We report a study on thin liquid films made of a cationic dimeric surf
actant referred to as 12-2-12. This kind of surfactant enables one to
form charged wormlike micelles whereas classical systems such as CTAB
or CPClO3 require added salt to form only screened wormlike micelles.
The disjoining pressure has been measured as a function of film thickn
ess at various surfactant and salt concentrations. In a dilute surfact
ant regime, we observed a stable common black film which undergoes a t
ransition to a Newton black film by addition of salt. This behavior, c
lassical with anionic surfactant, is shown for the first time with a c
ationic surfactant. A comparison with the monomeric surfactant DTAB is
presented showing the important role of the twin structure on film st
ability. In the concentrated regime or overlap regime, pressure oscill
ations have been measured. They correspond to a structuring effect in
the film. The number of oscillations increases with c, the concentrati
on, whereas the period decreases as c(-1/2). By comparison with the th
eory developed for charged chains, we propose to relate the observed s
tratification to the correlation length of semidilute solutions. SANS
experiments have been performed to determine the peak position depende
nce on concentration and temperature.