globular proteins; presumably, the changes in surface area covered per
molecule are smaller but take a longer time. It should further be noted that
very fast adsorption (high polymer concentration, intensive convection) may
leave insufficient possibility for unfolding of a polymer (protein) before the
surface is covered; after all, unfolding too takes time.
Figure 10.17a gives some examples of the adsorption rate of a protein
at various concentrations. Calculation according to Eq. (10.6) leads to
values of about 0.6 s, 1 min, 100 min, and 7 days for the decreasing
concentrations given. It is clear that the observed adsorption times are much
longer. The main reason must be complication 7 just mentioned.
Dynamic Surface Tension. For several kinds of practical
problems, the surface tension of a surfactant solution at very short time
scales is important. A case in point is foam formation, where the time scales
of the relevant processes often are of order 1 or 10 ms. Since determination
ofgin static experiments is not possible at such time scales, one often
determines what is called dynamic surface tension. Here the surface of a
surfactant solution is rapidly expanded, andgdynis measured as a function
of the expansion ratex¼dlnA/dt, whereAis surface area; the time scale
then is taken as the reciprocal ofx. A surface confined between barriers (as
depicted in Figure 10.2b) can be expanded by moving the barriers away
from each other. Expansion can also be induced by letting the solution flow
over the rim of a vertical cylinder at a certain rate. Generally, a steady state
(i.e., constantg) is obtained: the supply of surfactant from the bulk to the
FIGURE10.16 Illustration of various stages in the adsorption of flexible polymer
molecules onto an A–W interface. Highly schematic.