Wetting/nonwetting behaviors in a ternary amphiphilic systemLiJen Chen, ShiYow Lin, and JiaWen Xyu Citation: The Journal of Chemical Physics 104, 225 (1996); doi: 10.1063/1.470892 View online: http://dx.doi.org/10.1063/1.470892 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/104/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Phase behaviors of supercooled water: Reconciling a critical point of amorphous ices with spinodal instability J. Chem. Phys. 105, 5099 (1996); 10.1063/1.472354 Measurement of dynamic/advancing/receding contact angle by videoenhanced sessile drop tensiometry Rev. Sci. Instrum. 67, 2852 (1996); 10.1063/1.1147117 Wetting and prewetting transition in metallic fluid K–KCl solutions studied by second harmonic generation J. Chem. Phys. 104, 8777 (1996); 10.1063/1.471567 Experimental study of incompressible Richtmyer–Meshkov instability Phys. Fluids 8, 405 (1996); 10.1063/1.868794 The breakdown of asymptotic hydrodynamic models of liquid spreading at increasing capillary number Phys. Fluids 7, 2631 (1995); 10.1063/1.868711

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Wetting/nonwetting behaviors in a ternary amphiphilic systemLi-Jen ChenDepartment of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China

Shi-Yow LinDepartment of Chemical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan 106,Republic of China

Jia-Wen XyuDepartment of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China

~Received 1 March 1995; accepted 25 September 1995!

We consider the three-liquid-phase,a, b, and g, coexisting system water1n-tetradecane1diethylene glycol monohexyl ether and the densities of these three phases are in the orderra,rb,rg . An enhanced video pendant drop tensiometer is applied to measure the interfacialtensions to further reconfirm that a wetting transition occurs by theg phase at theab interface uponapproaching its lower critical end point. The mechanism of reappearance of suspended drops at theab interface is carefully observed by the enhanced video microscopy system. It is found that thesesuspended drops are made of theg phase, and are formed by a very slow process of aggregation ofvery small drops. The contact angleu spanned by theag andbg interfaces of ag phase suspendeddrop at theab interface is verified to be a quasiequilibrium property. In addition, the interfacialtension resulting from the profile of a suspended drop below theab interface is consistent with thetension ofbg interfacesbg by the pendant drop digitization method. A variety of wetting/nonwettingbehaviors as well as wetting transitions is also discussed. It is found that there is ab phase wettinglayer separating the air and thea phase. ©1996 American Institute of Physics.@S0021-9606~96!50201-7#

I. INTRODUCTION

In 1977, Cahn1 and Ebner and Saam2 independently in-troduced the concept of wetting transitions, which predictsthat a three-phase system might undergo an interfacial phasetransition from nonwetting to wetting as the system ap-proaches its critical end point. Since then, this subject hasinduced a substantial amount of theoretical and experimentalworks.3

In 1980, Moldover and Cahn4 successfully used the bi-nary mixture methanol1cyclohexane to demonstrate thephenomenon of critical point wetting. Their work was car-ried out at ambient temperature and the system was drivenaway from its critical point by adding water into the system.The denser methanol-richa phase exhibits a wetting transi-tion from an intruding layer to a suspended drop, or viceversus, at the interface between the gasa and the less densecyclohexane-richg phase, as schematically shown in Figs.1~e! and 1~f!. Note that an intruding layer~or a drop! of thedenserg phase suspended between the other two less denseaandb phases is maintained by strong surface forces. Sincethen, there had been several other gas–liquid–liquid equilib-rium binary systems5,6 applied to demonstrate theg phasewetting transition at theab ~gas–liquid! interface. In allthese studies,4–6 a contact angleu spanned by theag andbginterfaces of ag phase suspended drop at theab interfacewas directly measured as an unambiguous evidence for awetting transition. Obviously, zero contact angle stands forwetting behavior, and nonzero contact angle stands for non-wetting behavior.

In our previous study,7,8 we have found the existence of

two wetting transitions at two different liquid–liquid inter-faces in the water1n-tetradecane1diethylene glycol mono-hexyl ether~C6E2! system by simply tuning the system tem-perature. It is well understood9 that within a certaintemperature range, such a ternary amphiphilic mixture mayseparate into three coexisting liquid phases, namely, an oil-rich a phase, an amphiphile-richb phase, and a water-richgphase. When the temperature is increased towards its uppercritical consolute temperature, the middleb phase exhibits awetting transition at theag interface. On the other hand,when the temperature is decreased towards its lower criticalconsolute temperature, the lowerg phase exhibits anotherwetting transition from suspended drops to an intruding layerat the ab interface. Figure 1 schematically illustrates theevolution of interfacial behavior of this particular systemwith increasing the temperature.

