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Journal of Colloid and Interface Science 282 (2005) 142–148www.elsevier.com/locate/jcis

Effect of temperature on the surface phase behavior and micelle formof a mixed system of nonionic/anionic surfactants

Md. Nazrul Islam, Teiji Kato∗

Venture Business Laboratory, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan

Received 5 March 2004; accepted 13 August 2004

Available online 27 October 2004

Abstract

The adsorption and micellar behavior of diethylene glycol mono-n-tetradecyl ether (C14E2), sodium 3,6,9,12-tetraoxaoctacosa(TOOCNa), and their mixture at a 1:1 molar ratio have been studied by film balance, Brewster angle microscopy (BAM), andtensiometry at different temperatures. Themonolayers of pure C14E2 and itsmixture with TOOCNa show a first-order phase transition wita conspicuous cusp point in their respective adsorption isotherms. This is further confirmed by the observation of bright two-dimensiocondensed phase domains visualizedby BAM just after the appearance of the phase transition. It is interesting to note here that for C14Econdensed domains are observed up to 19◦C, while in the mixed system, they are observed up to 22◦C. To understand why in the mixesystem the domains are observed at higher temperatures than for pure C14E2, we have measured the temperature dependencyrium surface tension at�cmc (γcmc) values of both the pure and the mixed systems. Theγcmc values of pure C14E2 remain almost constawhile those of pure TOOCNa and its mixture with C14E2 decrease appreciably with increasing temperature. It is concluded thdegree of dehydration of the ethylene oxide (EO) chain reduces the head-group size of TOOCNa, which outweighs the combinethe repulsive interactions between the head-groups and the thermal motion of the adsorbed molecules. Furthermore, C14E2 beinto the TOOCNa monolayer reduces the electrostatic repulsions between the charged heads, and consequently, the adsorbedattain closer molecular packing. As a result, theγcmc values of both pure TOOCNa and its mixture with C14E2 decrease with incretemperature. This facilitates theformation of condensed domains in the mixed system athigher temperatures, whereas none of the individmembers can show any indicative feature of phase transition under thesame experimental conditions. 2004 Elsevier Inc. All rights reserved.

Keywords: Nonionic surfactant; Anionic surfactant; Phase transition; Brewster angle microscopy; Adsorbed monolayer; Critical micelle concentration

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1. Introduction

Surfactant systems used in commercial preparationsally consist of mixtures of surfactants, either because frothe economic point of view they can be produced at aatively lower cost or becausemixtures of dissimilar surfactants often show better performance with respect to interfacial properties such as surface tension[1–4], surfaceviscosity [5], and foam stability[5,6] than those of thepure systems. These changes in interfacial propertiesattributed to a higher degree of molecular packing and or

* Corresponding author. Fax: +81-028-689-6179.E-mail address: teiji@cc.utsunomiya-u.ac.jp(T. Kato).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.08.116

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ing of monolayers at the air–water interface. An importstep toward understanding the behavior of surfactantsmixture is to study the adsorption kinetics and morpholocal features of mixed monolayers at the air–water interfacIn this regard, Brewster angle microscopy (BAM)[7,8] isan excellent technique for in situ visualization of chanin surface morphology of a monolayer without the use oprobe impurity. Surfactant molecules dissolved in the bof an aqueous solution adsorb spontaneously at the solsurface and brings about a gradual increase in surfacesure with increasing adsorption time. When the surfacecentration attains a definite value at a definite temperathe pressure(π)–time(t) adsorption isotherm shows a shacusp point followed by a plateau region indicating a fir

Md.N. Islam, T. Kato / Journal of Colloid and Interface Science 282 (2005) 142–148 143

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order phase transition[9–17]. Although surface pressure dvelops directly from the spontaneous adsorption of thesolved molecules from the bulk to the air–water interfathe phase transition leading to the formation of condensedphase domains is governed by a delicate balance betweefavorable van der Waals interactions in the alkyl chainsthe opposing repulsive interactions in the head groups. Gerally surfactant molecules that show this indicative featurin the adsorbed monolayer have an alkyl chain of medlength and a relatively smaller head group[9–17]. For suchsystems, the critical surface pressure necessary for the ptransition (πc) values increases while the equilibrium surfapressure attainable at�cmc (πcmc) values decreases with increasing temperature. At adefinite temperature, theπc andπcmc values become equal; this is the maximum temperafor the appearance of a phase transition[16].

