92168.pdf

274 Emulsions and foams

100 200

Disjoining pressure/Nnrr^

Figure 10.4 Film thickness as a function of disjoining pressure for films of 10~^3 moi

dm~^3 aqueous sodium oleate containing NaCl at concentrations of (1) 1(T^4 mol dm~^3 ;

(2) 1(T^3 mol dnT^3 ; (3) 1(T^2 mol dm"^3 ; and (4) 10"' mol dm~^3 (After B.V. Deryagin

and A.S. Titijevskaya^225 )

effectively c. 6 nm thick. Similar conclusions have also been reached

by van den Tempel^227 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