Langmuir 1999, 15, 2879-2883
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Surface Phase Transitions at Liquid-Mixture/Solid Interfaces Isabelle Mazeas, Pascal Pe´lerin, Hamid Sellami, Ahmed Hamraoui, Rene´ Olier, and Mireille Privat* UMR 6521-CR4, De´ partement de Chimie, Universite´ de Bretagne Occidentale, 6 Avenue Le Gorgeu, BP 809, 29285 Brest Cedex, France Received September 9, 1998. In Final Form: December 21, 1998 In an adsorption study from a liquid mixture, the choice of a moderately hydrophilic silica and of a solute like 2,5-dimethylpyridine (2,5-DMP), known for its layering ability in dilute solution in water, allows the observation of several adsorption gaps on the isotherms established at different temperatures. These gaps may be attributed to surface phase transitions. Collected on a (T-Γ21) diagram, they constitute a surface phase diagram highly similar to the bulk water-2,5 DMP liquid-solid diagram at low temperature. It is the first example of a surface phase diagram at a liquid-mixture/solid interface.
Introduction A recent study of 2,5-DMP adsorption on silica from aqueous solutions showed a marked layering effect on adsorption isotherms at different temperatures.1 This result is quite new at liquid-mixture/solid interfaces whereas it has been known for long in the case of gas adsorption on solids2 and identified as successive surface phase transitions. For gas-solid systems, a more elaborated phase diagram exhibits gas-liquid-solid transitions3-5 for the lowest surface concentrations. Looking for the evidence of such transitions in dilute solutions in contact with a solid was thus quite logical. Such an approach constitutes a new attempt to show the generality of surface phase transitions and will add to the results already found in liquid mixtures at liquid vapor interfaces,6 in prewetting phenomena7,8 and in other phenomena associated with wetting,9 in electrodeposition of metal, and in many other fields.10 In the present paper, we first recall how surface transitions appear on adsorption isotherms, then we describe the experimental procedure, and we finally present and discuss our experimental results. Theory Surface phase transitions in systems at equilibrium can be described in the so-called surface phases in the same way as phase transitions are described in bulk.11 We consider a 1-2 liquid mixture where 1 is water and 2 is * To whom correspondence should be addressed. (1) Sellami, H.; Hamraoui, A.; Privat, M.; Olier, R. Langmuir 1998, 14, 2402. (2) Duval, X.; Thomy, A. C. R. Acad. Sci. Paris 1964, 295, 4007. (3) Thomy, A.; Duval, X. J. Chim. Phys. 1970, 67, 1101. (4) Suzanne, J.; Coulomb, J. P.; Bienfait, M. Surf. Sci. 1973, 40, 414; 1974, 44, 141. (5) Larher, Y. J. Chem. Soc., Faraday Trans. 1 1974, 70, 67. (6) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinka, S. K.; Gang, O.; Deutsch, M. Phys. Rev. E 1997, 55, 3164. (7) Hamraoui, A.; Privat, M.; Sellami, H. J. Chem., Phys. 1997, 106, 222. (8) Bonn, D.; Wegdam, G. H.; Kellay, H. Phys. Rev. Lett. 1992, 69, 1975. (9) (a) Valignat, M. P.; Fraysse, N.,; Cazabat, A.-M. Langmuir 1993, 9, 325. (b) Fondecave, R. The`se, Paris, 1997. (10) Beckmann, O.; Gerischer, H.; Kolb, D. M.; Lehmpfuhl, G. Symp. Faraday. Soc. 1977, 12, 51. (11) (a) Defay, R.; Prigogine I.; Bellemans, E.; Everett, D. H. Surface Tension and Adsorption; Longmans: London, 1966. (b) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1976.
