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

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 ( T p ), the thermodynamic properties of droplet aggregation and the interaction enthalpy (−Δ H φ ) 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 (Δ H f 0 ) of the close contact droplets by a combination of the droplet aggregation enthalpy (Δ H ag 0 ) and −Δ H φ . It was found that both (−Δ H φ ) and (Δ H f 0 ) varied with the concentration of PEG200 ( c PEG ) and had the extreme value at c PEG  = 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.