Polyelectrolyte-mediated surface interactions

www.elsevier.com/locate/cis

Advances in Colloid and Interface

Polyelectrolyte-mediated surface interactions

Per M. Claessona,b,*, Evgeni Poptosheva, Eva Blomberga,b, Andra Dedinaitea

aDepartment of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas vag 51, SE-100 44 Stockholm, SwedenbInstitute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden

Available online 19 March 2005

Abstract

The current understanding of interactions between surfaces coated with polyelectrolytes is reviewed. Experimental data obtained with

various surface force techniques are reported and compared with theoretical predictions. The majority of the studies concerned with

interactions between polyelectrolyte-coated surfaces deal with polyelectrolytes adsorbed to oppositely charged surfaces, and this is also the

main focus of this review. However, we also consider polyelectrolytes adsorbed to uncharged surfaces and to similarly charged surfaces, areas

where theoretical predictions are available, but relevant experimental data are mostly lacking. We also devote sections to interactions between

polyelectrolyte brush-layers and to interactions due to non-adsorbing polyelectrolytes. Here, a sufficient amount of both theoretical and

experimental studies are reported to allow us to comment on the agreement between theory and experiments. A topic of particular interest is

the presence of trapped non-equilibrium states that often is encountered in experiments, but difficult to treat theoretically.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Surface forces; Polyelectrolyte; Steric interaction; Double-layer force; Electrosteric force; Structural force; Depletion attraction; Polyelectrolyte

brush; Flocculation; Stabilization; Colloidal stability; Adsorption; Non-equilibrium state; Surface force apparatus; SFA; MASIF; Atomic force microscope;

AFM

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

2. Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

2.1. Polyelectrolytes at oppositely charged surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

2.1.1. Effect of polyelectrolyte bulk concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

2.1.2. Effect of ionic strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

2.1.3. Effect of polyelectrolyte charge density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

2.1.4. Effect of surface charge density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

2.1.5. Asymmetric systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

3. Polyelectrolytes at uncharged surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

4. Polyelectrolytes at similarly charged surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

5. Grafted polyelectrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

0001-8686/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cis.2004.09.008

T Corresponding author. Department of Chemistry, Surface Chemistry,

Royal Institute of Technology, Drottning Kristinas vag 51, SE-100 44

Stockholm, Sweden.

E-mail address: [email protected] (P.M. Claesson).

1. Introduction

Polyelectrolytes are used in a multitude of traditional

applications. For instance, for rheology control, as wet and

dry strength additives, as flocculating or dispersing agents,

Science 114–115 (2005) 173–187

P.M. Claesson et al. / Advances in Colloid and Interface Science 114–115 (2005) 173–187174

and for surface conditioning to mention but a few. More

recent applications include the use of polyelectrolyte multi-

layers to prepare e.g. core-shell particles and hollow

capsules for controlled drug delivery [1]. Many further

applications can be found in the recent bHandbook of

polyelectrolytes and their applicationsQ [2]. An increased

understanding of polyelectrolyte properties is also of great

importance in the biochemical area where DNA, proteins

and many polysaccharides are polyelectrolytes. Among the

polysaccharides one may mention hyaluronic acid that has

an important function in the lubrication of joints, and the

complex glucoprotein mucin that is the main building block

of the protective coating on many internal surfaces in the

body. Considering the immense importance of polyelectro-

lytes in technology and biology it is obvious that the body

of scientific literature concerned with the properties of

polyelectrolytes in bulk solution and at interfaces is

enormous, and has inspired the publication of several recent

books, see e.g. Refs. [2,3].

This review is considerably more focused and treats

surface interactions generated by adsorbing polyelectrolytes

or polyelectrolytes in solution. This subfield is also large,

and has seen a rapid development in understanding due to

both theoretical and experimental progress. The first direct

measurement of the forces acting between polyelectrolyte-

coated surfaces dates back to the pioneering work of

Luckham and Klein [4] who used the surface force

apparatus to determine the interactions between mica

surfaces coated with poly-l-lysine. Since that time a range

of other surface force techniques, including atomic force

microscopy and thin film pressure balance, have been

employed to shed light on polyelectrolyte-induced surface

forces. A review of some of the surface force techniques

available today can be found in the paper by Claesson et al.

[5]. The theoretical development has been in equally or

perhaps in even more rapid progress, and reliable lattice

mean-field methods, scaling methods, and simulations are

nowadays used to increase our understanding of polyelec-

trolyte-mediated forces. The interested reader is referred to

the excellent book by Fleer et al. [6] and some recent

publications [7–9].

This review focuses on forces between polyelectrolyte-

coated surfaces under approach and separation. This means

that we do not consider the development in understanding of

the frictional properties of polyelectrolyte-coated surfaces,

despite the fact that some important advances recently have

been made in this area [10–14]. Most of the work on

interactions between protein coated surfaces [15] are also

judged to be outside the scope of this review. We have

divided the discussion of the results into four main parts;

polyelectrolytes at oppositely charged surfaces, polyelec-

trolytes at uncharged surfaces, polyelectrolytes at similarly

charged surfaces, and grafted polyelectrolytes. In each of

these sections we start by describing the present theoretical

understanding. This is followed by a recapitulation of some

experimental results, and comments are made on the

agreement between experimental findings and theoretical

predictions.

2. Results and discussions

2.1. Polyelectrolytes at oppositely charged surfaces

Adsorption of polyelectrolytes to surfaces with an

opposite net charge is the most common case encountered

in research on interfacial properties of polyelectrolytes, it is

also in this area that polyelectrolytes find many applications

as e.g. strength additives in paper making, flocculating

agents in waste water treatment, and surface conditioning.

The main driving force for adsorption is electrostatic

attraction between surface charges and charged groups

along the polyelectrolyte backbone, including the entropic

gain due to release of counterions to the surface and the

polyelectrolyte that occurs during the adsorption event. This

situation is usually referred to as electrostatically driven

adsorption. However, in many cases there is also a non-

electrostatic affinity between the surface and the polyelec-

trolyte. The addition of salt screens electrostatic forces, both

the attraction between the surface and the polyelectrolyte

and the intrachain and interchain repulsion between the

polyelectrolytes. For the situation when the polyelectrolyte

adsorption is driven only by electrostatic forces the net

effect of increasing the ionic strength is that the adsorbed

amount decreases, which is due to the reduction in electro-

static polymer-surface affinity that facilitates replacement of

polyelectrolyte segments with small ions at the surface. This

situation is referred to as the screening-reduced adsorption

regime [6]. On the other hand, when the non-electrostatic

surface affinity is sufficiently high in the screening-

enhanced regime, the adsorption is found to increase with

ionic strength up to a maximum, above which the adsorbed

amount may decrease with increasing ionic strength. The

initial increase in adsorbed amount with increasing salt

concentration is due to screening of the repulsion between

the polyelectrolyte chains. This behavior is well understood

theoretically and has been described in detail in e.g. the

book by Fleer et al. [6] and in several publications [7,16–

18]. One example of screening-reduced adsorption is found

for starch adsorption to cellulose [19], whereas screening-

enhanced adsorption is found e.g. for chitosan on mica [20].

