Adsorption Experiments from Mixed Protein + Surfactant Solutions - Food Colloids: Self-Assembly and Material Science - page 214

the measurement of the maximum pressure in a bubble growing at the tip of a
steel capillary (0.25 mm i.d.) immersed into the liquid under study. The
apparatus allows measurements down to a few milliseconds of surface age.
To measure the dynamic surface tension at long adsorption times from
seconds to hours (or days) the profile analysis tensiometer PAT1 (Sinterface
Technologies, Germany) was used. The principle of this method is to determine
the surface tension of the studied solution from the shape of a pendant drop or
a buoyant bubble. 31 By means of an active control loop, the instrument allows
long-time experiments with a constant drop (or bubble) volume or surface area.
The equipment is also suitable for transient and harmonic relaxation studies,
yielding the dilatational elasticity and viscosity of the interfacial layer.
In order to measure the adsorption layer thickness and the adsorbed amount,
ellipsometry measurements have been performed with a Multiskope apparatus
(Optrel, Germany). The operation of this equipment and the procedure to
calculate the layer thickness and adsorbed amount have been described in detail
elsewhere. 32 Briefly, the Multiskope set-up consists of a conventional polarizer/
compensator/sample/analyser (PCSA) null-ellipsometer. A low-capacity laser
(532 nm) serves as the light source of beam diameter 0.5-1mm. The light beam
passes through a first quarter-wave plate to produce circularly polarized light.
Then the light is linearly polarized by a Glan Thompson prism mounted in a
rotatable divided circle, which can be read to very high precision. A second
quarter-wave plate and the analyser (a second Glan Thompson prism) are
mounted in a similar manner. The angle of incidence of the light is 501.A
photodiode serves as detector. Both incidence and reflection arms are motor-
ized and computer controlled. The highly precise computer-controlled motors
of polarizer and analyser allow exact determination for both optical elements of
the null positions, i.e., positions at which a minimum intensity of transmitted
light is registered. From these nul positions of the polarizer and analyser are
obtained the two ellipsometric angles, D and C. These angles characterize the
polarization state of the reflected light, which can be attributed to the prop-
erties of the reflecting surface. 33
14.4 Results and Discussion
The dynamic surface tension of lysozyme measured with the maximum bubble
pressure method is shown in Figure 1. We can see that it does not change
significantly during the experimental time period of 30 s, which suggests a long
induction period. There is much speculation in the literature on protein
adsorption as to what might be the origin of the induction period. For
lysozyme, Xu and Damodaran 34 proposed that there is a negative surface
excess of protein during the induction period and that the surface pressure
increases only after conformational rearrangements have occurred. However,
many authors have assumed that during the initial stages of adsorption the
surface concentration is not zero. Graham and Phillips 35 reported a non-zero
surface concentration for lysozyme from radio-labelling studies, and similar