Intercalation of methylene blue in n-alkanethiolated self-assembled monolayers: Versatile electrochemical platforms for characterizing surfactant adsorption on hydrophobic surfaces

A versatile electrochemical platform for characterizing the adsorption of neutral and positively charged surfactants on hydrophobic surfaces was established using methylene blue (MB) as the probe. As a rigid, planar and electroactive species, MB can intercalate inside the regular self-assembled monolayers (SAMs) of n-hexanethiol and exhibit well-defined electrochemical responses. The adsorption of surfactants on the hydrophobic SAMs through the intercalation interaction between the hydrophobic tails of surfactants and the SAMs might change the density of the SAMs and influence the electrochemical behaviors of MB, providing a simple but effective approach for characterizing surfactant adsorption on hydrophobic surfaces. As an example, the adsorptive behaviors of cetyltrimethylammonium bromide (CTAB), a positively charged surfactant, and Triton X-100, a neutral surfactant, on hydrophobic surfaces were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results showed that these surfactants generally experienced three different adsorptive behaviors: the monomer adsorption at low concentrations, the loose monolayer adsorption at intermediate concentrations and the dense monolayer adsorption at high concentrations. In the case of CTAB, a new additional submonolayer adsorptive behavior between the monomer and the loose monolayer adsorption was observed for the first time, due to its rather long hydrophobic tail.