Study of the Adsorption Mechanisms of Surface Active Agents at the Silica-Water Interface by Total Internal Reflection Raman Spectroscopy

This study presents the results related to the adsorption behavior of one cationic surface active agent at the silica-water interface and demonstrates the capabilities of the Total Internal Reflection (TIR) Raman sampling configuration to provide spectroscopic information on a molecular level and simultaneously serve as an experimental platform for studying thermodynamic and kinetic properties of molecules at the silica-water interface. This Raman spectroscopic technique takes advantage of an “evanescent electric field” that is generated at the silica-water interface in TIR mode with limited, TIR geometry specific, probing depth.

  A minor portion of the research work was related to the adsorption of low vapor pressure solvent molecules from the gas phase at the silica-gas interface, followed by studies at the silica-water interface that proved that neither short alkyl chain hydrophilic ionic liquids nor anionic surfactants adsorb in excess at the silica-water interface. 
   The primary goal of this work was to study and identify the adsorption mechanisms of a cationic surfactant at the silica-water interface. Spectral analysis and the relevant Raman signal intensities as function of surfactant concentration and surface excess provided the input data for evaluating the thermodynamic and kinetic parameters of benzyldimethylhexadecyl ammonium chloride. Adsorption isotherms were fitted according to the Langmuir model for the pure surfactant, and based on a modified Langmuir model for the surfactant with various  concentrations of magnesium chloride, on bare and on hydrophobic silica. The results revealed enhancement of thermodynamic and kinetic properties of adsorption, as function of electrolyte concentration; the results from the 2nd part of the research work further emphasized the effects of a hydrophobic silica surface on surfactant adsorption behavior; the hydrophobic surface properties promoted different type of surfactant aggregation at the interface, with attenuation of the adsorption driving mechanisms, but with retention of the intrinsic surfactant properties found at the bare silica surface.
  Spectral analysis of the Raman scattering intensities as function of concentration and time indicated that no significant reorientation of surfactant molecules takes place at the silica-water interface and the most likely aggregate structure at the surface is spherical at the bare silica surface, and hemi-micellar or hemispherical at the modified hydrophobic silica surface. The study concludes that cationic surfactant adsorption mechanisms can be modulated by substrate properties, electrolyte type, and the concentration levels of both surfactant and electrolyte.