Mobilities of Small Ions and Macroions in Electric Fields

2 MOBILITIES OF SMALL IONS AND MACROIONS IN ELECTRIC FIELDS A COMPARISON 

The fact that positive ions migrate toward the cathode and negative ions migrate toward the anode is so well known as to be virtually self-evident. It seems equally evident, therefore, that positively and negatively charged colloidal particles should display similar migrations. Indeed, this is the case. Because we are relatively familiar with the conductivity of simple electrolytes, we start our discussion of electrokinetic phenomena with a comparison of the mobilities of the particles in the small ion and macroion size domains. 

An isolated ion in an electric field experiences a force directed toward the oppositely charged electrode. This force is given by the product of the charge of the ion q times the electric field E  

In SI units, E is expressed in volt meter -1 and q in coulombs, so Fel is correctly given in newtons, since C V = J = N m. The cgs unit system, which is widely encountered in older references, requires dividing the right-hand side of Equation (1) by a factor of about 300 since 299.8 V = 1.0 statvolt. Use of Equation (1) is limited to situations in which the electric field at the ion is due to the applied potential gradient only, undisturbed by the effects of other ions in the solution (i.e., infinite dilution). 

An ion in an electric field thus experiences an acceleration toward the oppositely charged electrode. However, its velocity does not increase without limit. An opposing force due to the viscous resistance of the medium increases as the particle velocity increases  

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