Chemistry Reference
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Figure 2 Dynamic surface tension of lysozyme from buoyant bubbles at different bulk
concentrations:
J
,10
10
mol L
1
;
n
,2
10
10
mol L
1
;
,2
10
9
;
$
,
2
10
8
mol L
1
;
B
,5
10
8
mol L
1
;
K
,1
10
7
mol L
1
the apparent probing depth deeper. So, instead of the thickness of the actual
layer with adsorbed molecules, one obtains the thickness of this surface layer
and the adjacent sub-layer. With increasing lysozyme concentration, the dif-
ference between the refractive indices of water and the adsorbed layer becomes
larger, and so the adsorbed layer thickness can be evaluated more accurately.
The layer thickness up to a concentration of 2
10
5
M can be considered
more or less constant, after which it starts increasing. Figure 6 shows that the
adsorbed amount increases continuously throughout the whole concentration
range, although one can see that above 7
10
5
M the surface concentration
increases sharply. This is in agreement with the findings of Lu et al.
38,39
that, at
a concentration of 0.1 wt% (7
10
5
M), lysozyme rearranges its conforma-
tion from a side-on state to an end-on state. When SDS is added to a lysozyme
solution, the thickness of the adsorbed layer is not significantly changed, but
the refractive index and adsorbed amount increase slightly. This increase in
protein adsorption and in the overall protein + ionic surfactant adsorption is
confirmed by theory, although the theoretical model predicts an increase of the
overall adsorption only up to a concentration of 10
5
M.
The plateau which is observed for the equilibrium surface pressure isotherm
for lysozyme (Figure 7) and for the adsorption from ellipsometric measure-
ments (Figure 6) begins at 10
7
up to 10
6
M. In this concentration range
the adsorbed lysozyme molecules are still in a side-on conformation,
38,39
and at
7
10
5
M they rearrange to an end-on conformation. Of course, this could
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