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Intrusion and extrusion of water in highly hydrophobic mesoporous materials: effect of the pore texture
Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 265–272
Intrusion and extrusion of water in highly hydrophobic mesoporous materials: effect of the pore texture B. Lefevre a,∗ , A. Saugey b,c , J.L. Barrat d , L. Bocquet d , E. Charlaix d , P.F. Gobin b , G. Vigier b a
Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05, France Groupe d’Etudes de Métallurgie Physique et de Physique des Matériaux, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, France c Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon, 36 Avenue Guy de Collongues, 69134 Ecully Cedex, France Laboratoire de Physique de la Matière Condensée et Nanostructures, Université Claude Bernard, 6 rue Ampère, 69622 Villeurbanne Cedex, France b
d
Abstract Understanding of the physics of confined fluids is of major theoretical and practical interest. In the field of materials science, the most popular techniques used to characterise the texture of mesoporous supports are based on the analysis of the properties of a confined fluid:thermoporometry, adsorption technique and mercury intrusion porosimetry. In particular, in the last two techniques the pressure parameter is related to the dimension of the pores. The present study is dedicated to an original analysis of the forced-intrusion of a non-wetting liquid (water) in hydrophobic mesoporous materials presenting different pore topologies. Among these supports, MCM-41 type materials allowed to point out distinct mechanisms for the intrusion and the extrusion of water. Whereas the intrusion process obeys to the Laplace law of capillarity, the extrusion is found to be preferentially governed by the formation of the vapor phase by a mechanism such as nucleation. Therefore, the conventional interpretation given for the hysteresis observed in mercury porosimetry, based on wetting hysteresis and pore-blocking effects is not directly transposable to the case of water intrusion experiments. Data obtained on other supports such as silica gels and Controlled Pore Glass supports are also reported and discussed taking into account the results obtained in MCM-41 type materials. Finally, the mechanisms leading to hysteresis in both phenomena (sorption of a wetting fluid and forced intrusion of a non-wetting liquid) are shown to present strong analogies in model materials as well as in disordered and interconnected porosities. © 2004 Published by Elsevier B.V. Keywords: Non-wetting; Mesoporous; Porosimetry; Hysteresis; Hydrophobicity
1. Introduction Porous materials are involved in many industrial processes such as catalysis, membrane technology, chromatography. . . . Accurate information on their porous texture (pore size distribution, pore shape, specific surface area) as well as surface chemistry data are required in order to understand their specific properties and to optimize their performances. Among the numerous characterization techniques, adsorption methods and mercury intrusion porosimetry are widely used to investigate porous textures [1]. On one hand, physical sorption of gases is the most famous route to investigate porosity in the micropore and mesopore range [2]. In the case of mesopores, the capillary ∗ Corresponding author. Tel.: +33-4-6716-3448; fax: +33-4-6716-3470. E-mail address: [email protected] (B. Lefevre).
0927-7757/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2004.04.020
condensation phenomenon, corresponding to the gas to liquid phase transition shifted by confinement effects, is exploited to obtain the porous characteristics listed before. In such experiments, the adsorbate molecules strongly interact with the pore surface (wetting configuration). The amount of adsorbed molecules is measured as a function of the applied vapor pressure. Great attention has been paid to the theoretical and experimental study of this confined phase transition in the past decades [3–8]. Most of these investigations were based on adsorption isotherms obtained on Vycor type materials [9], or silica gels. The most significant improvements in the understanding and simulation of this phenomenon are mainly related to the development of ordered micelle templated materials in 1992 [10]. Thanks to this new family of mesoporous supports of well-defined and ordered porosity, adsorption and desorption mechanisms in model geometries are now well understood, as discussed in details recently by Ravikovitch et al. [11].
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On the other hand, mercury intrusion porosimetry enables to investigate mesoporosity and macroporosity. This technique relies on the intrusion of a non-wetting liquid (mercury) into a porous material. In a typical experiment, the amount of liquid intruded into the pores is measured as a function of the applied liquid pressure. In some cases, cyclic pressure scans exhibit both intrusion and extrusion of the confined liquid. This type of cycles also present a pressure hysteresis. These observations reveal the existence of strong analogies between capillary condensation (wetting configuration) and forced-intrusion (non-wetting configuration) in mesopores. Surprisingly, the thermodynamic aspects of the hysteretic behavior in forced-intrusion of mercury have not been treated in details in the literature. Most simulation efforts are focused on the interpretation of the pressure hysteresis in terms of wetting hysteresis and geometrical consideration of pore network effects [12–14], such as the percolation approach [15] or renormalization [16]. To our knowledge, no results have been reported yet concerning the intrusion and extrusion of mercury in model materials such as micelle templated supports, probably because of their low mechanical stability upon the high pressures required for mercury intrusion. Such missing data would be of great importance to achieve a better understanding of this confined phase transition. However, other results obtained by using water instead of mercury represent an alternative and interesting source of data. Indeed, Eroshenko et al. developed a powerful technique, referred to as “water porosimetry”, to investigate hydrophobic porous materials [17–22]. They showed that intrusion and extrusion cycles could be obtained with water as a non-wetting liquid in strongly hydrophobic mesopores. This technique allows, for example, to estimate the surface hydrophobicity by determining the advancing contact angle θ a from the analysis of the intrusion pressure by means of the Laplace–Washburn equation: PL = −
2γ cos θa Rp
(1)
where PL is the pressure of the liquid for which intrusion occurs, γ the interfacial tension between the liquid and the vapor, and Rp is the pore radius. Gusev [23] and Gomes et al. [24] developed experimental devices allowing the simultaneous measurement of the pressure, the volume variation and the heat exchange during water intrusion experiments. A fine thermodynamic analysis of the intrusion phenomenon allowed them to determine simultaneously the pore size distribution and the advancing contact angle from a single water intrusion experiment on modified silica gels. Anyway, the deep understanding of the hysteresis was still not clearly understood. One remarkable fact about these water intrusion/extrusion data is the existence of reproducible cycles in a rather small range of pressure:between atmospheric pressure and 70 MPa. This possibility allowed us to test water intrusion in hydrophobized MCM-41 type materials in a range of relatively small
pressures for which they present enough mechanical stability [25–27]. A detailed study of the intrusion and extrusion process of water in model MCM-41 type materials was carried out [28]. The main conclusions are summarized in the first part of this paper. Then, in view of these conclusions, a qualitative analysis of the results obtained on other siliceous hydrophobic porous materials is proposed to highlight the additional effect of the pore texture on the hysteresis.
