| 1. |
- Simonsson, Isabelle, 1990, et al.
(författare)
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The specific co-ion effect on gelling and surface charging of silica nanoparticles: Speculation or reality?
- 2018
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 0927-7757 .- 1873-4359. ; 559, s. 334-341
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Tidskriftsartikel (refereegranskat)abstract
- © 2018 Elsevier B.V. Based on extensive experimental investigations on many different oxide nanoparticles, it is now a well-established view that the counter-ions exhibit ion specific effects due to their high charge density and strong interaction with oppositively charged surfaces. On the other hand, studies regarding co-ion effects are scarcely reported in the literature. In this study we have measured the surface charge densities and gel-times of silica nanoparticles in a number of salts which have the same counter-ions but different co-ions, i.e. anions in this case. Gel-times were measured in LiCl, NaCl, NaNO3, NaClO4, NaClO3and Na2SO4as well as in KCl, KNO3, and K2SO4. We have seen clear correlations between the gel-times and the extent of ion pairing in the solutions; salts that have strong ion pairing exhibit longer gel-times than salts having highly dissociated ions. To better understand the mechanisms at work we have determined the surface charging of silica nanoparticles in these salt solutions and we have observed that the surface charging behavior of silica nanoparticles follows the trends seen in the gel-time studies. From our gel-time determinations and potentiometric measurements we can claim that there is a clear co-ion effect on the gelling and surface charging of silica nanoparticles.
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| 2. |
- Sögaard, Christian, 1990, et al.
(författare)
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Development and Evaluation of Polyether Ether Ketone (PEEK) Capillary for Electrospray
- 2019
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 4:1, s. 1151-1156
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 American Chemical Society. With the rapid development of nanotechnology, there is urgent need of characterizing techniques; especially determining the particle size distribution directly from solution. Dynamic light scattering is often used but presence of a small number of aggregates can greatly influence the size distribution. Electrospray scanning mobility particle sizer (ES-SMPS) is rapidly emerging as an alternative method in colloidal science. However, a major limitation is the use of silica-coated capillaries, which are negatively charged at pH > 3, and therefore making its use difficult for positively charged nanoparticles. In this work, we have developed the polyether ether ketone (PEEK) capillary for ES-SMPS, which removes this limitation because it carries no charge. We have shown that the new capillary not only produced equally good particle size distributions for negatively charged particles (SiO2, Au, and latex) as obtained with silica capillaries, but also precise particle size distributions for positively charged particles (TiO2). Moreover, the PEEK capillaries are much cheaper than the silica capillaries. Thus, the results shown in this paper strengthen the development of the ES-SMPS method as a versatile method for determining the particle size distributions of colloidal sols directly from solution.
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| 3. |
- Sögaard, Christian, 1990
(författare)
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From Silica Nano-Particles to Silica Gels and Beyond - Salt Induced Aggregation of Silica Nano-Particles and the Stability of Resultant Gels
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aqueous silica nanoparticle suspensions are widely available and used within a number of industries. A relatively new area of application is as a grouting material for sealing narrow fractures in tunnels. While silica sols/gels have been used successfully to grout sections of tunnels the continued reliability of the grouting requires knowledge of gel formation, long time stability and functionality. Although much research has been done for silica nanoparticle interactions with monovalent cations, the effect of anions and gelling in salt mixtures has not been thoroughly investigated. Ionic interactions with the silica surface were investigated by potentiometric titrations and gel time tests. The strength of cation interaction with the silica surface is found to be controlled by cation and anion interactions in bulk salt solution in the following order Cl- ≈ ClO4- < ClO3- < NO3- < SO4-. Anions to the left in the ranking lead to shorter gel times and higher surface charge density, indicating stronger cation-surface interactions. In salt mixtures with divalent and monovalent ions generally the cations interaction with silica surface follows the direct Hofmesiter series. However, there are considerable differences seen in the kinetics of gel formation. Strongly interacting cations in a mixture of monovalent cations and divalent cations, determine the gelling kinetics. For divalent cations an unexpected shift