alkaline pH value, as for most pigments present in the washing process the isoelectric point is below pH 10. The nonionic surfactant shows no influence on the electrophoretic mobility, whereas the anionic surfactant increases the negative surface charge of the pigment due to the adsorption. By the adsorption of the cationic surfactant the surface charge can be changed from a negative to a positive value during the adsorption process. This picture explains quite well the mode of action of different surfactant types for pigment removal in the washing process. As nonionic surfactants do not influence the electrostatic repulsion of pigment and fabric, their washing efficiency is mainly caused by the disjoining pressure of the adsorption layer. Anionic surfactants also increase the electrostatic repulsion, but usually have lower amounts adsorbed than the nonionic surfactants. Cationic surfactants show similar effects in the washing process as anionic surfactants, but in spite of this they are not suited for most washing processes due to their adverse effects in the rinse cycles. In these cycles, the positively charged surfaces (due to the adsorption of cationic surfactants) are recharged to negative values due to the dilution of the washing liquour and the consecutive desorption of cationic surfactants. As the different fabrics and pigment soils have different isoelectric points, positively and negatively charged surfaces are present in the washing liquor which leads to heterocoagulation processes and a redeposition of the already removed soil on to the fabric. Therefore, cationic surfactants are not used in the washing process, but only as softeners in the rinse cycle when no soil is present any more and a strong adsorption of cationic softener on the negatively charged fabric is desired.
4 COMPLEXATION AND ION
c(g/i)
Figure 3.16. Settling volumes of graphitized carbon black and kaolinite in sodium triphosphate solutions: 16°dH water hard-ness; 0.30 g/(10 ml) graphitized carbon black; 0.50 g/(10 ml) kaolinite (11)
- adsorption of water-soluble substances, e.g. dyes on the zeolite particles
- heterocoagulation of pigments and solid fats with the zeolite
- action as crystallization nuclei for sparingly soluble salts.
All of these effects support the mode of action of zeolites in the washing process. The most characteristic feature of zeolites is the ion exchange of the sodium ions in the crystal structure by calcium and magnesium ions. Figure 3.17 shows the ion-exchange kinetics of zeolites A and X for calcium and magnesium ions (13).
Calcium ions diffuse with a high rate into both types of zeolite, with a slight preference for the wider-pore
f(min)
Figure 3.17. Kinetics of ion exchange of calcium and magnesium ions for zeolite A and zeolite X: T = 25°C; ion concentration = 5 . 3 6 x l 0 ~3 mol/1; zeolite concentration = 1
gfi (13)
zeolite X. These differences are only evident for short times which are not of practical importance for the washing process. The ion-exchange kinetics are more strongly dependent on the pore size of the ion exchanger for magnesium ions. Despite the smaller ion radius at 25°C, the magnesium ion has a more stable and bigger hydration shell than the calcium ion and therefore penetrates more slowly into the pore system of the zeolite.
A comparison between the decrease of water hard-ness by ion exchange and washing performance is given in Figure 3.18 (14). A decrease of the water hardness from 16 down to 3-4°dH only slightly influences the detergency performance. Only a further decrease of the calcium ion concentration leads to a significant increase of soil removal from the fabric. Due to the fact that zeolite A is an ion exchanger, the calcium ion exchange Vs (cm3 )
Figure 3.15. Chemisorption of the triphosphate anion on aluminium oxide: (a) pH < isoelectric point; (b) pH = isoelectric point;
(c) pH > isoelectric point (5)
Graphite
Kaolin
is decreased by a high concentration of sodium ions, despite the high selectivity of the ion-exchange pro-cess. According to this, the detergency behaviour in the presence of sodium ions slightly decreases. The ion exchange of the zeolite can be described by the follow-ing equation:
Gca2+ = ^1 (3-1 2)
cCa2+ + 2 — c ^+ + 22C a 2 +
b\
where <2ca2+ is the exchanged amount of calcium ions,
<2m is the maximum exchanged amount of calcium ions, cCa2+ is the equilibrium concentration of calcium
ions, cL,A+ is the initial concentration of sodium ions, and b\ and b2 are constants.
Figure 3.19 shows a comparison of experimental data of the ion exchange with the calculated curves (15).
Both sets of data are in good agreement. With increas-ing sodium concentration, not only do the maximum exchanged amounts of calcium ions decrease, but also a higher calcium ion concentration is necessary to reach the equilibrium values.
Zeolites show significant adsorption properties regarding the washing process and conditions in the waste water. Figure 3.20 demonstrates the adsorption of a cationic dye (Methylene Blue) and an anionic dye (benzopurpurine) on to zeolite A (16). The cationic
Ccao (odH)
Figure 3.19. Comparison of calculated and measured isotherms of calcium ion exchange by zeolite A:T = 22°C; 1 h exchange time (15)
cCa2+x 103(mol/l)
Qcao (mg/9) QCa2+x103 (mol/g)
CzeoliteAta/')
Figure 3.18. Influence of NaCl on the water-softening effect and the washing performance of zeolite A: water-softening effect at measured 90°C after 15 min; washing performance measured at 90°C and 285 ppm water hardness on particulate-sebum-soiled cotton (14)
Water hardness (ppm) Reflectance (%)
Without NaCI
0.04 mol/l NaCI
Figure 3.20. Adsorption of dyes on zeolite A, at T = 23°C (16;
dye is strongly adsorbed on the negatively charged surface of zeolite A, whereas the anionic dye is only adsorbed on zeolite A which is extrated from a detergent formulation produced on a technical scale. This is due to a hydrophobization of the zeolite surface in the production process, which increases the interaction of the dye and the zeolite surface.
