Effect of the degree of grafting in hydrophobically modified inulin polymeric surfactants on the steric forces in foam and oil-in-water emulsion films

The interaction forces in foam and oil-in-water emulsion films stabilized by four inulin based graft copolymers with different degree of grafting of the dodecyl chains onto the inulin backbone were investigated. The adsorbing copolymers were found to form inulin-loops at the oil (air)/water interface, as the inulin-loop size decreased with increasing degree of grafting. Disjoining pressure isotherms (disjoining pressure, Π vs. equivalent film thickness, hw) were measured for the foam and emulsion films at sufficiently high NaCl concentration, whereby there is no electrostatic interaction in the films. Under these conditions, the stabilization of the films is due to interaction between the two layers of inulin-loops at oil (air)/water film interfaces. Transition to a stable Newton black film in the disjoining pressure isotherm for the emulsion films was observed for all the copolymers and the transition pressure significantly decreased for the films from the copolymer with the smallest inulin-loops. The stability against rupture of the foam films decreased with reducing inulin-loop size. The de Gennes’ scaling theory for interaction between polymer ‘brushes’ was fitted to the experimental data for the foam films with the inulin-loops layer thickness as a fitting parameter. The results were interpreted on the basis of inulin-loop size induced changes of the steric forces in the foam and emulsion films.