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  • 1. Hellrup, Joel
    et al.
    Holmboe, Michael
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Nartowski, Karol P.
    Khimyak, Yaroslav Z.
    Mahlin, Denny
    Structure and Mobility of Lactose in Lactose/Sodium Montmorillonite Nanocomposites2016In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 32, no 49, p. 13214-13225Article in journal (Refereed)
    Abstract [en]

    This study aims at investigating the molecular level organization and molecular mobility in montmorillonite nanocomposites with the uncharged organic low-molecular-weight compound lactose commonly used in pharmaceutical drug delivery, food technology, and flavoring. Nanocomposites were prepared under slow and fast drying conditions, attained by drying at ambient conditions and by spray-drying, respectively. A detailed structural investigation was performed with modulated differential scanning calorimetry, powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, scanning electron microscopy, microcalorimetry, and molecular dynamics simulations. The lactose was intercalated in the sodium montmorillonite interlayer space regardless of the clay content, drying rate, or humidity exposure. Although, the spray-drying resulted in higher proportion of intercalated lactose compared with the drying under ambient conditions, nonintercalated lactose was present at 20 wt % lactose content and above. This indicates limitations in maximum loading capacity of nonionic organic substances into the montmorillonite interlayer space. Furthermore, a fraction of the intercalated lactose in the co-spray-dried nanocomposites diffused out from the clay interlayer space upon humidity exposure. Also, the lactose in the nanocomposites demonstrated higher molecular mobility than that of neat amorphous lactose. This study provides a foundation for understanding functional properties of lactose/Na-MMT nanocomposites, such as loading capacity and physical stability.

  • 2. Holmboe, Michael
    et al.
    Bourg, Ian C.
    Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 2, p. 1001-1013Article in journal (Refereed)
    Abstract [en]

    The diffusion coefficients (D) of water and solutes in nanoporousNa-smectite clay barriers have been widely studied because of their importancein high-level radioactive waste (HLRW) management and in the isolation of contaminated sites. However, few measurements have been carried out at the high temperatures that are expected to occur in HLRW repositories. We address this knowledge gap by using molecular dynamics (MD) simulations to predict the temperature dependence of diffusion in clay interlayer nanopores, expressed as a pore scale activation energy of diffusion (Ea). Our sensitivity analysis shows that accurate prediction of pore scale Dand Eavalues requires careful consideration of the influence of pore size, simulation cell size, and clay structure flexibility on MD simulation results. We find that predicted Dvalues in clay interlayer nanopores are insensitive to the size of the simulation cell (contrary to the behavior observed in simulation of bulk liquid water) but sensitive to the vibrational motions of clay atoms (particularly in the smallest pores investigated here, the one-, two-, and three-layer hydrates). Our predicted DandEavalues are consistent with experimental data. They reveal,for both water and Na+, that Eaincreases by∼6 kJ mol−1with increasing confinement, when going from bulk liquid water to theone-layer hydrate of Na-montmorillonite.

  • 3. Holmboe, Michael
    et al.
    Jonsson, Mats
    Wold, Susanna
    Influence of γ-radiation on the reactivity of Montmorillonite towards H2O22012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 81:2, s. 190-194, p. 1001-1013Article in journal (Other academic)
    Abstract [en]

    Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH•, e-(aq), H•, H202, H2, H02•, H30+), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/Fe(III) content in montmorillonite and its reactivity towards hydrogen peroxide (H2O2) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/FeTot ratio of dispersed montmorillonite are increased from ≤ 3 to 25-30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/FeTot ratio and the H2O2 decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H2O2 additions, since the structural Fe(II)/FeTot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H2O2.

  • 4. Holmboe, Michael
    et al.
    Karin Norrfors, Knapp
    Jonsson, Mats
    Wold, Susanna
    Effect of γ-radiation on radionuclide retention in compacted bentonite2011In: Radiation Physics and Chemistry, ISSN 0969-806X, E-ISSN 1879-0895, Vol. 80, no 12, p. 1371-1377Article in journal (Refereed)
    Abstract [en]

    Compacted bentonite is proposed as an engineered barrier in many concepts for disposal of high level nuclear waste. After the initial deposition however, the bentonite barrier will inevitably be exposed to ionizing radiation (mainly γ) under anoxic conditions. Because of this, the effects of γ-radiation on the apparent diffusivity values and sorption coefficients in bentonite for Cs+ and Co2+ were tested under different experimental conditions. Radiation induced effects on sorption were in general more noticeable for Co2+ than for Cs+, which generally showed no significant differences between irradiated and unirradiated clay samples. For Co2+ however, the sorption to irradiated MX80 was significantly lower than to the unirradiated clay samples regardless of the experimental conditions. This implies that γ-radiation may alter the surface characteristics contributing to surface complexation of Co2+. With the experimental conditions used, however, the effect of decreasing sorption was not large enough to be reflected on the obtained Da values.

