Ds

Aa

b

c

d

a

ARRAA

KMPWIDCZID

1

moTmpsatelrssp

TT

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 391 (2011) 112– 118

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h omepa g e: www.elsev ier .com/ locate /co lsur fa

ynamics of adsorption of polyallylamine hydrochloride/sodium dodecylulphate at water/air and water/hexane interfaces

. Sharipovaa,b,∗, S. Aidarovaa, V.B. Fainermanc, A. Stoccob, P. Cernochb,d, R. Millerb

International Postgraduate Institute, “Excellence PolyTech” of Kazakh National Technical University, Almaty, KazakhstanMax-Planck Institute of Colloids and Interfaces, Potsdam, GermanyDonetsk Medical University, 16 Ilych Avenue, 83003 Donetsk, UkraineInstitute of Macromolecular Chemistry AS CR, v.v.i., Heyrovskeho nam. 2, 162 06 Praha 6, Czech Republic

r t i c l e i n f o

rticle history:eceived 7 January 2011eceived in revised form 19 February 2011ccepted 24 February 2011vailable online 5 March 2011

eywords:ixed adsorption layers

a b s t r a c t

The present work is devoted to the study of interactions between a cationic polyelectrolyte and an anionicsurfactant in the solution bulk and at the water/air and water/oil interfaces. For this purpose surface andinterfacial tensions, zeta potential, ellipsometry and dynamic light scattering (DLS) were used to describethe formation of complexes of polyallylamine hydrochloride (PAH) and sodium dodecyl sulphate (SDS).Surface and interfacial tension results are supported by zeta potential measurements and aggregation sizemeasurements via DLS. Due to electrostatic binding of dodecyl sulphate anions to the positive charges ofthe polyelectrolyte chains the resulting complexes become more hydrophobic. At the 1:1 ratio of PAH:SDS

olymer/surfactant mixturesater/oil interface

nterfacial tension isothermynamic interfacial tensionsomplexationeta potential

the complexes precipitate and become recharged due to the additional hydrophobic binding of SDS ions.The complexation leads to a depletion of surfactant and hence to an increase in surface and interfacialtensions.

© 2011 Elsevier B.V. All rights reserved.

nterfacial ellipsometryynamic light scattering

. Introduction

Mixtures of polymers and surfactants are increasingly used inodern technologies in order to make use of the specific behaviour

f both components providing very different interfacial properties.hat is why understanding the behaviour of polymer/surfactantixtures in bulk and at different interfaces is important. Therefore,

olymer/surfactant mixed interfacial layers are very intensivelytudied [1–29]. The interaction between a polyelectrolyte andn oppositely charged surfactant in aqueous solution and athe water/air interface was frequently studied with differentxperimental techniques like tensiometry, ellipsometry, dynamicight scattering, zeta potential measurement, neutron and X-ray

eflectometry, shear and dilational rheology [1–11]. Various mixedystems containing cationic polyelectrolyte and anionic surfactant,uch as polyethylene imine–sodium dodecyl sulphate [12–16],olydimethyldiallylaminechloride–sodium dodecyl sulphate

∗ Corresponding author at: International Postgraduate Institute “Excellence Poly-ech” of Kazakh National Technical University, Almaty, Kazakhstan.el.: +7 727 2927962.

E-mail address: altynay.sharipova@mpikg.mpg.de (A. Sharipova).

927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2011.02.052

[17–19], poly(2-(dimethylamino) ethyl methacrylate–sodiumdodecyl sulphate [20] poly(vinylamine) (PVAm) and sodium dode-cyl sulphate (SDS) [21], were studied at the water/air interface.For a number of oppositely charged polyelectrolyte/surfactantmixtures remarkable lowering of the surface tension was observed,in particular at low surfactant concentrations [22].

In addition, there are many studies of polymer/surfactant com-plexes at the water/air interface, however, only very few paperswere devoted to investigations of polyelectrolyte/surfactant mix-tures at water/oil interfaces [27]. These studies were performedat the water/octane interface using polystyrene sulphonate mixedwith cationic, anionic and non-ionic surfactants and differentinterfacial behaviour was observed. For mixed PSS/SDS (anionicsurfactant) systems, the co-adsorption of PSS at the water/air inter-face was explained by hydrophobic interaction between the alkylchains of SDS and the polymer [28].

