Langmuir 1994,10, 3730-3737
3730
Adsorption Behavior at the Interface between a Solid and a Preferentially Wetting Bulk Phase Close to a Liquid-Liquid Critical Point. Water-2,5-Lutidine against Silica M. Privat,* M. Amara, V. Bassiloua, R. Bennes, and E. Tronel-Peyroz URA 330, CNRS, Route de Mende, BP 5051, 34033 Montpellier Cedex, France Received February 2, 1994. In Final Form: June 6,1994@ Relative adsorption isotherms of 2,5-lutidinefrom the preferentiallywetting water-rich phaseC3, to silica have been measured at numerous temperatures between T, (critical temperature of the liquid mixture) and T , (wetting transition temperature in the diphasic liquid) and integrated versus chemical potential to obtain surface tensions between solid and liquid, A&. Extrapolationofboth types of curvesto the phase separation point makes it possible to compare data along the coexistence curve with a theoretical model. The divergence of the adsorption data at T,, but not at T,, is consistent with wetting behavior and is partially what could be expected from theory: the experimental adsorption enhancement can be identified by the criticalbehavior of the surface of a preferentiallywettingbulk phase. However, the sign of adsorption is different from the sign found from most of the theoretical sample models. Surface behavior in the adsorption and surface tension is thus partially proved, experimentally,to depend on wetting properties of the studied liquid phase in the diphasic and probably on the nature of the wetting transition.
Introduction Since the first theoretical work concerning wetting transitions by Cahn,l numerous theoretical papers have been published, and several reviews of the subject have a ~ p e a r e d . ~The - ~ idea is the following. In a diphasic system such as a liquid two-phase system, whose points are inside the coexistence curve far from the critical point, there can be incomplete wetting by both phases, p and y , against a third one a, which can be a vapor or a solid, as shown by Figure 1. When the temperature approaches the critical temperature suddenly at T = T,, one of the two phases, say/?, completely wets a, and wetting remains complete until T,, the lower critical temperature of the system, is reached. T, is called the wetting temperature: it is the temperature ofthe wetting transition. “he wetting film ofg existing in the two-phase system diphasic can be preformed as a “thick adsorption layer” in the single phase whose composition is close to the composition of the y phase, 7. This disappears when the composition of jj is far from that of y itself. Then, the adsorption becomes “normal”. Relatively few experiments have been done in relation to these theories which can deal with the solid-gas, solidliquid, or liquid-gas interfaces. In the case of the solidliquid interfaces, considered in this paper, a wetting transition has been shown by Moldover and Cahn,5 by Pohl and Goldburg,6aand by ourselves7 (our results are summarized in the Results and Discussion); complete
* To whom correspondence should be addressed at TJRA CNRS 322,Facult6 des Sciences de Brest, 6 avenue Le Gorgeu, 29275 Brest Cedex, France. Abstract published in Advance ACS Abstracts, September 1, 1994. (1)Cahn, J. W. J . Chem. Phys. 1977,66,3667. (2)Rowlinson, J. S.;Widom, B. Molecular Theoy of Capillarity; Clarendon Press: Oxford, 1982. (3)Sullivan, D. E.; Telo da Gama, M. M. In Fluid Interfacial Phenomena; Croxton, C. A., Ed.; Wiley: New York, 1986;p 45. (4) (a) Dietrich, S. In Phase Transitions and Critical Phenomena; Domb, C . , Lebowitz, J. L., Ed.; Academic Press: London, 1988;Vol. 12, p 1. (b) Ebner, C., Saam, W. F. Phys. Reu. Lett. 1987,58, 587. (c) Indekeu, J. 0. Phys. Rev. B 1987,36,7296. (5) Moldover, M. R.; Cahn, J. W. Science 1980,207,1073. (6)(a) Pohl, D. M.; Goldburg, W. I. Phys. Rev. Lett. 1982,48,1111. (b) Kreuser, H.; Woermann, D. J . Chem. Phys. 1992,97,7757. (7)(a)Amara, M.; Privat, M.; Bennes, R.; Tronel-Peyroz, E. J . Chem. Phys. 1993,98,5028. (b) Tenebre, L.Unpublished results. @
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wetting close to T,has also been checked by Kreuser and Woermann.6b Correlative studies of adsorption at such interfaces have been essentially made by two teams: Beysens’ and ours. Beysens and co-workers worked on a water-2,6-lutidine systematg adsorbed on silica microspheres, and their method was turbidimetry: they showed a thick layer of “2,6-lutidine” occurs on the surface when the phase separation is approached in the “water-rich” domain of the phase diagram. The work reported here and our earlier measurements were made in a water-rich solution of 2,6-lutidine1°J1or 2,5-lutidine12using ellipsometrylo and direct adsorption determination11J2 on silica surfaces (plane surfaces for ellipsometry, microspheres surface for adsorption measurements). Both kinds of results show that adsorption becomes stronger when the phase separation is approached and a sort of surface transition appears (break in logarithmic isotherms, aggregation of microspheres), which curiously ends a t T = T,, the wetting transition temperature (see, for instance, Figure lb). These results about adsorption