Soap and Water Surface Tension
Name: Michelle S.
Why does soap change the surface tension of water on wax
paper, typing paper,and paper towel?
Each of these types of paper are quite different substrates. Soap is one
of a class of chemical substances called surfactants, an acronym for surface
active agents. In general surfactants have a hydrophylic [water loving] end
and a hydrophobic [water hating] end. The soap [surfactant] tends to orient
on the surface of water so that the hydrophylic end is directed toward the
water phase and the hydrophobic end is oriented toward air or whatever
hydrophobic surface is in contact with the aqueous solution. This surface
orientation property greatly changes the surface properties of the water
In the case of wax paper -- very hydrophobic -- the hydrophobic end of the
surfactant adheres to, or is attracted to, this hydrophobic surface. However
this hydrophobic end is attached to the hydrophylic end, which is in turn,
buried in the water phase. As a result the water phase "wets" the wax paper,
coupled by the surfactant. In the absence of surfactant the water would
"bead up" on the wax paper. This coupling of hydrophobic and hydrophylic
phases by the surfactant is also the reason "soapy" water removes oily dirt
Typing paper, and other copier paper, is of course cellulose fiber. However,
it is usually surface treated with a variety of agents that make the paper
more receptive to the inks used in typing, copying, and computer printer
inks. These treatments tend to make the surface more hydrophobic. These
papers are also more porous than wax paper, so that is an issue too. In
these types of paper soap, or other surfactant, tends to make the paper more
receptive to the usually more hydrophobic inks. In actual practice this is a
more sophisticated because ink receptivity and what is called ink "holdout"
has to be more carefully balanced. You don't want the ink to stay completely
on the surface, but at the same time you don't want the ink to get sucked
into the paper too much either.
Paper towels need to be designed to absorb both water and oil -- paper
towels need to be porous, so those types of papers have lower fiber density,
do not have any surface treatment, but may contain surfactants that render
the cellulose fiber receptive to both oily and watery liquids.
So papers in general are highly formulated products to meet various
properties needed for the application for which they are intended.
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Update: June 2012