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.

中文翻译：

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在其自组装上的十八烷基膦酸分子结构的α-Al ₂ ö ₃（0001）†

十八烷基膦酸（ODPA）的自组装单层（SAM）的2-丙醇溶液上羟基化的α-Al形成₂ ö ₃（0001）的表面进行了研究原位并实时在固体/液体界面。C–H拉伸模式的总频率生成（SFG）的时间分辨振动光谱揭示了ODPA的烷基主链和末端甲基以及由在α-Al处存在2-丙醇分子引起的振动带的贡献₂ O ₃表面。SAM形成过程中，SFG光谱中的2-丙醇特征减弱。这是由于ODPA分子触发从2-丙醇解吸吸附的α-Al ₂ ö ₃（0001）表面，以便这些位点可以被ODPA分子占据。针对不同体积的ODPA浓度研究了SAM的形成，这大大改变了最终SAM的质量和覆盖范围。在SAM生长的初始阶段，SFG光谱受亚甲基的贡献支配，并指示ODPA分子的低分子序和覆盖率。对于ODPA≤2 mM的浓度，这种情况在合理的吸附时间内（约16小时）没有改变，而对于5和30 mM的浓度，则在吸附的前2小时内观察到了分子顺序和覆盖率的急剧增加。使用Langmuir吸附动力学的热力学分析提供了平衡常数和吉布斯自由吸附能，介于-24和-28 kJ mol ^-1之间。