Measuring Van der Waals Forces

Van der Waals forces are among the weakest known chemical interactions, yet they are everywhere around us, making things stick together. In the world of nanotechnology, where particles can be very close together, Van der Waals forces can become the dominant interaction.

Van der Waals forces occur between atoms because, although an atom has no net overall charge, it is made up of different charged particles which are all moving and those movements cause tiny differences in charge that fluctuate very quickly. An atom that is experiencing a momentary tiny positive charge will then cause an adjacent atom to experience a momentary tiny negative charge, the oppositely charged atoms are attracted to each other.

Scientists have been studying Van der Waals forces by passing atoms through a grating. The atoms are attracted to the bars which separate the grating's slits by Van der Waals forces, and, depending on the strength of the interaction, the path the atoms are following is changed. The scientists can measure the degree to which this path has changed.

Reference
Vincent Lonij, Catherine Klauss, William Holmgren, Alexander Cronin. Atom Diffraction Reveals the Impact of Atomic Core Electrons on Atom-Surface Potentials. Physical Review Letters, 2010; 105 (23) DOI: 10.1103/PhysRevLett.105.233202

Further Reading
Intermolecular Forces
Intramolecular Forces

Study Questions

1. What is meant by the term intermolecular force?
2. How is an intermolecular force different to a intramolecular force?
3. Name three intermolecular forces.
4. Place the intermolecular forces above in order of increasing strength.
5. Name the type of intermolecular force that is predominant in interactions between molecules of:
   
• water
• paraffin wax
• methane
• ethanol (ethyl alcohol)
• trichloromethane
• nitrogen
• ammonia
• tetrachloromethane
6. For each pair of substances below, which will have the higher boiling point?
   
• octane and water
• ammonia and nitrogen
• ethanol (ethyl alcohol) and octanol
• chloromethane and methane
• hydrogen bromide and hydrogen fluoride
7. Explain why Van der Waals forces are considered to be very weak while ionic bonds, which depend on the attraction of oppositely charged ions, are considered to be very strong.
   

Catalysis by Gold Nanoclusters

Since the early 1980s, experiments have indicated that gold nanoparticles exhibit unexpected catalytic activity towards many industrially important chemical reactions that involve activation of atomic bonds inside oxygen or hydrocarbon molecules. Room-temperature formation of carbon dioxide, CO₂, from carbon monoxide, CO, and oxygen molecule, O₂, is one of the most extensively studied processes. A number of different factors have been suggested to contribute to the ability of gold particles to activate the O-O bond, which is considered to be the key reaction step.

Finnish scientists recently exposed monolayer-thick gold clusters to a variable number of oxygen molecules. It was found that even one gold cluster can effectively adsorb multiple oxygen molecules at the boundaries of the cluster, simultaneously weakening, stretching, the O-O bond by transferring electrons to the oxygen molecules. Taking into account the effects of temperature and ambient pressure, the calculations predicted that the oxygen molecules will completely dissociate and the oxygen and gold atoms will form one-dimensional alternating chains at the cluster boundary. The oxygen atoms in these chains are negatively charged and the gold atoms positively charged, creating a system that is reminiscent of a one-dimensional gold-oxide chain. These chains are expected to be the highly catalytically active part towards conversion of carbon monoxide to carbon dioxide at room temperature.

At room temperature and pressure, it appears that gold can catalyse an oxidation reaction by first oxidizing itself to gold oxide, which seems to contradict the known properties of gold in the macroscopic level.

References

1. Pentti Frondelius, Hannu Häkkinen and Karoliina Honkala. Formation of Gold(I) Edge Oxide at Flat Gold Nanoclusters on an Ultrathin MgO Film under Ambient Conditions. Angewandte Chemie International Edition, 2010; DOI: 10.1002/anie.201003851
2. X. Lin, N. Nilius, H.-J. Freund, M. Walter, P. Frondelius, K. Honkala, H. Häkkinen. Quantum Well States in Two-Dimensional Gold Clusters on MgO Thin Films. Physical Review Letters, 2009; 102 (20) DOI: 10.1103/PhysRevLett.102.206801

Further Reading
Naming Compounds
Writing Formula
Balancing Chemical Equations
Oxidation States
Transition Metals
Energy Profiles
Reaction Rate

Study Questions:

1. Write a balanced chemical equation for the formation of carbon dioxide from carbon monoxide and oxygen.
2. For the reaction above, what other possible steps in the reaction mechanism could be rate determining steps?
3. Why do you think that scientists believe that the activation of the O-O bond is the key reaction step in the reaction mechanism for this reaction?
4. What is meant by the term catalysis?
5. Why is gold described as a catalyst for the reaction described in the article?
   
