Wetting films on a hydrophilic silica surface obtained from aqueous solutions of hydrophobically modified inulin polymeric surfactant

The thickness of wetting films on a hydrophilic silica surface was investigated using a microinterferometric technique. Aqueous solutions of hydrophobically modified inulin (INUTEC®SP1) at various concentrations, in the presence or absence of NaCl or Na2SO4, were studied. The equilibrium film thickness (heq) showed a complex dependence on INUTEC®SP1 concentration. At low electrolyte concentrations, heq decreased with an increase in INUTEC®SP1 concentration, reaching a minimum at 10−6 mol dm−3. However, at high electrolyte concentrations, this dependence became less pronounced. At any given INUTEC®SP1 concentration, the equilibrium film thickness decreased with an increase in electrolyte concentration as a result of the compression of the electrical double layer reaching a minimum value. After that, the film thickness showed a small decrease with further increase in electrolyte concentration. This indicates that the electrostatic component of disjoining pressure can be neglected, and the steric repulsion of the loops and tails of INUTEC®SP1 determined the film thickness.