2. Materials and methods Three types of materials presenting completely different pore morphologies were chosen: MTS type materials (MCM-41), Controlled-Pore-Glasses (CPG) and silica gels. MTS materials possess model porous texture resulting from the self assembly of surfactants and silicates. Among this family, MCM-41 present non-intersecting cylindrical pores with a low pore size distribution. In the present study, four supports of increasing pore size, referred to as MTS-1, MTS-2, MTS-3 and MTS-4 were synthesized according to a procedure described elsewhere [28]. The mesoporous texture of Controlled Pore Glasses results from a spinodal decomposition leading to a bi-continuous highly interconnected network of pores presenting a relatively low pore size distribution. One commercially available CPG was obtained from Aldrich (ref CPG-75–120). Finally, two silica gels, one referred to as “FK” (Fluka 60, Aldrich) and the other referred to as “HP” (PEP100, Hypersil) were also selected. In this type of support, the mesoporosity results from the aggregation of primary nanoparticles, leading to a disordered porosity with a relatively large pore size distribution. The nitrogen sorption isotherms of the 7 samples listed above are gathered in Figs. 1 and 2. All these supports were modified by grafting of octyldimethylchlorosilane by a pyridine-assisted reaction [29], except sample HP which is commercially available as a C18-functionalized stationary phase for chromatographic applications (grafting density is unknown for this support). A constant grafting density of approximately 1.3 chains per nm2 was determined for the four MTS samples and about 2 chains per nm2 for samples FK and CPG. Water intrusion experiments were performed on a specially designed apparatus described elsewhere [30]. About 2 g of degassed hydrophobized material were gathered with a large excess of water (compared to the corresponding disposable pore volume) into a shrinkable polymer container. In a typical experiment, the pressure was first continuously increased from the atmospheric pressure (Patm ) to 80 MPa by means of a mechanical increasing constraint, and then decreased back to Patm . The time required for a complete cycle was about 4 min, as no significant difference was observed in our experiments for 1 min to 1 h cycles. A pause of 10 min at Patm was respected between each run. The pressure of the liquid and the volume variations were simultaneously
-1
Vads , ml.g STP
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B. Lefevre et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 265–272
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0 1600 1400 1200 1000 800 600 400 MTS-4 200 0 0.8 1.0
P/P0
Fig. 1. Nitrogen sorption isotherms at 77 K of the MCM-41 type materials before surface modification. The adsorbed volume, Vads , is plotted as a function of the relative pressure of the vapor, P/P0 .
recorded. The volume values were corrected to eliminate the deformation and the compressibility contributions. The representation of the water pressure as a function of the corrected volume variation (per gram of bare silica) are denoted P/V curves. In these plots, the volume variations V reflect the cumulative intruded and extruded volumes of water into the pores during compression and decompression steps, respectively.
400
FK
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300 200 100
HP 500 400
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Vads , ml.g STP
0
300 200 100 0 0.0
CPG 0.2
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0.8
1.0
P/P0 Fig. 2. Nitrogen sorption isotherms at 77 K of (up) the silica gels FK and HP and (down) the Controlled Pore Glass, before surface modification.