6. What is meant by the term dissociate?
7. Describe how oxygen molecules can dissociate.
8. What is meant by the term oxidize?
9. Given the position of gold in the Periodic Table, what oxidation states are possible?
10. Give the formula for two possible oxides of gold.
11. Name each of the oxides above.
    

Xanthophylls

Plant pigments are an important source of non-toxic compounds for use as food or cosmetic colouring agents. The coloured xanthophylls in capsicum are important sources of pigments that can replace carcinogenic synthetic dyes.


Xanthopylls are very similar to carotenes but often contain hydroxyl groups.
Cryptoxanthin, shown to the right, is an example of a xanthophyll.

β-carotene, shown on the left, is an example of a carotene.

Until now, the common method for extracting red pigments from dried fruit of Capsicum has used hexane as the extraction solvent. Current extraction processes are limiting in that the red pigment can only be recovered from American paprika varieties or other mild cultivars.

A study from the New Mexico State University presents a process for the efficient extraction of these pigments using a "green chemistry" method that generates an oleoresin from dried capsicum fruit with virtually the same xanthophyll composition as the hexane extraction method.

Reference
Richard D. Richins, Laura Hernandez, Barry Dungan, Shane Hambly, F. Omar Holguin, and Mary A. O'Connell. A 'Green' Extraction Protocol to Recover Red Pigments from Hot Capsicum Fruit. HortScience, 2010; 45: 1084-1087

Further Reading
Functional Groups
Empirical and Molecular Formula
Molecular Mass (Formula Weight)
Percentage Composition
Oxidation of Alcohols
Intermolecular Forces

Study Questions

1. Circle the hydroxyl group in cryptoxanthin.
2. Write the molecular formula for cryptoxanthin.
3. What is the empirical formula for cryptoxanthin?
4. What percentage by mass of carbon is present in cryptoxanthin?
5. Write the molecular formula for β-carotene.
6. What is the empirical formula for β-carotene?
7. What percentage by mass of hydrogen is present in β-carotene?
8. Which molecule, β-carotene or cryptoxanthin, would be the most polar? Explain your answer.
9. If you were given a solution that contained either cryptoxanthin or β-carotene, how might you be able to decide which of the compounds is present in the solution? Explain your answer.
10. If cryptoxanthin were to react with a strong oxidizing agent, what product(s) might you expect from the reaction? Explain your answer.
    

Titanium Dioxide To Treat Water

Scientists have discovered that adding silicone to titanium dioxide increases its ability to degrade aerosol and water-borne viruses.

Titanium dioxide (titanium IV oxide) is a naturally occurring compound that is used as a pigment in white paint, in sunscreens, in food colouring (E171), and as a disinfectant, used to kill viruses and bacteria and decompose organics via photocatalysis.

Silicone sealant is used to join glass plates in aquaria, silicone grease and lubricants are widely used in the automotive industry, silicone is used in cookware, liquid silicone is used as a dry cleaning solvent, silicone in gel form is used in medicine, and silicones are found as ingredients in shampoos, hair conditioners and gels.

Silicones are polymers made up of silicon as well as carbon, hydrogen, oxygen and sometimes other elements. They have the general chemical formula [R₂SiO]_n, where R is an organic group. Silicone polymers have an inorganic silicon-oxygen backbone: -Si-O-Si-O-Si-O-Si- with organic side chains attached to the silicon atoms.

Silicones can be produced from the reaction between chlorosilanes and water:
nSi(CH₃)₂Cl₂ + nH₂O →[Si(CH₃)₂O]_n + 2nHCl

In places where they don't have water treatment plants, water is disinfected using the SODIS method in which water is exposed to sunlight for its heat and ultraviolet radiation, but it takes a very long time to make water safe to drink. Treating titanium dioxide with silicone before adding it to the water sample to be disinfected can drastically reduce the time taken to disinfect the water.

