Sodium dodecyl sulfate (SDS) has the structure below. The surface tension of water changes as SDS is added, as shown in the graph below.
(a) Why does the surface tension of water change as SDS is added?
The number of micelles increases, which allows the non-polar tails of SDS to clump together and not interact with the water
The SDS will segregate to the surface to replace the unsatisfied bonds of water molecules with the hydrophilic heads of the surfactant
The hyrophobic tails of the SDS interact with the water so that air doesn't have to
The SDS makes the solution turbid, which decreases the surface energy
(b) Which of the following images BEST represents how the molecules in solution will look at a molar concentration of 0.004?
(c) Which of the following images BEST represents how the molecules in solution will look at a molar concentration of 0.01?
a-2 OK
b-3 NOK
c-2 NOK
b and c are wrong
B-1
C-5
c 4
a 1
b 2
c 5
are the correct ones
a-2
b-1
c-5
To answer question (a), we need to understand how sodium dodecyl sulfate (SDS) interacts with water. SDS is a surfactant molecule that consists of a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. When SDS is added to water, it disrupts the hydrogen bonding between water molecules by replacing the water molecules at the surface with its hydrophilic head. This reduces the cohesive forces between water molecules at the surface, leading to a decrease in surface tension.
Among the answer choices given, the correct option is: The SDS will segregate to the surface to replace the unsatisfied bonds of water molecules with the hydrophilic heads of the surfactant.
Moving on to question (b), we need to determine the molecular arrangement at a molar concentration of 0.004. At this relatively low concentration, the majority of SDS molecules are dispersed as individual molecules in water. Therefore, the correct image should represent dispersed SDS molecules without any significant aggregation.
Among the images provided, the best representation for a molar concentration of 0.004 would be an image showing individual dispersed SDS molecules in solution.
Finally, for question (c), we need to identify the molecular arrangement at a molar concentration of 0.01. At this higher concentration, the SDS molecules start to aggregate and form spherical structures called micelles. In a micelle, the hydrophobic tails of SDS molecules clump together in the core, while the hydrophilic heads remain in contact with the surrounding water.
Among the images provided, the best representation for a molar concentration of 0.01 would be an image showing SDS molecules forming spherical micelles, with the hydrophilic heads facing outward and the hydrophobic tails clustered in the core.