Molecular Adsorption at Silica/CH₃CN Interface Probed by Using Evanescent Wave Cavity Ring-Down Absorption Spectroscopy:  Determination of Thermodynamic Properties

Evanescent wave cavity ring-down absorption spectroscopy is applied to measure the thermodynamic properties of the surface adsorption for neutral trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl) pyridinium (DP) and charged trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (DMP⁺I^-) at the silica/CH₃CN interface, where the interfacial density is determined by measurement of absorbance. The bulk concentration dependence of the surface density may be characterized with a Langmuir isotherm model, which yields saturated surface density, equilibrium constant, and free energy of adsorption of (7.0 ± 0.3) × 10¹³ cm^-2, (1.3 ± 0.2) × 10⁴ M^-1, and −23.5 ± 0.4 kJ/mol for DP and (8.9 ± 0.3) × 10¹² cm^-2, (2.6 ± 0.7) × 10⁴ M^-1, and −25.2 ± 0.6 kJ/mol for DMP⁺I^-, respectively. The surface density of the isolated silanol groups may then be estimated in terms of the molecular probe results. The absorption contribution from the bulk solution is a factor of 10¹−10² smaller than the total absorbance measured such that subtraction of the bulk contribution leads to negligible change of the thermodynamic properties. The DP is adsorbed to the SiOH sites by forming hydrogen bonds, while the DMP⁺ cation is bound to the SiO^- sites by electrostatic attraction. Surface forces are also probed by addition of triethylamine (TEA), which is competitive with DP for the silanol sites. When the TEA concentration is increased, the DP surface density is found to decrease, whereas the DMP⁺ surface density increases. The obtained thermodynamic properties are generally consistent with those measured by second harmonic generation spectroscopy. However, when a tetramethylammonium ((CH₃)₄N⁺Cl^-) salt is added, the DMP⁺ cation behaves differently between these two methods. Formation of an electrical double layer may account for the difference.