Sticky Surfactant?

 How often have you found that, no matter how hard you try, it is impossible to get that last bit of detergent out of the plastic bottle? Do you turn the bottle upside down and wait, letting the cleaning stuff flow to the bottom, but then find there is still some left no matter how hard you squeeze the bottle? Do you then try adding a bit of water and shaking it so that you can extract just a little bit more of it out of the bottle? And then, do you eventually give up and finally chuck the bottle away, still containing a very small amount of the cleaning product?
Well, if you find this sticky surfactant problem unsatisfactory, you aren't alone!
 The reason why it is so hard to remove ALL the detergent from the bottle is the same reason why the detergent makes a good cleaning product, that is, surfactant molecules have a long non-polar chain that attracts other non-polar substances like oils and grease, and a polar or ionic head that attracts other polar substances like water. So, if you pour a detergent, containing surfactant molecules, into a non-polar plastic container like polyethylene or polypropylene, the non-polar parts of the surfactant molecule will be attracted to the non-polar surface of the bottle making it hard to get all the surfactant molecules out of the bottle.

But scientists at The Ohio State University have now developed a way to make the plastic bottles so that ALL your shampoo or "liquid" detergent will flow out of the bottle. It involves spray-coating the surface of the plastic with a solvent and ultrafine silica nanoparticles. The solvent softens the plastic enabling the silica to be embedded in the  surface formed "Y" shaped channels a few micrometers high and a few micrometers apart. The branches of the "Y" shapes overhang the plastic surface at an angle of less than 90 degrees resulting in trapped air. Surfactant molecules are then in contact with air rather than plastic so that they can form spherical beads that will roll off.

The university hopes to further develop this process and license the coating technique to manufacturers, not just for shampoo bottles, but for other plastic products that have to stay clean, such as biomedical devices or catheters.

Reference: 

Ohio State University. "Shampoo bottle that empties completely, every last drop." ScienceDaily. ScienceDaily, 27 June 2016.

Further Reading:

Soaps
Detergents
Wetting
Intermolecular Forces
Nanoparticles and Nanotechnology
Molecular Formula
2-Dimensional Structural Formula
Condensed (semi-structural) formula
Skeletal Formula
Introduction to Functional Groups
Carboxylic Acids


Suggested Study Questions

1. A typical soap molecule, sodium stearate is shown below:
   
   • Draw the full 2-dimensional structural formula for this molecule
   • Write the condensed (semi-structural) formula for this molecule
   • Write the molecular formula for this molecule
2. Draw the skeletal formula for potassium stearate:
   • draw a ring around the functional group
   • name the functional group
   • describe the non-polar part of the molecule
   • describe the polar part of the molecule
3. Draw a structural formula for stearic acid.
4. Write a chemical equation for the neutralisation of stearic acid using sodium hydroxide in aqueous solution.
5. Describe how soap removes dirt during washing.
6. Sodium dodecyl sulfate shown below is a common surfactant molecule found in detergents
   
• Draw the full 2-dimensional structural formula for this molecule
• Write the condensed (semi-structural) formula for this molecule
• Write the molecular formula for this molecule
7. Draw the skeletal formula for potassium dodecyl sulfate:
   • draw a ring around the functional group
   • name the functional group
   • describe the non-polar part of the molecule
   • describe the polar part of the molecule
8. Explain why sodium dodecyl sulfate is classified as an anionic detergent.
9. Compare molecules of sodium dodecyl sulfate and sodium stearate
   • Desribe any similarities between the two molecules
   • Describe any differences between the two molecules
   • Explain how both molecules can be used to remove dirt during washing
10. Consider the problem of detergent sticking to the inside walls of the plastic bottle.
    • Describe the physical properties of the plastic bottle that enable this to happen
    • Use a diagram to help explain why the detergent molecules can "stick" to the plastic bottle
    • Use a diagram to explain why adding water to the not-quite-empty plastic bottle allows more of the detergent to be removed

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Capillary Action

Why does cola rise up a drinking straw?
Why does water creep up paper?
Why does a tee-shirt "soak up" sweat?