Finally, we note that theory predicts that the adsorbed

polyelectrolyte layer on an oppositely charged surface

should be very flat, since it is only next to the surface that

the segment–segment repulsion can be counteracted by the

segment–surface attraction [6]. In the screening-reduced

regime the adsorbed amount decreases with salt concen-

tration whereas the layer thickness increases [18].

Interactions between charged surfaces are, of course,

strongly influenced by adsorption of oppositely charged

polyelectrolytes. Due to the high affinity between the

polyelectrolyte and the surface it is found that addition of

P.M. Claesson et al. / Advances in Colloid and Interface Science 114–115 (2005) 173–187 175

minute amounts of polyelectrolyte leads to a neutralization

of the net surface charge and the removal of the electrostatic

double-layer force. In all surface force studies at low ionic

strength that we are aware of, increasing the polyelectrolyte

concentration has led to a small charge reversal and the

reappearance of the repulsive double-layer force. Thus,

limited recharging is the rule rather than an exception, for an

example see Ref. [21]. We note that the degree of

overcompensation is predicted to increase with increasing

non-electrostatic surface affinity. Experimentally it is

observed that a branched polyelectrolyte structure, such as

in poly(ethylene imine) results in a larger degree of

overcompensation as compared to the linear analogue,

poly(vinyl amine) [22], which can be understood from the

fact that a branched polyelectrolyte by necessity brings a

large number of charges to a relatively small surface area.

2.1.1. Effect of polyelectrolyte bulk concentration

As mentioned above, polyelectrolytes readily adsorb

onto oppositely charged surfaces, causing charge neutral-

ization followed by charge reversal upon increasing the

polymer bulk concentration. The results reported in Fig. 1,

obtained for polyvinylamine, PVAm, on glass surfaces

illustrate this point [23]. In the absence of polyelectrolyte

(uppermost curve) the interactions at large separations are

dominated by the electrostatic double-layer force resulting

from the negative net charge of the glass surfaces in

aqueous media. Addition of only 1 ppm PVAm to the

bulk changes the interaction profile dramatically (inset in

Fig. 1). The long ranged repulsion vanishes and the

surfaces experience an attraction at separations below 30

nm. The absence of electrostatic double-layer repulsion

0.01

0.1

1

10

0 10 20

F/R

(m

N/m

)

D (nm)

-1

-0.5

0

0.5

1

0

F/R

(m

N/m

)

Fig. 1. Force normalized by radius as a function of separation between two flame

Solid lines are DLVO-fits with constant surface charge boundary conditions. The in

representing the calculated van der Waals force. Data from Ref. [23].

indicates that the net negative surface charge has been

completely neutralized by the adsorbing cationic PVAm.

The observed long-ranged attraction is attributed to

polymer bridging. We note that the long range bridging

force has a mainly entropic origin and is due to

polyelectrolyte chains that cross the midplane. These

chains have the possibility of having a larger number of

conformations with low energy due to the fact that

different parts of the chain may experience electrostatic

attraction to different surfaces, i.e. the chains do not need

to be directly bound to both surfaces in order to generate

a bridging attraction of this type [8,24,25]. Increasing the

polymer bulk concentration above 1 ppm brings another

change to the force distance curves. A double-layer force

reappears and the apparent double-layer potential increases

with increasing PVAm concentration. Apparently, addi-

tional charges brought about by PVAm adsorption over-

compensates the surface charge, i.e. the surfaces become

net positively charged. Increasing the bulk concentration

above 10 ppm (not shown) does not bring any change to

the interactions, indicating that a plateau adsorption is

reached at around 10 ppm.

2.1.2. Effect of ionic strength

The ionic strength of the medium has a great influence on

the electrostatic interactions between a polyelectrolyte and

an oppositely charged surface. In low ionic strength

solutions, highly charged polyelectrolytes adopt extended

conformations and are fairly inflexible due to the strong

repulsion between charged monomers. As the salt concen-

tration is increased and the electrostatic intrachain repulsion

decreased, the polyelectrolyte becomes more coiled. This

30 40

2 ppm PVAm

5 ppm PVAm10 ppm PVAm

No polymer

10 20 30 40D (nm)

polished glass surfaces in the presence of cationic polyvinylamine (PVAm).

sert shows the interactions across a 1 ppm PVAm solution with the solid line


has been investigated in detail by Ullner and Woodward

employing Monte Carlo simulations [26]. Adsorption from

low ionic strength solutions is predicted and observed to

result in thin adsorbed layers and the adsorbed amounts are

most often close to that needed to neutralize the surface

charge. Consequently, the steric interaction due to compres-

sion of the adsorbed layers is of short range. Increasing the

ionic strength of the solution has the effect of screening

electrostatic interactions in the system resulting in: i) more

coiled polyelectrolyte solution conformations due to screen-

ing of intrachain repulsion; ii) reduction in the number of

directly surface bound segments and increased length and

fraction of loops and tails of the adsorbed polyelectrolytes.

The thickness of the adsorbed layer increases but the

adsorbed amount may increase or decrease depending on the

non-electrostatic polymer-surface affinity [18]. Thus one

expects that the range of the steric force will increase with

ionic strength, which also is observed experimentally

[20,25,27,28].

The interactions between polyelectrolyte layers formed

by adsorption from aqueous solutions of high ionic strength

are thus dominated by steric forces due to compression of

the extending polymer chains. Strictly speaking the repul-

sion is of electrosteric origin, since the extending tails and

loops are charged and compression results in confinement of

both the polymer chains and the associated counterions. One

example of this situation is illustrated in Fig. 2 that includes

data for the interactions between mica surfaces bearing

adsorbed layers of poly-l-lysine in 0.1 M KNO3 solution

[4]. On initial approach of the surfaces, electrosteric

repulsion becomes detectable at separations below 120 nm

(Fig. 2a). Polyelectrolyte segments extending from the

surfaces are, together with associated counterions, confined

Fig. 2. Force normalized by radius as a function of surface separation between two

at pH 3.5. Fig. 2a shows results from first approach (outer curve) and first sep

compressions and decompressions. The figure is adopted from Ref. [4] with perm

in the gap between the approaching surfaces. The reduction

in entropy upon compression is in many cases the main

cause of the repulsion. The forces measured on separation

are much less repulsive. Consecutive measurements at the

same contact position (Fig. 2b) are reproducible and also

exhibit only a short-range repulsion. During the first

approach, an irreversible (on the time scale of the experi-

ment) compression of the polyelectrolyte layers occurs.