Reference
Huma R. Jafry, Michael V. Liga, Qilin Li, Andrew R. Barron. Simple Route to Enhanced Photocatalytic Activity of P25 Titanium Dioxide Nanoparticles by Silica Addition. Environmental Science & Technology, 2010; 101231104927031 DOI: 10.1021/es102749e

Further Reading
Pure Substances and Mixtures
Elements and Compounds
Percentage Composition
Yield
Gravimetric Analysis
Water Analysis

Study Questions

1. What is the formula for titanium dioxide?
2. Calculate the percentage of titanium in titanium dioxide.
3. Calculate the percentage of oxygen in titanium dioxide.
4. A 100g sample of crude titanium oxide ore was known to contain 70% titanium dioxide. Calculate the mass of titanium present in this ore sample.
5. One method for obtaining pure titanium dioxide is to reduce the crude ore with carbon, then oxidize it with chlorine to produce titanium tetrachloride. The titanium tetrachloride is then distilled, then oxidized in a pure oxygen flame or plasma to produce pure titanium dioxide and regenerate the chlorine. Draw a flow chart to represent this purification process.
   
6. What property of titanium dioxide makes it useful as a pigment in white paint?
7. What is the formula for chlorosilane?
8. Given the general silicone formula [R₂SiO]_{n, calculate the percentage of carbon present in each of the following silicones:}
   
• R = methyl groups
  
• R = ethyl groups
• R = propyl groups
• R = butyl groups
9. Draw a structure for a silicone in which R are methyl groups.
   

Colouring Plastics

At a temperature of 30.1^oC and a pressure of about 73.8 bar, carbon dioxide goes into a super critical state that gives the gas solvent-like properties. In this state it can be introduced into polymers, acting as a "carrier" in which dyes, additives, medical compounds and other compounds can be dissolved:

• liquid carbon dioxide is pumped into a high-pressure container with the plastic components to be impregnated
• temperature and pressure are increased steadily until carbon dioxide reaches its supercritical state
• pressure increased to 170bar and the powdered pigment dissolves completely
• the dissolved pigment diffuses with the carbon dioxide gas into the plastic
• high-pressure container is opened, the carbon dioxide gas escapes but the pigment remains attached to the polymer

Tests have shown that nanoparticles of antibacterial agents can be impregnated into polycarbonate and that E-coli bacteria placed on the surface will be killed. Tests conducted with silica and with the anti-inflammatory active pharmaceutical ingredient flurbiprofen were also successful.

The process is suitable for use with amorphous or partially crystalline polymers such as nylon and polycarbonate, but cannot be applied to crystalline polymers.

Reference
Fraunhofer-Gesellschaft (2011, January 4). Impregnating plastics with carbon dioxide. ScienceDaily. Retrieved January 10, 2011, from http://www.sciencedaily.com­ /releases/2011/01/110103110206.htm

Further Reading
Temperature Conversions
Chemical and Physical Changes
Lewis Structures
Molecule Polarity
Intermolecular Forces
Ideal Gas Law

Study Questions:

1. Convert the information provided in the dot points in the article into a flow chart.
   
2. Give the molecular formula for carbon dioxide.
3. Draw a Lewis Structure (electron dot diagram) for carbon dioxide.
4. Is carbon dioxide a polar or non-polar molecule? Explain your answer
5. What type of solvents would be the best to use to dissolve carbon dioxide?
6. What type of solutes do you expect to dissolve in carbon dioxide? Explain your answer.
7. Given that 1bar is equivalent to 100kPa, convert the following pressures in bars to pressures in atmospheres:
   
   • 73.8bar
   • 170bar
8. At 25^oC and 1 atmosphere pressure, 1 mole of gas has a volume to 24.47L. Assuming ideal gas behaviour, what volume would this gas occupy at 30.1^oC and 73.8bar?
9. Calculate the volume of gas above at the same temperature but at a pressure of 170bar.
10. Calculate the volume per molecule of carbon dioxide gas in question in 9.
11. Do you think it is reasonable to assume ideal gas behaviour at this temperature and pressure? Explain your answer.
12. What do you think the term "supercritical state" as used in this article might mean?