314 Research Article Received: 26 November 2010 Revised: 26 January 2011 Accepted: 1 February 2011 Published online in Wiley Online Library: 31 March 2011 (wileyonlinelibrary.com) DOI 10.1002/mrc.2749 Saturation-dependent nuclear magnetic resonance relaxation of fluids confined inside porous media with micrometer-sized pores Marius Simina, Ruben Nechifor and Ioan Ardelean ∗ In the present study, we investigate the relationship between the relaxation rate and the filling factor in partially saturated porous media. The filling fluids are polar (water, acetone) and nonpolar (cyclohexane, hexane). The porous sample is a silica glass (Vitrapor#5) with the nominal mean pore size of d = 1 μm(±0.6 μm). All nuclear magnetic resonance relaxation experiments are performed at 20 ◦ C using a NMR instrument operable at 20 MHz proton resonance frequency. The experimental results are compared with a two-phase exchange model providing us information on the strength of surface relaxation and fluid distribution inside pores. These results will affect the NMR estimations about fluid content of porous media. Copyright c  2011 John Wiley & Sons, Ltd. Keywords: NMR; 1 H; surface relaxation; molecular exchange; porous media; partial saturated Introduction Surface and interface science is a vital component for many of our most important and complex technological and biological processes. Improvements in systems involving interfacial and colloidal phenomena, and the development of new applications (separation, catalysis, oil recovery) require information both about molecules location and dynamics inside different confining structures. [1] Owing to its completely non-destructive character, NMR is widely used to investigate both the structure and the dynamics of molecules in different states of aggregation. If the structure of molecules is usually revealed in the frame of the so-called high field NMR spectroscopy with the magnetic field generated by expensive superconducting magnets, the dynamics of molecules can be investigated with much more inexpensive equipments operating at low magnetic fields which can be even mobile. [2] NMR measurements of diffusion coefficients [3] and relaxation times [4] render quantitative data on the dynamics of confined molecules and the restrictions the confinement imposes on their translational and rotational mobility. NMR relaxation experiments are suitable in studying liquids in porous media mainly due to an observed enhanced relaxation rate under confinement conditions. [5,6] This behavior is generated by interactions of the probed molecule with the surface depending on porous sample composition, morphology, the nature of the filling fluid and its distribution inside pores. [5,6] Consequently, relaxation time measurements may provide information about molecules location inside porous media and were comprehensively discussed in the NMR logging applications. [7] In previous studies, the relaxation of liquids partially saturating porous media was examined as a function of the filling factor. [8 – 12] The experimental results were compared with a two-phase exchange model where molecules are exchanging their position between a bulk-like region and a surface region. The main assumption of the previous investigations was that the whole surface region of the porous medium participates in the exchange process with the bulk-like region and thus influences the measured relaxation time. Even if this assumption may be correct in the case of water-filled porous media or under special surface conditions it cannot be considered as a general statement. The present work will show that a better assumption is to consider the internal surface only partially participating in the exchange process. Thus, we will try to find a relationship between the relaxation time distribution, filling factor, the polar character of the confined molecules and the liquid morphology under partially saturated conditions. The results will affect the NMR estimations of fluid content inside porous media. The Effective Relaxation in Partially Saturated Pores In the case of porous media saturated with liquids, it is often considered that the observed relaxation rate is a weighted average between the relaxation rate of the bulk-like liquid and the surface relaxation rate of molecules confined inside a tiny layer of thickness λ that uniformly covers the internal surface S of porous media. [5 – 12] The thickness λ of the tiny layer is considered to be of the order of one intermolecular distance. [5,6] This representation is based on the assumption that the confined molecules experience short range interactions with the surface and a fast exchange process between the molecules in the surface layer and the remaining liquid in saturated pores (bulk-like region) is present. Consequently, the induced nuclear magnetization relaxes mono-exponentially in time, with a transverse relaxation rate given as the weighted ∗ Correspondence to: Ioan Ardelean, Physics Department, Technical University of Cluj-Napoca, 400114 Cluj-Napoca, Romania. E-mail: ioan.ardelean@phys.utcluj.ro Physics Department, Technical University of Cluj-Napoca, 400114 Cluj-Napoca, Romania Magn. Reson. Chem. 2011, 49, 314–319 Copyright c  2011 John Wiley & Sons, Ltd.