Surface and interfacial tensions can be used to determine whether one liquid wil

Question

Surface and interfacial tensions can be used to determine whether one liquid will spread over another, or form isolated drops on the surface.
The work of cohesion (WA) is the work required to separate a cylinder of pure liquid A into two cylinders. This requires the formation of two new surfaces of unit area and so WA = 2A where A is the surface tension of the liquid.
The work of adhesion (WAB) is the work required to separate two cylinders of liquids A and B joined by an interface of unit area, WAB = A + B – AB where AB is the interfacial tension between the two liquids. A liquid, A, will spread on another liquid, B, if the initial spreading coefficient S (= WAB – WA) is positive or zero, that is, if the liquid A adheres more strongly to liquid B than it coheres to itself.

At 20°C the interfacial tension between benzene and water is 35 mN m–1. For water = 72.75 mN m–1, and for benzene = 28.85 mN m–1.
Calculate:
(a) the work of cohesion for benzene and for water,
(b) the work of adhesion between benzene and water,
(c) the initial spreading coefficient for benzene on water. Will benzene spread over water?

((a) benzene: 57.7 mJ m–2, water: 145.5 mJ m–2 (b) 66.6 mJ m–2 (c) 8.9 mJ m–2)

Explanation / Answer

a) Work of cohesion= 2*Y

for Benzene it is 28.85 mN/m, work of cohesion = 2*28.85= 57.7 mN/m= 57.7*mN.m/m2= 57.5 mJ/m2

for water is it 2*72.75= 145.5 mJ/m2

b)work of adhesion = 72.75+28.85- 35=66.6 mN/m= 66.6 mJ/m2

c)spreading coefficient = 66.6-57.7 =8.9 mJ/m2 . Sicne it is +ve. Benzene adhers strongly to water than Benzene itself

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