Interfacial behavior of randomly charged sulfonated polystyrene (PSS) at the air/water interface

Abstract

Langmuir monolayers of randomly charged polystyrene sulfonated acid, PSSx, with various degrees of sulfonation (x) ranging from 4 up to 35% were studied. Three characteristic regimes of PSSx were determined, according to the balance between electrostatic and hydrophobic interactions in these ranges. Low charged PSS (4–5%) aggregates to irreversible films upon compression, leading to strong hysteresis. Moderately charged PSS (6–16%) is shown to less hysteretic. In contrast, highly charged PSS (35%) formed a reversible film due to dominant electrostatic interactions over hydrophobic interaction. Using in situ neutron reflectivity (NR) we observed that PSS with 35% sulfonation formed a highly stretched brush into the water subphase upon compression.

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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 660–665
Interfacial behavior of randomly charged sulfonated
polystyrene (PSS) at the air/water interface
Young-Soo Seoa, Farhan Ahmadb, Kwanwoo Shinc,∗, Ju-Myung Songd, Joon-Seop Kimd,
Miriam H. Rafailoviche, Jon Sokolove, Sushil K. Satijaf
aDepartment of Nano Science and Technology, Sejong University, Seoul 143-747, Republic of Korea
bDepartment of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
cDepartment of Chemistry and Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea
dDepartment of Polymer Science and Engineering, BK21 Education Center of Technology for Advanced Materials and Parts,
Chosun University, Republic of Korea
eDepartment of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794, United States
fNational Institute of Standards and Technology, Gaithersburg, MD 20899, United States
Received 20 November 2006; accepted 30 April 2007
Available online 2 June 2007
Abstract
Langmuir monolayers of randomly charged polystyrene sulfonated acid, PSSx, with various degrees of sulfonation (x) ranging from 4 up to 35%
were studied. Three characteristic regimes of PSSxwere determined, according to the balance between electrostatic and hydrophobic interactions
in these ranges. Low charged PSS (4–5%) aggregates to irreversible films upon compression, leading to strong hysteresis. Moderately charged PSS
(6–16%) is shown to less hysteretic. In contrast, highly charged PSS (35%) formed a reversible film due to dominant electrostatic interactions over
hydrophobic interaction. Using in situ neutron reflectivity (NR) we observed that PSS with 35% sulfonation formed a highly stretched brush into
the water subphase upon compression.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Polyelectrolytes; Langmuir monolayer; Polystyrene sulfonated acid
1. Introduction
A sound understanding of the interfacial behavior of poly-
electrolytes is necessary for numerous applications in thin film
coating technology, particularly those involving biomimetic
membranes[1,2],stabilizationofcolloids[3],electronicmateri-
als [4,5], and cell adhesion for biomedical applications [6,7]. A
variety of external parameters, such as salt concentration [8,9],
pH [10–12] and temperature [13] of the subphase and the nature
oftheadsorbingsurface[14,15]areimportantforcontrollingand
tuningtheinterfacialpropertiesofpolyelectrolytes.Theadsorp-
tion characteristics of polyelectrolytes on various surfaces have
been reported [4,5,16,17]. Especially, an air/water [18,19] inter-
face has been widely chosen due to its neutrality, smoothness
and strong surface attraction to polymers.
∗Corresponding author. Tel.: +82 2 705 8441; fax: +82 2 715 0797.
E-mail address: kwshin@sogang.ac.kr (K. Shin).
Adsorption of highly charged polyelectrolytes at air/water
interface has been extensively studied by Yim et al. [20], where
chain conformations of PSS90are dependent upon molecular
weight and salt concentration. They observed that the adsorbed
layer formed a bilayer profile comprising a 10–20˚A thick dense
layer at the air surface followed by loosely extended layer (i.e.,
brush-likeconformation)intothewater.Inthepresenceofahigh
saltconcentration,theextendedlayerthicknessincreaseddueto
the screening of the electrostatic charge on the chain. Behavior
oflesschargedPSShasalsobeenstudiedbyTheodolyetal.[21]
with a degree of sulfonation between 30 and 90%, where they
foundthatrateofadsorptionwasveryslow,partiallyirreversible,
and strongly hysteretic.
The goal of this research is to understand detailed conforma-
tion of randomly charged polystyrene sulfonic acid (PSSx) with
degreesofsulfonation(x)rangingfrom4to35%attheair/water
interface. The interfacial behavior of lower charged polymers
wouldbestronglyinfluencedbyhydrophobicinteractions,while
highlychargedpolymersactverydifferently,indicativeofstrong
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2007.04.095

