Extracting the Free Energy of Adsorption and the Base-Ionization Constant of Triethylamine at the Silica/CH3CN Interface Using Nonlinear Optical Molecular Probes

Abstract

By using reflection second-harmonic generation (SHG), the adsorption of triethylamine (TEA) at the silica/CH3CN interface has been studied with the help of two molecular probes, trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (DMP+I-) and trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl)pyridinium (DP). At the silica/CH3CN interface, DMP+ cations are adsorbed on SiO- sites by charge−charge interaction, while DP molecules are adsorbed on SiOH sites by forming hydrogen bonds. The saturation surface density is 3.10 × 1013 cm-2 for DMP+ cations and is 1.85 × 1014 cm-2 for DP molecules. The free energy of adsorption ΔGads is −25.5 kJ mol-1 for DMP+ cations and is −22.0 kJ mol-1 for DP molecules. The addition of TEA into the solution causes dramatically different changes in the interfacial densities of the two probes: The surface density of DP molecules is decreased by a factor of 5 when the TEA concentration reaches 2 mM, while that of DMP+ cations is increased by a factor of 5 under the same condition. From the interfacial behavior of the two molecular probes that are coadsorbed with TEA and TEAH+ at the silica/CH3CN interface, the free energy of adsorption and the base-ionization constant of TEA at the silica/CH3CN interface have been extracted, ΔGTEA = −20.8 kJ mol-1 and Kb = 0.0173.