3. Results and discussion 3.1. Model MCM-41 type materials Tens of reproducible cycles have been recorded on grafted samples MTS-1, MTS-2 and MTS-3. For sample MTS-4 only one intrusion could be recorded as no water extrusion took place during the decompression step, and even after several hours at Patm . For this material, spontaneous extrusion of water did not occur. The corresponding P/V curves are reported in Fig. 3. The mean intrusion and extrusion pressures are reported as a function of the mean pore radii for these materials on Fig. 4 in logarithmic scales. We can note that the mean intrusion pressure is a linear function of R−1 p . This confirms that the intrusion mechanism can be described by means of Eq. (1) for these series of model materials. The value of θ a , derived from the pre-factor ◦ (−2γ cos θ a ) of the R−1 p scaling law, is 120 , which is consistent with macroscopic measurements reported on the same type of grafted surfaces. This clearly demonstrates the validity of Eq. (1) for pore radius as small as 1.3 nm. On the contrary, the mechanism of extrusion is radically different:the mean extrusion pressure is far to obey the R−1 p scaling law. The decrease of the mean extrusion pressure with increasing pore radius is much more sudden. The trend extrapolated from the mean extrusion pressures recorded for samples MTS-1, MTS-2 and MTS-3 (see the dotted line in Fig. 4) suggests that the extrusion pressure for large pores may be inferior to Patm , which is consistent with the reported lack of extrusion for the large pores MTS-4 sample. The scaling of the extrusion pressure as a function of the pore mean pore radius Rp shows that the extrusion
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∆V , mm .g
Pressure , MPa
-400 -300 80 70 MTS-1 60 50 40 30 20 10 0 80 70 MTS-3 60 50 40 30 20 10 0 -400 -300
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0
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-1
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Pressure , MPa
268
0
∆V , mm .g 3
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Fig. 3. Water intrusion and extrusion cycles obtained at room temperature for the hydrophobized MCM-41 type materials. The applied pressure of the liquid is plotted as a function of the volume variation of the system. The values of pore volumes which can be deduced from the volume variations at the end of the intrusion steps are systematically smaller than the initial pore volumes determined by nitrogen sorption, mainly because of the presence of high amounts of grafted phase from which water is excluded.
pressure can not be described by means of Eq. (1). Therefore, the extrusion process has to be governed by another mechanism:the nucleation of a vapor bubble (cavitation) is necessary to generate two liquid/vapor menisci in a pore initially filled with liquid water only. The extrusion is experimentally observed when the probability for nucleation of the vapor phase (in each pore) becomes significant over the time of the experiment. Since the nucleation rate usually
Pressure , MPa
100 10 1 0.1 0.01 1
PINT PEXT Patm
(O)
2
3
4
obeys an Arrhenius law, we expect the extrusion pressure to vary much more rapidly with the pore radius Rp than the R−1 p scaling law corresponding to Eq. (1). We show in a more detailed and theoretical analysis [28] that the nucleation process of a vapor phase in a cylindrical capillary can quantitatively account for the values of the mean extrusion pressures reported here for the samples MTS-1, MTS-2 and MTS-3. We can conclude that, in a model cylindrical pore, intrusion and extrusion are not governed by the same mechanism. The description of the withdrawal of water in these materials using the Eq. (1) and assuming a receding contact angle θ r is therefore not adequate. Finally, from the tendency extrapolated from the experimental data of extrusion (dotted line in Fig. 4) we can define a critical pore size Rc for the extrusion of water to be observed at a given pressure PL ≥ Patm for some materials presenting the same hydrophobic properties as the MCM-41 type supports under study:this critical pore size stands approximately between around 4 nm.
5 6
Mean pore radius , nm Fig. 4. Logarithmic representation of the mean intrusion pressure PINT (䊉) and the mean extrusion pressure PEXT (䉬) as a function of the mean pore radius for the hydrophobized MCM-41 type materials. The horizontal dotted line represents the atmospheric pressure Patm . The symbol (䊊) represents the extrapolation of the extrusion tendency to the mean pore radius of the support MTS-4. (Mean pore radii of the grafted supports were determined by means of Broekhoff–De Boer theory from nitrogen sorption data).
3.2. Silica gels and CPG The water intrusion and extrusion curves related to these three supports are reported in Fig. 5. For the silica gel FK, stable intrusion and extrusion cycles were obtained according to the procedure described above. For the silica gel HP, only half of the first intruded water volume was recovered after extrusion under the same experimental procedure. As
B. Lefevre et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 265–272
∆V , mm .g
Pressure , MPa
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CPG
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Norm. ∆V , a.u. Fig. 6. Comparison between the water intrusion curves obtained by simulation of the intrusion of water according to the Laplace–Washburn equation into a 2D-connected network (-䊊-) and a non-interconnected medium (—). The same pore size distribution was applied in both cases and chosen arbitrary centered at 4 nm assuming a gaussian distribution (σ = 0.6 nm). The advancing contact angle θ a was set to 115◦ .
HP
30
(1)
20 10 0
Pressure , MPa
Pressure , MPa
3
269
(2) -400
-300
-200
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∆V , mm .g 3
-1
Fig. 5. Water intrusion and extrusion cycles obtained at room temperature for (up) the hydrophobized silica gel FK and Controlled Pore Glass support and (down) the hydrophobized silica gel HP.
