Spectroscopic and Computational Studies of Aqueous Ethylene Glycol Solution Surfaces

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Abstract
The combination of Monte Carlo, ab initio, and DFT computational studies of ethylene glycol (EG) and EG−water hydrogen-bonding complexes indicate that experimental vibrational spectra of EG and EG−water solution surfaces have contributions from numerous conformations of both EG and EG−water. The computed spectra, derived from harmonic vibrational frequency calculations and a theoretical Boltzmann distribution, show similarity to the experimental surface vibrational spectra of EG taken by broad-bandwidth sum frequency generation (SFG) spectroscopy. This similarity suggests that, at the EG and aqueous EG surfaces, there are numerous coexisting conformations of stable EG and EG−water complexes. A blue shift of the CH2 symmetric stretch peak in the SFG spectra was observed with an increase in the water concentration. This change indicates that EG behaves as a hydrogen-bond acceptor when solvated by additional water molecules. This also suggests that, in aqueous solutions of EG, EG−EG aggregates are unlikely to exist. The experimental blue shift is consistent with the results from the computational studies.