Debye Length

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Debye length is important characteristic of interfacial electric double layers. It serves as a measure of a distance between charged interfaces where electrostatic component of surfaces forces is important. Reduction of the Debye length due to increasing ionic strength leads to weaker electrostatic interactions and possible stability loss by dispersions and emulsions.

Transition from the fresh to sea water at the rivers deltas is striking example illustrating importance of this parameter. This change causes aggregation of particles floating in the river waters and consequent formation of the deposits at the rivers deltas.

There is a simple method suggested by Dukhin and Goetz and described in ISO Standard 13099-Part 1, 2012. http://www.iso.org/iso/catalogue_detail.htm?csnumber=52807

This parameter can be calculated from the conductivity K:

Debye

 

where ε is dielectric permittivity, and D is diffusion coefficient

The main uncertainty in this equation comes from the unknown effective diffusion coefficient D. However this parameter varies over limited range. For instance, the diffusion coefficients of most ions in aqueous solutions are similar and have values that at room temperature are in the range of 0.6×10-9 to 2×10-9 m2/s. The square root of this variation would yield uncertainty in scale of only tens percents. With regard to non-aqueous systems, recent studies indicate that ions there are not much larger either.

Application of this method fort ANY liquid requires capability of measuring conductivity within complete liquids conductivity range from 10-11 S/m up to 10 S/m. We have two instruments that cover this very wide dynamic range:

DT-700 for low conductivity range below 10-4 S/m

DT-900 for high conductivity range above 10-4 S/m

Software outputs this parameter for these instruments. Alternatively one can use Options OP004 and OP0041 for the Model DT-1202.