Comparison of NMR Cryoporometry, Mercury Intrusion Porosimetry, and DSC Thermoporosimetry in Characterizing Pore Size Distributions of Compressed Finely Ground Calcium Carbonate Structures

The implementation of an easy-to-apply NMR cryoporometric instrument for porous materials

Time-domain NMR has been extensively utilised to study various characteristics of fluid-saturated porous,materials for instance their mobility, dynamics, stiffness, viscosity and rigidity features, particularly for solid hydrocarbons, rubbers and other polymers. As a unique time-domain technique available for over 30 years, NMR cryoporometry (NMRC) may be used to obtain pore-size distributions of the measured samples. To accurately control the sample temperature, a Peltier thermo-electrically cooled variable temperature probe has been developed and integrated with a highly compact precision NMR time-domain relaxation spectrometer, therefore providing the community with a high-performance instrument for NMR Cryoporometry. To extend the application of aforementioned high-performance NMRC instrument into more senarios, we designed a series of light-weight, compact and integral models with optional NMR frequencies from 12 MHz up to 23 MHz. The measured sample temperature can be precisely controlled from about -60 °C to +80 °C, with an excellent temperature resolution of 10 mK or better near the probe liquid bulk melting point. Therefore, it offers a fairly wide NMRC pore-size distribution ranging from about 1 nm to 2 μm by using water as the probe liquid in the pores, significantly wider than is possible when applying generic NMR Spectrometers for NMRC. A preliminary example of NMR Cryoporometric measurements on two special cement samples is shown in the paper in which the measured pore scales as well as their repeatability are demonstrated. Furthermore, various nano-materials, such as MOF, zeolite and shale kerogen would be potential materials to study by using these new available NMRC instrument models. We aim to offer this technique as a quantitative and easy-to-apply unitary benck-top tool for an even wider range of porous material.

Pore Structure Characteristics and Controlling Factors of a Tight Sandstone Reservoir in the Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

Tight sandstone reservoirs have ultralow physical properties and strong heterogeneity, and there is a need to describe the corresponding pore structure characteristics systematically to promote research on unconventional reservoirs. The pore structure, controlled by the diagenesis and volcanic activity of the tight reservoirs in the third member of the Shahejie Formation (Es₃) of the Gaoshangpu structural belt in the Nanpu Sag, is studied by high-pressure mercury injection, nuclear magnetic resonance, and constant-rate-controlled mercury porosimetry. The results show that the Es₃ reservoir can be divided into three types: the pore radii of Type I reservoirs range from 120 to 180 μm, and the throat radii are larger than 1 μm, resulting in good pore connectivity; pore radii of Type II reservoirs are approximately 100 μm, and the throat radii range from 0.1 to 1 μm, resulting in moderate pore connectivity; and pore radii of Type III reservoirs are much smaller than 100 μm, and the throat radii are smaller than 0.1 μm, resulting in worst pore connectivity. The pore size of Type I reservoirs is most sensitive to compaction, and the pore connectivity is mainly controlled by carbonate cementation; the pore throat size and pore connectivity of Type II reservoirs are seriously affected by clay cementation, and pores are mainly formed by dissolution. However, the pore structure of Type III reservoirs is the worst among those investigated in this study but can be further improved by dissolution to a certain extent. Volcanic activity controls cementation and affects dissolution, thus changing the pore structure. A pore structure evolution model is established, which can provide a reference for future oil gas exploration.

Recent developments in flow modeling and fluid control for paper-based microfluidic biosensors

Over the last 10 years, researchers have shown that paper is a promising substrate for affordable biosensors. The field of paper-microfluidics has evolved rapidly in that time, with simple colorimetric assays giving way to more complex electrochemical devices that can handle multiple samples at a given time. As paper devices become more complex, the ability to precisely control different fluids simultaneously becomes a challenge. Specifically, automated flow control is a necessary attribute to make paper-based devices more useable in resource-limited settings. Flow control strategies on paper are typically developed experimentally through trial-and-error, with little focus on theory. This is because flow behavior in paper is not well understood and sometimes difficult to predict precisely. Additionally, popular theoretical models are too simplistic, making them unsuitable for complex device designs and application conditions. A better understanding of flow theory would allow devices conceived straight from theoretical models. This could save time and resources by reducing experimental work. In this review, we provide an overview of different theoretical models used to characterize imbibition in paper substrates and document the latest flow control strategies that have been applied to automated fluid control on paper. Additionally, we look at current efforts to commercialize paper-based devices along with challenges facing this industry.

