Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 3. Influence of electrolyte type on emulsion films

摘要

The interaction forces in emulsion films stabilized using hydrophobically modified inulin (INUTEC SP1) were investigated as a function of concentrations of electrolytes of different types (NaCl, Na2SO4, and MgSO4). At a constant disjoining pressure of 36 kPa, a constant temperature of 22 degrees C, and a film radius of 100 mu m, the film thickness, h(w), decreased with an increase in electrolyte concentration until a critical value, C-el,C-cr, was reached above which h(w) remained constant. C-el,C-cr decreased with an increase in electrolyte valency (C-el,C-cr = 5 x 10(-2) mol center dot dm(-3) for NaCl and 1 x 10(-2) mol center dot dm(-3) for Na2SO4 and MgSO4). The reduction in film thickness below C-el,C-cr could be accounted for by the compression of the electrical double layer. The Pi-h(w) isotherms below C-el,C-cr could be fitted using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (constant charge and constant potential cases were considered). At a certain pressure, the film jumped to a Newton black film. The pressure at the jump decreased with an increase in electrolyte valency as a result of the reduction of the electrostatic barrier. At electrolyte (NaCl, Na2SO4, or MgSO4) concentrations higher than C-el,C-cr, the jump occurred at a low pressure that was independent of the electrolyte type. The thickness of the Newton black film was independent of both the concentration and nature of the electrolytes studied. The results show clearly that the polyfructose loops and tails remain strongly hydrated both in water and in high concentrations of electrolytes of different types, and these results explain the high INUTEC SP1 emulsion stability against coalescence of emulsions prepared under such conditions.