1910
J . Phys. Chem. 1994,98, 191&1917
Wetting Transitions at Liquid-Liquid Interfaces in Three-Component Water Surfactant Systems
+ Oil + Nonionic
L.-J. Chen,' W.4. Yan, M.-C. Hsu, and D.-L. Tyan Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China Received: August 10, 1993; In Final Form: December 7 , 1 9 9 P
There is a controversial problem concerning the wetting properties in the three-liquid-phase region of the system C6E2, where CiEj denotes the nonionic surfactant poly(oxyethy1ene) alcohol water + n-hexadecane CiH2i+l(OCHzCHz),OH. In this study, both interfacial tension measurements and direct contact angle measurementsareused to further reconfirm that themiddle phaseofthissystemdoesexhibit a wetting transition a t the interface separating the upper and the lower phase as temperature is increased towards the upper critical consolute temperature. The effect of chain length of oil on the wetting transition is also discussed in the ternary system water n-alkane C& by using three different oils: n-tetradecane, n-hexadecane, and n-octadecane. The wetting transition temperature increases as the chain length of the n-alkane increases. As temperature is decreased toward thelower critical consolute temperature, the lower phaseof the system water n-tetradecane CsE2 exhibits another wetting transition at the interface separating the upper and the middle phase. For all three systems-water n-tetradecane C6E2,water n-hexadecane C6E2, and water n-octadecane + C&-the wetting transition temperatures determined from interfacial tension measurements are consistent with those determined from direct contact angle measurements. In addition, experiments on the fish-shaped phase diagrams of these three systems are also performed to locate the critical end points of these systems.
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1. Introduction
Within a certain temperature range, a ternary system water +oil+ surfactantmayseparateintothreecoexistingliquidphases; Le., an excess oil a phase and an excess water y phase coexist with a middle @ phase containing most of the surfactant and appreciable amounts of oil and water. When the surfactant of these ternary systems has a long chain length, the middle @ phase always forms a lenticular loop floating at the ay interface or a ring clinging to the glass walls (nonwetting), as shown in Figure la,b. While in thesystems with a surfactant ofshort chain length, the middle @ phase always forms a thin film (complete wetting) separating the other two phases a and y, as shown in Figure IC. For medium-chain-lengthsurfactants, Robert and coworkers' found that the system water + n-hexadecane + C6E2,where C,E, stands for the nonionic surfactant CHI(CH~)~.~ (OCHZCHI),OH, exhibits a wetting transition' from a nonwetting @phaseat low temperatures to a wetting @ phase at high temperatures, or vice versa. The wetting transition temperature at which the wetting transition occurs in the system water + n-hexadecane + C6E2is found to be 49.4 "C.1 This finding, however, has been questioned by Kahlweit and Busse.' These authors found that the middle @ phase of water + n-hexadecane C6Ezsystem exhibits a nonwetting behavior up to 55 OC, and at higher temperatures it is difficult to observe whether wettingor nonwetting occurs (i.e., they were not able to observethewettingtransition). Obviously, this isindisagreement with the result of Robert and co-workers.1 However, Aratono and Kahlweit' have studied the effect of amphiphilicity on wetting behavior in ternary water n-octane CiEj mixtures. These authors also confirmed that the @ phase ofsystemswith the medium-chain-lengthsurfactant,suchasCsE2 and CsE,, does exhibit a wetting transition. In this study, experimental results on interfacial tensions are used to further reconfirm that the middle @ phase of water n-hexadecane C6E2systemdoesexhibita wetting transitionat the ay interface. In addition, we also examine here the effect of chain length of n-alkane on wetting behaviors in a ternary water n-alkane CsEl mixture, by using three different oils: n-tetradecane, n-hexadecane, and n-octadecane.
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'Abstract published in AdDnncc ACS Abmtroels. January 15. 1994.
0022-3654/94/2098-1910$04.50/0
(a)
(C)
(b) Figore 1. Schematic illustration of the geometric shapc of the middle @-phasein three coexisting phases: (a) a lens (nonwetting), (b) a ring (nonwetting),and (c) a thin layer (wetting).
