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Dawn Chemical, Inc. Presents - The Chemistry of Cleaning©
THE SCIENCE OF SOIL REMOVAL
I. THE pH SCALE
Perhaps we should start our discussion with an explanation of the pH scale. It is very important to understand this pH scale because the knowledge that is locked within the mystery of this scale, once opened, can be logically used to solve the many cleaning problems that we will encounter.
What is pH? Good question! It is the measurement of the concentration of "hydrogen ions" (shown in chemical notation as H+) in relation to the concentration of "hydroxyl ions" (shown in chemical notation as OH-). An excess of H+ (hydrogen) ions over OH - (hydroxyl) ions makes a solution an acid. Conversely, an excess of OH - (hydroxyl) ions will make the solution alkaline (often referred to as basic).
All solutions that are made up of water can be measured for their pH. The pH scale runs numerically from 1 to 14. On this scale, a solution that has a pH from 0 to 6.9 is considered to be an acid. It is acidic because it contains a larger amount of hydrogen ions.
A solution that measures from 7.1 up to 14 on the pH scale is considered to be basic (or alkaline) in nature, because it has a larger amount of hydroxyl ions.
As you may have guessed, a pH of 7.0 is neutral because it contains equal amounts of hydrogen (H +) and hydroxyl (OH-) ions. Pure, unpolluted rainwater or distilled water is neutral, because it has a pH of 7.0.
It is interesting to note that the pH scale is a logarithmic scale. This means that a change of one pH unit indicates a ten fold increase in the concentration of hydrogen ions. For instance, a solution that has a pH of 2 has ten times as many free H+ ions as a solution with a pH of 3!
Non-aqueous liquids or solutions (solvents such as gasoline, mineral spirits, chlorinated safety solvents) have neither hydrogen or hydroxyl ions. They do not have a pH, because they do not contain any water. As we mentioned before, pH is a characteristic of water solutions only. Without any water, we cannot have a pH.
Soil is nothing more than matter in the right or wrong place. For example, grease in a frying pan is good; grease on the stove top is bad. Dirt in the family garden is good; dirt on your wife's white carpet is bad - very bad!
There are various types of soil:
1. Inorganic soil: Matter that was never "alive", and thus contain no carbon.
2. Organic soils: Matter that once "lived" and that does contain carbon.
3. Petroleum soils: Motor oils, axle greases, wax, gums and other products made from petroleum. These soils contain no water - in fact they repel water - and thus do not have a pH. They often require another petroleum based solvent to remove them.
4. Combination soils: These are soils that contain an inorganic plus an inorganic soil and/or a petroleum substance These soils are difficult to remove because they are hard to identify. Once identified, they usually require a combination type cleaner - alkalines and solvents or acids and solvents.
III. CLEANING AGENIS
Cleaning agents usually contain some combination of ingredients to help them do their job: remove unwanted soils! The following ingredients all have a specific job to do in a cleaner formula:
Cleaners can be formulated to do a specific job. Sometimes a cleaner will not contain many of these ingredients; sometimes it will contain almost all of them.
This long list of ingredients might give you the idea that a cleaner is a very complicated mixture, and indeed it is. A good cleaning solution must be complex, because it has much to do. "Cleaning" (or "soil removal") is a series of events whereby the soil is wetted, loosened, broken up, suspended, dissolved, dispersed, and prevented from re-depositing. Also the cleaner can be formulated to work on a specific soil or a specific surface. Ingredients can be added or deleted to make a special purpose cleaner.
Let us examine the different ingredients that go into a cleaner, keeping an eye on the specific job each ingredient will do.
All cleaners require some sort of liquid solvent. This solvent not only "dissolves" the soil, but also provides a medium in which soil can be suspended and carried away from the surface. Water is the oldest, least expensive and most widely used cleaning solvent known to man. In time, water will clean or remove just about every type of soil, be it organic, inorganic, petroleum or combination.
Other solvents may also be used, including petroleum distillates (such as mineral spirits) and chlorinated solvents (such as methylene chloride). These are especially effective on petroleum type soils, or where water could damage the item being cleaned - such as on a wood surface.
2. Surfactants: detergents or soaps
The word surfactant is short for "surface active agent". Surfactants work at the boundary layer (the interface) between the soil and the solvent. Each surfactant molecule has two chemical groups; one that is attracted to water (the hydrophile) and one that is attracted to soil (the hydrophobe).
Soaps have the same structures and work in the same way. The only real difference between soap and detergent is that soap is made from natural materials, animal fat and sodium hydroxide (lye).
3. Penetrating and wetting agents
Modern technology can produce many different types of surfactants by changing the chemical composition of the hydrophobic and hydrophillic ends of the molecule. By changing the chemical composition, we can create surfactants that have greater or lesser abilities in different areas:
a. Detergency: the ability to break the bond between soil and the
Often, a surfactant that is an excellent detergent is not a very good penetrating agent. To make the best cleaner possible, the formula may include a different surfactant that is an excellent wetting and penetrating agent. This second surfactant will work along with the detergent or soap to increase the cleaning ability.
Sometimes, many surfactants will be used in combination to create a cleaner with just the right balance of detergency, foaming, wetting, emulsifying, solubilizing and dispersing properties. Each surfactant contributes its own special abilities to the cleaner formula.
4. Chelating agents
Chelating agents "tie up" the hardness in water. Dissolved minerals in water are the cause of hardness. Rain and distilled water is pure and soft, but it quickly dissolves minerals (mainly calcium, magnesium, iron and silica) as it soaks into the ground and travels through rivers or lakes. As the water becomes loaded with dissolved minerals, it becomes hard. The hardness of water is measured in either grains of hardness or parts per million:
(1 grain of hardness = 17.1 parts per million, or ppm).
Hardness in water will hinder the cleaning ability of a cleaning solution. This is because the detergents and other active ingredients in the cleaner see the hardness minerals as "soil". These actives are used up by the hardness, and are not available to clean the soil we want them to.
This hardness problem is solved by adding chelating agents to the cleaner formula. These chelating agents can efficiently and effectively "tie up" the hardness minerals, leaving the rest of actives to work on the target soil. Because they are so efficient, it takes only a very small amount of chelating agent to eliminate a lot of hardness from the water.
Saponifiers are strongly alkaline chemicals that convert animal fats and oils into natural soaps. This is the very same chemical reaction that has been used to make real natural soaps for many centuries, and the conversion of fats to soap is called saponification. Once the fats and oils are converted to soap, they are soluble in water and can be easily washed away.
Builders give the cleaning solution "reserve strength" to enable it to withstand heavy soil loads. Various phosphate, carbonate silicate and citrate salts are used as builders in modern cleaning solutions.
THE LAW OF MASS CLEANING ACTION
We can see, then, that the type of soil usually dictates the type of cleaner that we will use to remove it.
But we must also remember to take the surface into account. For instance, water based cleaners can damage delicate wood surfaces, so a petroleum based cleaner - containing no water - is appropriate.
We must also pay attention to the aggressiveness of a cleaner. A marble surface would be attracted by a strong (aggressive) acid cleaner. If we need to use an acid cleaner to remove inorganic soil from marble, we must use a very mild (non-aggressive) acid cleaner.
The law of mass cleaning action expresses a relationship between time, action, concentration, and temperature in the process of removing soils. This laws states that if you decrease any one of these factors, we must increase one or more of the remaining factors in order to maintain equal cleaning ability.