Many of the extending segments have been forced to the

surface, which in turn leads to the reduced range of the

electrosteric repulsion. The consecutive force measurements

are thus conducted between flattened layers.

The general observation of a more long-ranged force

between polyelectrolyte-coated surfaces at higher ionic

strength is in accordance with theoretical predictions.

However, direct comparison between experiments and

theory is not straightforward. The reason is that theoretical

models describe the equilibrium (or quasi-equilibrium) case,

whereas trapped non-equilibrium states are prevalent in

experiments. One evidence for the slow equilibration time is

seen in Fig. 2, compression of the adsorbed layer results in

conformational changes that are not reversible over pro-

longed times. The presence of non-equilibrium trapped

states is due to rapid polyelectrolyte adsorption followed by

very slow reconformation on the surface due to the high

surface-segment affinity. Thus, very different results are

obtained if the polyelectrolyte is adsorbed at low ionic

strength and then the ionic strength is increased compared to

if the polyelectrolyte is adsorbed directly from a high ionic

strength solution [27,29]. Recently, the slow reconformation

of adsorbed polymers has been used to advantage by Tilton

and co-workers. They co-adsorbed polymers and surfactant,

rinsed away the surfactant and the polymer was left in a

mica surfaces across a 100 Ag/mL poly-l-lysine solution in 100 mM KNO3

aration (inner curve). Fig. 2b show force curves obtained on subsequent

ission.

-0.5

0

0.5

1

1.5

2

0 100 200 300 400 500

Distance (Å)

F/R

(m

N/m

)

Fig. 4. Force normalized by radius as a function of surface separation. The

forces were measured across 0.1 mM KBr solutions. The polyelectrolytes

used were PCMA (each segment carries a permanent positive charge, filled

circles), AM-MAPTAC-30 (30% of the segments carry a permanent

positive charge, the neutral AM segment is acrylamide, unfilled squares),

AM-CMA-10 (10% of the segments carry a permanent positive charge,

filled squares), and AM-MAPTAC-1 (1% of the segments carry a

permanent positive charge, unfilled circles). The arrows indicate inward

jumps due to the action of attractive surface forces. The vertical dashed

lines illustrate the thickness of the PCMA layers (left line) and the AM-

MAPTAC-30 layers (right line). The figure is adopted from Ref. [36] with

permission.


trapped non-equilibrium state, see e.g. Refs. [30–34]. Thus,

it was possible to modify the properties of the final adsorbed

polymer layer by varying the conditions during the

adsorption process.

2.1.3. Effect of polyelectrolyte charge density

Another determining factor for the strength of the

polyelectrolyte–surface interactions is the charge density

of the polyelectrolyte itself. As pointed out earlier, highly

charged polyelectrolytes form flat adsorbed layers in low

ionic strength solutions. Under the same conditions,

decreasing the charge density of the polyelectrolyte

(increasing intercharge distance) results in higher adsorbed

amounts and the formation of thicker adsorbed layers. This

is a consequence of the polymers striving to adsorb up to the

point where it neutralizes the surface charge of the substrate.

We note, however, that for low charge density polyelec-

trolytes on highly charged surfaces the steric repulsion

between adsorbing chains prevent full charge compensation

by the adsorbing polyelectrolyte. This has been demon-

strated to be the case for low and moderately charged

cationic polyelectrolytes on the highly negatively charged

mica surface [35,36]. In absence of strong non-electrostatic

interactions with the substrate, the uncharged portion of the

polyelectrolyte chain between two charged segments is

extending into solution in the form of loops and tails. The

adsorbed layer structure is schematically illustrated in Fig.

3. As outlined in the previous section, extended conforma-

tions lead to (electro)steric repulsion. Fig. 4 illustrates the

interactions between mica surfaces precoated with poly-

electrolyte with charge densities varying from 100% of the

monomers being charged (PCMA) to 1% of charged

monomers, AM-MAPTAC-1, across dilute electrolyte sol-

utions. (The AM-MAPTAC polyelectrolytes have a roughly

random distribution of the different segments). Decreasing

the polyelectrolyte charge density results in progressively

thicker adsorbed layers. The interaction between AM-

MAPTAC-1 coated surfaces displays a steric repulsion at

separations below 800 2. In this case the polymer is

attached to the surface with large loops and tails protruding

Fig. 3. Illustration of the conformation of low charge density (a) and high

charge density (b) polyelectrolytes adsorbed on oppositely charged

surfaces. The figure is adopted from Ref. [36] with permission.

into solution. We also note that the adhesion force between

the polyelectrolyte-coated surfaces decreases with decreas-

ing charge density of the polyelectrolyte. Clearly, the

presence of the uncharged segments, i.e. the increase in

distance between charges, counteracts the bridging attrac-

tion, which is in line with theoretical predictions based on

Monte Carlo simulations [24,25]. A summary of some key

findings is reported in Table 1.

2.1.4. Effect of surface charge density

We are not aware of any systematic experimental study

of how the charge density of the surface influences the

forces acting between polyelectrolyte-coated surfaces. How-

ever, from theoretical calculations it is clear that decreasing

the charge density of the surface will, for electrostatically

driven adsorption at low ionic strength, result in a reduction

of the adsorbed amount [6]. This general trend is also

observed, e.g. when comparing the amount of polyelectro-

lyte adsorbed on highly charged mica and weakly charged

cellulose [35]. Both from an experimental and theoretical

point of view it is less clear how the structure of the

adsorbed polyelectrolyte layer will develop as the surface

charge density is decreased. In such a case the surface will

become increasingly heterogeneous that makes theoretical

simulations and mean-field calculations less accurate.