In a system of three phases, a,0, and y at equilibrium, there are three interfacial tensions: u-,, umT,and up7, which are the tensions of the a@, ay, and By interfaces, respectively. The densities of these three phases are in the order p . < p o < pr. The interfacial behavior (wetting and nonwetting) is directly related to the interfacial tensions and can be interpreted by the wetting coefficient'.5 W, ~ a p a r
w B = T
where the subscript @ in W, stands for the wettability of the B phase at the ay interface. 0 1994 American Chemical Society
The Journal ofPhysica1 Chemistry, Vol. 98, No. 7. 1994 1911
Wetting Transitions at Liquid-Liquid Interfaces
Consequently, a wetting transition from a wetting to a nonwetting j3 phase corresponds to a transition of the wetting coefficient from W, = -1 to W, > -1. Alternatively, the @ phase exhibits a wetting transition when the relation between the interfacial tensions switches from Antonow’s rule to Neumann’s inequality, or vice versa. Therefore, the Occurrence of a wetting transition can he determined directly from interfacial tension environments. In this study, we present experimental results on the phase diagram, the interfacial tensions, and the direct contact angle measurementsvia an enhancedvideomicroscopy system for three systems: water + n-tetradecane C6E2,water + n-hexadecane + C6E2,and water + n-octadecane C6E2. The experimental procedures are described in the next section. The experimental resultsand further discussionaregiven in section 111. Thecritical consolute temperatures of these three systems are determined from the fish-shaped phase diagram. According to the results for the interfacial tensions and the contact angles, it is found that the wetting transition of the system water + n-hexadecane C 6 b , as well as of the other two systems, does indeed exist.
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Fiyre 2. Definition of wntact angle 0 spanned by the a8 and the 87 interfaces. Dashed lines stand for the tangent lines at the intersection of three interfaces.
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11. Experimental Section
Figure 3. Schematic illustration of a thin intruding layer of y phase separating the other two phases a and 0.
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Figure 4. Schematic setup of the image analysis equipment: (1) light source; (2) thermostat, (3) optical cell, (4) traveling microscope. ( 5 )
B/WCCDcamera.(6)monitor,(7)imagedigitizerproccssor,(S)personal wmputer.
According to the interfacial behavior of the middle @phase, the wetting coefficient can bc classified into three regions:) ( I ) W , 5 -1, i.e., uUv2 a d + up7, which implies that the ay interface is thermodynamically unstable. As a consequence, the 6 phase completely spreads across the interface between the two otherphasesaandy,asshownin Figure Ic,inordertominimize the total system energy; i.e., the middle 6 phase wets the ay interface. The contact angle 8 spanned by the a@ and the By interfaces, defined hy Figure 2, vanishes. When W, = -1, the three interfacial tensions satisfy Antonow’s rule:6 auv= am,+ CEV
(2) -1 < W, < I , Le., the three interfacial tensions are related by Neumann’s inequality:’ ae8- qV< am < a#, a,? Under this condition, the middle @ phase only partially wets the ay interface and forms either a lenticular droplet suspended at the ay interface or a ring clinging to the glass walls, as shown in Figure la,b; i.e., and @ phase exhibits a nonwetting behavior. Consequently, there exists a nonzero contact angle 8 for this lenticular droplet (or ring). (3) 1 5 W,, Le., c d 2 uw, which implies that the a@ interface is thermodynamicallyunstable. Under thiscondition, thesurface forces overwhelm the earth’s gravitational forces and a small amount of y phase of greatest density forms a thin intruding layer separating the two other phases a and 8, as shown in Figure 3, in order to minimize the system energy.
