Page 284 - Introduction to Colloid and Surface Chemistry

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
98 225 226
means of investigating the above forces ' " . Figure 10.3 shows
an apparatus used by Deryagin and Titijevskaya 225 . A flat liquid film
2
with an area of c. I mm 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
225
function of their thickness (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