Chemistry Reference
In-Depth Information
structures at higher concentrations. Another example of this type of behaviour is
the interaction between isopropyl guar and divalent cobalt ions.
As polymer concentrations are increased, non-Newtonian behaviour is ob-
served. This usually consists of shear thinning and extensional thickening, and
the higher the molar mass of the polymer, the more marked this behaviour
becomes. The viscoelastic response becomes increasingly marked at higher
concentrations. Chapter 5 discusses this behaviour in some depth.
3.5.7 Surfactant Solutions
Below the critical micelle concentration of a surfactant, the viscosity of the
solution should increase with increasing concentration of surfactant because
the anisometric shape of the molecules and electrostatic interactions if the
molecules are ionic. However, the concentrations are low so that extremely
accurate measurements are required to differentiate between the viscosity of the
solution and that of the solvent. Once the cmc is exceeded the system becomes a
dispersion of micellar clusters, often charged, and the rate of viscosity increases
with increases in concentration.
The shape of micelles is spherical close to the cmc and eqn (3.42) can be used
to determine the viscosity as the volume fraction of micellar dispersions is low.
For nonionic surfactants the intrinsic viscosity is the Einstein value of [Z]
ΒΌ
2.5.
If the surfactant is ionic, the micelles are charged with z-potentials typically in
the range 50-100mV as a large fraction of the charge is neutralized by strongly
bound counterions. The intrinsic viscosity is now given by eqn (3.59). The solid
curve in Figure 3.18 would be appropriate for a surfactant such a sodium
dodecyl sulfate and predicts that a value of [Z]
3.5 would be appropriate.
As the surfactant concentration becomes several orders of magnitude above
the cmc, the shape of the micelles becomes elongated and eventually complex
viscoelastic phases are formed. However, the phase behaviour is markedly
dependent on the components added. For example, a strongly viscoelastic
lamellar phase is formed by sodium dodecanoate at a concentration
B
25% w/v
in water, but this turns into a low-viscosity phase as dodecane is added giving a
low-viscosity microemulsion phase.
A surfactant phase that has attracted a significant amount of attention is
the worm-like micellar phase of which hexadecyl trimethyl ammonium sali-
cylate (HDAS) is the most studied model system.
39
These types of systems are
becoming useful as agents for drag reduction and as additives to concrete where
they act as thickeners. There are some similarities between these systems and
monodisperse polymer solutions in that they have a clear low-stress-viscosity
region and a sharp transition to a marked shear-thinning region. However, the
analogy is misleading as the micelles form infinite interconnected networks. The
reason for the sharp relaxation time is that the micelles break and reform under
shear and that the diameter of the micelle is highly monodisperse so that each
break takes the same amount of energy input as any other.
Interesting transient behaviour can be observed during the start-up on the
application of shear. Ouchi et al.
40
B
studied a 0.03mol L
1
HDAS solution
