Particle size effect on transverse NMR relaxation in aqueous char suspensions

doi:10.1016/S0927-7757(99)00002-3

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 157, Issues 1–3, 30 October 1999, Pages 177-183

https://doi.org/10.1016/S0927-7757(99)00002-3 Get rights and content

Abstract

A strong particle size influence on NMR transverse proton relaxation in aqueous suspensions of several newly synthesized carbon based chars is reported. The results are quantitatively interpreted in terms of a two-stage molecular exchange model. A porous cage effect leads to slow exchange between molecules inside and outside pores, in contrast to fast molecular exchange at the solid–liquid interface, where a familiar two-site formalism can be applied. The dependence of surface properties on the relaxation data was detected. The transverse NMR relaxation time of water protons on the paramagnetic surface of chars is estimated to be T_2s≈ 0.5×10⁻⁵ s, which is comparable with the EPR relaxation time of the free radical centers. Dynamic nuclear polarization (DNP) is used to estimate the paramagnetic shift Δv of water protons at the surface. The value Δv so obtained is much less than the transverse NMR relaxation rate of water protons on the paramagnetic surface $Δ v ⪡ T_{2s}^{−1}$ . This inequality can account for the independence of the NMR transverse relaxation times on the interval τ observed in CPMG measurements.

Introduction

An understanding of intermolecular interactions and molecular motion at the solid–liquid interface is of central importance to research concerning heterogeneous catalysis, fluid penetration of engineering plastics and ceramics and biological perfusion. Studies of molecular diffusion and relaxation processes in paramagnetic porous materials provide valuable information about surface relaxivity and spin dynamics at the solid–liquid interface [1], [2].

Over the past two decades, many different magnetic resonance methods have been used in studies of paramagnetic centers in carbonaceous solids as well as in studies of the surface diffusion of solvents adsorbed on charcoals [2], [3], [4], [5], [6], [7], [8]. EPR spectroscopy has shown that unpaired electrons responsible for the paramagnetism of chars exist in stable organic free radicals created during the heat-treatment carbonization [2], [3]. Surface diffusion of solvents adsorbed on charcoals already has been the subject of NMR relaxation studies [4], [5], [6]. An NMR chemical shift technique also has been applied to study the molecules adsorbed on charcoal and silica gel [7], [8]. Self-diffusion coefficients in porous media, measured by different kinds of pulsed-field-gradient NMR, have been the object of intensive theoretical and experimental studies [9], [10], [11], [12], [13]. In spite of significant progress in these studies, the electronic structure of char paramagnetic centers and the process of free radical formation during carbonization, as well as the state of water at the paramagnetic surface, are not yet fully understood. So far, no theoretical calculation related to the electronic structure has been carried out for carbon based chars.

Many reports have shown that the water proton relaxation rate in porous media depends on structural properties, such as porosity, grain and pore size, geometry and distribution and specific surface interactions, as well as on flow properties, such as permeability, tortuosity and dispersivity [14], [15], [16], [17], [18]. Using simple model systems, such as glass beads in water, it was shown that the two-site exchange model can be successfully used to account for relaxation data of solvent protons [15], [17]. However, the origin of enhanced relaxation in real porous structures, as well as the state of water on more complex surfaces, remains a matter of debate. The analysis of relaxation data is especially complicated for aqueous suspensions of paramagnetic porous particles because water protons are influenced by fast nuclear-electron cross-relaxation on the surface due to hyperfine interactions with the paramagnetic centers in the porous media. Corresponding NMR chemical shifts for molecules adsorbed on the paramagnetic surface usually are obscured by broad resonance linewidths. The purpose of this work is to furnish an understanding of molecular diffusion and spin dynamics at the solid–liquid interface in several newly synthesized carbon based chars suspended in water.

Section snippets

Experimental

The NMR relaxation measurements were performed at a Larmor ¹H frequency of 6 MHz at room temperature with a relaxometer NMR-07PC, which is a modification of a model previously described [19]. The Carr–Purcell–Meiboom–Gill (CPMG) technique was employed to measure water proton transverse relaxation times with a 180° pulse period, τ, from 10 ms down to 0.2 ms. Special software has been created to account for multi-exponential relaxation, in which the relaxation (echo-decay) curve was described as

Results and discussion

Chars in this study have a well-developed porous structure, leading to multi-exponential NMR magnetization decays and several relaxation times. Spin-echo decay analysis shows the presence of two basic components (Fig. 1) with different relaxation times which represent more than 90% of all water molecules in the sample. One of them represents 2–9% of water molecules, has comparatively short relaxation times T_p≈3–10 ms, and demonstrates a weak char particle size dependence. This component (Fig. 1

Conclusions

1.
The dependence of the transverse relaxation of water protons on particle size in aqueous suspensions of several newly synthesized carbon based chars can be interpreted quantitatively in terms of a two-stage molecular exchange model. A porous cage effect leads to slow exchange between molecules inside and outside pores in contrast to fast molecular exchange processes at the solid–liquid interface, where the usual two-site formalism can be applied. Owing to increasing surface-to-volume ratio, the

Acknowledgements

The research was supported in part by a grant from NATO (HTECH LG 972264) from Fogarty International Foundation (1RO3 TW00998-01) from NIH (RBC; GM51630, GM42208) as well as from the US Department of Energy (DE FG22-96 PC 96205) and used facilities of the Illinois EPR Research Centre (NIH P41-RR01811).

