Adsorption and Electrokinetic Properties of
Polyethylenimine on Silica Surfaces
Ro ´bert Me ´sza ´ ros,*
,†,‡
Laurie Thompson,
†
Martin Bos,
†
and Peter de Groot
†
Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, CH63 3JW, U.K.,
and Department of Colloid Chemistry, Lora ´ nd Eo ¨ tvo ¨ s University, P. O. Box 32,
Budapest 112, Hungary H-1518
Received December 7, 2001. In Final Form: February 15, 2002
The adsorption on silica wafers of a hyperbranched, high molecular weight polyethylenimine (PEI) was
investigated using reflectometry. The pH and ionic strength dependence of PEI adsorption kinetics and
that of the adsorbed amount were interpreted according to the complex balance of segment/segment and
segment/surface site interactions. The observed adsorption properties show significant deviations from the
recently studied features of polyvinylamine adsorption on cellulose coated silicas. The different behavior
can be attributed to the pronounced difference in the nonelectrostatic affinity of polyamines toward the
two different types of surfaces. The role of electrostatic interactions was also characterized by electrokinetic
measurements. Due to the adsorption of PEI, significant charge reversal and shift in the isoelectric point
of silica wafers occur. The potential-pH curves show a maximum, which can be interpreted qualitatively
by the adsorption characteristics of PEI. An attempt was also made to interrelate the adsorption and
electrokinetic data via comparison of different estimates of the diffuse double layer charge of the PEI/silica
system.
Introduction
Polyethylenimine-based polyelectrolytes are widely
used as adhesives, dispersion stabilizers, thickeners, and
flocculating agents as well as in the paper industry as
effective drainage and retention aids for paper fines,
pigments, fillers, and dyes.
1-3
In all of these applications
the function of PEI is largely determined by its adsorption
and electrokinetic properties.
The adsorption of weak polyelectrolytes on oppositely
charged surfaces depends on parameters including pH
and ionic strength, which can significantly change the
charge density of the polymer and the surface as well.
However, the nonelectrostatic surface affinity and the
solvent quality also play an important role.
4
The actual
balance of the interactions between the different con-
stituents of the bulk solution and the surface layer
(polymer segments, solvent molecules, surface groups, etc.)
determines the equilibrium properties of the system. An
additional feature of the adsorbed weak polyelectrolytes
that their charge density is adjusted in the adsorbed layer
due to the local electrostatic potential profile near the
surface.
5,6
As a consequence of this, a very complex diffuse
double layer develops the structure that has been described
using sophisticated theoretical simulations.
5,7
In the case
of branched or starlike polyelectrolytes the problem is
even more complicated since the adsorption of these
structures is less understood than that of the linear ones.
8
In addition to the obvious importance of equilibrium
adsorption, the reversibility of polyelectrolyte adsorption
is also an essential question.
9-11
For example, there is
strong evidence that the adsorption of some weak poly-
electrolytes shows significant hysteresis in a pH cycle
experiment.
12
Therefore, the mechanism and kinetics of
adsorption should also be carefully considered. The
mechanism of polyelectrolyte adsorption can generally be
viewed as a three-step process: transport from the bulk
to the surface, attachment to surface, and rearrangement
in the adsorbed layer. On the practical time scale of the
adsorption experiments the kinetics are mainly deter-
mined by the balance of the first two steps, which can be
readily monitored by the recently developed technique of
the stagnation point flow cell.
13
In this paper we present a comprehensive study of the
ionic strength and pH-dependent dynamic and static
adsorption properties of a hyperbranched polyethylenen-
imine on silica wafers. Our data will be compared with
the recent results of Geffroy et al. for the adsorption of
polyvinylamine (PVAm), which is a similar but linear
polyamine, on cellulose-coated silica wafer,
14
and an
attempt will be made to explain the differences. Finally,
electrokinetic measurements will be presented and the
* Corresponding author.
†
Unilever Research Port Sunlight Laboratory.
‡
Lora ´nd Eo ¨tvo ¨s University.
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10.1021/la011776w CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/11/2002