Agnes Haber

Noninvasive depth profiling of walls by portable nuclear magnetic resonance

AbstractA compact and mobile single-sided 1H NMR sensor, the NMR-MOUSE®, has been employed in the nondestructive characterization of the layer structure of historic walls and wall paintings. Following laboratory tests on a model hidden fresco, paint and mortar layers were studied at Villa Palagione and the Seminario Vescovile di Sant’ Andrea in Volterra, Italy. Different paint and mortar layers were identified, and further characterized by portable X-ray fluorescence spectroscopy where accessible. In the detached and restored fresco “La Madonna della Carcere” from the Fortezza Medicea in Volterra, paint and mortar layers were discriminated and differences in the moisture content of the adhesive that fixes the detached wall painting to its support were found in both restored and original sections. These investigations encourage the use of the portable and single-sided NMR technology for nondestructive studies of the layer structure and conservation state of historic walls.n FigureA mobile NMR machine measuring a depth profile into a painted, old wall to unravel the layers from mortar and paint

Small-scale instrumentation for nuclear magnetic resonance of porous media

The investigation of fluids confined to porous media is the oldest topic of investigation with small-scale nuclear magnetic resonance (NMR) instruments, as such instruments are mobile and can be moved to the site of the object, such as the borehole of an oil well. While the analysis was originally restricted by the inferior homogeneity of the employed magnets to relaxation measurements, today, portable magnets are available for all types of NMR measurements concerning relaxometry, imaging and spectroscopy in two types of geometries. These geometries refer to closed magnets that surround the sample and open magnets, which are brought close to the object for measurement. The current state of the art of portable, small-scale NMR instruments is reviewed and recent applications of such instruments are featured. These include the porosity analysis and description of diesel particulate filters, the determination of the moisture content in walls from gray concrete, new approaches to analyze the pore space and moisture migration in soil, and the constitutional analysis of the mortar base of ancient wall paintings.

Surface UV aging of elastomers investigated with microscopic resolution by single-sided NMR

Depth profiles taken from the surface of UV irradiated natural rubber sheets have been measured with microscopic resolution using a Profile NMR-MOUSE. An NMR observable related to the sum of the spin echoes in the Carr-Purcell-Meiboom-Gill pulse sequence was used to characterize the cross-link density changes produced by the action of UV radiation in each sheet. The aging process was investigated as function of irradiation time and penetration depth. An exponential attenuation law with a space dependent absorption coefficient describes the change in the NMR observable with penetration depth. An Avrami model is used to describe the dependence of the absorption coefficient on the aging time. The method can be applied to investigate the effect of various aging agents on the surfaces of elastomers.

Hydrate Shell Growth Measured Using NMR

Benchtop nuclear magnetic resonance (NMR) pulsed field gradient (PFG) and relaxation measurements were used to monitor the clathrate hydrate shell growth occurring in water droplets dispersed in a continuous cyclopentane phase. These techniques allowed the growth of hydrate inside the opaque exterior shell to be monitored and, hence, information about the evolution of the shells morphology to be deduced. NMR relaxation measurements were primarily used to monitor the hydrate shell growth kinetics, while PFG NMR diffusion experiments were used to determine the nominal droplet size distribution (DSD) of the unconverted water inside the shell core. A comparison of mean droplet sizes obtained directly via PFG NMR and independently deduced from relaxation measurements showed that the assumption of the shell model-a perfect spherical core of unconverted water-for these hydrate droplet systems is correct, but only after approximately 24 h of shell growth. Initially, hydrate growth is faster and heat-transfer-limited, leading to porous shells with surface areas larger than that of spheres with equivalent volumes. Subsequently, the hydrate growth rate becomes mass-transfer-limited, and the shells become thicker, spherical, and less porous.

