Surfactant—electrolyte interactions in concentrated water-in-oil emulsions: FT-IR spectroscopic and low-temperature differential scanning calorimetric studies

Abstract

Surfactant—electrolyte interactions have been studied in high phase ratio (0.64 to 0.91), water-in-oil (w/o) emulsions containing large concentrations (up to 85 M) of ammonium nitrate in the internal aqueous phase. Sodium nitrate was also incorporated in some emulsions. Two types of surfactant i.e. a non-ionic one, sorbitan mono-oleate (SMO) and a polymeric emulsifier, LZX, have been chosen for this investigation. Our studies reveal that the emulsion characteristics (primarily freezing behaviour) and the surfactant-electrolyte interactions in such w/o emulsions are strongly influenced by the nature of the emulsifier and the concentration of the salt. The interactions are predominantly of the hydrogen-bonding type. With SMO-based emulsions, an interesting observation is that the interactions reach a maximum in a steady manner around a 35 M ammonium nitrate concentration, as evident from shifts in the group frequencies in FT-IR spectroscopy and lowering of the freezing temperatures in differential scanning calorimetry (DSC) studies. Above this salt concentration, a reversal in freezing temperature is observed. Second-order effects on the freezing behaviour of SMO-based emulsions, owing to the nature of the solute in the dispersed phase, were also observed. However, in the case of the polymeric surfactant (LZX)-based studies it was observed in both the FT-IR spectroscopic and the DSC studies that the interactions reach a saturation level at much lower salt concentrations. Although the homogeneous nucleation temperature of the emulsion with pure water was around 234 K (as with SMO), in the DSC experiments emulsions with this emulsifier and with salt concentrations of 15 M and higher do not freeze even at temperatures down to 150 K. This effect of the surfactant on the freezing behaviour of concentrated w/o emulsions with dissolved electrolytes is being reported for the first time. Interfacial tension measurements also reveal differences in the behaviour of the two emulsifiers in the presence of electrolytes. All the results point towards greater interaction of the polymeric surfactant, which has a more polar head group, with the ionic solutes in the dispersed phase of the emulsion compared with the non-ionic surfactant. In general, batches containing the polymeric surfactant are found to possess better stability.