Raman scattering study of liquid ethylene glycol confined to nanoporous silica glasses

The Raman spectra of liquid ethylene glycol (EG) and EG confined to series of nanoporous silica glasses with pores ranging from 30.7 to 68.5 Å were recorded at 303 K. Using vibrational assignments of specific Raman bands to the trans and gauche conformers of EG allowed us to follow the effect of confinement on the equilibrium of these conformers. With decreasing pore size an increase in the trans conformer fraction was observed. This increase in the trans conformer fraction can be related to the hydrogen bonding of EG molecules to pore surface silanol groups of the silica glasses. In surface modified glasses where the silanol groups were replaced by more hydrophobic—O—Si—(CH 3 ) 3 groups the trans / gauche ratio does not change with increasing confinement, thus providing evidence for the dominant role of the hydrogen bonding ability of silanols in non‐modified glasses to change the conformer equilibrium. The plot of the fraction of the trans conformer F t as a function of the reciprocal of the pore radius 1/ R showed good linearity. This indicates that the two‐state exchange model can be applied to the analysis of the Raman data allowing the determination of the fraction of the trans conformer in the surface layer liquid. Assuming a surface layer thickness of one molecular diameter of EG one determines F t ≈0.97 in the surface layer liquid.