Intermolecular Forces and Liquid Solubility

Liquids mix or dissolve in one another when the attractions between the different molecules are about as strong as the attractions within each pure liquid, which is summarized by the rule “like dissolves like”: polar liquids mix with polar liquids and nonpolar liquids with nonpolar liquids. In brief: ethanol mixes with water in all proportions because both form hydrogen bonds, while hexane and water form two separate layers.

The ability of liquids to mix determines how paints are thinned, how stains are removed, why oil and vinegar separate in a salad dressing and how drugs are absorbed in the body. This article explains the rule in terms of intermolecular forces, uses the data for alcohols as a test case, and shows how to predict solubility and miscibility in practice.

What does “like dissolves like” mean?

For a substance to dissolve, the particles of the solute must separate from each other and become surrounded by solvent molecules. The solute–solvent attractions must therefore compensate for the loss of solute–solute and solvent–solvent attractions. This works when all the forces are of the same kind and strength:

• Polar and hydrogen-bonding liquids (water, ethanol, acetone) mix with one another.
• Nonpolar liquids (hexane, benzene, carbon tetrachloride, oils) mix with each other through dispersion forces.
• A polar and a nonpolar liquid usually do not mix: water molecules attract each other strongly by hydrogen bonds, and they cannot get much attraction in return from hexane.

Two liquids that mix in all proportions are miscible. Two liquids that separate into layers are immiscible. Partial solubility is common for liquids of intermediate polarity.

Why do some liquids not mix?

Mixing increases the disorder (entropy) of the system, which always favors mixing. Whether mixing occurs is decided by the balance between this entropy gain and the energy changes. If a polar molecule such as water has to give up strong hydrogen bonds and receives only weak dispersion attractions from a nonpolar molecule, the energy cost is large, and the entropy gain does not compensate for it.

There is an additional effect in water: nonpolar molecules disturb the network of hydrogen bonds, and water molecules must rearrange around them. This decreases the entropy of the water and further opposes the dissolution of nonpolar substances, which is known as the hydrophobic effect.

Examples of miscible and immiscible pairs

Iodine dissolves in water only to the extent of about 0.03 g per 100 g at 20 °C, but it dissolves much better in a solution of potassium iodide because it forms the triiodide ion, I₃⁻, which is responsible for the brown color of iodine tincture.

How does chain length affect the solubility of alcohols?

An alcohol has a polar –OH group, which forms hydrogen bonds with water, and a nonpolar hydrocarbon chain. As the chain gets longer, the nonpolar part dominates, and the solubility in water decreases.

Values from different sources differ somewhat. The pattern is a rule of thumb: when the polar group is balanced by about three or four carbon atoms, the alcohol is soluble, and longer chains lead to poor solubility. Branched alcohols are more soluble than their straight-chain isomers, since they are more compact.

What about solids and gases in liquids?

The same principle applies to every kind of solute.

• Vitamins: vitamins B and C have many –OH and N–H groups and are water-soluble; vitamins A, D, E and K are largely hydrocarbon and are fat-soluble. Fat-soluble vitamins are stored in body fat, whereas excess water-soluble vitamins are excreted in urine.
• Soaps and detergents: a soap molecule has a polar head and a long nonpolar tail. The tails dissolve in grease, and the heads interact with water, so grease is carried away in micelles.
• Gases: oxygen and nitrogen are nonpolar and only slightly soluble in water, but more soluble in nonpolar solvents.
• Dry cleaning and paint: nonpolar solvents remove oily stains, and water is used for sugar and salt stains.

How do you predict solubility?

1. Identify the main intermolecular forces in the solute and in the solvent.
2. Check whether the solute and solvent can form hydrogen bonds with each other.
3. Compare the polarities, and note whether the solute is ionic.
4. For chain molecules, compare the size of the polar and nonpolar portions.
5. Predict “soluble” when the forces are similar, “insoluble” when they differ greatly.

Worked example: three solutes in water and hexane

Predict whether glycerol (C₃H₅(OH)₃), carbon tetrachloride (CCl₄) and potassium nitrate are more soluble in water or in hexane.

• Glycerol has three –OH groups and forms many hydrogen bonds: soluble in water, not in hexane.
• CCl₄ is nonpolar and has only dispersion forces: soluble in hexane, and very slightly soluble in water.
• KNO₃ is ionic and is stabilized in solution by ion–dipole forces: soluble in water, insoluble in hexane.

Where does the rule come from?

The saying “similia similibus solvuntur”, which means “like is dissolved by like”, is attributed to the alchemists and early chemists, long before the nature of intermolecular forces was known. In the 20th century, Joel Hildebrand introduced the solubility parameter in 1936 as a quantitative measure of the cohesive forces in a liquid, and liquids with similar parameters tend to mix. Charles Hansen later divided it in 1967 into contributions from dispersion, polar and hydrogen-bonding forces, which is still used to select solvents for paints and polymers.

Which mistakes are common?

• Treating the rule as absolute: it is a guide, and there are exceptions and partial solubilities.
• Forgetting the hydrocarbon part of a molecule: a long chain makes even an alcohol insoluble.
• Saying that oil and water do not mix because of density: the layers form because of the forces; density only determines which one is on top.
• Assuming that all polar solvents mix: they do not always do so, for example nitromethane and water are only partially miscible.
• Confusing solubility with speed of dissolving: stirring and heating change the rate, not the final solubility at a given temperature.