Hence, in this section we have to rely on a small number

of studies where the same polyelectrolyte has been used on

Table 1

Some key characteristics of the forces between mica surfaces coated with

polyelectrolytes of various charge densities

Polyelectrolyte

charge density

(%)

Adsorbed

mass

mg/m2

Dominant

force at large

separations

Adhesion

force

(mN/m)

Distance at

force minimum

(nm)

100 1.0 Bridging 100–200 1

30 1.3 Bridging 2–5 3–4

10 2.0 Bridging 0.5–1 10

1 2.5 Steric 0 –

In all cases the uncharged segment was acrylamide. The charged segment

was either (2-acyloxyethyl)trimethylammonium, CMA, or the structurally

similar (3-methacrylamido)-propyltrimethylammonium, MAPTAC.


vastly different surfaces. The surfaces used include highly

negatively charged mica, moderately charged glass and

weakly charged cellulose. Fig. 5 contains some data on the

adhesion force between polyelectrolyte-coated surfaces in

dilute electrolyte solutions as a function of polyelectrolyte

charge density on different substrates. The data are obtained

using the same copolymers as described in Table 1, and

under conditions when no double-layer force is observed.

It is clear that the increase in adhesion force with

increasing charge density of the polyelectrolyte is observed

both on mica and on glass. It is also clear that for a given

polyelectrolyte the adhesion increases with increasing

charge density of the substrate, i.e. the strongest adhesion

is observed in the case of mica, followed by glass, and the

lowest adhesion is observed in the case of cellulose. This is

in line with theoretical predictions [8,25], and due to that

more polymer bridges can form between the more highly

charged surfaces. Despite this qualitative consistency

1

10

100

0 20 40 60 80 100 120

AD

HE

SIO

N F

OR

CE

/ R

AD

IUS

(mN

/m)

POLYELECTROLYTE CHARGE DENSITY (%)

Fig. 5. The magnitude of the adhesion force normalized by radius for

surfaces coated with polyelectrolytes of different charge densities on mica

(filled circles), glass (unfilled squares) and cellulose (unfilled circle). The

measurements were done at low ionic strength and under conditions when

no, or very weak, double-layer forces were acting between the surfaces.

between theory and experiments we feel that systematic

studies of the effect of the substrate surface charge density

on interactions between polyelectrolyte-coated surfaces

would be of high value.

In at least one case it has been observed that decreasing

the surface charge density results in formation of polyelec-

trolyte layers that extend far out into solution [37]. This is

illustrated in Fig. 6 that displays the forces acting between

one glass and one cellulose surface coated with the 10%

charged AM-MAPTAC-10 polyelectrolyte. Cellulose has

considerably lower charge density than glass under the

conditions of the experiment (0.5 and 1.8 mC/m2, respec-

tively). Adding increasing amounts of AM-MAPTAC-10

leads to the appearance of a progressively more long-ranged

steric repulsion. For comparison, the interaction between

two identical glass surfaces in presence of 50 ppm AM-

MAPTAC-10 is well described by the DLVO-theory. At

large separations the dominant interaction is the exponen-

tially decaying double-layer force. In contrast, the inter-

actions between one glass and one cellulose surface under

identical conditions generate a distorted S-shaped (elec-

tro)steric repulsion detectable at approximately 55 nm from

contact. Apparently AM-MAPTAC-10 forms much more

extended layers on the less charged cellulose surface than on

the more charged glass surface. This may not only be due to

differences in electrostatic surface affinity but also due to

differences in non-electrostatic surface affinity. In fact,

adsorption of AM-MAPTAC-10 to cellulose results in a

large overcompensation of the surface charge [35], demon-

strating the importance of the non-electrostatic surface

affinity. This means that repulsion between the excess

polyelectrolyte charges contribute to the extended layer

structure and thus the long range (electro)steric interaction.

0.01

0.1

1

10

0 10 20 30 40 50 60

F/R

(m

N/m

)

D (nm)

1 ppm 10 ppm

50 ppm

Fig. 6. Force normalized by radius as a function of surface separation

between one glass and one cellulose surface across a range of solutions

containing the cationic polyelectrolyte AM-MAPTAC-10 (10% of the

segments carry a permanent positive charge). Unfilled square represent the

force measured with no polyelectrolyte added. NaCl was added to a

concentration of 0.1 mM in all cases.


2.1.5. Asymmetric systems

The term asymmetric system is used here to describe the

interactions between one surface bearing an adsorbed

polyelectrolyte layer and one charged surface without any

adsorbed polyelectrolyte. The uncoated surface and the

polyelectrolyte are oppositely charged. Such systems have

been investigated theoretically using Monte Carlo simula-

tions and mean-field calculations [38,39]. It was found that

a long-range attraction is present in many cases, and

addition of salt can switch the long-range force from

repulsive to attractive [39]. The most long-range component

of the attraction arises from the feature that the mobile

counterions to the uncoated surface can penetrate into the

polyelectrolyte layer attached to the second surface (the

mobile counterions have the same sign of charge as the

polyelectrolyte). This results in entropic and enthalpic gains.

The entropic gain arises since the mobile counterions can

occupy a larger volume, whereas the enthalpic gain is due to

that the fact that counterions that enter the volume occupied

by the polyelectrolyte layer feel a net attraction to the

substrate surface. In fact, for the case where the adsorbed

polyelectrolyte layer neutralizes the substrate surface

charge, the penetration of mobile counterions within this

layer results in a charge reversal. At shorter separations an

attraction due to bridging becomes increasingly important.

The strength of the attraction increases with polyelectrolyte

flexibility and decreases with increasing salt concentration

[38].

One experimental force curve is illustrated in Fig. 7 that

depicts the forces between one glass surface neutralized by

PCMA adsorption and one bare glass surface. The attractive

force is detectable at separations below 50 nm. Approx-

imately 10 nm from contact the strength of the attraction

exceeds the stiffness of the force sensor and the surfaces

-2

-1.5

-1

-0.5

0

0.5

1

0 10

F/R

(m

N/m

)

D (nm)20 30 40 50 60


between one bare glass surface and one glass surface coated with the

cationic polyelectrolyte PCMA (each segment carries one permanent

positive charge). The forces were measured across a polyelectrolyte-free

aqueous solution containing 0.1 mM NaCl. The upper and lower lines are

DLVO-fits using constant charge and constant potential boundary

conditions, respectively.

jump into contact. It should be pointed out that the attraction

in this case is not a normal attractive double-layer force.

Indeed fitting force curves calculated using standard DLVO-

theory to the experimental data shows that the condition of

constant surface charge (upper solid line in Fig. 7) predicts

repulsion at distances below 50 nm. The fit with constant

surface potential boundary conditions describes the inter-

actions much better. However, some caution should be taken

when resorting to the constant potential model in asym-

metric systems. Interaction under constant surface potential

implies that ions are allowed to adsorb and desorb freely

when the separation between the surfaces changes. For an

interaction between charged surfaces with different magni-

tude of their charge, it predicts infinite charge densities at

D=0. This is clearly unrealistic, since real surfaces have

limited ion adsorption capacities. We note that despite this

complication one report show very convincing agreement

between measurements of forces between one positively

charged polyelectrolyte-coated surface and one negatively

charged bare surface and theoretically calculated attractive

double-layer interactions using constant potential conditions

[40]. In light of the findings from more refined theoretical

work [39], this is not too surprising. These theories predict

that penetration of mobile counterions into the polyelec-

trolyte layer causes a charge reversal of the polyelectrolyte-

coated surface, which is qualitatively similar to the situation

obtained in standard PB-theory using constant potential

boundary conditions and differently charged surfaces. The

underlying mechanism for the observed attraction is,

however, more sophisticated [39] than in the PB-model.

