Ss. Datwani et Kj. Stebe, Surface tension of an anionic surfactant: Equilibrium, dynamics, and analysis for Aerosol-OT, LANGMUIR, 17(14), 2001, pp. 4287-4296
The equilibrium and dynamic surface tension of sodium bis(2-ethylhexyl) sul
fosuccinate (Aerosol-OT) are studied as a function of concentration and ion
ic strength controlled by the addition of either the monovalent salt sodium
chloride or the divalent salt calcium chloride. These data are compared to
a surfactant mass-transfer model with a quasi-equilibrium treatment of the
electrostatics. The Davies adsorption isotherm and surface equation of sta
te relate the bulk concentration, surface concentration, and surface tensio
n. At equilibrium, the surface concentration increases with the ionic stren
gth of the electrolyte, so the surface tension reduces more strongly. The d
ata at all ionic strengths are well described by the Davies model. Because
the characteristic diffusion time scale increases as the square of the surf
ace concentration, an increasing equilibration time with ionic strength mig
ht be anticipated for this molecule. However, the time required for the sur
face tension relaxation observed in experiment is fairly insensitive to cha
nges in the ionic strength over the range of surfactant concentrations stud
ied for both monovalent and divalent electrolytes at fixed surfactant bulk
concentration. When these data are compared to a full integration of the su
rfactant transport equations, they are found to agree with a diffusion-cont
rolled mass-transfer mechanism. The key issue behind the apparently contrad
ictory behavior of increased adsorption resulting in lower equilibrium surf
ace tensions, while diffusion time scales remain essentially unchanged, is
the high surface activity of Aerosol-OT. Even at the most dilute concentrat
ions studied, Aerosol-OT adsorbs close to its maximum packing limit. The su
rface concentration increases weakly near this value with ionic strength. T
herefore, the diffusion time scale also changes weakly. Concomitantly, the
equilibrium surface tension changes strongly because it is highly sensitive
to small changes in surface concentration near this limit.