Polarization-Modulated Infrared Reflection Absorption Spectroscopic Studies of a Hydrogen-Bonding Network at the Air−Water Interface

Abstract

The hydrogen-bonding network formed between a triaminotriazine amphiphile (2C 18 TAZ, 1) and complementary barbituric acid (BA, 2) at the air-water interface is investigated by polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The molecular structure and orientation of the 1:1 hydrogenbonding network at the air-water interface is revealed in this study. Without the addition of BA to the subphase, the NH 2 scissoring of 2C 18 TAZ appeared in the spectrum as a broad negative absorption band between 1660 and 1605 cm -1 , indicating its perpendicular orientation to the air-water interface. When BA was added to the subphase, the NH 2 scissoring absorption band from the triaminotriazine moiety disappeared due to the complementary hydrogen bonding of BA to the 2C 18 TAZ monolayer. The formation of the rigid 1:1 hydrogen-bonding network also resulted in the disappearance 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 clearly seen. Particularly, the CdO 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 hydrogen-bonding network with the C-2 carbonyl positioned vertically toward the air, 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 complementary hydrogen bonding between two components. Furthermore, the effect of competitive polar organic solvents in subphase, such as DMSO, on the hydrogen-bonding network has also been observed in this study. Compared to the previous IRRAS studies on the similar monolayers, the sensitivity of PM-IRRAS is obviously improved. PM-IRRAS will likely become a powerful analytical technique for the characterization of molecular structure and orientation of Langmuir monolayers at the air-water interface.

Surface FTIR Techniques to Analyze the Conformation of Proteins/ Peptides in H2O Environment

“…During the measurement of IRRAS of a Langmuir monolayer at the air-water interface, a vacuum cannot be applied and pure N 2 or dry air purging will not remove the water vapor from the continuous H 2 O evaporation of the water subphase. Although methods of delicate humidity control have been reported recently [38,39], a method able to automatically remove the water vapor IR absorption was welcomed and PM-IRRAS was developed as a consequence [34,45,46]. PM-IRRAS is similar to IRRAS and the difference between them stems from the polarization, which is switched between s-and p-polarization at a set frequency in PM-IRRAS.…”

“…Thus, any isotropic vibration (i.e., the vibration with randomly distributed orientation) will disappear. The vibrations of water vapor disappear in the PM-IRRAS results due to its isotropic orientation in the air [34,45,46]. When the incident angle is above the Brewster angle, the selection rule of PM-IRRAS is very similar to the p-polarized IRRAS: the peak will be positive if the vibration is parallel to the air-water interface.…”

Supramolecular self-assembly study of a flexible perylenetetracarboxylic diimide dimer in Langmuir and Langmuir–Blodgett films

“…The linkage, melamine ring, used to connect two monomeric PDIs units, is well known for its hydrogen bonding with barbituric acid and forming face-to-face conformation for the connected two chromophores [43][44][45][46] and therefore provides possibility for ordering the molecules in solid film through hydrogen bonding [33,34,47,48]. The detailed synthetic procedures together with the structure characterization are described in supplementary material.…”

“…This supramolecular system was also studied at the air-water interface. In contrast to the traditional belief that hydrogen bonding is not so efficient in aqueous solution, studies by a few research groups demonstrated that a highly ordered and rigid linear network was formed between amphiphilic barbituric acid and melamine with its complementary components in Langmuir and Langmuir-Blodgett films [31][32][33][34]. Moreover, it was reported by Huo et al that such a linear hydrogen bonding network could have a dramatic effect on the photopolymerization of a diacetylene monolayer film and the properties of the resulting polydiacetylene film [35,36].…”

Anisotropic aggregation and phase transition in Langmuir monolayers of methyl/ethyl esters of 2,3-dihydroxy fatty acids

“…in an M-DHO molecule, it is easy to understand that M-DHO molecules can form various types of molecular aggregates via hydrogenbonding at the air-water interface (Fig. 10) [14][15][16][44][45][46][47][48]. Formation of intermolecular hydrogen bonds could lead to the anisotropic growth of LC domains.…”