Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility ...to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.
Multiphase aqueous-organic systems where a bicontinuous phase is in equilibrium with an excess organic and aqueous phase find various applications in industry. These systemsalso known as Winsor ...IIIare complex not only for the different phases that develop therein but also because they are multicomponent systems. In this work, we explore for the first time the use of a benchtop low-field single-sided NMR to determine the species distribution in Winsor III systems. The proposed methodology provides information at macroscopic and microscopic levels. In particular, we show the use of single-sided NMR to determine the phases’ dimensions and the species distribution in a polymer-based bicontinuous system. The phases’ dimensions and limits can be resolved with micrometric precision and are indicative of the bicontinuous phase stability. The species distribution is determined by means of spatially resolved NMR relaxation and diffusion experiments. It was observed that the salinity of the aqueous phase also impacts the species distribution in the bicontinuous system. Experiments show that the additive and the polymer are mainly located in the bicontinuous phase. As the salinity of the aqueous phase increases, the amount of organic components in the bicontinuous phase decreases as a consequence of the species distribution in the system. This influences the total amount of recovered organic liquid from the organic phase. The information is obtained in a relatively fast experiment and is relevant to the system’s possible applications, such as enhanced oil recovery (EOR). This methodology is not only circumscribed to its application in EOR but can also be applied to the study of any emulsion or microemulsion systems without sample size or geometry constraints.
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•Diffusion-relaxation correlation maps in single-sided NMR.•Reduced experimental time with increased sensitivity.•Saturation recovery-diffusion is faster than diffusion-transverse ...relaxation.
Diffusion-relaxation correlation experiments in nuclear magnetic resonance are a powerful technique for the characterization of fluid dynamics in confined geometries or soft matter, in which relaxation may be either spin-spin (T2) or spin-lattice (T1). The general approach is to acquire a set of bidimensional data in which diffusion is codified by the evolution of the magnetization with either Hahn or stimulated echoes (STE) in the presence of a constant magnetic field gradient. While T2 is codified by a Carr-Purcell-Meiboom-Gil (CPMG) sequence, T1 is either encoded by saturation or inversion-recovery methods. In this work, we analyse the measurement time of diffusion-relaxation times in single-sided NMR and show that T1-D acquisition is always shorter than D-T2. Depending on the hardware characteristics, this time reduction can be up to an order of magnitude. We present analytical calculations and examples in model porous media saturated with water and in a dairy product.
•Two-dimensional relaxation maps at low field NMR.•Kerogen and bitumen detection at low field NMR.•Simultaneous determination of liquid and solid components in unconventional reservoirs.•Acquisition ...of fast decaying signals enables the detection of solid components of rocks.
Quantification of organic matter and fluids contained in oil source rocks from shale plays is a fundamental goal for the petrophysical and geochemical assessment of the production potential of a well. Laboratory 1H nuclear magnetic resonance (NMR) is a fast, reliable, and non-destructive method widely used in the oil industry. Measurement of T1-T2 correlation maps have been found to be critical for identifying the presence of solid organic matter, liquid hydrocarbons, and brine in these formations. However, the inherent long echo times associated with standard laboratory equipment for large sample volume challenge the detection of contribution from kerogen and viscous bitumen. In this work, we use a commercial low field NMR rock-core analyzer where a novel T1- T2*&T2 pulse sequence is implemented to enable the detection of solid organic matter by acquiring the free induction decay after the first excitation pulse. This acquisition is followed by a train of refocusing pulses that generate multiple echoes from which liquid components are detected. A set of outcrop and well samples from the Vaca Muerta Formation in Argentina, with a varying amount of total organic content, were measured. The signal intensity assigned to solid matter was correlated with RockEval 6 pyrolisis. The novel pulse sequence presented here can be implemented in any commercial apparatus without the need for hardware modifications.
The ability to create extended porous networks, such as those composed of metal–organic frameworks (MOFs), with tailored hydrophilic/hydrophobic character is crucial for adapting such widely used ...supports to different applications. To achieve this goal, direct polymer inclusion has proven to be a suitable strategy, and functional composite materials with multiple additional properties have been obtained in such a way. We have explored, by means of nuclear magnetic resonance diffusion experiments, the effect of polymer capping using a conducting polymer (polyaminobencylaminePABA), which results in the positioning of −NH2 moieties on the otherwise eminently hydrophobic surface of Zn-based ZIF-8 MOF nanocrystals. Our results demonstrate that increasingly higher amounts of PABA can confer, aside from conductivity, an increased hydrophilic character to the porous network, while also allowing for the identification of different environments available for water molecule diffusion.
