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

The adsorption of sodiumdodecyl sulfate (SDS) on gold surfaces coveredwith self-assembledmonolayers (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 forcemicroscope 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 andan initially positively charged gold substrate with a 2-aminoethanethiol layer was a strong function of SDS concentration. The phenomenon of surface charge reversal (where theamount of negativeSDSequals the cationic surface charge)wasdirectly observed at an SDS concentration of about 1/1000 cmc. The surface electrostatic potentials of the surfactantadsorbed substrates were calculated by solving the complete nonlinear Poisson-Boltzmann equationwith 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 hexadecylmercaptanSAM-covered gold substratewas also examined tomimic the adsorption behavior of thehydrophobichemimicelle,which could formonthe2-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 forcemeasurements show that the formation of a compact and uniformhemimicelle or bilayer did not occur.