Polarization-modulated infrared reflection absorption spectroscopic studies of a hydrogen-bonding network at the air-water interface

The hydrogen-bonding network formed between a triaminotriazine amphiphile ( 2C(18)TAZ, 1) and complementary barbituric acid (BA, 2) at the air-water in terface is investigated by polarization-modulated infrared reflection absor ption spectroscopy (PM-IRRAS). The molecular structure and orientation of t he 1:1 hydrogen-bonding network at the air-water interface is revealed in t his study. Without the addition of BA to the subphase, the NH2 scissoring o f 2Cl(18)TAZ appeared in the spectrum as a broad negative absorption band b etween 1660 and 1605 cm(-1), indicating its perpendicular orientation to th e air-water interface. When BA was added to the subphase, the NH2 scissorin g absorption band from the triaminotriazine moiety disappeared due to the c omplementary hydrogen bonding of BA to the 2C(18)TAZ monolayer. The formati on of the rigid 1:1 hydrogen-bonding network also resulted in the disappear ance of one of the ring quadrant stretch absorption bands of the 2C(18)TAZ molecule. New bands which are attributed to the vibration of BA can be clea rly seen. Particularly, the C=O stretch from BA shows up in the spectra as two negative absorption bands around 1700 cm(-1). The negative signature of these two bands suggests that the BA molecules are oriented in the hydroge n-bonding network with the C-2 carbonyl positioned vertically toward the ai r, and the C-4 and C-6 carbonyls directed into the water subphase. This is consistent with formation of an assembly which optimizes the use of complem entary hydrogen bonding between two components. Furthermore, the effect of competitive polar organic solvents in subphase, such as DMSO, on the hydrog en-bonding network has also been observed in this study. Compared to the pr evious IRRAS studies on the similar monolayers, the sensitivity of PM-IRRAS is obviously improved. PM-IRRAS will likely become a powerful analytical t echnique for the characterization of molecular structure and orientation of Langmuir monolayers at the air-water interface.