a consequence, “reduced” hysteresis cycles were registered for the subsequent runs. Nevertheless, if the system (sample HP + water) was left overnight at Patm , extrusion effectively occurred and a further intrusion curve similar to the initial one was obtained. Such a low kinetic of extrusion was only observed for this sample. Finally, only one intrusion could be recorded on the CPG sample, as extrusion did not happen even after a storage of several days at Patm (as for sample MTS-4). In Fig. 7, the mean intrusion and extrusion pressure for these three supports are reported as a function of the mean pore radii in logarithmic scales, and compared to the data of the MTS support series. 3.3. Intrusion mechanism in interconnected pores During the intrusion of a non-wetting liquid in an interconnected porous network presenting a given pore size distribution, well-known pore blocking effects are expected. Their influence on the macroscopic intrusion curves (P versus V) can easily be determined by simulating injection experiments in model networks. An example is reported in Fig. 6. A porous network with a given gaussian pore size distribution was generated by means of a 2-D square-lattice network (each connection is linked to 4 pores). Then, an algorithm was applied to simulate the injection curve:for a given value of the liquid pressure PL , all the pores already connected to external liquid phase and with a pore radius such as Rp ≥ −2γ cos θ a /PL are invaded by the liquid. In Fig. 6, the result is compared to the injection in a net-
work presenting the same pore size distribution but where pore-blocking effects are absent. The effect of the connectivity is a “smoothing” of the intrusion curve at the beginning of the injection. If we now compare the experimental intrusion curves for the MCM-41 type materials (Fig. 3) and for the particular case of the silica gel FK, we can note the same trend at the beginning of the intrusion for the connected gel, which is not the case for MCM-41s, see Fig. 5. The existence of pore-blocking effects in the disordered and connected texture of the silica gel could account for this experimental result. The important point is that this basic injection model indicates that the mean intrusion pressure in the connected medium is practically not affected by network effects. The mean intrusion pressure can be obtained from Eq. (1), taking into account the mean pore radius of the sample. Actually, this is the reason why Fadeev et al. could validate the use of the Laplace–Washburn equation for the description of the mean intrusion pressure of water in a series of hydrophobized silica gels [20,21] by checking its linear dependency as a function of the inverse mean pore radius (R−1 p ). Some of their results [21] have been reported in Fig. 7 (symbols (+) and line A). In this figure, it appears that the values of the mean intrusion pressures in the silica gels FK and HP, and in the CPG support are close to the values measured in the MCM-41 type materials and therefore in quite good agreement with the previous R−1 p scaling law. Thus, in spite of very different pore geometry, the mean intrusion pressure in those hydrophobic media is governed by the same mechanism. Only the pre-factor of the R−1 p scaling law is different, due to slight differences in the surface chemistry.
4. Extrusion mechanism in interconnected pores The value of the mean extrusion pressure for sample FK is in very good agreement with the data of Fadeev et al [21].
B. Lefevre et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 265–272
Pressure, MPa
270
100
A
10
B
from ref. [21]
1 0,1 0,01 1
from ref. [21]
Patm 2
3
4
5 6
Mean pore radius , nm Fig. 7. Logarithmic representation of the mean intrusion and extrusion pressures as a function of the mean pore radii (similar plotting as Fig. 5):mean intrusion pressure (䊉) and mean extrusion pressure (䉬) for the MTS supports, respectively (䊐) and (䊊) for the silica gel FK, () and (⊕) for the silica gel HP (first intrusion) and () for the intrusion in the CPG support. The symbols (+) and (×) represent respectively the mean intrusion and extrusion pressure values extracted from the exhaustive study published by Fadeev et al in Fig. 3 of reference [21]. These data were obtained on silica gels chemically modified by means of octyldimethylchlorosilane.
See symbol (䊊) and line B in Fig. 7. Thus, the mean extrusion pressure in this series of silica gels obeys a R−1 p scaling law, in the same way as the mean intrusion pressure. The physics of the extrusion process in these connected media is therefore completely different from the one observed in the MCM-41 type supports. Actually, the extrusion of water from an interconnected pore network can occur if both following requirements are fulfilled: (1) the possibility for the vapor phase to form during the pressure release and (2) the possibility for the resulting menisci to propagate into the pore network from pore to pore to empty the porous medium, for values of the applied liquid pressure PL ≥ Patm . Note that in these supports, the first condition (nucleation of the vapor phase) is absolutely not necessary in all the pores of the network, contrary to the case of the non-interconnected MCM-41 supports. In fact, according to this interpretation, only a few preferential nucleation sites are required for the vapor phase to appear. Then, the extrusion can proceed exclusively by propagation of menisci if the second condition is fulfilled. Consequently, to interpret properly the extrusion mechanism in interconnected pores, one has to consider these two phenomena:nucleation and propagation. Nucleation is most probable to take place in a favourable environment such as the smallest pores and/or the most hydrophobic sites. Anyway, large pores for which extrusion by means of nucleation is impossible at PL ≥ Patm (in the time scale of the experiment), can be emptied by propagation of menisci from surrounding pores at PL ≥ Patm . The withdrawal condition (Patm ≥ −2␥ cos θ r /Rp ) being satisfied for all the pores, complete extrusion could then occur by menisci propagation from the initial nucleation sites. Of course, pore-blocking effects are also expected during the propagation, as detailed previously for the intrusion process.
