2.3.1 Ultrasound
Ultrasound waves as a kind of high-frequency mechanical waves propagate in
physical medium and generate transient bubbles of vapour which produces abrupt
implosion or cavitation. The cavitation effect will provide high energy to induce
certain chemical and physical changes due to the short timescale of cavitation
leading to temperature of up to 5000 K and pressure of several hundred atmospheres. Surprisingly, the self-assembly can take place under sonication and may
promote gel formation at lower concentration, higher temperature, faster rate than in
mild conditions, which is hard to achieve the resulting aggregates by other methods
[89, 96–98]. And ultrasound may be suitable for stimulating gelation in a kinetically
disfavoured self-assembly processes with a high energy barrier.
Solubilization is a common use of ultrasound through breaking up and dissolve
materials which are resistant to thermal treatment, and under certain circumstances,
provide possibility to gelation process. The ultrasound as a driving force can
equilibrate the aggregates between the thermodynamic product and initial kinetic
product. Anderson and co-workers present highly insoluble compounds uric acid
and melamine with stable p-stacking and strong hydrogen bonding, which partially
dissolve in water. Ultrasound-induced dissolution of aggregates under
non-equilibrium conditions allows the materials to reaggregate as a stable gel [99].
The effects on nucleation under sonication may further promote gelation. Under
ambient conditions, typically, the aggregation processes involve that a relatively
small number of nuclei made of the dissolved gelator proceed into dense, highly
branched spherulites. However, under sonication treatment, the population of nuclei
becomes larger due to increasing of fragments and disperses of the initial aggregates, which vastly accelerated fibre growth. From the microlevel, the environment
of separate assemblies is less supersaturated on average, and tend to assemble into
smaller, less branched and more interpenetrated spherulites which are better suited
to form gel [100, 101]. This phenomenon in gels of 27 (Scheme 2.5) has been
studied by Wang and co-workers. Exposure to ultrasound, the CGC of gels reduced
from 2.0 to 0.5% (w/v) and the gel strength increased by up to three orders of
magnitude compared to thermally generated gels (Fig. 2.19) [101]. In addition, the
degree of branching, average fibre diameter and thermodynamic stability of the
sonogels can be tuned via the temperature or concentration [98, 102, 103]. Yi and
co-workers reported peptide-based ALS gelator 28 (Scheme 2.5). In CH 3 CN gels,
the gel-to-gel process triggered by sonication with the transformation of the
core-shell microspheres into nanoballs, further cross-linked into nanofibers with
higher thermostability. In toluene gels, sonication displayed a cutting effect on the
fibrous morphology with changes of 100 nm fibre diameter into 20–40 nm
(Fig. 2.20) [104]. Amazingly, sonication-triggered morphology changes in the gel
state are mostly reversible, and the sonogels return to initial state upon heating–
cooling treatment without fatigue. And the gel-to-gel transition shows potential
applications in controlled release, functional expression and shape-memory
materials.
30
2 Supramolecular Gels
Ultrasound waves as a kind of high-frequency mechanical waves propagate in
physical medium and generate transient bubbles of vapour which produces abrupt
implosion or cavitation. The cavitation effect will provide high energy to induce
certain chemical and physical changes due to the short timescale of cavitation
leading to temperature of up to 5000 K and pressure of several hundred atmospheres. Surprisingly, the self-assembly can take place under sonication and may
promote gel formation at lower concentration, higher temperature, faster rate than in
mild conditions, which is hard to achieve the resulting aggregates by other methods
[89, 96–98]. And ultrasound may be suitable for stimulating gelation in a kinetically
disfavoured self-assembly processes with a high energy barrier.
Solubilization is a common use of ultrasound through breaking up and dissolve
materials which are resistant to thermal treatment, and under certain circumstances,
provide possibility to gelation process. The ultrasound as a driving force can
equilibrate the aggregates between the thermodynamic product and initial kinetic
product. Anderson and co-workers present highly insoluble compounds uric acid
and melamine with stable p-stacking and strong hydrogen bonding, which partially
dissolve in water. Ultrasound-induced dissolution of aggregates under
non-equilibrium conditions allows the materials to reaggregate as a stable gel [99].
The effects on nucleation under sonication may further promote gelation. Under
ambient conditions, typically, the aggregation processes involve that a relatively
small number of nuclei made of the dissolved gelator proceed into dense, highly
branched spherulites. However, under sonication treatment, the population of nuclei
becomes larger due to increasing of fragments and disperses of the initial aggregates, which vastly accelerated fibre growth. From the microlevel, the environment
of separate assemblies is less supersaturated on average, and tend to assemble into
smaller, less branched and more interpenetrated spherulites which are better suited
to form gel [100, 101]. This phenomenon in gels of 27 (Scheme 2.5) has been
studied by Wang and co-workers. Exposure to ultrasound, the CGC of gels reduced
from 2.0 to 0.5% (w/v) and the gel strength increased by up to three orders of
magnitude compared to thermally generated gels (Fig. 2.19) [101]. In addition, the
degree of branching, average fibre diameter and thermodynamic stability of the
sonogels can be tuned via the temperature or concentration [98, 102, 103]. Yi and
co-workers reported peptide-based ALS gelator 28 (Scheme 2.5). In CH 3 CN gels,
the gel-to-gel process triggered by sonication with the transformation of the
core-shell microspheres into nanoballs, further cross-linked into nanofibers with
higher thermostability. In toluene gels, sonication displayed a cutting effect on the
fibrous morphology with changes of 100 nm fibre diameter into 20–40 nm
(Fig. 2.20) [104]. Amazingly, sonication-triggered morphology changes in the gel
state are mostly reversible, and the sonogels return to initial state upon heating–
cooling treatment without fatigue. And the gel-to-gel transition shows potential
applications in controlled release, functional expression and shape-memory
materials.
30
2 Supramolecular Gels