EFFECT OF CHARGE-DENSITY FLUCTUATIONS WITHIN A DROPLET ON DIELECTRIC POLARIZATION OF IONIC MICROEMULSIONS

A statistical model of the dielectric polarization of ionic water-in-o il microemulsions is proposed. The model makes it possible to describe the effect of temperature and dispersed phase content on the static d ielectric permittivity behavior of the microemulsions at a region far below percolation. With the help of this model, the microemulsions for med with the surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) , have been analyzed. The studied systems are considered to consist of nanometer-sized spherical non-interacting water droplets of equal siz e with negatively charged head groups SO3-, staying at the interface a nd positive counterions Na+, distributed in the electrical diffuse dou ble layer of the droplet interior. It can be conjectured that two diff erent mechanisms, that provide an increase of the static dielectric pe rmittivity as a function of temperature, may take place. These may be attributed either to the aggregation of droplets or the temperature gr owth of polarizability of non-interacting and therefore non-aggregatin g droplets dispersed in oil. The results support the hypothesis that t he experimental temperature behavior of dielectric polarization far be low the percolation region is only due to the polarization of a single droplet and not to an aggregation. The droplet polarizability is prop ortional to the fluctuation mean-square dipole moment of a droplet. It is shown that this mean-square dipole moment and the corresponding va lue of the dielectric increment, depend upon the equilibrium distribut ion of counterions within a diffuse double layer. The density distribu tion of ions is determined by the degree of the dissociation of the io nic surfactant. The dissociation of the ionic surfactant in the system has been analyzed numerically. The relationship between the constant of dissociation and the experimental dielectric permittivity has been ascertained. (C) 1998 Elsevier Science B.V. All rights reserved.