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O: Fachverband Oberflächenphysik
O 18: Solid-liquid Interfaces II
O 18.8: Vortrag
Montag, 31. März 2014, 17:45–18:00, WIL A317
Self-Assembly of Alkyl Phosphonic Acids at α-Al2O3(0001)-Solvent Interfaces — Christian Meltzer, Wolfgang Peukert, and •Björn Braunschweig — University of Erlangen-Nuremberg; Institute of Particle Technology (LFG), Cauerstrasse 4, 91058 Erlangen
Molecular self-assembly of octadecylphosphonic acids (ODPA) on α-Al2O3 is studied with vibrational sum-frequency generation (SFG) in-situ and in real time. A detailed analysis of SFG spectra shows that both intensity and relative contributions of methylene and methyl bands are excellent indicators for coverage dependent changes in the molecular structure of ODPA self-assembled monolayers (SAMs). Concentrations between 0.1 and 5 mM ODPA in propanol lead to slow adsorption where a SAM is formed over several hours. For concentrations < 1 mM, SAMs with low coverage and possibly flat lying molecules are formed even after prolonged adsorption times. For concentrations >1mM an equilibrium can be reached, but vibrational spectra still show substantial contributions from gauche defects. Dewetting of these SAMs in N2 gas leads to an increase in SFG intensity that is accompanied by a substantial decrease in SFG contributions from gauche defects. After re-immersing of dried samples in 2-propanol with 5 mM ODPA, SFG spectra remain unchanged and are representative of well-ordered ODPA SAMs. From these observations we hypothesize that before de-wetting coverage and molecular order are already very high, but final improvements in molecular order are accomplished only at the solid-liquid-gas three phase line.