3. Polyelectrolytes at uncharged surfaces

The adsorption of polyelectrolytes on uncharged surfaces

is obviously different from the situation discussed previ-

ously since there is no electrostatic driving force for

adsorption. There is also always an entropic penalty for

the polymer to adsorb to a surface. Hence, the non-

electrostatic polymer-surface affinity must be sufficiently

high for any adsorption to occur. Due to the electrostatic

repulsion between the adsorbing chains, the adsorbed

amount is expected to be low at low ionic strengths, and

the polymers will adopt a very flat conformation on the

surface since it is only at the surface that the non-

electrostatic surface affinity can compensate for the unfav-

orable electrostatic repulsion [6]. We have not been able to

find any report describing interactions between adsorbed

homopolyelectrolyte layers on uncharged surfaces.

(Uncharged polar surfaces can for instance be prepared by

self-assembly of OH-functionalized thiols on gold [41],

whereas non-polar uncharged surfaces can be prepared by

self-assembly of methyl terminated alkyl thiols on gold

[42]). However, there are a few studies reporting on the

adsorption of heterogeneous polyelectrolytes on uncharged

non-polar surfaces [43–45]. In these cases the non-electro-

-10

-5

0

5

10

15

0 20 40 60 80 100 120 140

FOR

CE

/ R

AD

IUS

(mN

/m)

DISTANCE (nm)


between uncharged hydrophobised mica surfaces coated with low charge

density rat gastric mucin (on approach, filled circles and on separation,

unfilled circles) or high charge density pig gastric mucin (on approach,

filled triangles, on separation, unfilled triangles). The mucin concentration

was 0.1 mg/L and 10�4 M NaCl was used as background salt. Data from

Ref. [43].

0.01

0.1

1

0 10 20 30 40 50 60 70

FOR

CE

/ R

AD

IUS

(mN

/m)

DISTANCE (nm)

No added salt

1 mM NaCl

10 mM NaCl

100 mM NaCl

Fig. 9. The normalized force as a function of surface separation between

hydrophobic surfaces coated with amphipol layers across a range of NaCl

solutions. Two measurements are shown for each salt concentration.


static driving force for adsorption arises from interactions

between the surface and non-polar parts of the polyelec-

trolyte. In two of these cases, dealing with proteoheparan

sulfate [44] and h-casein [45], the protein molecules

resemble diblock copolymers with a non-polar anchoring

part and more strongly charged parts extending into

solution. This situation is clearly very different from that

described theoretically above.

It has also been found that mucin, a high molecular

weight anionic glycoprotein that adsorbs readily to most

surfaces, also adsorbs to uncharged non-polar surfaces. The

interactions between such negatively charged mucin layers

have been investigated [43]. It was found that the layer

thickness decreased with increasing charge density of the

mucin, but also for a highly charged mucin the range of the

interaction was considerable, extending to about 60 nm, see

Fig. 8. However, the layer was easily compressed to a

thickness of 7 nm, i.e. 3.5 nm per surface. This shows that

the amount of extending tails and loops was very small and

the inner more compact layer was indeed very thin

considering the very high molecular weight, 15�106 g/

mol, of the glycoprotein. When comparing with the radius

of gyration for the mucins in solution with the extended

layer thickness, it was found that the adsorbed layer

thickness is significantly smaller for both the low and more

highly charged mucin. Thus, the general conclusion from

the theoretical considerations of an adsorption in a thin layer

was confirmed. However, due to the complex structure of

mucin, no detailed comparison between theory and experi-

ments is possible. Furthermore, the forces obtained with the

low charged mucin are purely repulsive, whereas those

obtained with the more highly charged mucin are partly

attractive (Fig. 8). This shows that the mucin molecule

could either bridge over to the opposing surface or that the

hydrophobic patches in the mucin gave rise to a weak

attraction. The interaction between the surfaces coated with

low charged mucin is only weakly dependent on the excess

electrolyte concentration. Hence, steric forces predominate

the interaction for the low charged mucin system.

The interfacial behavior of a new class of amphipathic

molecules, amphipols, which consist of a polyacrylic acid

backbone with grafted octyl side chains has been studied by

employing surface force measurements. These previously

unpublished data show that for the case of a 35% charged

amphipol on neutral hydrophobic surfaces the long-range

part of the repulsion is electrostatic whereas steric inter-

action predominates at a surface separation below about 10

nm (at low ionic strength). The final thickness of the

condensed amphipol layer was 1–1.5 nm. A strong attractive

force between the non-polar chains of the amphipols was

observed upon separation.

The interactions between the adsorbed amphipol layers

can be modulated by changing the salt concentration in the

solution, due to screening of intra- and interpolymer

repulsion (Fig. 9). At low salt concentration, the formation

of a pseudobrush is preferred, comprised of a compact inner

layer and tails extending out from the surface. The distance

dependence of the force does not follow regular PB theory,

but is well described by the Pincus model for surfaces

coated by polyelectrolyte brushes [46]. Upon increasing the

salt concentration, the range of the force decreases

gradually, until at 100 mM NaCl the tails collapse due to

screening of the electrostatic forces, and a compact, about 1


nm thick, layer is formed. At this salt concentration the

adhesion force between the amphipol layers virtually

disappears, suggesting that the non-polar octyl chains

become hidden within the collapsed layer.

4. Polyelectrolytes at similarly charged surfaces

Polyelectrolytes only adsorb to similarly charged surfa-

ces if the non-electrostatic affinity is sufficient to overcome

the electrostatic repulsion and the entropic penalty of

adsorption. Thus, polyelectrolyte adsorption occurs most

easily at high salt concentrations and for low charge density

polyelectrolytes. For instance, mucin, which is a weakly

negatively charged polyelectrolyte, adsorbs on negatively

charged mica surfaces and in 30 mM NaNO3 generates

long-range repulsive forces of electrosteric origin.

