The effect of polyethylene glycols on the interaction and stability of AOT/water/isooctane microemulsions

Abstract

The influence of water-soluble polyethylene glycols PEG200 on the properties of sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) water-in-isooctane microemulsions has been investigated by conductivity measurement and isothermal titration microcalorimetry (ITC). The percolation temperatures (Tp), the thermodynamic properties of droplet aggregation and the interaction enthalpy (−ΔHφ) between droplets for AOT/water/PEG200/isooctane with different PEG concentrations and a fixed molar ratio of water to surfactant ω = 22.2 were determined. Based on a two-stage thermodynamic approach of the percolation, we obtained the fusion enthalpy (ΔHf0) of the close contact droplets by a combination of the droplet aggregation enthalpy (ΔHag0) and −ΔHφ. It was found that both (−ΔHφ) and (ΔHf0) varied with the concentration of PEG200 (cPEG) and had the extreme value at cPEG = 25 g L−1. This phenomenon was interpreted by the variation of interface rigidity due to the varied distributions of added PEG200 between the water pool and the surfactant interface. Keywords: Conductance percolation, Thermodynamic parameter, Interaction enthalpy, Fusion enthalpy, Isothermal titration microcalorimetry