O. Mondainmonval et al., FORCES BETWEEN EMULSION DROPLETS - ROLE OF SURFACE-CHARGES AND EXCESSSURFACTANT, Journal de physique. II, 6(9), 1996, pp. 1313-1329
We report direct measurements of the force distance profiles between e
mulsion droplets in surfactant solutions of various nature. We probe t
he evolution of double layer repulsive forces in mixture of ionic and
non ionic surfactants. In the absence of any micelles, the force inten
sity is shown to be significantly enhanced by the presence of non ioni
c surfactant. Then, by varying the ratio of ionic over non ionic surfa
ctant well above the critical micellar concentration, we investigate t
he behavior of forces in the presence of micelles of various charge de
nsities. The double layer repulsive force-distance profile is not affe
cted by the presence of neutral non ionic micelles. However, as soon a
s they get partially charged, a deviation from the classical electrost
atic profile is observed. In that case, we establish that the repulsiv
e force can be described by the sum of two contributions: one is the r
epulsive double layer repulsive force while the other is attractive an
d arises from the depletion of charged micelles. Because micelles and
droplets are identically charged, they repel each other and this repul
sion enhances the depletion force. This effect is accounted by conside
ring an effective larger droplet diameter. We present the empirical re
lation between this extra thickness and the Debye length. The simple a
ddition of the two contributions is shown to successfully describe the
force-distance profiles measured in mixed ionic-non ionic as well as
in purely ionic micellar solutions.