  • 5.
    Holmboe, Michael
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Department of Pharmacy, Uppsala University, Uppsala Biomedical Centre, Uppsala, Sweden.
    Larsson, Per
    Anwar, Jamshed
    Bergström, Christel A.S.
    Partitioning into Colloidal Structures of Fasted State Intestinal Fluid Studied by Molecular Dynamics Simulations2016In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 32, no 48, p. 12732-12740Article in journal (Refereed)
    Abstract [en]

    We performed molecular dynamics (MD) simulations to obtain insights into the structure and molecular interactions of colloidal structures present in fasted state intestinal fluid. Drug partitioning and interaction were studied with a mixed system of the bile salt taurocholate (TCH) and 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLiPC). Spontaneous aggregation of TCH and DLiPC from unconstrained MD simulations at the united-atom level using the Berger/Gromos54A7 force fields demonstrated that intermolecular hydrogen bonding between TCH molecules was an important factor in determining the overall TCH and DLiPC configuration. In bilayered systems, these intermolecular hydrogen bonds resulted in embedded transmembrane TCH clusters. Free energy simulations using the umbrella sampling technique revealed that the stability of these transmembrane TCH clusters was superior when they consisted of 3 or 4 TCH per bilayer leaflet. All-atom simulations using the Slipids/GAFF force fields showed that the TCH embedded in the bilayer decreased the energy barrier to penetrate the bilayer (ΔGpen) for water, ethanol, and carbamazepine, but not for the more lipophilic felodipine and danazol. This suggests that diffusion of hydrophilic to moderately lipophilic molecules through the bilayer is facilitated by the embedded TCH molecules. However, the effect of embedded TCH on the overall lipid/water partitioning was significant for danazol, indicating that the incorporation of TCH plays a crucial role for the partitioning of lipophilic solutes into e.g. lipidic vesicles existing in fasted state intestinal fluids. To conclude, the MD simulations revealed important intermolecular interactions in lipidic bilayers, both between the bile components themselves and with the drug molecules.

  • 6. Holmboe, Michael
    et al.
    Wold, Susanna
    Jonsson, Mats
    COLLOID DIFFUSION IN COMPACTED BENTONITE: MICROSTRUCTURAL CONSTRAINTS2010In: Clays and clay minerals, ISSN 0009-8604, E-ISSN 1552-8367, Vol. 58, no 4Article in journal (Refereed)
    Abstract [en]

    In Sweden and in many other countries, a bentonite barrier will be used in the repository for spent nuclear fuel. In the event of canister failure, colloidal diffusion is a potential, but scarcely studied mechanism of radionuclide migration through the bentonite barrier. Column and in situ experiments are vital in understanding colloid diffusion and in providing information about the microstructure of compacted bentonite and identifying cut-off limits for colloid filtration. This study examined diffusion of negatively charged 2-, 5-, and 15-nm gold colloids in 4-month diffusion experiments using MX-80 Wyoming bentonite compacted to dry densities of 0.6–2.0 g/cm3. Breakthrough of gold colloids was not observed in any of the three diffusion experiments. In a gold-concentration profile analysis, colloid diffusion was only observed for the smallest gold colloids at the lowest dry density used (estimated apparent diffusivity Da ≈5×10−13 m2/s). The results from a microstructure investigation using low-angle X-ray diffraction suggest that at the lowest dry density used, interlayer transport of the smallest colloids cannot be ruled out as a potential diffusion pathway, in addition to the expected interparticle transport. In all other cases, with either greater dry densities or larger gold colloids, compacted bentonite will effectively prevent diffusion of negatively charged colloids due to filtration.