When the system temperature is lower than 30 °C, thereare some suspended drops at theab interface, as schemati-cally shown in Figs. 1~b! and 1~f!. Very gently shaking thetest tube~or the optical cell! under the water thermostat willmake those suspended drops detach theab interface, thenfall down through the bulkb phase, finally hit thebg inter-face, and immediately merge into the bulkg phase. The mer-gence of suspended drops and bulkg phase does not causeany disturbance of phase equilibrium, since no cloudy phe-nomenon near thebg interface is observed and all threephases are still transparent also with a sharp, mirrorlikebginterface. It is, therefore, believed that those suspended dropsare made ofg phase.

In order to further confirm that these suspended drops

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are made ofg phase, we use the pendant drop digitizationtechnique10 to examine the interfacial tension from the pro-file of a suspended drop below theab interface. Whether thisinterfacial tension is consistent with the tension ofbg inter-face,sbg , becomes a legitimate test to identify ingredients ofsuspended drops. Our tension results show very good agree-ment, which strongly implies that the suspended drops aremade ofg phase.

The most intriguing behavior is that after elimination ofsuspended drops by gently shaking the test tube, these sus-pended drops will reappear at theab interface by simplyplacing the test tube in the water thermostat still after severalhours. The mechanism of the reappearance of suspendeddrops at theab interface attracts our attentions. In this study,we use an enhanced video microscopy system to carefullyexplore the mechanism.

It is found that the reappearance of suspended drops isthe phenomenon of extremely slow equilibration process.When the system reaches equilibrium, there should be nosuspended drops at theab interface. This observation in-duces another question on the contact angleu measured di-rectly from ag phase suspended drop at theab interface.Since this contact angleu is not an equilibrium property, itwould be questionable of using the contact angle to identifywetting transitions,4–6 moreover to determine the order ofwetting transitions.5,6 Therefore, we carry out the contactangle measurements via the enhanced video microscopy sys-tem. It is found that the contact angleu has very good agree-ment with that calculated from interfacial tensions accordingto the force balance at three-phase contact points.10 Besides,the interfacial tension obtained from the lower part of a sus-pended drop is also found to be consistent withsbg , as wementioned above. Consequently, it is believed that the con-tact angleu resulting from a suspended drop is aquasiequi-librium property due to the slow equilibration process. Thecontact angle is thus a direct and unambiguous evidence toidentify wetting transitions.

The wetting and nonwetting behaviors in the systemwater1n-tetradecane1C6E2 are much richer than previouslyexpected. In this manuscript a variety of wetting/nonwettingbehaviors as well as wetting transitions in this particular sys-tem, such as there is ab phase wetting layer separating theair anda phases, are also discussed.

II. EXPERIMENTS

N-tetradecane of 991% purity is a product of TokyoKasei Chemical Co. The nonionic amphiphile diethylene gly-col monohexyl ether~C6E2! of 99% purity is purchased fromAldrich Chemical Co. These chemicals are used as receivedwithout any further purification, and water is purified by aBarnstead NAPOpure II System with a specific conductanceless than 0.057mV/cm.

The mixture is prepared in a flask and placed in a waterthermostat with the temperature stability60.1 °C for severaldays to allow the system to reach equilibrium. Before andduring the equilibration process, the samples are shaken vig-orously several times to ensure a thorough mixing. Afterequilibrium is reached, all three phases are transparent withsharp, mirrorlike interfaces. Following equilibration, all threephases are carefully transferred by using a syringe and putinto a quartz cell of 26341343 mm inside dimensions. Thisquartz cell along with a cover is then placed in an air ther-mostat for several hours, sometimes up to 3 days, to ensurethe equilibrium is reached after transferring the sample fromthe flask to the quartz cell. The temperature stability of theair chamber, which is a part of the pendant drop tensiometryshown in Fig. 2, is better than60.02 °C.