Relaxation in nuclear magnetic resonance (NMR), both transverse and longitudinal, provides information on microscopic features of a wide variety of systems and may be used to monitor dynamic ...processes such as cementation, chemical reactions, gelatinization, and evaporation. Dynamic relaxometry, in combination with spatial resolution, is a useful technique that provides deep insight into complex systems evolution. In this work, we explore the range of applicability of single-sided NMR to determine the evaporation kinetics of fluid from porous media. We show that, due to technical experimental restrictions, the determination of the time-dependent amount of fluid in different voids as a function of the position is in general not feasible with transverse relaxation experiments. However, as opposed to common intuition, longitudinal relaxation experiments provide reliable and fast acquisition, compatible with the requirements needed to monitor a water evaporation process from a model oil-reservoir rock sample.
The diffusion of alkaline chlorides (LiCl, KCl, and CsCl) and water in mesoporous silica samples with pore sizes covering the range from micropores (2 nm) up to mesopores larger than 30 nm have been ...measured by resorting to a simple diffusional technique in the case of electrolytes and 1H NMR in the case of water. The morphology of the silica samples varies from a microporous structure, an interconnected network of pores, and typical mesoporous materials with ink-bottle pores, with increasing pore size. The release of electrolytes from the silica as a function of time exhibits two differentiated regimes, at short and long times, which correlates quite well with the size of the pores and that of necks of the pores, respectively. The diffusion of water inside the pores follows the same trend with pore size that the diffusion of electrolytes, indicating a coupling between the ions and water diffusional mobilities. The tortuosity effect on the diffusion of all studied electrolytes and water shows a monotonic slight increase with decreasing diameter for pores larger than 5 nm, while the tortuosity factor increases markedly for smaller pores. In microporous and mesoporous silica with pore sizes below 10 nm, the tortuosity factor of Li+ ion is much larger than those for K+ and Cs+ ions, since its diffusion is hindered by a stronger electrostatic interaction with the ionizable silanol groups on the pore wall; and also larger than that for water diffusion which it is retarded by a weaker hydrogen bond interaction with the silanol groups. The differences in tortuosity factors among alkaline chlorides and water become negligible for pore sizes larger than 10 nm. The spin–lattice relaxation time measurements of 1H-water and Li+ ions confirm this behavior.
The influence of molecular exchange in porous polymeric systems on experiments that simultaneously correlate and determine pore length scales and surface relaxivities is studied. Pore length scales ...are determined by taking advantage of the internal field gradients generated by the difference in magnetic susceptibility between the polymer matrix and the contained water, namely the Decay Due to the Internal Field experiment. Transverse relaxation is codified in the detection period thus enabling correlation with the pore length. For the hierarchical very well connected porous polymer system, the considerable molecular exchange of water between different environments renders signals in the two-dimensional maps that make the determination of the surface relaxivity a complex problem.
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•Two dimensional pore size/relaxation experiments provide surface relaxivity.•Porous polymer systems present a hierarchical well connected pore size distribution.•Water exchange between different pores appears as cross-peaks in d-T2 maps.
NMR is a fast, nondestructive, and noninvasive technique that can provide information about the pore structure of macroporous polymer beads and the dynamics of liquids confined in them. In this work, ...we describe the study of the pore structure of the macroporous polymer of ethylene glycol dimethacrylate and 2-hydroxyethyl methacrylate poly(EGDMA-co-HEMA) in the dry but also in the swollen state by measuring relaxation times of liquids contained in the polymer network. The results show that the pore architecture differs from the dry to the soaked state. The behavior of polar liquids during evaporation and deswelling dynamics is monitored and described. An internal migration of water from the swollen polymer mesh into expanding pores takes place. With this procedure it is possible to obtain information about the microscopic morphology behavior of the matrix during evaporation and deswelling. This information is of great interest with the aspect of possible and future applications for these types of materials.
Select nuclear spins in a solid isolated from the rest due to local fields reach equilibrium via multielectron spin interactions.
Disorder and many body interactions are known to impact transport and ...thermalization in competing ways, with the dominance of one or the other giving rise to fundamentally different dynamical phases. Here we investigate the spin diffusion dynamics of
13
C in diamond, which we dynamically polarize at room temperature via optical spin pumping of engineered color centers. We focus on low-abundance, strongly hyperfine-coupled nuclei, whose role in the polarization transport we expose through the integrated impact of variable radio-frequency excitation on the observable bulk
13
C magnetic resonance signal. Unexpectedly, we find good thermal contact throughout the nuclear spin bath, virtually independent of the hyperfine coupling strength, which we attribute to effective carbon-carbon interactions mediated by the electronic spin ensemble. In particular, observations across the full range of hyperfine couplings indicate the nuclear spin diffusion constant takes values up to two orders of magnitude greater than that expected from homo-nuclear spin couplings.
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