Soap molecules have two ends; one is a carboxylate, and the other is a
hydrocarbon chain. The carboxylate end of the molecule is hydrophilic (“hydro”
meaning water, “philic” meaning attracted to), while the hydrocarbon chain is
hydrophobic (meaning repelled by water). Nearly all substances are either hydrophilic
or hydrophobic; anything that will dissolve in water is considered hydrophilic.
Anything that won’t, is considered hydrophobic.
Naturally, oil and water don’t mix, because oil molecules are hydrophobic. Most
soil contains some variety of oil, which means that it will not dissolve in water without
the help of an emulsifier, such as soap. The hydrophilic end of the soap molecules
allows the soap to be water-soluble, despite the oils contained in it. The hydrophobic
end is the portion that allows the soap to grab onto the grease molecules.
Grease normally adheres to skin and clothing; however, the hydrophobic ends of
the soap molecules form micelles, or tiny clusters, which surround the grease
molecules. The grease is held by the hydrophobic end, while the hydrophilic end
allows the suspended grease particles to be rinsed away with clean water.
Water hardness is a factor that greatly impacts the efficacy of soap. Hard water
refers to water that has an overabundance of minerals, such as calcium, iron,
manganese, or magnesium. When the soap is used, some of the soap molecules
attach themselves to these minerals, instead of to the soil that is supposed to be
cleansed. This creates a twofold problem; there are fewer soap molecules available
for cleaning, and the molecules that react with the minerals create a hard scum on
fabrics, skin, or other surfaces.
Because most soap molecules are fairly similar, and not very versatile, many
commercial soaps now use synthetic surfactants or detergents, which, because of their
chemical composition, can be engineered to be effective in a variety of conditions.
The downfall of synthetic detergents, however, is that they can be harsh and drying to
skin – imagine washing your whole body with dishwashing liquid, or glass cleaner. It
works well on your kitchen or bathroom surfaces, but you wouldn’t want it all over
you.
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