S. Spange et al., CARBENIUM ION-PAIRS ON SILICA - UV-SPECTROSCOPIC AND ELECTROKINETIC MEASUREMENTS OF TRIARYLMETHYLIUM HALIDE-AEROSIL IN 1,2-DICHLOROETHANE/, Colloid and polymer science, 272(1), 1994, pp. 99-107
The interaction of carbenium ion pairs with silica is studied by means
of UV-spectroscopy, electrokinetic and adsorption measurements using
triphenylmethyl derivatives, (RC(6)H(4))(3)CX, (X = F, Cl, Br, SCN, OH
; R = Cl, I, H, CH3, C(CH3)(3), OCH3, N(CH3)(2)) in interaction with s
ilica particles suspended in 1,2-dichloroethane. The adsorption of tri
arylmethyl-halides onto silica is accompanied by the heterolytic disso
ciation of the tertiary carbon-halogen bond. The degree of ionization
depends on the basicity of the counter anion and acidity of the cation
, respectively. The influences of both concentration and structure of
triarylmethyl halides on the zeta-potential are discussed with regard
to steric and electronic factor. The zeta-potential values of the adso
rbates decrease significantly, as compared with the free silica surfac
e, from a certain triarylmethyl halide concentration where ionization
takes place, to a constant level which is characteristic of the carben
ium ion pair. These constant zeta-potential values of the adsorbates d
epend on the basicity of the counter ion and the sigma(p) Hammett cons
tants of the ring substitutents, with exclusion of sterically hindered
substituents and salt derivatives, e.g., crystal violet and malachite
green.