The interfacial behavior such as suspended drops is di-rectly observed through an enhanced video microscopy sys-tem of the pendant drop tensiometry. Figure 2 schematicallyshows the setup of the pendant drop tensiometry. The light ofa halogen lamp with a constant light intensity is collimatedby a set of planoconvex lens, then goes through the samplecell enclosed in a thermostatic air chamber, an objective lens~effective local length 60 mm, f/#7.1!, and finally forms animage of the sample onto a solid state CCD camera~MS-

FIG. 1. Evolution of qualitative interfacial behaviors of the systemwater1n-tetradecane1C6E2 with increasing the temperature. The expectedcondition for only a small amount of theb phase at thea–g interface isshown in the upper row, the condition for a substantial amount of thebphase shown in the lower row. The thickness of theg phase wetting layer in~a! and ~e! is exaggerated for illustration.

FIG. 2. Schematic setup of the pendant drop tensiometry and the videodigitization equipment; A, light source; B, pinhole; C, filter; CCD, videocamera; D, plano-convex lens; DA, D/A data translation image board; E,thermostatic air chamber, quartz cell, and suspending inverted needle; F,objective lens; FCG, frame code generator; FG, frame grabber; M, monitor;PC, personal computer; S, syringe; SP, syringe pump; SV, solenoid valve;VTR, video recorder.

226 Chen, Lin, and Xyu: Wetting in a ternary amphiphilic system

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4030, Sierra Scientific Co.!. All the images are recorded by avideo recorder~VO-9600, Sony! along with a frame codegenerator~FCG-700, Sony!.

An image of a suspended drop is then digitized into 4803512 pixels by an image digitizer processor~DT2861 Arith-metic Frame Grabber, Data Translation! installed in a per-sonal computer. For each pixel, eight bites are used todistinguish its grey level, that is, the grey level is rangingfrom 0 ~blackest! to 255~brightest! for each pixel. The imageis displayed by a monitor~Sony, PVM-1342Q! connected tothe personal computer. An edge detection routine is devisedto search for the profile of a suspended drop by interpolatingthe location where the gray level is equal to 127.5 along aspecific direction.11 The locations of pixels of a drop profileare stored as Cartesian coordinatesx andz, and then fitted tothe Young–Laplace equation,12

sS 1R11

1

R2D5DP,

whereR1 andR2 are the two principal radii of curvature ofthe interface,s is the interfacial tension, andDP is the pres-sure difference across the interface. According to the geom-etry of a pendant drop, the last equation can be recast as a setof three first-order differential equations interpreted by thespatial positionsx andz and turning anglef of the interfaceas a function of the arc lengths, as shown in Fig. 3. Thethree first-order differential equations are in the followingform:13

df

ds8521Bz82

sin f

x8,

dx8

ds85cosf,

dz8

ds85sin f,

where the dimensionless variablesx85x/R0 , z85z/R0 , ands85s/R0 , the capillary constantB5DrgR0

2/s, R0 is the ra-dius of curvature at the apex,Dr is the density differencebetween the fluid phases, andg is the acceleration due togravity.

Note that most of our pendant drops have no equator dueto the small capillary constant. It requires a new technique todetermine the capillary constant and the radius of curvature

at the apex. The details of the image data treatment of inter-facial tension calculations can consult Ref. 14. The densitymeasurements for each phase are performed by using avibrating-tube densiometer~Anton Paar DMA58!.

III. RESULTS AND DISCUSSION

A. Are the suspended drops at the ab interface madeof the g phase?

In our previous study,7 we gently shook the test tube toforce suspended drops detach theab interface and fall down.These drops disappear in the bulkg phase immediately whenthey hit thebg interface. It is believed that these suspendeddrops are made of theg phase because there is no distur-bance of phase equilibrium due to the mergence of drops andthe bulkg phase. In this study, a direct measurement of theinterfacial tension from the geometrical shape of suspendeddrops is used to further confirm that these suspended dropsare made of theg phase. If the interfacial tension resultingfrom the geometrical shape of suspended drops happens tobe the same as tensionsbg , this verifies that the ingredientsof suspended drops are made ofg phase.

Three different pendant drops are imaged and the imagesare digitized to calculate the interfacial tension.

~i! The suspended drops naturally appear at theab inter-face, as the photograph shown in Fig. 4.