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Materials. Of the hydrocarbons we used, n-tetradecane (99%) isaproduct ofTokyoKaseiChemicalCo.,n-hexadecane(99+%) is a product of Merck Chemical Co., and n-octadecane (99%) is a product of Sigma Chemical Co. The nonionic amphiphile diethylene glycol monohexyl ether (C6E2) of 99+% purity is purchased from Aldrich Chemical Co. All these chemicals are used as received without any further purification, and water is purified by a Barnstead NAPOpure I1 System. Procedure. Phase Diagram. The samples are prepared in a 1-cm-diameter glass test tube at a fixed water:oil weight ratio (l:l), with varying surfactant concentration. The samples are then placed in a water bath, whose temperature stability is better than *4 mK, for several hours, sometimes up to several days, to allow the system to reach equilibrium. To ensure a thorough mixing, the samples are shaken vigorously several times before andduring theequilibriumproccss. After equilibriumis reached,
thenumberofliquidphascsforeachsampleisrecordedatdifferent temperatures. The phase boundary is systematically searched for each surfactant concentration, by locating the temperature at which the number of liquid phases changes. Wetting Transitions. ( 1 ) Contact Angle Measurement: Enhanced Video Microscopy. The samples are placed in a water bath which is set at a temperature lying within the three-liquidphase region. The equilibration procedure described above is followed. After equilibrium is reached, all three liquid phases are transparent with sharp, mirror-like interfaces. Following equilibration, both the upper and the lower phases are carefully removed from the test tube by using a syringe and are transferred into an optical cell (1 X IO X 50 mm inside dimensions). Next, a very small drop of the middle 6 phase is added to the optical cell containing only the upper and the lower phases. The middle @ phase, if in the nonwetting regime, can be observed to form a ring clinging to the glass walls or to form a lens floating at the ay interface. This optical cell is also placed back into the thermostat for several hours, to ensure equilibrium is reached after transferring the samole from the test tube to the optical cell. Theinterfacial behavior,suchas that manifested by asuspended droplet, isdirectlyohservedthroughan enhancedvideomicroscopy system. Figure 4 schematically illustrates the setup of the enhanced videomicroscopy system. An image is directly taken from a B/W CCD camera (Sony, XC-77) through a traveling microscope and then digitized into 480 X 512 pixels by an image digitizer processor (Data Translation, DT2861) installed in a personal computer. The image is displayed by a monitor (Sony, PVM-1342Q) connectedtothepersonalcomputer.andinterfacia1 behavior, such as wetting and nonwetting behavior, can thus he directly observed.
1912 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
Chen et al.
TU,
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Solid
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20 '
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Temperature ("C)
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Figure 5. Fish-shapedphase diagram at constant watermalkane weight ratio (1:l) as a function of temperature for the systems (a) water n-tetradecane + c&, (b) water + n-hexadecane + C&, and (c) water + n-octadecane + C&.
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TABLE 1: Up r and Lower Critical Consolute Temperatures o/%e Water mAlkane + C& Systems upper critical temp Tu("C) this Robert and lower critical system work co-workers' temp TI("C) water + n-tetradecane + C6E2 46.23 46.3 9.86 water + n-hexadecane + CsE2 60.34 61.0 a water + n-octadecane + C&2 75.73 75.0 a a Lower critical consolute temperature does not exist.
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To search for the interfaces separating two different phases, an edge detection routine is coded by identifying the abrupt change of any gray levels of two nearest-neighbor pixels. The locations of pixels of interfaces are stored as Cartesian coordinates and then fitted by a polynomial equation for each interface. The tangent line of each interface right at the intersection point of
'30
35
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4 Tu, 45
50
55
60
E5
Temperature ("C)
Figure 6. Variation of the 8-phase contact angle B as a function of temperatureresultingfrom imageanalysis (circle) and interfacial tension calculation (star) for the systems (a) water + n-tetradecane + C&, (b) water + n-hexadecane + C&, and (c) water + n-octadecane + c&. three interfaces can be directly calculated from the polynomial equation. Finally, the contact angle B can be easily evaluated from the angle spanned by the two tangent lines, as shown in Figure 2. Consequently, a wetting transition from a nonwetting to a wetting /3 phase occurring at the cry interface can be found by directly observing the vanishing of the contact angle 6. ( 2 ) Interfacial Tension Measurement. In order to further confirm our observations via the enhanced videomicroscopy system, a spinning-drop tensiometer (Kruss SITE 04) is used to measure the interfacial tensions, which are used to locate a wetting transition when the relationship of three interfacial tensions switches between Antonow's rule and " n a M ' S inequality. Density measurements needed in interfacial tension calculations are performed by using a vibrating-tube densiometer (Paar DAM 512 & DAM 60).
Wetting Transitions at Liquid-Liquid Interfaces
(b)27"C
The Journal ofPhysica1 Chemistry, Vol. 98, No. 7, 1994 1913
(d)44.1nC
Figure 1. Photographs of the geometric shape of the middle @-phasefor the system water + n-tetradecane (a) 16.0, (b) 27.0, (c) 44.0, and (d) 44.1 ' C .
