Dynamics and Microstructures of Nicotine/Water Binary Mixtures near the Lower Critical Solution Temperature

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Physics

Background:

• Understanding liquid mixtures, especially near phase transitions, is crucial for chemical engineering and materials science.
• Nicotine/water mixtures exhibit complex behavior near their lower critical solution temperature (LCST).

Purpose of the Study:

• To elucidate the orientational dynamics and microscopic structures of nicotine/water binary mixtures near the LCST.
• To investigate the influence of water concentration on these dynamics and structures.

Main Methods:

• Optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy
• Nuclear magnetic resonance correlation spectroscopy (NMR COSY)
• First-principles calculations
• Molecular dynamics simulations

Main Results:

• Anomalous slowing of orientational dynamics was observed below the LCST as the phase transition concentration was approached.
• Intermolecular cross-peaks in NMR COSY spectra indicated persistent structures (milliseconds) at moderate concentrations and low temperatures.
• First-principles calculations showed that hydrogen bonding between nicotine's pyridine nitrogen and water lowers reorientation energy barriers.
• Molecular dynamics simulations revealed a non-monotonic trend in hydrogen bonding interactions with increasing water concentration, peaking at moderate levels.

Conclusions:

• Pair correlations contribute to the observed slowdown in OHD-OKE data.
• The distinct configurations of water molecules around nicotine's pyridine ring moieties drive the observed dynamics.
• Hydrogen bonding plays a critical role in the structural and dynamic behavior of nicotine/water mixtures near the LCST.