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Colloids and Surfaces A: Physicochem. Eng. Aspects 382 (2011) 181–185

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

tudy of dynamic interfacial tension of alkyl sulphates with different alkyl chainengths at the water/hexane interface

. Sharipovaa,b,∗, S. Aidarovaa, V.B. Fainermanc, 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, Ukraine

r t i c l e i n f o

rticle history:vailable online 21 September 2010

eywords:urfactant adsorption

a b s t r a c t

The adsorption behaviour of alkyl sulphates at the water/oil interface is rather different from that fre-quently studied at the water/air interface. First of all typical impurities in form of homologous alcohols,immanent for this type of surfactants at the surface of aqueous solutions, are not essential at the water/oilinterface due to their high solubility in the oil phase. In the interfacial layers the oil molecules, here hexane,

ater/oil interfacerop profile analysis tensiometryrumkin adsorption modeleorientation model

are incorporated into the structure, and have essential impact on the equation of state at low interfacialcoverage and in particular at the shorter alkyl chain surfactants. For the longer chain alkyl sulphates(NaC14SO4 and NaC16SO4) and at high interfacial coverage the hexane molecules are squeezed out ofthe adsorption layer due to the stronger hydrophobic interaction between the surfactants’ alkyl chains.The adsorption characteristics are discussed in the framework of the two well-known thermodynamics

n mo

Frumkin and reorientatio

. Introduction

Ionic surfactants play an important role in many processes ofractical importance, such as formation of emulsions and foams,otation, wetting and detergency. Among the ionic surfactantsodium dodecyl sulphate (SDS) is the most frequently studied,ainly at the water/air interface [1–11] but also at differentater/oil interfaces [3,4,12,13,15]. All other alkyl sulphates areuch less investigated and almost exclusively at the water/air

nterface [5,14].Lunkenheimer et al. [5] studied the surface tension isotherms

f the homologous series of alkyl sulphates from C7 to C14 at theater/air interface and it was shown that all adsorption parame-

ers and CMC reveal a pronounced even/odd effect. It was found thathe cross-sectional areas of the short-chain homologues decreaseith increasing chain length, reaching a minimum value for C10 and

11. However the cross-sectional areas of the longer-chain homo-ogues (Cn ≥ C12) increase again drastically with increasing chainength. This effect is attributed to the increasing electrostatic repul-ion within the adsorption layer caused by the increasing Debyeengths of the stronger surface-active homologues, hence adsorbing

t much lower bulk concentrations.

The adsorption of sodium decyl and dodecyl sulphate was stud-ed at very high areas per molecule at the hydrocarbon 0.1 M NaCl

∗ Corresponding author. Tel.: +49 3315679251.E-mail address: altynay.sharipova@mpikg.mpg.de (A. Sharipova).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.08.044

dels.© 2010 Elsevier B.V. All rights reserved.

interface for a series of n-alkanes (C6–C16), and an ideal gaseousbehaviour was observed in all systems studied [12]. Based on thecomparison of data obtained for both air/water and hydrocar-bon/water interfaces and on the basis of the major forces involvedin the adsorption process, the authors suggested that sodiumdodecyl sulphate dimers, which are present in the aqueous bulk,dissociate at the oil/water interface and that the monomer is theexclusive adsorbed species.

Rehfeld investigated the adsorption of sodium dodecyl sul-phate at the water/air and water/hexane, octane, nonane, decane,heptadecane, 1-hexene, 1-octene, cyclohexene, benzene, n-butylbenzene interfaces where he observed a correlation betweenthe concentration of sodium dodecyl sulphate at water/oil inter-face and critical micelle concentration for each homologous series[15]. In the presence of other organic liquids, the CMC increasedwith decreasing solubility, but the concentration of sodium dodecylsulphate at the water/oil interface increased.

The dynamic surface tension of sodium tetradecyl hexadecylsulphate at the water/air interface was studied systematically bythe maximum bubble-pressure method and a method was sug-gested for the calculation of the micelle dissociation rate constant k[14]. For the studied alkyl sulphates k increases with increasing con-centration and the dependencies of k on the concentration becomesmore striking for C > (10–30) CMC which was explained by a transi-

tion in the micelle shape and by strengthening of the intermolecularrepulsion in the micelles.

The target of this study is the description of the adsorp-tion behaviour of the series of sodium alkyl sulphates NaCnSO4

1 : Phys

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

ith alkyl chain lengths of 10, 12, 14 and 16 carbon atoms athe water/hexane interface. Systematic interfacial tension mea-urements have been performed and analysed by two differentdsorption models giving access to the main thermodynamicdsorption parameters.

