When liquid molecules (such as water, alcohols, or organic solvents) are introduced into aerogel nanopores measuring only a few nanometers across, their local thermodynamic states, intermolecular interactions, and dynamics undergo changes known as nanoscale confinement effects. Within these constrained spaces, proximity to solid pore walls exposes molecules to electrostatic potentials and steric restrictions imposed by surface functional groups, altering bond lengths, bond angles, and hydrogen-bonding networks. Investigating alterations in bonding configurations and associated infrared vibrational spectrum frequency shifts clarifies the physics of confined matter.

Taking water molecules confined within hydrophilic silica aerogel nanopores as an example, infrared and Raman spectroscopic analyses reveal frequency shifts in characteristic vibrational bands (such as hydroxyl stretching modes). The physical origin of this spectral shift lies in hydrogen bonding interactions formed between residual surface silanol groups and confined water molecules. These interfacial hydrogen bonds differ in bond strength from the average hydrogen-bonding network of bulk liquid water.

When localized surface electric fields combine with spatial geometric compression, internal chemical bonds experience asymmetric force fields that alter effective force constants. Specifically, if confinement weakens intermolecular hydrogen bonding, vibrational frequencies shift toward higher wavenumbers (blue shift); conversely, if confinement strengthens hydrogen bonding or induces polarization restructuring of electron clouds, vibrational frequencies shift toward lower wavenumbers (red shift).

Furthermore, because nanoscale confinement dimensions match liquid molecular cluster scales, confined liquids cannot form extended long-range hydrogen-bonding networks, adopting low-coordination local cluster configurations instead. This structural distortion alters molecular dipole moment derivatives, affecting infrared absorption cross-sections. Analyzing vibrational frequency shifts within nanoscale confinement provides spectroscopic insight into fluid structures inside aerogels.