Duschl Markus, Pohlmeier Andreas, Brox Timothy I, Galvosas Petrik, Vereecken Harry
Agrosphere (IBG-3), Forschungszentrum Jülich GmbH, Jülich, 52425, Germany.
MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, 6012, New Zealand.
Magn Reson Chem. 2016 Dec;54(12):975-984. doi: 10.1002/mrc.4486. Epub 2016 Aug 8.
Magnetic impurities are ubiquitous in natural porous media such as sand and soil. They generate internal magnetic field gradients because of increased magnetic susceptibility differences between solid and liquid phase in the pore space and because of the presence of magnetic centers. These internal gradients accelerate NMR relaxation rates and thus might limit the possibility of pore space characterization using NMR. In this study, we investigate the effects of coating the surface of natural sands by the antiferromagnetic iron oxyhydroxide goethite on NMR relaxation and diffusion properties. We found a non-quadratic dependence of the relaxation time distributions on the echo time indicating that the relaxation experiments were not performed in the fast diffusion limit, while the weak dependence on the external magnetic field strength is explained by the preponderance of the surface relaxation over the effect of diffusion in internal gradients. The surface to volume ratio of the pore space, determined by NMR diffusimetry ((S/V) ) remains approximately constant, whereas the same quantity, determined from gas adsorption ((S/V) ) increases proportional to the coating density. This is because gas adsorption measures surface roughness on sub-nanometer scale, whereas NMR diffusimetry averages over structures smaller than few microns. This has consequences for the calculation of the surface relaxivities. The usage of the (S/V) leads to constant values, whereas the usage of (S/V) leads to apparently decreasing relaxivities with increasing coating, which is unrealistic. Copyright © 2016 John Wiley & Sons, Ltd.
磁性杂质在诸如沙子和土壤等天然多孔介质中普遍存在。由于孔隙空间中固相和液相之间的磁化率差异增加以及磁性中心的存在，它们会产生内部磁场梯度。这些内部梯度会加速核磁共振弛豫速率，因此可能会限制使用核磁共振表征孔隙空间的可能性。在本研究中，我们研究了用反铁磁性羟基氧化铁针铁矿包覆天然砂表面对核磁共振弛豫和扩散特性的影响。我们发现弛豫时间分布对回波时间呈非二次依赖性，这表明弛豫实验并非在快速扩散极限下进行，而对外部磁场强度的弱依赖性则由表面弛豫在内部梯度扩散效应中占主导来解释。通过核磁共振扩散法测定的孔隙空间的表面积与体积之比((S/V))保持大致恒定，而通过气体吸附测定的相同量((S/V))则与包覆密度成比例增加。这是因为气体吸附测量的是亚纳米尺度的表面粗糙度，而核磁共振扩散法是对小于几微米的结构进行平均。这对表面弛豫率的计算有影响。使用(S/V)会得到恒定值，而使用(S/V)会导致弛豫率随着包覆量增加而明显降低，这是不现实的。版权所有© 2016约翰威立父子有限公司。
