54615002
Salt enhanced solvent relaxation and particle surface area determination via rapid spin-lattice NMR
Abstract This paper demonstrates the influence of surface charge chemistry on the application of nuclear magnetic relaxation measurements (NMR relaxometry) for the in situ determination of particle surface area, in the presence of high electrolyte concentration. Specifically, dispersions of titania, calcite and silica with and without 1 M KCl were investigated. The addition of salt, showed no significant change to relaxation measurements for titanium dioxide; however, a significant rate enhancement was observed for both calcite and silica systems. These differences were attributed to counterion layers forming as a result of the particles surface charge, leading to an increase in the relaxation rate of bound surface layer water. Further, changes appeared to be more pronounced in the silica systems, due to their larger charge. No enhancement was observed for titania, which was assumed to be due to the particles being at their isoelectric point, with no resulting counterion layer formation. Solvent relaxation was further used to successfully determine the surface area of particles in a dispersion using a silica standard reference material, with results compared to Brunauer-Emmett-Teller (BET) and spherical equivalent estimations. Two different dispersions of titanium dioxide, of different crystal phases, were shown to have NMR surface area measurements in good agreement with BET. Thus showing the technique was able to measure changes in surface charge when surface chemistry remained relatively similar, due to the reference silica material also being an oxide. In contrast, the NMR technique appeared to overestimate the calcite surface areas in reference to BET, which was assumed to occur due to both better dispersion in the liquid state of nanocrystallites and potential ion enhancement from the solubility of the calcite. These results highlight the potential of this technique as a fast, non-destructive and non-invasive method for dispersion analysis, but also show the competition between surface area and surface chemistry interactions on measured relaxation rates.
93208421
In situ characterisation of a concentrated colloidal titanium dioxide settling suspension and associated bed development: Application of an acoustic backscatter system
The ability to accurately characterise the settling of particulate suspensions is imperative in numerous industries for improved processing, optimisation and control. Characterisation is typically a non-trivial process, and in situ measurements that remove the restriction of requiring particular sample sizes are preferred. Here, for the first time, we explore the use of an acoustic backscatter system (ABS) to characterise key settling dynamics within a common colloidal mineral suspension, where backscatter attenuation is heightened due to the associated low scattering cross sections of the particles. Settling titanium dioxide dispersions with concentrations ranging from 0.05 to 3.00 vol% were successfully profiled using ABS transducers of 1, 2 and 4 MHz frequencies. This approach enabled the simultaneous visualisation of both the settling interface and sediment bed formation, generating sedimentation curves and elucidating settling velocities. Furthermore, backscatter attenuation was empirically correlated with the attenuation–concentration relationship established for homogenous dispersions, to obtain concentration profiles of the settling suspensions. The data depicted concentration changes as a function of time in a hindered settling suspension, and allowed observation of the segregation of a size polydisperse suspension. Data were compared with sedimentation data obtained via two common ex situ bench scale techniques. Critically, the acoustic backscatter method was validated as a powerful in situ characterisation tool for opaque concentrated heterogeneous dispersions, with the ability to provide concentration density information in conjunction with settling kinetics that are not easily attainable via other methods.
95871370
The use of gas adsorption for the characterization of porous solids
Abstract Although physisorption measurements are widely used for the determination of surface area and pore size distribution, the interpretation of the isotherm data is not always straightforward. The purpose of this review is to examine the scope and limitations of physisorption as a means of characterising porous solids. Microporous adsorbents, having pores no wider than ∼ 2 nm, generally give Type I isotherms; whereas mesoporous adsorbents, with pores in the range 2–50 nm, give Type IV isotherms. As a result of enhanced adsorbent—adsorbate interactions, narrow micropores (of molecular dimensions) are filled with physisorbed molecules at very low p/po; whereas cooperative adsorbate—adsorbate interactions control the filling of the wider micropores over a higher range of p/po. Physisorption in mesopores takes place in two stages: first monolayer—multilayer adsorption on the pore walls; second, capillary condensation with the formation of a curved liquid-like meniscus. In spite of the weakness of the underlying theory, the Brunauer—Emmett—Teller (BET) method remains the most useful means available for the determination of the surface area of mesoporous materials. Methods based on the Kelvin equation are used extensively for the computation of the mesopore size distribution, but in some cases this form of analysis may give a misleading picture of the pore structure. The presence of the hysteresis loop poses a special problem in the evaluation of the pore size distribution. Progress has been made by the application of various empirical methods of isotherm analysis (e.g. the αs-method): in this manner it is possible to utilise adsorption data obtained with non-porous reference materials. Nitrogen adsorption (at ∼ 77 K) is now generally accepted as the preferred standard technique for the routine determination of surface area and mesopore size distribution. However, nitrogen cannot be used to obtain a reliable estimate of the micropore volume or size distribution and a range of other adsorptive molecules is best employed for this purpose. Thus, recent work has shown that it is possible to assess the effective micropore size distribution with probes of different molecular size and analysing the isotherms in terms of the two stages of micropore filling.
