Surface Phase Diagrams: Evidence of Molecular Arrangements at the

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Surface Phase Diagrams: Evidence of Molecular Arrangements at the Aqueous-Solution/Solid Interface Dominique Andrieux, Abdelhaq Acharid, Marie-Claire Fritsch, Jose Marquez Garcia, Marcos Martin Martin, Anne-Marie Meár, Mustapha Sadiki, Jean-Pierre Huruguen, Rene´ Olier, and Mireille Privat* UMR 6521-CR4, De´ partement de Chimie, Universite´ de Bretagne Occidentale, 6 avenue Le Gorgeu, C.S.93837 - 29238 Brest Cedex 3, France Received July 30, 2004. In Final Form: September 21, 2004 Surface state changes described as phase transitions or simple molecular rearrangements have become a key issue in modern science. Indeed, they have an impact on the development of numerous (nano)technologies; they are also involved in biochemical and chemical mechanisms at the molecular level and also in environmental phenomena. At last, they have been at the origin of flourishing statistical descriptions that have illuminated new and very interesting aspects of surface behaviors. Here, to obtain still lacking coherent sets of experimental data on systems in which molecular interactions and thermodynamic properties are different, the adsorption behaviors of three aqueous mixtures in contact with a dense and homogeneous silica were studied versus concentrations and temperatures. Of course, these mixtures displayed very different bulk phase properties. Their stairlike isotherms are interpreted through surface phase diagrams; each of them is very similar to the corresponding bulk phase diagram. Their comparison gives new insights into the different surface states, the role of solvent in the surface, and the probable molecular mobility.

Introduction Surface state changes are frequently described as either phase transitions or simple molecular rearrangements. Their growing importance in modern science is due to their potential impact on the development of (nano)technologies, their involvement in chemical and biochemical molecular-level mechanisms, and environmental phenomena. A quick literature survey showed a serious lack of consistent data and even of coherent views; this incited us to conduct a systematic experimental study on three systems, cited below, of theoretical as well as practical interest. Surface (two-dimensional) phase transitions were early observed by means of surface pressure variations in the so-called Langmuir films,1-4 but this was long after the existence of surface films had been proven and used;5 they also exist at the solid surface in contact with an infinite fluid reservoir.6-13 Other studies have also identified them with the so-called “prewetting transition”.14-27 According * Corresponding author. Phone: (33) 2 98 01 64 93. Fax: (33) 2 98 01 65 94. E-mail: [email protected]. (1) Pockels, A. Nature 1891, 43, 437. (2) Lord Rayleigh. Philos. Mag. 1899, 43, 375. (3) Devaux, H. J. Phys. 1912, 699 (No. 2), 699. (4) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848. (5) Pliny the Elder. Naturalis Historia (http://penelope.uchicago.edu/ Thayer/E/Roman/Texts/Pliny_the_Elder/home.html). (6) Adamson, A. W. Physical Chemistry of Surfaces; Wiley-Intersciences, New York, 1990. (7) Duval, X.; Thomy, A. C. R. Acad. Sci. 1964, 295, 4007. (8) Thomy, A.; Duval, X. J. Chim. Phys. 1970, 67, 1101. (9) Suzanne, J.; Coulomb, J. P.; Bienfait, M. Surf. Sci. 1973, 40, 414; 1974, 44, 141. (10) Lahrer, Y. J. Chem. Soc., Faraday Trans. 1974, 70, 67. (11) Hamraoui, A.; Privat, M.; Sellami, H. J. Chem. Phys. 1997, 106, 222. (12) Sellami, H.; Hamraoui, A.; Privat, M.; Olier, R. Langmuir 1998, 14, 2402-2409. (13) Mazeás, I.; Pe`lerin, P.; Sellami, H.; Hamraoui, A.; Olier, R.; Privat, M. Langmuir 1999, 15, 1879-1883. (14) Cahn, J. W. J. Chem. Phys. 1977, 66, 3667-3672. (15) Ebner, C.; Saam, W. F. Phys. Rev. Lett. 1977, 38, 1486. (16) Finn, J. E.; Monson, P. A. Phys. Rev. A 1989, 39, 6402. (17) Rutledge, J. E.; Taborek, P. Phys. Rev. Lett. 1992, 69, 937.

to Cahn14 and all the theorists cited hereafter, such a prewetting transition is associated with critical points in the bulk as well as in the surface and then falls within fluid-fluid transitions. But of course, liquid-gas and solid-liquid equilibria28-31 as well as smectic-nematic transitions31,32 and others more specific of polymeric systems33,34 have also been observed at various interfaces. Surface phase transitions have also been theoretically described. Abundant literature is available about not only prewetting transitions but also transitions in confined systems. Indeed, refs 35 and 36 are partially devoted to (18) Bonn, D.; Kellay, H.; Wegdam, G. H. Phys. Rev. Lett. 1992, 69, 1975-1978. (19) Bonn, D.; Wegdam, G. H.; Kellay, H.; Nieuwenhuizen, Th. M. Europhys. Lett. 1992, 20, 235-239. (20) Kellay, H.; Bonn, D.; Meunier, J. Phys. Rev. Lett. 1993, 71, 2607. (21) Nacher, P. J.; Demolder, B.; Dupont-Roc, J. Physica B 1994, 975, 194. (22) Hamraoui, A.; Privat, M. J. Chem. Phys. 1997, 107, 6936-6944. (23) (a) Shim, H.; Chatain, D.; Wynblatt, P. Surf. Sci. 1998, 415 (3), 346-350. (b) Shim, H.; Wynblatt, P.; Chatain, D. Surf. Sci. 2000, 465 (1-2), 97-102. (24) Philipps, J. A.; Taborek, P.; Rutledge, J. R. J. Low Temp. Phys. 1998, 113 (5/6), 829. (25) Hensel, F.; Yao, M. Eur. J. Solid State Inorg. Chem. 1997, 34 (7/8), 861-870; Ber. Bunsen-Ges. Phys. Chem. 1998, 102 (2), 17981802. (26) Freyland, W.; Ayyad, A. H.; Mechdiev, I. J. Phys.: Condens. Matter 2003, 15, S151-S157. (27) Indekeu, J. O.; Posahennikova, A. I.; Ross, D.; Bonn, D.; Meunier, J. J. Phys.: Condens. Matter 2002, 14 (9), 1777-4783. (28) Motschmann, H.; Lunkenheimer, K. J. Colloid Interface Sci. 2002, 248 (2), 462-466. (29) Gilanyi, T.; Meszaros, R.; Varga, I. Langmuir 2000, 16 (7), 32003205. (30) Rayss, J. J. Colloid Interface Sci. 1983, 91 (2), 376-383. (31) Dong, J.; Mao, G. Langmuir 2000, 16 (6), 6641-6647. (32) Haidara, H.; Vonna, L.; Schultz, J. Langmuir 1996, 12 (13), 3351-3355. (33) (a) Subramanyam, R.; Maldarelli, C. J. Colloid Interface Sci. 2002, 253 (2), 377-392. (b) Ohta, A.; Matsubara, H.; Tatsubuko, M.; Takanori, T.; Norihiro, I.; Makoto, A. J. Colloid Interface Sci. 2002, 256, 411-417. (34) Kitano, H.; Fukui, N.; Ohhori, K.; Maehara, Y.; Kokado, N.; Yoshiuzumi, A. J. Colloid Interface Sci. 1999, 221 (1), 58-64. (35) Rowlinson, J. S.; Widom, B. Molecular Theory of Capillarity; Clarendon Press: Oxford, 1982.