This paper presents studies on the interactions in mixedsolutions containing a cationic polyelectrolyte and an anionic sur-

factant, and the impact on the adsorption at the water/air andwater/oil interfaces. Surface and interfacial tension, zeta poten-tial and dynamic light scattering measurements were performedto analyse the bulk and adsorption features of polyallylaminehydrochloride (PAH)/sodium dodecyl sulphate complexes.The

: Physi

pasls

2

ps≥Mewmw

2

opmwaw

a11sp

2

sPwfbeatici

2

Mbsca

2

a3�aa

A. Sharipova et al. / Colloids and Surfaces A

olymer polyallylamine hydrochloride alone is weakly surfacective and mainly used with other polymers like polystyreneulphonate or epichlorhydrine for multilayer formations via theayer by layer technique [29,30], however, it has not been muchtudied in respect to polyelectrolyte/surfactant complex formation.

. Experimental

Ultrapure Milli Q water (resistivity = 18.2 M�. cm) is used torepare aqueous solutions of surfactant and polyelectrolyte. Theurfactant sodium dodecyl sulphate (SDS, MW = 288.38 g mol−1,99%) and the polyelectrolyte polyallylamine hydrochloride (PAH,W = 56,000 g mol−1) were purchased from Sigma–Aldrich. All

xperiments were performed at room temperature of 22 ◦C. Hexaneas purchased from Fluka (Switzerland) and purified with alu-inium oxide and subsequently saturated with Ultrapure MilliQater.

.1. Sample preparation

The properties of polyelectrolyte/surfactant complexes dependn their preparation and can drastically change with the mixingrotocol [12]. To prepare polymer/surfactant complexes a standardixing protocol was used where polymer and surfactant solutionsith higher concentrations were diluted and mixed with each other

nd kept in an ultrasonic bath for 30 min. Freshly prepared solutionsere kept for 24 h and then used for the measurements.

The polymer concentration in the solution was kept constantt 10−2 mol/l of monomer units of the polymer, further given as0−2 molmono/l while the concentration of SDS varied between

× 10−6 mol/l and 3 × 10−2 mol/l. The natural pH of the complexolution was 4, without adjustment of the ionic strength. The sam-les for all measurements were prepared in the same way.

.2. Surface and interfacial tension measurements

The dynamic surface and interfacial tensions of the differentystems were measured by the drop Profile Analysis TensiometerAT-1 (SINTERFACE Technologies, Berlin, Germany) the principle ofhich was described in detail elsewhere [31,32]. Equilibrium sur-

ace and interfacial tension values reported in the isotherms haveeen obtained after a sufficient adsorption time to reach respectivequilibrium values. The interfacial tension of ultrapure MilliQ watergainst hexane was 51 mN/m and against air 72.5 mN/m at roomemperature (22 ◦C). For all investigations at the water/hexanenterface a drop of aqueous solution was formed in a glass cuvetteontaining pure hexane. For the water/air interface, aqueous dropsn air were formed.

.3. Zeta potential measurements

Zeta potential of the PAH/SDS complexes was measured using aalvern Zetasizer NanoZ apparatus. The instrument uses a com-

ination of laser Doppler velocimetry and phase analysis lightcattering in a technique called M3-PALS. Details of this methodan be found in [33]. Prior to the measurements the instrument waslways tested with the Malvern Zeta Potential Transfer Standard.

.4. Dynamic light scattering measurements (DLS)

Solutions were investigated by dynamic light scattering using

n ALV-7004 Multiple tau digital correlator equipped with a CGS-

Compact Goniometer system, 22 mW He–Ne laser (wavelength = 632.8 nm) and a pair of avalanche photodiodes operated in

pseudo-cross-correlation mode. All measurements were madet an angle of 90◦. The measured intensity correlation function

cochem. Eng. Aspects 391 (2011) 112– 118 113

g2(t) was analyzed using the REPES algorithm [34] performing theinverse Laplace transformation according to

g2(t) = 1 + ˇ

[∫A(t)exp

(− t

�

)d�

]2

= 1 + ˇ

[n∑

i=1

Aiexp(

− t

�i

)]2

where t is the delay time of the correlation function and an instru-mental parameter and yielding the distribution A(�) of relaxationtimes �.

The relaxation time � is related to the diffusion coefficient D andrelaxation (decay) rate G by the relation:

G = 1�

= Dq2

where q is the scattering vector defined as q = (4��/�)sin(�/2)where n is the refractive index of the solvent, and � is the scatteringangle.

The hydrodynamic radius Rh of the particles can be calculatedfrom the diffusion coefficient using the Stokes–Einstein equation:

D = kBT

6��Rh

where T is the absolute temperature, � the viscosity of the solvent,and kB the Boltzmann constant.

2.5. Ellipsometric study

Ellipsometric measurements were performed using a null-ellipsometer (Multiskop, Optrel, Germany) working with greenlaser light (� = 532 nm). The scheme of this apparatus and the pro-cedure of calculation of the thickness ı were described in detailselsewhere [35–37].

