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The Co-Structure Directing Agent (CSDA) Approach to Mesoporous Silica Formation – Exploring the Assembly Characteristics

Lin, Ruiyu LU (2016)
Abstract (Swedish)
This thesis is focused on the detail process of formation of mesoporous silica materials.
Mesoporous silica materials are one type of porous materials with a pore size between 2 and 5 nm (1 nm=0.000 000 001 m). The pore walls consist of silicon dioxide – silica (sand also consists of silica). The mesoporous silica materials are formed by amphiphilic molecules, a kind of molecule with one part that loves water and one part that hates water, and a silica source. Within the group of amphiphilic molecules, surfactants are mostly used. Surfactants are molecules that can have positively or negatively charged head groups, which attract water, and long organic tails, that repel water. Examples of surfactants in our daily life are shampoos and... (More)
This thesis is focused on the detail process of formation of mesoporous silica materials.
Mesoporous silica materials are one type of porous materials with a pore size between 2 and 5 nm (1 nm=0.000 000 001 m). The pore walls consist of silicon dioxide – silica (sand also consists of silica). The mesoporous silica materials are formed by amphiphilic molecules, a kind of molecule with one part that loves water and one part that hates water, and a silica source. Within the group of amphiphilic molecules, surfactants are mostly used. Surfactants are molecules that can have positively or negatively charged head groups, which attract water, and long organic tails, that repel water. Examples of surfactants in our daily life are shampoos and detergents. When surfactants are dissolved in water, the organic tails will cluster together and only leave the charged head groups outside in contact with the water. In this case, the aggregated surfactants can form different shapes with the tails inside and the head groups on the surface of the aggregate. The silica components will decorate these aggregates and give rise to an attraction that draws the aggregates together. Accordingly, the “mesopores” of the materials are filled with surfactants during their creation. When the surfactants are removed, the mesopores arise, and the mesoporous materials with silica walls are formed. The networks of mesopores are often well ordered in various structures. When we add acid or salt into the formation mixture of mesoporous silica materials, the structure of the materials can sometimes be changed. The process is like baking, where by adding different ingredients, such as baking powder or salt, the shape or texture of the pastries can be changed.
In the work of this thesis, two recipes were used. We call them two synthesis systems. Both systems contain a molecule that eventually gives rise to the creation of the silica wall. We call this molecule the silica source. In system 1, a surfactant with a large positively charged head group was used, while in system 2, a surfactant with a negatively charged head group was used. Moreover, a molecule consisting of a charged part and a silica part was added in the mixture. In this thesis, the latter type of molecule is called CSDA. In system 1, the charged part of the CSDA is negative, and is called the carboxylate group, in vinegar there is a lot of carboxylic groups. In system 2, the charged part is positive, with the chemical name quaternary ammonium group, which is a common component used in softeners. In the mixture, the CSDA will be in contact with the surfactant head group via the charged part and with the silica source via the silica part. After the formation, when the surfactants are removed, the charged groups of the CSDA will remain within the pores, therefore, the materials are functionalized with the charged groups of the CSDA directly in the synthesis.
In system 1, addition of acid can change the structures of the materials. In this work we aim to find reasons for the structural change. We found that addition of acid will change the building-up of the silica source. With low acid addition, the silica sources build up walls around the surfactant head groups very fast, whereas with a higher amount of acid, the silica walls build slower. If salt is added in the slower system, a different structure will be formed. This is because the added salt makes the building work even slower.
In system 2, we used a special type of instrument, called cryo-EM, to look at the system. This instrument allows us to visualize very small things (hundreds of nm in size) in frozen samples. So we can freeze samples after different formation times, and check what has been built up at these specific times. We found that in this system, the material first forms fibers, and then the fibers grow in width and form ribbons. The ribbons then twist and, with time, grow in width to eventually become helical ribbons that later merge into tubes. We also found that a mathematical model can be used to explain this shapechange process. (Less)
Abstract
We investigate the formation mechanism responsible for two specific systems of mesoporous silicas formed with the so-called co-structure directing agent (CSDA) route. The synthesis relies on the interactions between silica source (tetraethylorthosilicate, TEOS), surfactants and CSDA. The structures of the mesoporous silica materials were investigated mainly by small angle X-ray diffraction (SAXD), supplemented with transmission electronic microscopy (TEM). The surfactant/water system was investigated mainly by small X-ray scattering (SAXS).In system 1, a cationic surfactant (C18H37N+(CH3)2(CH2)3N+(CH3)3Br2, C18-3-1) and an anionic CSDA (carboxyethylsilanetriol, CES) are used. We have insight into the surfactant aqueous solution, the... (More)
