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Colloids and Surfaces A: Physicochem. Eng. Aspects 391 (2011) 130– 134

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

ilational rheology of polymer/surfactant mixtures at water/hexane interface

. Sharipovaa,b,∗, S. Aidarovaa, N. Mucicb, R. Millerb

International Postgraduate institute “Excellence Polytech” of Kazakh National Technical University, Almaty, KazakhstanMax-Planck Institute of Colloids and Interfaces, Potsdam, Germany

r t i c l e i n f o

rticle history:eceived 16 March 2011eceived in revised form 22 April 2011ccepted 28 April 2011vailable online 6 May 2011

eywords:

a b s t r a c t

The present work is devoted to the interaction in mixed solutions of the cationic polyelectrolyte PAH andthe counter-charged anionic surfactant SDS at the water/oil interface. For this purpose interfacial tensionand dilational rheology studies were performed to describe the formation of complexes of PAH and SDS.The dilational elasticity values depend on the concentrations of surfactant and polyelectrolyte and theirmixing ratio. As expected the concentration dependence of the dilational elasticities of SDS and PAH-SDSmixtures are similar in the SDS concentration range 10−5 M till 10−3 M (at a fixed amount of PAH) andhave a maximum which correlates with an observed minimum in the interfacial tension isotherm. The

ixed adsorption layersolymer–surfactant mixturesater/oil interface

nterfacial tension isothermheological propertiesilational elasticity

sharp decrease of the dilational elasticity values in a narrow range is presumably due to the destructionof two-dimensional rigid structures and the formation of microaggregates in the interfacial layer. Fromdilational viscosity measurements as a function of oscillation frequency one can conclude that withincreasing surfactant concentration the SDS dominates in the interfacial adsorption layer whereas thepolyelectrolyte–surfactant complexes remain in the bulk phase.

ilational viscosity

. Introduction

Understanding of the interfacial properties of polyelectrolyte–urfactants mixtures at water/oil interfaces is important due toheir industrial, technological and domestic applications. How-ver, progress in this field is rather slow mainly due to lackf suitable experimental techniques. Classical methods such asurface tension measurements have been used widely to deter-ine the surfactant–polymer complexes at the water/air interface

nd a strong synergistic lowering of the surface tension haseen observed [1–8]. However, surface tension measurementsf polymer/surfactant mixtures are often difficult to interpretnd that is why other techniques such as neutron reflectiv-ty, ellipsometry, Zeta-potential and dynamic light scattering

easurements have been additionally used to understand theolyelectrolyte–surfactant behavior [9–15]. The interfacial behav-

or of polyelectrolyte–surfactant mixtures at water/oil interfacesave only very recently been studied, since suitable experimentalools are available, and only few papers are so far devoted to this

ubjects [16].

Rheological properties are the main characteristics of theynamic properties of a surface layer. The dilation surface rheology

∗ Corresponding author at: International Postgraduate Institute “Excellence Poly-ech” of Kazakh National Technical University, Almaty, Kazakhstan.el.: +7 7272927962.

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.2011.04.035

© 2011 Elsevier B.V. All rights reserved.

allows to obtain additional information on the polyelectrolyte–surfactant complex formation in the surface layer and the measure-ments of dynamic dilational visco-elasticity can be used to studyevery single chemical and physical process in the system and pro-vide more information of the dynamics of polymer chains and theirinteraction with surfactant molecules at the interface, supposed themeasurements are made in a suitable frequency range. So far thenumber of investigated systems is rather limited [17–21,24,25,27].In [16], for example, the interfacial dilational visco-elasticity ofmixed polyelectrolyte/surfactant adsorbed layers at the water/oilinterface was discussed. The author investigated mixtures ofpolystyrene sulfonate and cationic, anionic and nonionic surfac-tants, respectively, at the water–octane interface. The experimentalresults show that different interfacial behaviors can be observedfor different types of surfactants. In the case of mixtures of PSSwith the cationic surfactant CTAB, the interfacial tension remainsconstant in a wide surfactant concentration range up to 10−4 mol/l.At the same time, the dilational elasticity decreases and the viscouscomponent increases in the presence of 100 ppm PSS. These resultsare in accordance with the classical behavior of oppositely chargedpolyelectrolyte–surfactant systems and can be explained wellby the Goddard model [28]. For PSS/anionic surfactant SDS sys-tems, the co-adsorption of PSS at the interface mediated throughhydrophobic interactions with the alkyl chains of SDS at lower

surfactant concentrations, according to the adsorption modelproposed by Noskov et al. [24], leads to the increase of interfacialtension and the decrease of dilational elasticity. On the other hand,the dilational viscosity increases due to the slowed down exchange

