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

Abstract

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.

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“…The fittings of all the SFG spectra are done using Eq. 1, where the intensity of an observed SFG signal is proportional to the second-order nonlinear susceptibility (χ(2)) and is given by 37,44,45 …”

“…Overcoming the limitations of the conventional techniques, sum frequency generation (SFG) vibrational spectroscopy has been accepted as a powerful technique to investigate the interfacial structure of molecules with excellent molecular and surface specificity at various interfaces. 17,19,32−37 This is a label-free technique that is primarily based on the second-order nonlinear optical process. It uses two pump laser beams in a collinear geometry, i.e., visible (ω Vis ) and tunable IR (ω IR ), which are overlapped in space and time at the interface to produce SFG signal (ω SFG = ω Vis + ω IR ) in the phase matching direction.…”

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“…SFG is a second-order nonlinear optical spectroscopic technique used to examine various interfaces to provide the molecular level insight about various interactions and the interfacial structure of the molecules residing at the interfacial region. − The SFG signal (ω SFG ) is generated after fulfilling the spatial and temporal overlapping of two input beams visible (ω VIS ) and the tunable infrared (IR) (ω IR ) at the air–aqueous interface. A schematic representation of SFG spectroscopy is given in Figure .…”

“…The interfacial molecular behavior is quite distinct due to the interfacial asymmetric environment as compared to the centrosymmetric molecular environment of the bulk phase. , The intensity of the SFG signal increases when the frequency of the input tunable IR radiation corresponds to the natural vibrational frequency of the interfacial molecule . The observed intensity of the SFG beam is proportional to the squared modulus of second-order nonlinear susceptibility (χ (2) ) and the intensity of both visible ( I VIS ) and IR ( I IR ) input pump beams, and it is given as eq . ,,, …”

“…The nonlinear susceptibility (χ (2) ) is given as where A q , ω IR , ω q , and Γ q represent the amplitude of the q th mode of vibration, the frequency of the tunable IR beam, the frequency of the normal modes of vibration of the molecule, and the line width of the transition, respectively. , …”