7 A Primer on Gels (with an Emphasis on Molecular Gels)
309
also provide insights into the validity of applying the Schröder-van Laar equation
[65] (Eq. 7.3), which assumes ideality in melting-nucleation phenomena during the
sol-gel phase transitions (i.e., the interactions between gelator and liquid molecules
is the same in the sol and gel phases) over a range of concentrations). In Eq. 7.3, x
is the gelator solubility under conditions of ideal solution behavior, T is the equilibrium temperature, R is the ideal gas constant, and ΔH M and T M –available from
DSC measurements—are, respectively, the molar melting enthalpy and the melting
temperature of the neat gelator. Because the assumption of ideality is frequently not
valid, the use of this equation is useful in a limited number of gel systems.
ln x = (H M /R)(1/T M − 1/T )
(7.3)
Thixotropic gels are especially interesting because they can be destroyed by application of excessive stress and reformed when the sample is left under conditions
within the linear viscoelastic region, which is determined by oscillatory rheological measurements of the moduli as a function of oscillation frequency and applied
stress. The values of the moduli, as a sample is cycled periodically between the
linear viscoelastic (LVR) and the destructive strain (DS) regions, can yield valuable insights into the dynamics of the isothermal reformation step and the degree
(if any) of irreversible destruction of the gelator structure upon repeated application
of destructive strain. In the example shown in Fig. 7.7 with the gelator (R)-12hydroxy-N-(2-hydroxyethyl)stearamide (HS–2–OH), there is no discernible loss of
gel structure after repeated exposure to destructive strain [66]. That is usually not the
case: the magnitudes of the moduli decrease as the number of cycles or the magnitude of the applied strain increases. The recovery time constant τ after cessation of
destructive strain can be calculated from Eq. 7.4 (where m is a constant that depends
on the gel being examined).
ln
− ln
G
(∞) − G
(t)
G (∞) − G (0)
= m ln t − m ln τ
(7.4)
7.7 The Role of the Liquid Component
Despite the gelator being a much smaller portion of a gel than the liquid, the properties
of the latter are frequently given less attention. To discern how to characterize the
properties of the liquid, it is necessary to consider the interactions between the two
components at various stages during gel formation. Unfortunately, methods to assess
gelator-liquid interactions in sol phases, for example, are not well developed. Thus,
most approaches concentrate on whether a particular liquid is gelated by a specific
gelator molecule and comparisons of the properties of the two.
Several of those approaches have been compared recently [67b]. Here, the one
of Hansen will be discussed as it applies to gels [67], comparing it with the more
309
also provide insights into the validity of applying the Schröder-van Laar equation
[65] (Eq. 7.3), which assumes ideality in melting-nucleation phenomena during the
sol-gel phase transitions (i.e., the interactions between gelator and liquid molecules
is the same in the sol and gel phases) over a range of concentrations). In Eq. 7.3, x
is the gelator solubility under conditions of ideal solution behavior, T is the equilibrium temperature, R is the ideal gas constant, and ΔH M and T M –available from
DSC measurements—are, respectively, the molar melting enthalpy and the melting
temperature of the neat gelator. Because the assumption of ideality is frequently not
valid, the use of this equation is useful in a limited number of gel systems.
ln x = (H M /R)(1/T M − 1/T )
(7.3)
Thixotropic gels are especially interesting because they can be destroyed by application of excessive stress and reformed when the sample is left under conditions
within the linear viscoelastic region, which is determined by oscillatory rheological measurements of the moduli as a function of oscillation frequency and applied
stress. The values of the moduli, as a sample is cycled periodically between the
linear viscoelastic (LVR) and the destructive strain (DS) regions, can yield valuable insights into the dynamics of the isothermal reformation step and the degree
(if any) of irreversible destruction of the gelator structure upon repeated application
of destructive strain. In the example shown in Fig. 7.7 with the gelator (R)-12hydroxy-N-(2-hydroxyethyl)stearamide (HS–2–OH), there is no discernible loss of
gel structure after repeated exposure to destructive strain [66]. That is usually not the
case: the magnitudes of the moduli decrease as the number of cycles or the magnitude of the applied strain increases. The recovery time constant τ after cessation of
destructive strain can be calculated from Eq. 7.4 (where m is a constant that depends
on the gel being examined).
ln
− ln
G
(∞) − G
(t)
G (∞) − G (0)
= m ln t − m ln τ
(7.4)
7.7 The Role of the Liquid Component
Despite the gelator being a much smaller portion of a gel than the liquid, the properties
of the latter are frequently given less attention. To discern how to characterize the
properties of the liquid, it is necessary to consider the interactions between the two
components at various stages during gel formation. Unfortunately, methods to assess
gelator-liquid interactions in sol phases, for example, are not well developed. Thus,
most approaches concentrate on whether a particular liquid is gelated by a specific
gelator molecule and comparisons of the properties of the two.
Several of those approaches have been compared recently [67b]. Here, the one
of Hansen will be discussed as it applies to gels [67], comparing it with the more