and about wetting apparently are contradictory. The wetting behavior of the two-phase system, for both water-2,6-lutidine6 and ~ater-2,5-lutidine,~“ is that the water-rich phase (called ,f?) preferentially wets the silica wall, a. Additional verifications of refs 6 and 7a by a sophisticated optical method have been performed but not yet published:7bclose to T,, the water-rich phase is always the wetting phase, and no reentrant wetting phenomenon has been observed, as some theories suggest is p ~ s s i b l e . ~However, ~~‘ in all cases it is lutidine (2,6 or 2,5) which preferrentially adsorbs. It is possible to theoretically explain this behavior in systems in which the surface has a short-range force favoring adsorption of lutidine but a long-range force that does not favor wetting by l ~ t i d i n eso , ~that ~ finally such an experimental result is less surprising than felt at first sight. (8) Beysens, D.; Esteve, D. Phys. Rev. Lett. 1986,54,2123. (9)Gurfein, V.;Perrot, F.; Beysens, D. Phys. Rev. A 1989,40,2543.
(10)Tenebre, L.; %vat, M.; Ghaicha, L.; Bennes, R.; Tronel-Peyroz, E.; Douillard, J. M. C. R . Acad. Sci. Paris, s6r. XI 1987,304,311. (11)Ghaicha, L.; Privat, M.; Tenebre, L.; Tronel-Peyroz,E.; Douillard, J. M. Langmuir 1988,4,1326. (12)Bennes, R.; Privat, M.; Tronel-Peyroz, E.; Amara, M. Langmuir 1991,7,1088.
0 1994 American Chemical Society
Water-2,5-Lutidine against Silica
Langmuir, Vol. 20,No. 10, 1994 3731 T
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. 01 .02 *L Figure 1. (a)Presentationofwettingtransition and prewetting transition according to Cahn in a binary liquid system. The critical point of the two liquid phases coexistence diagram is a lower one. j3 phases are rich in component 1,y phases are rich in component 2. Large dots represent systems whose wetting behavior or adsorption behavior is drawn beside and designed by an arrow. Wetting is studied against a hard wall a, as it is shown in the vessels inside the coexistence curve. For T > T, there is no complete wetting 1’.
,
at the a,j3,y interface; for T < T,, the j3 phase is completely wetting a, and there is a wetting transition at T = T,. Adsorption layers or wetting films arejn small lateral figures and are represented by diagonal lines more or less wide, according to the thickness of the layer or the film. ,8 and j j are the water-rich and lutidine-rich single phases. The thick wetting film of ,8 existing in the two-phase system can be preceded in a component-2-rich j j single phase by a thick adsorption appearing after a prewetting transition: the dashed line is the prewetting transition line. (a’)The corresponding wetting classes transition. Two sets of contact angles define two different wetting classes. Every wetting class corresponds to a wetting situation at the a,,8,y contact, as can be seen through the Young equations, so that the wetting class transition is strictly equivalent to the wetting transition of part a, since y always completely wets ,8 in contact with vapor. (b) Summary of the previous presentation of some isotherms of Figure 2. Part ,8 is an example of linearization obtained by plotting P L w , the relative adsorption of 2,blutidine on silica, versus log (x - xco), xco being the phase boundary molar ratio. A break appears between two slopes. Part a shows the breaks as asterisks forming the surface transition (S.T.) curve. which ends at T,: in this sense, T, is a characteristictemperature of the ,8($*) phase surface behavior. Concerning the enhancement of the adsorption close to the phase boundary, it has hitherto been inferred that this enhanced adsorption could be related to the prewetting transition,8-10 in spite of the fact that the prewetting transition should have been observed in the lutidine-rich part of the diagram, in which the “wetting P-like film” should be formed. As this enhanced adsorption is observed essentially on micropheres of silica, whose surfaces present a strong curvature, while wetting behaviour is observed on plane interfaces, the possibility of a discrepancy for wetting on the two types of interface has been raised. To explain the experimental results, two types of precautions must be taken. For a theoretical explanation, the literature has to be re-read carefully. Surprisingly never noticed, a hint already was present in the initial paper by Cahn,l who indicated that if the adsorption from a wetting phase that is completely-wetting bulk phase does not diverge when the phase separation is approached a t constant T, it must become infinite close to the critical point. An enhanced adsorption a t the interface between a solid and a bulk phase which perfectly wets this solid when T tends to T, along the coexistence curve is then to be expected. A n explicit calculation of these adsorption effects is made in the paper by Telo da Gama and Evans.13 The model used in this reference is more sophisticated than the one used by Cahn, but it does not contain, as does ref 4b, the forcerange effects necessary to describe the apparently con(13) (a) Telo da Gama, M. M.; Evans, R. Mol. Phys. 1983,48,687. (b)Tarazona, P.;Telo da Gama, M. M.;Evans, R. Mol. Phys. 1983,49, 283.