10.1021/la048067d CCC: $27.50 © 2004 American Chemical Society Published on Web 11/06/2004

Molecular Arrangements at Solution/Solid Interface

the former, whereas the latter are dealt with in, for example, refs 37-45 about gas adsorption on solids,37 adsorption of surfactants,38,39,42 polymers,40,43-45 and in particular proteins for which the understanding of interfacial behavior is paramount in biology, and others at different interfaces. Paradoxically, very few experimental observations in relation with these theories have been published despite the industrial and theoretical interest of this topic. Among them we will, here, refer to three examples.46-48 However, there is a puzzling discontinuity between the numerous experiments carried out for practical purposes and the more systematic theoretical studies. The relationship between the surface behavior and the bulk one, almost evident in theoretical studies, is nearly always missing in experimental ones. This link, which is mainly, but not totally, based on the molecular interactions, can only appear in experiments through a systematic study where different parameters, for example, the chemical nature and concentration of the components and temperature, play key roles. This is why we, here, studied three different systems with very different bulk phase properties, that is, different thermodynamic behaviors. To limit the number of changes and make easier the comparison from one system to another, all of them contained water (W) and the same solid surface: Aerosil 200, a simple pyrogenated silica. This compound was chosen for its surface homogeneity and reduced porosity,49,50 these qualities being absolutely necessary to observe surface phase changes. The solutes were successively 2,5-dimethylpyridine (2,5DMP), 2,4,6-trimethylpyridine (2,4,6TMP), and 2,3-dihydro-2,2-dimethyl-7-benzofuranylmethylcarbamate (carbofuran). To avoid the special effects due to the bulk solubility limit, in the first place we drew surface phase diagrams from the beginning of the adsorption isotherms and systematically compared them with the bulk ones in the form of liquid-solid equilibria. This gave us information about the nature of the surface phase and allowed us to supplement and generalize our previous results obtained on the water (W)-2,5DMP-Aerosil 200 system.13 Here, we also give the liquid-liquid bulk phase diagram of this mixture which is very similar to the 2,6DMP-water one.51-53 This dual set of phase diagrams, both in the surface and in the bulk, is absolutely unique because no (36) Dietrich, S. In Phase transitions and Critical Phenomena; Domb, C., Lebowitz, J. L., Eds.; Academic Press: London, 1988; Vol. 12, pp 1-218. (37) Ball, P. C.; Evans, R. J. Chem. Phys. 1988, 89, 4412. (38) van Roij, R.; Dijkstra, M.; Evans, R. J. Chem. Phys. 2000, 113 (17), 7689. (39) Barbero, G.; Evangelista, L. R. Phys. Rev. E 2002, 65, 031708. (40) (a) Szleifer, I.; Carignano, M. A. Macromol. Rapid Commun. 2000, 21 (8), 423-448. (b) Fang, F.; Szleifer, I. J. Chem. Phys. 2003, 119 (2), 1053-1065. (41) Omata, K. J. Non-Cryst. Solids 2002, 312; 481. (42) Lelidis, I.; Galatola, P. Phys. Rev. E 2002, 65, 010701. (43) Muller, M.; Binder, K.; Albano, E. V. Int. J. Mod. Phys. B 2001, 15 (13), 1867. (44) Muller, M.; Binder, K. Phys. Rev. E 2001, 63 (2-1), 021602. (45) Chajer, M.; Gujrati, P. D. J. Chem. Phys. 1998, 109 (24), 11018. (46) Wilkinson, N. J.; Alam, M. A.; Clayton, J. M.; Evans, R.; Fretwell, H. M.; Usmar, S. G. Phys. Rev. Lett. 1992, 69 (24), 3535. (47) Maniwa, Y.; Kataura, H.; Abe, M.; Suzuki, S.; Achiba, Y.; Kira, H.; Matsuda, K. J. Phys. Soc. Jpn. 2002, 71 (12), 2863. (48) Thommes, M.; Findenegg, G. H.; Schoen, M. Langmuir 2000, 11, 2137. (49) Ferch, H.; Fratzscher, H. Kautsch. Gummi 1967, 20, 578. (50) Papirer, E.; Balard, H. Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1995; Vol. 99, pp 479-502. (51) Perron, G.; Quirion, F.; Lambert, D.; Ledoux, J.; Ghaicha, L.; Bennes, R.; Privat, M.; Desnoyers, J. J. Solution Chem. 1993, 22, 107. (52) Andon, R. J. L.; Cox, J. D. J. Chem. Soc. 1952, 4601. (53) Cox, J. D.; Herington, E. F. G. Trans. Faraday Soc. 1956, 52, 926.

