Difference Between Propylene Glycol and Glycerin

Main Difference – Propylene Glycol vs Glycerin

Propylene glycol and glycerin often appear the same since they are colourless, odourless, sweet and syrupy. Although they share some physical properties, they have very distinctive features and it is very important to identify these compounds accurately due to the toxicity of propylene glycol. Glycerin is also called glycerol. It is used in the food industry, cosmetic productions, and pharmaceutical applications. But the applications of propylene glycol are limited due to its toxic behaviour. The main difference between propylene glycol and glycerin is that propylene glycol has two –OH groups whereas glycerin has three –OH groups.

Key Areas Covered

1. What is Propylene Glycol
      – Definition, Properties and Uses
2. What is Glycerin
      – Definition, Properties and Uses
3. What are the Similarities Between Propylene Glycol and Glycerin
      – Outline of Common Features
4. What is the Difference Between Propylene Glycol and Glycerin
      – Comparison of Key Differences

Key Terms: Glycerin, Glycerol, Propylene Glycol, ToxicityDifference Between Propylene Glycol and Glycerin - Comparison Summary

What is Propylene Glycol

Propylene glycol is a synthetic organic compound that has the chemical formula C3H8O2. The IUPAC name for this compound is propane-1,2-diol. It is an alcoholic compound. It has two –OH groups as functional groups. The molar mass of this compound is about 76.1 g/mol. At room temperature and pressure, it is a clear and colourless liquid. The density of this liquid is about 1.03 g/cm3.

Difference Between Propylene Glycol and Glycerin_Figure 1

Figure 1: Chemical Structure of Propylene Glycol

Propylene glycol is composed of an asymmetric (chiral) carbon atom. Therefore, this molecule exists as a pair of enantiomers. Since it is an alcohol, it is capable of forming hydrogen bonds. It is also totally miscible with water. When it is mixed with water, it disturbs the formation of ice. This leads it to be used as an anti-freezing agent.

It is viscous than water; it is considered as a syrup since it flows very slowly. The melting point of propylene glycol is about -59oC. Since the vapour pressure of propylene glycol is negligible, it does not evaporate at a considerable degree.

However, propylene glycol is toxic to us. But the consumption of trace amounts may not have any significant effects. If a large dose is ingested, it becomes toxic. Prolonged contact of propylene glycol with skin or eyes may cause injuries.

One of the major applications of propylene glycol is its use as a chemical feedstock for the production of unsaturated polyester resins. As propylene glycol is able to decrease the freezing point of water, it is used as a de-icing fluid in air crafts.

What is Glycerin

Glycerin is an organic compound that is composed of three –OH groups. It is an alcoholic compound. Therefore, it is grouped as a polyol. It is a colourless, odourless, sweet and syrupy liquid. It is non-toxic. The viscosity of glycerin is high and slowly flow. The IUPAC name for glycerin is propane-1,2,3-triol.

The chemical formula of this compound is C3H8O3. The molar mass is given as 92 g/mol. The density of glycerin liquid is about 1.2 g/cm3. The melting point of glycerin is about 17.8oC. The presence of –OH groups causes glycerin to form hydrogen bonds and to be completely mixed with water.

Main Difference - Propylene Glycol vs Glycerin

Figure 2: The Chemical Structure of Glycerin

Glycerin can be found as natural glycerin or synthetic glycerin. Natural glycerin can be found as triglycerides in plant and animal sources. Synthetic glycerin can be obtained from processing propylene.

Difference Between Propylene Glycol and Glycerin

Figure 3: Layers of Glycerine, Propylene Glycol, Ethylene Glycol and Water

Since it is non-toxic, glycerin is used in food industry as a solvent or a sweetener. It has been also found that glycerin is helpful in the preservation of food. Furthermore, glycerin is used in the pharmaceutical industry. Ex: cough syrups. Glycerol can be used as an anti-freezing agent due to its ability to form strong hydrogen bonds. 

Similarities Between Propylene Glycol and Glycerin

  • Propylene Glycol and Glycerin are liquids at room temperature.
  • Both are sweet and syrupy.
  • Both compounds are colourless and odourless.
  • Both are alcoholic compounds.
  • Both compounds can be used as anti-freezing agents due to their ability to form strong hydrogen bonds with water molecules.

Difference Between Propylene Glycol and Glycerin

Definition

Propylene Glycol: Propylene glycol is a synthetic organic compound that has the chemical formula C3H8O2.

Glycerin: Glycerin is an organic compound that has the chemical formula C3H8O3.

Number of –OH Groups

Propylene Glycol: Propylene glycol has two –OH groups.

Glycerin: Glycerin has three –OH groups.

IUPAC Name

Propylene Glycol: The IUPAC name of propylene glycol is propane-1,2-diol.

Glycerin: The IUPAC name of glycerin is propane-1,2,3-triol.

Molar Mass

Propylene Glycol: The molar mass of propylene glycol is about 76.1 g/mol.

Glycerin:  The molar mass of glycerin is about 92 g/mol.

Melting Point

Propylene Glycol: The melting point of propylene glycol is -59oC – a negative value.

Glycerin: The melting point of glycerin is 17.8oC – a positive value.

Toxicity

Propylene Glycol: Propylene glycol is considered as a toxic compound.

Glycerin: Glycerin is a non-toxic compound.

Conclusion

Due to the similar appearance and the sweet taste, it is often difficult to understand the difference between propylene glycol and glycerin. But it is very important to recognize a sample of propylene glycol from a sample glycerin because of the toxic effects of propylene glycol.

References:

1. Hendrickson, Kirstin. “Properties of Propylene Glycol.” LIVESTRONG.COM, Leaf Group, 14 Aug. 2017, Available here. Accessed 30 Aug. 2017.
2. “Glycerol.” Wikipedia, Wikimedia Foundation, 28 Aug. 2017, Available here. Accessed 30 Aug. 2017.
3. Busch, Sandi. “Glycerine Vs. Glycol.” LIVESTRONG.COM, Leaf Group, 22 June 2015, Available here. Accessed 30 Aug. 2017.

Image Courtesy:

1. “Propylene glycol chemical structure” (CC BY-SA 3.0) via Commons Wikimedia
2. “Glycerin Skelett” Von NEUROtiker – Eigenes Werk (Gemeinfrei) via Commons Wikimedia
3. “Layers of glycerine, propylene glycol, ethylene glycol and water” By LHcheM – Own work (CC BY-SA 3.0) via Commons Wikimedia

About the Author: Madhusha

Madhusha is a BSc (Hons) graduate in the field of Biological Sciences and is currently pursuing for her Masters in Industrial and Environmental Chemistry. Her interest areas for writing and research include Biochemistry and Environmental Chemistry.

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