Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 2. Emulsion films

摘要

The interaction forces in emulsion films stabilized using hydrophobically modified inulin (INUTEC SP1) were investigated by measuring the disjoining pressure of a microscopic horizontal film between two macroscopic emulsion drops of isoparaffinic oil (Isopar M). A special measuring cell was used for this purpose whereby the disjoining pressure Pi was measured as a function of the equivalent film thickness h(w). The latter was determined using an interferometric method. In this way Pi-h(w) curves were established at a constant INUTEC SP1 concentration of 2 x 10(-5) mol center dot dm(-3) and at various NaCl concentrations. At a constant disjoining pressure of 36 Pa, a constant temperature of 22 degrees C, and a film radius of 100 mu m, h(w) decreased with an increase in the NaCl concentration, C-el, and reached a constant value of 11 nm at C-el = 5 x 10(-2) mol center dot dm(-3). This reduction in film thickness is due to the compression of the electrical double layer, and at the above critical NaCl concentration any electrostatic repulsion is removed and only steric interaction remains. This critical electrolyte concentration represents the transition from electrostatic to steric interaction. At a NaCl concentration of 2 x 10(-4) mol center dot dm(-3) the Pi-h(w) isotherms showed a gradual decrease in h(w) with an increase in capillary pressure, after which there was a jump in h(w) from similar to 30 to similar to 7.2 nm when Pi reached a high value of 2-5.5 kPa. This jump is due to the formation of a Newton black film (NBF), giving a layer thickness of the polyfructose loops of similar to 3.6 nm. The film thickness did not change further when the pressure reached 45 kPa, indicating the high stability of the film. Pi-h(w) isotherms were obtained at various NaCl concentrations, namely, 5 x 10(-2), 5 x 10(-1), 1.0, and 2.0 mol center dot dm(-3). The initial thicknesses are within the range 9-11 nm, after which a transition zone starts, corresponding to a pressure of about 0.5 kPa. In this zone all films transform to an NBF with a jump, after which the thickness remains constant with a further increase in the disjoining pressure up to 45 kPa, with no film rupture. This indicates the very high stability of the NBF in the presence of high electrolyte concentrations. The high emulsion film stability (due to strong steric repulsions between the strongly hydrated loops of polyfructose) is correlated with the bulk emulsion stability.