face-to-face and edge-to-face geometrical motifs are prevalent for pairs of equivalent molecules, while eclipsed face-to-face motif is favoured by pairs with complementary donor and acceptor properties. For offset face-to-face motif, the plane
separation is 3.4–3.6 Å. The stabilizing energy is about 9 kJ mol
−1 for a pair of
benzene molecules, and the involvement of charged species increases the stabilizing
energy.
Furthermore, van der Waals, halogen bonding, cation-p and metallophilic (e.g.
aurophilic) interactions as well as solvophobic effect play important roles in the
formation process of supramolecular gels. Among them, the solvophobic effect is
related to functional moieties in the gelator molecule showing poor solubility in the
solvent, resulting in gelation.
In the following chapters, gels have been classified into five general catalogues
according to the driving force, supramolecular gels, metal–organic gels, dynamic
covalent gels, polymer gels and inorganic gels.
1.3 Characterization
Various techniques can be used to gain structural information on different scales.
There are two basic parameters, the critical gelator concentration (CGC) and
gel-to-sol transition temperature (T gel ), which are extensively used to characterize
the capacity of the gels. The CGC means the minimum concentration of the
gelator molecule to form a gel at a certain temperature (usually at room temperature). The popular visual identified method, inversion test, is adopted to
characterize a gel. The inversion test, also named bottom-up method, is the simple
visual way to assess the gels behaviour by dissolving a gelator molecule in a
certain solvent or mixed solvents to form clear solutions (sol form) under heating
process, and cooling the solutions to get a stable gel by using a thermocontrolled
oil bath. And then, T gel is determined, however, to a certain extent, and the
stability of the gel is dependent on the container size resulting in relatively low
accuracy of the T gel . So, the dropping ball method is developed. A metal ball is
placed onto the gel surface; after conversion to the sol form, the ball would be
dropped. The temperature is recorded as the T gel . Differential scanning calorimetry
(DSC) provides a direct instrumental method to measure T gel during the heating
and cooling process. When the sample undergoes gel-to-sol transition, more
energy will be needed to flow than that of the reference. During the cooling
process, the sample undergoes sol-to-gel phase transition, and less energy will be
needed to flow. So, the phase transition enthalpy can also be collected to take
insight into the thermodynamic behaviours of gels. The DSC data can help further
take insight into the intrinsic behaviour of the gels.
The structures and morphologies of gels have been elucidated by conventional
imaging techniques such as scanning electron microscopic (SEM), transmission
electron microscopic (TEM) and atom force microscopic (AFM) at the microscopic
level. The preparation of xerogels is pivotal for the study of the morphology of gels
4
1 Introduction
separation is 3.4–3.6 Å. The stabilizing energy is about 9 kJ mol
−1 for a pair of
benzene molecules, and the involvement of charged species increases the stabilizing
energy.
Furthermore, van der Waals, halogen bonding, cation-p and metallophilic (e.g.
aurophilic) interactions as well as solvophobic effect play important roles in the
formation process of supramolecular gels. Among them, the solvophobic effect is
related to functional moieties in the gelator molecule showing poor solubility in the
solvent, resulting in gelation.
In the following chapters, gels have been classified into five general catalogues
according to the driving force, supramolecular gels, metal–organic gels, dynamic
covalent gels, polymer gels and inorganic gels.
1.3 Characterization
Various techniques can be used to gain structural information on different scales.
There are two basic parameters, the critical gelator concentration (CGC) and
gel-to-sol transition temperature (T gel ), which are extensively used to characterize
the capacity of the gels. The CGC means the minimum concentration of the
gelator molecule to form a gel at a certain temperature (usually at room temperature). The popular visual identified method, inversion test, is adopted to
characterize a gel. The inversion test, also named bottom-up method, is the simple
visual way to assess the gels behaviour by dissolving a gelator molecule in a
certain solvent or mixed solvents to form clear solutions (sol form) under heating
process, and cooling the solutions to get a stable gel by using a thermocontrolled
oil bath. And then, T gel is determined, however, to a certain extent, and the
stability of the gel is dependent on the container size resulting in relatively low
accuracy of the T gel . So, the dropping ball method is developed. A metal ball is
placed onto the gel surface; after conversion to the sol form, the ball would be
dropped. The temperature is recorded as the T gel . Differential scanning calorimetry
(DSC) provides a direct instrumental method to measure T gel during the heating
and cooling process. When the sample undergoes gel-to-sol transition, more
energy will be needed to flow than that of the reference. During the cooling
process, the sample undergoes sol-to-gel phase transition, and less energy will be
needed to flow. So, the phase transition enthalpy can also be collected to take
insight into the thermodynamic behaviours of gels. The DSC data can help further
take insight into the intrinsic behaviour of the gels.
The structures and morphologies of gels have been elucidated by conventional
imaging techniques such as scanning electron microscopic (SEM), transmission
electron microscopic (TEM) and atom force microscopic (AFM) at the microscopic
level. The preparation of xerogels is pivotal for the study of the morphology of gels
4
1 Introduction