The surface excess of sodium dodecyl sulfate (SDS) in aqueous
solutions of SDS and the polymer poly(vinylpyrrolidone) (PVP) has been measured as a function of SDS and PVP
concentrations using neutron
reflection. Below the critical aggregation concentration (CAC) the
adsorption of SDS is increased by the
presence of PVP, indicating that the two components interact
cooperatively at the surface. Between the
CAC and the critical micelle concentration (CMC) of the surfactant
there is a slight depletion of SDS from
the surface. Comparison of coverages determined by neutron
reflection with those from earlier radiotracer
work indicates that, in the higher concentration range, PVP is bound to
the surfactant layer, creating a
region from which surfactant is depleted, which is further evidence for
a strong polymer/surfactant interaction
at the surface. Comparison of the effect of added PVP on the
surface tension with the neutron reflection
measurements indicates that, even below the CAC, the surfactant
complexes to the polymer to some extent
in the bulk solution. There are no measurable effects of the
polymer on the thickness of the surfactant
layer at any concentration. There is an indication that at the
surface the surfactant is slightly displaced
outward from water on addition of polymer, but accurate structural
determination of the mixed layer
proved too difficult to be certain of this result.
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