a solute (2,5-DMP in our experiments). Figure 1a shows how, at a given temperature, the chemical potential of component 2 behaves with respect to the surface composition x2σ in the case of a pure 2-solid demixing from a 1-2 liquid mixture. The chemical potential is here written as a surface parameter, µ2σ, but is equal to µ2, the chemical potential of 2 in bulk whereas x2σ is different from the mole fraction in bulk at equilibrium, x2. Figure lb depicts the surface molar free energy behavior under the same conditions. The three-dimensional Figure 2 shows how isothermal transitions collected at various temperatures lead to the surface phase diagram on the (T, x2σ) plane. The (T, x2) front plane displays the corresponding bulk phase diagram. On an isothermal horizontal plane, the x2σ curve represents the adsorption isotherm at this temperature. When starting reading such an isotherm at x2 ) 0, one should note that, at x2C, the position which gives the x2σ of demixing on the (T, x2σ) side plane, the isotherm jumps. The jump describes the coexistence of three phases in the studied system, one of them being the bulk while the two others are surface ones. It should be considered as a kind of triple point. Whenever x2 varies, it disrupts this coexistence and then ones observes a classical x2σ ) f(x2) curve. This three-dimensional diagram is useful to visualize a surface demixing on an isotherm in agreement with the laws of equilibrium thermodynamics. Reciprocally, observing such a jump on an experimental isotherm enables one to detect any change in surface phases. The (T, x2) plane of Figure 2 represents the bulk liquidsolid diagram of 1-2 binary mixture which exhibits a eutectic. This, in fact, constitutes only a part of the experimental system behavior as shown below, but it is easily handled for demonstration purpose. Considering the multiplicity of phases potentially exhibited by a binary mixture, one may wonder what kind of phases will be seen on surfaces at room temperature. Previous experiments1,7 showed that, for the water-2,5DMP-on-silica system, prewetting can be seen as a liquidliquid demixing identical to the bulk phase change close to the one where prewetting occurs whereas layering could be attributed to a kind of liquid-solid demixing identical to the possible bulk phase change that occurs at lower temperature. This process is observed in surface phase
10.1021/la981214l CCC: $18.00 © 1999 American Chemical Society Published on Web 03/23/1999
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Figure 2. (T, x2) plane showing the bulk phase diagram. On each isothermal plane, the x2σ ) f(x2) curve represents the adsorption isotherm. For example, on isotherm T1 the point C corresponds to the start of a vertical line representative of a phase transition, its projection on the (T, x2σ) plane giving a point of the surface phase diagram. The vertical line ends at D, which represents the pure 2 2D-crystal.
Figure 1. (a) Chemical potential variation of component 2 vs the surface composition, x2σ, at the temperature where demixing of pure 2 crystal occurs. The thick line represents µ2 equilibrium values, the thin line the metastable state and the dotted line the nonequilibrium values. x2sat is the saturation concentration of solution. The dashed line depicts the curve shape when there is no demixing. (b) variation of the molar free energy, g, vs x2σ at the same temperature. The symbols used for the lines are the same as before. g20lσ is the liquid molar free energy of pure liquid in the surface phase; g20Cσ is the same for the solid. At the surface saturation concentration, “sat”, the tangent gives on the vertical axis at the left-hand side µ1σsat, the surface chemical potential of water, and at the right-hand side µ2σsat, which is equal to g20Cσ because at saturation there is equilibrium between the surface crystal and the surface liquid mixture.
