Direct liquid crystal templating of mesoporous silica

Abstract

The work described in this thesis is concerned with the development of ordered
mesoporous silicas by direct templating from lyotropic liquid crystal phases of the
surfactants Pluronic PI23, Pluronic F127, CTAB and Brij 78. The factors affecting the
regularity, morphology, pore diameter and wall thickness of the templated mesoporous
silicas were examined by exploring the reaction composition space and plotting the
structural properties on TMOS/surfactant/water ternary diagrams. The silicas were
studied using the complementary techniques of small angle X-ray diffraction,
transmission electron microscopy and N? adsorption isotherms.

Mesoporous silicas with hexagonal (H,) pore morphologies templated from the Hi phase
of CTAB were prepared from fumed silica/CTAB/water/NaOH mixtures under
hydrothermal conditions. The mesostructure showed very good long-range order and
very narrow pore size distributions, with -3.4 mn diameter pores and -1.4 mn thick
walls.

In addition, mesoporous silicas were prepared using a sol-gel route from
TMOS/surfactant/0.5 M HCl(aq) mixtures at 45°C. The central regions of the ternary
diagrams of these mixtures were explored. Using Pluronic PI23, mesoporous silicas with
hexagonal (Hi) pore morphologies were formed with pore diameters in the range of 5 -
10 nm. With Brij 78 and Pluronic F127, mesoporous silicas with 3d cubic pore
morphologies were formed with pore diameters in the range of 2.9 - 4.5 mn and 4 - 1 0
nm for Brij 78 and Pluronic F127 respectively.

The mesoporous silicas produced via the sol-gel route had the most ordered structures
with a TMOS:water molar ratio in the range of 1:5 to 1:7, leaving little water present
after hydrolysis. For ordered silicas, the sum of the pore diameter and wall thickness is
roughly constant, where the pore diameter decreases with increasing TMOS content and
decreasing surfactant content there is commensurate increase in wall thickness.
Finally, 1,3,5 trimethyl benzene was used to swell the hydrophobic core of Pluronic PI 23
and F127 micelles, allowing the templating of mesoporous silica with pore diameters up
to 14 nm.

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Identifiers

ORCID for George Attard: orcid.org/0000-0001-8304-0742

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