: Physicochem. Eng. Aspects 391 (2011) 130– 134 131

poPthocrv

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2

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2

opmoaw

1g

2m

sPoiai5cv

3

aaccto

ili

0,10,011E-31E-41E-50

10

20

30

40

50

γ [m

N/m

]

C [mol/l]

A

C B

Fig. 1. Interfacial tension of PAH/SDS complex plotted versus SDS concentration at

a constant PAH concentration of 10−2 M ( ) PAH without SDS (�), SDS without PAH

A. Sharipova et al. / Colloids and Surfaces A

rocess of SDS between the interface and the bulk. In the casef PSS mixed with the nonionic surfactant Triton X-100 (TX100),SS may form a sub-layer contiguous to the aqueous phase withhe partly hydrophobic polyoxyethylene chains of TX100, whichas little effect on the TX100 adsorption layer and consequentlyn the interfacial tension. However, the possible relaxation pro-esses such as the fast exchange of TX100 between the proximalegion and the sub-layer can decrease the dilational elasticity andiscosity.

In this paper, we have investigated the interfacial dilationalisco-elasticity of the water-soluble mixture of polyallilaminehy-rochloride (PAH) and sodium dodecyl sulfate (SDS) adsorbed athe water/hexane interface.

. Experimental

Ultrapure Milli-Q water (resistivity = 18.2 M� cm) wassed to prepare all aqueous surfactant solutions. Sodiumodecyl sulfate (SDS, MW = 288.38 g mol−1, ≥99%) was pur-hased from Sigma–Aldrich and polyallyl amine hydrochlorideMW = 56000 g mol−1) from Aldrich. All experiments were per-ormed at room temperature of 22 ◦C. Hexane was purchasedrom Fluka (Switzerland) and purified with aluminium oxide andubsequently saturated with ultrapure Milli-Q water.

.1. Sample preparation

The properties of polyelectrolyte/surfactant complexes dependn their preparation and can drastically change with the mixingrotocol [1]. To prepare polymer/surfactant complexes a standardixing protocol was used where polymer and surfactant solutions

f higher concentrations were diluted and mixed with each othernd 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 constant at0−2 mol of monomer units of the polymer per litre at pH 4, furtheriven as 10−2 molmono/l.

.2. Interfacial tension, dilational elasticity and viscosityeasurements

The dynamic interfacial tension and dilational rheology of theystem were measured with the drop Profile Analysis TensiometerAT-1 (SINTERFACE Technologies Berlin, Germany) the principlef which was described in detail elsewhere [22,23]. Equilibriumnterfacial tensions reported in the isotherms have been obtainedfter a sufficiently long adsorption time to reach plateau values. Thenterfacial tension of ultrapure Milli-Q water against hexane was1 mN/m at room temperature (22 ◦C). Dilational elasticity and vis-osity were measured after reaching equilibrium interfacial tensionalues.

. Results and discussion

To characterize the adsorption behavior of PAH/SDS mixturest the water/oil interface the interfacial tension data for PAH, SDSlone and for the PAH/SDS mixture are shown in Fig. 1. The PAHoncentration was kept constant at 10−2 M, while the SDS con-entration was varied from 10−6 to 3 × 10−2 mol/l. The interfacialension of PAH 10−2 M is about 46 mN/m, indicating that PAH isnly very weakly interfacial active.

There are three interesting parts in the interfacial tensionsotherm to be discussed in more detail. There is a first part (A) atow surfactant concentration (≤7 × 10−5 mol/l), where no turbiditys yet observed in the presence of the polymer, the second part (B)

( ).

is mainly horizontal and covers the range until the curve for themixture merges with the isotherm of pure SDS, and the third part(C) refers to the maximum at the ratio n = CSDS/CPAH = 1.

The observed features in the interfacial tension isotherm ofaqueous PAH/SDS mixed solutions can be explained by the associa-tion of oppositely charged polyelectrolyte–surfactant complexes inaqueous solutions, implemented through electrostatic interactionswhich lead to a significant hydrophobicity of the polyelectrolytechains and a reduction of the electrostatic free energy of polyions.

The maximum in the interfacial tension isotherm can beexplained by the formation of coarse particles of the complexes,precipitating near the critical micelle formation concentration ofSDS. Removal of polyelectrolyte–SDS complexes reduces the con-centration of surface-active particles (aggregated macromoleculesdecorated by surfactants) in bulk and therefore at the interface.That is why the interfacial tension of the solution at the onset ofthe complex precipitation dramatically increases, but not up to thevalue of pure solvent.