Adsorption of highly charged polyelectrolytes on highly

charged surfaces of the same charge is hindered by

electrostatic repulsion that can be moderated by adding

multivalent metal ions. Berg et al. [47] showed that there is

no evidence for sodium polyacrylate (NaPAA) adsorption

Fig. 10. Force normalised by radius between two mica surfaces in aqueous solutio

and 2.8�10�3 M CaCl2 at pH 6.4 (x), and 490 ppm NaPAA, 3�10�5 M KBr, an

from DLVO theory assuming constant charge (upper curves) or constant potential

unstable region, from which the surfaces jump into adsorbed layer contact. The ju

layer thickness when 140 and 490 ppm NaPAA is present, respectively. Insert: The

at pH 6.1 (!) and after addition of 3.2�10�3 M CaCl2, no NaPAA, pH 6.1 (E).

Waals force for mica interacting across water. The figure is adopted from Ref. [4

on negatively charged mica surfaces. However, added Ca2+

associates with both mica and PAA, thus reducing the

electrostatic repulsion between the polyelectrolyte and the

surface, which leads to polyacrylic acid adsorption. More-

over, by adjusting the concentration of Ca2+ ions in solution

the extent of PAA adsorption on the surfaces can be tuned

such that mica surface charge undercompensation, neutral-

ization or overcompensation can be achieved, see Fig. 10.

Further, there exists a strong attraction between the

surfaces coated with PAA that has been suggested to be due

to formation of interlayer Ca2+ bridges [47]. In a later study

on divalent metal ion effects on PAA mediated mica surface

interactions Abraham et al. [48] showed that the extent to

which adsorption of PAA on mica is promoted, as well as

the conformation of the adsorbed PAA, depends on the

hydrated size of the cation. While highly hydrated ions, such

as Mg2+, induce a limited PAA adsorption and low coverage

of the surface, whereby favoring long-range bridging

attraction, ions with low hydration degree, such as Ba2+,

cause precipitation of PAA on the surfaces, eventually

leading to a very strong and long range repulsive surface

force.

ns of 1.1�10�4 M CaCl2 at pH 6.1 (!); 140 ppm NaPAA, 3�10�5 M KBr,

d 2.5�10�3 M CaCl2 at pH 6.0 (n). Solid lines represent forces calculated

(lower curves) conditions. The open symbols indicate data points within the

mps are indicated by arrows. The dotted and the dashed line represent the

force normalised by radius showed on a linear scale in 1.1�10�4 M CaCl2The solid line present on the attractive side represents the expected van der

7] with permission.


Guldberg-Pedersen and Bergstrfm studied interactions

between ZrO2 surfaces coated with PAA [49,50] using

AFM and showed that at high pH, where the sign of the

charge of the surface and polyelectrolyte is the same, the

adsorbed amount is low but the range of the measured

superposed steric and electrostatic repulsive forces is

large. This indicates a high degree of dissociation of the

anionic polyelectrolyte and that the polyelectrolyte chains

are stretched away from the surface. In contrast, flat

layers of PAA adsorbed on negatively charged Si3N4

surface under condition of pHNpHiep were found by Laarz

et al. [51]. The pronounced differences in PAA con-

formation on the two negatively charged mineral surfaces

were explained by differences in local surface–segment

attraction. It was argued that the local electrostatic

surface–segment attraction in the Si3N4/PAA system

allows PAA adsorption sufficiently close to pHiep. How-

ever, since the binding in this case is relatively weak a

subsequent relaxation into a flat polymer conformation is

possible. In contrast, the strong irreversible surface–

segment association in the ZrO2/PAA system prevents

PAA relaxation at the interface, and the conformation of

the adsorbed polyelectrolyte at the surface will be similar

to that in the bulk. Thus, the large surface–segment

affinity for this system is responsible for the formation of

extended layers, and this surface conformation may be

regarded as a trapped non-equilibrium state.

It is interesting to note that also non-adsorbing

polyelectrolytes influence the interaction between surfaces.

This is due to the fact that confinement changes the

structure of the polyelectrolyte solution and this change

induces a structural force. Experimentally this force is

observed as a decaying oscillatory force similar to, but

with longer range and smaller amplitude than, the

structural force observed in pure liquids [52,53]. The

well-known depletion attraction [54,55] can be viewed as

a special case, involving only one oscillation. The

depletion as such, which is present also for non-adsorbing

uncharged polymers, results from the confining effect of

the walls that reduces the number of available conforma-

tions of the polymer and thus reduces the chain entropy.

For similar charged systems repulsive electrostatic inter-

actions between the polyelectrolyte and the surface also

contribute to the depletion.

The forces due to confinement of macroions have been

investigated theoretically in a small number of publica-

tions [9,56,57]. For instance, Jfnsson et al. [9] employed

Monte Carlo simulations and density functional calcula-

tions and concluded that for spherical macroions, mimick-

ing spherical charged micelles, the oscillation period

scales with the bulk concentration to the power of �1/

3. Oscillating forces across micellar solutions confined

between solid walls [58] and between air–liquid interface

[59] have indeed been observed experimentally. The

predicted dependence of the oscillation periodicity on

the bulk concentration of micelles is also confirmed [60].

The oscillating force curve results from layering of

micelles in the gap between the surfaces and each

oscillation is due to the removal of one layer of micelles

[9,59]. Oscillating structural forces can be expected when-

ever the average distance between the charged micelles is

smaller than the range of the intermicellar repulsion [9].

Theoretically it has been predicted that the molecular

weight of the polyelectrolyte and the nature of the

confining walls will have very small influence of the

long-range structural force across polyelectrolyte solutions

[9]. The very limited influence of the polyelectrolyte

molecular weight is supported by experiments [61].

Further, the prediction that the nature of the surface,

similar charged, uncharged or oppositely charged to the

polyelectrolyte, has a limited effect on the nature of the

oscillating forces is also confirmed by experiments [62–

65]. Since the organization of the polyelectrolyte solution

is caused by the repulsion between polyelectrolyte chains

any variation that will reduce this repulsion, e.g. decreas-

ing the linear charge density of the polyelectrolyte or

increasing the ionic strength, will lead to a reduction in

the oscillating structural force, which is confirmed by both

theory [9] and experiments [62,66]. Simulations have

shown that linear polyelectrolytes are aligned preferen-

tially parallel to the confining walls [9], and experiments

have shown that the periodicity of the oscillations scale

with the bulk concentration to the power of �1/2

[63,67,68], i.e. with the same scaling law as the mesh

size in bulk polyelectrolyte solution [61,63]. On the other

hand, highly branched polyelectrolytes provide oscillations

which scale with the bulk concentration to the power of

�1/3 [69].

We use the results from Milling and Kendall [70] to

illustrate these forces. They used AFM to measure forces

between silica surfaces in solutions of NaPAA at pH 7.

