© Copyright 2000 American Chemical Society
OCTOBER 3, 2000 VOLUME 16, NUMBER 20
Letters Capillary Condensation and Desorption of Binary Mixtures of N2-Ar Confined in a Mesoporous Medium M. A. Alam,* A. P. Clarke, and J. A. Duffy H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, U.K. Received March 23, 2000. In Final Form: July 5, 2000 The adsorption and desorption of the binary system N2xAr1-x confined in the cylindrical 4 nm diameter pores of Vycor glass has been studied using positron annihilation spectroscopy. When the Vycor sample was suspended in the gaseous mixture, the gas-liquid and liquid-gas phase transitions of the confined fluid exhibit hysteresis which varies little with composition, indicating that the confined mixture behaves like a simple fluid. However, if bulk mixture surrounding the samples was allowed to condense, the liquidgas transition exhibited a much larger degree of hysteresis for the mixtures, although unchanged for either pure gas. The observed behavior indicates that bulk liquid condensed around the Vycor samples is able to interact with the confined fluid, causing the confined mixture to become Ar rich.
Introduction The phase behavior of fluids adsorbed into nanometerscale pores poses an interesting fundamental problem in physics and chemistry and is of immense technological importance. The effects of finite pore size, substrate fluid forces, and the disorder associated with the porous medium lead to (i) broadening of the first-order phase transitions,1 (ii) shifting of the location of phase boundaries in (P,T),1,2 (iii) hysteresis between the adsorption and desorption branches,1,2 and (iv) often different structure of the confined fluid.3 Although, many experimental techniques (probing largely macroscopic behavior) have been used in the study of fluid properties under such circumstances, * To whom correspondence should be addressed. E-mail:
[email protected]. (1) See, e.g.: Evans, R. J. Phys.: Condens. Matter. 1990, 2, 8989. (2) See, e.g.: Duffy, J. A.; Wilkinson, N. J.; Fretwell, H. M.; Alam, M. A. J. Phys.: Condens. Matter. 1995, 7, L27. Wilkinson, N. J.; Duffy, J. A.; Fretwell, H. M.; Alam, M. A. Phys. Lett. 1995, 204A, 285. Sokol, P. E.; Ma, W. J.; Herwig, K. W.; Snow, W. M.; Wang, Y.; Koplik, J.; Banavar J. R. Appl. Phys. Lett. 1992, 61, 777. Wilkinson, N. J.; Alam, M. A.; Clayton, J. M.; Evans, R.; Fretwell, H. M.; Usmar, S. G. Phys. Rev. Lett. 1992, 69, 3535. Gelb, L. D.; Gubbins, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. Rep. Prog. Phys. 1999, 62, 1573. (3) Brown, D. W.; Sokol, P. E.; Clarke, A. P.; Alam, M. A.; Nuttall, W. J. J. Phys.: Condens. Matter. 1990, 9, 7317.
many fundamental questions remain unanswered. In the case of a binary fluid mixture in confinement, added complexities may arise due to, for example, the nature of the miscibility of the components, different affinities of the constituents to the pore walls, different size of the molecules, and any propensity to phase separation at a certain critical composition and temperature. In recent years, the phase behavior of binary mixtures in confined geometry has been the focus of immense theoretical4-11 and experimental11-17 interest (4) Lee, J. C. Phys. Rev. B 1992, 46, 8648. (5) Schwartz, M.; Villain, J.; Shapir, Y.; Nattermann, T. Phys. Rev. B 1993, 48, 3095. (6) Lee, J. C. Phys. Rev. E 1995, 52, 6368. (7) Zhang, Z.; Chakrabarti, A. Phys. Rev. E 1995, 52, 2736. (8) Gelb, D.; Gubbins, K. E. Phys. Rev. E 1997, 55, R1290. (9) Akhmatskaya, E.; Todd, B. D.; Daivis, P. J.; Evans, D. J.; Gubbins K. E.; Pozhar, L. A. J. Chem. Phys. 1997, 106, 4684. (10) Ro¨cken, P.; Somoza, A.; Tarazona, P.; Findenegg, G. J. Chem. Phys. 1997, 108, 8689. (11) Ayappa, K. G. Langmuir 1998, 14, 880; Chem. Phys. Lett. 1998, 282, 59. (12) Wiltzius, P.; Dierker S. B.; Dennis, B. S. Phys. Rev. Lett. 1989, 62, 804. (13) Durian, D. J.; Abesuriya, K.; Watson S. K.; Franck, C. Phys. Rev. A 1990, 42, 4724. (14) Frisken, B. J.; Ferri, F.; Cannell, D. S. Phys. Rev. Lett. 1991, 66, 2754.