~ii ! Artificially generate theg phase suspended drops attheab interface. A steel needle is used to suck a smallamount ofg phase from the bottom bulkg phase inthe quartz cell. This needle is then pulled up and thetip of the needle is carefully leveled slightly above theab interface, as the photograph shown in Fig. 5. Thesyringe pump then slowly injects a drop ofg phasefrom the needle. Once this drop detaches the needletip, it would be trapped by theab interface due to thetension to form a suspended drop at theab interface,as the photograph shown in Fig. 5.

~iii ! The tip of the needle is first positioned right in themiddle of the bulkg phase and sucks in a smallamount of theg phase. The drop forming needle isthen pulled up and positioned in themiddle of thebulk b phase. The liquidg phase inside the needle is

FIG. 3. Coordinates for the profile of a pendant drop.

FIG. 4. A naturally appeared suspended drop ofg phase at theab interface.

227Chen, Lin, and Xyu: Wetting in a ternary amphiphilic system

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pushed out by the syringe pump to form a pendantdrop ofg phase hanging on the needle tip in the bulkb phase. This is the typical method of measuring theinterfacial tension by using the pendant drop method.The interfacial tensionsbg resulting from images ofthis pendant drop is considered as the reference valueto compare to that of the other two methods.

All these three different suspended drops are imaged atthree different temperatures and these images are digitalizedto calculate the interfacial tensions. The results are given inTable I, where the errors are obtained by averaging over atleast seven runs. Note that the results of the naturally ap-peared and artificially generated drops have a very goodagreement. Within the experimental deviations, the tensionsresulting from three different pendant drops are consistentwith one another, that strongly indicates the naturally ap-peared suspended drops are formed by theg phase.

There are two interfaces for each naturally appeared~orartificially generated! suspended drop at theab interface; thebg interface for the lower part of a drop and theag interfacefor the upper part. The former one is considered as a pendantdrop, while the latter one is treated as a sessile drop. Theinterface of the lower part of a suspended drop at theabinterface is already confirmed to be thebg interface by thependant drop digitization technique. It is plausible to expectto obtain the interfacial tensionsag from the image of theupper part of a suspended drop.

It is found however that the interfacial tension resultingfrom the image of the upper part of a suspended drop issystematically smaller than the interfacial tensionsag at dif-ferent temperatures. This systematic deviation is subject tothe fact that the observable entire sessile drop profile is rela-

tively short, for example, see Fig. 4. One can see in Fig. 6that the theoretical profiles of a sessile drop at different cap-illary constants are almost coincided with one another, espe-cially the curves near the apex, over a very wide range. Con-sequently, it is difficult to obtain an accurate result ofsag

from such a limited drop profile. It is therefore inappropriatefor us to apply the pendant drop digitization technique todetermine the interfacial tensionsag from the profile of theag interface of a suspended drop.

B. The mechanism of the reappearance of suspendeddrops at the ab interface

First of all, it is nature for us to propose a possiblemechanism for the reappearance of suspended drops ofgphase at theab interface, that is, hydrodynamic flow of thegphase along a wetting layer on the vertical walls. In otherwords, theg phase wets the container walls to form a path-way for theg phase to climb up all the way to theab inter-face. If theg phase wets the container walls all the way up totheab interface, the meniscus of thebg interface should bebending upwards along the container walls. On the contrary,we found that the meniscus of thebg interface is bendingdownwards, as shown in Fig. 7. Obviously, there is no path-way for theg phase to have a hydrodynamic flow to create

FIG. 5. The generation ofg phase suspended drops at theab interface.

TABLE I. Interfacial tension measurements ofsbg ~mN/m! resulting fromthree different pendant drops at three different temperatures.

25 °C 17 °C 13 °C

Naturally appeared 0.2260.01 0.07160.003 0.02160.002Artificially generated 0.2260.01 0.06660.004 0.02260.002g phase drop in 0.2160.01 0.07260.001 0.020860.0004bulk b phase

FIG. 6. Theoretical profiles of a sessile drop calculated from Young–Laplace equation for different capillary constants withR051.

FIG. 7. The menisci of theab interface and thebg interface against thequartz container walls.

228 Chen, Lin, and Xyu: Wetting in a ternary amphiphilic system

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suspended drops at theab interface. Therefore, this observa-tion, Fig. 7, rules out the possible mechanism of a hydrody-namic flow.

Note that theab interface is bending upwards in Fig. 7.In fact, there is ab phase wetting layer separating thea andgas phases. We will discuss in detail the wetting behaviors oftheb phase in the next section.