  • 7. Holmboe, Michael
    et al.
    Wold, Susanna
    Jonsson, Mats
    Porosity investigation of compacted bentonite using XRD profile modeling2012In: Journal of Contaminant Hydrology, ISSN 0169-7722, E-ISSN 1873-6009, Vol. 128, no 1–4, p. 19-32Article in journal (Refereed)
    Abstract [en]

    Many countries intend to use compacted bentonite as a barrier in their deep geological repositories for nuclear waste. In order to describe and predict hydraulic conductivity or radionuclide transport through the bentonite barrier, fundamental understanding of the microstructure of compacted bentonite is needed. This study examined the interlayer swelling and overall microstructure of Wyoming Bentonite MX-80 and the corresponding homo-ionic Na+ and Ca2 + forms, using XRD with samples saturated under confined swelling conditions and free swelling conditions. For the samples saturated under confined conditions, the interparticle, or so-called free or external porosity was estimated by comparing the experimental interlayer distances obtained from one-dimensional XRD profile fitting against the maximum interlayer distances possible for the corresponding water content. The results showed that interlayer porosity dominated total porosity, irrespective of water content, and that the interparticle porosity was lower than previously reported in the literature. At compactions relevant for the saturated bentonite barrier (1.4–1.8 g/cm3), the interparticle porosity was estimated to ≤ 3%.

  • 8. Holmboe, Michael
    et al.
    Wold, Susanna
    Jonsson, Mats
    García-García, Sandra
    Effects ofγ-irradiation on the stability of colloidal Na+-Montmorillonite dispersions2008In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 43, p. 86-90Article in journal (Refereed)
    Abstract [en]

    In many concepts forfinal storage of spent nuclear fuel bentonite will be used as an engineered barrier,mainly due to its inertness, plasticity and ability to retard transport of radionuclides by adsorption. In theevent of water-bearing fractures making contact with the bentonite barrier, generation and transport ofcolloidal particles will strongly depend on groundwater composition and the surface properties of thecolloidal particles. The bentonite barrier will unavoidably be exposed to ionizing radiation from the spentnuclear fuel but very little is known about effects of ionizing radiation on bentonite concerning colloidalstability. In this work we have studied the effect ofγ-radiation on the stability of dilute colloidal Na+-montmorillonite dispersions using a Cs-137γ-source (doses of 0–53.2 kGy).Aggregation kinetics and sedimentation experiments revealed significant radiation effects, evident asincreased colloid stability. The only rationale for this is aγ-radiation induced increase in surface potential.The effects appeared to depend on the Na+-montmorillonite concentration in the irradiated dispersions,indicating that the change in surface potential is induced by aqueous radiolysis products.

  • 9. Holmboe, Michael
    et al.
    Wold, Susanna
    Petterson, Torbjörn
    Effects of the injection grout Silica sol on bentonite2011In: Physics and Chemistry of the Earth, ISSN 1474-7065, E-ISSN 1873-5193, Vol. 36, no 17–18, p. 1580-1589Article in journal (Refereed)
    Abstract [en]

    Silica sol, i.e., colloidal SiO2, may be used as a low-pH injection grout for very fine fractures in the construction of deep geological repositories for radioactive waste in Sweden and in Finland. If the bentonite barrier encounters SiO2-colloid particles under conditions favorable for aggregation, there is concern that it will modify the bentonite barrier at the bentonite/bedrock interface. In this study qualitative experiments were performed with mixed dispersions of SiO2-colloids and bentonite or homo-ionic Na/Ca-montmorillonite. Samples were prepared at different colloid concentrations and treated under various conditions such as low and high ionic strength (0.3 M NaCl), as well as dehydration and redispersing. Free swelling and settling experiments were performed in order to qualitatively compare the conditions in which SiO2-colloids affect the bulk/macro properties of bentonite. In order to study specific SiO2-colloid/montmorillonite interactions and preferred type of initial aggregation, dilute dispersions of homo-ionic montmorillonite dispersions mixed with varying concentrations of SiO2-colloids were prepared and selected samples were characterized by PCS, SEM/EDS, AFM and PXRD. The results from this study show that bentonite and montmorillonite particles can be modified by SiO2-colloids when mixed in comparable amounts, due to dehydration or high ionic strength. Some indications for increased colloidal stability for the SiO2-colloid modified clay particles were also found. From the AFM investigation it was found that initial attachment of the SiO2-colloids in Na+ dominated samples seemed to occur on the edges of the montmorillonite layers. In Ca2+ dominated samples not subjected to excess NaCl, SiO2-colloid sorption onto the faces of the montmorillonite layers was also found. In all, contact between the bentonite barrier and ungelled Silica sol should preferably be avoided.

