300
R. G. Weiss
and other adhesives, materials for producing new morphs of pharmaceuticals [4],
and aluminum soaps that gelate hydrocarbons for ‘fracturing’ in oil wells [5] and for
producing napalm [6].
When the liquid components are aqueous or organic, the gels are referred to
hydrogels or organogels, respectively. IUPAC defines a gel as “a non-fluid colloidal
network or polymer network that is expanded throughout its whole volume by a
fluid” [7]. This definition not very useful and is incomplete because it does not
address the viscoelastic properties of gels. Also, it does not provide insights into the
gel network structures of molecular gels and responses to stress at different length
scales, from the macroscopic to the nanometric. Additionally, it avoids important
questions concerning how adaptable are gel systems to changes in their component
structures and environments. Only some of the factors listed below will be considered
here:
(1) How amenable is a class of molecular gelators to small structural modifications
and what are the consequences of those changes to the properties of the gels?
(2) How wide a range of liquid types can be gelated by a single gelator?
(3) What is the lowest concentration of a molecular gelator that can gelate successfully a liquid at or near room temperature [i.e., the critical gelator concentration
(CGC)]?
(4) How long can a gel be maintained at room temperature without phase separation?
(5) What is the temperature range over which a gel phase can be maintained?
Specifically, at what temperature does a gel ‘melt’ (T g ) and revert to its sol
phase?
(6) How strong or malleable is a particular gel? That is, what are its viscoelastic
properties?
Common features of ‘gels’ are their compositions, at least two components—one
of which is a solid and the other a liquid—their solid-like rheological behavior despite
being mostly liquid [8], (i.e., they are non-Newtonian fluids), and their continuous
microscopic structures with macroscopic dimensions that are permanent on the time
scale of an analytical experiment [9]. Mechanical damping in gels is small: tanδ =
G
/G
1, where G
is the storage modulus and G
is the loss modulus. The moduli
are, respectively, measures of the energy stored and dissipated in a material in which
a deformation has been imposed. Also, the oscillatory frequency dependence of the
viscosity of a viscoelastic material under shear stress (i.e., the complex viscosity, η* =
√
(G
2
+ G
2 )/ω) is a useful parameter to assess the viscoelasticity of a soft material.
In fact, each of these is useful in assessing quantitatively whether a sample meets
the viscoelastic criteria to be called a gel. A typical response of a gel to oscillatory
frequency changes is shown in Fig. 7.1 [10]. Although cavitational rheology is a
newer and potentially more useful method for assessing whether a sample is a gel
[11], it has not been used extensively.
It is important to emphasize that microscopically the liquid component maintains
many of its bulk viscosity and diffusion properties in the gel phase. The diffusion
constant of the liquid or of a solute dissolved in the liquid portion is slower than in
the absence of the gelator, but it is not inconsequential [12]. Only a small fraction
R. G. Weiss
and other adhesives, materials for producing new morphs of pharmaceuticals [4],
and aluminum soaps that gelate hydrocarbons for ‘fracturing’ in oil wells [5] and for
producing napalm [6].
When the liquid components are aqueous or organic, the gels are referred to
hydrogels or organogels, respectively. IUPAC defines a gel as “a non-fluid colloidal
network or polymer network that is expanded throughout its whole volume by a
fluid” [7]. This definition not very useful and is incomplete because it does not
address the viscoelastic properties of gels. Also, it does not provide insights into the
gel network structures of molecular gels and responses to stress at different length
scales, from the macroscopic to the nanometric. Additionally, it avoids important
questions concerning how adaptable are gel systems to changes in their component
structures and environments. Only some of the factors listed below will be considered
here:
(1) How amenable is a class of molecular gelators to small structural modifications
and what are the consequences of those changes to the properties of the gels?
(2) How wide a range of liquid types can be gelated by a single gelator?
(3) What is the lowest concentration of a molecular gelator that can gelate successfully a liquid at or near room temperature [i.e., the critical gelator concentration
(CGC)]?
(4) How long can a gel be maintained at room temperature without phase separation?
(5) What is the temperature range over which a gel phase can be maintained?
Specifically, at what temperature does a gel ‘melt’ (T g ) and revert to its sol
phase?
(6) How strong or malleable is a particular gel? That is, what are its viscoelastic
properties?
Common features of ‘gels’ are their compositions, at least two components—one
of which is a solid and the other a liquid—their solid-like rheological behavior despite
being mostly liquid [8], (i.e., they are non-Newtonian fluids), and their continuous
microscopic structures with macroscopic dimensions that are permanent on the time
scale of an analytical experiment [9]. Mechanical damping in gels is small: tanδ =
G
/G
1, where G
is the storage modulus and G
is the loss modulus. The moduli
are, respectively, measures of the energy stored and dissipated in a material in which
a deformation has been imposed. Also, the oscillatory frequency dependence of the
viscosity of a viscoelastic material under shear stress (i.e., the complex viscosity, η* =
√
(G
2
+ G
2 )/ω) is a useful parameter to assess the viscoelasticity of a soft material.
In fact, each of these is useful in assessing quantitatively whether a sample meets
the viscoelastic criteria to be called a gel. A typical response of a gel to oscillatory
frequency changes is shown in Fig. 7.1 [10]. Although cavitational rheology is a
newer and potentially more useful method for assessing whether a sample is a gel
[11], it has not been used extensively.
It is important to emphasize that microscopically the liquid component maintains
many of its bulk viscosity and diffusion properties in the gel phase. The diffusion
constant of the liquid or of a solute dissolved in the liquid portion is slower than in
the absence of the gelator, but it is not inconsequential [12]. Only a small fraction