Effect of Polymer Molecular
Weight and Solution pH
on the Surface Properties of Sodium Dodecylsulfate-Poly(Ethyleneimine)
Mixtures
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Abstract
The effect of polymer molecular weight and solution pH
on the surface
properties of the anionic surfactant sodium dodecylsulfate, SDS, and
a range of small linear poly(ethyleneimine), PEI, polyelectrolytes
of different molecular weights has been studied by surface tension,
ST, and neutron reflectivity, NR, at the air–solution interface.
The strong SDS–PEI interaction gives rise to a complex pattern
of ST behavior which depends significantly on solution pH and PEI
molecular weight. The ST data correlate broadly with the more direct
determination of the surface adsorption and surface structure obtained
using NR. At pH 3, 7, and 10, the strong SDS–PEI interaction
results in a pronounced SDS adsorption at relatively low SDS and PEI
concentrations, and is largely independent of pH and PEI molecular
weight (for PEI molecular weights on the order of 320, 640, and 2000
Da). At pH 7 and 10, there are combinations of SDS and PEI concentrations
for which surface multilayer structures form. For the PEI molecular
weights of 320 and 640 Da, these surface multilayer structures are
most well-developed at pH 10 and less so at pH 7. At the molecular
weight of 2000 Da, they are poorly developed at both pH 7 and 10.
This evolution in the surface structure with molecular weight is consistent
with previous studies, where for a molecular
weight of 25 000 Da no multilayer structures were observed
for the linear PEI. The results show the importance with increasing
polymer molecular weight of the entropic contribution due to the polymer
flexibility in control of the surface multilayer formation