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charge repulsion, causing less hysteresis and brush formation.
Weexaminedthephaseisotherm,hysteresis,andconformations
of the adsorbed monolayer using the Langmuir balance tech-
nique as well as in situ neutron reflectivity (NR) measurements.
From surface pressure–area analysis of Langmuir monolayer,
we were able to categorize these polyelectrolytes into three dif-
ferent regimes, i.e., hydrophobic, ionomer, and polyelectrolyte
regimes as a function of the degree of sulfonation. For the first
time, we were able to observe the reversible contraction and
stretching of polyelectrolytes with ionic contents x≥35% at the
air/waterinterfaceusinginsituNRmeasurement.Byvaryingthe
charge contents on the hydrophobic polystyrene from 4 to 35%,
we were able to provide experimental evidence, quantifying
the transition from dense/hysteretic to expanded/reversible with
increasing sulfonation, and the conditions for the submersion of
strands.
2. Experiment
2.1. Materials
Polystyrene was prepared by bulk free radical polymer-
ization. Poly(styrene-ran-styrenesulfonic acid) (PSS) with a
various degree of sulfonation ranging from 4 to 35% was pre-
pared by the sulfonation method developed by Makowski et al.
[22]. Deuterated PSS with 9 and 35% sulfonation were also
synthesized for use in neutron reflectivity studies. The complete
specifications of the samples are listed in Table 1.
2.2. Langmuir monolayer
Surface pressure (π)–area (A) isotherms were recorded at a
compression rate of 10mm/min using a KSV 2000 Langmuir
trough with two computer-controlled moving barriers. The total
area of the Langmuir trough was 772.5cm2, and it was placed
within a Plexiglass box. The Langmuir trough was placed on an
anti-vibration optical table. A 25mm wide platinum Wilhelmy
plate, suspended from a microbalance, was used to monitor
the surface pressure. The Wilhelmy plate was properly cleaned
by a high temperature treatment. Water with a resistivity of
18MDcm, was used as the subphase. The temperature was
maintained at room temperature (∼22◦C) using an external
water chiller.
Table 1
Characterization of the polystyrene sulfonated acids (PSSx) used in this study
Designation
x [mol%]
Mw[g/mol]
h-PSS4
h-PSS5
h-PSS6
d-PSS9a
h-PSS11
h-PSS14
h-PSS16
d-PSS35a
4.0
5.0
6.0
9.0
11.0
14.0
16.0
35.0
110k
100k
150k
169k
150k
300k
150k
252k
ad-PSS9and d-PSS35were synthesized in deuterated form for use in neutron
studies, and (C8D8)(1−x) mol%(C8D7–SO3H)x mol%.
Differently charged polyelectrolyte solutions, typically
1mg/mL, were prepared by dissolving accurately quantities of
dry materials in a 2:1 (v:v) chloroform/methyl alcohol mixed
solvent. In each experiment, a known volume of the solution
wascarefullyaddedtothewatersurfacein5–10?Lincrements,
using a Hamilton micro-syringe. To ensure the complete evap-
oration of the solvent, a sufficient time (∼30min) was allowed
before the start of the measurements. Hysteresis was performed
in the same manner as described earlier, but for four continu-
ous cycles, each having the same rate for the compression and
expansion cycle. Each expansion cycle was begun soon after
the compression cycle. The stability of different polyelectrolyte
monolayers with respect to time was studied by constantly
observing the change in surface pressure for a period of nearly
4–5h.
2.3. Neutron reflectivity
Specular neutron reflectivity was performed on a NG7 hor-
izontal liquid reflectometer [23] at the Center for Neutron
Research (National Institute of Standards and Technology,
Gaithersburg) using a wavelength, λ of 4.76˚A. A mini Lang-
muir trough was mounted on the reflectometer so that in situ
isotherms measurements could be made concurrently with the
neutron measurements. The slit settings were adjusted so as to
fixtheresolutionataconstantvalueofdq/q=0.03.Thehorizon-
talslitsweresetat30mm.Aone-dimensionalpositionsensitive
detector (PSD) having 255 channels oriented perpendicular to
thesamplesurfaceenabledthespecularintensityandanincoher-
entbackgroundscatteringtobesimultaneouslyrecorded.Details
of the NR experimental setup and data acquisition procedures
have been described elsewhere [24–26].
3. Results and discussion
3.1. π–A isotherms
Phase information for Langmuir films can be obtained by an
analysis of π–A isotherms where compressibility, phase tran-
sition, and stability of Langmuir films that are closely related
to interactions between molecules can be characterized. π–A
isotherms of random PSS copolymers with a variety of degrees
of sulfonation are shown in Fig. 1. In general, we were able
to observe a common plateau region in these phase isotherms,
signifying that these PSS experience phase transitions upon
compression.Wefirstfocusedondeterminingwhethertheincep-
tion of a plateau or its collapse occurs at a different surface
pressureforpolymerswithvariousdegreesofsulfonation.Espe-
cially, a plateau is believed to signify phase transition related to
solvation, submerging or packing of polymer chains.
PSS with 4 and 5% degrees of sulfonation show the highest
plateau (∼45mN/m) and collapse pressure (∼65mN/m), indi-
cating that they are highly compressible and aggregated at the
collapsepressureduetostronghydrophobicinteractionbetween
PS monomers. On the other hand, PSS with between 6 and
16%sulfonationshowamoderateplateau(∼30mN/m)andcol-
lapsepressure(∼37mN/m),wherehydrophobicinteractionsare