Not only an apparatus for the interfacial tension mea-surements, the video-enhanced pendant drop tensiometry isalso a powerful tool for investigating wetting/nonwettingphenomena as well as the mechanism of the formation ofsuspended drops. According to our observation, there are lotsof small drops suspended at theab interface due to transfer-ring the sample from the flask into the optical cell. Althoughthe g phase is more dense than theb phase, the densityinversion is simply maintained by strong surface forces.Some of these suspended drops at theab interface wouldcoalesce to form a large drop, which, if large enough, wouldbend theab interface downwards a little bit due to the gravi-tational forces, as the photograph shows in Fig. 8. Then allthose small suspended drops around this large drop wouldslide down and coalesce with the large drop. This large dropkeeps growing up by coalescing with small drops until it istoo heavy to hang at theab interface. It then falls throughthe bulkb phase, hits thebg interface, and merges into thebulk g phase. Once the large drop disappears, the remainingsmall drops would coalesce to form another large drop.Again this large drop would grow and eventually detach theab interface to merge into the bulkg phase.

This process keeps going on and on until all the remain-ing small drops are separated far from one another and nomore large drop is formed. These small suspended drops thenslowly shrink and finally disappear due to the diffusion pro-cess. The size of all these suspended drops is usually smallerthan 0.5 mm in diameter. The lifetime of these suspendeddrops at theab interface varies from hours to days. It isbelieved that the lifetime of drops is directly related to somephysical properties, for example, temperature, density differ-ence between theb andg phases, and the height of theabinterface above the upper surface of bulkg phase. It is wellknown that the relaxation time increases substantially as asystem is close to its critical end point. As expected, the

lifetime of suspended drops gets longer when the tempera-ture is closer to its lower critical consolute temperature.

The time scale for equilibration in our system is muchlonger than that of the system water1n-dodecane1C6E2,which was reported15 to reach its equilibrium within minutes.It should be pointed out that the rate of equilibration processfor partially miscible mixtures has also been estimated byKayseret al.16 These authors found that it takes months for awetting layer to reach its equilibrium thickness in cells withdimensions on the order of 1 cm.

Here, consider a contact angleu spanned by theag andbg interfaces of ag phase suspended drop at theab inter-face. In the previous studies, the contact angleu isexperimentally4–6 used to identify a wetting transition of theg phase occurring at theab interface, moreover to determinethe order of the wetting transition.5,6 Since theg phase sus-pended drop only appears in the stage of equilibration, thecontact angle obviously is not considered an equilibriumproperty. The validity of the contact angle as an evidence forwetting transitions is questionable. On the other hand, thetime scale of equilibration for our system is at least morethan 3 days. The contact angle might be somehow very closeto an equilibrium property due to such a considerably slowequilibration. Here we use the interfacial tensions to calcu-late the equilibrium contact angleu, and also perform thecontact angle measurements directly from ag phase sus-pended drop at theab interface. The comparison betweenthese two results provides a direct information whether thecontact angleu of suspended drops is severely deviated froman equilibrium property.

The enhanced video microscopy system is applied to im-age ag phase suspended drop at theab interface at twodifferent temperatures 25 °C and 17 °C. For example, thephotograph of a suspended drop at 25 °C is shown in Fig. 4.The contact angleu is then evaluated by image analysis andfound 134.5° at 25 °C, while 99.7° at 17 °C.

The pendant drop digitization technique is applied to dointerfacial tension measurements. The advantage of thismethod is that the measurement is performed under the con-dition of three coexisting liquid phases,a, b, andg phases,that ensures the system is at equilibrium. The results of ten-sion measurements are given in Table II. According to theforce balance at interfaces, the contact angleu can be di-rectly calculated from the interfacial tensions via10

cosu5sab2 2sag

2 2sbg2

2sagsbg.

FIG. 8. Drops ofg phase hanging at theab interface.

TABLE II. Capillary lengths~mm! and interfacial tensions~mN/m! result-ing from the pendant drop digitization technique.