Characteristics of the Micro-Nanopore System in a Pelitic Dolomite Reservoir: A Case Study of the Lower Cretaceous Xiagou Formation in the Qingxi Depression Based on Nuclear Magnetic Resonance Experiments

The Lower Cretaceous Xiagou Formation is an important tight oil reservoir in the Qingxi Depression of the Jiuxi Basin. The micro-nanopore system within the reservoir requires a comprehensive analysis to improve the production of tight oil there. Nuclear magnetic resonance (NMR) experiments have been widely used for the petrophysical characterization of sandstones and carbonates. In the present study, the NMR experiment was applied to obtain the characteristics of the micro-nanopore system and permeability in the Lower Cretaceous Xiagou pelitic dolomite reservoir. According to the distribution shape of the transversal relaxation time (T₂) obtained under the 100% water-saturated condition (S_w), the samples are divided into four groups: (i) group I, two obvious peaks (P1 and P2); (ii) group II, an obvious high peak of P1 at 0.1˜1.0 ms and a relatively low peak of P2; (iii) group III, an obvious high peak of P2 and a relatively low peak of P1; and (iv) group IV, three peaks. In general, the distribution shape of T₂ under the initial condition (S_ini) is unimodal, with all its peaks lower than those under the S_w condition. The NMR T₂ spectrum reflects the distribution of the rock pore radius. Most of the pore radius distributions are bimodal, and the main pore radius ranges from 10 nm to 70 nm. Three patterns can be identified and determined based on the distribution of the pore radius: I-unimodal distribution, II-bimodal distribution and III-trimodal distribution. The results indicate that the porosity in the Xiagou reservoir ranges from 1.17% to 6.89%, with an average of 3.33%. The permeability ranges from 0.03×10^-3 μm² to 22.56×10^-3 μm², with an average of 2.95×10^-3 μm².

Comprehensive NMR Analysis of Pore Structures in Superabsorbing Cellulose Nanofiber Aerogels

Highly porous cellulose nanofiber (CNF) aerogels are promising, environmentally friendly, reusable, and low-cost materials for several advanced environmental, biomedical, and electronic applications. The aerogels have a complex and hierarchical 3D porous network structure with pore sizes ranging from nanometers to hundreds of micrometers. The morphology of the network has a critical role on the performance of aerogels, but it is difficult to characterize thoroughly with traditional techniques. Here, we introduce a combination of nuclear magnetic resonance (NMR) spectroscopy techniques for comprehensive characterization of pore sizes and connectivity in the CNF aerogels. Cyclohexane absorbed in the aerogels was used as a probe fluid. NMR cryoporometry enabled us to characterize the size distribution of nanometer scale pores in between the cellulose nanofibers in the solid matrix of the aerogels. Restricted diffusion of cyclohexane revealed the size distribution of the dominant micrometer scale pores as well as the tortuosity of the pore network. T ₂ relaxation filtered microscopic magnetic resonance imaging (MRI) method allowed us to determine the size distribution of the largest, submillimeter scale pores. The NMR techniques are nondestructive, and they provide information about the whole sample volume (not only surfaces). Furthermore, they show how absorbed liquids experience the complex 3D pore structure. Thorough characterization of porous structures is important for understanding the properties of the aerogels and optimizing them for various applications. The introduced comprehensive NMR analysis set is widely usable for a broad range of different kinds of aerogels used in different applications, such as catalysis, batteries, supercapacitors, hydrogen storage, etc.

Fabrication of PES/PVP Water Filtration Membranes Using Cyrene®, a Safer Bio-Based Polar Aprotic Solvent

A more sustainable dialysis and water filtration membrane has been developed, by using the new, safer, bio-based solvent Cyrene® in place of N-methyl pyrrolidinone (NMP). The effects of solvent choice, solvent evaporation time, the temperature of casting gel, and coagulation bath together with the additive concentration on porosity and pore size distribution were studied. The results, combined with infrared spectra, SEM images, porosity results, water contact angle (WCA), and water permeation, confirm that Cyrene® is better media to produce polyethersulfone (PES) membranes. New methods, Mercury Intrusion Porosimetry (MIP) and NMR-based pore structure model, were applied to estimate the porosity and pore size distribution of the new membranes produced for the first time with Cyrene® and PVP as additive. Hansen Solubility Parameters in Practice (HSPiP) was used to predict polymer-solvent interactions. The use of Cyrene® resulted in reduced polyvinylpyrrolidone (PVP) loading than required when using NMP and gave materials with larger pores and overall porosity. Two different conditions of casting gel were applied in this study: a hot (70°C) and cold gel (17°C) were cast to obtain membranes with different morphologies and water filtration behaviours.

