Effect of chain length of some 2D and 3D alkanethiols on gold studied by DSC and voltammetry

Abstract

Striking similarities and differences in the behaviour of two-dimensional (2D) self-assembled monolayers (SAMs) on gold and corresponding three-dimensional (3D) SAMs on gold of two alkanethiols of different chain lengths, hexadecanethiol (HDM) and octadecanethiol (ODM) were investigated as a function of temperature. Cyclic Voltammetry (CV) of the 2D SAMs and Differential Scanning Calorimetry (DSC) of the 3D SAMs were used to study previously unexplained behaviour. CV revealed two transition temperatures in both the HDM and ODM 2D systems. In the DSC studies, both a lower and higher temperature transition was observed in the 3D SAMs of HDM but only one temperature transition was observed in the 3D SAMs of ODM. It was deduced that the lower temperature transitions in the gold-alkanoate nanoparticles were related to interaction between the methyl group chain ends. The lower temperature transition was observed in the 3D system of HDM but not that of ODM because the curvature in the 3D system that is absent in the 2D system reduces the interaction between the alkyl chain ends due to a longer chain resulting in an increased distance between the end groups. It was concluded that the lower transition temperature was attributed to the disordering of the alkyl chains starting from the end closest to the terminal groups.