2.3. The Soil Removal Process
2.3.3. Calcium-Containing Soil
The principles discussed in Section 2.3.2 also apply without exception to calcium-containing particulate soil. However, other important mechanisms, which deserve discussion, also exist. Salts of multivalent cations are almost always present in soils and on textile fiber surfaces. Examples are calcium carbonate, calcium phosphate, and calcium stearate. Cationic bridges, which are responsible for binding soil components chemically to fibers, also frequently form. This type of linkage can be due, for example, to the carboxyl groups commonly found in cotton as a result of oxidation, but such a linkage may also arise from the presence of reactive centers associated with metal oxides or from soaps derived from sebum. Problems stem principally from poorly soluble calcium salts, whose solubility is further diminished as the water hardness of the wash liquor increases. On the other hand, their solubility in distilled water is higher because of displacement of the solubility equilibrium. When calcium salts are dissolved from a multilayered soil deposit, cavities remain in the structure. These cavities loosen the deposit and facilitate its removal from the surface. Thus, one task of detergent com-ponents is to create the highest possible calcium ion concentration gradient between soil and aqueous phase during the washing process.
Figure 25 illustrates this principle, taking the wash effectiveness of a water-insoluble cross-linked polyacrylate as an example [4]. Apparently, virtually no wash effectiveness is achieved with an ion exchanger at a concentration sufficient to eliminate most of the water hardness; an effect is observed only at higher concentrations of ion exchanger.
This phenomenon can be greatly accelerated by additionally introducing an appropriate amount of water-soluble complexing agent.
Figure 25. Water softening (a) and soil removal S from cotton soiled with a dust/sebum mixture (b) resulting from a cross-linked water-insoluble polyacrylate at 90 C [4]
PhysicalChemistryoftheWashingProcess
Before slightly soluble cations can be dissolved from soil and fibers, adsorption of the complexing agent takes place on the surface, particularly in those areas that contain multivalent cations. In the course of subsequent desorption of water-soluble multivalent cation complexes, many of the soil – fiber bonds are broken, leading to a marked enhancement of the washing effect. Removal of cations from soil and fibers by adsorp-tion – desorpadsorp-tion processes and displacement of solubility equilibria are the most important phenomena that accompany the use of complexing agents and ion ex-changers in the washing process.
The mechanisms of action for complexing agents and water-insoluble ion exchangers are different. The two complement each other in their respective roles. Figure 26 shows the way in which a small amount of a water-soluble complexing agent can increase the washing effectiveness of the water-insoluble ion exchanger zeolite A. The effect results from an increase in the rate of dissolution of divalent ions from soil and fibers. The mode of action is depicted schematically in Figure 27. The water-soluble complexing agent serves as a carrier that transports calcium from the precipitate into the
water-Figure 26. Comparison of soil removal S of ze-olite A (a), sodium triphosphate (b), and a mix-ture of the two builders in the ratio zeolite A:
sodium triphosphate = 9 : 1 (c); results obtained for non-resin-finished cotton tested in a Launder-ometer
Wash time: 30 min with heating; temperature:
90 C; water hardness: 16 d (285 ppm)
Figure 27. Mechanistic scheme for the carrier effect [24]
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insoluble ion exchanger. The process is based on successive adsorption, desorption, and dissociation, and it accelerates the delivery of free calcium ions into solution.
The effectiveness of complexing agents and ion exchangers is related to the presence of calcium in the system, as is evident from Table 7. Two different soils and the cotton yarn to be studied were decalcified prior to applying artificial soil. No washing effect due to zeolite A could be observed within the method's limits of accuracy. This result can be taken as an indirect proof of the importance of dissolution of calcium from soil and fibers during the washing process.
The concentration of complexing agent and the temperature are generally the decisive factors in removing multivalent metal ions by a water-soluble complexing agent; the binding ability diminishes with increasing temperature.
Essentially the same relationships also apply when using a water-insoluble ion exchanger, although the temperature effect is usually reversed [32], [33].
Figure 28 shows the wash effectiveness of various zeolites in water with a hardness of 16 d (285 ppm CaCO3) as a function of concentration. It is apparent that the best results are achieved with zeolite A and the poorest with analcime.
