IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Tribology Wednesday Sessions
       Session TR+MM-WeA

Invited Paper TR+MM-WeA3
Friction Properties of Self-assembled Monolayers: Influence of Adsorbate Molecular Structure and Molecular Organisation

Wednesday, October 31, 2001, 2:40 pm, Room 132

Session: Nanotribology
Presenter: G.J. Leggett, UMIST, UK
Authors: N.J. Brewer, UMIST, UK
G.J. Leggett, UMIST, UK
Correspondent: Click to Email
The frictional properties of self-assembled monolayers (SAMs) of alkanethiols on gold are influenced by both the chemistry of the adsorbate tail groups and the organisation of adsorbate molecules within the monolayer. Studies of mixed self-assembled monolayers composed of adsorbates with contrasting terminal group chemistries and equal chain lengths have revealed a linear relationship between SAM composition and the coefficient of friction. However, for mixed SAMs formed from molecules with differing alkyl chain lengths, molecular organisation may be quite different from that observed for single component SAMs, leading to substantial changes in the coefficient of friction. Moreover, while methyl terminated SAMs exhibit markedly different frictional properties for adsorbates of different chain length, this is not the case for SAMs formed from hydroxyl and carboxylic acid terminated adsorbates. For these polar SAMs the coefficient of friction varies little between short and long chain adsorbates. This is attributed to hydrogen bonding between adsorbate terminal groups, which has a dominating effect on SAM stability and organisation. The friction-velocity behaviour of SAMs with polar terminal groups in contact with polar tips is quite different from the behaviour observed for the same materials in contact with non-polar tips, and the expected linear relationship between the friction force and the log of the velocity is not observed; instead the friction force rises rapidly and then reaches a plateau. Finally, friction properties of specific thiols adsorbed onto gold and silver surfaces are markedly different, reflecting changes in molecular organisation and providing valuable insights into the structures of these materials.