94298643
Rapid surface area determination via NMR spin-lattice relaxation measurements
Abstract Specific surface area is one of the most commonly used properties to describe a powder or porous solid. This property is generally measured using gas adsorption techniques. However, NMR spin-lattice relaxation measurements offer numerous potential advantages over sorption techniques. These include speed/sample throughput, elimination of time-consuming out-gassing, lower operating costs (i.e., no gases and liquid nitrogen), and the potential for on-line analysis of slurries. Despite these potential advantages, the application of spin-lattice relaxation measurements for surface area analysis has not been previously demonstrated. In this work, a theoretical and experimental approach has been developed for measuring the surface area of both solid powders and porous solids. Spin-lattice relaxation measurements were made of adsorbed water at 20 MHz and 303 K. Materials studied included three Cab-O-Sil fumed silica powders and three controlled pore glasses with various water contents. The variation of the observed spin-lattice relaxation decay time T1 was found to be linear with both the solids concentration and specific surface area, as predicted by the theory.
44993777
The use of nitrogen adsorption for the characterisation of porous materials
Abstract Problems, which may arise when low-temperature nitrogen adsorption is used for the characterisation of porous materials, are discussed in this review. Continuous or discontinuous manometric techniques can be employed for nitrogen adsorption measurements at 77 K. For pore structure analysis, the nitrogen adsorption–desorption isotherms should be determined over the widest possible range of relative pressure, while allowing for slow equilibration and other operational problems, particularly at very low pressures. In spite of its artificial nature, the Brunauer–Emmett–Teller (BET) method is still used for the determination of surface area. In principle, nitrogen isotherms of Types II and IV are amenable to BET analysis provided that pores of molecular dimensions are absent and that the BET plot is obtained over an appropriate range of the isotherm. An empirical method based on the application of standard adsorption data is useful for checking the validity of the BET-area. All the computational procedures for pore size analysis have limitations of one sort or another. The various assumptions include an ideal pore shape, rigidity of the structure and an oversimplified model (capillary condensation or micropore filling). The derived pore widths and pore volumes should be regarded as effective (or apparent) values with respect to the adsorption of nitrogen at 77 K.
93306555
From powders to sintered pieces: forming, transformations and sintering of nanostructured ceramic oxides
Abstract The quest to produce fully dense nanostructured ceramics has received much attention over the past 10 years. This short review highlights some of the progress made and indicates some of the avenues where further research should prove fruitful. The review concentrates mainly on the attempts to produce nanograined dense ceramics from transition aluminas, where the phase transformation into the thermodynamically stable crystallographic alpha alumina just above 1000 °C plays an important role. The review also restricts itself to traditional low pressure room temperature forming methods and sintering cycles without any applied stress. Highlights for other systems such as yttria and zirconia where dense nanograined ceramics have been successfully produced will also be described and discussed.
28670039
Relaxation NMR as a tool to study the dispersion and formulation behavior of nanostructured carbon materials
Solvent relaxation NMR has been used to estimate the surface areas and wettability of various types of nanostructured carbon materials in a range of solvents including water, ethanol, and tetrahydrofuran. We illustrate the application of the technique through several short case studies using samples including nanocarbon blacks, graphene oxide, nanographites, and porous graphenes. The technique is shown to give a good measure of surface area, correlating well with conventional surface area estimates obtained by nitrogen adsorption, transmission electron microscopy, or light scattering for the non‐porous samples. NMR relaxation has advantages in terms of speed of analysis and being able to use concentrated, wet, and opaque samples. For samples that are porous, two distinct surface areas can be estimated assuming the two environments ('inner' and 'outer') have the same surface chemistry, and that there is a slow exchange of solvent molecules between them. Furthermore, we show that differences in wettability and dispersability between samples dispersed in water, ethanol, and cyclopentanone can be observed, along with changes to the surface chemistry of the interface. Copyright © 2015 John Wiley & Sons, Ltd.
97860116
The use of solvent relaxation NMR to study colloidal suspensions
Solvent relaxation nuclear magnetic resonance has been widely used to study the interactions of polymers and surfactants with nanoparticles, an important area of research for use in a range of industrial formulations, especially with regards to competition effects between components. The ability of the solvent relaxation technique to distinguish between solvent molecules at the surface and those in the bulk solution has been used to obtain valuable information on the interfacial interactions and structure. We focus on systems containing combinations of polymer, surfactant and colloidal particles and illustrate how solvent relaxation measurements have addressed problems of stabilisation, flocculation and depletion in both academically and industrially relevant systems.
97244254
NMR investigations of phase transition in aqueous polymer solutions and gels
Abstract The use of NMR spectroscopy in investigations of phase transitions in aqueous polymer solutions and gels is reviewed. Results on this subject as obtained mostly for thermoresponsive polymers (e.g., poly(N-isopropylacrylamide) and its copolymers, poly(N-isopropylmethacrylamide) and its copolymers, poly(vinyl methyl ether)) from temperature dependences of 1H and 13C NMR spectra, spin–lattice and spin–spin relaxation times, diffusion coefficients and NMR images are discussed.
98008300
Self-diffusion and spin-spin relaxation in blends of linear and cyclic polydimethylsiloxane melts
Self-diffusion and spin-spin relaxation measurements have been performed on a series of blends of narrow-fraction linear and cyclic polydimethylsiloxane polymers in the melt by pulsed nuclear magnetic resonance techniques. The results obtained are compared to the viscosity behaviour on the same samples and discussed in terms of the reptation and Rouse theories commonly invoked for the dynamics of single-component systems. The dynamics of the blend samples are shown to deviate from the expected behaviour above a critical ring size which can be understood through a mechanism based on ring threading.
37179447
Effects of electrolytes on adsorbed polymer layers: poly(ethylene oxide)-silica system.
