After soil has been removed, it must be stabilized in the wash liquor, and redepo-sition of the removed soil must be prevented. This property of a wash liquor is known as its soil antiredeposition capability. Several mechanisms play a role in ensuring good antiredeposition characteristics.
2.4.1. Dispersion and Solubilization Processes
The most important factor in this context is the dispersion process. Nonspecific adsorption of surfactants and specific adsorption of complexing agents cause liquid soils to be emulsified and solid soils to be suspended or dispersed. Poorly soluble substances are solubilized by surfactant micelles as molecular dispersions. These mech-anisms have been discussed in detail in Sections 2.3.1 and 2.3.2.
Figure 31. Influence of textile fibers on soil re-moval S [4]
Detergents: a) 1 g/L Alkylbenzenesulfonate + 2 g/L sodium sulfate; b) 2 g/L sodium triphos-phate; c) 1 g/L alkylbenzenesulfonate + 2 g/L so-dium triphosphate
LaundryDetergents
2.4.2. Adsorption
Supplementary adsorption effects play an important role when zeolites are used in detergents. Such effects are not observed with detergents containing only the com-plexing agent sodium triphosphate.
Zeolites can significantly enhance washing effectiveness by serving as competitive substrates for the adsorption of molecularly dispersed soluble substances and colloidal particles. Their presence is particularly advantageous under extreme conditions involv-ing large quantities of particulate matter. Adsorption and heterocoagulation of the soil by zeolites substantially reduce redeposition of soil on the fabric, leading to a signif-icant increase in whiteness maintenance.
2.4.3. Soil Antiredeposition and Soil Repellent Effects
Soils and detergents also contain natural and synthetic macromolecules in addition to low molecular mass compounds. Proteins from blood and protein-containing food, for example, can be adsorbed onto textile fibers and must by some means be desorbed during the wash process.
Detergents often contain polymeric soil antiredeposition agents, whose role is to be adsorbed onto the substrate, thereby creating a protective layer that sterically inhibits redeposition of previously removed particulate soil. The desorption of proteins and the adsorption of antiredeposition agent in a single process represent competing phenom-ena, so that careful selection of the proper antiredeposition agent is required.
In a surfactant-free environment, the adsorption of most macromolecules onto a solid surface is effectively an irreversible process. The reason for this irreversibility is the vast number of points of contact that exist between macromolecule and substrate.
From a statistical standpoint, the number of bonds between the two is so large that adherence is ensured regardless of the strength of the bonds. Most of the observed binding results from weak hydrophobic interactions.
In a multicomponent system containing both surfactants and macromolecules, competitive adsorption is possible at the substrate surface. This permits the surfactant to successively destroy the individual points of contact binding the macromolecule, thereby displacing it from the interface. This mechanism is most often observed for nonionic surfactants. Anionic surfactants are often capable of forming polymer – sur-factant complexes, in the course of which the conformation of the macromolecule is altered in such a way as to reduce the extent of its attraction to the interface.
Figure 32 depicts the adsorption behavior of gelatin on glass both in the presence and in the absence of sodium n-dodecyl sulfate. The two adsorption isotherms are very different; in the presence of a surfactant, adsorption of the macromolecule is virtually eliminated. Even preadsorbed gelatin is desorbed from the surface by subsequent
PhysicalChemistryoftheWashingProcess
surfactant addition. This competitive adsorption phenomenon involving surfactants and macromolecules is of great importance in soil removal, as is the formation of macromolecule – surfactant complexes. Both significantly impair the desired adsorption of polymeric antiredeposition agents.
The adsorption of antiredeposition agents is usually a selective process dependent on the chemical constitutions of both fiber and polymer. For example, the soil antirede-position effect of carboxymethyl cellulose is rather limited on hydrophilic fibers such as cotton. Cellulose ethers, such as methylhydroxypropyl cellulose, or polymers from terephthalic acid and polyethylene glycol (soil repellent) are effective especially with more hydrophobic fibers such as polyester. Therefore, combinations of several anti-redeposition agents often must be used to ensure satisfactory results with mixed laundry. In this case the absolute amount of adsorbed substance is not the determining factor, but rather the extent to which adsorption confers hydrophilic characteristics, i.e., the change in surface characteristics relative to untreated fiber surfaces. This can be characterized either by measuring the kinetics of wetting (Fig. 33) or by observing the resulting differences in the wetting tension with respect to pure water (cf. Eqs. 1 and 2).
Figure 33 reveals clear differences in the time-dependent increase in weight when prewashed polyester is immersed in water [9]. The polyester fibers had previously been washed with different detergent formulations. Table 8 shows that carboxymethyl cellulose, a frequently used antiredeposition agent for cotton, has no effect on polyester.
Figure 32. Adsorption of gelatin on powdered glass (temperature 25 C)
a) Without sodium dodecyl sulfate; b) With so-dium dodecyl sulfate (gelatin – surfactant mass ratio 1 : 1.44)
Figure 33. Kinetics of wetting of polyester fibers by three different detergents. The increase in weight of the fibers during wetting by pure water is plotted as a function of the wetting time [9]
LaundryDetergents
Table8.Comparisonofchangesinpolyesterwettingtensionswithchangesinremissionforaheavy-dutydetergent(I)andalow-temperatureheavy-dutydetergent(II)forpolyester DetergentI,DetergentII,AntiredepositionagentChangeinChangein (in%detergentcharge)wettingtension*remission** Sodiumcarboxy-Methylhydroxy-HydroxyethylIIIIII methylcellulosepropylcellulosecelluloseDjv,Djv,DR,DR, g/Lg/LmN/mmN/m%% 7.44.50000–1 000.50011 7.44.50.500203–1 0000.502222 7.44.500.5014141716 00000.52221 7.44.5000.53250 * Djv=calculatedforadvancingcontactangleconditions. **DR=measuredafterthreewashes. PhysicalChemistryoftheWashingProcess
By contrast, methylhydroxypropyl cellulose and soil repellents cause the polyester surface to become considerably more hydrophilic. It is noteworthy that the effects are retained with both detergents, albeit to a somewhat reduced extent. Only with these formulations is a significant increase in the soil antiredeposition effect observed, as evidenced by the changes in percentage remission. Hydroxyethyl cellulose can be regarded as a representative of numerous polymers which, though they are readily adsorbed from aqueous solutions and are capable of showing considerable antirede-position activity in pure water, nonetheless lose most of their effectiveness in a detergent solution as a result of competitive adsorption and displacement by surfac-tants.