Video Transcript
As well as removing stains, what
other effect can anionic detergent molecules, such as those of alkylbenzene
sulfonates, have when added to stained items in water? (A) Precipitating out metal
cations. (B) Repairing any structural damage
on the surface of the stained item. (C) Softening clothes and other
fabrics. (D) Reducing the surface tension of
water. Or (E) penetrating and breaking
down soils.
Detergents are a class of
substances with cleaning properties. The cleaning properties of a
detergent are a result of their unique structure. Detergent molecules consist of a
polar, hydrophilic head region, which can interact with water molecules, and a
nonpolar, hydrophobic tail region, which can interact with nonpolar fats and
oils.
There are several types of
detergent molecules that are classified based on the structure of the head
region. The head region of an anionic
detergent is negatively charged and is often accompanied by a positively charged
counterion. Examples of anionic detergents
include alkylbenzene sulfonates, like sodium 4-dodecylbenzenesulfonate.
To determine what other effect
apart from removing stains anionic detergent molecules can have when added to
stained items in water, let’s clear some space and consider what occurs when a
generic anionic detergent is first added to water. When an anionic detergent is first
added to water, the counterion will dissolve and disperse throughout the water. The head of the detergent is
hydrophilic or water-loving, so the heads will be attracted to the water molecules
at the surface. But the tails are hydrophobic or
water-fearing, so they will stick up into the air.
Already, the detergent is having an
effect on the water. Water molecules are held together
by strong hydrogen bonding. The introduction of a detergent
interferes with the hydrogen bonding between the water molecules at the surface. This decreases the surface tension
of water. The decreased surface tension
allows water to wet clothing more easily and is the reason why water mixed with a
detergent can foam or produce lots of semistable bubbles.
From this discussion, we can see
that the other effect an anionic detergent can have when added to stained items in
water is answer choice (D), reducing the surface tension of water.