A fixed angle of incidence ϕ scheme was adopted; and the choiceof ϕ = 54.1◦ results from a compromise between accuracy and sen-sitivity (the air–water Brewster angle is 53.1◦). A circular troughwas used (diameter of 8 cm, depth of 3 cm) for the measurements.In order to minimize effects due to external vibrations on themeasurements, the trough was placed on top of an active vibra-tion isolation table. The data allowed us to obtain information onthe interfacial profile, in particular on the thickness and refractiveindex of the surface layer [38].

3. Experimental results

Due to the fact that surfactants can significantly change theconformation of polyelectrolyte macromolecules in aqueous solu-tions it was interesting to elucidate the effect of surfactants onthe adsorption kinetics and equilibrium properties of the mixedadsorption layers at the water/hexane interface.

Significant effects on the adsorption kinetics of macromoleculescan be caused by their association with surfactants or other sub-stances leading to a change in their hydrophilic–lipophilic balance.Surface and interfacial tension measurements at water/air andwater/hexane interfaces have been realized for polyallylaminehydrochloride–sodium dodecyl sulphate mixtures. As an example,the dynamic interfacial tensions of mixed aqueous PAH/SDS solu-tions at the water/hexane interface are plotted in Fig. 1.One cansee from Fig. 1 that with increasing SDS concentration kineticalcurves of interfacial tension of PAH/SDS mixed solutions change.Thus for the low SDS concentrations the adsorption equilibrium of

the mixed PAH/SDS solutions is reach slowly. With increasing SDSconcentrations the equilibrium interfacial tensions are reach faster.The observed features of the kinetics of interfacial tension reductionof PAH/SDS solutions can be explained by the association of oppo-sitely charged polyelectrolyte macromolecules and surfactant in

114 A. Sharipova et al. / Colloids and Surfaces A: Phys

0,0 2,0x103

4,0x10 3

6,0x10 3

8,0x10 3

1,0x10 4

1,2x10 4

0

5

10

15

20

25

30

35

40

45

50

55

t [sec]

γ [m

N/m

]

12

3

5

4

6

7

Fig. 1. Dynamic interfacial tensions of PAH/SDS mixture as a function of time; thec

c1

awabtidpc

awoPritptocftm

wfttw

tion of a second corona of surfactant ions around the first layer

oncentration of PAH monomer units was constant at Cp = 0.01 molmono/l; SDS con-

entrations c (mol/l) were: (�, 1) 10−6; ( ) 5 × 10−6; ( ) 5 × 10−5; ( ) × 10−4; ( ) 1 × 10−3; ( ) 5 × 10−3; ( ) 1.5 × 10−2.

queous solutions, implemented through electrostatic interactionshich leads to a significant hydrophobicity of the polycomplexes

nd the reduction of electrostatic free energy of polyions. This cane the reason for the accelerated interfacial tensions reduction inhe initial part of the curves because increase of the compactionn accordance to the Stokes–Einstein equation leads to increasediffusion rates. The ion exchange interaction between the cationicolyelectrolyte PAH and anionic SDS at the water/hexane interfacean be schematically shown in Fig. 2.

The adsorption kinetics of SDS at concentrations of 10−4 mol/lnd 7 × 10−3 mol/l alone and in presence of PAH at water/air andater/hexane interfaces is shown in Fig. 3. We can see that the rate

f adsorption layer formation is much slower in the presence ofAH. For example at a SDS concentration of 10−4 mol/l the equilib-ium value is reached after about 20 min, whereas for the complext needs more than 200 min to reach a plateau value. It means thathere is a deceleration of the interfacial tension decrease of mixedolyelectrolyte/surfactant solutions due to the slower diffusion ofhe polyelectrolyte/surfactant complexes, containing the majorityf SDS molecules. The equilibrium interfacial tension value at a con-entration of 10−4 mol/l for a pure SDS solution is 40 mN/m, whileor the complex PAH/SDS it is 27 mN/m, hence the mixed adsorp-ion layer (comprised of adsorbed complexes) reduces the tension

uch more efficiently.The adsorption layer formation by PAH/SDS complexes at the

ater/air interface differs from that at the water/hexane inter-

ace. One can see that the equilibrium surface tension value ofhe PAH/SDS mixture at 10−4 mol/l SDS concentration takes moreime at the water/air interface than at the water/hexane interface,hile the equilibrium interfacial tension value of the corresponding

Fig. 2. Scheme of interaction between PAH a

icochem. Eng. Aspects 391 (2011) 112– 118

SDS solution is reached instantly. For the mixed PAH/SDS solutionsthe surface tensions decrease slowly over a long period of time,which can be explained by the much lower molar concentrationand larger size of the polymer/surfactant complexes [39–41]. Atthe water/hexane interface the equilibrium interfacial tension isreached after 15,000 s whereas at the water/air interface it takesmore than 35,000 s. This can obviously be caused by the differ-ence in the polarity of the hydrophobic phase at the water/gas andwater/oil interface.

