Abstract
Film thickness of foams made of a pH sensitive surfactant with ion complexing properties, nonaoxyethylene oleylether carboxylic acid (AKYPO® RO 90 VG), has been determined in situ by small angle neutron scattering (SANS). The effect on the film thickness of pH variation and salt addition of nitrate salts with different cation valency (Na+, Ca2+ or Nd3+) was investigated and discussed in terms of the influent parameters stabilizing the foam such as surface properties and electrostatic effects. On one hand zeta potential was measured on the bulk micellar solutions in order to obtain information on surface charge. On the other hand tensiometry measurement at the water-air interface informed on static and dynamic properties of the surfactant such as the critical micellar concentration, area per surfactant head group and diffusion coefficient of the surfactant.It was observed that reducing the surface charge, i.e. by increasing the amount of neutral form of the surfactant over the charged carboxylate form, either by changing pH from high to low values or by adding a complexing multi-valent ion results in an increase in the film thickness. Accordingly, the variations in the surface coverage of the surfactant as well as in the cmc values indicate that the surfactant has a more pronounced surface activity in acidic condition or in the presence of a trivalent cation. Consequently the film thickness results in a subtle interplay between screening of the double layer in foam films (tranverse interaction) increased with ionic strength and cohesive forces between surfactants at the interface (lateral interaction) increased by surface ion complexation or surfactant protonation. The pH and specific salt effects revealed that foams produced by nonaoxyethylene oleylether carboxylic acid are of interest in the potential use of this surfactant in ion separation process based on foam such as ion flotation of multi-valent metal ions.