The available experimental results obtained for the three samples under study and other results extracted from [21], all of which are gathered in Fig. 7, indicate that the extrusion for all the silica gels (except sample HP) obey to a R−1 p scaling law. This means that the mean extrusion pressure can also be described in these materials by means of Eq. (1), taking into account the mean pore radius of the pore size distribution, and assuming the existence of a receding contact angle θ r . Thus, the important point is that the pressure hysteresis observed between intrusion and extrusion can be described by means of wetting hysteresis (θ = θa − θr ) as concluded earlier [20,21]. Actually, the existence of an initial stage in the extrusion process, i.e. the local formation of the vapor phase by nucleation, can not be underscored by such experiments because (1) the resulting volume variation is not significant enough to be detected, and because (2) the whole extrusion process is governed by the propagation of the menisci in the pore network. In other words, it is highly probable that the menisci generated by nucleation in the most favourable sites are unable to propagate as long as the applied pressure is to high to allow their motion in the surrounding pores. The macroscopic extrusion mechanism in these interconnected pore networks is then completely different from the one observed in the non-interconnected MCM-41 type materials. Following the previous arguments, the presence of nucleation sites is required in the first stage of the extrusion. As deduced from the tendency extrapolated from the MCM-41 data, a critical pore size Rc (around 4 nm) is expected for nucleation to take place. If the pore size distribution of a connected sample does not include any pore of radius Rp ≤ Rc then this initial stage of nucleation becomes highly improbable and, as a consequence, extrusion could be impossible. Now considering the case of the CPG support, we note that extrusion does not occur in this material. Indeed, its hydrophobic properties are quite similar to the other samples under study. Anyway, its pore size distribution is narrow (as revealed by nitrogen sorption and by the low dispersion of the water intrusion pressure) and centred around 5.6 nm. Therefore, the absence of small pores in which nucleation could be favoured is one possible explanation to account for this lack of extrusion. Finally, the specific case of sample HP, with a low kinetic of emptying at atmospheric pressure, has not been reported before to our knowledge. The previous arguments fail to describe completely this experimental observation, which suggest the contribution of an additional mechanism to the extrusion process. In the disordered structure of a silica gel, some breaks in the continuous liquid paths can occur during the extrusion due to connectivity effects, resulting in the entrapment of part of the confined liquid. This mechanism has been clearly identified in mercury intrusion porosimetry [13]. The presence of a residual amount of trapped water being thermodynamically unfavourable at Patm , pore drying by means of a vapor phase transport to the external liquid phase is a possible pathway to account for this result.
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5. Conclusion
Acknowledgements
The technique of water porosimetry applied to model micelle-templated materials of the MCM-41 type provided a unique opportunity to investigate the behavior of a non-wetting liquid confined in lyophobic mesopores of model geometry. We could then compared the hysteretic properties of the intrusion/extrusion phenomena in the ideal pore geometry of the MCM-41 type supports and in interconnected pore networks (silica gel and Controlled-Pore-Glass supports). For all these materials, the mean intrusion pressure was shown to be inversely proportional to the mean pore radius, which confirms the validity of the Laplace–Washburn equation down to extremely small pore radius (1.3 nm), regardless of the mesoporous texture. In model MCM-41 supports, the extrusion can not be described as the withdrawal of menisci with a receding contact angle according to the Laplace–Washburn equation. We show that the extrusion has to be governed by another mechanism:the nucleation of the vapor phase. For this geometry, the argument of wetting hysteresis is not relevant to explain the entire experimental pressure hysteresis in our water intrusion/extrusion experiments, which had not been clearly pointed out before. The mean extrusion pressure in these materials does not obey the Laplace–Washburn equation. It decreases much more rapidly with increasing pore radius than the R−1 p scaling law corresponding to this equation. During the pressure release, the pressure threshold required for extrusion appears to be related to the capacity for a stable vapor phase to form into the metastable confined liquid. From this series of model pore geometry, we defined a critical pore size Rc (around 4 nm):nucleation is impossible for pores of radius inferior to Rc . In interconnected media, the extrusion process is different. If the pore size distribution is large enough, the extrusion is governed by Eq. (1), in the same way as the intrusion. This is due to the fact that nucleation may occur in the smallest pores, then followed by propagation. Our experimental results on the silica gel FK are in good agreement with other data reported earlier [21]. For an interconnected medium presenting a low pore size distribution, if the mean pore size is too large, then nucleation is impossible, as observed for the CPG sample. Finally, whereas the intrusion can always by described by means of the Laplace–Washburn equation (applied to mean values of the intrusion pressure and the pore radius), nucleation effects are predominant for the extrusion process. Slow kinetic effects have also been reported, suggesting that vapor phase transports in the porous medium are expectable during the extrusion. All these results confirm that the phenomenon of forced-intrusion and extrusion of a non-wetting liquid such as water in mesoporous materials can be considered and treated as a confined phase transition.
We are indebted to T. Martin, C. Biolley, A. Galarneau, D. Brunel, F. Di Renzo, F. Fajula (Laboratoire des Matériaux Catalytiques et Catalyse en Chimie Organique, Montpellier, France) for providing us with most of the materials and for fruitful discussions as well as R. Denoyel (Centre de Thermodynamique et Microcalorimétrie, Marseille, France). B. Lefevre and A. Saugey are supported by the French Ministry of Defense (DGA).