The rate of adsorption layer formation can be estimated byv = −d/dt. For PAH/SDS mixtures at an SDS concentration of10−4 mol/l at the water/hexane interface the rate of adsorptionlayer formation is v = 0.02 mN s/m whereas at the water/air inter-face we have v = 0.003 mN s/m.

For a SDS concentration of 7 × 10−3 mol/l one can see the oppo-site situation. The establishment of the equilibrium surface tensionvalue of the PAH/SDS mixture is faster than at the water/hexaneinterface and one can see that the kinetics of the mixed adsorp-tion layer formation at the water/air interface is similar as for SDSalone while at the water/hexane interface it differs significantly,i.e. the complexes require more than 4 h to reach the adsorptionequilibrium.

Interaction of polyelectrolytes with surfactants is accompaniedby significant conformational transformations of the polyelec-trolyte chains. The electrostatic binding of ionic surfactants to apolyelectrolyte chains is typically accompanied by an increasedmacromolecular compaction. In contrast, binding of surfactants topolyelectrolytes via hydrophobic interactions leads to a looseningof the macromolecular coil and increase in the complex solubility[24].

The interfacial tension isotherms of PAH (�), SDS ( ) andPAH/SDS ( ) at constant PAH concentration as a function of SDSconcentrations are shown in Fig. 4.

One can see that the polyelectrolyte alone shows little surfaceactivity and does not strongly depend on the bulk concentration.But the addition even of small amounts of SDS to the polymer leadsto a significant decrease in interfacial tension.

There are three interesting parts worth to be discussed in moredetail. The first part (A) begins at low surfactant concentration(10−4 mol/l), where no turbidity (caused by the formation of largeraggregates) is yet observed. The second (B) begins when turbidity ofthe solution starts and continues until merging with the isothermof pure SDS. The third part (C) is a maximum in the isotherm abovethe 1:1 mixing ratio of PAH: SDS.

The third peculiarity is a maximum in the isotherm at the 1:1mixing ratio of monomer PAH:SDS. Such situations were observedfor most cationic polyelectrolytes mixed with anionic surfactantsand studied at the water/air interface [12–22]. In [42,43] it wasproposed that the first plateau is due to electrostatic binding ofSDS anions to the cationic polymers, and the second – the forma-

bound via hydrophobic interaction. Furthermore, since all chargesof the polymer capable for binding surfactants are saturation, alsothe formation of normal micelles of SDS begins.

nd SDS at the water/hexane interface.

A. Sharipova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 391 (2011) 112– 118 115

0.0 5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

0

10

20

30

40

50

60

70

γ [m

N/m

]

0.0 2.0x103

4.0x10 3

6.0x10 3

8.0x10 3

1.0x10 4

1.2x10 4

0

5

10

15

20

25

30

35

40

45

50

55

γ [m

N/m

]

(a)

(b)

F nce of)

simcisc

spwt

cpSrmfd

Fs

increasing C, deviates significantly from 360 (the value expectedfor the bare air–water interface) following a trend similar to the

t [sec]

ig. 3. Adsorption kinetics of SDS alone at water/hexane interface ( ) and in prese; SDS concentration of 1 × 10−4 mol/l (a) and 7 × 10−3 mol/l (b), respectively.

In the case of poly(ethyleneimine) (PEI) and sodium dodecylulphate complexes, at low surfactant concentrations the systems a thermodynamically stable, transparent solution of complex

olecules. The surfactants are bound in form of monomers to theharged amine groups, which results in an increasing degree ofonization of the PEI molecules as well as in decreasing charge den-ity and average size of the PEI/SDS complexes with increasing SDSoncentration [16].

The surface and interfacial tension of PAH/SDS complex arehown in Fig. 5. One can see that the curve shape of PAH/SDS com-lex at water/air and water/oil interface is the same i.e. that atater/air interface there is a plateau at low surfactant concentra-

ion and a maximum at the 1:1 mixing ratio of PAH:SDS.The maximum in the surface and interfacial tension isotherms

an be explained by the formation of coarse particles of the com-lexes, precipitating near the critical micelle concentration ofDS (8 × 10−3 mol/l). Removal of polyelectrolyte–SDS complexeseduces the concentration of surface-active particles (aggregated

acromolecules decorated with surfactants). That is why the sur-

ace tension of the solution at the onset of the complex precipitationramatically increases, but not up to the value of pure solvent.

