Paper Towels and Oil
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“Can you explain the effect now? With little prompting, students should reason that it
is again a matter of hydrogen bonding. Water molecules readily hydrogen bond to the
exposed –OH groups on the paper fibers while still hydrogen bonding to each other. Thus,
there is a sort of “leapfrogging” of water molecules climbing over one another to
hydrogen bond with more of the –OH groups in the paper and dragging others along
behind. This is the process of absorption on the molecular level.”Based on this explanation from this section, I’m not sure how to explain that paper towels also absorb hydrophobic oil? Is it not more due to the porosity and capillary action that paper towels work? But capillary action is a result of cohesion and adhesion, so the oil must be adhering somewhat to the paper towel, right?
Thanks for any explanation that you can give that can help this make sense to me so I can help it make sense to my kids!
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Thank you for your question Emily. It baffled me too. I pulled the following off the internet. I hope it helps.
2. Cellulose and Polarity:
Cellulose, the main component of paper towels, is a polymer made of sugar molecules.
These sugar molecules have polar (charged) regions.
Oil molecules are generally non-polar.
However, paper towels can still absorb oil due to the capillary action and the ability of cellulose to create a network that can trap oil molecules.
The porous structure of the paper towel allows oil molecules to penetrate and be held within the towel.
3. Adhesion and Cohesion:
Water molecules are cohesive (they stick to each other) and adhesive (they stick to other materials, like cellulose).
While oil is not as strongly attracted to cellulose as water, it is still able to be absorbed through the capillary action and the physical structure of the paper towel.
The oil molecules are trapped within the spaces of the paper towel, effectively removing the oil from the surface it was on. -
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Sorry to be so long getting back to you.
After researching your question on the internet, I have concluded that my description in the text falls short.
First, it can easily be demonstrated that vegetable oil is actively absorbed by a paper towel. Roll a piece of paper towel into a stick-shape and place it in a glass with bit of vegetable oil in the bottom. Over the next couple hours it will be seen that the the oil does wick up the paper towel one to two inches. Therefore, without doubt, oil on paper towel dose exhibit capillary action, but it is slower and not as “strong”. The oil will not rise as far on paper towel as does water.
It turns out that capillary action does not depend on hydrogen bonding. It only depends on the paper or other wick material not repelling the oil. (If the paper repelled the oil we would see the oil remaining in droplets on the paper surface.) Once the oil penetrates the tiny spaces among paper cellulose molecules, capillary action comes into play and here is the main point.
Capillary action only depends on the liquid molecules having a greater attraction to the solid (paper) surface than they do for each other. In other words, the oil molecules have a greater attraction to the cellulose molecules of the paper than they have for each other. It is this difference that facilitates capillary action, not hydrogen bonding by itself. With hydrogen boning the difference is great; hence very marked capillary action with water. With oil, the difference is very slight but is still there. Hence the oil does exhibit capillary action, albeit weakly.
I hope this helps, Emily. Thank you for your interest and questioning. It has gotten me learning something new myself.
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