111. Results and Discussion
The experimental results on the fish-shaped phase diagrams of the three systems water n-tetradecane + C6E2,water + n-hexadecane C6E2, and water + n-octadecane + C6E2are shown in Figure 5 , where the symbols 36, 26, and 1 6 stand for the three-, two-, and one-liquid-phase regions, respectively. The three-liquid-phase region is slightly enlarged as the chain length of n-alkane increases. The highest and the lowest temperatures of the three-liquid-phase region correspond to the upper and the lower critical consolute temperatures, respectively. Due to the freezing of the upper oil-rich phase a t low temperatures, the systems water n-hexadecane C6&and water + n-octadecane + C6E2do not have a complete fish-shaped diagram. Therefore, there exists no lower critical consolute temperature in these two systems. Table 1lists the upper and the lower critical consolute temperatures of these three systems, in accord with the results of Robert and co-workers.' In this study, we restrict the temperature within the three coexisting liquid phase region of the system H2O + n-alkane C6E2. According to the Gibbs phase rule, there are only two degrees of freedom for a three-coexisting-liquid-phaseternary system. If one more system parameter, for example, the system pressure, is fixed, there is only one degree of freedom left. In this study, all our experiments are performed in the three-liquid-phase coexisting region of the ternary system H 2 0+ n-alkane + C6E2 under atmospheric pressure. As a conesequence,properties such as contact angles, densities, and interfacial tensions, uniquely depend on temperature. We therefore simply adjust system temperature to search for the wetting transition at liquid-liquid interfaces in thethree-coexisting-liquid-phase region of thesystem H 2 0 + n-alkane + C6E2.
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+ CsE2 at four different temperatures:
The enhanced videomicroscopy system is used to measure the contact angle 0 in this three-coexisting-liquid-phase system with only a small amount of the middle 0 phase, a t different temperatures. Figure 6 shows the results for the contact angle 0 as a function of temperature for the three systems HzO n-tetradecane + C6E2.H 2 0 + n-hexadecane C6E2,and HzO + n-octadecane + C6Ez. The error bars in Figure 6 are obtained by averaging over several measurements. The contact angle 0 decreases monotonically as temperature increases, except in the low-temperature region of the system HzO + n-tetradecane C6E2. As mentioned above, a wetting transitionoccurs whenthe contact angle Ovanishesas temperature increases. The wetting transition temperatures of the mixtures H20+ n-tetradecane + C6E2,H 2 0+ n-hexadecane C6E2,and H20+ n-octadecane + C6E2are found from Figure 6 to be, respectively, 44.05.54.45. and 64.55 "C. As the chain length of the n-alkane decreases, the temperature difference between the wetting transition temperature and the upper critical consolute temperature decreases monotonically. One can expect the wetting transition temperature of the system HzO+ n-tridecane + C6E2, if it exists, to he very close to its upper critical consolute temperature. As one can see in Figure 6, the contact angle 0 decreases as temperature increases, and it suddenly drops to zero right a t the wetting transtion temperature. When an n-alkaneoflongerchain lengthisusedinthistemarysystem,thesuddendropping behavior of the contact angle 0 becomes less pronounced. Within the accuracy of our experiments, the wetting transition is found to be the first order, in accord with other previous studies.s.9 It should be pointed out that there exists experimental evidence10
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Chen et al.
1914 The Journal of Physical Chemistry, Vol. 98, No. 7,1994
F] (C)54.4"C
El
(d)54.5OC
(b)45'C
Figure 8. Photographs of the geometric shape of the middle @-phasefor the system water (a) 18.0, (b) 45.0, (c) 54.4, and (d) 54.5 "C.
