The prime cause of the surface shear viscosity is friction between
surfactant molecules; the cause of surface shear elasticity is attractive forces
between those molecules, leading to a more or less continuous two-
dimensional network. For a closely packed layer, the effects may be
substantial. For layers of small-molecule surfactants, however, the values of
ZSSa are generally immeasurably small, about 10^5 N?s?m^1 or less. For
adsorbed polymers, values between 10^3 and 1 N?m?s^1 have been
reported.
For example, for Na-caseinate at an A–W surface,
ZSSa& 2? 10 ^3 N?s?m^1 has been observed. Other experiments reveal that
the thickness of such an adsorbed layer equals about 10 nm. Interpreta-
tion ofZSSa as a bulk viscosity of a layer of that thickness would yield
Za¼ 2? 10 ^3 /10^8 & 2? 105 N?s ?m^2. Furthermore,G&3mg?m^2 is
observed for adsorbed Na-caseinate; this yields, for the average concentra-
tion of caseinate in a layer of 10 nm, a value of 30%(w/v). A Na-caseinate
solution of that concentration does indeed have a bulk viscosity of about
2? 105 N?s?m^2. It thus appears as if surface shear viscosity may be
interpreted as the bulk viscosity of a layer of unknown thickness. An
adsorbed layer that is thicker (compare, e.g., frames 2 and 5 in Figure 10.16)
or denser would thus yield a higher value ofZSSa.
Caseinate is a mixture of fairly flexible polymers. Most proteins are of
globular conformation, and their surface properties are not easy to
interpret. The values ofZSSa are much higher and tend to increase with the
age of the film. It may take a day to obtain a more or less constant value,
which is typically 0.1–0.5 N?s?m^1. However, the surface layer is clearly
viscoelastic, and the apparent viscosity obtained will strongly depend on
measurement conditions, especially the shear rate. Actually, it cannot
always be ruled out that the proteinaceous surface layer is subject to yielding
or fracture upon large deformation; this would imply that ‘‘slip’’ occurs in
the rheometer, leading to a greatly underestimated viscosity.
The increase in apparent viscosity with time points to slow
rearrangements of protein structure and possibly to the formation of
intermolecular bonds. A protein likeb-lactoglobulin, which contains an
22 SH group, is known to be subject to 22 S 22 S 22 bond reshuffling, leading to
bonds between molecules if these are close to each other. In an adsorbed
layer,ZSSa keeps increasing for days, leading to values well over 1 N?s?m^1.
Surface shear viscosity (and modulus) may thus tell us something
about conformation and thickness of adsorbed protein layers, and especially
about changes with time and with composition. The latter may involve the
addition of small quantities of an amphiphile, which tends to greatly reduce
ZSSa, or partial displacement of an adsorbed protein by another one.
However, a clear and simple theory is not available; combination with the
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