. Experimental

Ultrapure MilliQ water (resistivity = 18.2 M� cm) was used torepare all aqueous surfactant solutions. The surfactants sodiumecyl sulphate NaC10SO4 (MW = 260.63 g/mol), sodium dodecylulphate NaC12SO4 (MW = 288.38 g/mol), sodium tetradecyl sul-hate NaC14SO4 (MW = 316.43 g/mol), sodium hexadecyl sulphateaC16SO4 (MW = 344.48 g/mol) were synthesised in our labs and

espectively re-crystallised [5]. All experiments were performedt room temperature of 22 ◦C. Hexane was purchased from FlukaSwitzerland) and was purified with aluminium oxide and subse-uently saturated with Ultrapure MilliQ water.

Before doing experiment hexane was saturated with the respec-ive surfactant solution for 5 h and then the bulk phases separatedor the measurements. This protocol is typical for studies atater/oil interfaces because surfactant can be soluble in bothhases and this time is enough to transfer from one phase tonother to establish the distribution equilibrium. The experimen-al setup used to measure the dynamic interfacial tension of theifferent systems was the drop Profile Analysis Tensiometer PAT-1SINTERFACE Technologies Berlin, Germany) the principle of whichas described in detail elsewhere [16,17]. Equilibrium interfacial

ension values reported in the isotherms have been obtained after aufficient adsorption time to reach a plateau. This adsorption timeepends on the surfactant concentration and was 1 h for the highestoncentrations close to the CMC and 6 h for the lowest concen-rations. The interfacial tension of ultrapure MilliQ water againstexane was 51 mN/m at room temperature (21 ◦C). For all investi-ations at the water/hexane interface a drop of aqueous solutionas formed in a glass cuvette containing pure hexane.

. Theoretical models

There are quite a number of adsorption models, however, mostf the surfactants can be very well described by the Frumkin modelhich takes into account the lateral interactions between adsorbed

urfactant. In the following equation of state it is assumed that ω0the maximum molar area of the adsorbed surfactant) is equivalento the double molar area of solvent [2]:

= −2RT

ω0[ln(1 − �) + a�2] (1)

here ˘ is the surface pressure (the difference between the inter-acial tension of the pure water/hexane interface �0 and that ofhe solution/hexane interface �), � = ω·� is the surface coverage, �s the surface concentration, b is the adsorption equilibrium con-tant, a is the Frumkin interaction constant, R and T are gas lawonstant and absolute temperature, respectively. The pre-factor 2n the right hand side takes also into account that ω0/2 is the area ofhe solvent or counter ions. It was discussed in [18] that the molarrea surfactant ω can depend on ˘ .

= ω0(1 − ε˘�) (2)

here ω0 is the molar area of surfactant at ˘ = 0, and ε is thewo-dimensional relative surface layer compressibility coefficient,hich characterises the intrinsic compressibility of the molecules

n the surface layer. Several examples showed the importance ofhis property to describe properly the variation of adsorbed amountnd the dilational rheological behaviour of the surfactant adsorbedayer [19].

icochem. Eng. Aspects 382 (2011) 181–185

The following Frumkin isotherm takes into account the averageactivity of all ions present in solution (ionic surfactant and addedelectrolyte) [2].

b[c(c + c2)]1/2f = �

1 − �exp[−2a�] (3)

Here b is the adsorption constant, c and c2 correspond to thesurfactant and electrolyte concentrations, respectively. Withoutadditional salt i.e. c2 = 0, the isotherm (3) becomes the well-knownFrumkin isotherm. The activity coefficient f is defined as following:

log10 f = − 0.5115√

I

1 + 1.316√

I+ 0.0551 (4)

where I = c + c2 is the ionic strength for an 1:1 electrolyte expressedin mol/l.

Besides the Frumkin model we use also the reorientation modelfor the analysis of our data, which was first proposed in 1995 [20].The reorientation model assumes that two orientations of adsorbedsurfactant molecule co-exist at the surface, with different molarareas ω1 and ω2 (for definiteness we assume ω2 > ω1). This model inthe form given below also accounts for the non-ideality of entropy[20]. The equation of state reads

−˘ω10

RT= ln(1 − �) + � (ω − ω10) + a�2 (5)

with the adsorption isotherm (written for the adsorption in state1)

b1c = �1ω10

(1 − �)ω1/ω10exp

(− ω1

ω10(2a�)

)(6)

The total adsorption is given by � = � 1 + � 2, while � i are theadsorptions in one of the two states. Here, ω is the average molararea, b1 is the adsorption constants, and ω10 is the molar area of asurfactant molecule in state 1 for ˘ = 0.

