Characterisation of Microemulsions Using Conductivity, Viscosity and NMR

Characterisation of Microemulsions Using Conductivity, Viscosity and NMR 10.7.6.1 Conductivity Measurements 

Conductivity measurements may provide valuable information on the structural behaviour of microemulsions. In the early applications of conductivity measurements, the technique was used to determine the nature of the continuous phase. O/W microemulsions should give fairly high conductivity (which is determined by that of the continuous aqueous phase) whereas W/O microemulsions should give fairly low conductivity (determined by that of the continuous oil phase). 

At low Vw/Vo, a dear W/O microemulsion is produced with a high resistance (oil continuous). As Vw/Vo increases, the resistance decreases, and, in the turbid region, hexanol and lamdlar micelles are produced. Above a critical ratio, inversion occurs and the resistance decreases, producing O/W microemulsion. 

The system based on butanol shows a rapid increase in k above a critical water volume fraction value, whereas the second system based on hexanol shows much lower conductivity, with a maximum and minimum at two water volume fractions, 

In the first case (when using butanol), the curve can be analysed using the percolation theory of conductivity [18]. In this model, the effective conductivity is practically zero as long as the volume fraction of the conductor (water) is below a critical value (the percolation threshold). Beyond this value, k suddenly takes a non-zero value and increases rapidly with further increase in In the above case (percolating microemulsions), the following equations were derived theoretically. 

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