References [1] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Everett, J.H. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, Pure Appl. Chem. 66 (8) (1994) 1739. [2] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (4) (1985) 603. [3] L.D. Gelb, K.E. Gubbins, R. Radhakrishnan, M. SliwinskaBartkowiak, Rep. Prog. Phys. 62 (1999) 1573. [4] J. Crassous, E. Charlaix, J.L. Loubet, Europhys. Lett. 28 (1) (1994) 37. [5] F. Celestini, Phys. Lett. A 228 (1997) 84. [6] K.G. Kornev, I.K. Shingareva, A.V. Neimark, Adv. Colloid Interface Sci. 96 (2002) 143. [7] K. Morishige, H. Fujii, M. Uga, D. Kinukawa, Langmuir 13 (1997) 3494. [8] K. Morishige, M. Shikimi, J. Chem. Phys. 108 (18) (1998) 7821. [9] J.H. Page, J. Liu, B. Abeles, H.W. Deckman, D.A. Weitz, Phys. Rev. Lett. 71 (8) (1993) 1216. [10] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leononwicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. Mc Cullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [11] P.I. Ravikovitch, A.V. Neimark, Langmuir 18 (2002) 9830. [12] W.K. Bell, J. Van Brakel, P.M. Heertjes, Powder Technol. 29 (1981) 75. [13] C.D. Tsakiroglou, A.C. Payatakes, Adv. Colloid Interface Sci. 75 (1998) 215. [14] J. Kloubek, Powder Technol. 29 (1981) 75. [15] A. Gabrielli, R. Cafiero, G. Caldarelli, J. Phys. A: Math. Gen. 31 (1998) 7429. [16] D.A. Quenard, K. Xu, H.M. Künzel, D.P. Bentz, N.S. Martys, Mater. Struct. 31 (1998) 317. [17] V.A. Eroshenko, A.Y. Fadeev, Russian J. Phys. Chem. 70 (8) (1996) 1380. [18] V.A. Eroshenko, A.Y. Fadeev, Colloid J. 57 (4) (1995) 446. [19] A.Y. Fadeev, V.A. Yaroshenko, Colloid J. 58 (5) (1996) 654. [20] A.Y. Fadeev, V.A. Eroshenko, J. Colloid Interface Sci. 187 (1997) 275. [21] A.Y. Fadeev, V.A. Eroshenko, Mendeleev Chem. J. 39 (6) (1997) 109. [22] V.A. Eroshenko, A.Y. Fadeev, Mendeleev Chem. J. 40 (1) (1996) 116. [23] V.Y. Gusev, Langmuir 10 (1994) 235. [24] F. Gomez, R. Denoyel, J. Rouquerol, Langmuir 16 (2000) 4374. [25] T. Martin, B. Lefèvre, D. Brunel, A. Galarneau, F. Di Renzo, F. Fajula, P.F. Gobin, J.F. Quinson, G. Vigier, Chem. Comm. 1 (2002) 24. [26] B. Lefèvre, P.F. Gobin, T. Martin, A. Galarneau, D. Brunel, Mat. Res. Soc. Proc. 726 (2002) Q.11.9.6.
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[27] B. Lefèvre, P.F. Gobin, T. Martin, F. Fajula, in: S.H. Feng, J.S. Chen (Eds.), Frontiers of Solid State Chemistry, Proceedings of the International Symposium On Solid State Chemistry in China, Changchun, 9–12 August 2002, World Scientific Publishing Co. Pvt. Ltd., Singapore, 2002, p. 197. [28] B. Lefevre, A. Saugey, J.L. Barrat, L. Bocquet, E. Charlaix, P.F. Gobin, G. Vigier, J. Chem. Phys. 120 (2004) 4927.
[29] T. Martin, A. Galarneau, D. Brunel, V. Izard, V. Hulea, A.C. Blanc, S. Abramson, F. Di Renzo, F. Fajula, Stud. Surf. Sci. Catal. 135 (2001) 29. [30] B. Lefèvre, E. Charlaix, P.F. Gobin, P. Perriat, J.F. Quinson, in: 2nd International Conference on Silica Science and Technology, Proc.CdRom, Mulhouse, 3–6 September 2001.