Several research groups, including ours, have studiedsurface phase behavior of mixed surfactant systems aair–water interface[18–28]. These studies reveal that dpending on the nature of the surfactants in a binary mture, the surface phase behavior of the mixed monolayebe quite different from that of the pure systems. Sumquency vibrational spectroscopic studies[21–23] of phasetransitions in the mixed adsorbed monolayers of cationicfactants with dodecanol or tetradecane demonstrate thamonolayers of the mixed system are conformationallydered and are comparable to that of the pure alcohol,though the pure cationic surfactant or the hydrocarbonot capable of forming densely packed conformationallydered monolayers. In monolayer penetration experimeBarnes et al.[24] showed that hexadecyltrimethylamoniubromide (C16TAB) molecules can adsorb significantly ithe octadecanol monolayer, giving rise to an increase insurface pressure if the molecular area is held constantcently, Jain et al.[29] have studied the surface complexatiof C16TAB and a nonsurface-active anionic polyelectrolyteand observed that the complex undergoes a first-order ptransition at the air–water interface. Brewster angle micscopic study of the phase behavior of mixed monolayeranionic–nonionic systems reveals that the presence oionic surfactants has a profound effect on both adsorpkinetics and surface morphology of the monolayer, ethough the pure anionic surfactants are not capable of shing any indicative feature of phase transition[25–28].

Despite this experimental work, much remains tolearnt about the phase behavior of mixed monolayerwater-soluble surfactants. What has received less atteis the nature of changes in the surface and bulk propertiesurfactants in a mixture with changes in temperature. Tis particularly important form the viewpoint of both fundmental and applied aspects in selecting a suitable pasurfactants, which could show performance propertiesperior to those of pure systems alone. In the present wwe have demonstrated the adsorption and micellar behaof the C14E2–TOOCNa mixed system at different tempatures at a 1:1 mixing ratio. The selection of the surfac

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pair lies behind the structural similarity of their head groubearing EO units. In the present study, it will be shown tthe following thermal properties of both pure and mixed stems must be addressed in order to assess the performproperties of a surfactant mixture: (1) the effect of tempature on the equilibrium surface tension attainable at�cmc(γcmc), (2) the effect of temperature on dehydration ofhead group and on surface adsorption and bulk micellizaand (3) the effect of thermokinetic molecular motion onpacking of the adsorbed film. The last two effects havetagonistic natures with temperature change, and their relmagnitude will govern theγcmc and the surface concentrtion of the adsorbed molecules. From these effects we capiece together an explanation of how the surface andproperties of the surfactants in both pure and mixed systare influenced by the change in temperature, and whymixed system undergoes phase transition at higher temtures then for pure C14E2.

2. Experimental section

The surfactant C14E2 (CH3(CH2)13(OCH2CH2)2OH)was supplied by Nikko Chemical Co., Tokyo, Japan, whTOOCNa (CH3(CH2)15(OCH2CH2)3OCH2COONa) wassupplied by Lion Corporation, Tokyo, Japan, both with arity >99%, and were used without further purification. Tsolutions of the pure surfactants were prepared separateultrapure water of resistivity 18 M� cm (Elgastat UHQ–PSand their 1:1 molar ratio was obtained by mixing thequired volumes of the pure solution. The experimental sefor the study of adsorption kinetics and surface morpholwas equipped with a shallow Langmuir trough above wha Brewster angle microscope (BAM) was mounted. Theperiments were carried out by pouring a definite amounaqueous solution into the trough. The surface pressuremeasured by the Wilhelmy method using a small rectanlar glass plate. The plate was washed by a 1% HF solufollowed by ultrapure water prior to each measurement. Thtemperature of the surface was controlled precisely, usilarge number of integrated Peltier elements attached toback of the base plate of the trough, and was detectea platinum-wire temperature sensor of very small heatpacity, sealed in a piece of nickel-plated copper. To atequilibrium with the desired experimental temperature,solution was allowed to stand for about 25 min beforestart of the experiment. The molecules already adsorbethe surface during this span of time were removed by swing the surface with movable Teflon barriers at a conssweep rate controlled by a program. Under these conditithe surface concentration of the adsorbed molecules canconsidered to be zero; i.e.,π = 0 at t = 0. The increase insurface pressure was then followed with time and simuneously the change in surface morphology was observeBAM. Details of the trough merit[30] and the experimentaprocedure[15] have been reported previously.

144 Md.N. Islam, T. Kato / Journal of Colloid and Interface Science 282 (2005) 142–148

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Surface tension measurements of the solutions ofpure and mixed systems at different concentrations wereried out by a surface tensiometer (Krüss K-10) equipwith a platinum plate. The plate was cleaned by rinsing wultrapure water followed by flaming. The solutions wetransferred into a vessel that was thermostated by circulawater of desired temperature at a constant flow rate. In oto minimize the evaporation losses the vessel was covwith a lid for the experiments above 25◦C. The measurement was started with a dilute solution and the subseqconcentrated solutions were prepared by adding a previoprepared stock solution. Establishment of equilibrium wchecked by taking a series of readings after 15-min interuntil no significant change occurred.