AUS-e-TUTE has a new set of Surface Chemistry resources for Capillary Action (also known as capillarity, capillary motion, or, wicking). AUS-e-TUTE Members can log-in to use the new tutorial, game, test.

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Find out what you're missing at http://www.ausetute.com.au/membership.html
and register for membership at http://www.ausetute.com.au/register.html
There is a free sample tutorial on capillarity currently available at
http://ww.ausetute.com.au/capillarity.html

Keeping Glass Clear

Windows get "foggy" when water condenses on them. The glass of the window is colder than the surrounding air, and the air contains moisture which is gaseous water. When the gaseous water hits the surface of the cooler glass, the water turns into a liquid.
What happens next depends on the nature of the glass surface.

If the glass were perfectly flat, and perfectly clean, the strong adhesion between the water molecules and the hydrophilic glass would allow the water to spread out evenly over the surface of the glass so you would still be able to see through the glass.
If the glass is dirty it has an uneven cover of hydrophobic particles which reduces the adhesive forces between the water and the surface resulting in a multitude of small water droplets collecting on the surface which means it is harder to see through the glass.

Traditional ways of keeping the glass of car windows "fog free" include wiping the water off if it is on the outside, or turning on the heater or air-conditioner to increase the rate of evaporation if the "fog" is on the inside of the car.

From a chemistry point of view, we can see there are two possible approaches to making a coating for the glass surface so that it will stay "fog free":

• Make an extremely hydrophobic, transparent coating, then any water that comes into contact with the surface will not adhere at all and will just run off. This is the traditional approach used to create water repellent coatings and materials.
• Make an extremely hydrophilic, transparent coating so that any water than comes into contact with the surface will spread out evenly, making an extremely thin, transparent, layer over the surface. This is what scientists at A*STAR's Institute of Materials Research and Engineering (IMRE) have done. They have created a new technology, CleanClear, which is a durable and permanent ceramic coating that is transparent and superhydrophilic, which means it attracts water instead of repelling it.

The new transparent, superhydrophilic ceramic coating is anticipated to have many application; on car windows, glass cooking pot covers, hot food displays, it might even be used to keep your spectacles clear.

Reference:
The Agency for Science, Technology and Research (A*STAR). "Creating a water layer for a clearer view." ScienceDaily. ScienceDaily, 12 June 2014. www.sciencedaily.com/releases/2014/06/140612212430.htm.

Further Reading
http://www.ausetute.com.au/members/surfacetension.html
http://www.ausetute.com.au/members/wetting.html
http://www.ausetute.com.au/members/heatlatent.html

Suggested Study Questions

1. Write a balanced chemical equation to show the condensation of gaseous water on a glass surface.
2. Write a balanced chemical equation to show how liquid water evaporates off a glass surface.
3. Use (kinetic) particle theory of matter to explain what happens when water condenses and evaporates.
4. Define the terms hydrophilic and hydrophobic.
5. Define the terms adhesion and cohesion.
6. Explain why water forms spherical droplets.
7. Explain why glass is a hydrophilic surface.
8. Explain why dirty glass, glass covered with oily particles, is hydrophobic.
9. Explain how a water repellent coating on glass might work.
10. Discuss potential problems with using CleanClear on car windows, that is, what factors might reduce its effectiveness and why.

Wet, Wetting and Wettability

In Surface Chemistry, the concept of wetting or wettability is very important. Understanding wetting enables scientists to make water repellant fabrics, or, to make sponges that soak up oil spills, or to produce detergents that remove grease and oil in water.

Learn more about wetting and wettability with AUS-e-TUTE's news tutorial, game and test.
Members should log-in and go to the Surface Chemistry section for links to the new resources.

Not an AUS-e-TUTE Member?
There is a free tutorial available: http://www.ausetute.com.au/wetting.html