Adsorption Potentials of Dipolar Compounds

Replacement of gas by the nonpolar (e.g., hydrocarbon) phase (oil phase) has been sometimes used to modify the interactions among molecules in a spread film of long-chain substances. The nonpolar solvent/water interface possesses an advantage over that between gas and water in that cohesion (i.e., interactions between adsorbed molecules) due to dipole and van der Waals s forces is negligible. Thus, at the oil/water interfaces, the behavior of adsorbates is much more ideal, but quantitative interpretation may be uncertain, in particular for the higher chains, which are predominantly dissolved in the oil phase to an unknown extent. The oil phase is poured on the surface of an aqueous solution. Thus, the hydrocarbon, such as heptane or decane, forms a membrane a few millimeters thick. It is thicker than the adsorbed monolayer. Owing to the small difference in dielectric constant between the air and a hydrocarbon oil, the 

Adsorption of a dipolar substance at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water (Fig. 11). As seen in Fig. 11, the change in compensation voltage due to adsorption is the surface potential difference, usually called the surface potential or better the adsorption potential and often indicated unnecessarily by AV.  

The presence of the supporting electrolyte MX in the aqueous phases ensures the stability of the measurements. However, MX must not adsorb at the surface to avoid affecting the potential. 

Usually before adding a surfactant it is necessary to clean the surface of the water solution by siphoning off the top layers of water with a glass capillary until the constant compensation voltage is obtained. This is not done in the jet method. 

The surface potential change, besides the surface pressure, is the most important quantity describing the surface state in the presence of an adsorbed substance. However, the significance in molecular terms of this very useful experimental parameter still remains unclear. It is common in the literature to link A% with the properties of the neutral adsorbate by means of the Helmholtz equation  

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