10.1021/la0004505 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/31/2000
7552
Langmuir, Vol. 16, No. 20, 2000
among physicists and chemists. However, most of these studies have been concerned with the phase separation behavior in the liquid phase at or near the critical concentration. There appears to be no experimental study on the nature of the capillary condensation/desorption of a binary mixture in confinement and the interaction of the capillary condensed fluid with the bulk fluid if it is allowed to form in an experimental cell around the porous specimen. In this Letter we report the preliminary results of a comprehensive experimental study of the capillary condensation and desorption behavior of a simple binary mixture of nitrogen and argon in Vycor pores as a function of composition using a relatively new technique: positron and positronium annihilation spectroscopy.2,18 In the bulk, the Ar-N2 mixture is fully miscible at all compositions. Positron and Positronium Annihilation in Vycor Pores In a porous medium, such as Vycor glass, injected positrons thermalize in the glass matrix and a significant fraction forms and annihilates from a positronium (Ps) state (a electron-positron “hydrogen-like” bound state) within the pores.2 Three-quarters of these Ps will be in the form of ortho-Ps (o-Ps, parallel electron and positron spins) and one-quarter para-Ps (p-Ps, antiparallel electron and positron spins). In a vacuum, an o-Ps decays via the emission of 3γ photons and has a relatively long lifetime of ∼140 ns leading to a broad and roughly triangular photon energy distribution extending from 0 to 511 keV (≡moc2)2. p-Ps has a considerably shorter lifetime of ∼125 ps and annihilates into 2γ photons with a narrow energy distribution centered around 511 keV. In a small pore (pore radius ∼ a few nanometers , ortho-Ps mean free path) even a thermalized o-Ps will collide many times with the pore walls during its relatively long lifetime. This may result in the positron of the o-Ps annihilating with an electron other than its partner and of opposite spin from the pore wall, and thereby emitting 2γ photons instead of 3γ. Such a process is termed “o-Ps quenching".2 When the pores contain a fluid, the extent of o-Ps quenching will be governed by the local fluid density and will undergo significant increase at the gas w liquid transition. The presence of dense fluid within the pores may also lead to additional positron thermalization within the pores and thus may lead to an increase in the total positronium population. In the presence of the highdensity fluid this additional o-Ps fraction is expected to quench rapidly. Consequently, the precise balance of positronium formation and o-Ps quenching in confining geometry is likely to be complex and will depend on the presence, nature, and density of fluids. Nevertheless, in all cases, the ratio of 3γ to 2γ annihilations, N(3γ/2γ), provides a good local probe for phase transitions in confined geometry. If Ps is thermalized prior to annihilation, then the formation of additional p-Ps will also provide its own signature of phase transitions in the form of increased counts near the 511 keV annihilation photo peak. Experimental Details The adsorption and desorption experiments were performed in a purpose built pressure cell.19 Two identical Vycor specimens (∼6.5 mm diameter, ∼2.5 mm thickness) were sandwiched around (15) Dierker, S. B.; Wiltzius, P. Phys. Rev. Lett. 1991, 66, 1185. (16) Lin, M. Y.; Sinha, S. K.; Drake, J.; Wu, X. I.; Thiyagarajan, P.; Stanley, H. B. Phys. Rev. Lett. 1994, 72, 2207. (17) Frisken, B. J.; Cannell, D. S.; Lin, M. Y. Sinha, S. K. Phys. Rev. E 1995, 51, 5866. (18) Positron and Positronium Chemistry; Schrader, D. M., Jean, Y. C., Eds.; Elsevier: Amsterdam, 1988. (19) Clarke, A. P.; Fretwell, H. M.; Duffy, J. A.; Alam, M. A. J. Radioanal. Nucl. Chem. 1997, 211, 165.