References (26)

M.D. Hurlimann et al.
J. Magn. Reson. A
(1994)
P.P. Mitra et al.
J. Magn. Reson.
(1995)
R.L. Kleinberg et al.
J. Magn. Reson.
(1990)
S. Boyer et al.
Colloids Surf. A
(1994)
B.M. Odintsov et al.
Surf. Sci.
(1997)
K.S. Rothenberger et al.
H.L. Retcofsky et al.
F.P. Auteri et al.
Appl. Magn. Res.
(1994)
H.A. Resing
Adv. Mol. Relaxation Processes
(1967)
J.K. Thompson et al.
J. Phys. Chem.
(1965)

H.A. Resing et al.

J. Phys. Chem.

(1964)

S. Kaplan et al.

J. Chem. Phys.

(1973)

E.G. Derouane et al.

Catal. Rev.

(1972)

Cited by (5)

Nuclear magnetic resonance cryoporometry
2008, Physics Reports
Citation Excerpt :
Due to the regularisation in the Laplace inversion, the NMR relaxometry peaks were forced to be Gaussian in shape. Coal is a porous form of carbon that has been examined by NMR cryoporometry (Norinaga et al., 1999) and NMR relaxometry analysis (Hayashi et al., 2001; Odintsov et al., 1999). Water absorbed in Yallourn, Beulah Zap, and Illinois #6 coals was used to study the effect of drying on the porosity (Norinaga et al., 1999).
Nuclear Magnetic Resonance (NMR) cryoporometry is a technique for non-destructively determining pore size distributions in porous media through the observation of the depressed melting point of a confined liquid. It is suitable for measuring pore diameters in the range 2 nm–1 μm, depending on the absorbate. Whilst NMR cryoporometry is a perturbative measurement, the results are independent of spin interactions at the pore surface and so can offer direct measurements of pore volume as a function of pore diameter. Pore size distributions obtained with NMR cryoporometry have been shown to compare favourably with those from other methods such as gas adsorption, DSC thermoporosimetry, and SANS. The applications of NMR cryoporometry include studies of silica gels, bones, cements, rocks and many other porous materials. It is also possible to adapt the basic experiment to provide structural resolution in spatially-dependent pore size distributions, or behavioural information about the confined liquid.
Peculiarities in the water adsorption and morphology of carbon chars assessed by NMR
2001, Journal of Colloid and Interface Science
Proton spin–lattice relaxation and ¹H NMR spectroscopic data are used for studies of the water adsorption and morphology of several newly synthesized carbon-based chars. Two NMR lines have been observed in accordance with a difference between macropore and micropore water in a system with heterogeneous pore structure. The macropores occupy about the half of the char particle volume. In aqueous suspension fast molecular exchange between water in macropores and bulk water leads to an intense NMR line with a single spin–lattice relaxation time T₁, which exhibits a strong particle size effect. This effect has been used for the estimation of T₁ of water molecules in the vicinity of paramagnetic centers at the surface of chars, as well as for macropore size estimation. The kinetics of water vapor adsorption in chars as seen by ¹H NMR indicates that this process begins in micropores, which serve as primary adsorption centers. Water molecules in micropores exhibit specific properties which are characteristic of liquids in a small, restricted space, and they give rise to the observed accumulation as well as saturation effects.
Synthesis and comparative study on dispersion stability of binary suspensions of mixed and composite nanoparticles
2017, Nanoscience and Nanotechnology Letters
Sedimentation and resuspendability evaluation of pharmaceutical suspensions by low-field one dimensional pulsed field gradient NMR profilometry
2013, Pharmaceutical Development and Technology
Oil disperse systems study using nuclear magnetic resonance relaxometry (NMRR)
2013, Advances in Energy Research

View full text

Particle size effect on transverse NMR relaxation in aqueous char suspensions

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

J. Magn. Reson. A

J. Magn. Reson.

J. Magn. Reson.

Colloids Surf. A

Surf. Sci.

Appl. Magn. Res.

Adv. Mol. Relaxation Processes

J. Phys. Chem.

J. Phys. Chem.

J. Chem. Phys.

Catal. Rev.