Question

Describe the spreading behaviour you would expect when a drop of benzene is placed on a clean water surface. For the pure liquids the interfacial tensions (in mN m−1 ) are: • Air–water 72.8 • Air–benzene 28.9 • Benzene–water 35.0 However small amounts of each liquid slowly dissolve in the other and this affects the interfacial tensions which then become: • Air–water 62.2 • Air–benzene 28.8 • Benzene–water 35.0 [S0 = 8.1 mN m–1; Sf = –1.6 mN m–1]

Answer 1

Spreading Coefficient, S = Surface tension of water-Surface tension of Benzene-Surface tension of benzene-water

S,initial = 72.8-28.9-35 = 8.1 mN/m

S,final = 62.2-28.8-35=-1.6 mN/m

Spreading coefficient posiitive means Benzene spreads on water.

Spreading coefficient negative means Benzene doesn't spread on water.

Hence, initially Benzene spreads on water. After time, water solubilize in Benzene and spreading coefficient reduces to negative. Benzene finally is non-spreading on water after reaching equilibrium