Cryoporometry in Femtoliter Volumes by Confocal Raman Spectroscopy

The properties of porous material are largely dependent on the size, shape, and connectivity of the pores. Here, we present a method based on confocal Raman spectroscopy to quantify porosity using a cryoporometric approach. We show that the phase transition of water imbibed in porous silica can be accurately determined using two different, but complementary methodologies. The first one relies on integrating the temperature-dependent spectral intensities across the whole OH (H₂O) or OD (D₂O) stretching region. The second, more quantitative approach, deconvolutes the spectral contributions within the pores in terms of liquid and solid fractions. The results show the expected reciprocal dependence of the average phase transition point with pore size, as well as the typical hysteresis between the freezing and melting transitions. One of the key advantages of the confocal Raman approach is its high spatial resolution, with sampling volumes starting from just a few femtoliters, opening the possibility of mapping the structure in heterogeneous porous materials.

Characterization of Pore Throat Size Distribution in Tight Sandstones with Nuclear Magnetic Resonance and High-Pressure Mercury Intrusion

Characterization of pore throat size distribution (PTSD) in tight sandstones is of substantial significance for tight sandstone reservoirs evaluation. High-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) are the effective methods for characterizing PTSD of reservoirs. NMR T2 spectra is usually converted to mercury intrusion capillary pressure for PTSD characterization. However, the conversion is challenging in tight sandstones due to tiny pore throat sizes. In this paper, the linear conversion method and the nonlinear conversion method are investigated, and the error minimization method and the least square method are proposed to calculate the conversion coefficients of the linear conversion method and the nonlinear conversion method, respectively. Finally, the advantages and disadvantages of these two different conversion methods are discussed and compared with field case study. The research results show that the average linear conversion coefficients of the 20 tight sandstone core plugs collected from Yanchang Formation, Ordos Basin of China is 0.0133 μm/ms; the average nonlinear conversion coefficient is 0.0093 μm/ms and the average nonlinear conversion exponent is 0.725. Although PTSD converted from NMR spectra by the nonlinear conversion method is wider than that obtained from linear conversion method, the nonlinear conversion method can retain the characteristic of bi-modal distribution in PTSD.

Experimental Investigation of Pore Structure and Movable Fluid Traits in Tight Sandstone

Whether the variation of pore structures and movable fluid characteristics enhance, deteriorate, or have no influence on reservoir quality has long been disputed, despite their considerable implications for hydrocarbon development in tight sandstone reservoirs. To elucidate these relationships, this study systematically analyzes pore structures qualitatively and quantitatively by various kinds of direct observations, indirect methods, and imaging simulations. We found that the uncertainty of porosity measurements, caused by the complex pore-throat structure, needs to be eliminated to accurately characterize reservoir quality. Bulk water was more easily removed, while surface water tended to be retained in the pores, and the heterogeneity of pore structures was caused by the abundance of tiny pores. The rates of water saturation reduction in macropores are faster than those for tiny pores, and sandstones with poor reservoir quality show no marked descending of lower limits of movable pore radius, indicating that the movable fluid would advance exempted from the larger pores. This study suggests that the deterioration of reservoir quality is strongly affected by the reduction of larger pores and the aqueous phases tended to remain in the tiny pores in the forms of surface water.