The effects of various electrolytes on the adsorption of poly(ethylene oxide) onto silica have been studied. The salts were the chlorides of Na+, Mg2+, Ca2+, and La3+. The methods used were adsorption isotherms, found using a depletion method with phosphomolibdic acid, photon correlation spectroscopy, and solvent relaxation NMR. All the salts increased the particle-polymer affinity and adsorbed amount according to the adsorption isotherms, and a linear relationship was found between the initial slope of the isotherms and the ionic strength of the solution. Final adsorbed amounts were approximately 0.4-0.5 mg m(-2). The polymer layer thicknesses as found by PCS were of the same order as the radius of gyration of the polymer and increased with both the concentration and the valency of the salt due to increased adsorption. Solvent relaxation NMR showed that NaCl is too weak to have a noticeable effect on the polymer train layer, but the divalent salts clearly did increase both the strength of solvent binding close to the silica surface and the amount of PEO required to reach the maximum train density.
3140693
Sodium polyacrylate adsorption onto anionic and cationic silica in the presence of salts.
Sodium polyacrylate is well known for its application as a scale inhibitor in common household products, and the effects of both monovalent and divalent metal cations on its structure have been covered by a range of previous publications. In the present article, we extend this work by using solvent relaxation NMR to look at the adsorption of the polyelectrolyte onto both positively and negatively charged silica and how this is altered by calcium chloride. In the anionic case, we found that polyacrylate adsorption was predictably very weak, and interestingly, perhaps counterintuitively, it was further reduced by calcium ions. This is probably linked to NaPA-Ca2+ binding, which changes the conformation and charge of the polyelectrolyte. In contrast, NaPA adsorbs very strongly on cationic silica, to the point that precipitation often occurs, particularly on addition of salt.
33860072
Surfactant/Nonionic polymer interaction. A NMR diffusometry and NMR electrophoretic investigation.
The interaction between the nonionic polymer poly(ethylene oxide) (PEO) of molecular weight 20,000 and surfactants of various types [sodium dodecyl sulfate (SDS), dodecyl trimethylammonium bromide, octyl beta-D-glucoside, and potassium laurate] has been investigated in an aqueous solution at 25 degrees C by 1H NMR pulsed-gradient spin-echo self-diffusion techniques. The SDS/PEO study was further complemented by component-resolved 1H NMR-based studies of the electrophoretic mobility of PEO and the alkyl part of SDS under the same measurement conditions. Through such combined studies, a much more complete picture of the binding and aggregation processes becomes accessible.
93983993
NMR studies in surfactant and polymer-surfactant systems : micelle formation of sodium ω-phenyldecanoate and interaction with poly(ethylene oxide)
Abstract The micellization process of sodium ω-phenyldecanoate and the interaction between this surfactant and poly(ethylene oxide) (PEO) in aqueous solution have been studied, using 1 H NMR spectroscopy. The critical micelle concentration (CMC) and the micellar aggregation number of sodium ω-phenyldecanoate are found to be 0.0121 ± 0.0001 m and 26 ± 1, respectively. Upon micellization, large upfield (low frequency) chemical shifts are observed for the aromatic protons and the methylene protons near the terminal phenyl group, while the shifts of protons attached to methylene carbons near the headgroup are very small. These observations imply that the terminal phenyl group is on average located in the micellar interior. In the presence of PEO, two critical concentrations are observed, corresponding to, respectively, the formation of a polymer-surfactant aggregate and the coexistence of the polymer-surfactant aggregate and surfactant micelle. The first critical concentration, X 1 , is at 0.00885 ± 0.00004 m and is independent of the PEO molecular weight and concentration. The second critical concentration, X 2 , is dependent on PEO concentration. The binding ratio, ( X 2 − CMC)/ C PEO , is 0.44 ± 0.01 ω-phenyldecanoate ion per monomer unit of PEO. In the PEO/ω-phenyldecanoate system large 1 H aromatic ring current-induced shifts are observed for the surfactant protons near or on the terminal phenyl group and for the PEO protons. Very small shifts are observed for the surfactant protons near the head group. These observations suggest that the terminal phenyl group and most of the PEO are in close proximity, and on average in the interior of the polymer-surfactant aggregate.
895650
Competition between polymers for adsorption on silica: a solvent relaxation NMR and small-angle neutron scattering study.
The competition between poly(vinylpyrrolidone) and poly(ethylene oxide) for adsorption at the silica surface was studied by solvent relaxation nuclear magnetic resonance and small-angle neutron scattering. The additive nature of the NMR relaxation rate enhancement was used to observe changes in the train layer when the two polymers were in direct competition for an increasing weight percentage of silica. PVP is shown to displace preadsorbed PEO from the particle surface, and this was observed for a range of PVP molecular weights. SANS measurements were found to give detailed information on the adsorption of the polymers in the separate systems; however, only qualitative information on the adsorption of the polymers could be obtained from the mixed samples. At a total polymer concentration of 0.4% w/v with 1.1% w/v silica, the SANS data were consistent with PVP adsorbing at the surface and dPEO remaining in solution, in agreement with the NMR data.
94879820
NMR Solvent Relaxation in Studies of Multicomponent Polymer Adsorption
NMR spin-spin solvent relaxation measurements have been performed on a series of aqueous dispersions of silica with physisorbed poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP). Enhancements in the specific relaxation rate constant as a result of polymer adsorption were observed. Measurements of the relaxation rate constant as a function of the concentrations of silica and polymer in the system have been used to determine the amount of polymer trains adsorbed at the particle surface, at saturation. Studies of PEO-PVP-silica dispersions have shown that preadsorbed PEO is completely displaced when the silica concentration is small enough to allow the added PVP to saturate the surface. When the concentration of silica is increased beyond this point, then PEO and PVP are able to coexist on the particle surface.
