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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