Petrophysical characterization of high-rank coal by nuclear magnetic resonance: a case study of the Baijiao coal reservoir, SW China

To better apply nuclear magnetic resonance (NMR) to evaluate the petrophysical characterization of high-rank coal, six anthracite samples from the Baijiao coal reservoir were measured by NMR. The porosity, T 2 cutoff value, permeability and pore type were analysed using the transverse relaxation time (T 2) spectrum before and after centrifugation. The results show that the T 2 spectrum of water-saturated anthracite can be divided into a discontinuous and continuous trimodal distribution. According to the connectivity among pores, three T 2 spectrum peaks were identified at the relaxation times of 0.01-1.7 ms, 1.7-65 ms and greater than 65 ms, which correspond to the micropores (less than 100 nm), mesopores (100-1000 nm) and macropores (greater than 1000 nm), respectively. Based on the T 2 cutoff value, we divided the T 2 spectrum into two parts: bound fluid and free fluid. By comparing two classic permeability models, we proposed a permeability model to calculate the permeability of anthracite. This result demonstrates that NMR has great significance to the exploration of coal reservoirs and to the understanding of the development of coalbed methane.

Relationship between the Size of the Samples and the Interpretation of the Mercury Intrusion Results of an Artificial Sandstone

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.

Characterisation of pore structures of pharmaceutical tablets: A review

Traditionally, the development of a new solid dosage form is formulation-driven and less focus is put on the design of a specific microstructure for the drug delivery system. However, the compaction process particularly impacts the microstructure, or more precisely, the pore architecture in a pharmaceutical tablet. Besides the formulation, the pore structure is a major contributor to the overall performance of oral solid dosage forms as it directly affects the liquid uptake rate, which is the very first step of the dissolution process. In future, additive manufacturing is a potential game changer to design the inner structures and realise a tailor-made pore structure. In pharmaceutical development the pore structure is most commonly only described by the total porosity of the tablet matrix. Yet it is of great importance to consider other parameters to fully resolve the interplay between microstructure and dosage form performance. Specifically, tortuosity, connectivity, as well as pore shape, size and orientation all impact the flow paths and play an important role in describing the fluid flow in a pharmaceutical tablet. This review presents the key properties of the pore structures in solid dosage forms and it discusses how to measure these properties. In particular, the principles, advantages and limitations of helium pycnometry, mercury porosimetry, terahertz time-domain spectroscopy, nuclear magnetic resonance and X-ray computed microtomography are discussed.

Phase transitions in disordered mesoporous solids

Fluids confined in mesoporous solids exhibit a wide range of physical behavior including rich phase equilibria. While a notable progress in their understanding has been achieved for fluids in materials with geometrically ordered pore systems, mesoporous solids with complex pore geometries still remain a topic of active research. In this work we study phase transitions occurring in statistically disordered linear chains of pores with different pore sizes. By considering, quite generally, two phase change mechanisms, nucleation and phase growth, occurring simultaneously we obtain the boundary transitions and the scanning curves resulting upon reversing the sign of the evolution of the chemical potential at different points along the main transition branches. The results obtained are found to reproduces the key experimental observations, including the emergence of hysteresis and the scanning behavior. By deriving the serial pore model isotherm we suggest a robust framework for reliable structural analysis of disordered mesoporous solids.

Porous structure of ion exchange membranes investigated by various techniques

A comparative review of various techniques is provided: mercury intrusion porosimetry, nitrogen sorption porosimetry, differential scanning calorimetry (DSC)-based thermoporosimetry, and standard contact porosimetry (SCP), which allows determining pore volume distribution versus pore radius/water binding energy in ion-exchange membranes (IEMs). IEMs in the swollen state have a labile structure involving micro-, meso- and macropores, whose size is a function of the external water vapor pressure. For such materials, the most appropriate methods for quantifying their porosity are DSC and SCP. Especially significant information is given by the SCP method allowing measuring porosimetric curves in a very large pore size range from 1 to 10⁵nm. Experimental results of water distribution in homogeneous and heterogeneous commercial and modified IEMs are presented. The effect of various factors on water distribution is reviewed, i.e. nature of polymeric matrix and functional groups, method for membrane preparation, membrane ageing. A special attention is given to the effect of membrane modification by embedding nanoparticles in their structure. The porosimetric curves are considered along with the results of electrochemical characterization involving the measurements of membrane conductivity, as well as diffusion and electroosmotic permeability. It is shown that addition of nanoparticles may lead to either increase or decrease of water content in IEMs, different ranges of pore size being affected. Hybrid membranes modified with hydrated zirconium dioxide exhibit much higher permselectivity in comparison with the pristine membranes. The diversity of the responses of membrane properties to their modification allows for formation of membranes suitable for fuel cells, electrodialysis or other applications.

