Overlapping Double Layers and Interparticle Repulsion

7 Schematic representation of the overlap of two double layers when a pair of plates is brought to a surface separation h. 

The electrical force per unit volume is given by the product of the charge density times the field strength according to Equation (6)  

Combining these results with the criterion for equilibrium leads to the expression (dp/dx) 4- p (dip/dx) = 0 (72) 

This result shows that the condition for equilibrium is equivalent to requiring that 

Equation (78) is easily integrated between the following limits p = p0 (the outer reference pressure) at p = 0 and p = ph/2 at p = ph/2. Integration of Equation (78) gives 

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