The measured forces showed that in absence of added

salt, NaPAA was depleted from the surface. This caused

the development of an attractive depletion force at close

approach of the surfaces, and weak oscillating structural

forces at large surface separations with a scaling length

proportional to [polyelectrolyte concentration]�1/2.

As seen from Fig. 11, both the depletion layer

thickness and the correlation length systematically

decrease with increasing polymer concentration. The

magnitude of the oscillations, on the other hand,

increases. Force curves of the same type were observed

in AFM studies of sodium poly(styrenesulfonate) at silica

surfaces, [62] and with partially sulfonated polystyrene

(PSS) of various charge fractions [71]. Similar structural

forces across solutions containing the anionic polysac-

charide, carboxymethylcellulose, [62–64] and the acryl-

amide and acrylamido-methyl-propane sulfonate

copolymer [59] was found in surfactant stabilized thin

liquid films. The oscillation period of the oscillations

scaled as [polyelectrolyte concentration]�1/2, as predicted

by theory, and was close to the mesh size defined for

Fig. 11. Force divided by radius for a silica sphere interacting with a silica

plate at various NaPAA (Mn=33 kg/mol) concentrations at pH 7 in the

absence of background electrolyte. The figure is adopted from Ref. [70]

with permission.


semidilute polymer solutions. The reader interested in

structural forces in solutions containing polyelectrolytes is

also referred to two recent reviews (See Refs. [60,72]).

5. Grafted polyelectrolytes

The structure of polyelectrolyte brushes anchored to

surfaces [46,73], and how the structure is influenced by

polyelectrolyte charge density, chain length and graft

density has been theoretically investigated in detail [74].

The effects of ionic strength [74–76], multivalent ions

[77], the nature of the charged groups [75,78] (weak or

permanent charges), and solvent quality [79,80] have also

been considered in a large number of theoretical publica-

tions during the last 15 years. Publications treating the

effect of the elasticity of the underlying substrate to which

the polyelectrolytes are grafted, [81] or surface curvature

effects [82] can also be found. Some experimental results

describing the segment density profile for polyelectrolyte

brushes are also available in the literature, see for instance

Ref. [83]. One important feature is that most of the

counterions to the polyelectrolytes are incorporated within

the brush layer, which is a consequence of the strong

Coulombic attraction. When the polyelectrolyte brushes are

grafted onto neutral walls the brushes are highly extended,

mainly due to the excess osmotic pressure due to the

mobile counterions.

The forces acting between surfaces carrying grafted

polyelectrolyte chains have been predicted [46,84–87]. It

is typically found that the outermost part of the force

curve has a decay-length equal to that of a double-layer

force. This force is due to overlap of the diffuse ionic

clouds that extend beyond the edge of the brush layer. At

smaller separations, but starting prior to direct brush

layers contact, counterions are redistributing to become

further incorporated within the brush layer that responds

by contracting. At this stage the resulting force depends

crucially on the ionic strength of the surrounding solution.

Zhulina and co-workers distinguish between three cases,

the osmotic regime, the salt dominance regime, and the

quasi-neutral regime [84]. When the concentration of

counterions within the brush is larger than the concen-

tration of ions in bulk solution we have the osmotic

regime (the bosmotic brushQ). In this regime the brush

height increases with salt concentration [78]. The force is

mainly due to the osmotic pressure of mobile counterions

within the brush. In this regime the force (between flat

plates) vary with separation, D, to the power of D�1. On

the other hand, when the salt concentration in bulk is

larger than the concentration of counterions in the brush,

the salt dominance regime (the bsalted brushQ), the

osmotic pressure of the added salt contributes significantly

to the forces that now have a distance dependence of

D�2. In the salted brush regime the brush layer thickness

decreases with increasing salt concentration [78]. At very

high salt concentrations electrostatic forces become insig-

nificant and the force is governed by the osmotic pressure

of the polyelectrolyte segments. Under theta conditions

this results in a force varying with separation to D�3. It is

important to note that when the brush layer is compressed

the concentration of counterions within the layer will

increase. Hence, one may enter into different regimes as

the compression progresses, and the resulting force curve

may be rather complex. We further note that the effect of

salt is more complex when the polyelectrolyte brushes are

built from ionizable groups. In this case the salt

concentration also affects the degree of dissociation within

the brush layer, as discussed in Ref. [84]. One conse-

quence is that the osmotic regime is significantly smaller

compared to the situation with polyelectrolyte brushes

having permanent charges. The distance dependence of

the force in this regime is also altered and, due to

reduction in the charge density of the brush with

decreasing separation, now goes as D�0.5 [84].

Polyelectrolytes can form brushes on solid–liquid [88–

90] and air–liquid [91,92] interfaces through adsorption

[93] or by forming covalent bonds via chemical grafting

[94]. There are some reports on direct measurements of

the forces acting between brush-like polyelectrolyte layers

in the literature [88–90,93,95–99]. For instance, Kurihara

et al.[88,89] investigated interactions between anionic

chain-end-anchored monolayers of poly(metacrylic acid)

(PMA), a charge regulating polyelectrolyte, as a function

of pH and background salt concentration. It was

demonstrated that at low pH in water, where PMA is

close to charge neutral and adopts a hypercoiled

conformation, surface interactions could be described by

the diffuse electrical double-layer model as used for

normal solid surfaces. At higher pH and inert salt

Fig. 12. A universal force curve in the osmotic brush regime, incorporating

data for three different molecular weights. One force curve is shown for

each salt concentration. The separations are normalized by twice of the

equilibrium brush height at the corresponding salt concentration, and the

measured force F/R is normalised by the prefactor (2kkTjaN), which is

derived from scaling theory. The solid line is a fit based on equations from

the scaling theory (see Ref. [100]). The figure is adopted from Ref. [100]

with permission.