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alike combination of data is available in the literature. It leads to new interesting insights on surface behavior. Theoretical Aspects The prominent fact to be retained from the existing statistical theories is the role of factors such as bulk molecular interactions, bulk-surface interactions, surface potential, and statistical correlations; all of them are only evoked in classical thermodynamic analysis. However, classical thermodynamics helps one to understand why, under some constraints, adsorbed molecules have to reorganize into new phases. When surface demixing occurs from a semi-infinite solution, from a general point of view the most surprising fact is that the bulk remains obstinately one-phase. A system is generally composed of two domains, the bulk and the surface. Before surface demixing, it contains two phases, one in each of these domains. At equilibrium, each component has the same chemical potential in both phases. At surface demixing, three phases, that is, one bulk phase and two surface phases, are coexisting. Here, again, each component has the same chemical potential in all the phases. The reason no demixing takes place within the bulk, whereas the surface demixes despite alike chemical potentials, is that the variation of each µ versus the domain composition (classically the mole fraction of each component) is not the same because the surface composition is changing at a faster rate than the bulk population. For instance, the surface quickly becomes more solute-rich than the solution (Γ21, the relative adsorption of component 2 with respect to component 1, is then positive). So, in the case of the sole surface demixing, the use of the stability criterion according to van der Waals and Kohnstamm (ref 54, pp 232 and 240 or ref 55, p 938) leads to

∂µi/∂xi > 0

(1)

for the i component in the bulk and to

∂µiσ/∂xiσ > 0

(2)

for the i component in the σ surface domain. This is illustrated in Figure 1a for a solute denoted 2 and for demixing between a liquid mixture and a pure 2-crystal: when all the values of x2σ are successively taken, demixing occurs and persists as soon as ∂µ2σ/∂x2σ becomes and remains negative. On both sides of the negative slopes, there are two short domains where ∂µ2σ/∂x2σ is positive, but within the surface they may correspond to either a stable diphasic or a metastable monophasic. To get another form for the stability criterion, one may use the combination of chemical potentials that gives the molar Gibbs free energy:

g ) x1µ1 + x2µ2

(3)

At constant pressure, the stability criterion takes the following form, with respect to the 2-component:

(∂2g/∂x22)T,P > 0

(4)

which leads to the so-called common tangent construction depicted in Figure 1b. Between both points of contact, either (∂2g/∂x22)T,P is negative and the system is diphasically (54) Prigogine, I.; Defay, R.; Everett, D. H. Chemical Thermodynamics; Longmans: London, 1969. (55) (a) Berry, R. S.; Rice, S. A.; Ross, J. Physical Chemistry; Wiley: New York, 1980. (b) Widom, B. J. Phys. Chem. 1996, 100, 13197.

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Figure 1. Thermodynamic conditions for a surface phase change. (a) Variation of the chemical potential of component 2 with the surface composition (mole fraction) in the case of a liquid/pure solid 2 demixing. Equilibrium is observed as ∂µ2σ/ ∂x2σ > 0; stable equilibrium is depicted by a thick line, and the metastable one by a thin solid line. The horizontal thick line shows the equilibrium between the pure 2-crystal and the surface liquid phase of composition x2sat at saturation. The dashed line is the µ2 variation without demixing. (b) Variation of the molar Gibbs energy in the same case. There, the equilibrium criterion with respect to diffusion is (∂2gσ/∂x2σ2)T,P > 0. Stable equilibrium is represented with a thick line, and the metastable one with a thin solid line. Like above, the dashed line shows the g variation without demixing.

stable, or (∂2g/∂x22)T,P is still positive, which permits metastable monophasics. The two contact points correspond to the composition of the two phases at equilibrium: a liquid mixture and a pure 2-crystal in the chosen case study. The experimental construction of diagrams such as Figure 1 is impossible; the only thing that can be obtained through measurement of µi variations with the mixture composition is clues about the overall shape of diagrams. However, one can get the nature and composition of the two phases appearing at the tangency points (Figure 1b) and the ends of the horizontal segment (Figure 1a); finally, the collection of such data constitutes the so-called phase diagram. In the same way, experiments allow one to detect metastable states, either in the bulk or in the surface; this observation is a good criterion in phase change assessment. Experimental Section Systems. At atmospheric pressure and room temperature, carbofuran is solid and weakly soluble in water. 2,5DMP and 2,4,6TMP are liquid under ordinary conditions. Their mixtures with water exhibit, at least, a lower consolute point. W-2,5DMP system liquid-liquid coexistence had been studied by Andon and Cox52 and Cox and Herington.53 To our knowledge, until now the whole W-2,4,6TMP phase diagram as well as solid-liquid equilibria for W-2,5DMP were both unknown. In both systems, solid phases appear some tens of degrees below the water crystallization temperature. Bulk Phase Diagrams. In binary systems, the first step to determine phase diagrams is thermal analysis: it is done by varying the temperature and then detecting the phase changes on an appropriate parameter. The next step is the characterization of the phase nature. In the present study, only the simple thermal analysis was used. The solubility limit of carbofuran crystals was determined by letting crystals dissolve in water under gentle agitation, in a thermoregulated oven. Dissolution equilibrium was reached after about 3 weeks (it was determined by successive titrations of the supernatant), and the supernatant was analyzed using UV spectroscopy with a KONTRON Uvikon 930 spectrophotometer operating at 276 nm. The experiment was repeated at different temperatures. The liquid-liquid equilibrium for the 2,4,6TMP aqueous mixture was visually observed with the cloud point method and controlled by differential scanning calorimetry (DSC). The solid-liquid demixings for both W-2,5DMP and W-2,4,6TMP were determined by DSC with a DSC 92 device by