Interconnectivity imaged in three dimensions: Nano-particulate silica-hydrogel structure revealed using electron tomography

We have used Electron Tomography (ET) to reveal the detailed three-dimensional structure of particulate hydrogels, a material category common in e.g. controlled release, food science, battery and biomedical applications. A full understanding of the transport properties of these gels requires knowledge about the pore structure and in particular the interconnectivity in three dimensions, since the transport takes the path of lowest resistance. The image series for ET were recorded using High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM). We have studied three different particulate silica hydrogels based on primary particles with sizes ranging from 3.6nm to 22nm and with pore-size averages from 18nm to 310nm. Here, we highlight the nanostructure of the particle network and the interpenetrating pore network in two and three dimensions. The interconnectivity and distribution of width of the porous channels were obtained from the three-dimensional tomography studies while they cannot unambiguously be obtained from the two-dimensional data. Using ET, we compared the interconnectivity and accessible pore volume fraction as a function of pore size, based on direct images on the nanoscale of three different hydrogels. From this comparison, it was clear that the finest of the gels differentiated from the other two. Despite the almost identical flow properties of the two finer gels, they showed large differences concerning the accessible pore volume fraction for probes corresponding to their (two-dimensional) mean pore size. Using 2D pore size data, the finest gel provided an accessible pore volume fraction of over 90%, but for the other two gels the equivalent was only 10-20%. However, all the gels provided an accessible pore volume fraction of 30-40% when taking the third dimension into account.

Characterization of the Pore Structure of Functionalized Calcium Carbonate Tablets by Terahertz Time-Domain Spectroscopy and X-Ray Computed Microtomography

Novel excipients are entering the market to enhance the bioavailability of drug particles by having a high porosity and, thus, providing a rapid liquid uptake and disintegration to accelerate subsequent drug dissolution. One example of such a novel excipient is functionalized calcium carbonate, which enables the manufacture of compacts with a bimodal pore size distribution consisting of larger interparticle and fine intraparticle pores. Five sets of functionalized calcium carbonate tablets with a target porosity of 45%-65% were prepared in 5% steps and characterized using terahertz time-domain spectroscopy and X-ray computed microtomography. Terahertz time-domain spectroscopy was used to derive the porosity using effective medium approximations, that is, the traditional and an anisotropic Bruggeman model. The anisotropic Bruggeman model yields the better correlation with the nominal porosity (R² = 0.995) and it provided additional information about the shape and orientation of the pores within the powder compact. The spheroidal (ellipsoids of revolution) shaped pores have a preferred orientation perpendicular to the compaction direction causing an anisotropic behavior of the dielectric porous medium. The results from X-ray computed microtomography confirmed the nonspherical shape and the orientation of the pores, and it further revealed that the anisotropic behavior is mainly caused by the interparticle pores. The information from both techniques provides a detailed insight into the pore structure of pharmaceutical tablets. This is of great interest to study the impact of tablet microstructure on the disintegration and dissolution performance.

Porous structure of fibre networks formed by a foaming process: a comparative study of different characterization techniques

Recent developments in making fibre materials using the foam-forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure-characterization methods, both two-dimensional and three-dimensional techniques were used to explore the porous structure of foam-formed samples made with two different types of cellulose fibre. These techniques included X-ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X-ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two-dimensional imaging techniques. The two-dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam-fibre interaction.

Structure of perfluorinated membranes investigated by method of standard contact porosimetry

The results of investigation of various factors influencing water distribution in perfluorinated membrane structure by method of standard contact porosimetry are summarized. The Nafion membranes (Dupon de Nemoure, USA) and MF-4SK membranes ("Plastpolymer", Russia) were the objects of the research. The influence of production process and conditioning method on porosimetric curves of perfluorinated membrane is discussed. New results related to the porosity of perfluorinated membranes after reinforcing fabric introduction and processing by organic solvents are reported. The role of the modifying components of various nature in the shaping of transport channels in perfluorinated membrane is studied. The influence of polyaniline and hydrogen zirconium phosphate on water distribution in membrane structure is revealed. The correlation between the maximum porosity value of the membrane and its diffusion and electroosmotic permeability, as well as between the fraction of the gel pore volume and membrane selectivity is established. It allows the prediction of possible changes in the structural characteristics and also in the transport properties of the membranes under the influence of the modifying components of different types and various operating conditions.

