Characterization of Adsorption of Sodium Dodecyl Sulfate on Charge-Regulated Substrates by Atomic Force Microscopy Force Measurements

Abstract

The adsorption of sodium dodecyl sulfate (SDS) on gold surfaces covered with self-assembled monolayers (SAMs) of thiols made with either hexadecyl mercaptan or 2-aminoethanethiol hydrochloride was investigated by probing the surface charge. This was accomplished by determining the force between a modified (with a negatively charged silica sphere) tip of an atomic force microscope and the surface as SDS was adsorbed. The ionic nature of aqueous SDS solutions and the critical micelle concentration (cmc) in deionized water were determined by measuring the sudden change in diffuse double-layer thickness on micelle formation. The interaction between a silica probe and an initially positively charged gold substrate with a 2-aminoethanethiol layer was a strong function of SDS concentration. The phenomenon of surface charge reversal (where the amount of negative SDS equals the cationic surface charge) was directly observed at an SDS concentration of about 1/1000 cmc. The surface electrostatic potentials of the surfactant-adsorbed substrates were calculated by solving the complete nonlinear Poisson−Boltzmann equation with the knowledge of silica probe surface potentials. From the surface charge vs surfactant concentration data, the adsorption behavior of SDS was assessed. The interaction between the silica probe and the hydrophobic hexadecyl mercaptan SAM-covered gold substrate was also examined to mimic the adsorption behavior of the hydrophobic hemimicelle, which could form on the 2-aminoethanethiol surface. Considerably different surfactant adsorption behavior was found for the hydrophobic hexadecyl mercaptan SAM and the 2-aminoethanethiol surfaces. For the adsorption of SDS on an initially positively charged surface, quantitative force measurements show that the formation of a compact and uniform hemimicelle or bilayer did not occur.