Why are some oils solid while others are liquid?

Fats, oils, waxes and butters are natural materials that find use in improving the look and feel of skin and hair. They can provide shine, smoothness, supple feel, slip, and aid in combating the negative effects of low moisture.

The number and type of these materials available to cosmetic formulators are numerous. An excellent book on the subject that goes through all the different types can be found in the book Oils of Nature.

Different oil structures

A thing I find interesting about these materials is that some of them like Coconut Oil and Babassu Oil are solids at room temperature while others like Olive Oil or Sunflower Seed Oil are liquids. The reason has to do with the molecular structure of the compounds that make up these materials.

It is first important to understand that natural (and synthetic) oils, butters, waxes and fats are all mixtures of different molecules. There really is no such molecule as Coconut Oil. Rather, it is a blend of different fatty acids and triglycerides. The exact ratio of different compounds is what makes these materials have different properties and effects.

Composition of fats and oils

The materials we’re talking about are all classified as Triglycerides.

Triglycerides - These are compounds made up of relatively long sequences of Carbon, Hydrogen and Oxygen atoms. They are tri-esters of glycerin with three fatty acids attached. In plant derived ingredients these fatty acids have a number of carbons from at least 5 to more than 20.  Triglycerides are typically studied in Organic chemistry courses.  In fact, the term Organic chemistry refers to the study of compounds made up of Carbon.  

Fatty acids - These materials are also made up of carbon, hydrogen and oxygen.  They are a string of connected carbon atoms, surrounded by hydrogen atoms, and capped off with two oxygens and a hydrogen to make a carboxylic acid.  Some of these fatty acids are saturated which means they have the maximum number of Hydrogens and there are no double bonds between carbon atoms in the molecule. Some fatty acids are unsaturated which means there are some Carbon-Carbon double bonds.

It is the presence or absence of these double bonds that is responsible for whether something is liquid or solid at room temperature.

Solids and liquids

The characteristic that makes these materials solid or liquid at room temperature is the ability for the molecules to pack close together and Van der Waals forces.  Oils or fats made up of mostly saturated fatty acids are able to pack very close together and produce a regular structure. Think of it like stacking cards from a new deck of cards on top of each other. They make a nice even pile.  This configurations gives rise to the materials being solid at higher temperatures.

Coconut oil is made up of mostly saturated fatty acids, C12, C14, C16. It only has 7% unsaturated fatty acids. Babassu oil has 14% unsaturated fatty acids.

For materials that are liquid at room temperature, they have a much higher content of unsaturated fatty acids.  The reason this causes the material to be liquid is that the component molecules are not able to lay together nicely. The double bonds cause bends in the molecule which causes more irregular structures. Think of trying to stack a deck of cards in a pile with half of the cards having bends in them. This means that the materials can more easily slip into a liquid form.

An ingredient like Sunflower Seed oil is made up of 68% unsaturated fatty acids so it is liquid at room temperature. In fact, it has a solidifying temperature of around 18C. Olive oil is made up of 90% unsaturated fatty acids and is also liquid at room temperature. In general, the more double bonds in a mixture of fatty acids, the lower the solidifying temperature.

So, there you have it.  Ingredients that are liquid at room temperature are generally composed of a high level of unsaturated fatty acids. Materials that are solid at the same temperature have a low level of unsaturation. For those ingredients, you’ll need to formulate at a higher temperature.