demixing for gases in surfaces: the xenon triple point is at 164.2 K in bulk and at 100.1 K on graphite.11b Experimental Section Chemicals. 2.5-DMP was supplied by Aldrich and purified as previously described.7 Water was purified on a Milli Q device. Silica was Degussa Aerosil 200 prepared by pyrogenation. Its specific area measured by BET adsorption isotherms for nitrogen was 196 m2‚g-1, due to its high-division state. According to the supplier, silica particles had 3 OH groups for 100 Å2, a far lower number than with silica prepared by precipitation methods, and an average diameter of about 15 nm; there were thus 1017 particles in 1 g of aerosil. One should note that the silica porosity was rather low and the surface was smooth and very homogeneous.12,13 Our previous experiments have taught us that these properties of silica surface are very important in the observation of surface transitions. Isotherms. Water and 2,5-DMP in variable amounts were mixed to give about 20 cm3 of solution and then poured onto a given weight of silica powder. Most of our experiments were carried out on a 0.4-g silica sample; i.e., 4 × 1016 silica particles were dispersed in 20 cm3 to get a distance of about 64 nm between particles14 sufficient to avoid capillary condensation between them. To avoid losses of organic compound in the vapor, the tubes containing mixtures were nearly totally filled. They were gently shaken for 2 h at a preset temperature in order to reach adsorption equilibrium. After centrifugation at the same tem-
Figure 3. Schematic phase diagram of the water-2,5-DMP system from published results on the water-2,6-DMP system16 and for the water-2,5-DMP system.17 l corresponds to liquid, c1 to pure 1 crystal, c2 to pure 2 crystal, and C to the 1-1 compound, E1 and E2 are the two eutectic points, Tif is the i-component fusion point, Te1 is the first eutectic point temperature, and Te2 is the second one. perature and, for the most dilute supernatants, after filtration on 0.45 µm HVLP 02500 Millipore filters, the supernatant was analyzed by UV absorption spectroscopy at 267 nm. The relative excess Γ21 was calculated using the following conventional formula:
Γ21 ) n°(x2° - x2)/(ms(1 - x2))
(1)
Here n° is the total number of moles, x2° is the mole fraction of component 2 in solution before adsorption whereas x2 is its mole fraction after adsorption at equilibrium, m is the silica mass, and s is its specific area. In relation (1), n°(x2° - x2)m ) n2s which is the term traditionnaly used in applied studies.15 In dilute solutions eq 1 becomes (12) Ferch H.; Fratzscher, H. Kautsch. Gummi 1967, 20, 578. (13) Papirer, E.; Balard, H. In Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1995; Vol. 99, pp 479-502. (14) Privat, M.; Amara, M.; Hamraoui, A.; Sellami, H.; Mear, A.-M. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 626. (15) Schay, G. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, 1969; Vol. 2, p 155.
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Figure 4. Adsorption isotherms at different temperatures (a-d). The relative adsorption Γ21 of 2,5-DMP with respect to water is given vs x2, the mole fraction in bulk at equilibrium. Error bars are identical to the vertical diagonal of diamonds. The step labels and vertical segment labels defined in (d) are used in the text and in Figure 6. Horizontal dashed lines in (a)-(c) represent metastable steps. In (b) a dashed line represents also an isotherm beginning ignoring the two first steps.
Γ21 ) Γ2 - (x2/x1)Γ1 ≈ Γ2
(2)
where Γi ) ∫∞0 (Fi(z) - Fi(∞)) dz with Fi(z) the mole density in the direction perpendicular to the surface. Γ21 and n2s are relative values and Γi are excesses with regards to the bulk density of compound i.11a Great care was taken to estimate the errors on adsorption experimental values. Since in our previous study1 we had greatly overestimated errors, we consequently improved our calculations to take into account error compensation and put emphasis on the main error source namely the (x2° - x2) term. We also statistically checked these error values by calculating them on a set of data on the same plateau (Figure 4). These points came from various experiments, i.e. different initial solutions, different days, and different silica samples. Our results are presented as adsorption isotherms. Eleven temperatures were investigated, and above T ) 30 °C, the isotherms were the same as in ref 1, except at 42 °C which was not taken into account. At 20, 25, and 30 °C the beginnings of the isotherms were very carefully studied.