We investigate the formation mechanism responsible for two specific systems of mesoporous silicas formed with the so-called co-structure directing agent (CSDA) route. The synthesis relies on the interactions between silica source (tetraethylorthosilicate, TEOS), surfactants and CSDA. The structures of the mesoporous silica materials were investigated mainly by small angle X-ray diffraction (SAXD), supplemented with transmission electronic microscopy (TEM). The surfactant/water system was investigated mainly by small X-ray scattering (SAXS).In system 1, a cationic surfactant (C18H37N+(CH3)2(CH2)3N+(CH3)3Br2, C18-3-1) and an anionic CSDA (carboxyethylsilanetriol, CES) are used. We have insight into the surfactant aqueous solution, the formation kinetics (reactions of TEOS), and the electrostatic interactions (addition of salt or controlling the concentration of CSDA). Depending on the concentration of HCl in the synthesis, the structure is defined by Fm-3m (at high pH) and by Fd-3m (at low pH), with a gradual transition in the intermediate pH range. When salt is added in the Fd-3m synthesis, the Pm-3n structure is formed. The micellar sizes of C18-3-1 assembled in these three structures are the same. Using SAXD and 13C PT ssNMR, we followed the synthesis process of these three structures, and found that a fast process results in Fm3m, regardless of pH, and a slow process results in Fd-3m. When NaCl is added to the slow system (low pH) the formation is altered resulting in a material with the Pm-3n structure. We suggest that the materials strive to form the densest structure possible which structure that forms depends on the when the condensation of the silica network arrests the formation process. We also investigate the difference of this system with that of a reversed system (i.e. based on an anionic surfactant and a cationic CSDA). We find both similarities and differences between these two “mirroring” systems.In system 2, an anionic surfactant (N-myristoyl-L-alanine, C14-L-Ala) and cationic CSDA (3-aminopropyltriethoxysilane, APES) are used. The cryo-TEM and cryo-SEM images providedetailed information about the formation process of the material. A formation mechanism issuggested. The fibers grow in width. The ribbons twist, and with time grow in width, eventuallyforming helical ribbons. Later on the helical ribbons merge into tubes. This evolutionary progressof the configuration could be explained by the incompatible elastic sheet theory. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Prof. Dr. Lindén, Mika, Ulm University, Germany
organization
publishing date
type
Thesis
publication status
published
subject
keywords
mesoporous silica materials, co-structure directing agent, surfactants, mesostructures, scattering, NMR, TEM
pages
192 pages
publisher
Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry
defense location
Center for chemistry and chemical engineering, lecture hall B, Naturvetarvägen 14, Lund
defense date
2016-11-25 13:15
ISBN
978-91-7422-483-2
978-91-7422-484-9
language
English
LU publication?
yes
id
f15799a3-4eb7-4df5-9abf-e08237794392
date added to LUP
2016-10-31 15:28:47
date last changed
2016-11-24 21:48:02
@phdthesis{f15799a3-4eb7-4df5-9abf-e08237794392,
  abstract     = {We investigate the formation mechanism responsible for two specific systems of mesoporous silicas formed with the so-called co-structure directing agent (CSDA) route. The synthesis relies on the interactions between silica source (tetraethylorthosilicate, TEOS), surfactants and CSDA. The structures of the mesoporous silica materials were investigated mainly by small angle X-ray diffraction (SAXD), supplemented with transmission electronic microscopy (TEM). The surfactant/water system was investigated mainly by small X-ray scattering (SAXS).In system 1, a cationic surfactant (C18H37N+(CH3)2(CH2)3N+(CH3)3Br2, C18-3-1) and an anionic CSDA (carboxyethylsilanetriol, CES) are used. We have insight into the surfactant aqueous solution, the formation kinetics (reactions of TEOS), and the electrostatic interactions (addition of salt or controlling the concentration of CSDA). Depending on the concentration of HCl in the synthesis, the structure is defined by Fm-3m (at high pH) and by Fd-3m (at low pH), with a gradual transition in the intermediate pH range. When salt is added in the Fd-3m synthesis, the Pm-3n structure is formed. The micellar sizes of C18-3-1 assembled in these three structures are the same. Using SAXD and 13C PT ssNMR, we followed the synthesis process of these three structures, and found that a fast process results in Fm3m, regardless of pH, and a slow process results in Fd-3m. When NaCl is added to the slow system (low pH) the formation is altered resulting in a material with the Pm-3n structure. We suggest that the materials strive to form the densest structure possible which structure that forms depends on the when the condensation of the silica network arrests the formation process. We also investigate the difference of this system with that of a reversed system (i.e. based on an anionic surfactant and a cationic CSDA). We find both similarities and differences between these two “mirroring” systems.In system 2, an anionic surfactant (<i>N</i>-myristoyl-L-alanine, C<sub>14</sub>-L-Ala) and cationic CSDA (3-aminopropyltriethoxysilane, APES) are used. The cryo-TEM and cryo-SEM images providedetailed information about the formation process of the material. A formation mechanism issuggested. The fibers grow in width. The ribbons twist, and with time grow in width, eventuallyforming helical ribbons. Later on the helical ribbons merge into tubes. This evolutionary progressof the configuration could be explained by the incompatible elastic sheet theory.},
  author       = {Lin, Ruiyu},
  isbn         = {978-91-7422-483-2},
  keyword      = {mesoporous silica materials,co-structure directing agent,surfactants,mesostructures,scattering,NMR,TEM},
  language     = {eng},
  pages        = {192},
  publisher    = {Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry},
  school       = {Lund University},
  title        = {The Co-Structure Directing Agent (CSDA) Approach to Mesoporous Silica Formation – Exploring the Assembly Characteristics},
  year         = {2016},
}