Heterodyne-Detected Vibrational Sum Frequency Generation Study of Air Water Fluoroalcohol Interface: Fluorocarbon Group-Induced Structural and Orientational Change of Interfacial Water

The presence of fluorocarbon group in alcohols provides them unique and important physicochemical properties. Fluoroalcohol containing aqueous interfaces are used in diverse fields, such as liquid liquid phase separation, chemical reactions, and polymer industry, and so forth. However, the molecular level understanding of interfacial water in the presence of fluorinated alcohol is sparse. In this paper, we have applied phase-sensitive and surface-specific vibrational spectroscopic technique, heterodyne-detected vibrational sum frequency generation, to understand the structure and orientation of interfacial water in the presence of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). To realize the role of fluorination, the results of the air water HFIP interface have been compared with the corresponding hydrogenated alcohol, air water isopropanol (IP) interface. It is observed that the net orientation (H-down) and hydrogen bonding of water at the air water IP interface are similar to the neat air water interface. In contrast, water changes its net orientation (H-up) and becomes more inhomogeneous at the air water HFIP interface. On increasing the bulk pH (11), the interfacial water becomes increasingly ordered (H-up oriented) at the air water HFIP interface, whereas there is no significant change at the air water IP interface. At acidic pH (2.8), the interfacial water takes the same H-down orientation for both IP and HFIP, though the average H-bonding is stronger for HFIP. Thus, fluorination of IP changes the preferred orientation and structure of interfacial water, which may be useful in understanding chemical and biochemical processes that occur at fluoroalcohol-containing aqueous interfaces.