37366431
Ex situ NMR relaxometry of metal-organic frameworks for rapid surface-area screening.
Author(s): Chen, Joseph J; Kong, Xueqian; Sumida, Kenji; Manumpil, Mary Anne; Long, Jeffrey R; Reimer, Jeffrey A | Abstract: Taking a relaxed approach: The NMR relaxation behavior of imbibed solvent has been used to estimate the surface area of a variety of microporous metal-organic frameworks and zeolites. This NMR technique is amenable to automation and can expedite the characterization of microporous materials by identifying any nonporous or low-surface-area structures. Copyright © 2013 WILEY-VCH Verlag GmbH a Co. KGaA, Weinheim.
53581523
NMR relaxation and exchange in metal–organic frameworks for surface area screening
Abstract We describe a robust screening technique that correlates the surface area of metal–organic frameworks to the proton T2 relaxation behavior of imbibed solvent at low field (13 MHz). In frameworks with small pore sizes ( 1 nm) lead to corresponding increases in the rate of solvent exchange, as confirmed by T2 relaxation exchange (REXSY) experiments; increases in the pore size also increases the T2 of the pore-confined solvent. The combination of these two effects results in comparable rates of relaxation and exchange, which precludes the direct analysis of Laplace inversions. Thus, two- and three-site kinetics models were applied to extract porosity from relaxation decays, thereby improving the utility of the porosity screening tool.
50945798
A laboratory study of NMR relaxation times and pore coupling in heterogeneous media
Nuclear magnetic resonance (NMR) relaxation times of geologic materials are closely related to pore geometry. In heterogeneous media, however, the details of this relationship are poorly understood because of a phenomenon known as pore coupling, which arises when diffusing protons sample multiple pores before relaxing. Laboratory experiments allow us to explore whether surface geochemistry can influence pore coupling and how this process affects the observed relaxation-time distribution. Measurements of the NMR response for microporous silica gel packs, treated with varying amounts of surface-coating iron, demonstrate that samples with less iron exhibit stronger pore coupling than those with abundant iron. When pore coupling is strong, the relaxation-time distribution grossly misrepresents the underlying bimodal pore-size distribution of micropores and macropores. Specifically, the bimodal relaxation-time distribution becomes merged and the relative amplitude of the peaks fails to reflect the true macropore and micropore volume. A reduction in pore coupling, observed with increasing iron content, is attributed to a decrease in the distance protons are able to diffuse before relaxing. Basic parameters describing the shape of the relaxation-time distributions for this range of samples are well-predicted by a 1D analytical model. Experimental results conclusively demonstrate that surface geochemistry is an important factor determining the degree to which pore coupling occurs and illustrate how this phenomenon can affect the interpretation of NMR relaxation measurements in heterogeneous porous media.
92676298
Firing-Induced Microstructural Properties of Quasi-Diamagnetic Carbonate-Based Porous Ceramics: a 1H NMR Relaxation Correlation Study
This study deals with the application of two-dimensional proton nuclear magnetic resonance relaxometry (2D 1H NMR-R) to the characterization of porous ceramics nearly free of magnetic compounds. Different microstructural properties were obtained by firing a diamagnetic mixture of kaolin, calcium, and magnesium carbonate over a wide range of maximum temperatures (600–1100 °C) and firing times at the maximum temperature (soaking times) (0–10 h). The 2D 1H NMR-R method relies on the correlated measurement of 1H longitudinal (T1) and transverse (T2) relaxation times of pore-filling water by which the properties of the interconnected pore space may be investigated. In the absence of significant magnetic susceptibility effect due to para- and ferro-magnetic compounds, the 2D 1H NMR-R maps allow studying the conjoint effects on pore size distribution and inter-pore coupling due to the variations in both time and temperature of firing. The NMR experiments were performed with a low-field 1H NMR sensor, which allows non-destructive and in situ analysis. For ceramic specimens fired at 600 and 700 °C, the fraction of smallest pores increases with firing time at the expenses of those with intermediate size. The pore shrinkage occurring at this stage, and likely associated with the transformation of kaolinite in metakaolinite, is affected in a similar way by soaking time and firing temperature, in line with the concept of equivalent firing temperature. At temperatures from 800 to 1100 °C, the structural modifications involving interconnectivity and average pore size are driven primarily by firing temperature and, secondarily, by soaking time. The 2D 1H NMR-R results are confirmed by more traditional, but destructive, mineralogical, and structural analyses like X-ray powder diffraction, helium pycnometry, mercury intrusion porosimetry, and nitrogen adsorption/desorption method.
135700300
Pore size analysis from low field NMR spin–spin relaxation measurements of porous microspheres
Abstract
The porous structure characteristics of twenty-one porous microspheres with agarose framework were investigated based on the low-field nuclear magnetic resonance (LF NMR) relaxation time (T2) distribution measurements. The feasibility of the technique was confirmed by direct relationship between T2 and the mean pore size of the polymer networks (Rh) of agarose hydrogel which was established using the “Fiber-Cell” model. The expected pore radius distribution curve was obtained and the reliability was validated by comparing them with the results obtained by inverse size-exclusion chromatography. Thereafter, the technique was applied to characterize the pore size distribution (PSD) of soft microspheres and the pore size transformation of microspheres with or without grafted polymers in the process of protein adsorption. All of these results strongly support LF NMR as a promising, rapid and nondestructive technique for the determination of PSDs in many kinds of soft porous microspheres.
