Two-dimensional vibrational sum frequency generation (2D-VSFG) spectroscopic calculations and molecular dynamics simulations are employed to investigate the dynamics of interfacial water at the liquid–vapor interface of aqueous urea, trimethylamine N-oxide (TMAO), and ternary aqueous TMAO–urea mixtures. The 2D-VSFG spectra in the hydrogen-bonded OH stretch region reveal a slowing down of spectral diffusion from binary water–urea to binary water–TMAO and to ternary water–TMAO–urea mixtures. Calculations of the slope of the nodal line and frequency–time correlation functions show that the fast sub-picosecond dynamics is weakly dependent on the presence of the osmolytes, whereas the long-time spectral diffusion time scale increases significantly in the presence of TMAO, revealing the existence of a more rigid and heterogeneous interfacial hydrogen-bond network. Analyses of dangling OH dynamics through diagonal peak decay, cross-peak growth, and non-hydrogen-bond correlation functions demonstrate reduced dangling OH populations and hindered hydrogen-bonded to dangling interconversion at increasingly crowded interfaces in the presence of osmolytes. The orientational relaxation and hydrogen-bond lifetime calculations further reveal slower rotational dynamics and longer-lived hydrogen bonds in the interfacial systems containing TMAO. Together, these results establish a unified molecular level picture of osmolyte induced modulation of aqueous interfacial dynamics.
Negi et al. (Mon,) studied this question.