Difference Between Adhesive and Cohesive Forces

Key Difference – Adhesive vs Cohesive Forces
 

Adhesive forces cause dissimilar surfaces to get attracted to each other. Adhesive forces can be mechanical forces that cause molecules to stick together or can be electrostatic forces that exist between positive and negative charges. Cohesive forces exist between similar surfaces. Therefore, these forces cause the formation of clusters of similar molecules. Cohesive forces can be hydrogen bonds or Van der Waal forces. Hydrogen bonds occur between polar molecules containing terminal O-H, N-H, and F-H groups. Van der Waal forces exist between nonpolar molecules. The key difference between Adhesive and Cohesive Forces is that adhesive forces exist between dissimilar molecules whereas cohesive forces exist between similar molecules.

CONTENTS

1. Overview and Key Difference
2. What are Adhesive Forces
3. What are Cohesive Forces
4. Side by Side Comparison – Adhesive vs Cohesive Forces in Tabular Form
5. Summary

What are Adhesive Forces?

Adhesive forces are intermolecular forces that exist between dissimilar particles of surfaces. It is a macroscopic property of a material. Adhesion is the clinging of a surface to another surface where the two surfaces are different from each other. Therefore, adhesive forces are the attractive forces that exist between unlike molecules.

Adhesive forces can be mechanical forces or electrostatic forces. Mechanical forces cause particles to stick together whereas electrostatic forces cause attractions between two opposite charges; positive charges and negative charges. Apart from that, there are diffusive and dispersive forces that can cause adhesion or adhesive forces.

The strength of adhesive forces between two, unlike surfaces, depends on the mechanism by which the adhesion is going to take place and the surface area on which the adhesive forces act on. The surface energy of a material determines the ability to wet against each other. For example, a substance such as polyethylene has a low surface energy. Therefore, it requires special surface preparation prior to the formation of adhesive forces.

Let us consider an example regarding adhesive forces. A meniscus is the creation of the surface of a liquid that can be observed in a liquid when that liquid is filled to a container. For the formation of the meniscus, both cohesive and adhesive forces are required. There, adhesive forces between liquid molecules and the surface of the container cause the edges of the liquid to be located at a higher level than that of the middle of the liquid (formation of a concave).

What are Cohesive Forces?

Cohesive forces are intermolecular forces that exist between similar particles of surfaces. It is a macroscopic property of a material. Cohesion is the process of similar molecules sticking together. The intermolecular forces that cause cohesive forces can be hydrogen bonds or Van der Waal attraction forces. Cohesion can be defined as the tendency of liquid particles to separate.

Cohesive forces cause molecules of the same substance to unite, forming clusters. For example, rain falls as water droplets. That is due to the cohesive forces that exist between water molecules. These forces cause the formation of water molecule cluster. A water molecule has polarization; there is a partial positive charge and a partial negative charge in the same molecule. The positive charges of water molecules are attracted to the negative charges of the neighboring molecules.

What is the Difference Between Adhesive and Cohesive Forces?

Summary – Adhesive vs Cohesive Forces

Adhesive forces are attraction forces that can be either mechanical forces or electrostatic forces. Cohesive forces can be hydrogen bonds or Van der Waal attractions. The difference between adhesive forces and cohesive forces is that the adhesive forces exist between dissimilar molecules whereas cohesive forces exist between similar molecules.

Reference:

1.“Adhesion.” Wikipedia, Wikimedia Foundation, 20 Feb. 2018. Available here
2.“Cohesion (Chemistry).” Wikipedia, Wikimedia Foundation, 28 Feb. 2018. Available here
3.“Cohesive and Adhesive Forces.” Chemistry LibreTexts, Libretexts, 28 Aug. 2017. Available here

Image Courtesy:

1.’Meniscus’By PRHaney – Own work, (CC BY-SA 3.0) via Commons Wikimedia 
2.’Hg Mercury’By А – Own work, (Public Domain) via Commons Wikimedia