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Y.-S. Seo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 660–665
Fig. 1. Pressure–area per molecule (π–A) isotherms of PSSx(x=4–35%) at
air/water interface. The arrow indicates the onset pressure of d-PSS35in the
isotherm.
counterbalancedbyelectrostaticinteractionsbetweennegatively
charged monomers.
For PSS with 35% degree of sulfonation, very weak plateau
is shown in the range of 25–35mN/m appeared in the isotherm
and surface pressure increment upon compression is much less
thananyotherpolymer.ItseemsthataphasetransitionofPSS35
monolayer is slow down due to the electrostatic repulsion. Here
we define an onset area indicated by arrow as a point where
surface pressure start to increase in the isotherm. Even though
PSS35layer is compressed to three-fold from an onset area of
∼4200–1400˚A,compressedPSS35layersdonotreachneithera
strong plateau nor collapse point while the other polymer does.
This implies that highly stretched polymer chain of PSS35on
the water forms tethered brush-like structure under the water
subphase upon compression. This will be proved by neutron
reflectivity studies.
Therefore based on the isotherm analysis, behavior of the
compressed monolayer can be categorized by three distinct
regimes, referred to as hydrophobic, ionomer, and polyelec-
trolyte. This categorization is strongly dependent on the effect
of charge, which appears to dictate the phase transition trend.
Fig. 2. Hysteresis effect of PSSxmonolayers: (a) PSS4; (b) PSS6; (c) PSS11; (d) d-PSS35. The compression and expansion isotherms are denoted as follows: (—)
first compression, (···) first expansion, (−−−) second compression, and (–··–) second expansion. The arrows indicate hysteresis in the isotherms between the first
compression and second compression.

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Together with the effect of charge, the compositional factor in
PSS imparts an equal contribution to its phase isotherms.
3.2. Hysteresis in the isotherms
In order to determine the stability of the monolayer
and deduce information concerning the relative strength
between hydrophobic and electrostatic interactions for differ-
ently charged PSSs, a hysteresis study is necessary. For the
hysteresis measurements, we performed two compression and
expansion cycles for the same monolayer. Once the monolayer
was compressed to a certain surface pressure and then suddenly
released without any delay. We measured the hysteresis for a
complete range of PSS with 4–35% sulfonation, and four cases
are displayed in Fig. 2.
A high hysteresis was observed for the low charged PSS 4%
in Fig. 2a. After the first compression compressed films are
unable to expand again even though the pressure is released
(indicated by an arrow). Because of strong hydrophobic inter-
action between the monomers, irreversible aggregation occurs
upon compression. A rigid and slightly white-colored aggre-
gated layer after the first compression was observed with the
naked eye. This behavior can be categorized into a hydropho-
bic regime. As the degree of sulfonation increases, compressed
films were expanded somewhat, i.e., less hysteretic as shown in
Fig. 2b and c. For second compression–expansion cycle, how-
ever,hystereticbehaviorsbecomesimilartothecaseinFig.2a.It
isthoughtthatonceunchargedmonomersfromthefirstcompres-
sionaggregatesintoclusterforms,repulsiveinteractionbetween
the charged monomers around the clusters is insufficient for
overcoming a strong hydrophobic interaction between them.
This is somewhat different from the first case so that it can be
categorized into an ionomer regime.
PSS with 35% shows almost negligible hysteretic behav-
ior, at least in the first compression–expansion cycle, as shown
in Fig. 2d. The improvement in reversibility was due to the
dominance of electrostatic interactions over hydrophobic inter-
actions. It is now clear that the amount of charge in a polymer
chain has a strong influence on the reversibility. This behav-
ior is also different from the former two cases so that it can be
categorized by a polyelectrolyte regime.
3.3. In situ neutron reflectivity
In order to find whether polymer chains with ionized ions
are submerged in water, specular neutron reflectivity (NR) was
performed. NR data are very sensitive to the structure of the
deuterated chains, which are in contact with the lower SLD
medium, the water subphase (SLD=−5.6×10−7˚A−2).
Fig. 3a shows the neutron specular reflectivity of perdeuter-
ated PSS9Langmuir films at two different surface pressures.
The solid lines together with the symbols are the fits obtained
for the corresponding SLD profiles, which are shown in Fig. 3b.
EachdatasetinFig.3showsauniqueinterferencefringe,which
contains only the form factor for the film without a q−4decay.
Fitstothereflectivitydata(solidlines)wereobtainedfromatwo
layer model as shown in Fig. 3b, consisting of an upper layer
Fig. 3. (a) Neutron reflectivity data for a d-PSS9monolayer at the air/water
interface. The solid lines through the data are the best fits, and (b) are the
corresponding SLD profiles obtained from the fits. The x-axis z is taken from
air.
of floating d-PSS hemi-micelles on the subphase surface and a
lower layer of sulfonated corona chains submerged in the sub-
phase medium. The y-axis represents a SLD, the real part of the
refractive index (n). Note that the lower layer is not noticeable
in this profile, because most of the chains remain near water
surface.
The Parratt algorithm was applied for the fitting routine [27].
Details of this model, experimental techniques, and theoretical
formulae can be found in previous papers [9]. The parame-
ters, including thickness (d), roughness (σ), and SLD obtained
from the fits are summarized in Table 2. Each layer of poly-
mer chain was well fitted with the general Gaussian distribution
function for each interface used in this model of an independent
layer.
At low pressure, PSS9layer formed a 18˚A thick layer and
then by increasing the surface pressure to 30.0mN/m (close to
the plateau in the isotherm shown in Fig. 1) becomes 27˚A. It
is noted that the increased layer thickness is mainly due to a
increase in the upper layer, indicating that submersion of the
chargedmonomerarenotsignificantevenafterfullcompression.

