Critical thermal variation of displacement of alkanethiols in self-assembled films

A packed structure of hexadecanethiol (HDT) self-assembled monolayer (SAM) on the polycrystalline gold electrode was reportedly loosed at temperature more than T-C = 45 degreesC. In this paper, the looseness was examined through the displacement of some longer alkanethiols by propanethiol (PT) in ethanol solution. As a measure of the looseness, capacitance, C, by the ac impedance was obtained under cyclic scanning variations of temperature, T, between 0 and 70 degreesC. Curves of C vs. T at the HDT SAM showed no hysteresis in ethanol solution as well as HDT solution. However, C vs. T curves of the HDT SAM in the PT ethanol solution increased drastically at temperature (T-D = 53 degreesC) higher than T-C to approach gradually the capacitance value of the PT SAM, indicating the displacement of the HDT by PT. Values of T-D were independent of the chain length, whereas those of T-C decreased with a decrease in the chain length of alkanethiols. (C) 2002 Elsevier Science B.V. All rights reserved.