Binary liquid mixtures in porous solids.

The technique of nuclear magnetic resonance cryoporometry has been used to study the behavior of binary liquid mixtures of water and decane in porous sol-gel silicas. It was observed that the water preferentially adsorbed onto the silica surface and so was able to displace the decane from the pores.

Pore surface exploration by NMR.

A carefully chosen set of experimental techniques applied to porous media characterization provides results that can be much greater than the sum of the individual parts. The inter-relation and complementarity of a number of techniques will be considered. NMR cryoporometry provides a valuable method of pore size measurement. An NMR method that is more widely used to assess pore dimensions relies on relaxation time analysis of a liquid that fills the pores and the enhanced relaxation that occurs in a liquid at the solid/liquid interface. Thermoporometry, a method based on the application of Differential Scanning Calorimetry (DSC), is closely related to cryoporometry, but employs a different set of assumptions to evaluate pore size distributions. Comparison of the results obtained on the same samples using all these methods together with gas adsorption serves to validate the methods and provide significantly more information about surface-fluid interaction and the behavior of nano-scale material within pores than each method employed in isolation. Technique developments will be discussed and applications of these methods to ideal silicas will be used to illustrate their power, particularly in combination.

An evaluation of NMR cryoporometry, density measurement and neutron scattering methods of pore characterisation.

Sol-gel silicas with nominal pore diameters ranging from 25A to 500A were studied by NMR cryoporometry, and by neutron diffraction and small angle scattering from dry silicas over the Q range 8. 10(-4)A(-1) < or = Q < or = 17A(-1). Density and imbibation experiments were also performed. Geometric models of porous systems were constructed and were studied by both analytic techniques and Monte-Carlo integration. These models, combined with the information from the above measurements, enabled the calculation of the fully density corrected solid-solid density correlation functions G(r) for the sol-gel silicas, deduction of the (voidless) silica matrix density, measurement of the silica fraction in the grain and of the packing fraction of the silica grains and an estimation of the water equivalent residual hydrogen on the dried silica surface. In addition, the pore diameter D, pore diameter to lattice spacing ratio D/a, and pore and lattice variance sigma could also be measured. While the NMR cryoporometry pore diameter measurements for the sol-gel silicas show excellent co-linearity with the nominal pore diameters as measured by gas adsorption, and the calculated pore diameters from the measured neutron scattering show surprisingly good agreement with these measurements at large pore diameters, there is a divergence between the calibrations for pore diameters below about 100A.

Combined MR-relaxation and MR-cryoporometry in the study of bone microstructure.

MR-Relaxation (MRR) of 1H nuclei and MR-Cryoporometry (MRC) are combined to assess their feasibility and their potential in the study of bone microstructure. In principle, both techniques are able to give information on the structure of the pore space confining the fluids. Cow femur samples were carefully cored and cleaned in order to remove the natural fluids inside. For MRR analysis quasi-continuous distributions of T(1) and T(2) were obtained on samples fully saturated with water. Cyclohexane was used as a saturating fluid for MRC analysis. All T(1) and T(2) quasi-continuous distributions of water confined in bone samples are more than three decades wide, showing sufficient details to differentiate the samples. Pore size distributions obtained by MRC also differentiate the samples showing different characteristics of the pore space structure in the range of the highest sensitivity of the method (typically 3 to 100 nm, mesopore range). In particular, in samples where MRR shows a large fraction of signal with relaxation times below 10(2) ms, MRC indicates a large fraction of pore volume with pore sizes in the mesopore range.

Diffusion-weighted imaging of bacteria colonies in the STRAFI plane.

Imaging colonies of bacteria in water suspension using NMR requires that the water inside the bacteria can be differentiated from the surrounding water. This is commonly carried out by using diffusion-weighted pulsed field gradient techniques. However, it is also possible to use the diffusion sensitivity inherent in stray field imaging (STRAFI). In STRAFI, the subject to be imaged is normally moved along the axis of a superconducting magnet so that it passes through the sensitive slice. However, by moving the sample in the transverse direction and by using a long copper strip in place of a surface induction coil, a diffusion-weighted one-dimensional projection profile can be obtained across the sensitive slice. Profiles from a water phantom and from a bacteria suspension show convincing discrimination between intracellular and extracellular water.

Microdynamics and phase equilibria in organic nanocrystals.

The effects of confinement on various organic molecules have been studied by nuclear magnetic resonance (NMR) relaxation techniques (T2 and T1 rho) between room temperature and 77 K. Cyclohexane, pentadecane, squalane, and squalene have been constrained within 60 A porous silica, and the behaviour of the resulting nanocrystals has been compared to that of the bulk material. In all cases the molecular dynamics of the confined material were found to be significantly different from the bulk, and there is evidence to suggest that the behaviour is largely dependent on the size and shape of the molecules and the resultant structural disorder present when constrained within the pores.

Diffusion processes in confined materials.

The diffusion coefficient for cyclohexane confined within pores of diameter 40 to 500 A has been measured as a function of temperature between 296 and 180 K, and is compared to values obtained for the bulk material. A substantial liquid-like signal is observed in the region of the depressed freezing points and a diffusion coefficient is measurable in all samples to well below these temperatures. The diffusion data appear to be continuous over the freezing region. These observations suggest persisting molten layers at interfaces which exchange with crystals forming within the silica pores. The diffusion coefficient of the molecules in the surface layer is three orders of magnitude larger than in the plastic phase of bulk cyclohexane.

Susceptibility effects in unsaturated porous silica.

