100 Nano-Stories: Polar vs. Non-Polar Molecules!
Episode #45: Hydrophilic vs. Hydrophobic Aerogels!
Preface! ✨
It’s your favorite material science & nanotechnology enthusiast! A couple of days ago, I explained the meaning of applying pressure in an autoclave for supercritical drying using PSI (Pound-Force per Square Inch) and Bar Pressure! Although you do not need to know all of the content, I advise you to check it out!
But for today, we want to talk about the difference between a hydrophilic and hydrophobic molecule! To explain how we can make hydrophilic or hydrophobic aerogels at home, we have to understand what it means to be hydrophilic or hydrophobic!
Yes, I can see that you’re confused, reader! But watch me explain the terms in under 5 minutes! 😉
Hydrophilic vs. Hydrophobic Aerogels!
Fundamentals! 🔑
Hydrophilic means it will have polar molecules attached to the surface of the aerogel. These molecules are attracted to water because of the polar molecules.
Hydrophobic means it will have non-polar molecules attached to the surface of the aerogel. These molecules repel water because of the non-polar molecules.
But Carlos, what do the words “polar” and “non-polar” mean?
Polar molecules mean that the electrical charges of the molecule are not evenly distributed. This leads to electronegativity. But in aerogels, you can find OH groups attached to the surface of the aerogel. Anything that is directly attached to hydrogen (hydrogen-bonding) is a polar molecule.
Non-polar molecules mean that the electrical charges of the molecule are evenly distributed. No positive or negative charges are formed in the molecule. There is still electronegativity, but the electronegativity is low enough for them to repel water or have hydrophobicity. Anything that is an alkane (CH4), alkene (CH2 = CH2), or alkyne (H-C ≡ C-H)is non-polar because of its electronegativity.
Electronegativity! 🔑
To have a polar or hydrophilic molecule, the difference in electronegativity of two atoms must be equal to higher than 0.5.
To have a non-polar or hydrophobic molecule, the difference in electronegativity of two atoms must be lower than 0.5.
The electronegativity of an Oxygen atom is 3.44. The electronegativity of a Hydrogen atom is 2.1. The overall difference in electronegativity is 1.34, which is more than the required 0.5 difference in electronegativity to become a polar/hydrophilic molecule.
Essentially, any molecule that is composed of only oxygen and hydrogen (water) will have polar bonds.
The electronegativity of a Carbon atom is 2.5. The electronegativity of a Hydrogen atom is 2.1. The overall difference in electronegativity is 0.4, which is less than the required 0.5 difference in electronegativity. The difference in electronegativity results in a non-polar/hydrophobic molecule.
Essentially, any molecule that is composed of only carbon or hydrogen will have non-polar bonds.
Dipole Moments: Polar Molecule! 🔑
To figure out if a molecule is polar or non-polar, we can analyze its dipoles moment! Let’s use two photos to demonstrate the difference between a polar molecule and a non-polar molecule!
Since the electronegativity of the oxygen atom will draw in the hydrogen atoms, the dipole moments will face the oxygen atom.
But because the dipole moment of both hydrogen atoms is facing the oxygen atom, we can assume that the dipole moments cancel out! However, there is a piece of the dipole moments that I didn’t tell you about. There are hidden vectors that make water a polar molecule.
These vectors make something similar to a right triangle! When we add and cancel the vectors, we notice that only one pair of vectors remain: the dipole moments pointing straight up (the brown arrows).
The reason why the green and blue dipole moments are canceled out is that they are pointing towards each other. When dipole moments point towards each other, they will cancel out. The brown dipole moments are pointing straight ahead, but not towards each other, so they don't cancel out.
To summarize how to find a polar molecule with dipole moments:
- If you can make a right triangle between the atoms, you have more dipole moments to observe and cancel.
- Make a “right triangle” to reveal all the possible dipole moments possible.
- If a pair of vectors remain (or were not able to be canceled out), you end up with a net dipole moment. Net dipole moments result in a polar molecule.
Dipole Moments: Non-Polar Molecule! 🔑
Let’s use carbon dioxide as our test subject in figuring out the dipole moment of a carbon atom!
If you remembered from my passage on electronegativity, anything made out of carbon and hydrogen will be non-polar. However, we have oxygen instead of hydrogen, so this molecule is polar.
But Carlos, how can it be non-polar? 🤔
When we draw out the dipole moments of the molecule, we notice something unique about the way the dipole moments are facing the carbon atom!
The dipole moments of the oxygen atoms are pointing away from each other! This means that since they are pointing away from each other, they also cancel out!
Any pair of dipole moments that either face each other in the same direction or points away from each other in the opposite direction will result in the cancellation of dipole moments!
Since the molecular structure of CO2 won’t allow us to make a right triangle with the carbon atoms or oxygen atoms, there are no more dipole moments to find and cancel. Therefore, the dipole moment is zero, and the molecule is non-polar.
If the net dipole moment is zero (zero vectors left), you have a non-polar molecule.
If the net dipole moment is greater than zero (1 pair of vectors left), you have a polar molecule.
If the molecular structure/atoms have an angle of 180 degrees, you don’t have to look for more dipole moments (aka the “right triangle” dipole moment”)
If the molecular structure/atoms don’t have an angle of 180 degrees, there are more dipole moments (aka drawing out the “right triangle” dipole moment”)
Closing Thoughts! 💭
Whew! That was an exhausting article to write, reader! 😵
But it was an essential one to write! We can finally talk about the 3 ways to turn a hydrophilic aerogel into a hydrophobic aerogel in the next articles!
Until then, see you tomorrow! ✌🏽
Vocabulary! 📓
Hydrophilic Aerogel → Polar molecules are attached to the surface of the aerogel. Hydrophilic Aerogels will absorb water.
Hydrophobic Aerogel → Non-Polar molecules are attached to the surface of the aerogel. Hydrophobic Aerogels will repel water.
Polar molecules → The electrical charges of the molecule are not evenly distributed. The OH group is a polar molecule commonly found on the surface of hydrophilic aerogels. Polar molecules are hydrophilic.
Non-Polar molecules → The electrical charges of the molecule are evenly distributed. No positive or negative charges are formed in the molecule. All alkanes, alkenes, and alkynes are non-polar molecules because they are composed of carbon and hydrogen.
Electronegativity → The chance that a pair of elections will be attached to a certain atom. If the difference between the electronegativity is greater or equal to 0.5, the molecule is polar. If the difference in electronegativity in the molecule is less than 0.5, the molecule is non-polar.
Dipole Moment → The difference between positive and negative charges in a molecule. These vectors of dipole moments can determine if a molecule is polar or non-polar, even if a molecule has polar bonds.
Alkanes → Single Bonds that occur between carbon atoms in a molecule in organic chemistry.
Alkenes → Double Bonds occur between carbon atoms in a molecule in organic chemistry.
Alkynes → Triple Bonds that occur between carbon atoms in a molecule in organic chemistry.
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© 2021 by Carlos Manuel Jarquin Sanchez. All Rights Reserved.
