Ask A Scientist Physics Archive

name         Tillu J.
status       student
age          17

Question -   Why does the surface tension of water decrease on adding
detergents? What is the phenomenon behind decrease in surface tension of
water?
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Tillu J,:

In the solid and liquid phase, water molecules are bonded to each other by
virtue of an interaction between hydrogen atoms on one molecule and the
oxygen atom of another -- the process is referred to as "hydrogen bonding."
The result is a kind of "skin" on liquid water's surface.

Soaps and detergents have a polar head to which water is attracted and a
non-polar tail that is hydrophobic -- water repelling. When these substances
are present, they weaken the strength of the skin by interfering with
hydrogen bonding between water molecules because the polar end of the soap
or detergent is also attracted to the water.

Regards,
ProfHoff 582
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Dear Tillu,

I found the answer to your question in the book, Conceptual Physical
Science, second edition, by Hewitt, Suchoki, and Hewitt copyright 1999, p
443.

     "The surface tension of a liquid is defined as the energy required to
break through the surface.  Liquids in which there are strong molecular
interactions, such as water, typically have high surface tensions.  Surface
tension accounts for the spherical shape of liquid drops.  The surface
molecules of a liquid are pulled sideways, but attractions from underneath
also pull them down into the liquid.  This pulling of surface molecules into
the liquid causes the surface to contract and become as small as possible.
Guess which geometrical shape has the least surface for a given volume?
That's right-a sphere.  So we see why raindrops, drops of oil, and falling
drops of molten metal are all spherical.
     The surface tension of water is dramatically reduced by the addition of
soap or detergent.  Soap or detergent molecules tend to aggregate at the
surface of the water, where their non-polar tails stick out away from the
water.  At the surface, these molecules interfere with the dipole-dipole
interactions among water molecules, thereby reducing surface tension, often
as much as 90 percent."

I hope this helps.

Martha Croll
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The surface tension is determined by what molecules are present in the one
or so layers of molecules at the surface of the interface between a liquid
and its vapor or air from the atmosphere. If the attraction between these
molecules is strong, as is the case for water due to hydrogen bonding, the
surface tension will be higher. For water it is about 72 dyne x cm = 72 
erg / cm^2. I prefer the units of erg / cm^2 because it says that it takes 
72 ergs of energy to create a new square cm. of surface, which I find easier to
visualize. Now molecules that are surface active (surf-act-ants) or
detergents have two properties that cause them to reduce the surface tension
of water. First, there is an excess concentration of these molecules at the
interface (called the surface excess concentration) and the forces between
these molecules is smaller than, or interfere with, the bonding between the
water molecules at the surface. So it is easier (takes less energy) to
"stretch" the surface of the interface. It turns out that a lot, but not all
by any means, of such molecules have a hydrophilic polar end, and a
hydrophobic end. The polar end sticks into the water phase and the
hydrophobic end (often a hydrocarbon) sticks up into the vapor or air phase.
They are "standing on their heads"! Since these hydrocarbon ends are more
weakly bonded to one another they are easier to stretch. As the surface is
stretched and the surface density of the "tails" decreases there are new
ones below in the water phase ready to take their place on the surface. In
fact, within the water phase these surfactants form bodies called micelles
where the hydrophilic (water loving) heads point into the water phase and
the hydrophobic tails are on the "inside" of these microscopic globules.

Vince Calder
=====================================================
The surface tension of water exists because water molecules against the air
cannot easily enjoy the same water-water interactions that the water
molecules farther from the interface can.  A water molecule in the "bulk"
phase is not very restricted; it can move about and rotate without losing
these stabilizing "hydrogen bonds."  Water up against the air, on the other
hand, cannot move into the air phase without losing all its hydrogen bonds,
and it cannot rotate to point its hydrogen atoms toward the air without
losing some of them, for instance.  So the water molecules near the air
surface are constrained in their movements by these interactions.

A detergent molecule has one end that is attracted to water, and one that is
not.  Because water molecules are so attracted to each other, a detergent
molecule placed in water will find its water-loving end surrounded by water
molecules, and its other end "squeezed out" because the water molecules
would rather touch each other than it.  At the air-water interface, the
detergent molecules will line up so that their water-loving ends face the
water phase and their other ends face the air phase.

Water molecules near the water-loving end of a detergent molecule "feel
comfortable."  They can rotate about and make hydrogen bonds to the
detergent in a similar way as to other water molecules.  If the air-water
interface is completely lined with detergent molecules, then the water near
the interface will not be as constrained as it would be without the detergent.
This means that the surface tension is lower.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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