Why doesn't oil dissolve in water? And why does water tend to cause
oil molecules to clump?
Hydrophilic and Hydrophobic Substances(from Campbell
6, p. 46)
"Whether ionic or polar, any substance that has an affinity for water
is said to be hydrophilic (from the Greek hydro,
water, and philios, loving). This term is used even if the substance does
not dissolve-because the molecules are too large, for instance. Cotton, a
plant product, is an example of a hydrophilic substance that absorbs water
without dissolving. Cotton consists of giant molecules of cellulose, a compound
with numerous regions of partial positive and partial negative charges associated
with polar bonds. Water adheres to the cellulose fibers. Thus, a cotton towel
does a great job of drying the body, yet does not dissolve in the washing
machine. Cellulose is also present in the walls of water-conducting vessels
in a plant; you read earlier how the adhesion of water to these hydrophilic
walls functions in water transport.
There are, of course, substances that do not have an affinity for water.
Substances that are non-ionic and nonpolar actually seem to repel water;
these substances are termed hydrophobic (from the Greek phobos,
fearing). An example from the kitchen is vegetable oil, which, as you know,
does not mix stably with watery substances such as vinegar. The hydrophobic
behavior of the oil molecules results from a prevalence of nonpolar bonds,
in this case bonds between carbon and hydrogen, which share electrons almost
equally. Hydrophobic molecules related to oils are major ingredients of cell
membranes. (Imagine what would happen to a cell if its membrane dissolved.)"
How do detergents remove grease stains?
"Let's assume we have oily, greasy soil on clothing. Water alone will
not remove this soil. One important reason is that oil and grease present
in soil repel the water molecules. Now let's add soap or detergent. The surfactant's
water-hating (hydrophobic) end is repelled by water but
attracted to the oil in the soil. At the same time, the water-loving (hydrophilic)
end is attracted to the water molecules. These opposing forces loosen the
soil and suspend it in the water. Warm or hot water helps dissolve grease
and oil in soil. Washing machine agitation or hand rubbing helps pull the
soil free."(from the Soap
and Detergent Association's Soap Chemistry Page)