tradictory behavior of adsorption compared to the preferential wetting quoted in the preceding paragraphs. Nevertheless, as very few works present explicit calculations, we will use, keeping in mind its limitations, ref 13 as a theoretical illustration of the behavior of adsorption on a solid surface in contact with a preferentially wetting bulk phase which must be one of the generic effects contained by Telo da Gama and Evans’s model. In the case of an agreement between experimental results and these generic theoretical results, the enhanced adsorption already quoted might be attributed to a critical effect. An additional discussion will decide if this critical effect is due to complete wetting (asin the model of ref 13)or only to partial wetting (allowed by models of ref 4b), which both should present the same critical behavior. Experimentally, to avoid surface state effects, the silica microspheres used in this work were prepared by precipitation; such spheres are known to have a surface state very close to plane silica that has been water-washed after a bath in an acid mixture.20 After this listing of interpretation difficulties, but also (14) Sullivan, D. E. Phys. Reu. B 1979,20, 3991; J. Chem. Phys. 1981,74, 2604; Faraday Symp. 1982,16, 191. (15) Schay, G.In Surface and Colloid Science; Matijevic, E., Ed.; Wiley-Intersciences: New York, 1969. (16) Biais, J.;Odberg,L.;Stenius, P. J.J.Colloid Interface Sci. 1982, 86,350. (17)Andon,R. L. J.; Cox, J. D. J. Chem. SOC.1952,4601. (18) Durian, D. J.; Franck, C. Phys. Reu. B 1987,36, 7307. (19)Parfitt, G.D., Rochester, C. H. Adsorption from Solution at the SolidlLiquid Interface; Academic Press: London, 1983. (20) McDermott,D. C.;Lu, J. R.;Lee, E. M.;Thomas, R. K.; Rennie, A. R. Langmuir 1992,8,1204.
Privat et al.
3732 Langmuir, Vol. 10, No. 10,1994 of various facts we have about both properties of our physical system and theory, we can define some aims about the interpretation of our adsorption results. In a general way, we have to establish a convincing relation between adsorption behavior of the water-2,5-lutidine system and its wetting properties. In doing so, we have to elucidate the particular behavior of adsorption at the phase boundary. At last, we can profile some new experiments to confirm our conclusion and perhaps find some new phenomena. The paper is therefore arranged as follows: in the first part we summarize the paper ofTelo da Gama and Evans; secondly,we describe experiments for obtaining adsorption isotherms and data to calculate the surface tension between solid and liquid; and finally, recalling the wetting behavior of the system, we analyze our new results and compare them with the theory. In this work, we have confined our studies to solutions in contact with silica in the water-rich region, with respect to adsorption and surface tension.
Summary of the Theory A. Main Features of the Theory. As the phenomenon we are studying concerns two-liquid phases in equilibrium, the theoretical method starts with the writing of a density functional, appropriate to our system which splits into two phases. Interactions between pairs of particles of liquid are measured by “Yukawa-like”potential with an attractive part and a repulsive one. The interaction potential of an i species with the wall is taken into account through an additional parameter simply multiplying the attractive part of the pair potential. The chosen functional, first written by Sullivan,14 gives only longrange density profiles and monotonic ones: it fails to describe preferential adsorption of a component i from a j-rich bulk phase. The detailed form of the different interaction potentials, the mixing rule adopted in the case of the binary liquid, and the form given to the solid-fluid interaction allow considerable simplification in the mathematical treatment. In fact, the problem reduces to a singlesrderparameter problem with all the limitations it confers to the realism of the description of the phenomena. The wetting situation is defined by comparing the repulsive potential of the species i on the wall with the repulsive potential of the same species in the equilibrium bulk phases. Therefore, the wetting situations will depend on both the nature of the wall and the nature of the system and, of course, on the thermodynamic conditions, in particular, the temperature, wetting classes, and the “phase diagram” for wetting. Because of the form of the model, wetting by vapor is an essential criterion, and the wetting description of the liquid binary system in contact with a hard wall is made through the wetting at solid-liquid-vapor contact. This leads to the definition of wetting classes through a couple of wetting states of ,f3 and y against wall and vapor. We recall the definition of classes I and IIA in the Result and Discussion and show that the wetting transition experimentally observed at the a,P,y-contact and the wetting class transition are identical (see also ref 7a and Figure l a and la’). B. Generic and Specific Results of the Theory. We now consider what is general and what is specificfrom the theory. First, in a quite general way, the relative adsorption r1,2of a given component of the liquid mixture and also its sign depend on the interaction with the wall. The adsorption also depends on the wetting class: this can be inherent in the model used and actually corresponds with the wetting behavior of our real system.