29456906
An NMR technique for the analysis of pore structure: Application to mesopores and micropores
The use of NMR spin-lattice relaxation experiments on water, or other proton-containing fluids, in a porous solid as a pore structure/size measurement tool has been recently reported for porous solids with pore size greater then ≈5 nm. This approach has numerous advantages, as compared to nitrogen sorption/condensation and mercury porosimetry, including no network/percolation effects, no pore shape assumption is required, wet materials may be analyzed, and small specific pore volumes may be studied. Assumptions used to relate the measured spin-lattice relaxation time, T1, to the pore size limit the application of the method to pore sizes greater than 5 nm. In this work, we explore the extension of this NMR technique to porous solids with pores in the micropore and mesopore size ranges. 
 
By comparing previously published results of the relative distribution of surface- and bulk-phase water as a function of pore size (E. Almagor and G. J. Belfort, J. Colloid Interface Sci. 66, 146, (1978)) to predictions from various pore models, we have demonstrated that the “two-fraction, fast exchange” model may be applied to relate measured T1 to pore size in the mesopore and micropore size range if the pore geometry is known (or assumed). For pore size greater than 5 nm, pore geometry is not important. Comparisons between conventional techniques, such as nitrogen sorption and mercury porosimetry, and the results of NMR spin-lattice relaxation experiments are complicated by the fact that different pore structure parameters are measured with these methods. Despite this, agreement between the methods is quite good for the five sol-gel-derived materials which we have studied.
97849634
A NMR technique for the analysis of pore structure: Application to materials with well-defined pore structure
Abstract The application of NMR spin—lattice relaxation measurements of water contained in porous solids is investigated as a possible tool for the determination of pore size distributions. NMR methods have several potential advantages over conventional porosimetry/adsorption techniques including the study of wet porous solids, a wide range of sample sizes that may be studied, and the fact that no pore shape assumption is required. In principle, water contained in a pore will relax faster than bulk water. This decrease in the observed relaxation rate decay constant, T1, is directly related to the pore volume to surface area ratio (i.e., hydraulic radius) according to the “two-fraction, fast-exchange model.” However, the suitability of this model and NMR spin—lattice relaxation measurements for the determination of pore size distributions has not been previously demonstrated. Pore size distributions have been determined for two series of porous solids by mercury porosimetry and a NMR method. A series of four controlled-pore glasses with very narrow pore size distributions and a mean radius in the range 3.4 to 17.6 nm were studied at a frequency of 20 MHz and at four temperatures. By using four samples with similar surface chemistry/ pore shape, the validity of the two-fraction, fast-exchange model was assessed. Experiments were conducted using a 180°–τ-90° pulse sequence and the distribution of T1 was calculated using a nonnegative leastsquares routine (NNLS) presented in a previous paper. The two-fraction model described the change of mean T1 with pore radius at a given temperature. The change of the surface relaxation decay constant, T1s, fit an Arrhenius expression with an activation energy of 1.8 kcal/mole. Agreement between the mercury porosimetry and NMR derived pore size distributions is excellent. T1 measurements at both 20 and 300 MHz and 303 K were also made on a series of porous solids fabricated by pelleting submicron silica spheres. These solids exhibit afairly wide pore size distribution (⋍ one order of magnitude). The two-fraction, fast-exchange model satisfactorily described the relationship between pore size and T1. Agreement between mercury porosimetry and NMR pore size distributions is good. However, the NMR derived distributions consisted of a series of narrow peaks overlapping the single broad porosimetry peak. The surface relaxation effect, and, hence, the overall pore size sensitivity, is increased by a factor of 3 when the proton frequency is decreased from 300 to 20 MHz.
46576191
What is the surface specific area of porous cement-based material? A nuclear magnetic relaxation dispersion approach.
We propose a new NMR method to measure and follow the evolution of the surface specific area, Sp, of a porous cement-based material. This method, that does not require any preliminary drying process, uses nuclear magnetic relaxation dispersion (NMRD), the measurement of spin-lattice relaxation rate as a function of magnetic field strength or nuclear Larmor frequency. The method is applied for three different mortars samples prepared by mixing cement, sand, silica fume, water and superplasticizer with a water to cement ratio w/c=0.25, 0.38 and 0.65, respectively. The evolution of Sp grows linearly with the degree of advancement of chemical reactions measured by thermal heating and we evidence two relaxation processes independent of the w/c ratio.
94289876
Relaxation study of cement based grouting material using nuclear magnetic resonance
Abstract Aiming at actual condition of poor effect of hole sealing for the reason of poor cement paste fluidity in the process of coal mine gas drainage, by adding a water reducing agent, cement paste for hole sealing was produced. The changes of initial distribution, weighted average values and total relaxation signal intensity of transverse relaxation time ( T 2 ) of water in pure cement paste and water reducing agent added cement paste were studied with low field proton nuclear magnetic resonance (NMR). The results show that there are four peaks in T 2 distribution curves of cement paste: the first peak is related to the bound water in flocculation, the second and the third peaks are related to the water in flocculation, water reducing agent makes it extending towards the long relaxation time, increasing its liquidity, and the fourth peak is related to the free water. By using weighted average values of T 2 and total relaxation signal intensity, hydration process of cement pastes could be roughly divided into four stages: the initial period, reaction period, accelerated period and steady period. By analyzing the periods, it makes sure that the grouting process should be completed in the reaction period in the site, and the drainage process should be started in the steady period. The results have great guiding significance to the hole sealing and methane drainage.
