Adsorption of a Polyethoxylated Surfactant from Aqueous Solution to Silica Nanoparticle Films Studied with In Situ Attenuated Total Reflection Infrared Spectroscopy and Colloid Probe Atomic Force Microscopy

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Abstract
Polyethoxylated (PEO) surfactant adsorption to silica under aqueous conditions is an important physical process in a multitude of industries. Consequently, a considerable number of spectroscopic and other studies have been carried out to ascertain the molecular/structural details the adsorbed surfactant and the kinetics of PEO-surfactant adsorption. However the use of infrared spectroscopy to probe surfactant adsorption at the silica/aqueous solution interface has been limited due to the instability of silica particle films under aqueous conditions and the opacity of silicon prisms below 1300 cm-1 typically employed for these studies. The work presented here provides infrared spectroscopic measurements of silica particle films formed from differing suspension pH on a diamond internal reflection prism to probe silica film stability as a function of pH. Films formed from a suspension pH of 2.5 were found to be the most stable owing to a sol-gel transition of the colloidal suspension upon drying and the reduction in electrostatic repulsion between silica nanoparticles, creating a tightly packed nanoparticle film. Colloid probe atomic force microscopy (CP-AFM) was used to confirm the alteration of surface forces between silica nanoparticles as a function of pH. Particle films formed from pH 2.5 silica suspensions were formed in situ on an ATR-IR diamond prism and used to probe Triton X-100 adsorption from aqueous solution. The obtained infrared spectra revealed a critical surface aggregation concentration at a solution concentration of 0.14 mmol L-1, and that Triton X-100 forms discrete micelles at the silica surface with the polyethoxylated head group preferentially adopting a helical conformation. Most intriguingly, a breakup of the silica particle film was observed at the critical micelle concentration of the surfactant. This is believed to be due to repulsive steric forces arising between the PEO corona of the surfactant micelles formed at the silica surface, as confirmed by CP-AFM measurements.
Funding Information
  • Ministry of Business, Innovation and Employment
  • University of Otago

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