Kitchen science

Water can't do the cleaning job alone. Enzymes, abrasives and more help with the dirty work.

By Times staff writer

© St. Petersburg Times, published May 6, 2000

How do detergents get your clothes or surfaces clean?

Here's a simple explanation, offered by the Soap & Detergent Association.

Let's start with water, which possesses a property called surface tension that causes it to bead up on surfaces. That slows the wetting of the surface. How many times have you spilled some water on fabric and it just beads up and sits there and doesn't penetrate the fabric?

So the first goal in cleaning is to reduce the surface tension of water so it can spread and wet the surface in question. Chemicals that do this are called surfactants. They make water wetter.

Surfactants perform other important cleaning functions. They loosen dirt. They emulsify it -- that is, they disperse it in water -- and they hold soil in suspension (so it doesn't settle back on the item being cleaned) until it can be rinsed away.

So the perfect detergent will have two properties: (1) Part of it will be water-loving, i.e., it will reduce the water's surface tension so it can wet the surface being cleaned; and (2) Part of it will be attracted to oil and grease, to do the actual cleaning.

Detergent surfactants were developed in response to a shortage during World Wars I and II of animal and vegetable fats and oils, which are used to make soaps. Scientists also wanted to develop a substance that worked well in hard water to make cleaning more effective. Detergents today are made from petrochemicals and/or oleochemicals (derived from fats and oils).

Petroleum and fats and oils contain hydrocarbon chains that are repelled by water but are attracted to oil and grease in soils. Other chemicals are used to create the water-loving part of the surfactant, to break that surface tension.

So how do these chemicals work to clean, say, a dirty item of clothing?

Three types of energy are needed, interacting in proper balance.

Water alone will not remove the soil on clothing. The grease and oil repel the water molecules.

So we add soap or detergent (chemical energy). The surfactant's grease-loving part will be attracted to the oil in the soil, and the water-loving part will be attracted to the water molecules.

These opposing forces loosen the soil and suspend it in the water. Warm or hot water (thermal energy) helps dissolve grease and oil in soil. Washing-machine agitation or rubbing by hand (mechanical energy) helps pull the soil free.

For household cleaners, "use a water temperature that feels good to your hands," said a spokeswoman for the Soap & Detergent Association. "These products are formulated to work with room-temperature water. That's not to say, if you had really greasy soil, you might find you had to put less effort into cleaning with hotter water. But as a general rule, room temperature water is fine."

Scientists can add a variety of ingredients to household cleaners to increase their effectiveness in performing specific jobs. Abrasives -- small particles of minerals -- contribute to the mechanical effectiveness of scouring cleansers. Acids are often added to tub, tile, sink and toilet-bowl cleaners to dissolve calcium and metal salts. Anti-microbial agents can destroy bacteria and viruses and act as disinfectants.

Enzymes are proteins that break down soil into simpler forms that can easily be removed by the cleaner. Some enzymes work well on starch, some on oily soil, some on protein. You'll often see enzyme cleaners marketed for clothes washing.

Polymers dry to a thin film that protects a surface and provides a shine, desirable in floor-care products. Solvents help to remove grease without leaving a residue. These are the "No rinsing needed!" products we use to clean windows or walls.

For a more detailed explanation of cleaning, visit the Soap & Detergent Association's Web site: http://www.sdahq.org. (For the consumer version, click on "Cleaning Products Overview" and then on "Chemistry." For the professionals, click on "Household Cleaning Products" and then on "Fact Sheet Notebook."

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