The surface coverage is given by � = ω� = ω1� 1 + ω2� 2, and themolar area in state 1 again assumed to depend on the surface pres-sure

ω1 = ω10(1 − ε˘�) (7)

4. Experimental results

Interfacial tension measurements at water/hexane interfaceshave been realized for four alkyl sulphates with alkyl chain lengthsfrom 10 to 16. As we have used ionic surfactants the solubility inhexane is negligible and no transfer of surfactant into the oil phasehas to be considered. As an example, the dynamic interfacial ten-sions of aqueous solutions of NaC12SO4 against hexane are plottedin Fig. 1.

As expected from diffusion controlled adsorption kinetics withincreasing concentration the rate of adsorption becomes larger andhence the equilibrium values of interfacial tension were reachedfaster.

For the data interpretation two thermodynamic models wereapplied, the Frumkin Ionic compressibility and reorientation mod-els. The experimental results together with the correspondingtheoretical curves obtained are shown in Figs. 2–4. With both mod-els, the experimental data are fitted very well which provides thethermodynamic adsorption parameters of the studied surfactants.

The isotherms are shifted towards lower concentrations withincreasing alkyl chain length and the shift for each CH2–CH2becomes smaller and less than a factor of 10 which means that the

Traube factor is smaller than observed for the water/air interfacein [2].

Parameters of adsorption layers obtained with the Frumkin ioniccompressibility and reorientational models at the water/hexane

A. Sharipova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 382 (2011) 181–185 183

Fig. 1. Dynamic interfacial tensions of NaC12SO4 at water/hexane interface for dif-ferent bulk concentrations c [mol/l]: (1) 10−5; (2) 5 × 10−5; (3) 7 × 10−5; (4) 10−4;(5) 3 × 10−4; (6) 5 × 10−4; (7) 10−3; (8) 3 × 10−3; (9) 5 × 10−3; (10) 6 × 10−3; (11)7 × 10−3.

Fig. 2. Interfacial tension isotherms of NaCnSO4 plotted versus the concentration

cF

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cfihbtta

Fig. 3. Interfacial tension isotherms of NaCnSO4 plotted versus the concentration

c (�) n = 10; ( ) n = 12; ( ) n = 14; ( ) n = 16; dotted lines correspond to thereorientation model.

TP

(�) n = 10; ( ) n = 12; ( ) n = 14; ( ) n = 16; dotted lines correspond to therumkin model.

nterface are shown in Table 1. As one can see that parametersbtained with two models are close to each other.

For the Frumkin ionic compressibility model the intrinsicompressibility coefficient was fixed at ε = 0.008 mN/m for all sur-actants which allows surfactants to get smaller molar area withncreasing surface coverage. This value will not be showed in detailere because it influences mainly on the dilational rheological

ehaviour of the surfactants. At low surface coverage, the surfac-ant can use the maximum free space at the interface leading to ailt of the alkyl chain [19]. Due to this possibility of orientation, therea of the surfactant will change as a function of the surface pres-

able 1arameters of adsorption layers obtained with the reorientation and Frumkin ionic comp

NaCnSO4 ω0 (m2/mol) ω01 (m2/mol) ω2 (m2/

Frumkin Reorientation Reorien

n = 10 6.5E+5 4.0E+5 1.5E+6n = 12 6.0E+5 4.0E+5 2.0E+6n = 14 6.0E+5 3.9E+5 3.0E+6n = 16 5.6E+5 3.6E+5 3.0E+6

Fig. 4. Adsorbed amount � of NaCnSO4 plotted versus the concentration c (�) n = 10;( ) n = 12; ( ) n = 14; ( ) n = 16, as calculated with the Frumkin model.

sure and surface coverage. For the reorientation model the intrinsiccompressibility coefficient was fixed at ε = 0.003 mN/m for all sur-factants. Its value is smaller than for the Frumkin model, as in thereorientation model a transfer from the state of large molar areainto the state of smaller surface area takes place.

The adsorption constant b for both models is more or less close toeach other and increases with the alkyl chain length. The parametera which expresses the strength of interaction between the adsorbed

alkyl sulphate molecules in the adsorption layer is the same in thetwo models. Positive values of a obtained for the longer chainscan be explained by the hydrophobic forces dominating over theelectrostatic repulsion forces.

ressibility models at the water/hexane interface.

mol) a b (l/mol)

tation Frumkin Frumkin Reorientation

0 6.8E+03 8.8E+030 3.1E+04 5.6E+040.5 9.0E+04 1.6E+050.6 2.2E+05 3.4E+05

184 A. Sharipova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 382 (2011) 181–185

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ig. 5. Adsorbed amount � of NaCnSO4 plotted versus the concentration c (�) n = 10;) n = 12; ( ) n = 14; ( ) n = 16, as calculated with the reorientation model.

