Particulate Soils
Solid particles such as clay, alumina, silica, iron, and other metal oxides are present in particulate soils, deposited mostly from air suspensions (dust). They are soluble neither in water nor in organic solvents. They usually exhibit a large surface area, on which the oils and greases absorb very strongly. Particulate soils contribute significantly to the difficulty of removing oily and greasy soils because they contribute to their rigidification and, sometimes, they act as catalysts in the oxidation/crosslinking of unsaturated triglycerides.
Since they are not water-soluble, the particulate soils can be redeposited on surfaces that have been cleaned. It is accordingly important to keep such soils effectively dispersed in the washing liquid.
It is noteworthy that particulate soils are not necessarily mineral. Some biopolymers such as starch remain insoluble in water during the dishwashing operation and behave actually as particulate soil.
Particulate soils almost always occur with other soils such as oily and greasy soils. The particulate soils contribute to the toughness of the soil deposit, and the grease acts as a cement, binding the particles together. The first step, just after wetting, is to attack the oily-greasy component. The particulate soils are then made available. The best way to clean particulate soils is to use a surfactant that is able to adsorb efficiently at the water-solid particle interface, to reduce the interfacial tension and, accordingly, to reduce the adhesion forces binding the particles together. This can be achieved with an anionic surfactant; in which case the surface of the solid particles is made more negative and electrostatic repulsion can occur between adjacent particles.
Since particulate soils are not water soluble, they have a tendency to redeposit in the later stages of the washing operation.
Oxidizable or Bleachable Soils
Some soils (wine, blood, fruit juices, grass, tea, coffee, etc.) are highly colored, the color resulting either from a series of conjugated double bonds or from porphyrinic structures (wine). These soils can be oxidized by hypochlorite, hydrogen peroxide, or peracids, leading in most cases to colorless substances. The oxidized substances may not be removed by the cleaning operation, but they are no longer visible. It must also be pointed out that even if the soil is not completely removed during the cleaning operation, it is usually broken into smaller pieces, which can be removed in the next wash.
In laundry, bleachable soils are treated with an oxidant. The oxidant can be based on either hypochlorite or oxygen. Sodium hypochlorite, the most common bleaching agent, is widely used in North America as a laundry additive. It is inexpensive, very efficient, and delivers a sanitation benefit. Unfortunately, hypochlorite can damage colors and the fabric itself. The formulation of finished cleaning products with sodium hypochlorite is very difficult because the hypochlorite reacts very rapidly with many organic materials.
Oxygen bleaches have been preferred in Europe and are now being adopted worldwide. The most common system is based on hydrogen peroxide, which is a much milder oxidant than hypochlorite. Sodium perborate tetrahydrate or monohydrate is used as a source of hydrogen peroxide. Since its molecular weight is lower, perborate monohydrate is more concentrated in hydrogen peroxide, but it is a brittle solid. It is difficult to formulate in a powdered detergent because it generates a lot of dust. Perborate tetrahydrate, although less rich in hydrogen peroxide, is more appropriate for the formulation of powders because the beads exhibit good mechanical resistance.
In contact with water, perborate dissolves slowly and releases hydrogen peroxide. The fall bleaching capacity of hydrogen peroxide is generated only at 70 °And above. Effective bleaching can be achieved at lower temperature by using bleach activators. The most common one is tetraacetylethylenediamine (TAED).
Peracetic acid is the actual low temperature bleaching agent. It does an effective job even at 40 °C. At higher temperatures, peracetic acid can decompose to regenerate hydrogen peroxide. Moreover, peracetic acid can react with hydrogen peroxide in the presence of colloidal catalysts such as silica. To prevent this undesired reaction, the addition of phosphonate-based stabilizers is recommended.
The activated perborate bleaching system is quite easily formulated in powder detergents. The bleaching system can remain active after several months of aging if the atmosphere is not too humid.
Preformed Peracids can also be used as bleaching agents. Magnesium monoperoxyphthalate is an example. Such a molecule is already a peracid and is accordingly active at 40°C. The peracids are in general harder to stabilize because they are more reactive than activated systems. They can also be responsible for spot bleaching (local discoloration) if solubilizetion is not fast enough.
Hypochlorite bleaches usually are used in automatic dishwashing mainly for reasons of Sanitization, efficacy in removal of colored stains (e.g., tea, coffee), and ability to break down protein soil.
Proteins and Starchy Soils
Proteins and starch are polymeric materials that can resist conventional cleaning. They act as glue for other soils, making cleaning more difficult. A typical example is macaroni and cheese. These soils are best addressed by enzymatic cleaning. Proteolytic and amylolytic enzymes are currently used for this purpose in modern automatic laundry detergents.
Proteins and starches are present in significant quantities in food stains. The moderate water solubility of these polymers makes them difficult to remove with classical techniques. Proteases and amylases are enzymes able to hydrolyze proteins and starch, respectively. Such enzymes are currently used in laundry detergents and even in automatic dishwashing detergents.
The selection of a protease is challenging: since the enzyme itself is a protein, it is subject to self-destruction. To prevent such autohydrolysis, sophisticated stabilizing systems have been developed. An elegant way to proceed to stabilize the enzyme structure inbuilt liquid laundry detergents is to use a borax-glycol complex to reduce the water activity.
Second-Order Challenges
Soil Redeposition
Once the soil has been detached from the substrate, it is either “solubilized” inside the micelles, emulsified, or, in the case of solid particles, dispersed as a suspension in the washing liquid. The emulsions and dispersions of solids are not stable, and often the soil redeposits on the cleaned items or on parts of the washing machine. Redeposition can be assessed by measuring the loss of reflectance (whiteness) of white fabrics after several cumulative washes with soiled items.