Andrieux et al. SETARAM. Such a measurement is particularly difficult in the case of liquid mixtures; this is especially true for the systems under study because of persistent metastable states. However, satisfactory diagram shapes were obtained from the melting DSC diagrams. Adsorption Isotherm Measurement. The classical gravimetric method was used. Samples of silica powder (0.4 g) were weighted into centrifuge tubes; 10 g of aqueous solution of carbofuran or 20 g of aqueous solution of 2,5DMP or 2,4,6TMP was added and smoothly agitated during the time needed to reach equilibrium. According to previous adsorption kinetic experiments, we let adsorption develop for 2 h, this time being sufficient for the three systems to reach equilibrium. During agitation, the samples were kept in a thermoregulated oven, whose temperature was known at more or less 0.2 K. Prior to adsorption determination, silica powder was removed from the suspensions by centrifugation at 2000g in a FirLabo centrifuge until getting a clear supernatant, which was then titrated by UV spectrometry. We operated at 267 nm for 2,5DMP, 253 nm for 2,4,6TMP, and 276 nm for carbofuran. To express the relative adsorption of solute 2 with respect to solvent 1, let n0 be the total number of moles in the sample, x20 the initial mole fraction of the solute, and x2 the equilibrium mole fraction of the supernatant after adsorption. It gives

Γ2,1 ) (1/x1)n0(x20 - x2)/ms where s is the specific area of silica, m is the mass of silica, and x1 is the mole fraction of water in the supernatant. Γ2,1 must clearly be identified with the relative adsorption of solute 2 with respect to solvent 1 as defined by Gibbs; it combines the Gibbs’ excesses of both solute and solvent56a,b (see appendix 1). Chemicals. 2,4,6TMP (99% pure) and 2,5DMP (96% pure) supplied by Aldrich were colored and contained water; both were, therefore, purified by distillation. Carbofuran certified for laboratory use, 99.9% pure, was supplied by Dr. Ehrenstorfer, GmbH. 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 because of its high division state. According to the supplier, silica particles had a mean diameter of about 15 nm and carried three OH groups per nm2; it is worth noting that this number is much lower than the one in silica prepared by precipitation methods. There were, thus, 1017 particles in 1 g of Aerosil. Most of the experiments were carried out on silica samples of 0.4 g mixed with about 20 cm3 of solution, or 0.2 g for 10 cm3. A rough calculation, which implied several particle diameters in relation with the number of adsorbed molecule layers, showed that the shortest distance between two particles fell within 64.0 and 40.4 nm: these lengths correspond to no adsorption and 40 layers, respectively. Because the latter was never attained over the present study, silica particles were quite free in the solution, and particles were at distances sufficient to avoid capillary condensation.57 In addition, one should note that the silica porosity, measured by BET adsorption with different gases, was very low and that the mean radius of the pores was 6.688 nm (mesoporosity) with nearly no microporosity. According to the supplier, the distribution of particle diameters has a maximum at d ) 10 nm and the particles are almost spherical. These data together with the very small microporosity and the mean size of the mesopores show that silica exhibits dense and regular beads with pieces of homogeneous surfaces whose curvature radius is large with respect to the diameter of the studied organic molecules. Our previous experiments together with those made by pioneer colleagues7,8 have taught us that these adsorbent properties are of key importance in the observation of surface transitions. The surface state of silica highly depends on pH. At pH higher than 8.0, it even dissolves in water with, however, very slow kinetics. The pKa values of both pyridine derivatives are (56) (a) Defay, R.; Prigogine, I.; Bellemans, E.; Everett, D. H. Surface Tension and Adsorption; Longmans: London, 1966. (b) Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces; Wiley VCH: New York, 2003. (c) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous Solution; Butterworth: London, 1965. (57) Privat, M.; Amara, M.; Hamraoui, A.; Sellami, H.; Meár, A. M. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 626.


Figure 2. W-2,4,6TMP liquid-liquid solubility curve (in weight fraction), determined by the cloud point method and verified by DSC, versus temperature. Circles correspond to the first method and squares to the second. [pKa]2,5DMP ) 6.40 and [pKa]2,4,6TMP ) 7.43.56c In this study, the natural pH was around 9.5 for 2,5DMP and for 2,4,6TMP and did not fluctuate much (as shown by control measurements). So, we could consider that changes in concentration caused no effect on the surface state of silica.

Results and Discussion Bulk Phase Diagrams. Figure 2 shows the liquid/ liquid equilibrium for the W-2,4,6TMP system. It is very similar to that of the dimethylpyridines52 and has a lower critical point of coordinates:

Tc ) 6.0 ( 0.3 °C

xc ) 0.045 ( 0.010 (weight percent XC ) 28%) (5)

Figure 3 depicts the liquid/solid diagrams for W-2,5DMP (Figure 3a), W-2,4,6TMP (Figure 3b), and W-carbofuran (Figure 3c). As expected, the diagram produced by W-2,5DMP looks very similar to the one reported for W-2,6DMP,51 with a 1-1 compound (50 mol %) and two eutectics, E1′ and E2′. W-2,4,6TMP exhibits a more complicated pattern, with clearly 1-3, 1-2, 2-3,

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and 1-1 compounds (25, 32, 42, and 50 mol %) and maybe more in concentrated solutions; eutectics seem to be six, and in addition, structural changes are noticeable for the solids around the 50 mol % (parallel horizontal lines). Because of very persistent hysteresis,64 the determination of both diagrams was very difficult. Compared to what we know, the W-carbofuran diagram seems simpler; however, we explored neither the occurrence of different solid compounds nor those of different solid structures, and then the so-observed solubility limits may have been metastable ones. However, adsorption isotherms were measured above these limits for consistency in our determinations. It is worth noting that these equilibria were observed at ordinary temperatures, whereas the two first systems required us to work at much lower temperatures. Isotherms. Figure 4 shows several new versions of isotherms at 30 and 38 °C for the W-2,5DMP-Aerosil 200 system already published12 and two new isotherms measured at 60 and 65 °C. Figure 4a,b clearly shows particularly well-formed vertical steps along with several different metastable plateaus at different locations. Around the mean values of adsorption in the diagram, the magnitude of the error is the third part of the dot size in the figure and almost the sixth part at the beginning of the curve. It is worth noting that the adsorption scale on both parts of the figure, due to the large values of Γ21 close to the bulk solubility limit (shown by a vertical dashed line on the figure), is unusually small (between 0 and 5 × 10-9 mol cm-2 instead of 0 and some 3 to 5 or 6 × 10-10 mol cm-2). Figure 4c,d shows a magnification of the starting section of both curves at a more classical scale where steps clearly appear because they are far much higher than the error bars. The repeatability of each plateau was tested with the difficulties inherent to metastable phenomena. It is obvious that plateaus depicting stable surface states exhibit more frequent experimental points than metastable ones. These observations are totally compatible with the assessment of

