The Water-Replacement Model of Bockris, Devanathan and Muller

The Water-Replacement Model of Bockris, Devanathan and Muller 

The isotherm derived by Bockris, Devanathan, and Muller (BDM) is centered on the role of water at the interface. 

A fundamental premise of the BDM isotherm is that water molecules can take up only one of two positions at the interface with the dipole vector either in the direction of the field or in the opposite direction. In other words, the angle (p between the direction of the field and the orientation of the dipole can either be zero or k, and the energy of interaction between the water dipoles and the field is either pF or pF, (which equals -pF) depending on its orientation. 

Considering only the energy of interaction of the water dipole with the surface charge density, all the water molecules should have their positive ends facing the surface when it is negative and vice versa, with a sharp transition at the E. There are, however, several factors that modify this transition, making it gradual. 

1) First, there is the entropy term. A situation in which all dipoles are oriented in one direction represents the minimum entropy for this system. Completely random orientation corresponds to maximum entropy. Thus, enthalpy and entropy affect orientation in opposite directions, as is often found in chemistry. 

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