9047953
A 1H NMR relaxation study of hydration water in polyelectrolyte mono and multilayers adsorbed to colloidal particles
Abstract The adsorption of polyelectrolytes to charged surfaces can be applied to build up multilayers of polyelectrolytes of alternating charge by layer-by-layer assembly. Here, poly-(allylamine hydrochloride) (PAH) is adsorbed to colloidal latex particles in aqueous solution, and to particles precoated with four layers of polyelectrolytes. The method of 1H NMR transverse relaxation is applied to monitor the hydration water in such layers, where relaxation rates R2 serve as a parameter describing the average immobilisation of water molecules. Electrophoretic and relaxation measurements are performed as a function of polymer coverage, surface precoating and salt content in the solution. While the zeta-potential increases monotonously with initial polymer concentration due to a charge reversal from a negatively to a positively charged surface, pronounced structures occur in the R2 curve for monolayers, which are attributed to the contributions arising from different water populations. For adsorption to multilayers, the electrophoretic curves are shifted to higher concentrations, and relaxation rates show an unexpectedly high water immobilisation. The electrophoretic and relaxation data are consistent with a molecular picture of rodlike chains in the absence of salt, and a coil shaped structure for adsorption from salt solution. In both cases, the properties of the fifth layer differ significantly from the first one, this is attributed to the entanglement of adsorbing chains in the precoated layers, leading to larger adsorbed amounts.
17924695
Changes in Specific Surface as observed by NMR, caused by saturation of Chalk with porewater bearing divalent Ions
Nuclear Magnetic Resonance (NMR) spectrometry has proved to be a good technique for determining the petrophysical properties of reservoir rocks; such as porosity and pore size distribution. We investigated how pore water rich in divalent ions affect the NMR signal from chalk with two different depositional textures. We compared two cases. The first experiments on outcrop chalk with high salinity brines showed that saturation with divalent ions (Mg 2+ ,Ca 2+ and SO4 2- ) cause major shifts in the T2 distribution curve, probably due to precipitation in the pore space. In a second set of experiments, fluid samples where precipitation takes place were found to show shifts in the T2 relaxation curve due to the creation of crystals. We were able to identify how differences in the rock texture and precipitants within the pore space may affect the transverse relaxation time by altering the surface-to-volume ratio of the pore space. The results of this work could benefit the ongoing study on the optimization of the water composition for Enhanced Oil Recovery (EOR) methods and shed light on how it can affect the mechanical and physical properties of the rock.
97751760
On-line Surface Area Measurement of Concentrated Slurries using low field spin-lattice relaxation NMR
A method for the rapid on-line determination of surface area and solids content in flowing concentrated slurries using low field NMR spin-lattice relaxation measurements has been developed and demonstrated. The relationship between flow and spin-lattice relaxation time (T 1 ) of protons in water at 20 MHz was examined using aqueous copper sulfate solutions. The ability to measure surface area and solids concentration in both stagnant (stopped flow) and flowing systems via NMR was demonstrated using several different concentrated aqueous titania and glass slurries (20 to 80 weight percent) for which the dried powder surface area was previously determined via nitrogen adsorption/BET analysis and the solids content determined gravimetrically. Surface areas were also calculated from particle size analysis and found to vary by up to an order of magnitude from the adsorption and NMR results
96972702
The influence of pH and NaCl on the zeta potential and rheology of anatase dispersions
Abstract The rheological behaviour of anatase dispersions was compared with the ζ potential, when the pH (from 3 to 10) and sodium chloride concentration (from 0 to 1 mol dm−3) was varied over a wide range. The isoelectric point, pHiep, is shifted to higher values with increasing salt concentration until a charge reversal was no longer observed at a concentration of 1 mol dm−3 NaCl. The rheological properties were analyzed in the form of the critical yield stress and the shear stress and viscosity at arbitrarily selected high and low shear rates, respectively. The maximum of the rheological properties was found to correlate with to the pHiep. The electrokinetic and rheological features of the system were related to a recent theory of Hunter et al. [J. Colloid Int. Sci. 28 (1968) 250] by plotting the yield stress against the square of the ζ potential for each salt concentration. The acidic branch, i.e. when pH pHiep), however, the curves did not show the expected linear behaviour. Also when comparing the influence of pH and sodium chloride concentration on the rheology, it was found that on the acidic yield stress branch the dependency of pH and salt concentration was very different from the one on the alkaline branch. On the acidic branch, low ionic strengths shared a common straight line. On the contrary, the alkaline branch had separate linear dependencies for each salt concentration. Moreover, the lines on the alkaline side seemed to be quite parallel for medium salt concentrations.
