Self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on gold surface using a lyotropic liquid crystalline medium

Abstract

We have studied the self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on gold prepared in a lyotropic liquid crystalline medium using electrochemical techniques. We have used thiols of different alkyl chain lengths viz., C 5, C 8 and C 10 which exhibit nematic liquid crystalline order in bulk. They are dispersed in a hexagonal lyotropic liquid crystalline phase, consisting of a non-ionic surfactant, Triton X-100 and water. This medium provides a highly hydrophobic environment to solubilize the thiols and later facilitate their delivery to the gold surface to form a monolayer. The electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy were used to evaluate the barrier property and ionic permeability of these monolayers on gold surface. We have compared our results with that of the corresponding monolayers prepared using dichloromethane as a solvent. We find from our studies that the monolayers prepared using the hexagonal liquid crystalline phase show a better electrochemical blocking ability towards the redox reactions and exhibit very low ionic permeability. From the impedance studies, we have determined a surface coverage value of >99.9% for the monolayer on Au surface for all the thiol molecules studied in this work. We have also proposed a model for the likely processes involved in the formation of monolayer from the liquid crystalline medium. This is the first example in literature of self-assembled monolayer formation on Au by a compound exhibiting bulk nematic phase that is dispersed in a lyotropic liquid crystalline medium.