The key difference between adhesion and cohesion is that adhesion is the attraction between substances or molecules that are not similar whereas cohesion is the attraction between the molecules or substances that are similar.
There are various phenomena to explain the things we observe in our daily lives. Although sometimes we do not focus on these little details, they are the ones that help to keep life on earth. Adhesion and cohesion are two such phenomena. Though they sound similar, they are completely different terms.
What is Adhesion?
Adhesion is the attractive force between two types of molecules, which are different from each other. For example, the attraction between the water molecules with the walls of xylem vessels is adhesion. Because of this force, water can travel through the xylem in plants. Moreover, these are intermolecular forces.
There are five mechanisms to explain the mechanism of adhesion as followings:
- Mechanical adhesion
- Chemical adhesion
- Dispersive adhesion
- Electrostatic adhesion
- Diffusive adhesion
In mechanical adhesion, adhesive material holds the surface by filling into the holes or pores in it. In chemical adhesion, chemical bonds form, and they can be either ionic or covalent bond. If the bonds are ionic, then electrons may donate or attract, or else electron sharing may present in covalent bonding.
Other than these, intermolecular bonds like hydrogen bonds can participate in holding two materials together. If the two materials are held together due to Van der Waals forces, then we can explain that mechanism by dispersive adhesion.
Moreover, when there is a slight (permanent or temporal) charge separation in a molecule, we say the molecule has become polarized. The opposite charges tend to attract each other; hence, there are attractive forces between the molecules. An electron passing in conductive material may cause electrical charge differences. The charge differences can cause attractive electrostatic forces between materials. We call it electrostatic adhesion.
When two types of molecules are soluble in each other, they can move into the other surface; thus, it results in diffusive adhesion. The strength of the adhesion forces depends on the mechanism; as to how it happens. For instance, if the contacting surface area is very larger, the strength of Van der Waals forces is higher. Therefore, the strength of dispersive adhesion forces is higher.
What is Cohesion?
Cohesion is the intermolecular force between two similar molecules. For example, water molecules have intermolecular attraction forces between them. This property of water allows water molecules to travel with consistency. We can explain the shape of the raindrops or the existence of water droplets rather than single molecules by the cohesion.
Further, the hydrogen bonding ability of water molecules is the main reason behind the cohesion forces of water molecules. Each water molecule can form four hydrogen bonds with other water molecules; thus, the collection of attraction forces is much stronger. Electrostatic forces and Van der Waals forces between similar molecules also cause adhesion. However, adhesion due to Van der Waals forces is somewhat weaker.
What is the Difference Between Adhesion and Cohesion?
Adhesion is the attractive force between two types of molecules, which are different from each other and cohesion is the intermolecular force between two similar molecules. Therefore, the key difference between adhesion and cohesion is that the adhesion is the attraction between substances or molecules which are not similar whereas cohesion is the attraction between the molecules or substances that are similar.
Moreover, another significant difference between adhesion and cohesion is that the adhesion includes electrostatic attractions while cohesion includes Van Der Waal forces and hydrogen bonding. For instance, cohesion is between two water molecules and adhesion is between water molecules and the walls of xylem vessels.
Summary – Adhesion vs Cohesion
Adhesion and cohesion are two types of intramolecular forces. The key difference between adhesion and cohesion is that adhesion is the attraction between substances molecules which are not similar whereas cohesion is the attraction between the molecules or substances that are similar.
1. Libretexts. “Cohesive and Adhesive Forces.” Chemistry LibreTexts, National Science Foundation, 26 Nov. 2018. Available here