Molecular structure of octadecylphosphonic acids during their self-assembly on α-Al₂O₃(0001)

The formation of octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) from 2-propanol solutions on hydroxylated α-Al2O3(0001) surfaces was studied in situ and in real time at the solid/liquid interface. Time-resolved vibrational spectra from sum-frequency generation (SFG) of C-H stretching modes revealed contributions from ODPA's alkyl backbone and the terminal methyl group as well as vibrational bands that originated from the presence of 2-propanol molecules at the α-Al2O3 surface. 2-Propanol signatures in SFG spectra decreased during SAM formation. This is due to adsorption of ODPA molecules which trigger desorption of 2-propanol from the α-Al2O3(0001) surface, so that these sites can be occupied by ODPA molecules. SAM formation was studied for different bulk concentrations of ODPA which changed substantially both the quality and the coverage of the final SAM. At initial stages of SAM growth, SFG spectra are dominated by methylene contributions and are indicative for a low molecular order and coverage of ODPA molecules. For concentrations of ODPA ≤2 mM this situation did not change within reasonable adsorption times (∼16 h) while for 5 and 30 mM concentrations a dramatic increase in molecular order and coverage within the first 2 h of adsorption is observed. Thermodynamic analysis using Langmuir adsorption kinetics provided equilibrium constants and the Gibbs free energy of adsorption between -24 and -28 kJ mol-1.