Nuclear magnetic resonance proton line widths have been studied as the saturation level of water in a porous silica system is varied. Two silica samples were used with nominal pore sizes of 60 and 140 A. It was found that the line width increased with saturation level for both systems; this is consistent with the saturation process suggested by Allen et al. At low saturation levels the peak shift, from bulk water, increased with the saturation level reaching a maximum at filling factors of approximately 40% and 20% for the 60- and 140-A samples, respectively, after which point it began to decrease. Analysis is currently under way to try to model this system to determine whether these results are also consistent. It is anticipated that further analysis will give information on the pore morphology of the system.

A three-dimensional NMR imaging scheme utilizing doubly resonant gradient coils.

A 3DFT gradient-echo technique has been developed which, in conjunction with series-resonant gradient-coil circuits, can produce three-dimensional NMR images with an echo time of less than 100 microseconds. The method involves a read-gradient waveform composed of two sinusoids of different frequencies. This is an improvement on previous imaging sequences using a single sinusoid where only half of k space was sampled and where the second half was calculated using conjugate symmetry. The inaccuracies involved in the necessary "cut and paste" of k space inevitably lead to artifacts in the final image. The important features of the new method are that with suitable phase encoding all octants of k space are sampled, the RF pulse is applied when the gradients are all zero, and the echo forms when the gradient is essentially constant. This method will allow more extensive application of solid imaging techniques to biological samples in vivo.

Pore size distribution mapping.

Pore size distribution mapping has been demonstrated using NMR cryoporometry in the presence of a magnetic field gradient. This novel method is extendable to 2D and 3D mapping. It offers a unique nondestructive method of obtaining full pore-size distributions in the range 3 to 100 nm at any point within a bulk sample.

Phase equilibria of absorbed liquids and the structure of porous media.

The phase equilibria of water within porous silica has been studied by proton and deuteron magnetic resonance. Proton signal amplitude as a function of filling factor was measured. These protons arise from the proton-deuterium equilibrium that is established between the liquid and the absorbed layer on the pore wall. The results for temperatures below 0 degree C show a maximum as a function of filling factor, theta. This suggests that the pores fill from either a surface layer or from the crevices and interstices into the center. Another experiment used cryoporometry to study the size of crystals formed within the pores as a function of theta and leads to the same conclusion.

A 7Li and 19F NMR relaxation study of LiCF3SO3 in plasticised solid polyether electrolytes.

7Li and 19F NMR relaxation time (T1, T2, T1 rho) measurements have been used to probe the dynamics of LiCF3SO3 dissolved in an amorphous co-polymer poly(ethylene oxide-co-propylene oxide), and in particular the influence of the plasticising agents propylene carbonate and dimethyl formamide. The changes in relaxation behaviour of 19F and 7Li with increasing plasticiser concentration are very different, as is the effect of each plasticiser. These differences can be explained qualitatively in terms of the interaction between the plasticiser and the ions. Preliminary 7Li T1 rho measurements reveal two components at low temperatures.

NMR studies of molecular mobility and diffusion in porous systems.

The uptake, partitioning, and release of ingredients such as water, oil, surfactant, and ions are important factors to understand and control in the design and manufacture of detergent and personal products. Although conventional pulse NMR (PNMR) spectroscopy continues to be used to analyse bulk molecular mobility and phase composition, more recently MR imaging techniques have created unique opportunities for gaining spatial information about these processes in ways that are noninvasive and potentially quantitative. This paper describes the evaluation of MRI and associated PNMR techniques to study transport in three relevant cases: ion diffusion (e.g., fluoride) in concentrated dispersions, oil transport through powders, and water ingress into porous powders (zeolite). Results are presented to illustrate the potential of multiple pulse and gradient echo MRI methods for dealing with the short T2 scenarios that represent a common problem in quantitative imaging of water in solid-containing composites involving, for instance, zeolite, or silica. Pore-size characterisation results are also presented.

Ingress of water into zeolite 4A powder plugs.

One dimensional profiles of the concentration of water absorbed from vapour diffusing into compacted type 4A zeolite powder have been obtained by broadline NMR imaging. After an induction period of approximately 6 h, a region of full hydration advances linearly with time into the zeolite plug. This behaviour is typical of Case II diffusion. A simple numerical simulation gives good agreement with the experimental results.

The characterization of porous solids by NMR.

We are using a novel NMR method, that has been developed in our laboratory and employs the depression of the freezing point (delta Tm) of confined liquid within porous media, to investigate the effect of pre-drying silica with average pore diameter ranging from 60 A to 1000 A. Cyclohexane was the confined liquid. Pre-drying was found to affect only the smallest pores. A study to compare partially filled and over-filled samples showed that the average delta Tm for partially filled samples is greater than for over-filled. We have also investigated the use of water as the absorbed liquid and compared results with those obtained from cyclohexane studies. Reasonable agreement was found but cyclohexane was more sensitive. The method is fast and is suitable for monitoring pore size distributions in the range of 50-1000 A.

Diffusion of fluids in confined geometry.

Pulsed field gradient NMR measurements of restricted self-diffusion of three nonspecific liquids are reported in well characterised samples with average pore diameters ranging between 35 A and 500 A. Structural information has been obtained in the long-times and small "wave-vectors" regime, using an exact analytic solution for the related problem of "thin" semi-permeable barriers. Experimental results suggest that the permeability of a porous medium depends on the porosity (connectivity) of the pore space and the molecular size of the self-diffusing molecules.