Figure 3. Liquid-solid solubility curves. (a) DSC-determined W-2,5DMP showing two eutectics E1′ and E2′ and one definite compound 1-1 at 50 mol %. Squares and triangles correspond to two different peaks on a DSC curve at a given total W-2,5DMP mixture composition, that is, to two different physical forms. (b) DSC-determined W-2,4,6TMP showing six eutectics E1, E2, E3, E4, E5, and E6 and several definite compounds, at least 1-3, 1-2, 2-3, and 1-1 (25, 32, 42, and 50 mol %). (c) W-carbofuran, determined by titration of solutions in equilibrium with carbofuran crystals. Concentrations of solutions in equilibrium with solid crystals are denoted “s” for “solubility (limit)”.

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Figure 4. Adsorption isotherms of 2,5DMP from aqueous solution onto silica (Aerosil 200). (a) Complete isotherm at 30 °C. The liquid-liquid solubility limit is shown with a vertical line. Around the mean values of adsorption in the diagram, the error bar order of magnitude is half of the dot size in the figure, and almost the quarter at the beginning of the curve. Vertical segments between plateaus indicate surface phase changes. Superposed parallel plateaus correspond to a “hysteresis”, that is, to the existence of metastable surface states, comparable to supercooled states in bulk, or even to “pre-solid”. The dashed frame shows the part of the figure magnified in part c, and the dotted frame shows the part magnified in part e. (b) The same at 38 °C. The dashed frame shows the part of the figure magnified in part d. (c and d) Magnified beginnings of the isotherms at 30 and 38 °C, respectively. Both series of points in Figure 3c (squares and diamonds), obtained by two different experimenters at different times, show the reproducibility of the plateaus. (e) Magnification of a “vertical step” on the isotherm at 30 °C. Two series of points, obtained under very different conditions (several years later for crosses, with differently prepared silica and 2,5DMP and another experimenter) clearly show the very good reproducibility of both plateaus and vertical steps. (f and g) Complete isotherms at 60 and 65 °C, respectively. The curves still show vertical steps and plateaus, but their layout is very different from those observed at the other temperatures.

experimental data quality from error bars. Figure 4c also illustrates a reproducibility test of the starting section plateaus through two independent series of points: white squares depict experimental data produced by a different experimenter 4 years before the black diamond ones. Figure 4e illustrates another reproducibility test concerning both plateaus and vertical parts where white diamonds depict experimental data produced by a different experimenter 4 years before the cross ones. The mixtures for diamond points were made of Aldrich 2,5DMP and Aerosil 200 freshly synthesized, whereas those for cross points consisted of another 2,5DMP prepared by Epsilon Chimie

(Brest, France) and an older stock of silica previously ovendried at 190 °C for 1 week. Both reproducibility tests are quite convincing. All the systems concerned with this study were tested in a similar way. Such tests provide convincing evidence of the reality of the stairlike shape of the isotherms. In addition, it is worth noting that more than 10 experimentalists have contributed to this study. Figure 4a-e shows results previously published under a different form in refs 12 and 13. On the other hand, Figure 4f,g presents new isotherms, at 60 and 65 °C. Together with Figure 4a,b and the other ones contained in the above references, all these figures demonstrate, through stairs


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Figure 5. Adsorption isotherms of 2,4,6TMP from aqueous solution onto silica (Aerosil 200) at five temperatures. Error bars at the tops of the curves are nearly twice the triangle perpendicular height while at the bottom they correspond to a third of this height. Part f is a magnification of the starting part of the 10 °C isotherm. Error bars at the tops of the curves are nearly three times the triangle perpendicular height while at the bottom they correspond to this height. Error bars allow the drawing of numerous and very long plateaus. Such magnifications have been used to draw the plateaus at the other temperatures. Conversely to Figure 4, metastable points (hysteresis) are extremely frequent, which makes the identification of vertical steps almost impossible.

shifting and variations of plateau lengths and horizontality, how temperature strongly affects the shape of isotherms. Figure 7a is issued from magnifications of figures such as Figure 4c,d; but because it was made from magnification at the start of isotherm, it, thus, does not show the surface phase diagram close to the 2,5DMP solubility limit. Figure 5a-e describes, at five temperatures, the 2,4,6TMP isotherms where data are represented by triangles. Error bars at the top of the curves are nearly twice the triangle perpendicular height while at the bottom they correspond to a third of this height. Figure 5f is the magnification of the starting section of Figure 5e at a more classical scale (10 °C isotherm). Error bars at the

top of the curves are nearly three times the triangle perpendicular height while at the bottom they correspond to this height. Comparison with Figure 4 highlights the presence of numerous plateaus and metastable states in Figure 5. As a result of the close vicinity of the plateaus and to the numerous metastable points, it is almost impossible to draw the vertical steps, but a thorough examination of these plateaus evidences the presence of many different surface states. Despite the quite disordered aspect of the experimental points, the error bars on each point and the repeatability of the plateaus together with their reproducibility through many experiments carried out by four different experimentalists allow the present discussion.

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Figure 6. Adsorption isotherms of carbofuran from aqueous solution onto silica (Aerosil 200) at four temperatures. Error bars are nearly similar to the triangle perpendicular height. Vertical lines indicate the solubility limits, and the horizontal lines depict the adsorption plateaus. Some of these plateaus seem essentially metastable (dashed horizontal lines); several vertical steps clearly appear. Note the very low values of adsorption compared to those of the other systems.