Letters
Figure 1. Isobaric adsorption (downward arrow, solid symbols) and desorption (upward arrow, open symbols) cycles for pure argon at 3 bar in 4 nm diameter pores of Vycor glass. The gas S liquid transitions of the bulk fluid surrounding the samples are observed at 98.5 K. The solid lines are drawn to guide the eye. a 20 µCi 22Na source. The gases were mixed in the required proportions in an external large reservoir connected via a capillary tube to the experimental pressure cell. Heating and cooling of the cell were achieved with a closed cycle He cryocooler and a resistive heater, computer controlled via a “Eurotherm” temperature controller (temperature stability ∼(0.1 K). The pressure was computer controlled with the aid of high accuracy transducers. The Vycor samples were commercially acquired and contained cylindrical pores of diameter ∼4 nm ((0.05 nm) occupying ∼30% of the total glass volume.20 The gases used were 99.999% purity nitrogen and argon and mixtures of N2xAr1-x, where x is given in mole fractions. The total pressure in the cell was maintained at 3 bar throughout the experiments. The phase behavior was studied via measurements of the annihilation energy spectra (N(E)) collected at each temperature point. Each spectrum was analyzed in terms of the N(3γ/2γ) parameter, defined as the ratio of the number of counts in a 3γ annihilation region (340-490 keV) to a 2γ region (491-540 keV). Although it needs to be remembered that there will be some overlap of 2γ and 3γ annihilation events near the photopeak region (around 511 keV), for the purpose of monitoring the phase transitions via N(3γ/2γ) one is only concerned about the relative changes in this parameter, and the overlap is of no great significance. The geometry of the experiment was chosen in such a way as to facilitate the monitoring of the pore as well as the bulk transitions in the same experiment. For experimental details, the reader is referred to refs 2 and 21.
Results and Discussion For the purpose of illustrating the nature of the experiments, in Figure 1 we show the adsorption (solid symbols) and desorption (open symbols) isobars at 3 bar for pure Ar in Vycor pores and in bulk. During the cooling cycle, we observe a relatively sharp drop in N(3γ/2γ) at around 101.5 K due to gas-liquid transition in the pores in response to increased o-Ps quenching as a result of the appearance of a much denser fluid. The bulk gas-liquid transition occurs at a lower temperature and is indicated by the rise in N(3γ/2γ) at ∼98.5 K. This is in line with the expectation of the pore capillary condensation occurring at a higher temperature than that for the bulk liquefaction. The fact that the 3γ/2γ annihilation ratio increases at the bulk gas w liquid transition in contrast to its decrease in the pore gas w liquid transition lies in the geometry of the experiment and associated nature of the formation and quenching behavior of positronium and is discussed in (20) Corning, Vycor Brand Thirsty Glass Moisture Getters Data sheet 7930; Materials Department, Corning Glass Works: Corning, NY, 1967. (21) Clarke, A. P. Ph.D. Thesis, Bristol, 1997.
Letters
Figure 2. Temperature vs composition phase diagrams for binary Ar/N2 mixtures in 4 nm diameter pores of Vycor glass at a fixed pressure of 3 bar: (a) adsorption phase diagram; (b) desorption phase diagram of the confined fluid where the desorption process commenced prior to the liquefaction of the bulk fluid surrounding the Vycor samples (reversal of the temperature at point A in Figure 1); (c) desorption phase diagram for confined fluid which was allowed to interact with the bulk liquid prior to the commencement of the desorption transition (reversal of the temperature at point B in Figure 1). The transition temperatures are taken as the midpoints of the sharp changes in the N parameter (see Figure 1) of the confined fluid. The solid lines are drawn to guide the eye.