for the possibility of a second-order wetting transition, which is, however, still controversial. Note that the inside thickness of the optical cell we use is only 1 mm. Onemaysuspect thatthecontactangle0wouldbedistorted by the wall effects. For example, Aratono and Kahlweit' found that in the system H20 n-Octane CsE4 the contact angle 0 of a lens of the I3 phase floating in the center of a quartz cuvette of IO-" inside thickness is somewhat smaller than that of a ring-shaped middle phase in a quartz cuvette of I-mm inside thickness. To verify whether the wall effects distort the contact angle 0, six cells of different inside thickness ranging from 1 to IO mm are used, and it is found that at 30 "C the contact angle has no significant deviations between these cells of different thickness. Figures7, %and9 showthephotographs takenatfourdifferent temperatures, includingtemperatures both higher and lower than the wetting transition temperatures of three systems H 2 0 + n-tetradecane C6Ea H 2 0 + n-hexadecane + C6E2,and H20 + n-octadecane+ C6E2,respectively. Ohviously, when thesystem temperature is higher than the wetting transition temperature, the 0 phase forms a thin layer separating the other two phases 01 and y. as shown in, e.g., Figure 8d. For temperatures lower than the wetting transition temperature, the fact that the contact angle 0 increasesas decreasingtemperaturecan beeasily observed in, e.g., Figure 8a+, Besides these contact angle measurements, the existence of a wetting transition is further confirmed by performing measurements on interfacial tensions. The experimental results for interfacial tensions as a function of temperature are shown in Figure IO. As we mentioned above, the Occurrence of a wetting transition can also be verified by the wetting coefficient W,
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+ n-hexadecane + GE2 at four different temperatures:
switching from W, = -1 to W, > -1. The wetting ccefficient as a function of temperature for three ternary systems is shown in Figure 11. One can see in Figure 11 that the wetting coefficient decreases as temperature increases,and eventually it drops down to -1 at a particular temperature: the wetting transition temperature. According to the experimental results for wetting coefficients (or interfacial tensions), the wetting transition temperatures for three systems H 2 0 n-tetradecane + C6E2, H 2 0 n-hexadecane C6E2,and H 2 0+ n-octadecane + C6E2 are found to he 44.04.54.80, and 64.53 OC, respectively,in accord with our results of direct contact angle measurements. The contact angle 9 can also he directly calculated from the interfacial tensions via
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cos 0 =
aq
2
-0.0
2
-a,?
2
2"B% according to the force balance at interfaces.' The calculated results of the contact angles 0 from the interfacial tensions are consistent with those of the direct contact angle measurements from the enhanced video microscopy system, as shown in Figure 6. Note that the results calculated from the interfacial tensions exhibit more scattering and have larger experimental deviations. Since the ultralow interfacial tensions ampand upr appear in the denominator of the above equation used to evaluate contact angle 8, a small experimental deviation on interfacial tension measurementswould indeedcause such largedeviationsin thecontact angle calculation. For the system H20 n-hexadecane + C6E2, we do observe the wetting transition by both direct contact angle measurements and interfacial tension measurements. Table 2 lists all stud-
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The Journal of Physical Chemistry, Vol. 98. No. 7, 1994 1915
Wetting Transitions a t Liquid-Liquid Interfaces
(d)64.6*C
(b)5S°C
Figure 9. Photographsof the geometric shape of the middle &phase for thc system water 45.0, (b) 55.0, (c) 64.5, and (d) 64.6 ' C .
TABLE 2 Comparison of the Results for the Wetting Transition Temperature in the System Water + n-Hexrdecane + C&* Obtained from This and Other Studies source
method
Robert and coworkers' Kahlweit and Bussel Smith and Covatch'l Abillon et a1,I2 this work
direct eye observationand enhanced video microscopy direct eye observation direct eye observation interfacial tension measurement interfacial tension measurement enhanced video microscopy
a
T.. P C ) 49.4
no transition 45.14 5 3 9 54.80 54.45
Estimated directly from Figure 4 in ref 12.
ies,'.]."." to the best of our knowledge, with their results on the wetting transition in this particular system. It is believed that the discrepancies between these results are due to experimental uncertainties, such as the presence of different impurities in the system. Note that our results for interfacial tensionsareconsistent with those of Abillon et al.," which were obtained by using the light-scattering method, which is different from the one we use (spinning drop tensiometer). Figure 12 shows a comparison of the results for the contact angle between our work and other studiesl.12 for the system water + n-hexadecane + C6E2. Note that our wntact angle measurements agree well with those of Abillon et aLi2calculated from interfacial tension data. The results of Robert and co-workers, however, arewnsistently lower than ours. It is believed that this discrepancy is due to the different techniques of wntact angle measurements. Robert and co-workers measured the wntact angle directly from a photograph, which might cause some systematical deviations. It should be pointed out that thecurvature
+ n-oetadecane+ CsEz at four differenttemperatures: (a)
near the three-phase wntact point changes dramatically. This is bard to ohserve directly from a photograph but much easier to detect from the image analysis technique we use here. According to the critical point wetting theory of Cahnll and of EbnerandSaam," when a three-phasesystem is brought close to a critical end point, a transition from a nonzero wntact angle (nonwetting) to a zero wntact angle (wetting) must occur'. The above results show that the wetting transition occurs as temperature is increased toward the upper critical consolute temperature of the system. It is natural to ask whether another wetting transition occurs as temperature is decreased toward the lower critical consolute temperature of the system. However, it is found that in the system H20 n-tetradecane +CsEzthemiddle@phasealwaysformsanonwettinglensfloating a t the oly interface as temperature decreases all the way down to the lower critical consolute temperature. This observation conflicts with that of Robert and Shukla,l who found that the system exhibits another @-phasewetting transition close to its lower critical temperature. It is believed that the"wetting" layer of the @-phaseobserved by Robert and Shukla is induced by adding a superfluous amount of the @ phase into the system. In our previous paper," we have already reported that there does exist another wetting transition in the system HzO + n-tetradecane C6E2 as temperature decreases. The most intriguing behavior is that this transition, identified as a y-phase wetting transition, is different from the@-phasewetting transition described in Figure 1. The y-phase wetting transition is a transition between suspending beads of the y phase at the a@ interface and an intruding y layer separating them and @ phases, as schematically illustrated in Figure 13. It should he pointed
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Chen et al.