in the Hofmeister series was observed for Mg2+ at pH > 8. It is expected that Mg2+ due to its strong hydration should follow the direct Hofemister series as Li+ does i.e., weakly interacting with silica surface due to strong hydration, than Ca2+, but this is not the case. However, at pH < 8 the direct Homeister series was observed. The plausible explanation for this unusual strong interaction of Mg2+ with negatively charged silica surface compared to Ca2+ is its ability to polarize the hydrating water molecules leading to strong interaction with silica surface. The effect of temperature and particle size on the aggregation behaviour is investigated using gel time tests, rheological measurements, and electrospray scanning mobility particle sizer. Smaller average particle size and increased temperature lead to faster aggregation due to increased Brownian motion causing higher number of particle collisions in the sols. The formation of a gel network is sudden, leading to an exponential increase in complex viscosity. The average number of particles contained in an aggregate of average size at the vii gel point was found to be three, indicating that large numbers of particles are not incorporated in the gel network at the gel point. To test the long-time stability of silica gels new test equipment was designed and constructed. Waters of different ionic composition and pH were pushed through gels and leachates were collected for maximum 488 days and were analysed by inductively coupled plasma atomic emission spectroscopy for metal concentrations. It was found that much of the salt such as NaCl used to generate the gels exits with the water. The amount of salt exiting follows the direct Hofmeister series for monovalent cations i.e., Na+ leaching more than K+. Increased pH of the water entering the gels does not lead to increased silica dissolution since the silica gels buffer the water down to pH ≈ 9-10. Using a simple numerical method the collected data is used to predict the lifetime of the grouted silica gels. The lifetime is calculated to between 200 and 400 years depending on different factors such as flow rate through the gels and salt used to form the gels.
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| 4. |
- Sögaard, Christian, 1990, et al.
(författare)
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Hofmeister effects in the gelling of silica nanoparticles in mixed salt solutions
- 2021
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 0927-7757 .- 1873-4359. ; 611
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the nature of specific interaction of ions with the charged silica nanoparticles is vital not only to produce gels for applications such as grouting but also for determining their long term stability. Interaction of silica nanoparticles in single salt solutions has been thoroughly investigated but in mixed salt solutions is rarely investigated. In this work we have investigated the gelling of silica nanoparticles in the mixtures of monovalent ions as well as in the mixtures of divalent and monovalent ions. To gain an understanding of the interaction of ions with the charged silica surface at molecular level we have performed molecular dynamics (MD) simulations. Our overall goal was to find out if in salt mixtures ions silica interactions follow the Hofmeister series or not and how ion specific interactions may change when chaotropic ions are successively replaced by kosmotropic or vice versa. The gelling results show that generally monvalent ions in salt mixtures follow the Hofmeister series, e.g., the gel times in the mixtures of lithium and sodium chlorides are much longer than in the mixtures of lithium and potassium chlorides. On the other hand, gel times in the salt mixtures containing divalent ions do not follow the expected Hofmeister series e.g. mixtures of magnesium and sodium chlorides show shorter gel times than that of calcium and sodium chlorides. However, pH dependent gelling revealed that at pH values less than 9 gelling in these mixtures follow the normal Hofmeister series i.e., longer gelling time in magnesium and sodium chlorides than in calcium and sodium chlorides. This reversal of Hofmeister series for divalent and monovalent salt mixtures at pH > 9 and normal Hofmeister series at pH < 9 is reported for the first time in literature. Such a revesal at pH> 9 is explained due to enhanced surface charge, ordring of surface water layer which leads to enhanced ion specificity of strongly hydrated ions such as Mg2+. Moreover, in mixtures having the same divalent salt but different monovalent salts such as magnesium chlorides mixtures with lithium, sodium and potassium chlorides a normal Hofmeister series prevails. MD simulations results revealed that Mg2+ ions retain their strong hydration shell while interacting with the oppositely charged silica surface which means that the shorter gelling times obtained in magnesium salts mixtures are not due to inner sphere complexation of magnesium with the silica surface. Instead magnesium interacts with the silica surface through its hydrating water molecules.
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| 5. |
- Sögaard, Christian, 1990, et al.
