The effect of electrolytes on emulsions stabilized by nonionic surfactants

Abstract

The objective of this study was to investigate the effect of high electrolyte concentrations on the stability of oil-in-water- emulsions stabilized by nonionic surfactants.

In chapter 1 several stability mechanisms are briefly outlined and the distinction between coalescence and flocculation of an emulsion is explained.

The structural characterization of the nonionic surfactants used (Synperonic NPE series) is described in chapter 2. These surfactants consist of a hydrophobic nonylphenol part, a less hydrophobic propylene oxide part (PO) and a hydrophilic ethylene oxide moiety (EO). Various samples differing with respect to the length of the EO part have been studied. Consistent results for the molecular masses have been obtained by pmr- and uv- spectroscopy. It has been demonstrated that the hydrophobic nonyl group of the surfactant is branched in different ways but there is no significant preference for any of the aliphatic nonyl isomers in the ethoxylation process.

In chapter 3 attention is paid to the properties of surfactant solutions by studying the surface- and interfacial tension, density, heat capacity, light scattering and viscosity. It was found that Synperonic NPE surfactant solutions have properties similar to other ethoxylated surfactant solutions and that the solvent quality of the EO-moiety plays the main role in the solution behavour of NPE surfactants. With respect to the effect of concentrated electrolytes it was established that there are two opposing trends which lead to a maximum in the micelle interaction coefficient as a function of salt concentration.

Chapter 4 is devoted to the adsorption behaviour of Synperonic surfactants and to the flocculation of dispersions stabilized with these surfactants. The adsorption was found to depend on the nature of the adsorbent, which had consequences for the stability of the dispersions. Polystyrene latices and emulsions could be stabilized with Synperonic NPE surfactants in electrolyte solutions, whereas
pyrogenic silica could not. The PS latex flocculated under theta conditions for poly ethylene oxide indicating that the ethylene oxide moieties of the adsorbed surfactants protrude into the aqueous solution.

Free liquid film thickness measurements are treated in chapter 5. For comparison purposes also a number of experiments has been carried out with Synperonic NP, a surfactant similar to NPE but without the propylene fraction. The measured film thicknesses were in good agreement with layer thicknesses obtained from the reduction in the streaming potential in a narrow capillary. This agreement supports the correctness of the two different techniques. The film thickness depended strongly on the ethoxy chain length and did not change with increase of surfactant concentration in the range studied. From thickness measurements of films stabilized by binary mixtures of surfactants it was demonstrated that fractionation did not occur in the free liquid films. The film thickness could be analyzed using a simple model in which the ethoxy chain behaved as a terminally adsorbed chain with dimensions similar to a coil trapped in a tube. The effect of electrolytes obeyed laws similar to those observed in chapter 3 and was entirely attributable to the interaction of the EO parts of the molecule.

Chapter 6 deals with the stability against coalescence of emulsions stabilized by Synperonic NPE 1800 in the presence of electrolytes. All emulsions studied were very stable according to criteria given in the literature, only in the course of several weeks some coalescence could be observed. The trend as a function of electrolyte concentration and temperature could be described with the film rupture theory of Vrij and Overbeek, but the observed effect of the oil volume fraction upon coalescence could not be explained with this theory.

Finally in chapter 7 the conclusion is obtained that with the present system the influence of electrolytes on the flocculation and coalescence are entirely different processes. Flocculation is due to interaction between the ethylene oxide moieties; with respect to this phenomenon the effect of electrolytes is similar to that observed in solutions of the surfactants and in the thin films.