1916 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
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&
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* 0.8
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o.03
Temperature ("C) Figure 10. Variation of interfacial tension as a function of temperature for the systems (a) water + n-tetradecane + C&, (b) water + n-hexadecane + C&, and (c) water + n-octadecane + C&.
Figure 11. Variation of wetting coefficient as a function of temperature for the systems (a) water + n-tetradecane + CsE2, (b) water + n-hexadecane + C&, and (c) water + n-wtadecane + C&.
out that this is the first system, to our knowledge, observed to exhibit both a j3-phase and a y-phase wetting transition while approaching its upper and lower critical end points, respectively. According to our direct contact angle meas~rements,'~ we have found that the y-phase wetting transition is also a first-order transition, which is consistent with previous experimental results,8s9 as well as with the theoretical prediction of the Ginzburg-Landau m0de116.1~for water + oil surfactant mixtures. Note that a hysteresis of the thickness of a wetting layer was found18 in the cyclohexane methanol system, consistent with the assumption of a first-order transition. It is plausible to expect the occurrence of a y-phase wetting transition as temperature decreases in the other two systems HzO n-hexadecane C6E2 and H2O n-octadecane + C6E2, since the system H20 n-tetradecane C& exhibits such a transition as temperature decreases. In the system H2O + n-hexadecane
C6E2, we do observe the suspending beads of the y phase at the a@ interface, as seen in the photographs of Figure 14. As temperature decreases, however, the freezing of the oil-rich a phase occurs before these suspending beads collapse into an intruding layer. For the system H2O n-octadecane C6E2, the suspending beads of y phase on the aj3 interface are not even observed. Therefore, no y-phase wetting transition is observed as temperature decreases in both systems H2O n-hexadecane + C6E2 and H2O + n-octadecane + C&2. Alternatively, a y-phase wetting transition can be interpreted by the language of interfacial tensions. When a y-phase wetting transition occurs, the relationship between the interfacial tensions exhibits a transition between uap < bay + Up7 and Ua@ = uay u # ~Le., , a transition of the wetting coefficient between W,= 1 and Wp< 1. Figure 1 l a shows that the y-phase wetting transition temperature for the system H2O n-tetradecane C6E2 is 16.80
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The Journal of Physical Chemistry. Vol. 98, No. 7. 1994
Wetting Transitions at Liquid-Liquid Interfaces
I
H20+n-C16+CaE1
A i , ,
~ b ~ ' " " ' ' " " 30' ' ' ' " ' 3'11' ' " ' ' ' ' 21
42
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5k.
T e m p e r a t u r e ("C) Figure 12. Comparison of wntact angle B for the system water n-hexadecanc + C ~ E between I several studies.
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a
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a
(a)
1917
1
BO
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I
@)
Figwe 13. Schematic illustration of a y-phase wetting transition from (a) an intruding y layer (wetting) to (b) suspending beads of they phase (nonwetting). or vice versa.
TABLE 3 Wetting Transition Temperatures of Three Water *Alkane C&, Systems Determined from Both Interfacial Tension Measurement and Direct Contact Angle
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Measurement
system water + n-tetradcane + C ~ E I water + n-hexadecane + C6Ez water + n-madecane + CsEz
wetting transition temr, T, ('0 interfacial contact angle measurement measurement 44.04 54.80 64.53
54.45 64.55
In summary, our experimental results for interfacial tensions
and contact angles have reconfirmed that the system water + n-hexadecane C6Ezdoes exhibit a @-phasewetting transition as temperatureincreases toward thatoftheupper criticalconsolute point. Theexperimental resultsfor the@-phasewetting transition temperatures for three mixtures water n-tetradecane + CsE2, water n-hexadecane C6Ez, and water + n-octadecane C6E2are summarized in Table 3. T h e most intriguing behavior is that as temperaturedecreases toward that of thelower critical consolute point thesystem water + n-tetradecane+ CsEzexhibits a y-phase, instead of a &phase, wetting transition.
