ol mono-

teractionsrder phasesed (LC)

2E1/C14E4al micellesurfactantsture. Theinteractione mixture.dicating anits in theirnd micelle

Journal of Colloid and Interface Science 289 (2005) 581–587www.elsevier.com/locate/jcis

Adsorption and micellar properties of a mixed systemof nonionic–nonionic surfactants

Md. Nazrul Islam, Teiji Kato∗

Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan

Received 4 April 2005; accepted 20 May 2005

Available online 15 July 2005

Abstract

We study the surface adsorption and bulk micellization of a mixed system of two nonionic surfactants, namely, ethylene glycn-dodecyl ether (C12E1) and tetraethylene glycol mono-n-tetradecyl ether (C14E4), at different mixing ratios at 15◦C. The pure C14E4monolayer cannot show any indicative features of phase transition because of both hydration-induced and dipolar repulsive inbetween the bulky head groups. On the other hand, the monolayers of pure C12E1 and its mixture with C14E4 undergo a first-otransition, showing a variety of surface patterns in the coexistence region between the liquid expanded (LE) and liquid condenphases under the same experimental conditions. For pure C12E1, the domains are of a fingering pattern while those for the C1mixed system are found to be compact circular and small irregular structures at 2:1 and 1:1 molar ratios, respectively. The criticconcentration (cmc) values of both the pure and the mixed systems were measured to understand the micellar behavior of thein the mixture. The cmc values of the mixed system were also calculated assuming ideal behavior of the surfactants in the mixexperimental and calculated values are found to be very close to each other, suggesting an almost ideal nature of mixing. Theparameters for mixed monolayer and micelle formation were calculated to understand the mutual behavior of the surfactants in thIt is observed that the interaction parameters for mixed monolayer formation are more negative than those of micelle formation, instronger interaction between the surfactants during monolayer formation. It is concluded that since both the surfactants bear EO uhead groups, structural parity and hydrogen bonding between the surfactants allow them to be closely packed during monolayer aformation. 2005 Elsevier Inc. All rights reserved.

Keywords: Brewster angle microscopy; Monoionic surfactants; Adsorbed monolayers; Critical micelle concentration

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ter-ichide

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1. Introduction

One of the most important features of surfactants is toutstanding ability to adsorb onto surfaces. The adstion alters the properties of surfaces, which is relevantheir uses in a wide variety of household cleaning produpaints, cosmetics, and pharmaceuticals. Extensive stuhave been dedicated to mixed surfactant systems, becone of the suitable ways to improve the surface or infacial properties is to add one surfactant to another whcan show superior performance upon mixing and prov

* Corresponding author. Fax: +81 28 689 6179.E-mail address: teiji@cc.utsunomiya-u.ac.jp(T. Kato).

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.05.072

se

more favorable properties than the individual ones[1–6].Depending on the nature of the surfactants in a mixturesurface and micellar properties of a mixed system canmain either inside or outside of those of the pure surfact[7–11], which are governed by the coulombic ion–dipoleteractions or hydrogen bonding between the head groHowever, it should be borne in mind that unfavorable strtural parity can lead to inferiority instead of superiorityperformance, even in cationic–anionic mixed systems, slack of head group compatibility and, apart from head-head interactions, a closer molecular packing in micelle

in monolayers is not possible[11].

In recent years, there has been an increasing interest intwo-dimensional (2-D) surface phase behavior and bulk mi-

and

met-aceuiteemselpuired tothetamra-

f theure

tantfac-ouldhostivity

c-tos ofd filmonedfor-s ofn ofmi-(FM

no-ter

ac-s atvide

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rena-

e ben-

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582 M.N. Islam, T. Kato / Journal of Colloid

