The influence of salt in the formation of carboxylic functionalized mesoporous silica

Abstract

Popular summary
This master thesis was focused on the influence of salt, such as NaCl and CaCl2, in the formation
of carboxylic functionalized mesoporous silica materials.
Mesoporous silica materials are one type of porous materials with a pore size between 2 and 5
nm (1݊݉ = 0.000000001 ݉ ). The walls of the pores in this kind of materials consist of silicon
dioxide, which is called silica (sand). The mesoporous silica materials, or rather the mesoporous
silica particles, 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. The surfactants are molecules with positive or negative
charged head groups, which love water, and long organic tails, that hate water. Examples of
surfactants in our daily life are shampoo and detergent. When the surfactants have been
dissolved in water, the tails that hate water will get together and only leave the charged head
groups outside in contact with the water. In this case, the grouped surfactants can form different
shapes with the tails inside and the head groups on the surface of the shapes. The silica source
will build up walls around these shapes. 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 adding acid or salt into the formation mixture of
mesoporous silica materials, the structure or the shape of the materials (particles) can sometimes
be changed. The process is like baking, when adding different things, such as baking powder or
salt, the shape or the texture of the pastries can be changed.
In the work of this thesis, a surfactant with positive charged head group was used. Moreover,
one kind of molecule consists of a negative part and a silicon part was added in the mixture. The
negative part of this molecule is called carboxylic group, which is the major component of
vinegar. In the mixture, this molecule will be in contact with the surfactant head group via the
carboxylic part and in contact with the silica source via the silicon part. After the formation,
when the surfactants are removed, the carboxylic groups will remain within the pores, therefore,
carboxylic functionalized mesoporous silica (CFMS) materials are formed. Addition of acid can
change the structures of the CFMS materials. The shapes of the particles can change depending
on the structure. In this work we show that addition of salt can change the structures and the
shapes of the CFMS particles. Additionally, the particle sizes increase when the amount of salt
increases. Some of the CFMS particles have interesting and well-defined shapes, as shown in
Figure 1. This means that salts can be used as control chemicals in the formation.

Abstract

Carboxylic group functionalized mesoporous silicas (CFMSs) were synthesized using cationic
Gemini surfactant (C18-3-1) as structure director and an anionic co-structure directing agents
(CSDA). Well ordered mixture of CCP (Fm3തm) and HCP (P63/mmc), and cubic Fd3തm structures
were obtained. Non-uniform morphologies were observed.
This thesis has been focused on the salt influence on the formation of CFMS materials and how
the material properties are affected. Monovalent NaCl and divalent CaCl2 have been added to the
syntheses.
The structure, the morphology and the particle size of the CFMSs were investigated with small
angle x-ray diffraction (SAXD) and scanning electronic microscopy (SEM). Addition of salt
influenced the structure transformation within a limited pH range. Further the morphology and
the particle size was influenced in the entire pH-range investigated. Moreover, well defined
morphologies are more easily obtained with a high salt concentration. The influence of NaCl and
CaCl2 showed qualitative similar but quantitative different results.

• author: Lin, Ruiyu
• supervisor: Viveka Alfredsson
• organization: Department of Chemistry
• course: KEMN10 20121
• year: 2012
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Nanokemi
• language: English
• id: 3458435
• date added to LUP: 2013-02-11 11:33:43
• date last changed: 2013-02-11 11:51:04