Video Transcript
The following image shows a cartoon
structure of sodium 4-dodecylbenzenesulfonate. What arrangement will these
molecules form when highly concentrated in water?
Sodium 4-dodecylbenzenesulfonate is
an example of anionic detergent. A detergent is a class of
substances with cleaning properties that has a hydrophilic head and a hydrophobic
tail. Detergents are classified by the
head region. An anionic detergent has a
negatively charged head region. This head region is represented by
the orange circle in the cartoon structure. The hydrocarbon chain attached to
the head region is the hydrophobic tail. In the cartoon structure, the blue
wavy line represents the tail. The sodium ion is a counterion,
which balances out the charge of the head region. The counterion is not explicitly
shown in the cartoon structure.
Using the cartoon structure, we
need to determine what arrangement anionic detergent molecules will have when highly
concentrated in water. To think about this further, let’s
clear some space. To better understand what happens,
when a detergent is added to water, let’s break down the terms hydrophilic and
hydrophobic. Hydrophilic can literally be
translated to water loving, and hydrophobic can literally be translated to water
fearing. So when a detergent is first added
to water, the molecules tend to accumulate at the surface of the water, with the
hydrophilic heads attracted to the water molecules and the hydrophobic tails
sticking into the air.
When the concentration of the
detergent is increased or the detergent is mixed with the water, the detergent
molecules will group together to form spheres called micelles, where the hydrophilic
heads are on the outside of the sphere in contact with the water and the hydrophobic
tails are inside the sphere insulated from the water. Looking at the answer choices, we
can see that the figure which best represents a micelle is the figure shown in
answer choice (A), which shows the molecules arranged in a circle with the heads
forming the outside of the circle and the tails on the inside of the circle.
So the arrangement of sodium
4-dodecylbenzenesulfonate molecules when highly concentrated in water is best
represented by the figure shown in answer choice (A).
