An adsorption model is proposed which assumes a different partial
molar surface ω for adsorbed molecules
depending on the degree of saturation of the adsorption layer.
Equations for an adsorption isotherm and a
surface tension isotherm are derived for the adsorption of one single
surfactant or of a mixture of surfactants.
The model assumes that the molecules may adsorb in two different
states, i.e. a large ω at smaller surface
pressures Π and a smaller ω at large surface pressures Π.
Adsorption data of
(N-n-hexadecyl-N,N-dimethylammonio)acetic acid bromide at the air/solution interface
are used to demonstrate the application of
the theory. The experimental results are in very good agreement
with the model.
Thermodynamic equations for describing protein adsorption layers
at liquid/fluid interfaces are derived as a
generalization of a theory published recently [J. Colloid
Interface Sci. 1996, 183, 26].
In this new theory the
nonideality of enthalpy (Flory−Huggins' parameter) and entropy of
mixing are taken into account, and also
the effect of the electric charge of the protein molecules on surface
pressure is considered. The model is
verified by experimental dynamic and equilibrium surface tension data
for HSA solutions obtained from
pendent drop experiments (ADSA). The derived isotherm is in good
agreement with the experimental data.
The values of the isotherm parameters surface area per molecule,
electric charge of the HSA molecule, and
the adsorption layer thickness are close to values obtained by other
methods in literature. It follows that
HSA molecules undergo almost no denaturation at the solution/air
interface and occupy a surface area of
about 50 nm2, independent of the packing in the adsorption
layer, which is in agreement with the concept of
a triple-domain structure.
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