It is worth noting that the other isotherms displayed in Figure 5 and determined at four other temperatures exhibit alike shapes and systematic temperature-induced changes with, at the start of isotherms, well-differentiated plateaus further moving closer and closer with rise in temperature. Figure 6 presents the isotherms of W-carbofuranAerosil 200 at four temperatures: 35, 30, 20, and 10 °C. The curve at 25 °C was published in ref 58. Error bars are roughly the triangle perpendicular height. All curves show a very clear vertical step, which we previously attributed to a phase change.58 But, the horizontal parts on each curve (except maybe at 25 °C) are split into, at least, two lines, which indicates the existence of several surface states. In addition, at low concentrations, one should also note a vertical step, whose height decreases with rising temperatures; at just above 25 °C it seems to vanish to be replaced, at 30 °C, by a higher one observed at a different location in the final phase diagram (see Figure 7c). Finally, the curve at 35 °C seems to show only inflections instead of vertical steps. Surface Phase Diagrams. To draw the surface phase diagrams, we used the method already described in ref 13, with slight changes to take into account the specificity of our new data. The method used can be easily followed from Figure 4b and the corresponding part of Figure 7a. In these figures, numbers and Greek letters show isotherm plateaus and vertical parts, respectively. For a given temperature, for example 38 °C, the R, β, γ, δ, ... vertical parts taken on the isotherm are reported as segments on the same horizontal line parallel to the abscissa axis of a (T-Γ21) plane (Figure 7a). The same operation is done for other temperatures along the vertical T axis. Then, segments are joined by their starts, for instance all the points 1 of R segments. This gives one line on the (T-Γ21) diagram. Then they are joined by their ends. The operation is repeated for the other segments β, δ, γ, .... This procedure (58) Meár, A. M.; Privat, M. Pestic. Sci. 1998, 53, 172.

allowed us to draw the phase coexistence limits and, thus, the surface phase diagram. As done for the W-2,5DMP-Aerosil 200 system (for the more dilute studied solutions only, i.e., in fact not too close to the solubility limit) and whenever possible, we plotted horizontally on a T-Γ21 diagram the vertical steps taken from the isotherms. In the case of carbofuran we used the two evident vertical steps of the isotherms. Because other vertical steps in the carbofuran isotherm would need too many experimental points, we took into account the split of plateaus by simply drawing a horizontal segment equal to the interplateau distance, ignoring the exact location of the vertical part, because it is not necessary to construct the diagram. We did the same for the 2,4,6TMP isotherms because the plateaus were too close to allow differentiation between metastable points on one hand and vertical steps on the other hand. Then, the “step” ends were joined together. However, some checks were needed to keep drawing consistent with the details of original isotherms. For instance, the first steps in Figure 6c,d are decreasing with T rise and become almost null at 25 °C (ref 58); thus, it is not sensible to attribute the existence of the Figure 6b first step (at 30 °C) to the same kind of phase change as for the other first steps. Building the surface phase diagram may be assimilated to a jigsaw game whose last drawing results from a series of “try and correct” operations. Uncertainties, essentially due to the metastable behavior of the systems, make the study more complex. Then, at last, the most consistent surface phase diagram appears. Some additional comments need to be made prior to the analysis of the obtained diagrams. First, the surface composition is given by the Γ21 parameter, which is all but a surface mole fraction (see appendix 1); this leads to a distortion in the diagram shape with respect to the shape it would have had in the case where x2σ had been used. Second, one must be aware that the sum of interactions


Langmuir, Vol. 20, No. 25, 2004 11019

Figure 7. Surface phase diagrams drawn from the three sets of isotherms similar to those of Figure 4b,d, 5, and 6, either from vertical steps (parts a and c, 2,4DMP and carbofuran) or from the distances between horizontal plateaus (part b, 2,4,6TMP). (a) W-2,5DMP-Aerosil 200 system. Two eutectic points E1′ and E2′, one definite compound between them, and two solid solution lines appear roughly below Tw, the wetting temperature for the liquid/liquid diphasic of the system. Above Tw, another phase behavior is noticeable and may correspond to some mesophases (more mobile ones). This diagram needs to be compared to the bulk phase one in Figure 3a and to the more complicated pattern of Figure 8. On Figure 7a and Figure 8, similar points are located by letters A-C. (b) W-2,4,6TMP-Aerosil 200 system. On the left-hand part of the figure, transition lines are easily drawn, but only some tie lines (vertical steps on isotherms) are available. The top of the drawing is purely speculative, but shows how these phase changes could be assimilated to the bulk phase changes at low temperatures (Figure 3b). Like in Figure 3b, the right part of the figure is very complicated, and no drawing was attempted. (c) W-carbofuran-Aerosil 200 system. The surface phase diagram seems to exhibit two eutectic points E1′′ and E2′′, one definite compound between them, and two solid solution lines. It is a bit more complicated than the current available knowledge about the bulk diagram (Figure 3c), whose reflection in part c is probably the second curve from the right-hand side (solid/liquid solubility limit when a solid solution exists). The isotherm at 35 °C no longer shows vertical steps but exhibits points of inflection, which are drawn on the figure. To better assess the similarities between Figure 7c and Figure 8, similar points are located by letters A-C.

in the interface creates a “surface field” unexplained in experiments of this kind and constituting a new variable with respect to the bulk. Third, but this is not the least point, the representations of Figure 7 have to be read while keeping in mind that, there, the surface always contains water even if Γ21 is approximately equal to Γ2 according to eq A9 because x2/x1 is very small in very dilute solutions. However, Γ1 can be very large. At Γ21 ≈ 0 there is, in fact, water on silica that may be strongly linked to the solid. Figure 7a-c respectively shows 2,5DMP, 2,4,6TMP, and carbofuran surface diagrams. The three figures share two common characteristics. First, the simplicity, or complexity, of the surface phase diagram reflects the same features as the bulk solid-liquid diagram, and some obvious similarities in the shape should be noticed. Second, each of the surface phase diagrams exhibits transitions very close to the null concentration; this feature is missing in the bulk diagrams. It suggests the presence of solid solutions on the surface.13 The equivalent in the bulk is schematized in Figure 8. Generalization about our first observation is quite satisfactory and, as already pointed out, brings to the forefront a very interesting view of the surface behavior in this kind of system. The isotherm starts tangentially to the x2 abscissa axis (with 2 being the solute): so, its shape indicates that, at first, water is more attracted or already attached to the silica surface whose affinity for water is well-known. Solute comes onto the surface only when the bulk concentration is increased;