cellization of mixed surfactant systems because stoichioric association of surfactants can strikingly alter both surfand micellar behavior of the molecules, which can be qdifferent from those of the pure systems. This interest stin part from the fact that this type of understanding can hone to address some crucial facets of what necessary reqments are to be fulfilled to achieve better performance anwhat extent the mixed system could show superiority topure constituents under a set of conditions. Besides, conination in surfactants, whether occurring in original prepation or later, can be properly addressed from the study oadsorption and micellar behavior of surfactants in both pand mixed systems. Therefore, it is fundamentally importo study the 2-D surface phase behavior of mixed surtant systems to select a suitable surfactant pair that cshow desired performance properties. In this regard, aof surface-sensitive techniques such as neutron reflec[12,13], sum frequency vibrational spectroscopy[14–17], el-lipsometry[18,19], and infrared reflection absorption spetroscopy[20–23]have been widely used to gain insight inthe phase behavior, the tilt orientation, the constituentthe adsorbed species, and the thickness of the adsorbeat the air–water interface. Sensitive as the above-mentitechniques may be, none of them can provide direct inmation as to changes in surface morphological featurethe monolayer with changes in the surface concentratiothe molecules. In this connection, the development ofcroscopic techniques such as fluorescence microscopy[24,25] and Brewster angle microscopy (BAM)[26,27] hasopened the door to in situ visualization of changes in molayer morphologies of amphiphilic systems at the air–wainterface. The advantage of using FM and BAM to charterize the changes in surface morphology of monolayerthe air–water interface is that these techniques can prostraightforward evidence of changes in surface morphowith changes in experimental temperature[28–37], impuritycontents[38–40], and subphase pH[41,42], as well as inter-nal textures of the condensed phase responsible for diffetilt orientation of the molecules within a domain if an anlyzer is placed in the reflected beam path[34–37,41,42].

In the present work, we have studied the surface phashavior and micelle formation of a mixed system of two noionic surfactants, namely, ethylene glycol mono-n-dodecylether (C12E1) and tetraethylene glycol mono-n-tetradecylether (C14E4), at different mixing ratios by film balance aBAM as well as their micellization in the bulk of the aqueosolution by surface tensiometry. Here we have showneven though nonionic surfactants in mixtures with minimintermolecular interactions have the lowest synergism,hancement in properties can fairly be observed with mixtuof similar structures. Since both the surfactants bear eene oxide (EO) units in their head groups, steric hindrabetween them should be a minimum during monolayer

micelle formation. Of the two surfactants used in the presentstudy, C12E1 forms fractal patterns in the adsorbed mono-layers, while C14E4 cannot show any feature indicative of a

Interface Science 289 (2005) 581–587

-

-

t

)

t

-

two-phase coexistence state upon adsorption at the air–winterface. However, it can fairly change the adsorptionnetics and surface morphology as well as the bulk miceproperties when mixed with C12E1. It is to be noted tthe mixed system at the bulk C12E1/C14E4 molar ratio2:1 shows a much lower equilibrium surface tension tthe pure constituents. The observation of compact circdomains at this mixing ratio suggests that the line tensof the LE–LC interface has appreciably increased upon ming because of an increase in the van der Waals interabetween the alkyl chains. Besides, interaction parametertween the surfactants for both adsorption and micellizawere calculated to gain insights into their mutual behavduring monolayer and micelle formation.

2. Experimental

2.1. Materials

The nonionic surfactants C12E1 (CH3(CH2)11OCH2CH2OH) and C14E4 (CH3(CH2)13(OCH2CH2)4OH) were pur-chased from Nikko Chemical Co. Tokyo, Japan with a puof >99%, and were used as received. The solutions ofsurfactants were prepared in ultra-pure water of resisti18 M� cm (Ulgastat UHQ-PS) separately, and were miin an appropriate volume ratio to obtain the desired molatio. The surface pressure–time(π–t) curves were measurein a shallow Langmuir trough (2 mm depth) with a surfaarea of 41× 16 cm equipped with a Wilhelmy balance andtemperature-controlling system. Mounted above the trowas a Brewster angle microscope (BAM) to observe thsitu surface morphology of the adsorbed film. A beamp-polarized light from a 20-mW He–Ne laser was directo the air–water interface at the Brewster angle. Under scircumstances, the coexistence of two phases with diffesurface densities was readily observed.