Second, and this result is general, the evolution of l?1,2 with temperature, and for a given interaction parameter with the wall, also strongly depends on wetting. This adsorption at the interface between the wall and the preferentially wetting bulkphase diverges to +or --m when T tends to T,(one is moving along the coexistence curve). It happens equally with a one-phase system, for example along the critical isochore. This could be in fact predicted by any other theory.’ Among the same theorists, their paper about interfaces between binary liquids and vapor, using a quite different functional and interaction model, shows exactly the same b e h a ~ i 0 r . l ~ ~ The adsorption at the interface between the wall and the bulk phase, which at T T, only partially wets the wall and at T =- T, does not wet at all, diverges to infinity (+ or -1 when T tends to T c ~the , temperature of the class transition, which has to be identified with T,, the temperature of the wetting transition. The adsorption value is infinite for T > T,. This is shown also by ref 13b. Thus, there is a considerable difference in the behavior of adsorption at the interface between the wall and one of the two liquid bulkphases,according to the wettingability of the considered liquidphase: both diverge at aparticular temperature, but one at T , and the other at T,. The divergence, according to several authors, is due, for the wetting phase always in contact with the solid, to the divergence of the correlation length when T tends to T,. The sign of divergence may be positive or negative: it depends on the sign of the adsorption. If for example Tzl is positive, which means that species 2 preferentially adsorbs, Tzl will diverge to +m at T,or T,, according to the wetting ability of the considered liquid phase. But l?12 = -XI/XZ l?zl will diverge to --m: the divergence of adsorption is then a generic result, its sign can be specific of the model, and anyway, it is specific to the adsorption definition. A less general result is that a transition from class I to class IIA is accompanied by an extremum of both surface tensions whose position on the temperature scale depends on the wetting ability of the phase (Figure 6b). Although the authors of the model strongly suspect this to be an artefact of the model, it is worthwhile examining the corresponding experimental behavior, because T , is, surprisingly, a temperature characteristic of the behavior of the adsorption a t the interface between the solid and the bulk phase which perfectly wets this solid in the twophase system in our experiments (Figures l b and la).
Experimental Section (1)AdsorptionMeasurements. The relative adsorption of 2,5-lutidine on silica with respect to water, I&, was measured from 15 to 54.3 “C in monophasic water-rich phases. Samples of 20 cm3 of solution were shaken with an average of 3.5 g of silica powder at least 1day at the working temperature. After centrifugation, the supernatant was analyzed by UV absorption spectroscopy. The relative surface excess was calculated by a conventional method,15 and the error bars were calculated for each measurement. The silica powder, prepared by a precipitation technique, was kindly supplied by RhGne-Poulenc. This silica had a specific area of 40 m2g-l, confirmed by BET of nitrogen. The silica was more than 99%pure and consisted of an agglomerate of nonporous spheres having a mean diameter of 0.13 pm as measured by electron microscopy. For the sake of experimental reproductibility, we confined our experiments to the same silica stock. This placed limitations on the number of experiments we were able to undertake. We have focused our attention on measurements close to the coexistence curve; consequentially, data for weak compositions are limited. The coexistence curve is known from the literature17and has been confirmed several times. (2) ActivitiesMeasurements-Chemical Potentials. The chemical potential of a species in a given solution was deduced
Water-2,5-Lutidine against Silica
Langmuir, Vol.10,No.10,1994 3733
by the analysis of a known volume of a sample of vapor in equilibrium with the studied liquid mixture. The sample of vapor was removed by a syringe from the top of a closed vessel, kept at constant temperature. The vapor was analyzed by chromatography. At each temperature, identical samples of vapor of pure water, on the one hand, and pure 2,5-lutidine,on the other, were also injected. The ratio of the peaks for 2,5-lutidinein the mixture and for pure 2,5-lutidine gives the activityuL of lutidine at the given temperature (it is identical to the ratio of vapor pressures). The activity of water was determined in the same way.16 Precision was about 2%. A relative chemical potential is then available from ,uL- p o L ( r ) = RT In a,