  • 10. Tinnacher, Ruth M.
    et al.
    Holmboe, Michael
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States.
    Tournassat, Christophe
    Bourg, Ian C.
    Davis, James A.
    Ion adsorption and diffusion in smectite: Molecular, pore, and continuum scale views2016In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 177, p. 130-149Article in journal (Refereed)
    Abstract [en]

    Clay-rich media have been proposed as engineered barrier materials or host rocks for high level radioactive waste repositories in several countries. Hence, a detailed understanding of adsorption and diffusion in these materials is needed, not only for radioactive contaminants, but also for predominant earth metals, which can affect radionuclide speciation and diffusion. The prediction of adsorption and diffusion in clay-rich media, however, is complicated by the similarity between the width of clay nanopores and the thickness of the electrical double layer (EDL) at charged clay mineral-water interfaces. Because of this similarity, the distinction between 'bulk liquid' water and 'surface' water (i.e., EDL water) in clayey media can be ambiguous. Hence, the goal of this study was to examine the ability of existing pore scale conceptual models (single porosity models) to link molecular and macroscopic scale data on adsorption and diffusion in compacted smectite. Macroscopic scale measurements of the adsorption and diffusion of calcium, bromide, and tritiated water in Na-montmorillonite were modeled using a multi-component reactive transport approach while testing a variety of conceptual models of pore scale properties (adsorption and diffusion in individual pores). Molecular dynamics (MD) simulations were carried out under conditions similar to those of our macroscopic scale diffusion experiments to help constrain the pore scale models. Our results indicate that single porosity models cannot be simultaneously consistent with our MD simulation results and our macroscopic scale diffusion data. A dual porosity model, which allows for the existence of a significant fraction of bulk liquid water-even at conditions where the average pore width is only a few nanometers-may be required to describe both pore scale and macroscopic scale data.

  • 11. Tournassat, Christophe
    et al.
    Bourg, Ian C.
    Holmboe, Michael
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sposito, Garrison
    Steefel, Carl I.
    Molecular dynamics simulations of anion exclusion in clay interlayer nanopores2016In: Clays and clay minerals, ISSN 0009-8604, E-ISSN 1552-8367, Vol. 64, no 4, p. 374-388Article in journal (Refereed)
    Abstract [en]

    The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is particularly important in the case of long-lived anionic radioisotopes (129I , 99TcO4 , 36Cl) which interact with clay minerals primarily by anion exclusion. For example, models of ion migration in clayey media do not agree as to whether anions are completely or partially excluded from clay interlayer nanopores. In the present study, this key issue was addressed for Cl using MD simulations for a range of nanopore widths (6 to 15 Å) overlapping the range of average pore widths that exists in engineered clay barriers. The MD simulation results were compared with the predictions of a thermodynamic model (Donnan Equilibrium model) and two pore-scale models based on the Poisson-Boltzmann equation under the assumption that interlayer water behaves as bulk liquid water. The simulations confirmed that anion exclusion from clay interlayers is greater than predicted by the pore-scale models, particularly at the smallest pore size examined. This greater anion exclusion stems from Cl being more weakly solvated in nano-confined water than it is in bulk liquid water. Anion exclusion predictions based on the PoissonBoltzmann equation were consistent with the MD simulation results, however, if the predictions included an ion closest approach distance to the clay mineral surface on the order of 2.0 0.8 Å. These findings suggest that clay interlayers approach a state of complete anion exclusion (hence, ideal semi-permeable membrane properties) at a pore width of 4.2 +/- 1.5 Å. 

