Chemistry Reference
In-Depth Information
When surfactant is added, the adsorption mechanism may be changed, as
well as the rheological characteristics of the surface layer.
3-11
The pro-
tein
surfactant interactions depend mostly on two factors - the nature of
the surfactant (non-ionic, ionic, and zwitter-ionic) and its concentration in the
solution bulk phase. For protein + non-ionic surfactant systems, the adsorp-
tion is competitive,
9,12-24
whereas for protein + ionic surfactant systems the
mechanism is totally different. Here several concentration ranges have to be
distinguished in which different kinds of interactions dominate. So, Coulombic
interactions are observed at low surfactant concentration, whereas hydro-
phobic interactions become more important with increasing surfactant concen-
tration.
In the present work, the adsorption behaviour of the globular protein
lysozyme with and without surfactant will be described. First, we consider
the dynamic surface tension for lysozyme, presented as surface tension versus
time curves at the air
water interface and compared with the theoretical
model. Secondly, we discuss mixed systems of lysozyme at a constant concen-
tration (7
10
7
mol L
1
) with varying concentrations of the anionic surfactant
SDS. The equilibrium surface tension versus concentration isotherms for lyso-
zyme alone and with SDS present will be analysed and compared with the
behaviour of a theoretical model.
14.2 Theoretical Approach
14.2.1 Thermodynamic Model
The theory of protein adsorption with changing partial molar area has
been described in detail elsewhere.
25
It is based on the idea of Joos and
Serrien,
26
according to which the protein component adsorbs at the inter-
face with two different molar areas. This model has been generalized
to allow protein molecules to exist in multiple states in the interfacial
layer.
Our model assumes that protein molecules can exist in a number of states of
different molar area, varying from the maximum value (
o
max
) at very low
surface coverage (low surface pressure) to the minimum value (
o
min
) at high
surface coverage (high surface pressure). The molar areas of two 'neighbouring'
conformations differ from each other in terms of the molar area increment
o
0
,
chosen to be equal to the molar area of the solvent molecules. If the total
number of possible states of an adsorbed protein molecule is n, the molar area
in the ith state is
o
i
¼
o
1
+(i
1)
o
0
and the maximum area is
o
max
¼
o
1
+
(n
1)
o
0
, where
o
1
¼
o
min
c
o
0
.
The thermodynamic model is based on Butler's equation of the chemical
potentials of the ith state of the protein molecule within the interfacial layer and
in the solution bulk.
27,28
The equation of state is based on a first-order model,
accounting for non-ideal entropy and enthalpy of mixing for the surface
Search WWH ::
Custom Search