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
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.