(1)
where p~ is the chemical potential of 2,5-lutidine,,&(T) is the standard chemical potential,R and T have their usual meaning, and U L is the (relative) activity of lutidine in a symmetrical reference system. (3) Surface Tensions. Surface tension data were not direct experimental values but were deduced from by integrating the Gibbs formula:
d1beingthe surface tension between solid and the solution where the lutidine activity is U L . Formula 2 has been graphically integrated between XL = 0 and the chosen value of X L to obtain the variation of d1between silica in contact with pure water and mixtures with lutidine at composition XL:
Ad' = - R T s r L w d(ln a,)
(3)
However, formula 3 is not directly usable in a numerical or graphicalcalculation, because In U L tends to infinityfor X L tending to zero. We therefore performed the calculation using the form
Ad' = -RTsCLWlaL)da,
(4)
and plotting TLW/uL versus U L . This is possible because the ratio rLw/uL tends to a finite value as XL tends to zero. As it can be seen in Figure 4,the value of A d at coexistence,Ad&, essentially
depends on the shape of the middle curve. (4) Chemicals. 2,5-Lutidine(2,5-dimethylpyridine;96%)was obtained from Aldrich, and distilled twice immediately before use, to give a product which was 99.5%pure. It was stored in closed containers. The 0.5%impurities were probably isomers of lutidine. The water was first purified on an ion-exchange column and then distilled from potassium permanganate in sulfuric acid. All glass containers were washed with a freshly prepared sulfochromic mixture and then amply rinsed with distilled water.
Results and Discussion (1) Previous Results on Wetting. The wetting transitions studied in ref 7a occur in the two-phase liquid system, at the contact between a (solid),B, and y (liquids). It has been shown to be from class I to class IIA. Class I is the situation observed for T,< T < T,. y completely wets the interface between a and vapor (no wetting of a by vapor), and /3 completely wets the interface between a and vapor (no wetting of a by vapor): a a v = Gay a,, and a,, = a@+ up, (a for the interfacial tension, v for vapor). The consequence is that /3 completely wets the interface between a and y : a,, = a@ up, (this is true because up,
+
= ajy
+ a,'").
+
Class IIA is the situation observed for T > T,. y always completely wets the interface between a and vapor (no wetting of a by vapor), and B only partially wets the interface between a and vapor (partial wetting of a by = Day + a, and uav< a@ US,. The consequence vapor): is that /3 then partially wets the interface between a and y , Day < a@+ up,: there has been a wetting transition (or
+
10
15
1 0%
Figure 2. Adsorptionisotherms set at different temperatures. rskwis the relative adsorption of 2,5-lutidine with respect to water, XL is the mole fraction of lutidine: ( x ) 54.3"C; (+) 47 "C;(0)40 "C; (-1 31.3 "C;(0)25 "C;(*) 19.2 "C; (4) 15 "C. The dashed lines mark the phase separation composition for each at "coexistence", temperature. The greatest value of rLw, becomes lower when T increases: they can be read off on the figure only at 54.3,47,and 40 "C.
a wetting class transition) at Tw,which is called the wetting transition temperature for the a,/3, y contact line and the a,/?, v contact line. All this is experimentally checked and analyzed in ref 7a. Figures la,a' summarizes, in a somewhat realistic way, the wetting situation at the different phase contacts, what the terms class I and class IIAmean, and the identity between the wetting transition and the wetting class for our system. (2)Adsorption. A. Results. Adsorption isotherms at different temperatures are shown in Figure 2. For the sake of clarity, error rectangles have been drawn only for the four lowest temperatures. Measurements at 19.1 "C have high precision, but because of the limited silica stock, those at other temperatures have greater errors because the silica quantity used for these temperatures is smaller. Typically the error rectangles are those illustrated for 25 and 31.3 "C. At 15 "C, additional errors arise from the inaccuracy in determining rfW at high equilibrium concentrations. A cursory examination of the figure shows that, at low temperatures, very large values are attained for l?fW when the coexistence concentration is approached (beyond the borders of the figure for 15 "C and even 31.3). At 54.3, 48,and 40 "C the value "at coexistence" is far lower, as it is seen in the figure. These results show, in a far more detailed way, a behavior similar to the adsorption of some surfactants, that are only partially miscible (ref 19, p 128). This behavior has never been explained before.
Priuat et al.