3. Results and discussion

3.1. Adsorption kinetics and surface morphology

Surfactant molecules dissolved in the bulk of the aquesolution adsorb spontaneously, showing gradual increassurface pressure(π) with increasing adsorption time(t). Af-ter a definite period of time, theπ–t adsorption profile showa sharp cusp point followed by a plateau region, whicindicative of a first-order phase transition in the adsormonolayer[8–17]. Fig. 1 shows the adsorption kineticspure C14E2 and TOOCNa, and their 1:1 mixture at 15◦C.The isotherm of pure TOOCNa shows no cusp point, sgesting that the adsorbed molecules are homogeneousltributed at the air–water interface. The repulsive interactbetween the charged head groups, as well as higher deghydration around the EO chain, do not allow the molecuto accumulate in the form of a densely packed monolaythe water surface[27,28]. On the other hand, the isothermof C14E2 and its mixture with TOOCNa show well-defincusp points, indicating LE–LC phase coexistence of thesorbed monolayers. Comparing the adsorption behaviothe mixed system with that of the pure systems, we can

Fig. 1. π–t adsorption kinetics of 2.0 × 10−5 M solutions of (I) C14E2,(II) TOOCNa, and (III) their 1:1 mixture at the air–water interface at 15◦C.

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the effect of TOOCNa on the adsorption of C14E2 at an estage of the adsorption process. For the mixed systemcusp point occurs at lower surface pressure and moves tolier time than for that of pure C14E2. This suggests that bthe surfactants adsorb simultaneously at the water surFig. 2shows theπ–t adsorption kinetics of the mixed systeat 15, 19, and 22◦C. The rate of adsorption increases wincreasing the temperature of the experiment. The cufor 15 and 19◦C have not reached their equilibrium valuwithin the time span of the experiment. The cusps aredistinctive for 19 and 22◦C; nevertheless there are regioof two-phase coexistent states, as can be observed by B

Fig. 3 shows the BAM images of C14E2 and its mture with TOOCNa at different temperatures. As shown

Fig. 2. π–t adsorption kinetics of 2.0 × 10−5 M 1:1 mixed solutionof C14E2 and TOOCNa at different temperatures: (I) 15, (II) 20,(III) 22 ◦C. The arrows in the respective isotherms indicate the positiothe appearance of condensed-phase domains at the air–water interfac

Fig. 3. BAM images of pure C14E2 ((A) 15 and (B) 19◦C) and its 1:1mixture with TOOCNa ((C) 15 and (D) 22◦C ) observed after the phastransition. The bar in image (A) indicates 100 µm.

Md.N. Islam, T. Kato / Journal of Colloid and Interface Science 282 (2005) 142–148 145

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the figure, for pure C14E2, the domains are found tospiral with a number of sharp bends and hooks at 15◦C(image (A)). Heckl et al.[31] observed that monolayersL-α-dimyristoylphosphatidic acid form dendritic domainslow pH, but if the pH of the subphase is raised to aboutspiral domains are formed, which has been ascribed to thionization of the head groups of the amphiphile. In a pvious paper[16], we have reported that the condensedmains of ethylene glycol mono-n-tetradecyl ether, whichbears only one EO unit in its head group, are circulalower temperatures and display a fingering pattern at higtemperatures. Line tension of the interface is believed tothe governing factor for the domain shapes over the sied temperature range. Because of the introduction ofmore EO unit to the head, C14E2, the molecules adsoat the water surface, experience an appreciably highegree of repulsive interaction, showing strip-like structuthat become longer and spiral with increasing adsorptime. Circular domains of uniform brightness are obserat 19◦C (image (B)), which is the maximum temperatufor the appearance of the phase transition. Formation ofcular domains at higher temperatures seems contradicsince line tension decreases with increasing temperature. Texplain the observed behavior we need to take into accthe effect of temperature on dipole–dipole repulsionshydrophobicity of the surfactant simultaneously. Detailsthe observed behavior have been described in our pous reports[32,33]. Briefly, the dipolar repulsion betweethe molecules decreases with increasing temperature sincrandom thermal motion of the molecules opposes thedency of their dipoles to line up in an electric field. Othe other hand, dehydration around the EO unit increathe hydrophobicity[34,35], and imparts to the moleculea longer-chain-like character. As a result, the cohesivedency between the molecules increases with increasing temperature. This results in an increase in the line tensiothe interface that probably overshadows the repulsiveteractions between the molecules showing circular domat higher temperatures. On the other hand, for the msystem, irregularly shaped small nuclei are observed aand 19◦C. Image (C) shows the representative domainsserved at 15◦C. It is important to note here that in the mixesystem the domains are observed up to 22◦C (image (D)).This suggests that repulsive interactions between the mcules substantially decrease to cause the condensatithe mixed monolayer where none of the pure systemscapable of showing any indicative feature of phase trsition. These changes in surface morphology suggespresence of both the surfactants in the adsorbed moners. It has been reported that a large value of line tenfavors large compact domains, which would be circulatwo-dimensional isotropic systems, while electrostaticpulsions favor irregular-shaped domains or elongated stures, when the domain area is large, to minimize the electrostatic repulsions[36–38]. Therefore, the irregular structureobserved for the mixed system in the temperature range

