The volumetric properties of water/AOT/isooctane microemulsions with small-size droplets

The densities of water/AOT/isooctane microemulsions in a wide concentration range with the prepared molar ratios. of water to AOT being 1, 2, 3, 4, 6 and 8 were measured at 303.15 K. The apparent volumes of the quasi-pure solute (water/AOT), the values of the volumetric interaction parameter a and the apparent specific volumes of water and AOT of the small-size droplets were deduced and compared with that of the large-size ones. It was found that the precisely determined volumetric properties in the microemulsion systems were capable to detect the structure transition and the stability boundary of the microemulsion droplets. A thermodynamic model was proposed to analyze the stability boundary of the microemulsion droplets, to investigate the mechanism of their instability and to discuss the different behaviors between the small-size and large-size microemulsion droplets.