Interfacial tension results are supported by Zeta potential andDLS measurements [26] where due to the electrostatic bindingof SDS anions with cations of the polyelectrolyte–polymer chainsbecome more hydrophobic. The complex size in the bulk have beenchanged from 300 nm at the ratio CSDS/CPAH = 0.001 to less than100 nm at the ratio CSDS/CPAH = 1 and at this ratio the complexesbecome hydrophobic and compact enough to precipitate and thezeta potential shows that a recharge of the complex occurs, i.e.the complex becomes negatively charged. Due to depletion of theadsorbed layer the interfacial tension increases sharply [26].

Interfacial tension measurements are not very sensitive tochanges in the surface layer structure. However, their rheologicalproperties disclose for example interface association or reorgani-zation processes. Therefore, interfacial dilational visco-elasticitymeasurements were done, which can provide more informationon the interfacial surfactant–polyelectrolyte complexes.

The dilational elasticity refers to the variation of interfacial cov-erage and inter-molecular interaction caused by any changes ofthe interfacial area while the dilational viscosity is directly relatedto the relaxation processes within the surface layer and with theadjacent bulk phases.

Figs. 2 and 3 show the dependences of interfacial dilationalelasticity plotted versus 1/period (frequency) for SDS solutions inabsence and presence of PAH 10−2 M at the water/hexane interface.

132 A. Sharipova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 391 (2011) 130– 134

Fig. 2. Dilational elasticity of SDS at water/hexane interface plotted versus 1/period

(

0

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dP

Fs

(

(

0,100,080,060,040,020,000

5

10

15

20

25

30

35

40

45

ε [m

N/m

]

f [1/s]

Fig. 4. Dilatational elasticity of SDS (�) and PAH/SDS mixture (�) at the SDS con-centration of 7 × 10−5 M at water/hexane interface.

1E-31E-41E-50

2

4

6

8

10

12

14

ε [m

N/m

]

frequency); SDS bulk concentrations c [M] are: (�) 0.00003; ( ) 0.00005; ( )

.00007; ( ) 0.00052; ( ) 0.001; ( ) 0.004; ( ) 0.005.

ne can see that the elasticity values of PAH/SDS solutions areignificantly higher than those of pure surfactant solutions.

It is evident from Figs. 2 and 3 that the values of dilational elastic-ty of PAH/SDS mixtures are much higher than those of SDS alone.n particular, the ε values for SDS are below 10 mN/m, whereashose for PAH/SDS are in the range between 20 and 80 mN/m. Thisndicates a strengthening of the Rehbinder structural–mechanicalactor. This is clearly demonstrated by the curves shown in Fig. 4hich shows the dilatational elasticity of SDS (�) and a PAH/SDSixture (�) at the SDS concentration of 7 × 10−5 M. The dilational

lasticity of the mixture is about 10 times higher which indicates aignificant strengthening of the mixed interfacial adsorption layersuilt up by surfactants and polyelectrolyte. In contrast, individu-lly each of these components does not show high strengtheningroperties.

Figs. 5 and 6 show the concentration dependence of interfacialilational elasticity for SDS solutions in absence and presence ofAH 10−2 M at the water/hexane interface.

ig. 3. Dilational elasticity of PAH/SDS complexes plotted versus 1/period at con-tant PAH concentration of 10−2 M at different SDS concentrations c [M]: (�) 10−5;

) 3 × 10−5; ( ) 5 × 10−5; ( ) 7 × 10−5; ( ) 10−4; ( ) 3 × 10−4; ( ) 5 × 10−4;

) 7 × 10−4; ( ) 3 × 10−3; ( ) 5 × 10−3; ( ) 7 × 10−3; (+) 10−2.

C [mol/L]

Fig. 5. Dilational elasticity of SDS solutions plotted versus the SDS concentration at

different frequencies [Hz] ( ) 0.005; ( ) 0.01; ( ) 0.02; ( ) 0.04; ( ) 0.05; () 0.1.

0,011E-31E-4

0

10

20

30

40

50

60

70

80

90

ε [m

N/m

]

C [mol/L]

Fig. 6. Dilational elasticity of PAH/SDS complexes plotted versus SDS concentrationat a constant PAH concentration of 10−2 M and different frequencies [Hz]: ( ) 0.005;

( ) 0.01; ( ) 0.02; ( ) 0.04; ( ) 0.05; ( ) 0.1.

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

0,040,020,000

20

40

60

80

100

120

η [s

*mN

/m]

f[1/s]

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ig. 7. Dilational viscosity of SDS (�) and mixture PAH/SDS ( ) plotted versus/period at the concentration PAH 10−2 M and SDS 7 × 10−5 M.