Due to the negatively charged zeolite surface at alkaline pH values, cationic surfactants are strongly adsorbed on to zeolite A (Figure 3.21). For mixtures of cationic and nonionic surfactants, a strong increase of the adsorbed amounts is observed in a certain con-centration range (17). Because of hydrophobic inter-actions between the adsorbed cationic surfactants and nonionic surfactant molecules, additional nonionic sur-factant molecules are probably adsorbed in a second layer from mixtures. These effects have an impact on the behaviour of zeolites in waste water.
In modern detergents, zeolites are used in combi-nation with water-soluble complexing agents or poly-carboxylates. The dissolution of calcium by zeolite A is enhanced by complexing agents which specifically adsorb on calcium-containing particles and subsequently desorb after sequestering calcium ions. Even small amounts of water-soluble complexing agents increase the dissolution rate of calcium carbonate by zeolites to the extent that the dissolution rate approaches that of the water-soluble complexing agent alone. This increase is particularly pronounced over the range of small com-plexing agent concentrations and with short reaction times. As the water-soluble complexing agents act as carriers for the transfer of calcium from the precipi-tate to the water-insoluble ion exchanger, this process is known as a "carrier-effect" in the literature.
cDACx 105(mol/l)
Figure 3.21. Mixed adsorption of cationic and nonionic surfac-tants on to zeolite A:T = 25°C, DAC, ditallowalkyl dimethy-lammonium chloride; NP8, nonylphenoloctaglycol ether (17)
A different effect occurs with the use of polycarboxy-lates in combination with zeolites. Small amounts of polycarboxylates or phosphonates can retard the precip-itation of sparingly soluble calcium salts such as CaCO3
(the "threshold effect"). As they behave as anionic poly electrolytes, they bind cations (counterion conden-sation), and multivalent cations are strongly preferred.
Whereas the pure calcium salt of the polymer is almost insoluble in water, mixed Ca/Na salts are soluble, i.e.
only overstoichiometric amounts of calcium ions can cause precipitation. Polycarboxylates are also able to disperse many solids in aqueous solutions. Both disper-sion and the threshold effect result from the adsorption of the polymer on to the surfaces of soil and CaCO3
particles, respectively.
The stabilization of sparingly soluble salts such as CaCO3 in a colloidal state is one of the possible
Qx106 (mol/g) cx106(mol/l)
Benzopurpurin Ox107 (mol/g) Methylene Blue Qx105 (mol/g)
Methylene Blue
Zeolite A extracted from detergent
Benzopurpurin (zeolite A)
Na2CO3 (g/l)
Figure 3.22. Precipitation inhibition of calcium carbonate by polycarboxylates as a function of temperature and soda concen-tration (3.04 x 10~3 mol/1 calcium ions): (1) 105 mg/1 polycar-boxylate; (2) 210 mg/1 polycarboxylate (18)
effects of polycarboxylates in detergents. The advantage is that, in contrast to ion exchange or complexation, the concentration of the cobuilder can be much lower than the calcium concentration in the washing liquor.
Thus, small amounts of threshold-active compounds could be used as cobuilders, even in soda-based laundry detergents.
The effect, however, is strongly dependent on the experimental (or washing) conditions, i.e. temperature, and soda and cobuilder concentrations. Figure 3.22 (18) illustrates the range of effectiveness of polycarboxy-lates in a carbonate-containing system for typical cen-tral European conditions of water hardness (3.04 x 10"3 mol/1 Ca2 +). The results are based on turbidity measurements. The appearance of a CaCO3 particle larger than approximately 0.2 jim within 30 min was taken as an indicator of the threshold effect. The soda concentrations in the test include the hydrogen carbon-ate content of the tap wcarbon-ater as well as the soda con-tent of the detergent. The results show that for typical German phosphate-free, heavy-duty detergents, polycar-boxylate is no longer threshold-active at temperatures above 400C. This is valid even more for higher carbon-ate concentrations, i.e. purely soda-based detergents.
For zeolite A and soda-containing products, the participation of zeolite A in the elimination of calcium ions during the washing process has to be taken into account. For typical test concentrations, the amount of coarsely dispersed CaCO3 is reduced in the presence of zeolite A over the whole range of washing temperatures. The effect of polycarboxylate on the total amount of precipitation is strongly dependent on the presence of zeolite A. In the absence of zeolite, precipitation is inhibited only below 400C.
With increasing temperature, the precipitated amounts strongly increase. In this case, on addition of CaCO3, polycarboxylate is precipitated as calcium salt, as can be seen from the respective measurements of the residual concentrations of water-soluble polycarboxylate. In contrast, the amount of precipitates in the presence of zeolite A and polycarboxylate is negligibly low, and the residual concentration of water-soluble polycarboxylate is as high as in the zeolite A/polycarboxylate systems without soda.
These results can be explained by the binding of calcium ions by zeolite A and by polycarboxylate in its water-soluble form. This is possible because the calcium ion concentration of the water is lowered by zeolite A.
Thus, Ca2+ is no longer in excess of polycarboxylate and thus formation of the insoluble calcium salt of polycarboxylate is no longer possible.