Intrusion and extrusion of water in highly hydrophobic mesoporous materials: effect of the pore texture B. Lefevre a,∗ , A. Saugey b,c , J.L. Barrat d , L. Bocquet d , E. Charlaix d , P.F. Gobin b , G. Vigier b a
Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05, France Groupe d’Etudes de Métallurgie Physique et de Physique des Matériaux, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, France c Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon, 36 Avenue Guy de Collongues, 69134 Ecully Cedex, France Laboratoire de Physique de la Matière Condensée et Nanostructures, Université Claude Bernard, 6 rue Ampère, 69622 Villeurbanne Cedex, France b
d
Abstract Understanding of the physics of confined fluids is of major theoretical and practical interest. In the field of materials science, the most popular techniques used to characterise the texture of mesoporous supports are based on the analysis of the properties of a confined fluid:thermoporometry, adsorption technique and mercury intrusion porosimetry. In particular, in the last two techniques the pressure parameter is related to the dimension of the pores. The present study is dedicated to an original analysis of the forced-intrusion of a non-wetting liquid (water) in hydrophobic mesoporous materials presenting different pore topologies. Among these supports, MCM-41 type materials allowed to point out distinct mechanisms for the intrusion and the extrusion of water. Whereas the intrusion process obeys to the Laplace law of capillarity, the extrusion is found to be preferentially governed by the formation of the vapor phase by a mechanism such as nucleation. Therefore, the conventional interpretation given for the hysteresis observed in mercury porosimetry, based on wetting hysteresis and pore-blocking effects is not directly transposable to the case of water intrusion experiments. Data obtained on other supports such as silica gels and Controlled Pore Glass supports are also reported and discussed taking into account the results obtained in MCM-41 type materials. Finally, the mechanisms leading to hysteresis in both phenomena (sorption of a wetting fluid and forced intrusion of a non-wetting liquid) are shown to present strong analogies in model materials as well as in disordered and interconnected porosities. © 2004 Published by Elsevier B.V. Keywords: Non-wetting; Mesoporous; Porosimetry; Hysteresis; Hydrophobicity
1. Introduction Porous materials are involved in many industrial processes such as catalysis, membrane technology, chromatography. . . . Accurate information on their porous texture (pore size distribution, pore shape, specific surface area) as well as surface chemistry data are required in order to understand their specific properties and to optimize their performances. Among the numerous characterization techniques, adsorption methods and mercury intrusion porosimetry are widely used to investigate porous textures [1]. On one hand, physical sorption of gases is the most famous route to investigate porosity in the micropore and mesopore range [2]. In the case of mesopores, the capillary ∗ Corresponding author. Tel.: +33-4-6716-3448; fax: +33-4-6716-3470. E-mail address: [email protected] (B. Lefevre).
0927-7757/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2004.04.020
condensation phenomenon, corresponding to the gas to liquid phase transition shifted by confinement effects, is exploited to obtain the porous characteristics listed before. In such experiments, the adsorbate molecules strongly interact with the pore surface (wetting configuration). The amount of adsorbed molecules is measured as a function of the applied vapor pressure. Great attention has been paid to the theoretical and experimental study of this confined phase transition in the past decades [3–8]. Most of these investigations were based on adsorption isotherms obtained on Vycor type materials [9], or silica gels. The most significant improvements in the understanding and simulation of this phenomenon are mainly related to the development of ordered micelle templated materials in 1992 [10]. Thanks to this new family of mesoporous supports of well-defined and ordered porosity, adsorption and desorption mechanisms in model geometries are now well understood, as discussed in details recently by Ravikovitch et al. [11].
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On the other hand, mercury intrusion porosimetry enables to investigate mesoporosity and macroporosity. This technique relies on the intrusion of a non-wetting liquid (mercury) into a porous material. In a typical experiment, the amount of liquid intruded into the pores is measured as a function of the applied liquid pressure. In some cases, cyclic pressure scans exhibit both intrusion and extrusion of the confined liquid. This type of cycles also present a pressure hysteresis. These observations reveal the existence of strong analogies between capillary condensation (wetting configuration) and forced-intrusion (non-wetting configuration) in mesopores. Surprisingly, the thermodynamic aspects of the hysteretic behavior in forced-intrusion of mercury have not been treated in details in the literature. Most simulation efforts are focused on the interpretation of the pressure hysteresis in terms of wetting hysteresis and geometrical consideration of pore network effects [12–14], such as the percolation approach [15] or renormalization [16]. To our knowledge, no results have been reported yet concerning the intrusion and extrusion of mercury in model materials such as micelle templated supports, probably because of their low mechanical stability upon the high pressures required for mercury intrusion. Such missing data would be of great importance to achieve a better understanding of this confined phase transition. However, other results obtained by using water instead of mercury represent an alternative and interesting source of data. Indeed, Eroshenko et al. developed a powerful technique, referred to as “water porosimetry”, to investigate hydrophobic porous materials [17–22]. They showed that intrusion and extrusion cycles could be obtained with water as a non-wetting liquid in strongly hydrophobic mesopores. This technique allows, for example, to estimate the surface hydrophobicity by determining the advancing contact angle θ a from the analysis of the intrusion pressure by means of the Laplace–Washburn equation: PL = −
2γ cos θa Rp
(1)
where PL is the pressure of the liquid for which intrusion occurs, γ the interfacial tension between the liquid and the vapor, and Rp is the pore radius. Gusev [23] and Gomes et al. [24] developed experimental devices allowing the simultaneous measurement of the pressure, the volume variation and the heat exchange during water intrusion experiments. A fine thermodynamic analysis of the intrusion phenomenon allowed them to determine simultaneously the pore size distribution and the advancing contact angle from a single water intrusion experiment on modified silica gels. Anyway, the deep understanding of the hysteresis was still not clearly understood. One remarkable fact about these water intrusion/extrusion data is the existence of reproducible cycles in a rather small range of pressure:between atmospheric pressure and 70 MPa. This possibility allowed us to test water intrusion in hydrophobized MCM-41 type materials in a range of relatively small
pressures for which they present enough mechanical stability [25–27]. A detailed study of the intrusion and extrusion process of water in model MCM-41 type materials was carried out [28]. The main conclusions are summarized in the first part of this paper. Then, in view of these conclusions, a qualitative analysis of the results obtained on other siliceous hydrophobic porous materials is proposed to highlight the additional effect of the pore texture on the hysteresis.
