Vibrational Spectra in Nanoconfined Liquids
Vibrational, e.g., infrared or Raman, spectroscopies are frequently used to characterize nanoconfined liquids. The aim is to used the changes in the spectra upon confinement to shed light on the environment of the liquid molecules. This raises two important questions that we are currently addressing:
- How does the vibrational spectrum of a nanoconfined liquid differ from that of the bulk liquid?
- What do these changes reveal about the liquid structure and dynamics?
We are studying this problem in both model confining frameworks and atomistic hydrophilic and hydrophobic silica pores. Molecular dynamics simulations are particularly useful in this work as they permit the direct correlation between frequency shifts and molecular structure and dynamics. Thus, we can develop an accurate, detailed picture of the molecular-level liquid properties and see if and how they are manifested in the vibrational spectra.
An example of our work in this area is CH3CN confined in nanoscale (~2.4 nm diameter) hydrophilic (-OH terminated) silica pores. A comparison of our simulated infrared spectra, which are generally consistent with experimental measurements, for the bulk and confined acetonitrile are shown below:
A new blue-shifted peak appears upon confinement which can be attributed to CH3CN molecules hydrogen-bonded to the surface OH groups. Otherwise the changes to the spectra are relatively modest. This is surprising given the small pore size. Indeed, examination of the confined acetonitrile structure and dynamics indicate dramatic changes: molecular layering, orientational ordering, slowed, multiple time-scale spectral diffusion and reorientation, and dynamical heterogeneity. Yet, the infrared spectrum is relatively insensitive to all of these effects. (See J. Phys. Chem. A 113, 1922 (2009) for additional details.)
Our current work is investigating new avenues for obtaining even more detailed molecular-level mechanistic insight into the vibrational frequencies and exploring what nonlinear vibrational spectroscopies can reveal about the structure and dynamics of nanoconfined liquids.
Christine M. Morales and Ward H. Thompson,
Journal of Physical Chemistry A 113, 1922-1933 (2009). Abstract
"Simulations of Infrared Spectra of Nanoconfined Liquids: Acetonitrile Confined in Nanoscale, Hydrophilic Silica Pores"
Shenmin Li, Tricia D. Shepherd, and Ward H. Thompson,
Journal of Physical Chemistry A 108, 7347-7355 (2004). Abstract
"Simulations of the Vibrational Relaxation of a Model Diatomic Molecule in a Nano-confined Polar Solvent"