92168.pdf

Emulsions and foams 273

affect the equilibrium film thickness that a balance between the above

forces would otherwise determine.

Experimental studies on non-draining, horizontal liquid films in

which the equilibrium film thickness is measured as a function of ionic

strength and applied hydrostatic pressure (or suction) provides a

means of investigating the above forces^98 '^225 "^226. Figure 10.3 shows

an apparatus used by Deryagin and Titijevskaya^225. A flat liquid film

with an area of c. I mm^2 is formed between cups A and B which are

connected via tube C to equalise bubble pressures. The pressure in

the bubbles in excess of that in the liquid film is calculated from the

manometer reading A/z. Deryagin called this excess pressure the

disjoining pressure. An elaborate optical device (not shown) permits

measurement of the film thickness.

Ah T"JE» — «"

Figure 10.3 Apparatus for measuring the disjoining pressure of free films as a

function of their thickness^225 (After B.V. Deryagin and A.S. Titijevskaya)

Figure 10.4 shows the results of some measurements on aqueous
sodium oleate films. The sensitivity of the equilibrium film thickness
to added electrolyte reflects qualitatively the expected positive
contribution of electric double layer repulsion to the disjoining
pressure. However, this sensitivity to added electrolyte is much less
than that predicted from electric double layer theory and at high
electrolyte concentration an equilibrium film thickness of c. 12 nm is
attained which is almost independent of the magnitude of the
disjoining pressure. To account for this observation, Deryagin and
Titijevskaya have postulated the existence of hydration layers