The nature of physical adsorption of surface-active cations of some organic inhibitors of acid corrosion of metals

Abstract

The central atom of each of these surface-active cations, which confers their adsorption properties on them, is the nitrogen atom. The coordination valence of cationic nitrogen in N-substituted pyridine compounds is four, and the nitrogen atom carries practically the entire positive charge of the ion. The magnitude of this charge, which determines the adsorptive power of organic cations and, consequently, their surface-active properties, depends on the affinity of the substituting components (radicals and functional groups) to electrons, on the proneness of these components to ~-electron coupling, and on the place of substitution. Owing to the electro-affinity effect, electrons in the pyridine center are displaced toward the nitrogen atom [2-4], so that the negative charge density near this atom is larger than in other regions of the aromatic ring. The displacement of pyridine cation electrons is intensified by the positive charge produced in the nitrogen atom as a result of the coupling of its unshared electron pair with the N-substituting radical-cation [5-7]. For both these reasons, electrons in the pyridine center are strongly drawn toward the nitrogen atom. The n- and ~r-electron coupling, the difference in the affinity to electrons, and the establishment of a double virtual bond [8], all cause migration of electrons from the substituting groups to the pyridine center. Consequently, the substituting groups determine both the magnitude of the positive charge of the nitrogen atom in pyridine cations and the polarizability of the cation in the electric field of a metal surface, i.e., they determine its adsorbability. In respect to their affinity to electrons, radicals and groups form a series in which -CHa < -OH [6, 9, 10]. The affinity of the phenyl radical to electrons is higher than that of the methyl radical, so that the latter confers a negative charge on a benzene ring [6, 11, 12] as a result of which a certain dipole moment is produced in a toluene molecule [6, 13, 14]. A similar relation between affinities is indicated by the effect of these and several other substitutes in the 2-, 3-, and 4-positions on the basic properties of pyridine [3]. Since the electron affinity of the phenyl radical is higher than that of the hydroxy group (see the properties of phenyl [3, 7, 9, 13-17], the affinity series may be extended: -CH 3 < -OH < -C6H 5, It is known [2, 3, 7, 10, 14-18], that the -OH group in aromatic compounds serves as a donor of electrons, the aromatic radical intensifying the acid properties of this group, whereas in alkyl compounds (alcohols), for instance, this group acts as an acceptor of electrons (with hydrocarbon groups and, mainly, the methyl CHs-grou p acting as donors) and is weakly acidic in character. The donor properties of the hydroxy group are markedly pronounced even in phenyl, in which the n-electrons of oxygen become coupled with the ~r-electrom of the aromatic center. As a result of this coupling, oxygen becomes positively charged which, in turn, increases the probability of protons being detached from oxygen atoms. When the -OH group is fully ionized, the negative ionic charge of a phenolate ion extends over the entire coupling system, as a result of which the negative charge of oxygen is reduced and so is, consequently, the tendency of the phenolate ion to recombine with the proton. The processes under review are also affected by the fact that the ionization of phenyl molecules is strongly dependent on the pH of the medium: The first signs of ionization are observed at pH = 6, and it becomes complete at pH = 11.8 [19]. This is in agreement with the fact [20] that phenyl (dissolved in ethyl alcohol which behaves as a base in respect to phenyl) is more strongly adsorbed than benzene on charged mercury surfaces. It shifts the zero charge potential more strongly toward the negative potential values, reduces the electrocapillary maximum more strongly, and is more readily desorbed from negatively charged mercury surfaces than benzene which, naturally, is associated with the presence in the molecule of a polar hydroxy group. A similar phenomenon is observed in aqueous solutions of sulfuric acid [21]. When the -OH group is introduced into an aromatic center, the adsorbability of aromatic compounds on positively charged metal surfaces increases in the order benzene-phenyl-r esorcin, the shift of the zero charge potential toward negative values increasing, and adsorbability on negatively charged metal surfaces decreasing, in the same order.