2.2. Adsorption kinetics and surface morphology

The experiments were carried out by pouring a defiamount of aqueous solution (either of the pure surfactantheir mixtures) into the trough. To attain equilibrium withe experimental temperature the solution was allowestand for about 25 min before starting the experiments.molecules already adsorbed during this span of time wremoved by sweeping the surface by the movable Teflonriers. Under these conditions, the surface concentratiothe adsorbed molecules can be considered to be zeroπ = 0 att = 0. The increase in surface pressure was thenlowed with time, and simultaneously the change in surfmorphology was monitored by BAM. The recorded imag

were then treated by image-processing software to maximizethe contrast and to correct the distortion of the images causedby the oblique glancing of the microscope. Details of the ex-

d and

tra-10)

aysrnerintoy cirflowtion,pro-the

s al-these-nt

thet a

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diedn-mwithini-und

rfaceeeire

noto-seac-facean-. Asho-

asethereakec-

derdedased-thefaceand,isthat

f therac-sivecultsult,the

sitionthe

gherof

M.N. Islam, T. Kato / Journal of Colloi

perimental procedure[31] and set-up[43] are found to beelsewhere.

2.3. Surface tension

Surface tensions of the solutions of different concentions were measured by a surface tensiometer (Krüssequipped with a platinum plate. The plate was alwcleaned by heating it to a red-orange color with a gas buprior to use. The amphiphile solutions were transferreda vessel of depth 3 cm. The vessel was thermostated bculating water of the desired temperature at a constantrate. The measurement was started with a dilute soluand a previously prepared concentrated solution wasgressively added into the vessel by a pipette to obtainsubsequent concentrated solutions. Sufficient time walowed to attain the equilibrium surface tension. Thenestablishment of equilibrium was checked by taking aries of readings after a 15-min interval until no significachanges occurred.

3. Results and discussion

3.1. Adsorption kinetics and surface morphology

Fig. 1shows the equilibrium surface tension values ofC12E1–C14E4 mixed system at different mixing ratios atotal concentration of 2.0 × 10−5 M at 15◦C. This concen-tration is slightly less than the cmc value of C12E1, whit is higher than that of C14E4 at this temperature. Hoever, the cmc values of the mixed system over the stuC12E1/C14E4 mixing ratios are found to lie below this cocentration. It is evident in the figure that the equilibriusurface tension of the mixed system gradually decreasesincreasing the mole fraction of C14E4 and shows a mmum when the C12E1/C14E4 molar ratio reaches aro

Fig. 1. Dependence of surface tension as a function of mole fraction ofC12E1 for the mixture of 2.0×10−5 M solutions of both C12E1 and C14E4at 15◦C.

Interface Science 289 (2005) 581–587 583

-

2:1. This suggests that the monolayer attains better suactivity at this mixing ratio.Fig. 2 shows the representativπ–t adsorption kinetics of pure C12E1, C14E4, and thmixtures at 15◦C. It clearly turns out from the figure that thsurface pressure of the C14E4 monolayer increases monously with time until the value attains equilibrium. Becauof strong hydration-induced and dipolar repulsive intertions between the bulky head groups as well as low surviscosity of the adsorbed film, the C14E4 monolayer cnot achieve high-density states at the air–water interfacea result, the adsorbed molecules distribute themselvesmogeneously, showing no indicative features of two-phcoexistence state at the air–water interface. On the ohand, C12E1 and its mixture with C14E4 show a sharp brpoint in the course of adsorption process in their resptive isotherms (shown by the arrows), indicating a first-orphase transition between a lower density liquid expan(LE) phase and a higher density liquid condensed (LC) ph[27–41]. At a 2:1 mixing ratio, the cusp point corresponing to theπc value necessary for the phase transition formixed monolayer appears at an early time and low surpressure compared to that of pure C12E1. On the other hat a 1:1 mixing ratio, the cusp point for the mixed systemfound to appear at higher surface pressure compared toof pure C12E1. At a definite temperature,πc is governedby the balance between the van der Waals interaction ohydrophobic alkyl chains and the opposing repulsive intetion of the head groups. Usually a higher degree of repulinteraction between the bulky head groups makes it diffito achieve high-density states in the monolayer. As a recloser molecular packing with a consequent increase inπc value is necessary for the appearance of a phase tranwith increasing bulkiness of the head group. Therefore,phase transition for the mixed system appearing at a hiπc value at this mixing ratio is the outweighing influence

