Real-time measurement of the crystal violet adsorption behavior and interaction process at the silica–aqueous interface by near-field evanescent wave

Abstract

The interfacial adsorption and interaction of crystal violet (CV) at the silica–water interface was real-time measured based on a total-internal-reflection-induced near-field evanescent wave (TIR-NFEW). A silica optical fiber (SOF) was employed as a charged substrate for CV adsorption and as a light transmission waveguide for evanescent wave production for the investigation system. According to the change of evanescent wave intensity, the CV adsorption behavior could be real-time monitored at the silica–aqueous interface. The Langmuir adsorption model and two kinetic models were applied to obtain the related thermodynamic and kinetic data, including the adsorption equilibrium constant (K_ads) of (5.9 ± 1.5) × 10⁴ M⁻¹ and adsorption free energy (ΔG) of −21.6 ± 0.6 kJ mol⁻¹. Meanwhile, this method was shown to be able to isolate the elementary processes of adsorption and desorption under steady-state conditions, and gave an adsorption rate constant (k_a) and desorption rate constant (k_d) of 2089 ± 6.96 M min⁻¹ and 0.35 ± 0.0012 min⁻¹ for a 15 rpm flow rate. The surface interaction process was revealed and the adsorption mechanism proposed by a molecular orientation adsorption model with three-stage-concentration, indicating that CV first adsorbed on Si–O⁻ sites through electrostatic attraction, then on Si–OH sites through hydrogen bonding, and lastly on the surface through van der Waals forces with different CV concentrations. This study can provide a molecular-level interpretation of CV adsorption and provides important insights into how CV adsorption can be controlled at the silica–water interface.