RELATIVE ROLE OF SURFACE INTERACTIONS AND TOPOLOGICAL EFFECTS IN NUCLEAR-MAGNETIC-RESONANCE OF CONFINED LIQUIDS

Citation
Jp. Korb et al., RELATIVE ROLE OF SURFACE INTERACTIONS AND TOPOLOGICAL EFFECTS IN NUCLEAR-MAGNETIC-RESONANCE OF CONFINED LIQUIDS, The Journal of chemical physics, 101(8), 1994, pp. 7074-7081
Citations number
23
Categorie Soggetti
Physics, Atomic, Molecular & Chemical
ISSN journal
00219606
Volume
101
Issue
8
Year of publication
1994
Pages
7074 - 7081
Database
ISI
SICI code
0021-9606(1994)101:8<7074:RROSIA>2.0.ZU;2-Z
Abstract
The relative roles of surface and topological effects on the nuclear r elaxation rates T-1(-1), T-2(-1), and T-1 rho(-1) of polar or nonpolar liquids in porous sol-gel silica glasses are identified via their ver y different pore size and frequency dependences. On the basis of theor y, experimental relaxation rates, and molecular dynamics simulations f or the modeled porous systems, the 1/T-i's are interpreted in terms of a linear combination of bulk, confinement, and surface effects: 1/T-i = 1/T-ibulk + a(i)/R(2) + b(i)/R, where R is the average pore size an d a(i) and b(i) are given in terms of the usual relaxation parameters of the studied molecular species. This simple expression which allows the determination of the relative roles of surface and topological eff ects has been used to fit the observed H-1 NMR relaxation rates as a f unction of pore size and frequency for methylcyclohexane, nitrobenzene , pyridine, and toluene both for nonmodified and surface modified poro us silica glasses. Using this method, the surface (alpha 1/R) and pure geometrical (alpha 1/R(2)) relaxation contributions are evaluated and the surface and translational correlation times are calculated. More generally, the experimental data allows us to explain the following se emingly paradoxical results obtained for confined liquids: (i) The pur e confinement effect is independent of the polarities of the liquid mo lecules in pores and is very sensitive to the frequency. (ii) The find ing of the frequency variation of T-1(-1) and T-1 rho(-1) both for pol ar or nonpolar liquids confined to small pores, shows that the geometr ical confinement effects dominate over the surface interaction effects at low frequency and for small pores.