Temp.~°C! sab sbg sag sbg1sag aab

11 0.51660.007 0.002560.0001 0.51160.004 0.514 0.7213 0.50460.008 0.020860.0004 0.48160.001 0.502 0.7215 0.43160.001 0.043760.0002 0.426060.0004 0.47017 0.40060.009 0.07260.001 0.401060.0008 0.473 0.6625 0.28560.003 0.2160.01 0.37160.003 0.581 0.58

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The calculated results of the contact angleu from interfacialtensions are 130.2° at 25 °C and 95.9° at 17 °C, in very goodagreement with the results of direct contact angle measure-ments from the enhanced video microscopy system. Al-though the calculated contact angle from tensions is slightlysmaller than that of the direct measurement, the deviationbetween these two is within the experimental uncertainty.

Obviously, the contact angleu of a g phase suspendeddrop at theab interface is identical to its equilibrium valuecalculated from interfacial tensions. In addition, the interfa-cial tension obtained from the lower part of a suspended dropat theab interface happens to besbg , as we mentioned inthe last section. These evidences strongly indicate that theslow equilibration would not drive the system~the g phasesuspended drops! deviating from its equilibrium properties.Therefore, the contact angleu of theg phase suspended dropis considered aquasiequilibriumproperty. Consequently, thecontact angleu should be a valid evidence to verify wettingtransitions.

Besides theg phase suspended drops at theab interface,there are also small drops lying on thebg interface, as thephotograph shown in Fig. 9. This kind of hanging drops hasalready been observed previously in the other system water1ethanol1benzene1ammonium sulfate.17 Similarly, thesesmall drops can be removed by gently shaking the cell,which makes the small drops detach thebg interface, buoyup through the bulkb phase, hit theab interface, and disap-pear in the bulka phase. There is no sign of disturbance ofphase equilibrium due to the coalescence of these smalldrops into the bulka phase. All three phases are still trans-parent with sharp and mirrorlikeab interface. Consequently,it is believed that these small drops on thebg interface aremade ofa phase.

However, these drops lying on thebg interface are toosmall to apply the pendant drop digitization technique tocalculate the interfacial tension.18 The size of drops on thebg interface is relatively small due to the large density dif-ference. As one can see in Table III that the density differ-ence between thea andb phases,Drab , is larger than thatbetween theb andg phases,Drbg .

In addition, there are some small drops lying on the bot-tom of the container, as the photograph shown in Fig. 10,

similar to the previous observation in the system sodium do-decylsulphate11-butanol1water1NaCl1heptane.19 Thesesmall droplets are very firmly attached onto the quartz sur-face. It is impossible to remove these drops by gently shak-ing the cell, unlike theg ~or a! phase suspended drops at theab ~or bg! interface. These drops are always attached on thebottom of the container over the whole experimental period,up to a week. In order to identify the ingredients of thesesmall drops attached on the bottom of the container, a dye,brilliant blue, is added into the system. The middleb phaseimmediately changes the color from transparent to blue,while the other two phasesa andg remain transparent. In-terestingly, all those small drops attached on the bottom ofthe container also become blue. Consequently, it is believedthat these drops are made ofb phase.

C. Wetting and nonwetting behaviors in the systemwater1n -tetradecane 1C6E2

Consider the three-liquid-phase coexisting systemwater1n-tetradecane1C6E2 with only a small amount of themiddleb phase. At different temperatures, theb phase wouldform either a lens~nonwetting behavior! suspended at theaginterface or a thin film~wetting behavior! separating theaandg phases. Theb phase exhibits a wetting transition fromnonwetting to wetting at theag interface, when the tempera-ture approaches its upper critical consolute temperature.

On the other hand, when the temperature approaches itslower critical consolute temperature, theg phase exhibitsanother wetting transition from nonwetting to wetting at theab interface. It is of great interest that the change in wettingbehaviors is experimentally observed in a ternary am-phiphilic system. The evolution of wetting behaviors in the

FIG. 9. Drops ofa phase lying on thebg interface.

TABLE III. Density differences~g/cm3! in the three-liquid-phase region ofthe system water1n-tetradecane1C6E2.

Temp.~°C! Drab Drbg Drag

11.00 0.205 11 0.021 83 0.226 9413.00 0.199 93 0.028 32 0.228 2517.00 0.187 06 0.043 18 0.230 2425.00 0.173 55 0.059 24 0.232 79

FIG. 10. Drops ofb phase lying on the bottom of the container.

230 Chen, Lin, and Xyu: Wetting in a ternary amphiphilic system

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system water1n-tetradecane1C6E2 as a function of tem-perature is schematically illustrated in Fig. 1.

