Salt enhanced solvent relaxation and particle surface area determination via rapid spin-lattice NMR

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.

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title: Effects of electrolytes on adsorbed polymer layers: poly(ethylene oxide)-silica system.
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title: NMR Solvent Relaxation in Studies of Multicomponent Polymer Adsorption
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title: Ex situ NMR relaxometry of metal-organic frameworks for rapid surface-area screening.
abstract: Author(s): Chen, Joseph J; Kong, Xueqian; Sumida, Kenji; Manumpil, Mary Anne; Long, Jeffrey R; Reimer, Jeffrey A |  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.

title: NMR relaxation and exchange in metal–organic frameworks for surface area screening
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title: A laboratory study of NMR relaxation times and pore coupling in heterogeneous media
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title: Firing-Induced Microstructural Properties of Quasi-Diamagnetic Carbonate-Based Porous Ceramics: a 1H NMR Relaxation Correlation Study
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title: Pore size analysis from low field NMR spin–spin relaxation measurements of porous microspheres
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title: An NMR technique for the analysis of pore structure: Application to mesopores and micropores
abstract: 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,  (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.

title: 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.

title: What is the surface specific area of porous cement-based material? A nuclear magnetic relaxation dispersion approach.
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title: Relaxation study of cement based grouting material using nuclear magnetic resonance
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title: 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.

title: Changes in Specific Surface as observed by NMR, caused by saturation of Chalk with porewater bearing divalent Ions
abstract: 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.

title: On-line Surface Area Measurement of Concentrated Slurries using low field spin-lattice relaxation NMR
abstract: 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

title: 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.  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.

title: Mineral—solution equilibria in sparingly soluble mineral systems
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title: On-line tracking of the coating of nanoscaled silica with titania nanoparticles via zeta-potential measurements.
abstract: 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.

title: Titanium dioxide nanoparticles in food and personal care products.
abstract: 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).

title: Colloidal particles in competition for stabilizer: a solvent relaxation NMR study of polymer adsorption and desorption.
abstract: 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.

title: Characterization of Multiple Hindered Settling Regimes in Aggregated Mineral Suspensions
abstract: 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...

title: Surface area determination via NMR: Fluid and frequency effects
abstract: 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).

title: 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.

title: Synthesis, Characterization, and Photocatalysis of Well-Dispersible Phase-Pure Anatase TiO2 Nanoparticles
abstract: 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.

title: Size Particle Effects on Lithium Insertion into Sn-doped TiO2 Anatase
abstract: 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...

title: High surface area crystalline titanium dioxide: potential and limits in electrochemical energy storage and catalysis.
abstract: 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.

title: Influence of surface modifiers on the structure of precipitated calcium carbonate
abstract: 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)...