+
+
+
for the system water + n-hexadecane + C6El taken from the enhanced vidwmicrmwpy systemat twodifferenttemperatures: (a) I8.0and (b) 22.0 OC.
References and Notes ( I ) Robert, M.;Jcng. J. F. J. Phys. Frmce 1988,49,1821. Chcn. L.4.; Jeng. 1.-F.; Robert. M.; Shulda. K. P. Phys. Reo. A 1990,42,4716. Robert,
44.05
OC, where the wetting coeftfcient reaches 1.0 from below as temperature decreases.
+
(b)22'C
Figure 14. Photographs of suspended y-phase beads at the up interface
+
Acknowledgment. One of us (L.-J.C.) is grateful to Prof. M. Robert for his comments on the manuscript. W e a r e indebted to Prof. Shi-Yow Lin for his technical support on the image analysis of contact anglemeasurements. This work was supported by the National Science Council of Taiwan, Republic of China under the grant numbers NCS80-0402-E002-13, NCS81-0402EOO2-15, and NCS82-0402-E002-215.
M.; Shulda, K. P. Fluid Phose Equilibria 1992 79, 241. (2) For a review, scc: Sullivan, D. E.; Teio da Gama, M. M. In Fluid brer/oeiol Phennommo; Croxton, C . A,. Ed.; Wilcy: New York, 1985. p 45. (3) Kahlweit. M.; Busse, 0. J. Chem. Phys. 1989.91, 1339. (4) Aratono. M.; Kahlweit, M. 1.Chem. Phys. 1991,95,8578;199297. 5027
( 5 ) Rowlinson, J. S.; Widom, E. Molecular Theory of Copilloriry; Clarcndon: Oxford. 1982. (6) (a) Antonow, G. N. J. Chim. Phys. 1907,5,372; Kolloid-Zeit 1932
59,7; 1933.64.336. (b) Adam, N. K.The PhysicsondChemisfryofSwfmes, 3rd 4.;Oxford University Press: Oxford, 1941; pp 7, 214.215. (7) (a) Neumann, F. Vorlesungen ubcr die Thcorie der Copillarifa; Wangerin, A,, Ed.; Teubncr: Lcipzig, 1894: Chapter6,Scct. I, cspsiallypp. 161,162. (b)Buff,F.P.;Saltsburg.H.J.Chem.Phyr.1957.26.23.(e)Buff, F. P. Encyclopedia ofphysies; Flugge. S.. Ed.; Springer: Berlin, 1960: Vol. IO, Sect. 7, pp 298, 299. (8) Schmidt, 1. W.; Moldover, M. R.J. Chem. Phys. 1983, 79, 379. (9) Dietrich, S. Wetting Phenomena. Inphase l"mnSi1ionrandCri~icol Phenomena; Domb, C., Lcbowitz, J. L., Eds.; Vol. 12; Academic Press: New York, 1987. (IO) Trejo, L. M.; Gracia, 1.; Vsrca, C.; Robledo, A. Europhys LM. 1988, 7.537. (11) Smith, D. H.; Covatch, 0. L. J. Chrm. Phys. 1990,93,6870. (12) Abilion,O.;Lec,L.T.;Langcvin,D.;Wong,K.Physie~A1991,172, 209. (13) Cahn, J. W. 1. Chcm. Phyr. 1977.66.3667, 1977.38, 1486. (14) Ebner. C.; Saam, W. F. Phys. RN. hff. (15) Chen, L.-J.;Yan, W.-J. J . Chcm. Phys. 1993, 98.4830. (16) Gomppcr. G.; Schick. M. Phys. RN. LLrf. 1990,65, 1116. (17) Shulda. K. P.; Robert, M. J. Sfor. Phys. 1991, 335. 1053. (18) Bonn, D.; Kcllay. H.; Wcgdam, G. H. Phys. Reo. Lcff. 1992, 69. 1975.
