Emulsifying properties of neutral and ionic polymer surfactants based on dextran
Résumé
The emulsifying properties of neutral and anionic polymer surfactants derived from dextran (a neutral polysaccharide) are described. The kinetics of interfacial tension lowering by the amphiphilic polymers is first characterized by the use of semi-empirical equations. These equations allow the determination of the equilibrium value of surface tension by extrapolation to infinite time. Oil-in-water emulsions are prepared by sonication in the presence of the polymers previously dissolved in the aqueous phase. The average droplet size of the emulsions is determined immediately after preparation and followed during several weeks. Ageing of the emulsions is shown to result from molecular diffusion in accordance with previous work. The increase of droplet size with time is correctly depicted by the theoretical equation derived by Lifshitz, Slyozov and Wagner (LSW). It seems that creaming has no real effect on emulsion ageing up to 1 μm. The influence of oil nature is clearly related to the physical properties of the oil (solubility in water, diffusion coefficient, interfacial tension), a typical consequence of molecular diffusion. An increase in the amount of hydrophobic groups fixed on the polymer leads to slower ageing. An increase in the anionic group content of the polymer has the reverse effect. Ionic strength has a significant effect on ageing of emulsions stabilized by dextran derivatives bearing ionic groups. Increasing ionic strength in the continuous phase gives rise to slower emulsion ageing. An attempt is made to rationalise all these facts by relating emulsion ageing to the data of interfacial tension measurements. The results follow approximately the tendency predicted by the LSW equation