Thermodynamic study of adsorption of nickel complexes with Schiff bases from acetonitrile solutions on uncharged graphite surface

Abstract

Adsorption of nickel N, N-ethylenebis(salicylideneimine) complex from acetonitrile solution on expanded natural graphite powder at 2733313 K is studied.The chemistry of conducting polymers and molecular modification of electrode surfaces are thriving branches of modern chemical science. In the last decade, a number of new complexes of transition metals with Schiff bases have been synthesized [134]. These polymeric compounds demonstrate high electron conduction, stability, and electrochromic and catalytic properties [3]. The polymerization is always accompanied by adsorption of monomers on the electrode surface. However, to our knowledge, there are no published data on the adsorption of the complexes indicated. Therefore, we examined in this study the adsorption of nickel N, N-ethylenebis(salicylideneimine) complex [Ni(SalEn)] on graphite.Experimental data on adsorption of organic compounds on metal surfaces are often described by the Frumkin isotherm (1).where G and G max are the current and limiting numbers of moles of a surfactant that can be adsorbed on unit surface area (mol); a and B, attraction and adsorption constants; c org and X org , concentration (M) and mole fraction of the surfactant; and S n is the total number of moles contained in 1 l of the solution.Since the common case is that c org < 0.1 M, it may be accepted that S n is equal to the solvent concentration in the solution. It is known thatwhere DG 0 A is the standard Gibbs energy of adsorption, corresponding to the asymmetric standard state in the bulk of solution [6]. At a = 0, the Frumkin isotherm transforms to the Langmuir isotherm [7]: 1/c org = B G max /G 3 B. (3) From experimental dependences of G on c org , we can calculate the adsorption term B, and then, using Eg. (2), find the standard Gibbs energy of adsorption DG 0 A . In a previous communication [6], we suggested to calculate parameters G max , a, and B of the Frumkin isotherm using the Origin software package. If the symmetric standard state is chosen for the bulk of solution, the Frumkin isotherm should be represented as [6]: ÄÄÄÄÄÄ exp 32a ÄÄÄ = B`g org X org . Here B`= exp [3DG 0 ads /RT ] is the dimensionless parameter; DG 0 ads , standard Gibbs energy of adsorption, corresponding to the symmetric standard state; and g org , activity coefficient of the surfactant in the bulk of solution.According to [6], if c org < 0.1 M, thenwhere g E org is the activity coefficient of the surfactant at infinite dilution.The activity coefficient g E org is the characteristic of the solution, which can range from unity to 10 3 310 5 , depending on the nature of a surfactant and solvent [8]. The parameter DG 0 A in Eq. (5) depends both on the properties of the surfactant solution and on the metal3surfactant binding energy, while the parameter