The structure of the electrical double layer in the presence of adsorbed aliphatic molecules

Abstract

The equation previously deduced for the calculation of the adsorption of small polar molecules, such as thiourea, at the mercury/solution interface, has been extended to polar molecules containing straight-chain aliphatic groups. It is shown that the change in the dielectric constant, consequent on the adsorption of such molecules with their hydrocarbon group towards the mercury, is the distinguishing feature of such systems. By making allowance for this dielectric constant change, an equation which accounts for the peculiarities of the differential capacity curves, such as the adsorption peaks and the troughs in the vicinity of the electrocapillary maxima, is derived. John Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia (Communicated by J. A. V. Butler, F.B.S.—Received 10 November 1961). By using established values for the dielectric constants of water and of the hydrocarbon group, it is shown that the lowest value of the capacity for any aliphatic straight chain molecule should be around 5 or 6 u C in aqueous solution as found by a large number of investigators. The equation derived is applied to published data for the capacity of the mercury/aqueous sodium perchlorate interface in the presence of n -amyl alcohol. The calculations which involve successive approximations have been carried out with an electronic computer. The adsorption curves for n-amyl alcohol thus obtained are found to agree with other available data. The adsorption is shown to obey the Langmuir isotherm at the electrocapillary maximum. Isotherms applicable when the interface is charged are shown to be directly related to the Langmuir isotherm. The free energy of adsorption at the electrocapillary maximum is shown to be calculable from the isotherms for charged interfaces. The variation of the free energy of adsorption of n -amyl alcohol has been calculated and is shown to be similar to that observed with butanol.