(författare)
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Silica sol as grouting material: a physio-chemical analysis
- 2018
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Ingår i: Nano Convergence. - : Springer Science and Business Media LLC. - 2196-5404. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- At present there is a pressing need to find an environmentally friendly grouting material for the construction of tunnels. Silica nanoparticles hold great potential of replacing the organic molecule based grouting materials currently used for this purpose. Chemically, silica nanoparticles are similar to natural silicates which are essential components of rocks and soil. Moreover, suspensions of silica nanoparticles of different sizes and desired reactivity are commercially available. However, the use of silica nanoparticles as grouting material is at an early stage of its technological development. There are some critical parameters such as long term stability and functionality of grouted silica that need to be investigated in detail before silica nanoparticles can be considered as a reliable grouting material. In this review article we present the state of the art regarding the chemical properties of silica nanoparticles commercially available, as well as experience gained from the use of silica as grouting material. We give a detailed description of the mechanisms underlying the gelling of silica by different salt solutions such as NaCl and KCl and how factors such as particle size, pH, and temperature affect the gelling and gel strength development. Our focus in this review is on linking the chemical properties of silica nanoparticles to the mechanical properties to better understand their functionality and stability as grouting material. Along the way we point out areas which need further research.
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| 6. |
- Sögaard, Christian, 1990, et al.
(författare)
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Temperature and Particle-size Effects on the Formation of Silica Gels from Silica Sols
- 2022
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Ingår i: Silicon. - : Springer Science and Business Media LLC. - 1876-990X .- 1876-9918. ; 15:8, s. 3441-51
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Tidskriftsartikel (refereegranskat)abstract
- Silica nanoparticles (silica sols) based gels have increasingly been used as alternative grouting material for sealing the small fractures in the tunnel walls. Gelling of silica nanoparticles at room temperature has been investigated thoroughly but gelling at different temperatures scarcely investigated. At the same time temperature is one of major factor which can affect the long-term stability of grouted silica. In this work we have investigated the gelling of three different types of silica sols (Levasil CS40-213, Levasil CS40-222, and Levasil CS30-236) having different particle sizes, in 0.28 M NaCl at 10, 20 and 30 ?. Aggregation process, starting from the addition of salt to the gelling point, was monitored by measuring the time dependent particle size distribution. Electrospray scanning mobility particle sizer (ES-SMPS) was used to measure the aggregating. These measurements were complemented by rheological measurements in order to get a relationship between changes in aggregate structure and in the viscosity of silica suspension. Data from the temperature dependent gel time measurements were used to calculate the activation energy. At room temperature, silica sols with smallest average particle size showed the shortest gel times whereas the sols with the largest particle size showed the longest gel time. However, at increasing temperature shorter gel times were seen for all the sols. Temperature dependent rheological measurements showed similar trends in viscosity changes as seen for gel times i.e., increased temperature leads to quicker increase in the viscosity and a sharp increase in viscosity near the gelling point. Our calculations of fractal dimensions showed that in the gel network there are still many free particles which continuously incorporated into the gel network. Apparent activation energies calculated for CS40-213, CS40-222, CS30-236 were 13.40, 23.36 and 41.45 kJ/mol, respectively. These values are lower than values reported for silica in the literature. Moreover, temperature dependent zeta potential measurements show that zeta potential get less negative as temperature increase. The above mentioned measurements are at odd what has been reported in literature but we have provided plausible explanation of these results.
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| 7. |
- Sögaard, Christian, 1990, et al.
(författare)
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The long term stability of silica nanoparticle gels in waters of different ionic compositions and pH values
- 2018
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 1873-4359 .- 0927-7757. ; 544, s. 127-136
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Tidskriftsartikel (refereegranskat)abstract
- The use of silica nanoparticles for grouting underground tunnels offers an environmentally friendly option compared to organic grouting materials. Silica sols are commercially available and when mixed with an accelerator (salt) they form gels in a predetermined time. While much research has been focused on the practical implementation of silica sols in grouting as well as on the development of physical parameters such as viscosity and strength development, little is known about the long term stability of the resultant silica gels. When placed in rock fractures, parameters such as pH and ionic composition of groundwater may affect the long term stability and functionality of the gels. In this article we use a newly designed test equipment to simulate the behaviour of silica gels when water passes through the gel structure for up to 488 days. The pH and ionic composition of the water is varied to simulate environments that can be experienced by gels used for grouting applications. Results in the form of ionic composition, volume, and pH of leached water were used to evaluate and predict the lifetime of silica gels. The overall results show that several factors such as water flow and the nature of salt, so called accelerator used for gelling have significant effect on the gel life time. Furthermore, it is shown that the accelerator ions leach from the gels; however, the extent to which they are released from the gel depends upon the salt type. From these results we have predicted the lifetime of the 100 mL gels used in our experiments by using a simple numerical model. The predictions show that the total dissolution time for 100 mL gels are up to hundreds of years.
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