Like Dissolves Like: Solvents, Solutes, and Phospholipids

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In summary, polar and ionic molecules can dissolve in water due to dipole-dipole interactions and hydrogen bonds. Nonpolar molecules do not dissolve because there is no attraction between the solvent molecules. The tails of phospholipids do not mix with the water because they disrupt the ordered structure of water molecules, but they can form micelles or bilayers in agitation. This is due to the decrease in entropy when water molecules bond with hydrophobic molecules.
  • #1
Cheman
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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. :-)
 
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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! :)
 
  • #3
Monique said:
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 :)
 

1. What does the phrase "like dissolves like" mean?

The phrase "like dissolves like" refers to the principle that substances with similar polarities will dissolve in each other. In other words, polar substances will dissolve in polar solvents, and nonpolar substances will dissolve in nonpolar solvents.

2. How does the polarity of a solvent affect its ability to dissolve solutes?

The polarity of a solvent determines its ability to dissolve solutes because polar solvents are attracted to other polar molecules, while nonpolar solvents are attracted to nonpolar molecules. This means that polar solvents will dissolve polar solutes, and nonpolar solvents will dissolve nonpolar solutes, following the principle of "like dissolves like".

3. Can you give an example of "like dissolves like" in action?

An example of "like dissolves like" is the dissolution of table salt (a polar substance) in water (a polar solvent). The polar water molecules are attracted to the ions in the salt and are able to dissolve it, while nonpolar solvents such as oil would not be able to dissolve salt.

4. How do phospholipids play a role in the "like dissolves like" principle?

Phospholipids are molecules with both polar and nonpolar regions. This makes them amphipathic, meaning they can interact with both polar and nonpolar substances. In cell membranes, phospholipids line up with their polar heads facing outwards towards the aqueous environment, while their nonpolar tails face inwards, creating a barrier between the polar and nonpolar substances.

5. Can "like dissolves like" be applied to all solutes and solvents?

No, the "like dissolves like" principle is not a universal rule and there are exceptions. For example, some substances have both polar and nonpolar regions, making it difficult to determine which solvent they will dissolve in. Additionally, the size and shape of molecules can also affect their solubility in different solvents.

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