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When Bubbles Attack!
By Aziz Ullah, Ph.D., MBA
October 13, 2010
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In cleaning processes, gases are sometimes employed to achieve desired results.
These gases include ammonia, which is available as ammonium hydroxide; oxygen, which is available as ozone and hydrogen peroxide — or, in solid form, is available as sodium percarbonate and sodium perborate.
Carbon dioxide is employed in the dry cleaning industry, in stain removal, and is claimed to clean carpet better.
Sulfur dioxide, available as sodium bisulfite, is used in reducing stains; chlorine gas, as in chlorine bleach, is extensively used to remove stains, brighten white colors and bring luster to Oriental rugs made with wool fiber.
In addition, the gases can act as bleaches, germicides and sanitizers or, sometimes, as solvents as in the case of carbon dioxide — a supercritical fluid — for dry cleaning.
Let us examine the role of individual gases.
Ammonia
Ammonia is a gas that, when dissolved in water, yields ammonium hydroxide.
It has an unpleasant odor and is alkaline in nature. It helps saponify oils — converting them to soap — helping them solubilize in water.
It will attack copper and copper-based metals, giving a bluish discoloration.
It can turn red dyes blue and make some dyes bleed.
In concentrated form, ammonia can turn silk and wool yellow.
With care, a dilute solution can be employed to remove dyes that have bled into surrounding areas.
Ammonia is the quickest and safest alkali to neutralize acid discoloration of dyes.
Being a gas, ammonia is volatile and will evaporate; it will react with acids to form salts that can be easily flushed out with water.
Some commercial cleaning products containing ammonia are labeled as "self neutralizing."
Bleaching
Before we proceed any farther, let us talk briefly about bleaching.
The term "bleach" can be defined as to induce any change toward a lighter shade of a surface.
A bleaching effect can occur through mechanical, physical and/or chemical means.
Chemical bleaching is used for the removal of non-washable soils and is accomplished by oxidizing or reducing the colors.
The oxidizing bleaches are more permanent.
The reducing bleaches, such as sodium bisulfite, may make many dyes lose color, but later return to their colored state as a result of subsequent air oxidation.
Bleachable colors are generally of vegetable origin and include anthocyanin dyes — colors that range from red to blue and are derived from cherries, blueberries, currants — and curcuma dyes from curry and mustard.
The brown tannins found in fruit, tea and wine stains arise from the reaction of polyphenols with protein.
The brown colors in coffee, tea and cocoa include polymers like humic acids.
The green dye chlorophyll and the red betanin from beets are pyrrole derivatives, as are urobilin and urobilinogen derived from the degradation of hemoglobin and discharged in urine.
Carrot and tomato stains contain carotenoid dyes.
Synthetic dyes, found in fruit punches as well as hair coloring agents and ink, are all bleachable.
Oxygen
Hydrogen peroxide is a colorless, slightly acidic liquid — due to acid stabilizers.
Brown bottle peroxide available through pharmacies is a three percent solution of hydrogen peroxide dissolved in water and contains 10 volume of oxygen. A 30 percent solution of hydrogen peroxide contains 100 volume of oxygen.
To accelerate the bleaching effect of hydrogen peroxide, the solution can be brought to the alkaline side. This is generally accomplished by the use of ammonia.
Cellulosic fibers such as cotton require a higher pH; this can be accomplished by the use of sodium carbonate and sodium hydroxide, and the release of oxygen is controlled by the addition of sodium silicate.
Another oxidizer, sodium perborate, yields about 10 percent oxygen and releases oxygen only at higher temperatures. Some additives can be added to accelerate oxygen release at lower temperatures.
Sodium percarbonate yields about 14 percent oxygen and does not require higher temperatures to release oxygen.
There are other compounds besides the ones mentioned that release oxygen, such as the persulfates. The oxygen containing compounds are generally named with a "per" pre-fix.
Chlorine
Chlorine bleach, or sodium hypochlorite, has little cleaning value but it is a powerful oxidizer, especially in an acidic to neutral state. Commercial chlorine bleach is packaged at 12 percent active chlorine content and the retail market bleach has 5-6 percent chlorine.
The actual amount of chlorine is lower depending on age, purity and conditions of storage, during which chlorine is lost.
Powdered chlorinated trisodium phosphate combines the sanitizing and oxidizing properties with the detergent properties of trisodium phosphate. Sometimes, it is formulated with surfactants for better detergency.
