Adsorption of Methylene Blue onto Silylated Silica Surfaces, Studied Using Visible Attenuated Total Reflection Spectroscopy with a Slab Optical Waveguide

Abstract

The behavior of methylene blue (MB) molecules at silica/water and silylated silica/water interfaces was examined using visible attenuated total reflection spectroscopy with a slab optical waveguide (SOWG). An alkyl silane coating changed the adsorbed form of MB on the surface from a dimer (lambda max = 600 nm, bare silica surface) to a monomer (lambda max = 670 nm), and the carbon number of the silylation reagent influenced the amount of adsorption and the orientation of the molecule. Moreover, the addition of an anionic surfactant, dodecylbenzenesulfonate (DBS), caused the deposition of MB/DBS ion pairs, which gave an identical attenuated total reflection (ATR) spectrum to that of the dimer. Linear dichroism measurements revealed that the ion pairs were adsorbed onto the silylated silica surface randomly in terms of the orientation angle of MB, while the MB monomer was strongly oriented, i.e., the direction of the transition moment of MB roughly parallels the surface plane. This difference in the orientation angles of the adsorbed species can be utilized for their selective detection using polarization SOWG measurements.

Kinetics and mechanism of the adsorption of methylene blue from aqueous solution on the surface of a quartz cuvette by on-line UV–Vis spectrophotometry

“…Thus, the only plausible explanation for the observations is the reversible adsorption of the dye on the quartzwall of the cuvette. [13,14]The detrimental effect of hydrophobization of the surface on the adsorption clearly verifies that the surface needs to be hydrophilic for bonding MBefficiently (Fig. S12).…”

“…[13,14] None of our experimental results suggest a deviation from monolayer adsorption, thus the isotherm measured on aquartz cuvette was evaluated accordingly. First, the isotherm was fitted to the Langmuir modelrestricting the adsorption of MB to one molecule per one active site on the surface.…”

“…[11]The adsorption of MBon advanced materials is typically characterized by spectroscopic methods, as well as direct thermodynamic measurements in order to explore the binding mode and the orientation of the molecule on the surface. [12,13,14] The thermodynamic and UV-Vis spectroscopic features of MB adsorbed on silica surfacearewell characterized. [13,14,15,16,17] There seems to be a consensus in the literature that MB forms a monolayer on silica when adsorbed from solutions of high micromolar concentrationsat short contact times.…”

“…[12,13,14] The thermodynamic and UV-Vis spectroscopic features of MB adsorbed on silica surfacearewell characterized. [13,14,15,16,17] There seems to be a consensus in the literature that MB forms a monolayer on silica when adsorbed from solutions of high micromolar concentrationsat short contact times. The UV-Vis spectrum of adsorbed MB shows a blue shift at around 600 nm, which has beenattributed to dimer formation on the surface.…”

Dynamic Analysis of Adsorption-Desorption Equilibrium of Sublimed Aromatic Molecules Using Polarized Optical Waveguide Spectroscopy

“…17,18 OWG spectroscopy can trace any change of the transition moment of interface molecules depending on time of using polarization for incident light. 19 In general, two types of polarized light have been used, namely, a transverse magnetic mode (TM) and a transverse electric mode (TE) having perpendicular and parallel amplitudes to the waveguide, respectively. It is known that the component molecules can be excited by polarized incident light parallel to the transition moment of the molecules.…”