0.10.011E-31E-41E-51E-6

0

10

20

30

40

50

γ [m

N/m

]

C [mol/L]

A

C B

ig. 4. Interfacial tension isotherms of PAH (�), SDS ( ) and PAH/SDS ( ) at con-tant PAH concentration (Cp = 0.01 molmono/l) as a function of SDS concentrations.

t [sec]

PAH (Cp = 0.01 molmono/l) at water/air interface (�) and water/hexane interface (

A sudden sharp increase in surface tension belonging to theformation of PVAm/SDS complexes of nearly zero zeta potentialand to the appearance of precipitates was also seen in PDM-DAAC/SDS [44] and PVAm/SDS mixed systems [21]. With increasingSDS concentration a sharp increase in the surface tension of PDM-DAAC/SDS solutions was attributed to a partial depletion of thepolymer/surfactant complex from the surface and the subsequentdecrease in surface tension due to increased SDS adsorption offree SDS monomers [44]. The frequently observed phase separationat intermediate mixing ratios of surfactant and polyelectrolyte isinterpreted as a result of the aggregation of the electrically neutralpolyelectrolyte/surfactant complexes [43–47].

Ellipsometry measures the changes of the polarization states(phase and amplitude) of light reflected form an interface. InFig. 6 we plotted the ellipsometric parameter , which is thephase change, as a function of the surfactant concentration C. With

◦

change in surface tension (Fig. 6). At the same time the ellipsomet-ric parameter � , which describes the amplitude change, remainsapproximately constant � = 1·69◦. The highest concentration stud-

0.011E-31E-41E-51E-6

0

10

20

30

40

50

60

70

80

γ [m

N/m

]

C [mol/L]

Fig. 5. Surface/interfacial tension isotherms of PAH/SDS mixed solutions plottedvs. SDS concentration at a constant PAH concentration of Cp = 0.01 molmono/l; (�)correspond to water/hexane interface, ( ) corresponds to water/air surface.

116 A. Sharipova et al. / Colloids and Surfaces A: Phys

1E-31E-41E-51E-6

350

352

354

356

358

360

362

Δγ

Δ [o

]

30

40

50

60

70

γ [mN

/m]

Ff

isi

sett

Fd

C [mol/L]

ig. 6. Ellipsometric parameter , measured at ϕ = 54.1◦ , and surface tension asunctions of SDS concentration at Cp = 0.01 molmono/l.

ed was C = 10−3 mol/l which corresponds to the limit of negligiblecattering effects (due to the increase of concentration and turbid-ty) and applicability of the method.

The experimental values were fitted in the framework of a

ingle homogeneous interfacial layer characterized by two param-ters: the refractive index nl and the layer thickness ı. Withinhe approximation of the model used, it is also possible to extracthe surface concentration “seen” by ellipsometry: � = �ı, where

and � are the volume fraction of the system in the interfacial

ig. 7. (a) Model for the PAH/SDS complex at the water/air interface; (b) SDS concentrationependence of adsorption layer thickness at fixed Cp = 0.01 molmono/l.

icochem. Eng. Aspects 391 (2011) 112– 118

layer and the density, respectively. In a good approximation, thesingle interfacial layer of thickness ı should describe the poly-mer/surfactant system formed in the air–water interfacial region(see Fig. 7a). Hence the refractive index nl should be describedby the Wiener effective medium approximation [48]: n2

l= An2

A + SDSn2

SDS + PAHn2PAH + W n2

W , where i are the volume fractionsof the different species in the interfacial layer, nA is the refractiveindex of air and nW = 1.33 is the refractive index of water. The refrac-tive indices of the media are known, while the volume fractions(and the location of the polymer/surfactant system) are unknown.To simplify the problem we consider that the layer of thickness ı iscomposed just by SDS and PAH. In the fitting routine, the refractiveindex nl was fixed and the thickness was adjusted. We consider twoboundaries of the refractive index of the polymer/surfactant inter-facial layer (n2

l= SDSn2

SDS + PAHn2PAH), thus considering SDS = 1

(nl = nSDS = 1.43) and PAH = 1 (nl = nPAH = 1.38) [49,50]. The resultsare plotted in Fig. 7b.