3734 Langmuir, Vol. 10,No. 10,1994
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Figure 3. Adsorption at the interfacebetween silica a and the wetting bulk phase /3*, also called q*. (a) Curve of the absorption of lutidine at coexistence, versus temperature. Dots are values obtained by direct graphical extrapolation of the isotherms, stars are obtained after linearizationaccording to ref 12. (b)Transcriptionof the results of Telo da Gama and Evans in a T,-T scale. The nomenclature is that of the authors; an asterisk indicates the wetting bulk phase q*. (c) The same as in part (a) but the adsorption is one of water versus 2,Nutidine. T h e asterisk recalls that /3*, also called q*, is the wetting bulk phase. Adsorption from the wetting phase has a similar behavior in theoretical and experimental results: it tends to infinity at T = T,.
B . Comparison with the Theory. The values of r2w obtained by extrapolation of isotherms of Figure 2 and related data are illustrated by Figure 3a. The extrapolation was made either graphically or by using the delinearization method described in ref 12: these last values are drawn as stars. The considerable error bars for the lowest temperatures are due to extreme extrapolations of a very steep curve. The characteristic of Figure 3a is that tends to very large values when T tends to T,,the critical temperature of the system. In Figure 3c, we plotted R& = - (xw/x,)Tfw for an easier comparison of our results with Figure 3b: in both figures, the signs of r& and r12 are the same. In Figure 3b we replot the results of Telo da Gama and (Figure 7) on a shifted T scale (T, - T): our system is a lower consolute point system, theirs is an upper
Gw
consolute point system, and this type of representation unifies the figures. (What happens at T, and T, is read in the same order.) Although, in comparison with the adsorption behavior a t the interface between this solid and the bulk wetting phase, the nature of the wetting class transition is not important, we have chosen to plot a transition from class I to class IIA. experimentally the wetting transition in the water-2,5-lutidine system is between these same wetting classes (ref 7a and part 1of this section). Nomenclature of phases in Figure 3b is that of Telo da Gama and Evans, placed into commas to avoid confusion with ours and with a n asterisk added to specify the preferentially wetting phase y(*). We have called our preferentially wetting phase fi (a is the solid in both nomenclatures), and we have also added an asterisk to recall its wetting property. In the following,"y*" (Teloda
Langmuir, Vol. 10, No. 10,1994 3735
Water-$5Lutidine against Silica
Table 1. Correspondence between the Different Bulk Phase Symbols in the T w o Liquid Phase Systema in ref 13 class transition 1-11 A
ref 13 quoted in this work
B
“B”
Y
“Y”
less dense bulk phase, nonwetting
unified symbols
d
Y less dense bulk phase, nonwetting
B
the densest bulk phase, preferentially wetting the solid
the densest bulk phase, preferentially wetting the solid a
in ref 7 and in this work
@*
unified symbols with specification
d ( “ r )ref 13 d ( y ) ref 7 d*(“y”) ref 13
d*@ ref 7
Asterisk refers to the wetting phase.
Gama) and B* (ours) will be called q5* (wetting phase); “P,, (Telo da Gama) and y (ours) will be called 4 (nonwetting phase). “wetting phase” has to be read as “thebulk phase which, in the two-phase system, perfectly wets a (and thus, isolates the other liquid phase from the solid)”.Table 1gives the correspondence between the different symbols. According to the model, component 2, when it is pure, has the greatest density and surface tension, and the weakest vapor pressure. This would lead to the identification of 2 as water in the real system. It is then consistent to assign the theoretical q5* densest phase (“y”) to the real 4* phase (water-rich, so denser), B. However, a discrepancy arises when one tries to compare what is actually adsorbed. In theory, q5* is rich in the same component that is preferentially adsorbed, but in the experiment, q5* is rich in water and lutidine is preferentially adsorbed.
