Simulations of Microscopic Wetting Phenomena on Self-Assembled Monolayers.
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SIMULATIONS OF MICROSCOPIC WETTING PHENOMENA ON SELF-ASSEMBLED MONOLAYERS. JOSEPH HAUTMAN AND MICHAEL L. University of Pennsylvania,
KLEIN
Philadelphia,
PA.
19104-6323
Recently developed methods of fabricating layers of long-chain molecules with functional tailgroups have provided new possibilities in the design and modification of surface properties. (1) An ui.derstanding of the relationship between the detailed molecu.ar structure and macroscopic properties, the such as wetting and adhesion, is needed to facilitate design of surface characteristics for specific applications optical devices and substrates such as coatings, adhesives, for binding biomolecules. [1,2] Various experimental techniques have been employed to characterize self-assembled monolayers (SAMs) formed by the chemisorption of thiol molecules (SH(CH2)nX) on metallic surfaces. These include contact angle measurements (wetting) [5] x-ray electron diffraction, and ellipsometry, L3,4] infrared scattering, (6] normal and Fourier-transform desorption spectroscopy, [7-9] temperature-programmed x-ray studies, [10] helium beam diffraction[ll] and Molecular dynamics techniques photoelectron spectroscopy. [3] have also been used to investigate the microscopic structure The results for of these self-assembled monolayers.[12,13] layers terminated by methyl (X=-CH 3 ) and polar (X=-OH) groups On the appear to be consi.,tent with experimental data. (111) Au surface the neutral methyl tailgroups form a triangular structure commensurate with the structure of the This result is consistent chemisorbed sulfur headgroups. [11) and Scanning with He beam diffraction experiments Tunneling Microscope images [14]. On the other hand, the layers having polar -OH or -CN tailgroups were observed to reconstruct. In the former case the -OH tailgroups formed a disordered network of hydrogen bonds in the plane of the monolayer surface. This complex surface structure results from the competition between tailgroup interactions and the With the larger -CN fixed spacing of chemisorbed headgoups. tailgroup the competing interactions lead to a different structure in which the tethered -CN groups adopt a distorted triangular lattice incommensurate with the underlying (13] This leads to an unusual terracing headgroup lattice. of the exposed surface of the layer as can be seen in Figure 1. The measurement of wetting properties has been valuable for characterizing the various types of self-assembled monolayer surfaces Ehat have been fabricated. The wetting process and the accompanying interfacial reconstruction can be studied by extending molecular dynamics simulations to With include overlayer of solvent molecules.
Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
304
Figure 1. The spheres represent time averaged positions the atoms in a self-assembled monolayer consisting of chain molecules with tailgroups consisting of charged C (black) and This view emphasizes the terracing of the N (white) atoms. exposed surface.
the neutral and nonfor example, a water ove
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