(apparent) equilibrium values ofg, i.e., after no further change ing is
observed. At far shorter time scales the relation betweenpandGmay be
quite different, primarily because of the conformational changes occurring
upon adsorption. This is borne out by some results given in Figure 10.18,
which givesgdynvalues as a function of expansion rate. Forb-casein, the
values at which gdyn become constant agree well with the time scales
calculated from Eq. (10.6): 156 and 25 s for 0.1 and 0.25 g/L, respectively.
This means that changes in conformation must have been complete within
about 10 s. Forb-lactoglobulin, it took far longer to reach the finalgvalue,
although the adsorption time should also have been about 25 s; it appears
that the conformational change needed about 10^3 s.
In conclusion, it may take fairly long times for protein conformations
at an interface to become constant, be it after adsorption or after a change in
physicochemical conditions. The same holds for protein composition at the
interface if competition between proteins can occur. Moreover, the
FIGURE10.18 Dynamic surface tension (gdyn) of some protein solutions as a
function of expansion rate (d lnA/dt). 1,b-lactoglobulin, 0.25 g/L. 2,b-casein, 0.1 g/L.
3, same, 0.25 g/L. (Approximate results by courtesy of H. van Kalsbeek, A. Prins.)