Like Dissolves Like: Solvents, Solutes, and Phospholipids

[Cheman]

Considering solvents and solutes on a molecular level, it makes sense to say that ionic and polar molecules will dissolve in water, due to its polarity and slight charge. However, why is it that covalent substances (eg - lipids) will not dissolve in water? Also, why is it that covalent substances (eg -oil) will easily dissolve other covalent substances? (eg - different oil)
As well as this, as conserns phospholips, how come all of the tails point up - surely as the head is attracted to water it would be possible for the covalent part to become mixed with in it?
Thanks in advance. :-)

 

[Monique]

Well, polar substances or substances that form hydrogen bonds mix with water because they can undergo dipole-dipole interactions or hydrogen bonds respectively. These interactions are just as strong as the interaction of the water molecules among themselves, so the foreign substance is not forced out of solution.

Nonpolar liquids can mix with each other because there is no tendency of the solvent molecules to attract among themselves and squeeze out the solute molecules :)

And why the tail of the phospholipids don't mix together with the head into the water? Because the tail disrupts the ordered lattice of water molecules and is thus forced out of solution and will stick out of the water as you mention. But if you agitate the solution, what will happen is that micelles form with the tails all facing inwards and the heads all facing outwards to the water (or a bilayer like the cell membrane).

I hope that answers your question, welcome to the forums! :)

 

[Monique]

And why the tail of the phospholipids don't mix together with the head into the water? Because the tail disrupts the ordered lattice of water molecules and is thus forced out of solution.

Actually I realize that is not entirely correct as stated, when a water molecule is next to a hydrophobic molecule, it is more restricted in motion and has fewer neighbours with which it can interact because it cannot form hydrogen bonds with the hydrophobic molecule. Bonding to fewer water molecules results in a more ordered water structure, which decreases the entropy of the system, which is energetically unfavorable :)