Fig. 2. π–t adsorption kinetics of 2.0 × 10−5 M solutions of C12E1 (I),

C14E4 (II), and their mixtures at 2:1 (III) and 1:1 (IV) molar ratios at 15◦C.The arrows in the isotherms indicate the position of the appearance of thephase transition during the adsorption process.

and

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–LCpat

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eadin-rac-

(A)

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584 M.N. Islam, T. Kato / Journal of Colloid

Table 1The cmc,βM, andβσ values of the surfactants at different mixing ratios

αC12E1 cmc× 105 M XMC12E1 βM αC12E1 XC12E1 βσ

0.67 1.12 0.37 −0.46 0.67 0.40 −0.600.50 0.92 0.25 −0.48 0.50 0.26 −0.590.33 0.79 0.15 −0.46 0.33 0.18 −0.60

the repulsive interaction between the bulky head groupC14E4. This is reflected in the mole fraction of the surftants in the mixed monolayer where C14E4 dominates wa value of 0.73 (Table 1).

Fig. 3shows the BAM images of pure C12E1 and its mture with C14E4 at different mixing ratios at 15◦C. PureC14E4 cannot show higher density condensed-phasemains at the air–water interface because of strong dipas well as hydration-induced repulsive interactions betwthe bulky head groups. On the other hand, the monoers of C12E1 and its mixture with C14E4 show intereststructures in the LE–LC transition region at the same tperature. The domains of pure C12E1 are found to assufingering pattern at this temperature (image A). This chacteristic pattern in the condensed-phase domains ocbecause of the decrease in the line tension of the LEinterface. Images B and C show the condensed phaseterns of the mixed system at 2:1 and 1:1 mixing ratios ofsurfactants, respectively. At 2:1 mixing ratio, the domaare found to be circular, suggesting that the line tensiothe LE–LC interface appreciably increases upon mixingC14E4 with C12E1. This could be due to the outweighinfluence of the longer hydrocarbon chain of C14E4, siline tension increases with increasing length of the hydrobon chain. However, at the same time it may be the influeof hydrogen bonding between the head groups of the sutants. Besides, C14E4, being inserted into the monolayC12E1, can escape the electrostatic and hydration-indrepulsive interactions between the bulky head groups, sthe presence of four EO units provides the head groupconsiderable flexibility. As a result, the mixed monolaycan attain closer molecular packing and gives compactcular domains at this mixing ratio. At a 1:1 mixing ratithe domains are found to be small irregular structureshas been reported that a large value of line tension falarge compact disklike structures in two dimensions, whrepulsive interactions between the molecules favor smaregular structures or spiral ones when the domain arelarge, to minimize the repulsive interactions by maximing the intermolecular distance[44,45]. Therefore, the appearance of large compact domains at the 2:1 mixing ris the result of an increase in the line tension of the LLC interface. Since C14E4 bears four EO units in its hgroup, increases in its ratio in the mixture appreciablycrease both hydration-induced and dipolar repulsive inte

tions between the molecules, resulting in the appearance osmall irregular structures in the mixed monolayer at the 1:1molar ratio.

Interface Science 289 (2005) 581–587

-

s

-

Fig. 3. BAM images observed after the phase transition for pure C12E1and its mixture with C14E4 at 2:1 (B) and 1:1 (C) molar ratios at 15◦C.The bar in image (A) indicates 100 µm.

