Influence of Crown Ethers on the Electric Percolation of AOT/Isooctane/Water (w/o) Microemulsions

Abstract

A study was carried out on the influence of different crown ethers on the electric percolation of AOT/isooctane/water microemulsions. The crown ethers used were chosen on the basis of two fundamental criteria: (a) the different sizes of the molecules, where variation is found in the external size as well as the size of the cavity, and (b) the different solubilities of the ethers in water. In all cases we observed a dual behavior of the crown ethers with regard to the percolative phenomenon. At low additive concentrations we observed how the presence of the crown ethers caused an increase in the percolation temperature of the microemulsions, whereas at high additive concentrations there was a reduction in the percolation temperature causing the percolation threshold of the system to move forward. This dual behavior allowed us to define the compensation concentration, which corresponds with the crown ether concentration at which there is no effect on the percolative phenomenon. We observed a correlation between the effect exerted by the crown ethers and the size of the cavity. This shows the importance of the capacity to complexate Na+ and solubilize it in the interface and the continuous medium on the electric percolation. We also observed a correlation between the effect of the crown ethers on the percolation temperature and their external size. This shows the importance of their inclusion in the interface on the percolative phenomenon. Such an inclusion modifies the properties of the AOT film, facilitating the exchange of matter between droplets. We also obtained a satisfactory multiparametric correlation between the logarithm of the compensation concentration, the logarithm of the distribution parameter of the crown ether between water and 1-octanol, and the number of oxygen atoms in the crown ether. This correlation shows that the effect of the crown ethers on the electric percolation is due to its size and capacity to sequester ions, as well as to its solubility in the interface of the microemulsion.