Ancient Roman wall paintings mapped nondestructively by portable NMR

The stratigraphies of decorated walls in ancient Herculaneum, Italy, were analyzed by single-sided 1H NMR. A large version of the NMR-MOUSE® with a maximum penetration depth of 25xa0mm was used to map proton density profiles at different positions of the Mosaic of Neptune and Amphitrite showing considerable differences between different tesserae and the mortar bed at different times of the year. In the House of the Black Room, different mortar layers were observed on painted walls as well as different proton content in different areas due to different moisture levels and different conservation treatments. The proton density profiles of the differently treated areas indicated that one method leads to higher moisture content than the other. Untreated wall paintings from different times were profiled in a recently excavated room at the Villa of the Papyri showing two different types of mortar layer structures which identify two different techniques of preparing the walls for painting. Reflectance Fourier mid-infrared spectroscopy and in situ X-ray fluorescence measurements complemented the NMR measurements and provided additional insight into the identification of organic coatings as well as the nature of the pigments used, respectively. The information acquired nondestructively by NMR is valued for elaborating conservation strategies and for identifying different schools of craftsmen who prepared the mortar supports of the wall paintings.

Moisture dynamics in wall paintings monitored by single-sided NMR†

The durability of historic wall paintings is highly dependent on environmental influences such as moisture ingress, salt crystallization and temperature changes. A fundamental understanding of dynamic transport processes in wall paintings is necessary to apply suitable conservation and restoration methods to preserve such objects with high cultural value. Non‐invasive, mobile‐NMR techniques with single‐sided sensors, such as the NMR‐MOUSE®, enable to monitor the moisture content, transport and apparent diffusion constants in wall paintings. We investigated this technique by experiment and modeling to correlate salt crystallization, moisture transport and local diffusion in wall‐painting samples. Moreover, the influence of different painting techniques (fresco and secco) and conservation/consolidation methods on moisture transport and diffusion is discussed. The results are compared with results from field measurements on real fresco paintings in Casa del Salone Nero and the Villa of the Papyri, Herculaneum, Italy. Copyright

Shear-induced emulsion droplet diffusion studies using NMR

HYPOTHESISnShear-induced droplet diffusion of flowing hard spheres is relatively well understood and has been extensively studied both experimentally and via simulations. The same however is not true of soft spheres, specifically emulsions, despite their broad and extensive industrial relevance. Here we seek to demonstrate that appropriate NMR techniques can be used to quantitatively measure shear-induced droplet diffusion. Limited literature indicates that dilute dispersions of soft spheres experience significantly larger shear-induced droplet diffusion relative to otherwise equivalent hard sphere suspensions. Here we explore whether this effect persists to high concentrations.nnnEXPERIMENTSnNuclear Magnetic Resonance (NMR) pulsed field gradient (PFG) techniques were used to measure shear-induced droplet diffusion for capillary flow of various water-in-oil (w/o) emulsions in a direction transverse to flow. Two adaptations were necessary - the acquired signal was analyzed so as to quantitatively distinguish restricted molecular diffusion within the emulsion droplets from shear-induced diffusion of the droplets, whilst flow-compensated PFG pulse sequences were shown to be necessary to account for any erroneous effects due to flow. A range of w/o emulsions were considered to enable measurement of shear-induced droplet diffusion as a function of both water content and mean shear rate. The surfactant content of these emulsions was adjusted such that they presented similar (stationary) emulsion droplet size distributions (DSD) which were also measured using NMR PFG techniques.nnnFINDINGSnThe droplet shear-induced diffusion data for the emulsion systems were compared against relevant results from the literature. Consistent with predictions for dilute systems, significantly greater droplet diffusion was measured relative to hard sphere suspensions at all concentrations, and a quadratic dependence was found between droplet diffusion and mean droplet size. For more concentrated emulsions, a peak in the droplet diffusion-concentration relationship was observed for the first time in emulsions, prior to the onset of emulsion inversion.

Relaxation Exchange in Nanoporous Silica by Low-Field NMR

Abstract Investigating the 2D-T2-T2-relaxation exchange of inter-stitial water in a packing of sedimented Stöber-silicate spheres, we come the conclusion that contrary to its behaviour in macro-pores, water confined in nano-pores of silica exhibits enhanced diffusivity. The 2D-experiments, performed at different temperatures, reveal a temperature-dependent bimodal relaxation distribution and two-site relaxation exchange. Our recently introduced kinetic multi-site exchange model is applied to derive the according exchange rates. The resulting Arrhenius plot produces an exchange activation energy of 7 kJ/mol, which is well below the hydrogen bond energy or the activation energy for self-diffusion of water in the bulk. A possible hopping-mechanism as the source of enhanced proton-diffusion in nanoporous silica is discussed, as well as its significance to mass transfer in porous media.