We also perform interfacial tension measurementsaround theg phase wetting transition temperature by thependant drop digitization technique. The results of tensionmeasurements at five different temperatures are given inTable II. Within the experimental error, the interfacial ten-sions obey the Antonow’s rule,20 sab5sbg1sag , forT511 °C and 13 °C, which implies theg phase forms anintruding layer separating thea andb phases. On the otherhand, the tensions satisfy the Neumann’s inequality,21

sab,sbg1sag , for T515 °C, 17 °C, and 25 °C. As a con-sequence, there is ag phase wetting transition occurring at atemperature in-between 13 °C and 15 °C. The occurrence ofa g phase wetting transition is consistent with our previousobservation. Although this wetting transition temperature issomehow slightly lower than our previous finding 16 °C re-sulting from the spinning drop tensiometer.7

In fact, the occurrence of two wetting transitions at twodifferent interfaces in a given system is theoretically pre-dicted by a lattice model of microemulsions22,23 under thecondition of the system substantially far from the tricriticalpoint. The temperature difference, which is an index of howfar a system deviated from a tricritical point, between theupper and lower critical consolute temperatures is around36 °C, relatively far from the tricritical point, for the systemwater1n-tetradecane1C6E2. While a system is sufficientlyclose to the tricritical point, this lattice model22 predicts thatthe middleb phase would exhibit a wetting transition asapproaching either critical end point. That is, ab phase wet-ting transition occurs at theag interface, instead of ag phasewetting transition at theab interface, as the temperature isdecreased towards the lower critical end point. This predic-tion has recently been observed in the systemwater1n-octane1C5E2,

24 whose temperature difference be-tween the upper and lower critical consolute temperatures isaround 16 °C, comparatively close to the tricritical point.

We also perform the tension measurements for the inter-faces between the air and each liquid phase by the pendantbubble digitization technique at four different temperatures.According to the results shown in Table IV, there is abphase wetting layer separating the air and thea phase, sincethe interfacial tensions obey the Antonow’s rule,sa air5sb air1sab , over the temperatures ranging from 11 to25 °C we measured.

The b phase intruding layer between the air andaphases is too thin to be observed by naked eyes, even withthe help of our enhanced video microscopy system. This dif-

ficulty can be resolved simply by adding a dye, brilliant blue,into the system. The brilliant blue dissolves in the middlebphase to change its color from transparent to blue, while theother two phasesa andg remain transparent. It is found thatthere is a thin blue layer separating the gas and thea phase.This observation directly indicates the existence of ab phasewetting layer separating the air and thea phase, in accordwith our interfacial tension measurements.

Finally, consider the wettability of theb phase on thequartz container. As one can see in Fig. 11, the meniscus ofthe air–liquid interface, more precisely theb–air interface,rises up to the wall dramatically. We also found that theoutside of the container is always wet due to the wettabilityof the b phase on the quartz surface. It is suspected thatwhetherb phase would form an intruding layer separatingthe bulka phase and the quartz container walls.

According to the hydrostatic picture, theab interfacerises up to the wall a distance equal to the capillary length,aab5(2sab/gDrab)

1/2, at which point theab interface istangent to the wall.10 The capillary lengthaab , listed inTable II, is around 0.7 mm for temperatures lower than17 °C, in accord with the thickness of meniscus~black band!of theab interface in Fig. 11. It is very likely, as one can seein Fig. 11, that there is a thinb phase adsorbed layer intrudesbetween the bulka phase and the quartz container walls dueto the effects of the long-ranged dispersion and van derWaals forces between the wall and the fluids. Currently weare in the process to verify this point in our laboratory.

ACKNOWLEDGMENT

This work was supported by the National Science Coun-cil of Taiwan, Republic of China under Grant No. NSC84-2214-E002-004.

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TABLE IV. Surface tensions~mN/m! resulting from the pendant drop digi-tization technique.

Temp.~°C! sa air sb air sg air sb air1sab

11 26.9260.03 26.660.1 27.1313 27.4260.05 26.660.1 27.0717 26.7060.05 26.160.1 27.060.2 26.525 26.1060.08 25.8060.05 26.0060.08 26.1

FIG. 11. The menisci of theab interface and thebg interface with only asmall amount of thea phase.

231Chen, Lin, and Xyu: Wetting in a ternary amphiphilic system

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232 Chen, Lin, and Xyu: Wetting in a ternary amphiphilic system

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