Surface Tension vs Interfacial Tension
Both surface tension and interfacial tension are effects based on liquids. Both these effects take place due to the unbalanced intermolecular forces between the liquid or solutions molecules. We observe these effects in the day to day life in the form of many events such as, forming of droplets, immiscibility of liquids, capillary action, soap bubbles, and tears of wine and even the floating of the water strider. Both these actions play a major role in our daily activities without us even knowing they exist. For an example, you wouldn’t be able to mix an emulsion mixture, if it is not for these theories.
Consider a liquid, which is homogeneous. Every molecule in the central parts of the liquid has exactly same amount of force pulling it to every side. The surrounding molecules are pulling the central molecule uniformly on every direction. Now consider a surface molecule. It has only forces acting upon it toward the liquid. The air – liquid adhesive forces are not even nearly strong as the liquid – liquid cohesive forces. So, the surface molecules get attracted towards the center of the liquid, creating a packed layer of molecules. This surface layer of molecules acts as a thin film on the liquid. If we take the real life example of the water strider, it uses this thin film to place itself on the surface of the water. It slides on this layer. If it is not for this layer, it would have drown immediately. Surface tension is defined as the force parallel to the surface perpendicular to a unit length line drawn on the surface. The units of surface tension are Nm-1. Surface tension is also defined as energy per unit area. This also gives surface tension new units Jm-2.
Interfacial tension is defined only to immiscible liquids. As the name suggests it applies to the interface of the two immiscible liquids. The same theory of surface tension applies to this too. The only difference between interfacial tension and surface tension is the liquid – liquid interface instead of the liquid – air interface. Interfacial tension can be used to describe immiscibility of these two liquids. Consider the interface between the liquids. The molecules on the first surface have forces acting upon it from the first liquid and from the surface molecules of the second liquid and vice versa. If the force on the surface molecules from the first liquid (cohesive forces) equals the force from the second surface (adhesive forces) these two liquids will mix. If these forces are unequal these liquids will not mix.
Difference between surface tension and interfacial tension
The main difference between these two is the places where it occurs. Surface tension is defined to a single liquid surface, whereas the interfacial tension is defined to the interface of two immiscible liquids. Surface tension is actually a derivation of interfacial tension where force from the second surface is negligible or zero.