There are other chlorine products, such as the isocyanurates, with active chlorine content of about 60-90 percent that has little or no application in the textile cleaning industry.
Many use chlorine bleach as a lustering agent during the washing or finishing process of hand-woven wool Oriental carpet.
Carbon dioxide
Carbon dioxide is a gas that converts from a solid to a gas at minus 109.3 degrees Fahrenheit.
It forms a weak acid called carbonic acid with a pH of 3.7 when dissolved in water.
This acid, being unstable, cannot be isolated as a pure liquid or solid.
Carbonic acid rapidly decomposes back to carbon dioxide and water, which are much more stable.
Carbon dioxide is rapidly absorbed by most alkaline solutions, whether in the living body or a detergent solution, and converted to a salt.
Scientists have known since the late 19th century that certain materials acquire unique properties when heated and pressurized. These liquids are called supercritical fluids. Although these fluids resemble liquids, they are technically not liquids; they may have the density of a liquid, but behave as a gas.
Water becomes a supercritical fluid when subjected to a temperature of 705 degrees Fahrenheit and a pressure of 3,200 pounds per square inch (PSI).
Supercritical water can simultaneously behave both as a polar — like water — and a non-polar solvent — like oil.
For instance, water will dissolve normally insoluble organics like oils and waxes completely. Mixing oxygen with supercritical water will oxidize nearly anything.
One of the most astonishing properties is that a flame can burn within supercritical water. Carbon dioxide, at 60 degrees Fahrenheit and 700 PSI, becomes a gentle but a very effective supercritical solvent — this property makes it very effective in dry cleaning.
The "DryWash" process, developed by the U.S. Department of Energy''s Los Alamos National Laboratory, Hughes Environmental Systems Inc. and Global Technologies LLC, cleans with liquid carbon dioxide, which is not the same as cleaning with a carbonated water system.
This process offers an effective yet environmentally friendly alternative to potentially toxic solvents such as perchloroethylene.
Carbon dioxide, either in its liquid or its supercritical state, has been used in the past to clean electronic, mechanical and optical equipment.
It is widely used to extract caffeine from coffee.
DryWash is the first process that uses carbon dioxide as a cleaning solvent for most fabrics, furs, leathers and sequined materials.
Carbon dioxide is being used in a high-pressure liquid state — supercritical — at 50-60 degrees Fahrenheit and 700 PSI to dry clean clothes; this replaces perchloroethylene.
Instead of a rotating tub, as in conventional dry cleaning, it is the clothes that rotate while the basket remains fixed.
During the cleaning cycle, the carbon dioxide helps remove three types of soils: Soil soluble in carbon dioxide, soil soluble in water and soil insoluble in both carbon dioxide and water — pigment soil.
At the end of the cleaning cycle, the liquid carbon dioxide drains from the cleaning vessel into storage tanks that are connected to a still equipped with a heater. The carbon dioxide is converted to a gas in the still and the dirt collects at the bottom.
The clean carbon dioxide is recondensed into the storage for the next cycle.
Carbon dioxide in carpet cleaning
The Internet is full of stories on how to remove stains by employing club soda, which contains carbon dioxide; I do not know how well it works.
One company employs carbon dioxide gas and detergent in carpet cleaning. This company acquired a patent in 1980, but that patent has expired.
This patent was later modified for in-situ generation of carbon dioxide by use of a carbonate salt and an acid. A detergent is also employed in this system.
A patent does not mean that the process is scientifically sound — remember, there are more than 4,500 patents on the mouse trap alone.
The original patent for the carbon dioxide-employing system called for detergent mixture comprising of a surfactant, alkaline builder and a volatile organic solvent.
The cleaning solution made from the detergent mixture is carbonated with carbon dioxide.
Among the several claims made by this method are:
1.
Carpet cleaned by this method are left sparkling clean and dry in one to two hours
2.
The secret is the use of effervescent, carbonating cleaning solutions
3.
Soaps, detergents or harsh chemicals are not needed — the patents calls for a detergent system — to accomplish great cleaning results because of the power of crystal-clear carbonating bubbles
4.
The carbonated cleaning solutions have the ability to penetrate textile fibers and dissolve and/or lift both inorganic and organic materials from fibers.
One other company is advocating the use of carbon dioxide in their cleaning system.
Before we delve into how effective carbon dioxide is in removing soil from textiles, let us take a look at the composition of soil and what is effective in its removal: Soils include:
1.
Water-soluble soils, which include inorganic salts, sugar, perspiration, etc.
2.