Table 7. Wash experiments in a calcium-free system [31]
Soil Wash medium* Remission, %
80.2 % Osmosed kaolin, 16.5 % carbon black, H2O 65.5
3.3 % black iron oxide
H2O + 2 g zeolite 4 A/L 66.0
89.7 % Osmosed kaolin, 5.9 % carbon black, H2O 59.5
2.9 % black iron oxide, 1.5 % yellow iron oxide
H2O + 2 g zeolite 4 A/L 59.0
* Water is distilled.
Figure 28. Soil removal of various zeolites [32]
a) Zeolite A; b) Faujasite; c) Desmine; d) Sodalite;
e) Analcime
Apparatus: Launder-ometer; water hardness:
16 d; temperature: 95 C; wash time: 30 min with heating; fabric: non-resin-finished cotton PhysicalChemistryoftheWashingProcess
In addition to the previously described reasons for removing divalent alkaline earth ions, their interaction with other detergent components must also be considered. For example, soaps form poorly soluble salts with calcium, as do many synthetic surfac-tants, and these can be deposited on fibers. This phenomenon is extremely common with detergents in which soap is the key surfactant, when no strongly complexing agents such as sodium triphosphate are present. Precipitation of relatively insoluble surfactant calcium salts has the additional disadvantage that it causes a severe reduc-tion in the active surfactant concentrareduc-tion and, thus, to generally deteriorated con-ditions for soil removal.
Figure 29 illustrates how surfactants, salts, and complexing agents complement each other in the removal of soil. The negative influence of calcium is eliminated by sodium triphosphate or other complexing agents. The magnitude of the indirect counterion effect of sodium ions is made clear when sodium sulfate is added to alkylbenzenesul-fonate (curve b).
Polycarboxylates are widely used in detergents as cobuilders [34], [77] – [79]. They are able to retard precipitation of sparingly soluble calcium salts such as calcium carbonate and calcium phosphate (threshold effect) when used in small concentrations. As anionic polyelectrolytes they bind cations (counterion condensation), whereby multivalent cations are strongly preferred. Polycarboxylates can also disperse solids in aqueous solution. Both dispersion and threshold effect result from adsorption of the polymer on the surface of soil and CaCO3particles, respectively. Stabilization of sparingly soluble salts such as CaCO3in a colloidal state by polycarboxylates occurs at substoichiometric concentrations of the cobuilder in the washing liquor, which is an advantage compared to ion excahnge or complexation. Thus, small amounts of threshold-active compounds can be used as cobuilders even in laundry detergents with a high content of sodium carbonate. The effect, however, is strongly dependent on the washing conditions, such as temperature, soda ash, and cobuilder concentration. Figure 30 illustrates the range
Figure 29. Soil removal S from wool as a function of water hardness at 30 C [4]
a) Sodium alkylbenzenesulfonate (0.5 g/L);
b) Sodium alkylbenzenesulfonate (0.5 g/L) to-gether with sodium sulfate (1.5 g/L); c) Sodium alkylbenzenesulfonate (0.5 g/L) together with so-dium triphosphate (1.5 g/L)
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of effectiveness of a polycarboxylate (AC) in a carbonate-containing system for typical European conditions of water hardness (3.04 10-3mol/L Ca2+) [35]. The appearance of CaCO3particles larger than ca. 0.2 mm within 30 min was taken as an indicator of the threshold effect. The results show that polycarboxylates are no longer threshold-active above 40 C. This holds even more for higher carbonate concentrations, i.e., with detergents containing soda ash as the sole builder.
For zeolite A and soda ash containing products the participation of zeolite A in the elimination of calcium ions during the washing process must be taken into account. In contrast to the results obtained in the absence of zeolite A, the precipitation in the presence of zeolite A and the polycarboxylate AC is negligibly low at temperatures above 40 C. These results can be explained by the binding of calcium ions by zeolite A and by AC in its water-soluble form. This is possible because the calcium ion concen-tration of the water is reduced by zeolite A. Thus, Ca2+ is no longer in excess with respect to AC, and the formation of an insoluble calcium salt of polycarboxylates, which decreases the threshold activity, is no longer possible. According to this mechanism, polymers with high carboxylate content and relatively high molecular weight should be used in combination with zeolite A.