51807097
Mineral—solution equilibria in sparingly soluble mineral systems
Abstract Recent solubility, electrokinetic, spectroscopic and flotation studies in our laboratory on apatite and calcite in aqueous electrolytes indicate complex mineral—solution interactions which can alter surface characteristics of these minerals markedly in the presence of each other. In the case of systems containing such sparingly soluble minerals, understanding of the mineral—solution chemical equilibria under different physico-chemical conditions becomes important for developing schemes for processing them. The chemical equilibria in heterogeneous systems involving salt-type minerals are investigated in this work. Stability relations have been analysed on the basis of bulk equilibria for calcite, apatite, magnesite, calcite—apatite, calcite—dolomite, apatite—dolomite, gypsum—anhydrite, calcite—gypsum, calcite—apatite—dolomite and calcite—magnesite— dolomite systems. The role of atmospheric carbon dioxide, which under most laboratory experimental conditions cannot be avoided, has been emphasised. Theoretical results have been correlated with experimental data reported in the literature.
46042398
On-line tracking of the coating of nanoscaled silica with titania nanoparticles via zeta-potential measurements.
Nanoscaled spherical silica particles were directly coated with the titania nanoparticles by means of a heterogenic coagulation. Silica was prepared by the Stöber method, titania by a hydrolysis-condensation reaction of tetrapropylorthotitanate under acidic conditions. The on-line tracking of the coating process was performed by measuring the change in zeta potential during the gradual addition of a titania sol to the spherical silica particles. Silica particles of various sizes were used to determine the consumption of the titania sol in the dependence upon the particle size. The coated and uncoated particles were characterized by zeta-potential measurements, acoustic attenuation spectroscopy, dynamic light scattering, and scanning electron microscopy.
4056289
Titanium dioxide nanoparticles in food and personal care products.
Titanium dioxide is a common additive in many food, personal care, and other consumer products used by people, which after use can enter the sewage system and, subsequently, enter the environment as treated effluent discharged to surface waters or biosolids applied to agricultural land, incinerated wastes, or landfill solids. This study quantifies the amount of titanium in common food products, derives estimates of human exposure to dietary (nano-) TiO(2), and discusses the impact of the nanoscale fraction of TiO(2) entering the environment. The foods with the highest content of TiO(2) included candies, sweets, and chewing gums. Among personal care products, toothpastes and select sunscreens contained 1% to >10% titanium by weight. While some other crèmes contained titanium, despite being colored white, most shampoos, deodorants, and shaving creams contained the lowest levels of titanium (<0.01 μg/mg). For several high-consumption pharmaceuticals, the titanium content ranged from below the instrument detection limit (0.0001 μg Ti/mg) to a high of 0.014 μg Ti/mg. Electron microscopy and stability testing of food-grade TiO(2) (E171) suggests that approximately 36% of the particles are less than 100 nm in at least one dimension and that it readily disperses in water as fairly stable colloids. However, filtration of water solubilized consumer products and personal care products indicated that less than 5% of the titanium was able to pass through 0.45 or 0.7 μm pores. Two white paints contained 110 μg Ti/mg while three sealants (i.e., prime coat paint) contained less titanium (25 to 40 μg Ti/mg). This research showed that, while many white-colored products contained titanium, it was not a prerequisite. Although several of these product classes contained low amounts of titanium, their widespread use and disposal down the drain and eventually to wastewater treatment plants (WWTPs) deserves attention. A Monte Carlo human exposure analysis to TiO(2) through foods identified children as having the highest exposures because TiO(2) content of sweets is higher than other food products and that a typical exposure for a US adult may be on the order of 1 mg Ti per kilogram body weight per day. Thus, because of the millions of tons of titanium-based white pigment used annually, testing should focus on food-grade TiO(2) (E171) rather than that adopted in many environmental health and safety tests (i.e., P25), which is used in much lower amounts in products less likely to enter the environment (e.g., catalyst supports, photocatalytic coatings).
38973230
Colloidal particles in competition for stabilizer: a solvent relaxation NMR study of polymer adsorption and desorption.
The competitive adsorption of poly(vinylpyrrolidone) onto silica and alumina-modified silica particles was studied using solvent relaxation nuclear magnetic resonance. The additive nature of the measured relaxation rate enabled predictions to be made of the relaxation rate in different polymer adsorption scenarios. Preferential adsorption of the poly(vinylpyrrolidone) onto the unmodified silica particles occurred when there was insufficient polymer in the system to coat the entire available surface area. Desorption was also found to occur when the polymer was initially adsorbed upon the alumina-modified particle and silica particles were added.
67769146
Characterization of Multiple Hindered Settling Regimes in Aggregated Mineral Suspensions
Aqueous suspensions of magnesium hydroxide are shown to exhibit low ζ-potential behavior and highly complex settling dynamics. Two distinct regimes of hindered settling behavior are observed on either side of a threshold concentration, ϕ*, of 2.38% v/v, which is considerably below the gel point, ϕg, observed at 5.4 ± 1.6% v/v. The low-concentration regime was characterized by a very large Richardson and Zaki exponent of 146, a factor of 10 larger than that of the high-concentration regime. Michaels and Bolger analysis of the low-concentration regime implies settling governed by large, low-density macroaggregates of 138–147 μm diameter and low intraaggregate packing fractions on the order of 0.05, which is in good agreement with in situ particle characterization undertaken using particle vision and measurement (PVM) and focused-beam reflectance measurements (FBRM). The large macroaggregates must undergo some shear densification within the higher-concentration hindered settling regime in order for the suspe...
