Hello dear you must have noticed that while performing the practical of titration in chemistry, liquid solution filled in burette and pipette forms concave or convex meniscus inside it. This is due to the force of surface tension that acts on the surface of liquid.
The shape of meniscus (concave/convex) depends upon the magnitude of cohesive and adhesive forces between molecules of liquid, solid and other surfaces in contact. The tangent drawn to concave/convex meniscus forms certain angles with the solid surface in contact which gives rise the concept of angle of contact.
In this article we are going to illustrate the concept of angle of contact on the basis of forces due to surface tension.
Let’s discuss the concept.……………!
Angle of contact:
The angle between the tangent to the liquid surface at the point of contact &solid surface inside the liquid is called as the angle of contact between the solid & liquid.
Suppose small quantity of liquid kept on plane solid surface, it depends upon forces at solid-liquid & air interface.
Let ‘θ’ be the angle of contact
T1 – surface tension at liquid–solid interface.
T2 -surface tension at air –solid interface.
T3 -surface tension at liquid– air interface.
For drop to be at equilibrium force due to solid- air interface should have to balance other two forces.
In equilibrium of the drop,
∴ T1 + T3 cosθ = T2
∴ T3 cosθ = T2 – T1
∴ cosθ = (T2 – T1)/T3
⇒ When. T2 > T1 then cosθ is positive then θ is acute. For a liquid which partially wets the solid, the angle of contact is acute.
e.g. θ = 40o for kerosene – glass
⇒ When, T2 < T1 then cosθ is negative then θ is obtuse. For a liquid which does not wet the solid, the angle of contact is obtuse.
⇒ When, (T2 – T1) ≈ T3 then cosθ ≈ 1 than θ is nearly equal to zero.
⇒ When, (T2 = T1), cosθ = 0, then θ = 90o. For a liquid which wets the solid.
θ = 90o for pure water – silver
⇒ When, (T2 – T1) > T3 then cosθ > 1.which is impossible & equilibrium will not formed & liquid spread over a solid surface & drop would not formed.
Some important properties of angle of contact…………..!
1) The angle of contact is constant for a given liquid solid pair.
2) For a liquid which partially wets the solid, the angle of contact is acute.
e.g. acute angle θ = 40o (kerosene in contact with glass)
3) For a liquid which does not wet the solid, the angle of contact is obtuse.
e.g. obtuse angle θ = 128o (mercury in contact with glass)
4) For a liquid which completely wets the solid, angle of contact is equal to zero.
e.g. zero angle θ = 0o (water in contact with glass)
5) The angle of contact of a given liquid-solid pair, is depends on the nature of liquid & solid.
6) Angle of contact varies with temperature. As temperature increases, angle of contact decreases and vice versa.
7) Angle of contact varies with addition of impurity. Angle of contact increases with addition of soluble impurities and decreases when insoluble impurities are added.