  • 12. Yang, Guomin
    et al.
    Neretnieks, Ivars
    Holmboe, Michael
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Atomistic simulations of cation hydration in sodium and calcium montmorillonite nanopores2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 8, article id 084705Article in journal (Refereed)
    Abstract [en]

    During the last four decades, numerous studies have been directed to the swelling smectite-rich clays in the context of high-level radioactive waste applications and waste-liners for contaminated sites. The swelling properties of clay mineral particles arise due to hydration of the interlayer cations and the diffuse double layers formed near the negatively charged montmorillonite (MMT) surfaces. To accurately study the cation hydration in the interlayer nanopores of MMT, solvent-solute and solvent-clay surface interactions (i.e., the solvation effects and the shape effects) on the atomic level should be taken into account, in contrast to many recent electric double layer based methodologies using continuum models. Therefore, in this research we employed fully atomistic simulations using classical molecular dynamics (MD) simulations, the software package GROMACS along with the CLAYFF forcefield and the SPC/E water model. We present the ion distributions and the deformation of the hydrated coordination structures, i.e., the hydration shells of Na+ and Ca2+ in the interlayer, respectively, for MMT in the first-layer, the second-layer, the third-layer, the fourth-layer, and the fifth-layer (1W, 2W, 3W, 4W, and 5W) hydrate states. Our MD simulations show that Na+ in Na-MMT nanopores have an affinity to the ditrigonal cavities of the clay layers and form transient inner-sphere complexes at about 3.8 Å from clay midplane at water contents less than the 5W hydration state. However, these phenomena are not observed in Ca-MMT regardless of swelling states. For Na-MMT, each Na+ is coordinated to four water molecules and one oxygen atom of the clay basal-plane in the first hydration shell at the 1W hydration state, and with five to six water molecules in the first hydration shell within a radius of 3.1 Å at all higher water contents. In Ca-MMT, however each Ca2+ is coordinated to approximately seven water molecules in the first hydration shell at the 1W hydration state and about eight water molecules in the first hydration shell within a radius of 3.3 Å at all higher hydration states. Moreover, the MD results show that the complete hydration shells are nearly spherical with an orthogonal coordination sphere. They could only be formed when the basal spacing d001 ≥ 18.7 Å, i.e., approximately, the interlayer separation h ≥ 10 Å. Comparison between DFT and MD simulations shows that DFT failed to reproduce the outer-sphere complexes in the Stern-layer (within ∼5.0 Å from the clay basal-plane), observed in the MD simulations.

  • 13.
    Yeşilbaş, Merve
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Holmboe, Michael
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Residence times of nanoconfined CO2 in layered aluminosilicates2019In: Environmental Science: Nano, ISSN 2051-8153, Vol. 6, no 1, p. 146-151Article in journal (Refereed)
    Abstract [en]

    Nanoconfinement of CO2 in layered aluminosilicates contributes to the capture and release of this greenhouse gas in soils. In this work, we show that the residence times of CO2 in montmorillonite are lowered by 15 min for each 1 degrees C increment in temperature during venting. Molecular simulations showed that activation energies of release are no more than half of the experimentally derived value of 34 kJ mol(-1). This raised the possibility of additional processes limiting CO2 mobility in real materials, including (chemi)sorption at reactive sites or frayed edges or defects. The residence times (approximate to 1616 min at -50 degrees C to approximate to 6 min at 60 degrees C) for some of the driest (approximate to 1.4 mmol H2O per g) montmorillonites that can be produced at ambient temperatures are readily lowered by inclusion of additional water. They are, in turn, prolonged again as the water content and interlayer spacing become smaller through venting. These efforts showed that soil-building clay minerals will lose their propensity to dynamically exchange CO2 as temperatures continue to rise, yet they may retain CO2 more efficiently in cold seasons as soils will become depleted in moisture content.

  • 14.
    Yeşilbaş, Merve
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Holmboe, Michael
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Cohesive vibrational and structural depiction of intercalated water in montmorillonite2018In: ACS Earth and Space Chemistry, E-ISSN 2472-3452, Vol. 2, no 1, p. 38-47Article in journal (Refereed)
    Abstract [en]

    The vibrational spectral profiles of Na- and Ca-montmorillonite (MMT) of controlled water layer populations (nW) was extracted by chemometric analysis of new Fourier transform infrared (FTIR) spectroscopy data and validated by mixed-layer modeling of previously published X-ray diffraction data. These efforts resolved FTIR spectral profiles of 0W, 1W, and 2W interlayers, which can now be used to explore the distinct hydration states of MMT. These spectral profiles reflect water populations organized around interlayer cations (Na+, Ca2+), interacting with siloxane groups of the basal face of the interlayer, and with other bound and “free” water molecules. This cohesive description of water-bearing clays provides the link needed to relate vibrational to structural attributes of these geochemically important materials.

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