As one can see the adsorbed amount and the thickness ofmixed adsorption layer increase with SDS concentration while thethickness ı changes little and does not exceed 3 nm. This valuecorresponds to the adsorption of polymer/surfactant complexes,where the polymer is in an extended conformation at the interface.A similar behaviour was observed in PEI/SDS systems where thethickness was about 2 nm over the entire SDS concentration range

at pH 3 [13].

To approach the dynamics of formation of PAH/SDS micellarsystems a series of in situ observation were provided with theassistance of the non-invasive zeta potential measurements anddynamic light scattering. The obtained results are presented in

dependence of adsorbed amount at fixed Cp = 0.01 molmono/l; (c) SDS concentration

A. Sharipova et al. / Colloids and Surfaces A: Physi

0,0300,0250,0200,0150,0100,005

-100

-50

0

50

100

150

ξ [m

V]

Ft

FpiiimiFSc

(ciTc

mttcc

Fc

C [mol/L]

ig. 8. Dependence of the zeta-potential of PAH/SDS complexes on SDS concentra-ion at constant PAH Cp = 0.01 molmono/l.

igs. 8 and 9, respectively. The charges carried by the formed com-lexes were determined by measurements of the zeta-potential (�)

n samples prepared by mixing stock solutions of PAH and SDSn different ratios. For that, the final total concentration of PAHn the individual samples was keep constant at 10−2 mol/l of the

onomeric unit, while the final total concentration of SDS var-ed between 3 × 10−2 and 5 × 10−3 M. The results are showed inig. 8. A clear sigmoidal shape of the curve of the �-dependence onDS concentration is observed, with an inflection point at the SDSoncentration of 10−2 M.

The high absolute values of the horizontal parts of the curve� ∼ 100 mV and � ∼ −80 mV) confirm good stability of the studiedomplexes in solutions. The positive � at low SDS concentrationss an evidence of the positive charges of the polymer in solution.his observation is in good correspondence with the polycationicharacter of the PAH chains.

With increasing concentration of the negatively charged SDS

olecules bound by electrostatic interaction to the PAH polyelec-

rolyte, the charges of the polycation are gradually neutralized. Athe equimolar concentration of SDS and PAH (monomeric unit con-entration) the charges formed by amine and sulphate groups areompletely neutralized and the charge of the resulting complexes

0.10.011E-31E-41E-51E-6

0

50

100

150

200

250

300

350

400

450

Rh [nm

]

C [mol/L]

ig. 9. Dependence of hydrodynamic radius Rh of PAH/SDS complexes on SDS con-entration at constant PAH Cp = 0.01 molmono/l.

cochem. Eng. Aspects 391 (2011) 112– 118 117

is zero. This process is confirmed by the inflection point in the �dependence at the SDS bulk concentration of 10−2 mol/l, where the� ≈ 0 mV (the isoelectric point).

The negative values of � at SDS concentrations above 10−2 mol/limply additional adsorption of excess molecules of SDS onto/intothe neutralized PAH macromolecules via hydrophobic interaction.

The behaviour described above was confirmed also by DLS mea-surements of the prepared samples, as demonstrated in Fig. 9. Inthe graph the dependence of the hydrodynamic radius (Rh) of par-ticles on the total SDS concentration in the solution are shown. Thehydrodynamic radius was calculated from the diffusion coefficientusing the Stokes–Einstein equation.

We observe a gradual decrease of Rh with increasing SDS con-centration, up to the isoelectric point at equimolar concentrationsof PAH and SDS. This result is consistent with the idea of mutualneutralization of the charges of the polyelectrolyte molecules insolution by SDS, where polymer particles of more compact struc-ture are formed. In this situation the complexes have a maximumhydrophobicity and can form larger aggregates (Fig. 9). This is thesituation at which also the interfacial tension increases sharply, dueto a depletion of the adsorbed layer.

The described results are in good agreement, e.g. with publishedexperiments of Yohannes et al. [51] and Holappa et al. [52] where inpolyelectrolyte complexes of poly(ethylene oxide)-b-poly(sodiummethacrylate) and poly(methacryloyl oxyethyl trimethylammo-nium chloride) a similar behaviour for Rh was observed.

For increasing SDS concentrations the excess SDS molecules canbind to the surface of the polyelectrolyte/SDS complexes, whichcauses an increase in the negative charge of the observed particles.

Yoshida [53] studied the self-assembly of polyallylaminehydrochloride through an electrostatic interaction with SDS. It wasfound that PAH formed monodisperse spherical aggregates in thepresence of SDS. SDS supports the self-assembly of PAH below itsCMC. It was found that the aggregates had a micellar structure witha dodecyl sulphate cores and AH shells which can be an exampleof the formation of spherical nanoparticles using a polyelectrolyteand an oppositely charged ionic surfactant [53].

