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.
II. SOIL
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. a. Scale and lime deposits, such as water spots. b. Rust, corrosion,
and oxidation. c. Minerals and rock formation.
2. Organic soils: Matter that once "lived" and that does contain carbon.
a. Body oils and animal fat. b. Carbohydrates and proteins. c. Mold and
yeast. d. Bacteria and animal waste.
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 AGENTS
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:
1. Solvent
2. Surfactants: detergents or soap
3. Penetrating and wetting agents
4. Chelators
5. Saponifiers
6. Builders 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.
1. Solvent
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).
In a cleaning solution, the hydrophobic end of the surfactant molecule
orients toward the soil. Many surfactant molecules will attack the soil,
breaking it up into small pieces and completely surrounding it. The hydrophillic
ends of the surfactant molecules project into the solvent, causing the
soil to be broken up, removed from the surfaces, lifted and suspended
into the cleaning solution. 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 surface.
b. Penetrating and wetting: allows water to surround soil particles that
would otherwise repel the water. c. Foaming: creation of bubbles that
lift dirt from the surface. d. Emulsifying: the ability to break up greasy
petroleum soils into small droplets that can be dispersed thoroughly.
e. Solubilizing: dissolving a soil so that it is no longer a solid soil
particle. f. Dispersing: spreading the minute soil particles throughout
the solution - to prevent them from sticking to a mop, bucket or back
onto the cleaned surface. 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.
5. Saponifiers
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.
6. Builders
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. Inorganic
soil is cleaned with acid cleaners Organic soil is cleaned with alkaline
cleaners Petroleum soil is cleaned with petroleum based cleaners Combination
soil is cleaned with combination cleaners 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. For product information e-mail us at [email protected]
or use our Information Request Form
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