The maximum molar area ω0 is not constant for all surfactants inoth models and decreases with increasing chain length, obviouslyue to the increased van der Waals interactions.

Rehfeld studied NaC10SO4 at the water/hexane interface and,sing the Gibbs adsorption equation, calculated the adsorbedmount � and molar area ω, obtaining the values 3.7E−6 mol/m2

nd 4.5E+5 m2/mol, respectively [15]. As one can see the obtainedesults are close to those results presented in Table 1.

The adsorbed amount � of NaCnSO4 plotted versus the con-entration c for n = 10, 12, 14 and 16 calculated with Frumkin andeorientation models are presented in Figs. 4 and 5. � increasesith the surfactant concentration and alkyl chain length which

eads to higher surface coverages. For the reorientation modelhe adsorbed amount � of NaCnSO4 levels off on a plateauFigs. 6 and 7).

In general from the results obtained with the two models itpossible to divide the studied surfactants into 3 cases. First foraC10SO4 the molar area ω is the largest and the adsorbed amount

and adsorption constant b are the lowest among all samples.he electrostatic repulsion forces are prevalent because the chain

s short and there is little interaction between the adsorbed alkylulphate molecules in the adsorption layer. The large molar areaan be explained by the presence of intercalated solvent moleculeshat can be seen as a kind of competitive adsorption between the

ig. 6. Molar area ω of NaCnSO4 plotted versus the concentration c (�) n = 10; ( )= 12; ( ) n = 14; ( ) n = 16, as calculated with the Frumkin model.

Fig. 7. Molar area of NaCnSO4 plotted versus the concentration c (�) n = 10; ( )n = 12; ( ) n = 14; ( ) n = 16, as calculated with the reorientation model.

surfactant and the hexane molecules. Hence, a short chain surfac-tant will not be able to replace completely the solvent moleculesfrom the interface due to weak self-attractive interactions leadingto a smaller adsorbed amount for a saturated layer. Lu et al. studiedthe structure of surface films at water/air interface formed in sys-tems containing alkanes and cationic surfactants and found partialpenetration of the surfactant chains by the oil. They were able todetermine the individual distributions and relative positions of oiland surfactant molecules in the direction normal to the interface[21–24]. However, a direct comparison with our data on surfactantadsorption layers at the water/alkane interface is not suitable.

In the second group of the alkyl sulphates, NaC12SO4 andNaC14SO4, the molar area is lower than for NaC10SO4, and theadsorbed amount � and adsorption constant b are larger. The affin-ity of the surfactants to the oil phase increases with the alkylchain length. Electrostatic repulsion forces are still prevalent butthe molar areas decreased which mean that surfactant–surfactantinteractions between the adsorbed alkyl sulphate molecules in theadsorption layer are going to be dominated and the longer-chainsurfactants are able to replace the solvent molecules from the inter-face, at least at larger surface coverage.

Finally for NaC16SO4 the molar area is the lowest, the adsorbedamount � and adsorption constant b are the largest. Here the alkylchain length becomes long enough to let the hydrophobic forcesdominate over the electrostatic repulsion forces which also leadsto an interaction parameter a > 0.

Hydrophobic bonding, owing to the loss of structured water sur-rounding the hydrocarbon oil chains at the interface, is present asthe surfactant molecules penetrated into the oil phase and interactwith the hydrocarbon molecules. This force is present only at theoil/water interface [12].

5. Conclusion

In the present study the interfacial tension isotherms of alkylsulphates with different alkyl chain lengths (from C10 to C16) at thewater/hexane interface were studied. These isotherms are analysedin the frames of thermodynamics with Frumkin and reorientationmodels. The isotherms are shifted to lower concentrations withincreasing alkyl chain length, however, and the resulting Traube

factor is smaller than observed for the water/air interface. Adsorp-tion behaviour of the surfactant molecules depends on the alkylchain length, i.e. for the shorter chain surfactants the electrostaticrepulsion forces dominate while for longer chains the attractive

: Physi

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A. Sharipova et al. / Colloids and Surfaces A

urfactant-surfactant interactions prevail. The well-known SDS,aC12SO4, is in the transition range between the two cases.

cknowledgements

A.S. is grateful to the DAAD for a research grant (A/08/77714).he financial support by DFG projects is also gratefully acknowl-dged (Mi418/16-2 and Mi418/18-1).

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