Foam and wetting films from rhamnolipids produced by Pseudomonas aeruginosa BN10

Microscopic foam films and wetting films on SiO2–glass surface formed from aqueous solutions of rhamnolipid with two rhamnose groups (R2) have been studied. The wetting (receding) contact angles θr of the solution on the glass surface have been measured, using the sessile drop method. The receding contact angles θr versus rhamnolipid concentration CS curves showed a maximum at θr ≈ 18° at concentrations about 2 × 10−5 mol dm−3 R2 and a decrease in θr reaching values of 7° at 8 × 10−5 mol dm−3. The addition of electrolyte (NaCl) decreases the θr-values, respectively, decreases the hydrophobicity of the SiO2-glass surface. Microinterferometric methods of assessment of foam and wetting films are used which allow precise determination of the film thickness at various R2 and electrolyte concentrations. The measurements of the effect of the surfactant concentration on foam film thickness (h) show that the increasing adsorption of rhamnolipid ions leads to increase of the surface charge and potential at the air/liquid film interfaces until they reach plateau region at 5 × 10−6 mol dm−3 R2. At the same concentration a minimum in h(CS) for wetting films is obtained. The action of additional repulsive surface forces in symmetrical and asymmetrical thin liquid films is found. The obtained results show that the surface forces are determined not only by the structure and organization of the adsorbed layers but also by the molecular state of the bulk solution.