3.2. Critical micelle concentration and the interactionparameters for mixed monolayer and micelle formation

Fig. 4shows the representativeγ – logC plots for C12E1,C14E4, and their 1:1 mixture at 15◦C. It is clear in the fig-ure that theγ values decrease sharply with increasing bconcentration(C) of the pure and mixed surfactant systemAt a definite concentration, theγ – logC plots show a sharpbreak point followed by a plateau region of constantγ val-ues, known as the critical micelle concentration (cmc) ofsystem. As seen clearly, the cmc and the equilibrium surtension at�cmc values of the mixed system lie in betwethose of the pure systems. When two surfactants in a mixshow ideal behavior, the surface as well as bulk prope

fof the mixed system fall in between the solution propertiesof the pure systems, while nonideality of the mixed sys-tem leads them outside the solution properties of the pure

d and

ixed

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M.N. Islam, T. Kato / Journal of Colloi

Fig. 4. Surface tension(γ ) versus logC plots of pure (I) C12E1 and (II)C14E4, and (III) their 1:1 mixture at 15◦C. The pointsC1, C2, andC arethe concentrations used to calculate the interaction parameters for mmonolayer formation.

Fig. 5. Experimental and calculated cmc values of the mixed system aferent molar ratios at 15◦C.

systems. Therefore, the observed data indicate that thesurfactants behave almost ideally during micellization. Creported that if two surfactants in a mixture behave ideathe cmc of the mixed system at any mixing ratio can beculated by the expression[46]

(1)1

C′m

= a1

C′1

+ (1− a1)

C′2

,

wherea1 is the mole fraction of the surfactant 1 in the mture,C′

1, C′2, andC′

m are the cmc values of surfactants 1and their mixture, respectively. Both the experimentalthe calculated cmc values for the mixed system at diffemixing ratios are plotted inFig. 5. The cmc values calculateusing Eq.(1) are found to be very close to the experimenones over the studied mixing ratios. This clearly sugg

that both the surfactants behave almost ideally during micel-lization. The interaction parameters for both micellizationand adsorption at the air–water interface were calculated to

Interface Science 289 (2005) 581–587 585

gain insights into their mutual behavior in the mixed stem. The interaction parameterβM values for micellizationin the mixed system were calculated with the help ofequations[1]

(2)(XM

1 )2 ln(α1C

M/XM1 CM

1

)

(1− XM1 )2 ln

[(1− α1)CM/(1− XM

1 )CM2

] = 1

and

(3)βM = ln(α1C

M/XM1 CM

1

)

(1− XM1 )2

,

whereXM1 is the mole fraction of surfactant 1 in the tot

surfactants in the micelle, andCM1 , CM

2 , and CM are thecmc values of surfactant 1, surfactant 2, and their mixtrespectively, at a mole fraction ofα1 of surfactant 1. Usually the interaction between two different surfactants, aconsequently, the interaction parameters are dominatethe nature of interactions between their head groups. Bemixing there exist strong self-repulsive interactions betwindividual components. This self-repulsion is substantiareduced upon mixing of a suitable surfactant with anotgiving rise to a decrease in theβM values. In the presenstudy, small negative values are observed for micellizatindicating that there exists a weak interaction betweensurfactants. This attractive interaction during micellizatmay be due to hydrogen bonding between the head grof the surfactants.

Let us examine the nature of interaction for mixed molayer formation at different mixing ratios of the surfactanThe interaction parameters for monolayer formation wcalculated with the help of the equations[1]

(4)X2

1 ln(α1C/X1C1)

(1− X1)2 ln[(1− α1)C/(1− X1)C2] = 1

and

(5)βσ = ln(α1C/X1C1)

(1− X1)2,

whereX1 is the mole fraction of surfactant 1 in the totmixed monolayer,C1, C2, and C are the solution phasconcentrations of surfactant 1, surfactant 2, and their mture, respectively, at a mole fraction ofα1 of surfactant 1to produce a definite surface tension. The concentrationthe pure and mixed surfactants used to calculate the intetions parameters are shown inFig. 4. For the C12E1–C14Emixed system, theβσ values calculated by the use of Eqs.(4)and (5)are found to be a bit more negative than those forcelle formation, suggesting that the interaction betweenmolecules in the monolayer is stronger than that in thecelle. This is because of linearity of the hydrophobic groof the surfactants organized in a planar air–water intersince if the steric interaction between the head groupweak, reduction in the electrostatic repulsion has a gre

effect at the planar air–water interface than at the convex mi-celle surface[1]. Besides, the hydrophobic alkyl chains canmore easily accommodate themselves at the planar air–water

and

lin-

r ofe-xed

in-

E4thevity,rac-lineh ofyeresnedby

luepuren theatioelytionules. In

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.