detail elsewhere.2,22 Warming the cell causes the bulk liquid to boil at 98.5 K, the same temperature as the adsorption transition. The confined fluid desorbs at ∼103 K, leading to a hysteresis of ∼2 K between the confined absorption and desorption transitions, as expected. The widths of the confined phase transitions are consistent with those predicted by theoretical studies1,23 and those seen in mass adsorption studies on fluids in confined geometries.24 In our experiments, the transition widths remained constant for all compositions. Plotting the loci of the rapid changes (steps) in N(3γ/2γ) allows us to construct a temperature vs composition phase diagram for adsorption and desorption transitions of the binary mixture under investigation. To assess the effects of any interactions between the pore and bulk liquids surrounding the specimens on the desorption behavior, we performed two series of adsorption/desorption experiments for a number of compositions of the binary mixture: (i) In the first set, the cooling cycle was stopped at a temperature above the bulk liquefaction transition, as labeled A in Figure 1. The heating cycle then commenced from this point. (ii) In the second set, the cell was cooled further, to point B in Figure 1, liquefying the bulk fluid, to facilitate interaction between the bulk and pore liquid prior to pore desorption. The heating cycle then commenced from this temperature. A temperature vs composition phase diagram for the binary mixture was derived from these series of isobars and is shown in Figure 2. During the cooling cycles, both data sets (curve a) necessarily exhibit the same behavior. Apart from the mixture containing 10% mole fraction of argon, the capillary adsorption transition temperature increases monotonically with increasing argon fraction until the gas-liquid transition temperature of confined pure Ar is reached. This indicates that no preferential condensation of either of the fluids occurs during the pore transition. (22) Duffy, J. A. Ph.D. Thesis, Bristol, 1995. (23) Ball, P. C.; Evans, R. Langmuir 1989, 5, 714. (24) Everitt D. H. In The Solid Gas Interface; Marcel Dekker: New York, 1967; Vol. 2, p 1055. Machin, W. D. Langmuir 1994, 10, 1235.
Langmuir, Vol. 16, No. 20, 2000 7553
The desorption behavior, on the other hand, depends strongly on the prehistory of the condensation transition. First, we describe the desorption transitions where the pore liquid did not have any interaction with bulk fluid prior to desorption (curve b). In this case the capillary condensation and desorption lines run roughly parallel to each other with a hysteresis ∆T ∼ 2 K (ca. the same as for pure argon and nitrogen) between the adsorption and desorption temperatures for all compositions. This is roughly what is expected for a single component fluid in pores of the size under study from considerations of Kelvin equation like behavior. This would imply that the condensing mixture in the pore behaves like a singlecomponent fluid. Further, we observe no evidence of any preferential desorption of one of the components of the mixture. On the other hand, if the bulk mixture is allowed to liquefy and interact with the pore fluid prior to the desorption process, the pore desorption shows an entirely unexpected behavior. The pore desorption transition (curve c) temperatures rise sharply as a function of Ar fraction of the original gas mixture and reach the transition temperature for pure Ar at an Ar molar fraction of only ∼0.45, beyond which it remains constant at close to the transition temperature for Ar. In view of the desorption line (curve b) (no interaction of the pore liquid with the bulk liquid prior to desorption of the pore liquid), this would indicate an Ar enrichment of the pore liquid during its interaction with the bulk liquid. Given the fact that there is no evidence of preferential adsorption or desorption of one or the other fluid in the pores without the interaction with the bulk fluid, such an Ar enrichment is curious and can only be due the interaction of the pore liquid with the bulk. The pore desorption temperature is at least 4-5 K above the bulk desorption point. There are typically eight datum points between the two (bulk and confined) evaporation processes, which is ∼6 h in terms of experimental time, and the observed behavior cannot be explained in terms of a lack of attainment of equilibrium conditions. Therefore, the interaction between the pore and bulk liquids that leads to the Ar enrichment of the pore liquid must occur during either the condensation or the desorption of the bulk fluid. We would like to note that, in our experiments, there are no changes in the desorption behavior for a single-component fluid irrespective of whether the pore fluid is allowed to interact with the bulk liquid or not. Thus the observed behavior above cannot be an artifact of the experiment. In conclusion, the adsorption and desorption of the binary system N2xAr1-x confined in the cylindrical 4 nm diameter pores of Vycor glass has been studied using positron annihilation spectroscopy. Unless liquid is formed around the sample, the gas-liquid and liquid-gas phase transitions of the confined fluid exhibit hysteresis which varies little with composition, indicating that the confined mixture behaves like a simple fluid. However, if bulk mixture surrounding the samples is allowed to condense prior to pore desorption, the liquid S gas transition in the pores exhibits a much larger degree of hysteresis, which indicates an Ar enrichment of the confined fluid. This implies an interdiffusion between the bulk liquid condensed around the Vycor samples and the pore liquid. In light of the fact that there is no evidence of preferred condensation of Ar in the pores, this is an unexpected behavior. Acknowledgment. Generous financial support from the EPSRC UK is gratefully acknowledged. LA0004505