96385330
Surface area determination via NMR: Fluid and frequency effects
Recently, the use of NMR spin-lattice relaxation measurements has been proposed as a tool for the determination of specific surface area. The use of this approach, which has several advantages and disadvantages as compared to conventional methods, was demonstrated using relaxation measurements of water at 20 MHz for two materials with similar surface chemistry. In principle, the surface-enhanced spin-lattice relaxation rate is a function of proton frequency, fluid, temperature and surface chemistry. In this paper, we describe relaxation measurements at 303 K using a range of fluids (water, methanol, ethanol, cyclohexane) at two proton frequencies (20 and 300 MHz) for a series of samples with different surface areas from three different porous solid groups (controlled pore glass, mixed ester cellulose membranes and carbon blacks). Surface area of samples studied were in the range of 4.4 to 179.0 m2/g. A three-fold increase in the sensitivity of the surface area determination is noted when the frequency is decreased from 300 to 20 MHz. In general, surface area sensitivity increases with fluid type as (cyclohexane ⪡ water < methanol < ethanol).
53491133
Preparation of high surface area TiO (anatase) by thermal hydrolysis 2 of titanyl sulphate solution
Abstract Highly crystalline, ultra fine TiO2 (anatase) having high surface area has been prepared by thermal hydrolysis of titanyl sulphate solution and characterized using B.E.T surface area measurements, XRD and chemical analysis. The dependence of surface area on concentration of staffing solution, temperature of hydrolysis, duration of boiling and calcination temperature were also studied. As the boiling temperature, duration of boiling and calcination temperature increased, the surface area of TiO2 formed decreased significantly. On increasing calcination temperature, the crystallite size of TiO2 also increased and gradually the phase transformation to rutile took place. The onset and completion temperatures of rutilation were 700 and 1000°C, respectively.
97271471
Synthesis, Characterization, and Photocatalysis of Well-Dispersible Phase-Pure Anatase TiO2 Nanoparticles
High-purity anatase TiO2 nanoparticles were prepared using an improved sol-hydrothermal method. The as-prepared sample was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), and UV-vis diffuse reflectance spectra. TEM results showed that the average particle size of all TiO2 particles was calculated to be () nm. The XRD analysis indicated that the present sample was fully crystallized and appeared to be highly phase-pure anatase. The BET analysis showed that the as-prepared sample had a very large specific surface area of 186.25 m2/g. The photocatalytic performance of TiO2 nanoparticles was evaluated by photocatalytic degradation of X-3B and X-BR solutions. The degradation results revealed that the as-prepared TiO2 showed slightly higher photocatalytic activities than P25. Whereas, the as-synthesized TiO2 can settle down and be separated easily after the photocatalytic reaction finishes.
197177408
Size Particle Effects on Lithium Insertion into Sn-doped TiO2 Anatase
Lithium insertion into Sn-doped TiO2 anatase was studied in order to clarify the mechanism responsible for the first plateau observed at 1.75 V vs Li. One of the other aims of this study was to get deeper insight into the process responsible for the appearance of a first domain observed at a small amount of lithium (before the plateau) and a third domain between 1.7 and 1.2 V (after the plateau). Pure and Sn-substituted anatase were synthesized by several synthetic methods with different precursors and solvents. These materials are expected to have different electrochemical properties because crystallite size modifies the Li-insertion process. Their electrochemical behavior is discussed in order to establish a relationship between the materials properties and the electrochemical performances. Three processes have been identified from both X-ray diffraction and 119Sn Mossbauer spectroscopy: (i) topotactic insertion into the Li-poor compound LixTiO2, with the x value depending on the particle size; (ii) a t...
13930948
High surface area crystalline titanium dioxide: potential and limits in electrochemical energy storage and catalysis.
Titanium dioxide is one of the most intensely studied oxides due to its interesting electrochemical and photocatalytic properties and it is widely applied, for example in photocatalysis, electrochemical energy storage, in white pigments, as support in catalysis, etc. Common synthesis methods of titanium dioxide typically require a high temperature step to crystallize the amorphous material into one of the polymorphs of titania, e.g. anatase, brookite and rutile, thus resulting in larger particles and mostly non-porous materials. Only recently, low temperature solution-based protocols gave access to crystalline titania with higher degree of control over the formed polymorph and its intra- or interparticle porosity. The present work critically reviews the formation of crystalline nanoscale titania particles via solution-based approaches without thermal treatment, with special focus on the resulting polymorphs, crystal morphology, surface area, and particle dimensions. Special emphasis is given to sol-gel processes via glycolated precursor molecules as well as the miniemulsion technique. The functional properties of these materials and the differences to chemically identical, non-porous materials are illustrated using heterogeneous catalysis and electrochemical energy storage (battery materials) as example.
95465167
Influence of surface modifiers on the structure of precipitated calcium carbonate
A calcium carbonate (CaCO3) sorbent synthesized in the presence of surface modifiers has recently been developed in this laboratory for the purpose of enhancing the reactivity toward SO2 at high temperatures (800−1150 °C). The calcium carbonate particles are produced by precipitation from an aqueous suspension of calcium hydroxide by bubbling pure CO2. The operating parameters such as suspension concentration, gas flow rate, and concentration of surface modifiers are varied to study their influence on the particle size, surface area, and pore volume. The synthesized CaCO3 particles exhibit dramatically high SO2 reactivity and sorbent utilization. The increased reactivity is primarily due to the development of an open initial pore structure of the sorbent brought about by the action of surface-active agents, compared to the conventional calcium-based sorbents. ζ-potential measurements indicate that addition and subsequent adsorption of anionic surfactants neutralize the high positive surface charge (30 mV)...