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

The orientational dynamics and microscopic structures of nicotine/water binary mixtures near the system's lower critical solution temperature (LCST) were elucidated using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy, nuclear magnetic resonance correlation spectroscopy (NMR COSY), first-principles calculations, and molecular dynamics simulations. Water concentrations were investigated from zero to close to pure water. At temperatures below the LCST, OHD-OKE experiments measured an anomalous slowing as the phase transition concentration was approached. At moderate concentrations and low temperatures, intermolecular cross-peaks between nicotine and water molecules were observed in the COSY spectra, demonstrating the formation of structures that persist for milliseconds. These results suggest that pair correlations contribute to the slowdown in the OHD-OKE data at moderate water concentrations. First-principles calculations revealed that intermolecular hydrogen bonding coordination between nitrogen atoms in pyridine moieties and water lowers the energy barriers for the reorientations of the two nicotine rings. Atomistic simulations demonstrate that with increasing water concentration, hydrogen bonding interactions between pyridine moieties and water molecules first increase and then decrease with a maximum at moderate water concentrations. These experimental and computational characterizations of the dynamics of nicotine molecules are attributed to the distinct configurations of water molecules around the pyridine ring moieties in nicotine molecules.