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22◦C is the outweighing influence of the repulsive intertion over the line tension.

3.2. cmc and γcmc

Spontaneous adsorption of surfactant molecules frombulk to the air–water interface brings about a sharp falthe γ values with increasing the bulk concentration (C) ofthe solution. At a definite concentration, theγ –logC plotshows a characteristic break point followed by a flat regwith almost constant values, known as the cmc of thetem.Fig. 4shows the representativeγ –logC plots of C14E2,TOOCNa, and their 1:1 mixture at 15◦C. It is seen thaboth the cmc and theγcmc values of the mixed system ain between those of the pure systems. This typical beior has been observed for some anionic–nonionic surfacsystems previously[4,19]. Fig. 5 shows the cmc values othe pure and mixed systems at different temperatures.cmc values of pure C14E2 remain almost constant withe studied temperature range. This behavior is attribto the compensation of the favorable dehydration effec

Fig. 4. Surface tension (γ ) versus logC plots of pure (I) C14E2,(II) TOOCNa, and (III) their 1:1 mixture at 15◦C.

Fig. 5. Effect of temperature on the equilibrium surface tension value�cmc of (I) C14E2, (II) TOOCNa, and (III) their 1:1 mixture.

146 Md.N. Islam, T. Kato / Journal of Colloid and Interface Science 282 (2005) 142–148

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the head group by the opposing thermal solubility effect[34]. On the other hand, the cmc values of TOOCNaits mixture with C14E2 decrease gradually with increastemperature. Usually the cmc–temperature data for ionicfactants fit on a U-shaped curve showing a minimum aroroom temperature[39–43]. In contrast to the usual behaviothe cmc values of TOOCNa are found to decrease gradwith increasing temperature. Since TOOCNa bears three Eunits in its head group, the dehydration effect increasespreciably the hydrophobicity of the molecules, which probly overcome the repulsive interactions between the chahead groups. Furthermore, hydration-induced repulsiveteractions between the molecules decrease with increasintemperature. As a result, micellization becomes favorableshowing a gradual decrease in cmc values with incring temperature. The decrease in the cmc values formixed system is the result of the outweighing influenceTOOCNa over C14E2 in micellization.

Fig. 6 shows the equilibrium surface tension values�cmc (γcmc) of pure C14E2, TOOCNa, and their 1:1 miture at different temperatures. It is clearly seen that theγcmcvalues of C14E2 do not change appreciably with changetemperature. On the other hand, theγcmc values of TOOCNaand its mixture with C14E2 are found to decrease withcreasing temperature suggesting a better molecular pain the monolayers. A decrease in theγcmc values with in-creasing temperature seems contradictory, since an incin temperature imparts a higher degree of thermal moand chain flexibility to the adsorbed molecules. In thisgard, two temperature-dependent factors need to be tinto account to explain the observed behavior of the pand mixed systems. These are (1) an increase in thehydration of the EO chain, which increases the hydropbicity and decreases the head-group size of the moleculewith increasing temperature, and (2) an increase in molecular motion, which results in a poorer packing of the adsormolecules and a consequent increase in the occupied mcular area with increasing temperature. These two facoppose each other in the course of temperature chang

Fig. 6. Effect of temperature on the cmc of pure (I) C14E2, (II) TOOCand (III) their 1:1 mixture.

