The double layer and ion adsorption at the interface between two immiscible solutions: Part III. Long chain alkyl-trimethylammonium halides at partition equilibrium between water and nitrobenzene

Abstract

The surface excesses of alkyltrimethylammonium halides have been evaluated from interfacial surface tension measurements carried out at the partition equilibrium between water and nitrobenzene. The model considered is constituted by two space charge layers on either side of a central compact layer which contains alkyltrimethylammonium ions arranged as in an adsorbed monolayer. The value of the electrical potential at the junction plane between the two phases depends on the degree of dissociation within the inner layer and on the number of specifically adsorbed ions. If one assumes complete dissociation it is possible to establish the variation of the adsorption isotherms with the chemical and electrical parameters and thus determine the value of the free energy of adsorption with the number of CH 2 groups in the alkyl chain. The results obtained do not indicate an important effect due to the counter ion, which would tend to substantiate the hypothesis of a total dissociation within the central layer. Furthermore the fact is stressed that the knowledge of the isotherm depends on the magnitude of the potential drop within the compact layer, the value of which cannot be determined by purely thermodynamical measurements.