Oil-in-water emulsion films stabilized by polymeric surfactants based on inulin with different degree of hydrophobic modification

Oil-in-water emulsion films stabilized by hydrophobically modified inulin polymeric surfactants with different degree of substitution, DS, of alkyl chains on the inulin backbone, were studied. Four different graft co-polymers with increasing DS were used. Equivalent film thickness, hw, and disjoining pressure, Π, were measured at different NaCl concentrations and 2 × 10−5 mol dm−3 concentration of polymers. At a constant capillary pressure of about 36 Pa hw decreases with increase of NaCl concentration till a critical concentration, Cel,cr, of 5 × 10−2 mol dm−3 is reached above which the thickness remains constant indicating a transition from electrostatic to steric stabilization. For the four polymeric surfactants this dependence was virtually the same except for the polymer with the highest DS where above Cel,cr a jump in the thickness to a Newton Black Film (NBF) was observed. For a wide range of NaCl concentrations (up to 2 mol dm−3) the measured disjoining pressure isotherms, Π vs. hw, showed in all cases a jump to NBF at a critical film thickness and critical disjoining pressure, which again indicates a transition from electrostatic to steric stabilization. This transition occurred at lower film thickness and lower capillary pressure as the DS was increased. A reduction in disjoining pressure at the transition point occurs with increase of degree of substitution. The results were discussed in relation to loop-size induced changes in steric repulsion.