4. Conclusion

The present work is devoted to the study of interaction of mixedsolutions of cationic polyelectrolyte PAH and anionic surfactant SDSin solution bulk and at the at water/air and water/oil interfaces.For this purpose surface and interfacial tensions, electrophoreticmobility (zeta potential) and dynamic light scattering were used todescribe the formation of complexes of PAH and SDS.

There are three interesting parts worth to be discussed in moredetail. The first part (A) begins at low surfactant concentration(10−4 mol/l), where no turbidity (caused by the formation of largeraggregates) is yet observed. The second (B) begins when turbidity ofthe solution starts and continues until merging with the isothermof pure SDS. The third part (C) is a maximum in the isothermabove the 1:1 mixing ratio of PAH:SDS.The observed features ofthe kinetics of interfacial tension reduction of PAH/SDS solutionscan be explained by the association of oppositely charged polyelec-trolyte/surfactant complexes in aqueous solutions, implementedthrough electrostatic interactions which leads to a significanthydrophobicity of the complexes and the reduction of electrostaticfree energy of polyions. This can be the reason for the acceleratedsurface and interfacial tensions reduction in the initial part of the

curves because increase of the compaction in accordance to theStokes–Einstein equation leads to increased diffusion rates.

Surface and interfacial tension results were supported by zetapotential measurements and aggregation size measurements viaDLS where due to the electrostatic binding of SDS anions to

1 : Phys

thpisra

A

b

R

[[[

[[[

[

[

[[[

[

[

[[[

[

[[[[

[

[

[[[[

[

[[[[[[

[

[[[[[

18 A. Sharipova et al. / Colloids and Surfaces A

he cations of the polyelectrolyte polymer chains become moreydrophobic and above the 1:1 ratio of PAH: SDS the complexesrecipitate and get recharged due to additional hydrophobic bind-

ng of SDS ions. Due to the depletion of the adsorbed layer theurface and interfacial tensions increase sharply. The ellipsomet-ic results show that the polymer is in an extended conformationt the interface.

cknowledgements

The work was financially supported by a DAAD Grant (AS) andy the COST actions P21 and D43.

eferences

[1] P.L. Dubin, D.R. Rigsbee, L.M. Gan, M.A. Fallon, Macromolecules 21 (1988) 2555.[2] M. Almgren, P. Hansson, E. Mukhtar, J. van Stam, Langmuir 8 (1992) 2405.[3] K. Thalberg, J. van Stam, C. Lindbald, M. Almgren, B. Lindman, J. Phys. Chem. 95

(1991) 8975.[4] E.B. Abuin, J.C. Scaiano, J. Am. Chem. Soc. 106 (1984) 6274.[5] A. Asnacios, D. Langevin, J. Argillier, Macromolecules 29 (1996) 7412.[6] A.V. Gorelov, E.D. Kudryashov, J.C. Jacquier, D.M. McLoughlin, K.A. Dawson,

Physica A 249 (1998) 216.[7] A. Svensson, L. Piculell, B. Cabane, P. Ilekti, J. Phys. Chem. B 106 (2002) 1013.[8] K. Hayakawa, J. Kwak, J. Phys. Chem. 86 (1982) 3866.[9] K. Hayakawa, J. Kwak, J. Phys. Chem. 87 (1983) 506.10] K. Hayakawa, K. Santerre, Macromolecules 16 (1983) 1642.11] K. Hayakawa, J. Kwak, J. Phys. Chem. 88 (1984) 1930.12] K. Tonigold, I. Varga, T. Nylander, R.A. Campbell, Langmuir 25 (2009)

4036–4046.13] J. Penfold, I. Tucker, R.K. Thomas, J. Zhang, Langmuir 21 (2005) 10061–10073.14] H. Wang, Y. Wang, H. Yan, J. Zhang, R.K. Thomas, Langmuir (2006) 1526–1533.15] J. Penfold, I. Tucker, R.K. Thomas, D. Taylor, J. Zhang, X. Zhang, Langmuir 23

(2007) 3690–3698.16] R. Meszaros, L. Thompson, M. Bos, I. Varga, T. Gilanyi, Langmuir 2003 (19) (2003)

609–615.17] D. Vaknin, S. Dahlke, A. Travesset, G. Nizri, S. Magdassi, Phys. Rev. Lett. 93 (2004)

218302.18] J. Merta, P. Stenius, Colloids Surf. A 149 (1999) 367.19] E. Tarade, Y. Samoshina, T. Nylander, B. Lindman, Langmuir 20 (2004) 1753.20] M. Moglianetti, P. Li, F.L.G. Malet, S.P. Armes, R.K. Thomas, S. Titmuss, Langmuir

24 (2008) 12892–12898.21] A. Mezei, K. Pojjak, R. Meszaros, J. Phys. Chem. 112 (2008) 9693–9699.

