Thermal Study of Accumulation of Conformational Disorders in the Self-Assembled Monolayers of C-8 and C-18 Alkanethiols on the Au(111) Surface

Prathima, N and Harini, A and Rai, Neeraj and Chandrashekara, RH and Ayappa, KG and Sampath, S and Biswas, SK (2005) Thermal Study of Accumulation of Conformational Disorders in the Self-Assembled Monolayers of C-8 and C-18 Alkanethiols on the Au(111) Surface. In: Langmuir, 21 (6). pp. 2364-2374.

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Abstract

The thermal stability of short alkanethiol $CH_3(CH_2)_7SH (C_8)$ and long $C_1_8$ self-assembled monolayers (SAMs) is investigated using grazing angle reflection-absorption infrared spectroscopy, cyclic voltammetry, and molecular dynamics simulation. We track the disordering of SAM by untilting and gauche defect accumulation with increasing temperature in the 300-440 K range, a range of interest to tribology. Molecular dynamics simulation with both fully covered and partially covered $C_6, C_8, and C_1_8$ monolayers brings out the morphological changes in the SAM, which may be associated with the observed thermal stability characteristics. The molecular dynamics simulations reveal that short-chain $C_6$ and $C_8$ alkanethiols are more defective at lower temperature than the long-chain $C_1_8$ alkanethiol. With increasing temperature disorder in the SAM, as reflected in both untilting and gauche defect accumulation, tends to saturate at temperatures below 360 K for short-chain SAMs such that any further increase in temperature, until desorption, does not lead to any significant change in conformational order. In contrast the disorder in the long-chain $C_1_8$ SAM increases monotonically with temperature beyond 360 K. Thus, in a practical range of temperature, the ability of a SAM to retain order with increasing thermal perturbations is governed by the state of disorder prior to heat treatment. This deduction derived from molecular dynamics simulation helps to rationalize the significant difference we have observed experimentally between the thermal response of short- and long-chain thiol molecules.

Item Type:	Journal Article
Publication:	Langmuir
Publisher:	American Chemical Society
Additional Information:	Copyright for this article belongs to American Chemical Society.
Department/Centre:	Division of Mechanical Sciences > Chemical Engineering Division of Mechanical Sciences > Mechanical Engineering Division of Chemical Sciences > Inorganic & Physical Chemistry
Date Deposited:	03 May 2005
Last Modified:	25 Jan 2012 06:07
URI:	http://eprints.iisc.ac.in/id/eprint/3095

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