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Overview
Webster
defines soil as "to make dirty or unclean on the surface; to foul;
to dirty; to defile; as, to soil a garment with dust." For our
purposes, we will view soil as foreign matter that happens to be in the
wrong place. For example, yogurt is food, but if it's ground into carpeting,
it is considered soil.
What makes a cleaner work? How much chemistry is involved in removing
grease from a stove top or grit from a concrete floor? The answer to these
and other questions lies within words like surfactant, solvent, chelating
agent and builder. Understanding the basic elements of a cleaner's effectiveness
against different types of soil is essential to the "Chemistry of
Cleaning."
Soil
Types
Soil
can be broken down into three broad categories: organic, inorganic and
combination.
Organic soils encompass a broad range and include food soils such as
fat, grease, protein, and carbohydrate, living matter such as mold,
yeast and bacteria and petroleum soils such as motor oil, axle grease
and cutting oils. Most of the time organic soils are best removed using
alkaline cleaners or solvents.
Inorganic soils include rust, scale, hard water deposits and minerals
such as sand, silt and clay. Oftentimes acids are used to remove inorganic
deposits such as rust and scale. Minerals are often cleaned with general
purpose cleaners.
Combination soils often present the toughest challenge for a cleaner
since the soil contains both organic and inorganic components. Proper
identification is critical. Most combination soils are removed with
a very concentrated, highly built cleaner that also contains solvent.
Surfactants
A
surfactant is the most important part of any cleaning agent. The word
surfactant is short for "Surface Active Agent." In general,
they are chemicals that, when dissolved in water or another solvent,
orient themselves at the interface (boundary) between the liquid and
a solid (the dirt we are removing), and modify the properties of the
interface.
How does a surfactant work? All have a common molecular similarity.
One end of the molecule has a long nonpolar chain that is attracted
to oil, grease, and dirt (the hydrophobe). Another part of the molecule
is attracted to water (the hydrophile). The surfactant lines up at the
interface as diagrammed below. The hydrophobic end of the molecule gets
away from the water and the hydrophilic end stays next to the water.
When dirt or grease is present (hydrophobic in nature) the surfactants
surround it until it is dislodged from the boundary. Notice in diagram
4 that the dirt molecules are actually suspended in solution.
It should be noted that a surfactant can be either a soap or a synthetic
detergent. Soaps have been used for centuries because they are made
from natural materials such as animal fat and lye. Synthetics have only
become available over the last 40 years. Soaps are still commonly used
in personal hygiene products because of their mildness. Synthetic detergents
are the surfactants of choice for almost all other cleaning agents.
Chelating
Agents
Soil
removal is a complex process that is much more involved than just adding
soap or surfactant to water. One of the major concerns we have in dealing
with cleaning compounds is water hardness. Water is made "hard"
by the presence of calcium, magnesium, iron and manganese metal ions.
These metal ions interfere with the cleaning ability of detergents.
The metal ions act like dirt and "use up" the surfactants,
making them unavailable to act on the surface we want to clean.
A chelating agent (pronounced keelating from the Greek word claw) combines
itself with these disruptive metal ions in the water. The metal ions
are surrounded by the claw-like chelating agent which alters the electronic
charge of the metal ions from positive to negative (see diagram below.)
This makes it impossible for the metal ions to be precipitated with
the surfactants. Thus, chelated metal ions remain tied up in solution
in a harmless state where they will not use up the surfactants.
Some common
chelating agents used in industrial cleaning compounds include phosphates,
EDTA (ethylene diamine tetra acetate), sodium citrate, and zeolite compounds.
Household cleaning agents, such as laundry soap, used phosphate type
chelating agents heavily in the 1950's and 60's. In the 1970's, phosphate
bans were imposed because phosphates entered back into the environment
unchanged through sewerage works and caused oxygen depletion in waterways.
Luckily, alternative chelating agents like EDTA have been developed
as phosphate substitutes.
Builders
Detergents,
as we have learned so far, consist of surfactants and chelating agents.
Remember, surfactants remove dirt from a soiled surface and chelating
agents are used to surround unwanted metal ions found in cleaning solutions.