Figure 8. Schematic phase diagram showing, besides eutectic points and 1-1 compound, solid solution areas denoted 1, 2, 2′, and 3.

the surface phase diagram is in favor of solute dissolution in “surface solid water”. Afterward, solute becomes in sufficient abundance to form either another solid, likely a “definite compound”, or a “liquid solution” depending on the temperature. The names given to these surface phases are, of course, only analogies with the bulk phases, but they may reflect changes in the mobility of water and solute molecules on the surface along with some tendency to surface aggregation between both. Figure 7b displays the 2,4,6TMP surface phase diagram. Despite the very numerous adsorption data contained in the isotherms, this diagram is the least achieved among the three ones. Only four isotherms have been completed, and one is only partly known. However, the existing points seem to show the beginning of a phase diagram very similar in shape to the bulk phase one, except, as already

11020


mentioned, when x2 , x1 (very dilute solutions). We only drew the left part of the figure, the right one being far too complicated and incomplete. As a consequence, the bulk diagram was convincingly determined only on its left part. Then, a close examination of Figure 7c about carbofuran highlights a more complex surface phase diagram than the one issued from our knowledge of the bulk diagram; in Figure 7c the equivalent of the latter is probably the second curve from the right-hand side (solid/liquid solubility limit whenever a solid solution exists). The transitions described by the left part of the figure likely occur, or would occur, in the bulk below 0 °C. Examination of the 2,5DMP diagram in Figure 7a shows that its lower part is very similar to that in Figure 3a (except for a general twist attributable to the surface composition variable, whose nature is very different from that of a mole ratio, as already explained). On the other hand, the upper part is obviously different. Large zones of new structured phases, whose shape recalls the mesophases of either soaps or other surfactants, seem to exist.60 Gel-like or liquid-crystal-like phases should exist on the surface at a high temperature, but this needs further confirmation. It is certain, and quite worth being noted, that this radical change in structure occurs at the wetting transition temperature, Tw, of the system, or in its more or less close vicinity, depending on the bulk composition. The wetting transition is observed in the liquid/liquid bulk equilibrium. Here, the W-rich phase completely wets between silica and the 2,5DMP-rich phase for Tc < T < Tw, but this is not true when T > Tw; Tw is highly dependent on the silica surface state. Figure 7a shows that any temperature increase strongly affects the silica surface in contact with the solution, and this is very likely, even in a demixed solution. The most radical change occurs at Tw and must be at the origin of the wetting transition. But a similar behavior is observable in Figure 7c about carbofuran. The temperature, about 35 °C, at which the bulklike behavior stops agrees with a previous report59 about the same system: a wetting transition at the silica/ solution/vapor contact had been observed “between 25 and 40 °C”. Namely, at and below 25 °C, close to the solidliquid coexistence line, the solution completely wets silica in contact with vapor, whereas at 40 °C, the contact angle is zero (no other values for the contact angle have been measured). A first analysis of the isotherm aspect led us to localize the transition “around 30 °C”. According to Figure 7c, it would fall within 30 and 35 °C, and then the surface structural change obviously causes the wetting transition. Finally, it is worth noting that at very dilute bulk concentrations (x2 < 0.01 in general and commonly x2 < 0.005, that is, x2 , x1) similar to the ones used in our three experiments, the Γ21 values are commonly assimilated, as already noted, to solute surface concentrations (Γ21 ≈ Γ2), and then reasoning is often focused only on molecule 2. However, the phenomena described in this study evidenced strong implication of both solute and solvent in surface behavior. Probably because of the surface confinement and more specifically of the reduced molecular mobility, the phenomena we named “surface changes” are similar to the phase changes observed in the bulk at far lower temperatures. So, the adsorbed solute behavior cannot be assimilated to a simple condensation of a molecular “gas” as historically done in Langmuir films. (59) Meár, A. M. Thesis, Universite´ de Bretagne Occidentale, Brest, France, 1997. (60) Laughlin, R. G. The aqueous phase behaviour of Surfactants; Academic Press: London, 1994; p 111.

Andrieux et al.

In fact, the role of silica, considered as the wall in adsorption studies and theoretical considerations, needs to be understood in its whole complexity. First, surface changes governed by interactions with a wall can be evidenced only on homogeneous surfaces; in the case of heterogeneity, the influence of the several surface microdomains with different properties leads to a mean effect which suppresses any observation of a first-order transition. The second key feature about the wall is its chemical nature.61 In material systems, chemistry mainly rules the molecular interactions and, among them, those between the wall and the adsorbed particles of concern. The wall being an oxide favors a strong interaction with the π systems contained in all the solutes and semilabile hydrogen atoms of water. The observation of changes in surface phases will, indeed, depend on the hydration state of the substrate: with increasing hydration of the surface, the steps on the isotherms tend to vanish. Conclusion The determination of several adsorption isotherms on three systems, all made of aqueous solutions in contact with Aerosil 200, allowed us to plot surface phase diagrams, still confined in the most dilute solutions under study. Whereas liquid/solid bulk diagrams for each system were appreciably different, the surface diagrams looked similar and showed that the surface-induced molecular confinement leads to structures similar to those obtained in the bulk through a noticeable reduction in temperature. In every surface diagram, one can detect the presence of structured “ice-like” or “solid-solution-like” water, which constitutes the most apparent effects of the existence of solvent at the surface. Among the systems that exhibit wetting transitions against silica, passing the wetting temperature corresponds to a noticeable change in the nature of phases. However, this aspect of phase change phenomena deserves a more detailed study than the present one. The unavoidable experimental method used in our study needs to be completed by physical methods allowing identification of the nature of the surface phases and confirmation of the phase transitions. NMR experiments are presently carried out. In our previous study about carbofuran, we showed that solubility limits and wetting transitions could play a role in environmental events implying the adsorption of such an insecticide on soils.58 In the same way, the solubility limit of some pollutants can affect the water surface content and the passing of the pollutants to the atmosphere through the surface by formation of aerosols.58,62 According to other adsorption experiments, it seems that not only temperature changes but also configuration changes in Aerosil 200 surfaceadsorbed polymers induce modifications in the formation of composite materials.63 The elucidation of such mechanisms should be helpful to control these different types of formation. Theoretical analyses would be very constructive. Appendix 1 Given the importance of water in the interpretation of our results, it is worth recalling the significance of the (61) Huruguen, J. P.; Amara, M.; Meár, A.-M.; Hamraoui, A.; Olier, R.; Privat, M. J. Phys.: Condens. Matter 2001, 13, 4939. (62) Sadiki, M.; Quentel, F.; Elleóuet, C.; Huruguen, J. P.; Jestin, J.; Andrieux, D.; Olier, R.; Privat, M. Atmos. Environ. 2003, 37, 3551. (63) Derkaoui, N.; Grohens, Y.; Privat, M. Unpublished results. (64) Hysteresis is a term widely used by theorists (see ref 37), but this use of the word is probably improper. It refers, in fact, to the existence of metastable states, similar to supercooled liquids, whose stable state is solid crystal, for example, liquid water at T ) -10 °C.


