The structure of water-DMF binary mixtures probed by linear and nonlinear vibrational spectroscopy

Attenuated total reflectance Fourier transform infrared spectroscopy and sum frequency generation (SFG) vibrational spectroscopy have been employed to probe the molecular structure of N,N-dimethylformamide (DMF) and water mixture by varying the concentration of DMF. From the bulk studies, we observed a gradual decrease in the intensity with a continuous blue shift in the OH-stretch region with the increase in the DMF concentration. In contrast, no significant blue shift in the OH-stretch region is noticed from the SFG spectra collected from the air-aqueous binary mixture interface as a function of DMF concentration. However, the impact of DMF is found to be disruptive in nature toward the existing hydrogen bonding network of the pristine water at the interfacial region. Interestingly, in the CH-stretch region, the vibrational signatures of the DMF molecule show blue shifts, as proposed in earlier studies. We have calculated the molecular tilt angle of the methyl group of the DMF molecule as a function of DMF concentration. For the case of neat DMF, the observed tilt angle is ∼17.7° with respect to the surface normal. The value of tilt angle decreases with the decrease in DMF concentration and reaches a value of ∼1.7° for a mole fraction of 0.5, and it further increases with the decrease in DMF concentration. It achieves a value of ∼20° for the dilute DMF mole fraction of 0.05 in the binary mixture. This indicates that DMF molecules at the air-binary mixture interface are placing their methyl groups more toward the normal for the intermediate DMF concentrations.