A study of the hydrophobic properties of alkanethiol self-assembled monolayers prepared in different solvents

We have measured the interfacial capacitance of an alkanethiol self-assembled monolayer (SAM) and its barrier properties adsorbed in different non-aqueous solvents by means of electrochemical impedance spectroscopy and cyclic voltammetry. Our impedance results show that the permeability of the SAM depends to a large extent on the solvent used as the adsorption medium. Monolayers of alkanethiol are highly impermeable when formed in solvents like hexane and chloroform, compared to those formed in solvents like ethanol, DMF, acetonitrile, hexadecane and toluene. Our studies lead us to believe that a thin hydrophobic layer separates the alkanethiol SAM and the aqueous electrolyte when the monolayer is prepared in hexane and chloroform. This layer acts as a series capacitor to the dielectric film of the alkanethiol monolayer. This tends to lower the effective interfacial capacitance and also enhances the uncompensated solution resistance of the monolayer coated gold electrode. Our experimental results may indicate a large-scale depletion of water density as predicted by the theory of Lum Chandler and Weeks (LCW theory, J. Phys. Chem. B 103 (1999) 4570), or the presence of nanobubbles or cavities at the highly hydrophobic interface.