¹³C NMR Relaxation Studies of Pyridine and Pentafluoropyridine Liquids Confined to Nanopores of Porous Silica Glasses

Abstract

A dynamic nuclear magnetic resonance (NMR) study of the polar liquids pyridine and pentafluoropyridine (PFP) confined to porous silica sol−gel glasses is reported. The 13C NMR spin−lattice relaxation times, T 1, of ortho (C2), meta (C3), and para (C4) carbons were measured in glasses with pore radii ranging from 30 to 94 Å, over a temperature range of −22 to +20 °C. The experimental data were analyzed in terms of the two-state, fast exchange model, and the surface-layer relaxation times, T 1s, were calculated. On the basis of surface enhancement factors, T 1b/T 1s, where T 1b is the relaxation time of the bulk liquid, we conclude that fluorination considerably weakens the interaction of pyridine with the glass surface. This is consistent with pyridine serving as a hydrogen bond acceptor for the surface. By analyzing the relaxation differences in C2, C3, and C4, we find that the reorientation of confined pyridine is highly anisotropic, due to directional hydrogen bonding between the N atoms of pyridine and the OH groups on the glass surface. In contrast, we see no confinement effect on the rotational anisotropy of PFP.

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

“…In analogy with expressions used in NMR 3,4 and Raman experiments, 5 -8 one can write F t,obs D F t,bulk C 2ε R F t,s F t,bulk 1…”

“…The sol-gel porous glass also represents an excellent material to study the surface effects at the molecular level as the surface area to volume ratio can be varied over a wide range. Since one can prepare porous silica glasses with well defined pore sites in the range 20-150Å, this allows one to investigate the properties of the liquids confined to a series of glasses with different pore sizes and, provided that the two-state model 3,4 is applicable, one can determine the dynamics or structural characteristics of the surface layer liquid. This is a unique aspect of these experiments as only by finding out how some property of the confined liquid scales with the pore radius R can one obtain information about the surface layer liquid.…”

Structure and dynamics of liquid methanol confined within functionalized silica nanopores

“…5 Nanoporous silicate glasses have proven to be the excellent media for studying confinement effects upon the structure and dynamics of trapped liquid phases. A variety of experimental techniques has been recently implemented to analyze these issues including NMR, [6][7][8][9][10] Rayleigh 11,12 and neutron 13,14 scattering, and several spectroscopic methods, such as Raman, [15][16][17] dielectric, [18][19][20][21] and optical Kerr effect. [22][23][24][25][26] New methods in chemical synthesis allow subnanometric control of the pore linear dimensions and, at the same time, an accurate "tuning" of the characteristics of the interactions prevailing between the solid walls and the trapped liquid phases.…”

Effect of Confinement on the Density and Diffusivity of Organic Molecules in Single-Digit Nanopores Relative to Bulk Fluid Behavior

“…Such systems include carbon nanotubes, , polymer nanochannels, , and silicon nanochannels . A variety of experimental techniques have been implemented to analyze confined fluid properties, including nuclear magnetic resonance (NMR), − neutron diffraction, and quasi-elastic neutron scattering. , Of these techniques, NMR has been used extensively to study the diffusion , and relaxation properties of confined molecules; − however, previous data sets focused on a handful of molecules. Further, additional relationships between the diffusivity and other thermodynamic or molecular properties were not presented. , One commonly investigated compound, cyclohexane, serves as a useful benchmark system for comparison of different experimental measurement approaches and the diversity of results that are obtained. − One study using a diaphragm cell found the ratio of diffusion in bulk to the pore ( D bulk / D pore ) of cyclohexane to be 7.2 in a 2.9 nm porous silica, while another study using pulse-field gradient (PFG) NMR determined the value to be 4.64 and 2.4 , for 2 and 5 nm pores, respectively.…”