concentrations, an increase in both the range and

magnitude of surface interactions was observed. These

variations were attributed to the ionization of PMA that

forces the polyelectrolyte to adopt a more extended

conformation. On the whole, interactions between such

weak polyelectrolytes grafted to surfaces are complex and

affected by structural changes of the polyelectrolytes, salt-

promoted ionization, and condensation of counterions. For

instance, at elevated pH, when the chains of PAM are

strongly ionized, addition of salt causes, on the one hand,

further increase in chain ionization, and on the other hand,

screening of electrostatic interactions. These two phenom-

ena cause opposite trends in surface interactions. The

former effect was found to dominate and cause increased

repulsive forces at low pH and strong compressions. At

high pH and weak compression, the latter effect was

found to dominant and the repulsive force decreases with

increasing salt concentrations. Later, interactions between

hydrophobically anchored strongly charged polyelectrolyte

brushes as a function of salt concentration were studied

by Abraham et al. [99] who used hydrophobic–hydro-

philic diblock poly(tert-butyl methylacrylate-b-poly(gly-

cidyl methacrylate sodium sulfonate) (PtBMA-b-PGMAS)

as a polyelectrolyte. They showed that the measured

interactions are due to a combination of electrostatic and

steric forces. A decrease in the range of the measured

force with increase in added salt concentration was found

as result of electrostatic screening. However, the added

salt effect is less pronounced than the one observed for

regular charged surfaces. At low salt conditions, the long-

range interactions can be quantitatively described by

classical electrical double-layer theory, whereas long-range

forces with added salt deviate from those expected using

classical considerations and the brush structure has to be

taken into account. Kelley et al. [93] who employed

poly(4-tert-butylstyrene)-sodium poly(styrene-4-sulfonate)

(PtBS-NaPSS) studied interactions between such polyelec-

trolyte covered surface and a Si3N4 tip as a function of

salt concentration using AFM. These authors observed a

decrease in repulsive force range with increasing salt

concentration and attributed this to a shrinking of the

brush layer due to screening of electrostatic interactions.

In a further study, Balastre et al. [100] studied hydro-

phobically grafted NaPSS brushes on mica surfaces at

various salt concentrations and showed that no exponen-

tially decaying force could be detected, at any of the salt

concentrations. Thus long-range double-layer interactions

were not of prime importance. There were two regimes of

polyelectrolyte brush behavior observed. In the bsaltedQbrush regime, the interaction range shifted to progres-

sively larger distances with the decreasing salt concen-

tration due to decreasing screening and increasing

electrostatic excluded-volume repulsion. In the bosmotic

brushQ regime the measured forces between the brush

layers showed almost no dependence on salt concen-

tration. The authors succeeded in fitting the data for

polymers of different molecular size into one general

force curve profile for the bosmoticQ regime and one for

the bsaltedQ brush regime, see Figs. 12 and 13. The fits

were based on equations from the scaling theory and

corroborated the validity of this theory in both regimes.

In investigations using a polypeptide poly(l-glutamic

acid) (PLGA) [98,95] it was demonstrated that the forces

between polypeptide brushes could be characterized by the

combination of long-range electrostatic and short-range

steric repulsion. A significant correlation between the

secondary structure of the polypeptides and the steric force

was also found. Hayashi et al., using polyelectrolyte brush

layers formed by PLGA and poly(l-lysine) (PLL), inves-

tigated the dependence of surface interactions on polyelec-

trolyte degree of polymerization, solution pH and salt

concentration [97]. They showed that the thickness of the

brush layers agrees well with the extended length of the

polyelectrolytes, is independent of salt concentration and

scales with the degree of polymerisation. The effective

charge density of the brush layer is much smaller than the

density of ionized groups in the brush, demonstrating that

the small counterions are incorporated in the brush layer as

predicted by theory. The elastic compressibility modulus

calculated from the surface force data demonstrated that for

brush layers of PLGA and PLL the modulus increases with

increasing ionization degree and decreases with increasing

salt concentration because of a decrease in the osmotic

Fig. 13. A universal force curve in the salted brush regime, incorporating

data for three molecular weights. One curve is shown for each salt

concentration. The separations are normalized by twice of the equilibrium

brush height at the corresponding salt concentration, and the measured force

F/R is multiplied with the prefactor a2/3Cs2/3/(2kkTj5/3a4/3N) which is

derived from the scaling theory. The solid line is a fit based on equations

from the scaling theory (see Ref. [100]). The figure is adopted from Ref.

[100] with permission.


pressure of the counterions. Further, it is worth noticing a

work of Gelbert et al. [96] who probed collapse of strong

and weak polyelectrolyte brushes by noise analysis of a

scanning force microscope cantilever. It was demonstrated

that polyelectrolyte brushes posses an increased dynamic

compressibility when they are in a near-collapsed state.

For further reading on the fascinating topic of inter-

actions between polyelectrolyte brushes we refer to a short

overview by Guenoun et al. [101] of available theories on

planar polyelectrolyte brushes, spherical polyelectrolyte

brushes and experimental results on interactions between

charged brushes obtained until year 2000.

6. Summary

The theoretical framework for describing polyelectrolyte

adsorption, structure of adsorbed polyelectrolyte layers and

forces between polyelectrolyte-coated surfaces is well

developed and seems to describe many experimental results

satisfactory. In fact, in many cases we find that more

experiments are needed in order to fully test the detailed

theoretical predictions. A major difficulty is the fact that

trapped long-lived non-equilibrium states are prevalent. To

describe such situations the theoretical models need to be

further developed, and the design of the experimental

studies must be carefully considered. The possibility of

using trapped non-equilibrium states to produce surface

coatings with appropriate properties for applications is

exiting, and up to now not much has been done in this

area. It is thus a promising research field for the coming

years.

We also notice a lack of experimental results where the

effect of the surface properties on polyelectrolyte adsorption

and the resulting surface forces has been investigated in a

systematic manner. Such studies should be feasible utilizing

the possibilities of forming mixed self-assembled mono-

layers using e.g. functionalized alkylthiols on gold sub-

strates. Too little is also known about the effect of

multivalent mobile ions, both as counterions to the substrate

surface and to the polyelectrolyte. The increased electro-

static coupling is expected to strongly affect both the

polyelectrolyte-surface affinity and the conformation of the

polyelectrolyte. This may be of particular importance for

polyelectrolytes of biochemical origin.

The forces acting between oppositely charged surfaces

also need further consideration, and the predictions based

on modern theories have not yet been carefully tested. This

is another challenge for the future, which is of high

relevance for understanding and controlling the properties

of multilayer assemblies prepared by e.g. the layer-by-layer

deposition method. A complication in experimental studies

is the possibility of material transfer between the two

surfaces, which may cause the layer structure and surface

interaction to change after each contact between the

adsorbed layers.

Finally, the lubricating properties of polyelectrolyte

layers have, with a few recent exceptions, not been

investigated in great detail. This is another important issue,

not the least in the area of biopolymers. In this area, the

experimentalists seem to be ahead of the theoreticians. It

would also be of considerable interest to investigate

lubricating properties of polyelectrolyte–surfactant mix-

tures, which can form well-structured surface aggregates.

Thus, despite the impressive progress that has been made in

understanding forces between polyelectrolyte-coated surfa-

ces during the last 20–30 years, many important challenges

remain, both theoretically and experimentally.

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Polyelectrolyte-mediated surface interactions

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