Question

Lab Intermolecular Forces: Cleaning Up an Oil Spill

Option 1 – Chemical Agents: To speed the process of oil removal from the ocean interface, researchers use oil dispersants or surfactants, (specialized chemical agents) to break up the spill. These chemical function in the same way that dish soap functions when we wish to break up grease/oil when washing dirty dishes. Basically, these chemicals work to mix oil and water better than normally possible so that the oil sinks deeper into the water column. These chemicals are often called “Surfactants” which is short for “Surface Active Agents”.   These are long molecules that have a water loving (hydrophilic) end that is polar, and an oil loving (oleophilic) end that is non-polar. An example of a surfactant molecule that is used in soaps is sodium stearate

The molecules arrange themselves so that the hydrophilic tails are in the water and the oleophilic heads are in the oil, and they do this by lining up at the interface between the oil slick and the water. However, if enough surfactant is added the surfactant molecules begin to move into the water. These surfactant molecules form mini spherical “cells” with their oleophilic tails pointing to the center of the sphere, so that their tails do not need to be adjacent to water molecules (see Fig 7). When the surfactant molecules move into the water, they take oil droplets with them, which sit in the oleophilic centers of these “cells”. Consequently the oil slick is broken up. The cells are called micelles, and the minimum concentration of surfactant needed to ensure that these micelles begin to form is called the critical micelle concentration (CMC). Dispersing the oil in this way buries it a little deeper in the water, which means that surface slicks won't float toward shorelines as readily. However, mixing these chemicals into water has long been a controversial process, as they have proved toxic to some marine organisms.

Option 2 – The Boom: Another method used for cleaning up oil is to surround the oil slick with something called a containment boom. Basically, a containment boom is just a large float that surrounds the slick. As the boom is pulled inwards, a boat can skim the oil off the top of the water, the oil slick shrinks, until finally it is completely cleaned up. Although it is possible to clean a slick by this method, in the case of large spills the boom is mainly useful for containing oil slicks while other clean up methods are used.

Options 3 – Sand: Oil can also be caused to clump by pouring absorbent sand on it. When looking for sorbents to absorb or adsorb oil from the water, the substance should attract oil (be oleophilic) but repel water (be hydrophobic). Sand is made up mainly of silicon dioxide (SiO₂) in the form of quartz (see figure 8). This forms a grid of non-polar molecules with plenty of open spaces within the solid structure for the non-polar oil molecules to enter as the sand sinks to the bottom of the ocean. The oil is basically adsorbed onto and into the sand, which drags the oil to the bottom of the ocean in sandy clumps.

During today’s lab you will simulate an oil spill and compare and contrast three methods of clean up; using a boom, using absorbent sand and using dispersants. You will also consider how each of these methods relies on intermolecular forces.

*Questions:

1. In the chemical dispersant method, if the surfactant used is sodium stearate,

a. what is the name of the dominant intermolecular force that attracts the hydrophilic end of the surfactant molecules to the water molecules?

b. What is the name of the dominant intermolecular force that attracts the hydrophobic end of the surfactant molecules to the oil molecules?

2. In the sand method, what is the name of the dominant intermolecular force that draws the oil molecules into the spaces inside the three dimensional quartz structure formed by the SiO₂ molecules?

3. Why is the strength of the intermolecular forces between SiO₂ molecules in the sand and oil molecules comparable to (or possibly greater than) the strength of the intermolecular forces between the oil molecules, such that clumps of oil can be drawn into the sand?

4. Of the three techniques used in lab, compare, contrast and rank them from least to most useful and briefly explain your reasoning.

Answer 1

1a) The hydrophilic ends of the surfactants are polar. Water molecules are polar because of the O-H dipoles. Therefore, the interaction taking place between the hydrophilic end of the surfactant molecule and a water molecule will be ion-dipole or dipole-dipole interaction and/or hydrogen bonding. The O-H group in water in polarized with the O atom bearing a slight negative charge and the H atom having a slight positive charge. Hydrophilic ends of surfactants contain polar groups like hydroxyls (-OH), carboxyls (-COOH) or esters (-COOR). The polar ends can attach to dipolar water molecules by hydrogen bonding or by dipole-dipole attraction.

b) The hydrophobic ends of the surfactants usually contain long hydrocarbon chains. The predominant interaction between the hydrophobic chains and the oil molecules will be Vander Waal’s forces or dispersion forces or a combination of both.

2. The cavity inside quartz is non-polar where the oil molecules are “sucked in”. The predominant interaction seems to be Vander Waal’s forces or/and dispersion forces.