Molecular Orientations of Myristic Acid Derivatives with Different Perfluoroalkyl Chain Lengths at the Air/Water Interface Evaluated by Heterodyne-Detected Sum Frequency Generation Spectroscopy

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Surface-related properties of perfluoroalkyl compounds such as hydrophobicity and adhesion force can have a close relationship with the orientational order of the molecules at the surface. Myristic acid (MA) derivatives with perfluoroalkyl (R f ) groups have been known to form a well-ordered monolayer at the air/gold substrate interface. In this study, molecular orientations of the MA derivative monolayers at the air/water interface were investigated by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. We estimated the molecular orientation of MA derivative monolayers with a marker band observed at around 1370 cm –1 . The R f chain length and the surface pressure dependence of the molecular orientation of four MA derivatives were examined. The marker bands were observed except for the derivative with the shortest R f chain at the higher surface pressure, whereas the marker bands were observed only for the two derivatives with relatively longer R f chain lengths at the lower surface pressure. The observation indicates that the longer the R f chain is and the higher the surface pressure is, MA derivatives tend to adopt a standing orientation closer to perpendicular to the interface.