-
The theory ofref 13 also predicts that r1,2 has a negative divergence as T T,, measured at the interface between the solid and the bulk q5* phase (* for wetting). This disagrees with the experimental results, which show a positive divergence of r1,2 rLw as T T,, reflecting enhanced adsorption of lutidine. Therefore, we conclude that the divergence of r1,2 as T T , is a “natural critical” effect, but we observe also a discrepancy in sign between the theory and the experiment. This could be due to the approximate nature of the theory of rep3and particularly t o the fact that the theory is “one-component-like”,but this remains to be proved. However, if this is admitted, one must also admit that the critical behavior of lutidine adsorption, measured on silica microspheres, is closely related to the wetting behavior of the 4* phase (water-rich),which perfectly wets plane walls of silica, when in equilibrium with the 4 phase (lutidine-rich,nonwetting). If, by some perverse inversion of the wetting properties, in passing from a macroscopic silica wall to silica microspheres q5 (lutidine-rich) would become the preferentially wetting phase on microspheres, the expected adsorption behavior in q5* (water-rich), then nonwetting because of the supposed wetting inversion would be a divergence for T T,, which is clearly not the case. These experimental results about adsorption seem to prove that the water-rich phase can be the wetting phase, as shown by direct wetting experiments (ref 7a,b), while lutidine is preferentially adsorbed on silica, as shown by direct adsorption measurements (on microspheres instead of plane walls). Moreover, these experimental results are the first experimental demonstration of the divergence of adsorption, from a preferentially wetting phase, when T tends to T,. The possibility that the water-rich phase only partially wets the silica considered in the Introduction will be rejected in the analysis of section 3. C. Comment about the Prewetting Transition in the Silica- Water-2,5-Lutidine System. This analysis of adsorption isotherms clearly shows that the behavior of adsorption of 2,5-lutidine from water t o silica in water-
-
+
-
-
rich phases is directly connected to the wetting behavior of this phase: perfect wetting for T < T,. It is then illusory to seek a prewetting phenomenon at this interface, as has been done before.8-12 Prewetting can be viewed as an enhanced adsorption, preceding by a single phase whose concentration is close to a phase boundary, the formation of the continuous macroscopic wetting film which appears in the two-phase system. The enhancement observed in the water-rich phase, which is spectacularly revealed by a two-slope linearization in our previous parametrization approach12(Figure lb), is typically a critical phenomenon, linked to the divergence of adsorption when T tends to T,and when the coexistence curve is approached: it is known that a critical behavior can be detected far from the critical point at a distance that depends on the extension parameters of the scaling law of the studied phenomenon. The break in slope may be explained by the linearization process, taking ln(x - XCO) as variable (ref 19, p 2911, and probably simply reveals a change in the shape of the isotherm, smoother in the representation of Figure 2. However, there is another experimental observation that is still unexplained. The results of ref 12 show that an important change occurs at T = T,, since the same formula cannot describe isotherms for both T < T , and T > T , (see Figure lb). Beysen’s experiments show something very similar: for T > T,, adsorption characteristics become insufficient to make silica microspheres aggregate. A n explanation follows in section 3C. (3)Surface Tension. A. Calculation and Results. The calculations of A@‘ have been performed according to formula 4 by graphical integration (Figure 4) and errors evaluated. Results, given in Figure 5, are confined to the highest temperatures so as to have a readable Au scale. Error bars have acceptable values if they are compared with direct experimental errors on liquid-vapor surface tensions. A@‘ appears as a lowering of surface tension compared to the pure water-silica interface: this is normal, since 2,5-lutidine is preferentially adsorbed. This lowering is important at low temperatures, becomes a minimum for T = 47 and 48.8 “C (in fact, within errors, both curves can be considered identical), and becomes greater again for T = 54.3 “C. B. Comparison with the Theory. Figure 6a gives the values of A@‘ extrapolated t o coexistence with their error bars. A point has been added at T = 10 “C, below T,, corresponding to a temperature at which the system is always monophasic: the value of A d ‘ is taken for x = x , and was obtained by extreme extrapolation, and this accounts for the considerable error bar. Dashed lines indicate T, and T,. The striking point is that the curve has a maximum for a temperature close to T,. For comparison, Figure 6b presents the results obtained by Telo da Gama and Evans (ref 13, Figure 8). For consistency with our previous discussion, we have chosen the result correspondingto the case of a class I-class IIA transition, and similarly we have changed the temperature scale t o T,-T. The nomenclature of phases is that of the authors, but placed in inverted commas and with an
Privat et al.
