Potential difference between two contacting phases

In general, an electrostatic potential arises between two phases when they are brought into contact. We deal with the electrostatic potential in two condensed phases being in contact. The electrostatic inner potential, of a condensed phase (solid or liquid) is given by the sum of the outer potential, V(o , associated with the electric charge, a, of the condensed phase and the surface potential, X(dip associated with the surface dipole as shown in Eqn. 4—1 and Fig. 4-5  

For noncharged phases (o = 0), the outer potential i )(o a)is zero and the inner potential equals the siuface potential (4 = X(dip)). As described in Chap. 2, the siuface potential arises from the surface dipole due to a spread-out of electrons from the smface in the case of metals and due to a dipole orientation of water molecules at the siuface in the case of aqueous solutions. 

When two condensed phases A and B are brought into contact, as shown in Fig. 4-6, the surface charge, o, the inner potential, , and the outer potential, tf(o), of the initially separate two phases are changed to values characteristic of the joined solid A/B on the other hand, the surface potential, Xcd ), at the free surface of each phase remains unchanged except, of course, for the joining surface. The outer potential, if, of the free surface of the two phases is given by Eqn. 4-2, whether or not the two phases are in contact  

Bringing two phases into contact, as in Fig. 4-6, produces an electric charge o on the free surface of the two phases, which is different from its initial value o hence, the outer potential of the two phases changes from its initial value to its final value, giving rise to a change in the difference of the outer potential between the two phases from its initial value 4t a/b before the contact to its final value 4 a/b after the contact as shown in Fig. 4-6. 

An electrostatic potential difference, called the inner potential difference, arises across the interface of two contacting phases. This inner potential difference consists of a potential gA/ac) due to an interfacial charge (charge on both sides of the interface), Oa/b aiid a potential gj /snip) due to an interfacial dipole, dipA/B, as shown in Eqn. 4-3  

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Fig. 4-6. llie inner potential, 4, and the outer potential, tf, of two condensed phases A and B before and after their contact d4 )= inner (outer) potential difference between two contacting phases o = surface or interface charge dip = surface or interface dipole. 

The inner potential difference between two contacting phases is cafied in electrochemistry the Galvani potential difference, and the outer potential difference is called the Volta potential difference. The outer potential difference corresponds to what is called the contact potential between the two phases. We call, in this test, the inner potential difference across an interface the interfacial potential. 

The outer potential difference between two contacting phases can be measured because it is a potential difference between two points in the same vacuum or gas phase outside the free surfaces of the two phases. On the other hand, the inner potential difference can not be measured, because the potential measuring probe introduces its interfacial potential that differs with the two phases and thus can not be canceled out this gives rise to an unknown potential in the potential measurement. 

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