The chelating process, though very effective, is not always necessary
and adds to the cost of formulating detergents. Builders are often a
good alternative.
Builders are added to a cleaning compound to upgrade and protect the
cleaning efficiency of the surfactant(s). Builders have a number of
functions including softening, buffering, and emulsifying.
Builders soften water by deactivating hardness minerals (metal ions
like calcium and magnesium. They do this through one of two ways:
Sequestration - holding metal ions in solution.
Precipitation - removing metal ions from solution as insoluble
materials.
Builders, in addition to softening, provide a desirable level of alkalinity
(increase pH), which aids in cleaning. They also act as buffers to maintain
proper alkalinity in wash water.
Finally,
builders help emulsify oily and greasy soil by breaking it up into tiny
globules. Many builders will actually peptize or suspend loosened dirt
and keep it from settling back on the cleaned surface. Below are three
of the most common builders used in today's heavy-duty detergents. A
short description of each follows.
Phosphates, usually sodium tripolyphosphate (STPP), have been
used as builders extensively in heavy-duty industrial detergents. They
combine with hardness minerals to form a soluble complex which is removed
with the wash water. They also sequester dissolved iron and manganese
which can interfere with detergency.
Sodium carbonate (soda ash) is used as a builder but can only
soften water through precipitation. Precipitated calcium and magnesium
particles can build up on surfaces, especially clothing, and therefore
sodium carbonate is not used in laundry detergents.
Sodium silicate serves as a builder in some detergents when used
in high concentrations. When used in lower concentrations, it inhibits
corrosion and adds crispness to detergent granules.
Solvents
Detergents,
as we have learned so far, consist of surfactants, chelating agents
and builders. Remember that surfactants are designed to remove dirt
from a soiled surface. Chelating agents and builders are added to the
formula to keep water hardness from interfering with the cleaning process.
Water makes up a large percentage of most liquid cleaner formulas. It
is not uncommon for water-based detergents to contain 50% water or more.
Some ready-to-use formulations may contain as much as 90% to 95% water!
With this much water present in a cleaner, why do they work so well?
Water can be considered an active ingredient that actually adds to the
detergency of cleaners. It performs several very important functions
in liquid cleaners. Most importantly, it adds to the "detergency"
of a cleaner. Water acts as a solvent that breaks up soil particles
after the surfactants reduce the surface tension and allow the water
to penetrate soil (water is commonly referred to as “the universal
solvent”).
One can visualize how this works if you think of your own clothes washing
machine. Think about what would happen if you were to add a cup of detergent
to your washer and wash a load of clothes with no other water added.
Your clothes certainly would not come out clean! Water is necessary
for the laundry detergent to work properly.
Water also aids in the suspension and anti-redeposition of soils. Once
the soil has been dissolved and emulsified away from the surface, we
want to prevent it from being redeposited. Water keeps the soil suspended
away from the clean surface so that it can be carried away easily during
the rinsing process. It is clear that without this water, our cleaning
formulas would be much less effective.
In addition to water, other chemical solvents are often added to cleaners
to boost performance. Compounds such as 2-Butoxyethanol (butyl), isopropyl
alcohol (rubbing alcohol) and d-Limonene are all considered solvents.
Their main function is to liquefy grease and oils or dissolve solid
soil into very small particles so surfactants can more readily perform
their function.
Preservatives
A
preservative is nothing more than a substance that protects soaps and
detergents against the natural effects of aging such as decay, discoloration,
oxidation and bacterial degradation. Synthetic detergents are preserved
differently from soaps as we will see.
In soaps, preservatives are used to forestall the natural tendency to
develop rancidity and oxidize upon aging. Butylated hydroxdytoluene
(BHT) and stannic chloride are commonly used in this application. Also
used in small amounts is EDTA.
In detergents, preservatives are used to prevent bacteria from spoiling
the solution. Methyl paraben and propyl paraben are very common for
this application. Detergents would not be preserved if they weren't
biodegradable. Bacteria found in air, waste treatment systems and in
soil decompose the surfactants and other ingredients found in our cleaners
once they enter into the environment.
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