Results and Discussion Bulk Phase Diagram. Figure 3 shows the schematization of the bulk phase diagram drawn from the known
diagram16 of the water-2,6-DMP mixture and from experimental points determined in our laboratory for the water-2,5-DMP system.17 This diagram is characterized by a 1-1 compound, C, and two eutectics, E1 and E2. These particular points being excepted, for any given x2 composition of the system considered as a whole there are two crystallization temperatures: the temperature at which the first crystal appears and the eutectic temperature. Adsorption Isotherms. Figure 4 presents detailed isotherm beginnings at 20, 25, 30, and 48 °C. The isotherm at 48 °C is a magnification of a curve previously published.1 It takes into account only the two first steps already mentioned in this paper and labeled here 1 and 2. The adsorption value of step 1 corresponds to a saturated layer of 2,5-DMP molecules flat on the surface whereas along step 2 it corresponds to the equivalent of two saturated layers. These steps are also visible on the isotherms (16) Perron, G.; Quirion, F.; Lambert D.; Ledoux, J.; Ghaicha, L.; Bennes, R.; Privat, M.; Desnoyers, J. E. J. Solution Chem. 1993, 22, 107. (17) Maze´as, I. Me´moire de DEA, Brest, unpublished results, 1998.
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Figure 5. Detailed study of the 25 °C isotherm vertical part between steps 1 and 2.
recorded at 20, 25, and 30 °C. It should be noted that error bars are quite compatible with the existence of these steps and that these steps are reproducible from one temperature to another one even though their reading may be difficult because of the occurrence of other points that draw a second curve probably characteristic of a metastable phenomenon. At very low concentrations these experiments are quite easily carried out, but they become more complicated as soon as steps appear because of the occurrence of metastable extra points. Sometimes, either two parallel steps or one curve with steps and another one without any step can be drawn. We previously pointed out1,7 that this metastability is one of the best indications of surface phase changes. Moreover, one should note that for some specific compositions of the solution the successive points lie on a vertical line as illustrated in Figure 5. Finally on a single isotherm we must consider both “horizontal” parts or steps labeled -1, 0, 1, 1′, and 2 and vertical parts labeled R, β, γ, and δ as shown on the 48 °C isotherm (Figure 4). Vertical parts evidence surface phase changes whereas horizontal parts may indicate surface saturation but, often, simply separate two surface transitions and may not be strictly horizontal. Surface Phase Diagram. To draw the surface phase diagram we collected the vertical parts on the 11 isotherms at our disposal, i.e. the four isotherms presented in Figure 4 and the others from ref 1. The estimation of their height is rather subjective but possible. For a given temperature, these heights are reported as segments on the same horizontal line parallel to the segments abscissa axis of a (T-Γ21) plane, i.e., R, β, γ, and δ (Figure 6). The operation is repeated for other temperatures along the vertical T axis. Joining the start of all the first segments altogether, i.e., all the R segments, gives one line on the (T-Γ21) diagram. Then their ends are joined in the same way. The same procedure is used for the other segments β, γ, δ .... This allows us to draw the phase coexistence limits and thus the surface phase diagram. However, some comments must be made. First, the surface composition is given by the relative parameter Γ21, which is all but a surface mole fraction. This leads to a distortion in the diagram shape with respect to the shape it would have had if x2σ were used. Second, one must be conscious that the sum of interactions in the surface creates a “surface field”, which is a new variable with respect to the bulk and which is nowhere explicited. Third and not the least, the representation of Figure 6 must be read keeping in mind that there the surface always contains water even if Γ21 is approximately equal to Γ2 according to eq 2 because x2/x1 is very small in dilute solution. However, Γ1 can be very large. At Γ21 ≈ 0 there is in fact water on silica that may be strongly linked to the solid. Finally, the surface phase diagram is very similar to the bulk one presented in Figure 7. One can notice some similarities with Figure 3: one compound and two eutec-
Figure 6. Surface phase diagram obtained from segments R, β, γ, and δ of Figure 4 and other isotherms from ref 1. Horizontal lines such as R, β, γ, and δ segments at 48 °C are nothing but vertical segments of the isotherm (see Figure 4d). Blank sections at each temperature correspond to the steps such as segments -1, 0, 0′, 1, and 2 at 48 °C. The borders of R, β, γ, and δ segments linked together give the limit of phase coexistence and show a surface phase diagram very similar to Figures 3 and 7. The dashed lines are only speculative.
