Good Example Of Why Are Butters Solid And Oils Liquid?how Molecular Composition Affects The Melting Point Of Fats And Oils Research Paper

Introduction

It is puzzling to know why oils are liquid while butter is solid when they are both oily and greasy. Not all oils are solid in room temperature or in the fridge. By studying chemistry we get to understand why such phenomenon occurs, which is not only restricted to oils and fats but to almost the entire materials we find on Earth.

Discussion

Butter is solid and gets harder when left in the fridge. By understanding the states of matter, we have come to understand that there are three phases of matter: solid, liquid, and gas. We have also learned that when a solid becomes liquid, it is called melting and the reverse is freezing. When butters are put in a skillet over a medium flame, they slowly melt and become liquid. If left to cool in a fridge, they solidify quite easily. Oils are different as they are liquids yet may also solidify when left in fridge for a long time. Oils of different compositions (palm, coconut, canola, corn, etc.) also behave differently as some are more prone to solidifying and others not.
What composes oils and fats? If we are to search for references, oils and fats are composed of triglycerides and free fatty acids. Triglycerides are large molecules composed of three fatty acids attached to a glycerol.
There are three attachment sites for fatty acids in the glycerol backbone. These three positions can be attached with fatty acids of different lengths and degree of saturation. These are the reason why we have saturated and unsaturated fatty acids. Saturated fatty acids do not contain unsaturation or double bonds. To simplify things, let us take butter, coconut oil and canola oil as an example, with their fatty acid composition and melting points. Butter has a melting point of 32-35 °C. Its fatty acid composition is comprised of 61% saturated and 33% unsaturated fatty acids. The saturated fatty acids of butter are also comprised of the following: 26% palmitic, 12% myristic, 11% stearic, 4% lauric, 3% butyric, 3% capric, and 2% caproic acids. The unsaturated fatty acids for butter include 28% oleic, 3% palmitoleic, and 2% linoleic acids. As for coconut oil, it has a melting point of 24 °C. Coconut oil’s fatty acid composition is 90% saturated and 9% unsaturated. The 90% saturated fatty acids part is composed of 48% lauric, 16% myristic, 9% palmitic, 8% caprylic, 7% capric, and 2% stearic acids. On the other hand, the 9% saturated part is composed of 7% oleic and 2% linoleic acids. Lastly, in the case of canola oil, its melting point is -10 °C. The fatty acid composition of canola oil is 6% saturated and 92% unsaturated. The saturated fatty acids of canola oil are represented by 4% palmitic and 2% stearic acids. Moreover, the unsaturated fatty acids are further divided into 56% oleic, 26% linoleic, and 10% linolenic acids (Gunstone, 1996).
Since room temperature is around 25°C or slightly higher if in the tropics, butter remains solid while coconut oil might have some hazing or the appearance of solidifying. Canola oil, on the other hand would remain as liquid even when placed in the fridge. The underlying reason for this phenomenon is the composition of the three. Butter and coconut oil have higher melting points because of the large amount of saturated fatty acids. Saturated fatty acids are linear, thus they line up side-by-side of each other, in which intermolecular forces of attraction are stronger. We know that Van der Waals dispersion forces are stronger when molecules are linear and larger/longer. In the case of butter vs coconut oil, butter has a higher melting point because it contains high amounts of myristic, palmitic, and stearic acids.
These fatty acids are large molecules comprising of 14, 16 and 18 carbons long. The majority of coconut oil however is composed of lauric and myristic acid, a 12 and 14-carbon long fatty acid. For canola oil, despite it containing 18-carbon long unsaturated fatty acids, they exhibited very low melting points. The reason for this is that unsaturated fatty acids are not linear in shape but rather appear crooked or curved. Since we know that shape influences the dispersion forces and IMFA influences melting point, is it then obvious why canola remains liquid even at cold temperatures.

Conclusion

Molecules have different characteristics. These differences are not only associated with the nature of the atoms that comprise them or their sizes but also their shapes. Molecules that tend to be more linear in shape have stronger Inter Molecular Forces of Attractions (IMFA) – which means that they have more surface area to interact with other molecules. Those with branched shapes, on the other hand, tend to have lesser IMFA. The main result of such differences is that those with linear shapes tend to be in the solid state, while those with branched shape tend to be in their liquid state.

References

Clark, J. (2000). Intermolecular Bonding – Van der Waals forces. Chemguide. Retrieved from: http://www.chemguide.co.uk/atoms/bonding/vdw.html
Gunstone, F. (1996). Fatty Acid and Lipid Chemistry. Blackie: London. Adapted in online database. Retrieved from: http://web.pdx.edu/~wamserc/C336S12/fat.pdf.
Nazir, D.J., Moorecroft, B.J., & Mishkel, M.A. (1976). Fatty acid composition of margarines. American Journal of Clinical Nutrition, 29, 331-339. Retrieved from: http://ajcn.nutrition.org/content/29/4/331.full.pdf
Przybylski, R. (no date). Canola oil: Physical and Chemical Properties. Canola Council. Retrieved from: http://www.canolacouncil.org/media/515239/canola_oil_physical_chemical_properties_1.pdf