It is evident from Figs. 5 and 6 that the values of dilationallasticity depend on the concentration of surfactant and polyelec-rolyte. The shape of the curves of dilational elasticities of SDSnd PAH/SDS mixtures are quite similar in the concentration rangeetween 10−5 M and 10−3 M. Note the curves of dilational elas-icities of PAH/SDS mixtures have a maximum which correlatesith the minimum in the interfacial tension isotherm. A quanti-

ative analysis for the mixed system is not possible as respectiveodels do not exist. Therefore, we can discuss the dilational rheol-

gy results here only qualitatively, i.e. by compare of the layers inependence of the SDS concentration in absence and presence ofAH.

The high dynamic surface elasticity for the mixture at low sur-actant concentrations indicates hydrophobic interactions betweenolyelectrolyte segments with surfactant molecules which can leado two-dimensional heterogeneities in the adsorption layer [24].

Further increase in the concentration leads to an increase in dila-ional elasticity values up to a concentration of 9 × 10−3 M and then

sharp decrease of dilational elasticity values. The sharp decreasen a narrow range is presumably due to the destruction of a two-imensional rigid structure and the formation of micro-aggregates

n the interfacial layer.Recently similar abrupt decreases of the dynamic surface elas-

icity in a narrow concentration range have been discovered at theater/air interface for many other systems containing complexes

f conventional surfactants with synthetic polyelectrolytes, such asDMDAC/SDS [17], poly(vinylpyridinium) chloride/SDS (PVP/SDS)18], poly(acrylic) acid/DTAB (PAA/DTAB) [19], polymethacryliccid/DTAB (PMA/DTAB) [19], and PAMPS-AA/DTAB [20].

Measurements of the dynamic elasticity of an adsorptionayer of poly(vinylpyridinium chloride)/sodium dodecylsulfatePVP/SDS) complexes indicate a structure transition when theurfactant concentration approaches the concentration of PVPonomers and show that the rate of relaxation processes in

olyelectrolyte/surfactant adsorption layers is determined by theroperties of the polymer chain [25].

The interfacial tension sharply increases (cf. Fig. 1) and strength-ning of the mixed interfacial adsorption layers deteriorate at theatio n = CSDS/CPAH = 1 due to recharge, compaction and precipita-ion of the complexes.

The viscous component of the dilational rheological properties

s the main characteristics of the dynamic properties of a surfaceayer. In surfactant systems, when the interface is perturbed, dif-erent processes can occur, which contribute to the reequilibrationf the system. Among these mechanisms, there is diffusion in the

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cochem. Eng. Aspects 391 (2011) 130– 134 133

bulk phases and kinetic processes inside the adsorbed layer, such asreorientation, aggregation, and other rearrangements of the layeror of the molecular structure.

Fig. 7 shows the dependence of interfacial dilational viscosityplotted versus 1/period (frequency) for SDS solutions in the pres-ence and absence of PAH 10−2 M at the water/hexane interface atthe SDS concentration of 7 × 10−5 M.

It is seen that the dilational viscosity of PAH/SDS mixtures forsmall rates of mechanical perturbation is much higher, but withincreasing frequency the dilational viscosity of PAH/SDS complexbecomes identical to that of SDS. We can assume that this differenceis caused by a release and binding of SDS molecules within thePAH/SDS surface layer.

4. Conclusion

The present work is devoted to the interaction of mixed solu-tions of the cationic polyelectrolyte PAH and anionic surfactant SDSat the water/oil interface. For this purpose interfacial tension anddilational rheology studies were performed to describe the forma-tion of complexes of PAH and SDS.

The dilational elasticity values depend on the concentration ofsurfactant and polyelectrolyte. The shape of the dilational elastici-ties of SDS and PAH/SDS mixtures are similar in the concentrationrange 10−5 M till 10−3 M and have a maximum which correlateswith the minimum in the interfacial tension isotherm. Furtherincrease in the concentration leads to an increase of the dilationalelasticity up to a mixing ratio of n = CSDS/CPAH = 1 and then a sharpdecrease of the dilational elasticity is observed. This sharp decreasehappens in a narrow range presumably due to the destruction ofa two-dimensional rigid structure and the formation of micro-aggregates in the interfacial layer. Interfacial shear rheology isunder way to elucidate this idea.

The dilational viscosity of PAH/SDS mixtures for small frequen-cies of perturbation is much higher than for SDS alone. This effectcould be caused by a release/binding relaxation of SDS moleculeswith the polymer at the interface.

Acknowledgements

The work was financially supported by a DAAD grant (AS) andby the COST action D43.

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