Figure 7. Phase diagram, more general than the diagram of Figure 3, displaying solid-solution areas denoted 1, 2, 2′, and 3.
tics. But, there are also something like “solid solutions” on both sides of the whole diagram and on both sides of the 1-1 compound C. This fact is very interesting because it indicates that water on the surface (for small Γ21) is very structured although slightly mobile and that this structure is not destroyed by the addition of the first 2,5DMP molecules. It is necessary to add a rather important amount of 2,5-DMP to make this structure melt. But, further adsorption leads to another structure denoted the “compound like” or its solid solutions. Their fusion is accompanied with dilution by 2,5-DMP while x2 is increasing. This phenomenon in fact corresponds to the step 1 previously mentioned. It seems that, along step 2, there is another surface solid solution. This point has not yet been investigated. Figures 6 and 7 show that the concept of monolayer or bilayer linked to an adsorption step must be moderated. Indeed, steps 1 and 2 seem to correspond to very structured layers. But, step 1 must be “liquidlike”, i.e. less rigidly set, whereas step 2 is rather “solidlike”. The structuration of “liquids” at interfaces may have implications in biology, in catalysis, in material chemistry,
Transitions at Liquid-Mixture/Solid Interfaces
in environmental problems, and in many other areas. The results presented above show how simple temperature and concentration effects may modify water in a very thin layer, maybe in membranes and certainly on a solid surface. Then can biological processes be the same on icelike or liquidlike surface layers? In catalysis it is known that crystal surface structure influences chemical reactions; would not the physical state of physically adsorbed molecules such as water and/or other compounds act as well? The deposition of metal on well-defined faces of metal crystals by electrochemistry in the underpotential region is known to lead to surface phase transitions or surface reconstruction10 whose theoretical modelization is under progress.18 Could new phases, useful in material prospect, be built by temperature effects? Could different surface phases be used to induce preferential orientation when grafting molecules on solid surfaces? Could these subtle differences between the surface state of solids in contact with liquid mixtures at different temperatures explain contradictory values19 found in adsorption of pesticides on soils in the equatorial area and in Europe? Could they explain different wetting behaviors of liquid soil pollutants according to temperature? A rich research challenge seems to be open.
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Conclusion Careful adsorption measurements, very numerous in a short concentration range, led to the observation of successive phase demixings on all the adsorption isotherms for the surface of silica in contact with aqueous solutions of 2,5-DMP. Collected on the same graph, these surface changes constitute a surface phase diagram similar enough to the bulk one to allow a sort of surface phase identification. The choice of the silica type, very well defined, whose surface is smooth, homogeneous, and rather poor in -OH groups, as well as the choice of the organic molecule, already known to give surface demixings, is certainly essential to permit these observations. Already known in Langmuir films and at the solid-gas interface, the surface phase diagram existence seems now established at the liquid-mixture/solid interface and allows interesting prospects in several application fields. LA981214L (18) (a) Zhang, J.; Sung, Y.-E.; Kickvold, P. A.; Wieckoski, A. J. Chem. Phys. 1996, 104, 5699. (b) Ocko, B. M.; Wang, J. X.; Wandlovski, T. Phys. Rev. Lett. 1997, 79, 1511. (19) (a) Moreale, A. E.; Van Bladel, R. Med. Fac. Landbouww. Rijkuniv. Gent. 1981, 46, 281. (b) Khan, A. A.; Singh, R. P. Colloid Surf. 1987, 24, 33. (c) Me´ar, A.-M.; Le Saint, J.; Privat, M. Ecotoxicol. Environ. Saf. 1996, 35, 163.
