Mizukami Masashi, Moteki Masashi, Kurihara Kazue
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan.
J Am Chem Soc. 2002 Oct 30;124(43):12889-97. doi: 10.1021/ja027141g.
Adsorption of ethanol onto silica surfaces from ethanol-cyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long-range attraction was found between the silica (glass) surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol % at room temperature. At 0.1 mol % ethanol, the attraction appeared at a distance of 35 +/- 3 nm and turned into a repulsion below 3.5 +/- 1.5 nm upon compression. Half of the attraction range agreed with the adsorption layer thickness estimated from the adsorption excess amount by assuming that the adsorption layer was composed only of ethanol. This indicated that the observed long-range attraction was caused by the contact of opposed adsorption layers of ethanol on the silica surfaces and that the sharp increase of repulsion at shorter distance was caused by the overlap of structured ethanol clusters adjacent to the surface. ATR-FTIR spectra demonstrated that ethanol adsorbed on the silica (silicon oxide) surfaces formed hydrogen-bonded clusters (polymers). Practically no ethanol clusters were formed on the hydrogen-terminated silicon surface. These results indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol hydroxyl groups. At higher temperatures (30-50 degrees C), the range and the strength of attraction decreased owing to the decrease in the hydrogen-bonded clusters monitored by FTIR spectroscopy, reflecting the nature of hydrogen bonding. The range and the strength of the attraction also changed when the ethanol concentration increased: The long-range attraction started to decrease at 0.6 mol % ethanol at room temperature and disappeared at 1.4 mol % while the adsorption excess amount remained almost constant as did the FTIR peak intensity of the hydrogen-bonded OH group of adsorbed ethanol. In the bulk solution, ethanol clusters appeared at 0.5 mol % ethanol; thus, this change in the attraction could be accounted for in terms of the exchange of ethanol molecules between the surface clusters and bulk clusters. The novel self-assembled structure of alcohol on the surface, found in this study may be called a "surface molecular macrocluster" because the hydrogen-bonded clusters extend to distances of ca. 20 nm longer than the typical sizes of common clusters, 2-4 nm, of alcohol (e.g., ethanol).
采用胶体探针原子力显微镜、吸附过量等温线测量以及衰减全反射（ATR）模式的傅里叶变换红外光谱（FTIR）相结合的方法，研究了乙醇从乙醇 - 环己烷二元液体中吸附到二氧化硅表面的情况。发现在室温下，当乙醇浓度在0.1 - 1.4 mol%范围内时，二氧化硅（玻璃）表面之间存在异常长程的吸引力。在乙醇浓度为0.1 mol%时，吸引力出现在35±3 nm的距离处，压缩时在低于3.5±1.5 nm处转变为排斥力。吸引力范围的一半与通过假设吸附层仅由乙醇组成，根据吸附过量量估算出的吸附层厚度一致。这表明观察到的长程吸引力是由二氧化硅表面上相对的乙醇吸附层的接触引起的，而在较短距离处排斥力的急剧增加是由与表面相邻的结构化乙醇簇的重叠引起的。ATR - FTIR光谱表明，吸附在二氧化硅（氧化硅）表面的乙醇形成了氢键簇（聚合物）。在氢终止的硅表面上几乎没有形成乙醇簇。这些结果表明，簇的形成除了涉及乙醇羟基之间的氢键相互作用外，还涉及表面硅醇基团与乙醇羟基之间的氢键相互作用。在较高温度（30 - 50℃）下，由于FTIR光谱监测到的氢键簇减少，吸引力的范围和强度降低，这反映了氢键的性质。当乙醇浓度增加时，吸引力的范围和强度也会发生变化：在室温下，乙醇浓度为0.6 mol%时，长程吸引力开始下降，在1.4 mol%时消失，而吸附过量量以及吸附乙醇的氢键合OH基团的FTIR峰强度几乎保持不变。在本体溶液中，乙醇浓度为0.5 mol%时出现乙醇簇；因此，这种吸引力的变化可以用表面簇和本体簇之间乙醇分子的交换来解释。在本研究中发现的表面上醇的新型自组装结构可能被称为“表面分子大簇”，因为氢键簇延伸的距离约为20 nm，比醇（如乙醇）常见簇的典型尺寸2 - 4 nm长得多。