Characterization of pore structure of a strong anion-exchange membrane adsorbent under different buffer and salt concentration conditions

The quantitative characterization of pore structure of Sartobind Q, a strongly basic membrane anion exchanger that is formed by cross-linked cellulose support and a hydrogel layer on its pore surface, was made combining the results obtained by several experimental techniques: liquid impregnation, batch size-exclusion, inverse size-exclusion chromatography, and permeability. Mercury intrusion and nitrogen sorption porosimetry were carried out for a dry cellulose support membrane in order to get additional information for building a model of the bimodal pore structure. The model incorporated the distribution of the total pore volume between transport and gel-layer pores and the partitioning of solutes of different molecular weights was expressed through the cylindrical pore model for the transport pores and random plane model for the gel layer. The effect of composition of liquid phase on the pore structure was investigated in redistilled water, phosphate and Tris-HCl buffers containing up to 1M NaCl. Evident differences in the bimodal pore structure were observed here when both the specific volume and size of the hydrogel layer pores significantly decreased with the ionic strength of liquid phase.

AFM-porosimetry: density and pore volume measurements of particulate materials

We introduced the novel technique of AFM-porosimetry and applied it to measure the total pore volume of porous particles with a spherical geometry. The methodology is based on using an atomic force microscope as a balance to measure masses of individual particles. Several particles within the same batch were measured, and by plotting particle mass versus particle volume, the bulk density of the sample can be extracted from the slope of the linear fit. The pore volume is then calculated from the densities of the bulk and matrix materials, respectively. In contrast to nitrogen sorption and mercury porosimetry, this method is capable of measuring the total pore volume regardless of pore size distribution and pore connectivity. In this study, three porous samples were investigated by AFM-porosimetry: one ordered mesoporous sample and two disordered foam structures. All samples were based on a matrix of amorphous silica templated by a block copolymer, Pluronic F127, swollen to various degrees with poly(propylene glycol). In addition, the density of silica spheres without a template was measured by two independent techniques: AFM and the Archimedes principle.

Pore morphology and interconnectivity in a mesoporous/macroporous polyhedral silica foam material

The pore system of a highly swollen, block-copolymer-templated, polyhedral silica foam material is investigated by a combination of transmission electron microscopy, nitrogen sorption, and NMR cryoporometry. The adsorption-desorption hysteresis and melting-freezing hysteresis data recorded by the respective methods provide pore volume and access channel sizes that virtually coincide for the two used methods. This provides a consistent picture where polyhedral foam cells of 60-70 nm diameter are interconnected by cylindrical access channels with several characteristic sizes for the latter.

Use of a void network model to correlate porosity, mercury porosimetry, thin section, absolute permeability, and NMR relaxation time data for sandstone rocks

The Pore-Cor void network model is used to construct stochastic realizations of the void structures of five sandstone samples of varying lithography. A close match was achieved to experimental porosity and mercury intrusion curves. The samples were resin impregnated and the fragments of voids revealed in thin sections photographed by backscatter electron microscopy at two magnifications. The sizes of these pore fragments matched those derived from a simulated microtoming of the network model much more closely than the sizes derived from the traditional capillary bundle approximation. Absolute permeabilities of the network were calculated by finding the flow capacity of the entire flow network, based on parametrized Navier Stokes equations with Klinkenberg correction, applied to each pore-throat-pore arc. A match to the experimental trend was obtained, although the network model considerably underestimated the experimental values. The results were also compared with the semiempirical equations of Thomson et al. and Kozeny and Carmen modified to accept thin section image analysis. Finally, the simulated pore and throat size distributions were compared to proton NMR transverse (T2) spin-echo relaxation times. Although the shapes of the distributions differed markedly, the mean values trended together. The capillary bundle approximation, however, gave a poor match to the NMR data.

Pore size distributions of biodegradable polymer microparticles in aqueous environments measured by NMR cryoporometry

NMR cryoporometry is a unique method permitting the investigation of pores in the microporous and mesoporous regimes for samples in aqueous environments. Here, we apply the technique to porous biodegradable polymer microparticles designed as devices for drug delivery in depot formulations. The results indicate that structural features too small to be captured in surface and fracture images obtained by SEM are able to be accessed using the technique, and that the evolution of pore structure can be studied for several days as the particles swell and degrade in the aqueous environment.