[

[

[[

icochem. Eng. Aspects 391 (2011) 112– 118

22] D. Taylor, R.K. Thomas, J. Penfold, Adv. Colloid Interface Sci. 132 (2007)69–1107.

23] J. Koetz, S. Kosmella, T. Beitz, Prog. Polym. Sci. 26 (2001) 1199–1232.24] S. Aidarova, PhD Thesis, Moscow, 1992, p. 39.25] K. Musabekov, S. Aidarova, K. Abdiev, 31st IUPAC Macromolecular Symposium,

Merseburg, 1987, p. 175.26] K. Musabekov, V. Palmer, T.I. You Tzun-Sin, 32nd IUPAC Congress, Stockholm,

1989, p. 144.27] Hong-Bo Fang, Colloid. Polym. Sci. 287 (2009) 1131–1137.28] B.A. Noskov, G. Loglio, R. Miller, J. Phys. Chem. 108 (2004) 18615–18622.29] G.V. Rama Rao, T. Konishi, N. Ise, Polymer 42 (2001) 6817–6821.30] J.M.C. Lourenco, P.A. Ribeiro, A.M. Botelho do Rego, F.M. Braz Fernandes, A.M.C.

Moutinho, M. Raposo, Langmuir 20 (2004) 8103–8109.31] G. Loglio, P. Pandolfini, R. Miller, A.V. Makievski, F. Ravera, M. Ferrari, L. Lig-

gieri, Novel methods to study interfacial layers in studies, in: D. Möbius, R.Miller (Eds.), Studies in Interface Science, vol. 11, Elsevier, Amsterdam, 2001,pp. 439–484.

32] S.A. Zholob, A.V. Makievski, R. Miller, V.B. Fainerman, Adv. Colloid Interface Sci.134–135 (2007) 322.

33] A. Mezei, R. Meszaros, Langmuir 22 (2006) 7148–7151.34] J. Jakes, Collect. Czech. Chem. Commun. 60 (1995) 1781–1797.35] D. Ducharme, A. Tessier, R.M. Leblanc, Rev. Sci. Instrum. 58 (1987) 571.36] M. Harke, R. Teppner, O. Schulz, H. Motschmann, H. Orendi, Rev. Sci. Instrum.

68 (1997) 3130.37] R. Miller, V.B. Fainerman, A.V. Makievski, J. Krдgel, D.O. Grigoriev, F. Ravera, L.

Liggieri, D.Y. Kwok, A.W. Neumann, in: J. Sjöblom (Ed.), Encyclopaedic Hand-book of EmulsionTechnology, Marcel Dekker, New York, 2001.

38] J.L. Keddie, Curr. Opin. Colloid Interface Sci. 6 (2001) 102.39] H. Frish, R. Simha, J. Chem. Phys. 27 (1957) 702–706.40] H. Frish, R. Simha, J. Phys. Chem. 58 (1954) 507–512.41] A. Silberberg, J. Phys. Chem. 66 (1962) 1972–1983.42] T.T. Nguyen, B.I. Shklovski, J. Chem. Phys. 114 (2001) 5905–5916.43] H. Schiessel, R.F. Bruinsma, W.M. Gelbart, J. Chem. Phys. 115 (2001)

7245–7252.44] E. Staples, I. Tucker, J. Penfold, N. Warren, R.K. Thomas, D. Taylor, Langmuir 18

(2002) 5147.45] P. Hansson, Langmuir 17 (2001) 4167–4180.46] R.J. Allen, P.B. Warren, Langmuir 20 (2004) 1997–2009.47] M. Skepo, P. Linse, Macromolecules 36 (2003) 508.48] D.E. Aspnes, Thin Solid Films 89 (1982) 249.49] http://www.molecularstation.com/wiki/SDS.

50] C.L. Schauer, M. Chen, M. Chatterley, K. Eisemann, E.R. Welsh, R.R. Price, P.E.

Schoen, F.S. Ligler, Thin Solid Films 434 (2003) 250–257.51] G. Yohannes, S. Holappa, S.K. Wiedmer, T. Andersson, H. Tenhu, M. Riekkola,

Anal. Chim. Acta 542 (2005) 222–229.52] S. Holappa, T. Andersson, L. Kantonen, Polymer 44 (2003) 7907–7916.53] E. Yoshida, Colloid. Polym. Sci. 288 (2010) 1321–1325.