Coadsorption of water molecules and third-particles

Adsorption of third particles other than water molecules on metal electrodes influences the microstructure and the electrochemical activity of the electrode interface. For example, the interface of metal electrodes usually acts as a Lewis add in the adsorption of water molecules, but its Lewis add-base property is altered by the adsorption of third partides. Electronegative particles such as oi en molecules, if adsorbed, increase the local Lewis acidity of interfacial metal atoms around the adsorption sites whereas, electropositive particles such as sodium atoms, if adsorbed, increase the local Lewis basicity around their adsorption sites. Furthermore, the adsorption energy of water molecules is altered by the coadsorption of third partides on metal electrodes. 

the dissociative adsorption of hydroi l radicals increases with increasing coadsorption of electropositive potassium atoms on the platinum surface. It has also been reported that coadsorption of electronegative oxygen molecules accelerates the adsorption of hydroxjd radicals on the surface of copper, silver and nickel [Thiel-Madey, 1987]. 

In electrochemistry, the effect of acidic anions on the electrode interface is important. The interaction of the adsorbed water molecules is relatively weak with perchlorate ions (Cl04 ) and fluoride ions (F ), intermediate with chloride ions (Cl ) and sulfate ions (S04 ), and relatively strong with phosphate ions (P04 ) and bromide ions (Br ). In a series of halogen anions, fluoride ions (F ), which interact weakly with the metal surface, are adsorbed as hydrated ions (H3O F ) chloride and bromide ions (Cl and Br ), which interact strongly with the metal surface, are adsorbed as dehydrated ions (Cl M and Br M). 

In general, the contact adsorption of deh3drated anions changes the interfacial lattice structure of adsorbed water molecules, thereby changing the interfadal property. For example, the clean surfaces of metallic gold and silver, which are hydrophobic, become hydrophilic with the contact adsorption of dehydrated halogen anions. 

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