Langmuir, Vol. 20, No. 25, 2004 11021

term Γ21, determined by the gravimetric method in the present study. It will be shown to be the relative adsorption of component 2 (here the solute) with respect to component 1 (here water, the solvent). Relative Adsorption. Γ21 was introduced by Gibbs as a thermodynamic parameter characterizing adsorption; this parameter is of paramount importance because it fits well thermodynamic relations and is the only one that can be exactly experimentally determined. Indeed, many other experimental parameters depend on the adsorption but can lead to its evaluation only through models. From a model describing a liquid mixture L of components 1 and 2 in contact with a plane solid S, the contact area being A, Gibbs first defined the excesses as follows:

Γ21 ≈ Γ2 - Γ1(c2L/c1L)

(A7)

Γ21 ≈ Γ2 - Γ1(n2L/n1L)

(A8)

Γ21 ≈ Γ2 - Γ1(x2/x1)

(A9)

Γ1 ) (1/A)(n1 - c1LVL - c1SVS) L

L

S

S

Γ2 ) (1/A)(n2 - c2 V - c2 V )

(A1) (A2)

where VL and VS are the volumes of the liquid and the solid, respectively, with both being assumed homogeneous up to a hypothetical plane assumed to be located within the interfacial zone (in fact, heterogeneous). Component i has the concentration ciL in the liquid bulk and ciS in the solid bulk; ni is the total number of i molecules in the system. The excesses depend on the position of the hypothetical plane, but correctly combining the excesses will provide another quantity independent of this position. If V is the total volume of the system, VS ) V - VL and

Γ1 ) (1/A)[n1 - c1LV + (c1S - c1L)VL]

(A3)

Γ2 ) (1/A)[n2 - c2LV + (c2S - c2L)VL]

(A4)

In Equation A9, x2 and x1 are the mole fractions of components 2 and 1 in the liquid bulk, respectively. One should note that, with the same approximation c1S ≈ 0 ≈ c2S, the following relations are got from eqs A1 and A2:

Γ1 ) (1/A)(n1 - c1LVL) ) (1/A)(n1 - n1L) (A10) Γ2 ) (1/A)(n2 - c2LVL) ) (1/A)(n2 - n1L) (A11) Gravimetric-Method Adsorption Parameter. Substituting eqs A10 and A11 in eq A9 gives the following relations:

Γ21 ) (1/A)[(n2 - n2L) - (n1 - n1L)(x2/x1)]

(A12)

Γ21 ) (1/A)[(n2 - n2L) - (n1 - n1L)(n2L/n1L)]

(A13)

Γ21 ) (1/A)[n2 - n1(n2L/n1L)] ) (1/A)[n2 - n1(x2/x1)]

Equation A14 is ready for use with the experimental parameters issued from the gravimetric method. Using the mass m of silica as used in a given experiment and the specific area s of the solid, A ) ms. The total matter quantity n0 can be estimated from weights, as well as the initial (before adsorption takes place) mole fraction x20. The final mole fraction x2 can be calculated from the supernatant content. One should be reminded that x20 + x10 ) x2 + x1 ) 1. So,

Eliminating VL by multiplying eq A3 by (c2S - c2L)/(c1S - c1L) and subtracting eq A3 from eq A4 lead to

Γ21 ) (1/ms)(1/x1)[n2x1 - n1x2] ) (1/ms)(1/x1)[n0x20x1 - n0x10x2]

Γ2 - Γ1[(c2S - c2L)/(c1S - c1L)] ) (1/A)[n2 - c2LV -

) (n0/msx1)[x20 - x2]

(n1 - c1LV)(c2S - c2L)/(c1S - c1L)] (A5) The right-hand side of the expression is obviously independent of the position of the hypothetical Gibbs plane, and so the left-hand side expression is also independent; it defines the relative adsorption of component 2 with respect to component 1:

Γ21 ) Γ2 - Γ1[(c2S - c2L)/(c1S - c1L)]

(A6)

This is the parameter determined by the gravimetric method as shown below. In general, the components of the liquid do not really enter the solid. So, c1S ≈ 0 ≈ c2S and

(A14)

(A15)

Equation A15 is the expression actually used to calculate Γ21 from the experiments. It is now clear that the adsorption parameter obtained from eq A15 and then from the experiments is the relative adsorption of Gibbs, which is a combination of both excesses, solute and solvent. The hypothesis Γ1 ) 0, sometimes used to interpret some experimental data, is absolutely extra-thermodynamical; it has never been used by Gibbs and is missing in eq A15. So, the presence and role of solvent in the phenomena described in the above study are correctly discussed from the experimentally determined Γ21. LA048067D

Surface Phase Diagrams: Evidence of Molecular Arrangements at the

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