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their relative magnitude determines the changes inγcmc val-ues with increasing temperature. Theγcmc values of C14E2are found to be almost constant within the studied tempture range. This is because the decrease in head-groucaused by the dehydration of the head group is offset byincrease in thermal motion of the adsorbed molecules wiincreasing temperature[34]. On the other hand, theγcmcvalues of TOOCNa and its mixture with C14E2 decreasepreciably with increasing temperature. This occurs becauof the dehydration around the EO chain, which apprebly reduces the head-group size, and outweighs the cbined effect of repulsive interactions between the charhead groups and the thermokinetic molecular motion. Csequently, adsorbed molecules gain closer packing inmonolayers of pure TOOCNa and the mixed system, shing decreases inγcmc values with increasing temperatuSuch behavior has been observed previously for ethoxynonionic surfactants containing a longer EO chain inhead group[34,35].

3.3. Phase transition and its dependence on πcmc and πc

We are particularly interested in the temperature depdency of surface-phase behavior of the monolayer atair–water interface. In the C14E2 monolayer, the condenphase domains are observed up to 19◦C, while in the mixedsystem, they are observed up to 22◦C. Lines I and III ofFig. 7 show that theπc values of both pure C14E2 anits mixture with TOOCNa increase linearly with increaing temperature. However, theπc values of the mixed system are found to be much lower than the correspondvalues of pure C14E2. Usuallyπc values decrease with increasing length of the hydrocarbon chain[16,44], while itis expected to increase with increasing the bulkiness ohead group. The decrease in theπc values for the mixedsystem is probably the influence of the longer hydrobon chain of TOOCNa molecules over the head group.the other hand, theπcmc values (deduced by subtractin

Fig. 7. Effect of temperature on critical surface pressure,πc, necessary fothe phase transition (I for C14E2 and III for mixture) and equilibrium sface pressure attainable at�cmc,πcmc (II for C14E2 and IV for mixture).

Md.N. Islam, T. Kato / Journal of Colloid and Interface Science 282 (2005) 142–148 147

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the γcmc values from the standardγ values of water) ofC14E2 show a slight decreasing trend (line II) while thofor the mixed system initially increase and then decreabit with increasing temperature (line IV). Theπc andπcmcvalues coincide at 19 and 22◦C for C14E2 and for C14E2TOOCNa mixed systems, respectively, which are the mmum temperatures for the observation of condensed-pdomains. Above this temperature, phase transition ispossible, as the hypotheticalπc values outweigh the correspondingπcmc values of the systems. Besides, decreain cmc values (for the mixed system) with increasing teperature suggest a higher degree of micellar stabilitythe bulk. Therefore, molecules prefer micellization in thebulk to adsorption at the air–water interface. Even thothe TOOCNa molecules bear a negative charge alongthree EO units in the head group, these EO units distathe charge from the hydrocarbon chain and thereby redtheir ionic character. Besides the presence of the threeunits in the head group provide considerable flexibilitythe head group. Because of this flexible nature, the termpart of the head group can escape the electrostatic repulbeing situated below the head group of C14E2 molecin the mixed system. Furthermore nonionic surfactantsing inserted into the monolayer of ionic surfactants sepathe charge and thereby reduce the electrostatic repuls[27,45]. Therefore, the observed behavior of the mixed stem is the result of the insertion of C14E2 molecules ithe TOOCNa monolayer. Collectively, all these factors caa decrease in the pressure necessary for the phasesition, and consequently condensed domains in the mmonolayer form at higher temperatures compared to thapure C14E2.

4. Conclusions

In the present study, we have presented the effect ofperature on the cmc,γcmc, and surface morphological features of the adsorbed films of C14E2, TOOCNa, andC14E2–TOOCNa mixed system. A cusp point followeda plateau region is observed in the adsorption isotheof both pure C14E2 and its mixture with TOOCNa. A vriety of condensed phase domains surrounded by themogeneous liquid expanded phase are visualized by Bjust after the appearance of the cusp point. Pure TOOcannot show any indicative feature of phase transitionrespective of the bulk concentration because of the strepulsive interactions and a higher degree of hydratiothe head group. On the other hand, for C14E2 and its mture with TOOCNa the condensed domains are obseup to 19 and 22◦C, respectively. The cmc values of boTOOCNa and its mixture with C14E2 decrease while thfor C14E2 remain almost constant within the studied teperature range. Even though increases in temperature ima higher degree of thermal motion and chain flexibilitythe molecules adsorbed at the water surface, theγcmc val-

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ues of TOOCNa and its mixture with C14E2 decrease wincreasing temperature. This is because the decrease in hegroup size caused by the dehydration of the head groumuch more pronounced than the combined effects ofrepulsive interactions between the head groups and themal motion of the molecules. As a result, the monolaattains closer packing, and phase transition in the msystem occurs at higher temperatures compared to thpure C14E2.

Acknowledgment

The authors appreciate very much the financial supfrom the Venture Business Laboratory of Utsunomiya Uversity.

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