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Table 2
Fitting parameters from NR of for d-PSS9and d-PSS35at the air/water interface
π (mN/m)
t1(˚A)
t2(˚A)
σ1(˚A)
σ2(˚A)
σ3(˚A)SLD1(10−6˚A−2) SLD2(10−6˚A−2)
Fig. 3
3.0
30.1
16
22
2
5
4.0
3.4
7.5
6.5
4.4
5.8
5.6
6.1
4.5
2.9
Fig. 4
5.1
22.3
36.0
8
11
11
13
2
2
3
–
4.8
5.1
6.1
4.4
12.7
18.2
18.0
–
4.3
4.4
4.5
–
5.6
5.9
6.0
5.6
4.2
4.8
6.2
–
48.0
af(z)=A×10−6(1−(z/B)2)C, A=0.64, B=60, C=1
aThe SLD profile of the second layer was obtained from the brush density profile described in Ref. [28].
Fig. 4a shows NR data obtained for d-PSS35. The solid lines
inFig.4aarethefitsobtainedwiththemodelshowninFig.4b.At
low π, total layer thickness of 10˚A is considerably thinner than
that of PSS9. This may be because the highly charged chains
were fully stretched and flattened at the air/water interface.
As π increases, the total film thickness increases progressively
to 14˚A for π=36mN/m. Interestingly, at π=48mN/m, NR
Fig.4. (a)Neutronreflectivitydataofd-PSS35monolayerspreadattheair/water
interface. The solid lines through the data are the best fits. An arrow indicates
the presence of diffused chains submerged into the water subphase and (b) is
the corresponding SLD profiles obtained from the fits.
profile in Fig. 4a shows that the oscillation curve develops
a kink at qz −0.04˚A−1(marked by the arrow). The profile
fits only to a parabolic bottom layer with layer thickness of
approximately>60˚A, with the parabolic layer profile given
by SLD2(1−(z/d2)2), where SLD2and d2are the scattering
length density at the interface of the upper and bottom layer,
respectively, and d2is the thickness cutoff for the bottom layer
[28].
Therefore the bottom layer seems fully extended into the
water and the upper layer remains almost constant, function-
ing as floating points on the water surface. This transition might
occur at the plateau in π–A isotherm as shown in Fig. 1. More-
over, the transition is in fact reversible and reproducible, as
observed in the hysteresis curves shown in Fig. 2d.
In summary partially solvated PSS chains with sulfonation
higher than 35% are reversibly submerged into the water sub-
phase with increasing surface pressure at the air/water interface.
To our knowledge, this reversible brush-transition of poly-
electrolytes at an air/water interface has not previously been
observed.
4. Conclusion
Langmuir monolayers of randomly charged PSSxwith var-
ious degrees of sulfonation (x) ranging from 4 up to 35%
were studied. The randomly charged PSS were found to be
amphiphilic and stable at the air/water interface. We observed
reversible contraction and stretching of polyelectrolytes with
ionic contents x≥35% at the air/water interface using in situ
NR measurements. We could finally categorize those poly-
electrolytes into three different regimes, hydrophobic (<5%),
ionomer (6–16%), and polyelectrolyte (≥35%) as a function of
the degree of sulfonation.
Acknowledgements
This work was supported by the Korea Research Foundation
(KRF-2005-042-D20046).
References
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Press, Boca Raton, 1996.