Pigments, metal oxides, carbonates, silicates, humus, carbon black, etc.
3.
Animal fats, vegetable fats sebum — oily substance exuded from the skin — mineral oil and wax
4.
Proteins such as blood, egg, milk and skin residue
5.
Carbohydrates such as starch
6.
Bleachable dyes such as fruit, vegetables, wine, coffee and tea.
Soils are strongly bonded to the carpet. The most difficult soils to remove include pigments like carbon black, inorganic oxides, carbonates and silicates, which tend to become embedded in the fibers.
Other problem soils include greases, oils, fats, waxes, proteins and certain dyes. All these tend to be present as mixed soils.
Soil removal is helped by certain factors; its removal is enhanced by increases in detergency, mechanical input, wash time and temperature.
How soils are removed
Fats and related substances can be removed from textiles by three methods:
1.
By emulsification with detergent
Detergents clean by:
a.
Decreasing water''s surface tension, making it a better wetting agent
b.
Converting greasy and oily dirt into micelles that become dispersed in detergent-containing water
c.
Keeping the oil micelles in suspension, thereby preventing them from coalescing back to large globules of oil that could be redeposited on a clean surface.
2.
By saponification — converting to soap — from the alkalinity of the cleaning solution
3.
By extraction with an organic solvent.
There are several issues with the use of carbon dioxide in cleaning solutions.
1.
Gases including carbon dioxide follow Henry''s Law; they are more soluble in water at higher pressure and less soluble at higher temperatures. Heating the unpressurized cleaning system will accelerate the release of the gas and purge the gas from the system. Carbon dioxide gas, though somewhat soluble in cold water, has very limited solubility in warm water and is even less soluble in hot water. At room and higher temperatures, and without adequate sustained pressure, it will just bubble through and escape.
2.
Cleaning solutions clean best on the alkaline side. The builder salts, which synergize detergents, are alkaline, and for them to function properly, they must be kept alkaline. Oils and fats in carpet are saponified by the alkalinity in the cleaning solution and thus much more easily removed. Carbon dioxide, when dissolved in water, produces an acid, which will react with the alkalinity present in the cleaning solution. If too much carbon dioxide is passed through, the cleaning solution could lose its alkalinity and the anionic surfactants start losing their ability to act as detergents when their pH level falls to the acidic side. This is like the reverse of saponification. Thus, by reducing alkalinity, carbon dioxide interferes with the cleaning process.
3.
There is no scientific reason that, by introduction of carbon dioxide in the cleaning system, it will reduce the amount of water required. Also, carbon dioxide does not form any azeotropes — a constant boiling mixture — with water, which means that before the carpet dries, any carbon dioxide will be expunged and the remaining water will evaporate at its normal rate like any other wet carpet.
Carbon dioxide, as a carbonated solution, does not exhibit any properties that will enhance soil removal in a detergent-based system.
Carbon dioxide has no affinity for the common soil; it has no synergy with the surfactant system.
When heat is employed in a system in which carbon dioxide is introduced in-situ or externally, it will purge out faster and, if the system is an alkaline system, it will be absorbed and lower the pH of the system, in turn reducing the efficacy of the builders and the detergent system.
Gaseous carbon dioxide has little ability to clean in a water-based system devoid of a detergent system. In other words, it is the detergent that is doing the cleaning and, from a marketing point of view, the credit is given to carbon dioxide.
If carbon dioxide, through purging, is the prevailing mechanism, then no detergent or soap is needed in the system but, without a surfactant, it cannot clean effectively.
Bubbling gas thorough a detergent-based cleaning solution may impress a layman, but it does nothing for the cleaning and may even be counterproductive.
Gaseous carbon dioxide added to the detergent cleaning solution will tend to neutralize the beneficial effect of the inorganic builders; it will not help with cleaning, but will add to the cost.
Up to now, I have not come across any peer-reviewed independent studies in scientific journals that show the advantages of gaseous carbon dioxide as to whether or not it enhances the detergency.
A detergent system that relies on carbon dioxide not only increases the cost, but also interferes with the detergency.
Gases play an important and effective role in the stain removal, cleaning and maintenance of carpet — but sometimes the role is overblown.
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Aziz Ullah, Ph.D., MBA, is president of Fabpro Manufacturing, a leading formulator of carpet and upholstery cleaning products. He is a member of the American Chemical Society, senior member of the American Association of Textile Chemists and Colorists, and a member of The Textile Institute (UK). He can be reached at www.fabpro.com
