title: pH dependence of the crystal violet adsorption isotherm at the silica-water interface.
abstract: The pH-dependent adsorption isotherms for the charged chromophore crystal violet, CV(+), have been measured with three different bases by a free-running cavity implementation of evanescent wave cavity ring-down spectroscopy. The ratio of the maximal absorbance measurements at pH 5.10 and 9.05 is consistent with a Q2:Q3 silanol site ratio of 72.8:27.2. The adsorption isotherms have been interpreted in terms a cooperative binding adsorption allowing more than one ionic species to bind to each silanol group. The surface concentration is consistent with a silanol charge density of 1.92 +/- 0.55 nm(-2) and a total neutralized interface layer structure extending 9 nm from the surface. Binding constants and stoichiometric coefficients are derived for CV(+) to both the Q2 and Q3 sites. A variation of the adsorption isotherm with base is observed so that the isotherm at pH 9.05 adjusted with ammonium hydroxide sets up a competitive acid-base equilibrium with the SiOH groups with only 49% of the surface silanol sites dissociated. The implications for functionalized surfaces in chromatography are discussed.
authors.affiliation: School of Biological and Chemical Sciences, University of Exeter, Stocker Road, Exeter EX4 4QD, UK.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Fisk
authors.forename: Robin
authors.initials: R
authors.lastname: Batten
authors.forename: Glenn
authors.initials: G
authors.lastname: Jones
authors.forename: Josephine P
authors.initials: JP
authors.lastname: O'Reilly
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Shaw
mesh_terms: D000327:Adsorption; D005840:Gentian Violet / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D015394:Molecular Structure; D012821:Silanes / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D012821:Silanes; D014867:Water; C082343:silanol; D012822:Silicon Dioxide; D005840:Gentian Violet