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The knowledge of the interactions and the stability boundaries of microemulsion droplets is important not only for explanation of the phase behaviors of those complex fluids but also for various applications,such as drug carriers,cosmetic materials,and microreactors.These interactions are generally thought to be attractive resulted mainly from the mutual interpenetration of the surfactant tails and described by a simple attractive square-well potential model,while the investigation of the stability boundary requires measurements of some properties of microemulsions in an extremely diluted concentration region.We proposed experimental designs for measurements of the real dilution heat of the microemulsion droplets by isothermal titration microcalorimetry(ITC)and the critical microemulsion concentration of the droplets by vibrating-tube densimetry.By using these methods,we investigated the dilution enthalpies and the apparent specific volumes of the droplets for a series of water/AOT/oil microemulsions with various oils and molar ratios R of water to AOT.It was found that the solvents appear to play important roles in varying the droplet interactions from the positive to the negative,which reveals the importance of the entropy contribution for the stability of the microemulsion systems.A thermodynamic model was proposed to analysis the apparent specific volume data,which confirms maximum value of R being about 65 for the stable AOT/water/isooctane microemulsion system.Keywords: interaction of microemulsion droplets; dilution enthalpy; stability boundary; critical microemulsion concentration,apparent specific volume.