3736 Langmuir, Vol. 10, No. 10, 1994 A os' mNm5( 51 -1
i
r
- 4
I
0 I
0,l
03
i
J
31
-. I
-1
i4.3'1 1
Figure 4. Evolution of the ratio r&/aL according to temperature. Surfaces of these curves gives A@', the interfacial tension between solid and liquid (cf. eq 4). asterisk added to indicate the preferentially wetting phase in the diphasic. Surface tension for the model varies with temperature in a manner opposite of the real system, decreasing in the one-phase system monophasic and until T , in the wetting phase of the two-phase system, instead of increasing. The variation of surface tension with temperature depends on the nature of the system and is not of fundamental physical significance. Both parts a and b of Figure 6 have then the same characteristic; concerning of course the wetting bulk phase surface, they have an extremum for T = T,. In the theory, this extremum is due to the transition being from class I to IIA and a special form of the density profile at T,. If the theoretical results of ref 13a are artefacts in this respect, as suggested by the authors, one must conclude that the extremum in A d at T , is perhaps a more general physical characteristic than expected from the theory and is related to the fact that T , is also the wetting temperature ofp phase between a and vapor (see ref 7a and the Introduction). We must also consider the possibility that the extremum in Figure 6a, which is substantiated by a single point (at 54.3 "C), is illusory: it would be far preferable to have more values for higher temperatures. However, uncertainties are small enough, due to the favorable shape of TLW/aL curves (Figure 41,to give us some confidence in the existence of this extremum. C . The Role of T , in the Surface Behavior of the Wetting #* (p) Phase. According to formulas of ref 12, the set of "breaks" detected in our linearized isotherms can be drawn on a phase diagram as an "adsorption transition line" whose shape is very similar to a "prewetting" line (Figure lb). It is now clear that adsorption measured on silica in the water-rich phase of our system cannot be connected to prewetting, but rather to a critical phenomenon, it is
3
5
7
103xL
Figure 5. Interfacial tensions between silica obtained by integration of the isotherms through curves as in Figure 4. Curves are interrupted by the phaseseparation: (*) 40-'C; (0) 43.3 "C; (-1 47 "C; ( x ) 48.8"C; (0)54.3"C. necessary to understand why the apparent transition, which is possibly a passing to a critical behavior, stops at T = T,. An explanation can be found in the extremum shown in Figure 6a. Our "transitions" detect a change in the shape of the isotherms. A@' shown in Figure 6a are the results of integration of such isotherms. A change in the shape of the isotherms has an influence on Ad', and a drastic change can result in an inversion in the sense of variation of A@' at coexistence. This change can be seen in Figure 4,where a change in curvature is more strongly drawn at 54,3 "C than for example at 40 "C. In Figure 7, isotherms are plotted in a rLw =fix - xco)representation (dots corresponding to the same x - xco for different temperatures are at equal distance of phase separation): curves y and z cross twice, while it is impossible to obtain such a double crossing with w and y , when a superposition by translation is attempted. Thus, a change in the form of isotherms for T > T , can explain both the disappearance of the break in the linearized isotherms and the extremum in values of A d at coexistence. In addition, the role played by T , in the adsorption behavior is consistent with the existence on the surface of the silica microsphere, of the same wetting transition as the plane surface: wetting by the waterrich phase on these microspheresmust then be considered as complete. This a posteriori justifies our discussion about the prewetting transition in section 2B.
Conclusion The analysis of a set of isotherms experimentally determined at the interface between a solid of high energy (silica) and a binary liquid system (water-2,5-lutidine) has made it possible to verify the divergence of adsorption
Water-2,BLutidine against Silica
Langmuir, Vol. 10, No. 10, 1994 3737
A us' mNm'
-10
- 20
Figure 6. Interfacial tension along the coexistence curve, obtained by extrapolation of curves such as in Figure 5: (a) experimental values; (b) theoretical values according to Telo da Gama and Evans, in a T,-Tscale. The preferentially wetting bulk phase is indicated by an asterisk. An extremum exists on the preferentially wetting phase curve, at T = T,.
along the coexistence curve, however, with a discrepancy in sign. A comparison has been made only in water-rich phases, which are completely wetting for temperatures close to T,. The partial correspondence of experimental and theoretical results makes clearer the connection between such adsorption measurements and wetting properties of the system. Previous resultss-10J2concerning the enhancement of adsorption a t the surface of wetting bulk phases can now be clearly related to the critical behavior of such a surface: divergence of adsorption for T,. A hint is given to explain other experimental results (ref 19, p 128). The modification of the enhancement process at T = T, might
Figure 7. Isotherms of Figure 1,drawn versus z - XCO, mole fraction in lutidine minus the fraction of coexistence. This representation allows the comparison of the shape of the isotherms at equal distances of coexistence. A change of shape occursbetween 47 and 54.3"C, which might explain the change in the linearization (ref 12)and in the aggregation process (refs 8 and 9) which appears at T = T,: (v) = 25 "C; w = 40 "C;y = 47 "C; z = 54.3 "C.
be related to the nature of the wetting transition (transition from wetting classes I to IIA)and its effect on the shape of the surface tension curves. This last point needs to be verified with considerably more experimental isotherms. The study of the interface with respect to the nonwetting phase now becomes of paramount importance: the surface behavior should be quite different.
Acknowledgment. We are very grateful to Dr. C . F'ickett, from the Institute for the Nitrogen Fixation a t Brighton, for having kindly reread and sometimes rewritten our manuscript.
