This thesis aims to understand the self-assembly behaviour of light-responsive cationic azobenzene photosurfactants in different local environments to investigate their ability to form lyotropic liquid crystal phases and to optimise their efficiency as micellar catalysts.
To begin with, in an effort to provide a better understanding of the techniques used, Chapters 2 and 3 provide a theoretical background and experimental details on the techniques used, respectively. The first part of this thesis will investigate the effect of the position of the light-responsive azobenzene chromophore on the self-assembly properties of cationic azobenzene trimethylammonium bromide surfactants (AzoTABs). In Chapter 4, the design, synthesis and characterisation of a series of five AzoTABs will be described. The concept of the packing parameter will be examined for AzoTABs and the photoisomerisation properties will be studied below the CMC. In Chapter 5, the physico- chemical properties derived from the position of the azobenzene core will be investigated in water, below and above the critical concentration of micelle formation, to understand the nanoscale organisation of AzoTAB photosurfactants, with and without light exposure. The validation of the packing parameter for AzoTABs will also be discussed. Following this, Chapter 6 will focus on concentrated solutions of AzoTABs to identify the key structural parameters that drive the formation of lyotropic liquid crystal phases. These chapters aim to provide a general understanding and prediction of the structure-property relationships for AzoTAB photosurfactants to enable the optimum molecular structure for targeted applications to be selected. The optimum molecular structure was selected to compete with the conventional cetyltrimethylammonium bromide (CTAB) surfactant in micellar
27 catalysis, to prove the benefit of using light-responsive surfactants over non-responsive surfactants. Micellar catalysis enables chemical reactions, usually achieved in organic solvents, to be performed in water. Micelles formed by the self-assembly of surfactants behave as nanoreactors in water, in which the chemical reaction may be performed. Chapter 7 will determine as a proof-of-concept whether AzoTABs photosurfactants can be beneficial for micellar catalysis compared to conventional non-responsive surfactants. The aim is to select optimal AzoTAB structures, based on the understandings obtained in Chapter 4 and 5 and to evaluate the benefit of photoisomerisation on the recovery of the product. The self-assembly-efficiency relationship of the reaction will also be discussed. Finally, Chapter 8 summarises the main findings of this thesis and give general directions for future work arising from these studies.
28
1.8.
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32
2.1
Scattering Techniques
Scattering techniques are the method of choice to investigate the nanoscale organisation of colloidal solutions and thin films and to gather precise information on the size, shape and structure of their constituting components. Scattering corresponds to the change of path of incident particles, such as photons, neutrons, and electrons, after collision with an object.1 In this section, the fundamental scattering theory of light, neutrons and X-ray scattering is reviewed, with special attention placed on the instrumentation and scattering techniques used in this thesis.
2.1.1 Inelastic and Elastic Scattering
It is important to separate scattering events between inelastic scattering, where there is an exchange of energy, and elastic scattering, where energy is conserved. In inelastic scattering, the loss or gain of energy after interaction of the particles with matter results in excitation of the internal energy levels of atoms. While this allows the electronic structure of the material to be determined, it is not usually used to obtain the atomic structure. Therefore, inelastic scattering will not be further discussed in this thesis.
In elastic scattering, the incident particles interact with the material without loss or gain of energy. The atoms within a material oscillate at the same frequency as the incoming particle wave and the scattered waves contain information about the structural organisation of the material. Figure 2.1 shows a schematic representation of an elastic scattering event, where an incident particle with a wavevector is scattered from a sample, with a wavevector . The momentum transfer, M, associated with the energy transfer, that results from this event is expressed as:
= 2 − × ℎ 2× ℎ = 2 × ℎ (2.1) where h is Planck’s constant (6.63 × 10-34 J s) and is the scattering vector which can be expressed as:
| | = – (2.2)
33
=2 (2.3)
where
λ
is the wavelength of the incident ray. If the elastic scattering occur through an angle of 2θ
, simple trigonometry leads to:= | | = 4 2 (2.4)
where n is the reflective index of the particle. Bragg’s law equation of diffraction
= 2 2 (2.5)
yields the final expression
= 2 (2.6)
where d is the distance between two neighbouring scattering objects.
Figure 2.1. Schematic representation of an elastic scattering event. is the incident wavevector from a source generating a particle with wavelength λ, is the scattered wavevector resulting from the interaction with a sample at a scattering angle θ and is the scattering vector observed at the detector.
2.1.2 Properties of Light, Neutron and X-ray Radiation
Light and X-rays are electromagnetic radiation, which means they are defined by their wavelengths and related to energy by Planck’s equation:
=ℎ (2.7)