

With graphite, the results are exactly opposite to those observed with kaolin clay (Fig. 47). Sodium triphosphate alone shows no effect and is thus analogous to sodium sulfate (curves a and b), whereas sodium n-dodecyl sulfate has a very significant stabilizing effect. Because n-dodecyl sulfate is sensitive to water hardness, the dispersing effect increases with diminishing hardness; hence, the sediment volume decreases (both curves c: 16 and 0 d). Although neither electrolytes nor sequestering agents alone are capable of stabilizing the particulate soil graphite, they are capable of exerting a positive influence, either indirectly through an electrolyte effect (curve d) or in hard water as a result of fixation of alkaline-earth ions (curve e). With sodium triphosphate, seques-tration is accompanied not only by an electrolyte effect, but also by a pH effect. An increase in the hydroxide ion concentration and accompanying hydroxide ion adsorp-tion causes increased electrostatic repulsion and, hence, better dispersion (reduced sediment volume). The indirect effect of sequestering agents on the dispersion of hydrophobic particulate soil applies also in principle to emulsification of water-insol-uble greasy soil.

A number of additional factors must be considered when selecting a sequestering agent for detergent use. Several important criteria have been listed in Section 3.2. The extent to which evaluation of a sequestering agent can vary depends on how the several criteria are weighted, as is apparent from Table 14.

Figure 47. Sediment volume Vsof graphite as a function of active ingredient concentration [4]

a) Sodium sulfate; b) Sodium triphosphate;

c) Sodium n-dodecyl sulfate; d) Sodium n-dodecyl sulfate + 1.5 g/L sodium sulfate; e) Sodium n-dodecyl sulfate + 1.5 g/L sodium triphosphate

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In document 23550908 Laundry Detergents (Page 74-79)