Page 285 - Introduction to Colloid and Surface Chemistry

274 Emulsions and foams

100 200
Disjoining pressure/Nnrr^

3
Figure 10.4 Film thickness as a function of disjoining pressure for films of 10~ moi
4
3
3
dm~ aqueous sodium oleate containing NaCl at concentrations of (1) 1(T mol dm~ ;
3
2
3
3
3
(2) 1(T mol dnT ; (3) 1(T mol dm" ; and (4) 10"' mol dm~ (After B.V. Deryagin
225
and A.S. Titijevskaya )
effectively c. 6 nm thick. Similar conclusions have also been reached
227
by van den Tempel from experiments with oil droplets in aqueous
sodium dodecyl sulphate plus sodium chloride.
Film rupture
In addition to film drainage, the stability of a foam depends on the
ability of the liquid films to resist excessive local thinning and rupture
which may occur as a result of various random disturbances. A
number of factors may be involved with varying degrees of
importance, depending on the nature of the particular foam in
question.
Gibbs-Marangoni surface elasticity effect

This is an important stabilising effect in foams which are formed from
solutions of soaps, detergents, etc. If a film is subjected to local
stretching as a result of some external disturbance, the consequent
increase in surface area will be accompanied by a decrease in the
surface excess concentration of foaming agent and, therefore, a local
increase in surface tension (Gibbs effect). Since a certain time is