Dimethylpiperazine on Air-Water Interface: Vibrational Sum frequency Generation study of a cyclic tertiary amine

Abstract

Dimethyl piperazine (DMP) is a weakly basic, cyclic, tertiary diamine. Being a saturated 6-membered ring, it may attain a number of energetically close conformational orientations in the ground state. In a particular environment, its conformational distribution depends on the stabilising factors present in the surroundings. Though soluble in water, it is expected to have substantial surface propensity due to presence of multiple –CH2 and –CH3 bonds. Detection of Bohlmann bands in the C–H stretch region of the Vibrational Sum Frequency Generation (VSFG) spectrum revealed that comparatively higher energy chair conformer with one axial and one equatorial –CH3 bond is present on the pure liquid DMP-air interface. At the air-aqueous solution interface, those bands were found to disappear as the lone electron pair on N-atom is engaged in hydrogen bonding with water molecule. Due to this hydrogen bonding, the peak of “dangling free OH” bonds disappeared from VSFG spectrum of water molecules at air–water interface. Alongwith this hydrogen bonding, the hydrophobic nature of the –CH3 bonds of DMP molecules cause disorganisation of native hydrogen bonded network of water molecules in the air–water interface. Structure and orientation of DMP is expected to be pH-responsive since the chemical conditions of the surrounding medium directly affect the extent of protonation of the amine group. Upon addition of HCl to the bulk, protonated DMP ions, too, could be detected on the air-solution interface by their VSFG spectrum which were considerably different from unprotonated DMP. As the bulk pH of solution changed towards more acidic, spectral features of VSFG spectra of DMP on the surface showed significant change, implying variation in the surface pH, as structure and orientation changes at different degrees of protonation. Since the VSFG spectral features of DMP was found to be pH-responsive, it can be utilised to ascertain the pH of any surface that has DMP absorbed on it. pH-sensitivity of the DMP molecules on air–water interface also suggest that orientation of headgroups will have some effect on the surface orientation of cationic surfactants with amine based headgroups.