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05

586 M.N. Islam, T. Kato / Journal of Colloid

interface than in the cramped interior of spherical or cydrical micelles, giving a more negativeβσ value comparedto βM.

We are particularly interested in the phase behaviosurfactant mixtures at different mixing ratios. Our BAM rsults reveal that at the 2:1 ratio, the C12E1/C14E4 misystem forms compact circular domains, suggesting ancrease in the line tension of the LE–LC interface.Table 1shows that at this molar ratio the mole fraction of C14in the monolayer is 0.60. Probably, at this mixing ratio,surfactants in the monolayer attain a better surface actiwhich gives rise to an increase in the van der Waals intetions between the hydrophobic alkyl chains. Since thetension of the interface increases with increasing lengtthe alkyl chain, the insertion of C14E4 into the monolaof C12E1 positively contribute to the line tension and givcompact circular domains at the water surface. As mentioearlier, at 1:1 molar ratio the monolayer is dominatedC14E4 with a mole fraction of 0.73, showing a higher vaof πc necessary for the phase transition than that of theC12E1 because of relatively stronger repulsions betweebulky head groups. With a further increase in the molar rof C14E4 in the bulk of the solution, the domains completdisappear, because a higher degree of repulsive interacbetween the bulky head groups does not allow the molecto achieve high-density states at the air–water interfaceaddition to this, equilibrium surface pressure cannot cwith the increasing trend of theπc values with increasingthe molar ratio of C14E4. It is well known that condensdomains are formed only when the line tension of theterface and the surface viscosity of the condensed p[47] are favorable for the system. Hühnerfuss reportedthe surface viscosity of adsorbed molecules decreasesincreasing hydrophilic character of the molecules[48–50].Since C14E4 bears four EO units in its head group, increin the proportion of C14E4 molecules give rise to a decrein the surface viscosity, which in turn renders the possibof domain formation unfavorable. Thus, the overall effecthe increase in the repulsive interactions between the bhead groups, as well as the decrease in the surface viity with increasing proportion of C14E4, does not allow tmolecules to cohere in the form of condensed-phasemains when the C12E1/C14E4 molar ratio is 1:2 or high

4. Conclusions

In the present work, we have studied the adsorptionmicellar behavior of two nonionic surfactants at differemixing ratios at 15◦C. Pure C14E4 cannot show any featuindicative of two-phase coexistence because of strong dlar and hydration-induced repulsive interactions betweenbulky head groups. On the other hand, pure C12E1 an

mixture with C14E4 show interesting surface patterns de-pending on the mixing ratios of the surfactants. The domainsof C12E1 have a fingering pattern, while those for the mixed

Interface Science 289 (2005) 581–587

s

-

systems are found to be circular and irregular structureC12E1/C14E4 mixing ratios of 2:1 and 1:1, respectivelyis interesting to note here that the mixed system at themolar ratio shows much higher surface activity than the pconstituents under the same experimental conditions.cmc values of the mixed systems at different mixing ralie in between those of the pure systems, suggesting alideal behavior of the surfactants during micellization. Tcmc values of the mixed system were calculated by theproach of ideal mixing of the surfactants. The interactparameters of mixed monolayer and micelle formation walso calculated to gain insights into the mixing behaviorthe surfactants. In both the cases, small negative valuesgest that the surfactants behave almost ideally during mlayer and micelle formation. The interaction parametersmixed monolayer formation are found to be more negathan those for micelle formation, suggesting that in the pner monolayer surface the molecules face less difficultincorporating themselves than at the convex micelle surresulting in more negativeβσ values.

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d and

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0)

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n.

90

6)

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