The physicochemical phenomenon involved is the following. At short distances, even nonpolar solid particles attract each other, thanks to van der Waals interactions. This type of interaction decreases with the sixth power of the distance between the particles.
The result is an energy minimum at short distances. This minimum is so deep that the particles cannot be separated by mechanical energy. Usually referred to as the primary minimum, this minimum corresponds to irreversible flocculation or coagulation. This attraction is not limited to two particles. A large number of particles can agglomerate to form a big floc that deposits on the washed items. There are two basic ways to stabilize a dispersion in an aqueous liquid and to prevent (or delay) redeposition: electrostatic repulsion and steric stabilization.
Electrostatic Repulsion
If the particles are electrically charged, electrostatic repulsion can create an energy barrier to flocculation/coagulation. Electrostatic repulsion drops with the square of the distance. Its magnitude depends on the amount of charge on each particle, the dielectric constant of the solvent, and the ionic strength.
It is not a good idea to rely on electrostatic repulsion to prevent redeposition because ionic strength is not a parameter under our control. Stains can include a significant amount of electrolytes, and builders, contained in most detergents, are electrolytes.
Steric Stabilization (“Colloid Protection”)
Steric stabilization offers a better potential for the control of redeposition. The principle is to deposit a large molecule (a surfactant or a polymer) on the surface of the particle or of the substrate to be protected from redeposition.
Some polymer segments called trains stick to the surface (thanks to van der Waals or electrostatic interactions), and other segments called loops interact with water. The tails of the molecule usually interact with water. This process is dynamic the trains can desorb and the loops can adsorb.
The anti-redeposition agent must exhibit the right balance between water solubility and adsorption efficacy. If the water solubility is too high, the proportion of loops is too high and the adsorption is not efficient; on the other hand, if water solubility is too low, there are almost no loops and the molecule is deposited as a thin layer on the surface of the particle.
The result of the expansion of the loops in the water phase is an increase of the effective volume of the particle. If two particles come together, the loops of their adsorbed polymers begin to interact. In the interaction volume, the polymer concentration becomes too high, resulting in an osmotic pressure (water molecules want to enter the interaction volume and pull the particles apart). There is also an entropic effect: compression significantly reduces the configurational entropy of the polymer. At a distance significantly longer than the onset of van der Waals attraction, a new energy barrier, corresponding to the compression of the polymer loops, develops to prevent flocculation.
The control of redeposition is significantly more complex than this simple theoretical approach would lead one to believe. The adsorption efficacy depends on the nature of the surfaces, on the composition of the water phase, on temperature, and so on. The use of polymer adsorption for suspension stabilization is very delicate. Particularly when too small an amount of a high molecular weight polymer is used, interparticle bridging can occur, speeding up the flocculation.
To prevent redeposition on cotton fabrics, carboxymethylcellulose appears to be the most efficient material. On polyester fibers, hydroxyethyl-or hydroxypropyl cellulose gives better results.
Encrustation
Automatic laundering can generate fabric encrustation, particularly in areas where the water is hard. The fabric encrustation can be organic (most often soap deposits) or mineral (lime scale or other insoluble calcium salts). Mineral deposits can be responsible for fabric graying and wear, as well as a surface roughening effect.
The problem of fabric encrustation becomes critical with the reduction of sodium tripolyphosphate in areas where the water hardness reaches or exceeds 300 ppm (as calcium carbonate). In several European countries, for example, the level of encrustation can reach very high levels! Fabric encrustation is related to the ratio between calcium ions and tripolyphosphate (TPP). If the Ca2+/TPP ratio is not greater than 1, a water-soluble Ca-TPP complex is formed and almost no encrustation is observed. This happens in areas of soft water or, in hard water. if the amount of TPP is high enough. If the Ca2+/TPP ratio exceeds 1, a water-insoluble Ca2-TPP complex is formed, which can deposit on the fibers. This occurs if water hardness is increased or the level of TPP is reduced.
The quality of the phosphate builder is also essential. Higher encrustation levels can also be observed if TPP is degraded to pyrophosphate and orthophosphate, calcium orthophosphate being water-insoluble.
Non-Phosphate (no-P) built formulations can be based on sodium citrate (calcium salt soluble) or, more often, zeolite 4A. Zeolite particles can also deposit on fabrics if the suspension is not stabilized properly, but the level rarely exceeds 5%. The problem is more apparent on dark items, on which white zeolite deposits are more apparent. In “no-P” built products it is a good idea to add several percent of a low molecular weight sodium polyacrylate or acrylate-based copolymer. Such polymers present a moderate complexation power for calcium and magnesium, but they mainly act as scale inhibitors. Rather than sequestering the alkaline earth cation, they reduce the rate of crystallization of their insoluble salts (mainly calcium carbonate) by interfering with crystal growth. Depending on concentration and temperature, sodium polyacrylate polymers can delay calcium carbonate precipitation for several hours. The effect is essentially kinetic, and precipitation will eventually develop.
Sourcess:
- Leigh, A.G., U.S. Patent 4,225,452, 1980.
- McCrudden, J.E., et al., U.S. Patent 4,154,695, 1979.
- Boskamp, J.V., U.K. Patent GB- 2,079,305, 1980.
- Feighner, G.C., J. Am. Oil Chem. Soc., 61(10), 1645 (1984).
- Hunter, R.J., Foundations of Colloid Science, Vol. I, Clarendon Press, Oxford, 1989.
- Guy Broze, Colgate-Palmolive, Research & Development Inc., Mechanisms of Soil Remal, Avenue du Pare Indemel B-4041 Milmort (Herstal), Belgium.
- Robert Lange, Detergents and Cleaners, A Handbook for Formulators, 1994.
- Tadros, T.F., Surfactants, Academic Press, London, 1984.