2. Materials and methods Three types of materials presenting completely different pore morphologies were chosen: MTS type materials (MCM-41), Controlled-Pore-Glasses (CPG) and silica gels. MTS materials possess model porous texture resulting from the self assembly of surfactants and silicates. Among this family, MCM-41 present non-intersecting cylindrical pores with a low pore size distribution. In the present study, four supports of increasing pore size, referred to as MTS-1, MTS-2, MTS-3 and MTS-4 were synthesized according to a procedure described elsewhere [28]. The mesoporous texture of Controlled Pore Glasses results from a spinodal decomposition leading to a bi-continuous highly interconnected network of pores presenting a relatively low pore size distribution. One commercially available CPG was obtained from Aldrich (ref CPG-75–120). Finally, two silica gels, one referred to as “FK” (Fluka 60, Aldrich) and the other referred to as “HP” (PEP100, Hypersil) were also selected. In this type of support, the mesoporosity results from the aggregation of primary nanoparticles, leading to a disordered porosity with a relatively large pore size distribution. The nitrogen sorption isotherms of the 7 samples listed above are gathered in Figs. 1 and 2. All these supports were modified by grafting of octyldimethylchlorosilane by a pyridine-assisted reaction [29], except sample HP which is commercially available as a C18-functionalized stationary phase for chromatographic applications (grafting density is unknown for this support). A constant grafting density of approximately 1.3 chains per nm2 was determined for the four MTS samples and about 2 chains per nm2 for samples FK and CPG. Water intrusion experiments were performed on a specially designed apparatus described elsewhere [30]. About 2 g of degassed hydrophobized material were gathered with a large excess of water (compared to the corresponding disposable pore volume) into a shrinkable polymer container. In a typical experiment, the pressure was first continuously increased from the atmospheric pressure (Patm ) to 80 MPa by means of a mechanical increasing constraint, and then decreased back to Patm . The time required for a complete cycle was about 4 min, as no significant difference was observed in our experiments for 1 min to 1 h cycles. A pause of 10 min at Patm was respected between each run. The pressure of the liquid and the volume variations were simultaneously
-1
Vads , ml.g STP
-1
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0.4
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0 1600 1400 1200 1000 800 600 400 MTS-4 200 0 0.8 1.0
P/P0
Fig. 1. Nitrogen sorption isotherms at 77 K of the MCM-41 type materials before surface modification. The adsorbed volume, Vads , is plotted as a function of the relative pressure of the vapor, P/P0 .
recorded. The volume values were corrected to eliminate the deformation and the compressibility contributions. The representation of the water pressure as a function of the corrected volume variation (per gram of bare silica) are denoted P/V curves. In these plots, the volume variations V reflect the cumulative intruded and extruded volumes of water into the pores during compression and decompression steps, respectively.
400
FK
-1
Vads , ml.g STP
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300 200 100
HP 500 400
-1
Vads , ml.g STP
0
300 200 100 0 0.0
CPG 0.2
0.4
0.6
0.8
1.0
P/P0 Fig. 2. Nitrogen sorption isotherms at 77 K of (up) the silica gels FK and HP and (down) the Controlled Pore Glass, before surface modification.
3. Results and discussion 3.1. Model MCM-41 type materials Tens of reproducible cycles have been recorded on grafted samples MTS-1, MTS-2 and MTS-3. For sample MTS-4 only one intrusion could be recorded as no water extrusion took place during the decompression step, and even after several hours at Patm . For this material, spontaneous extrusion of water did not occur. The corresponding P/V curves are reported in Fig. 3. The mean intrusion and extrusion pressures are reported as a function of the mean pore radii for these materials on Fig. 4 in logarithmic scales. We can note that the mean intrusion pressure is a linear function of R−1 p . This confirms that the intrusion mechanism can be described by means of Eq. (1) for these series of model materials. The value of θ a , derived from the pre-factor ◦ (−2γ cos θ a ) of the R−1 p scaling law, is 120 , which is consistent with macroscopic measurements reported on the same type of grafted surfaces. This clearly demonstrates the validity of Eq. (1) for pore radius as small as 1.3 nm. On the contrary, the mechanism of extrusion is radically different:the mean extrusion pressure is far to obey the R−1 p scaling law. The decrease of the mean extrusion pressure with increasing pore radius is much more sudden. The trend extrapolated from the mean extrusion pressures recorded for samples MTS-1, MTS-2 and MTS-3 (see the dotted line in Fig. 4) suggests that the extrusion pressure for large pores may be inferior to Patm , which is consistent with the reported lack of extrusion for the large pores MTS-4 sample. The scaling of the extrusion pressure as a function of the pore mean pore radius Rp shows that the extrusion
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∆V , mm .g
Pressure , MPa
-400 -300 80 70 MTS-1 60 50 40 30 20 10 0 80 70 MTS-3 60 50 40 30 20 10 0 -400 -300
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0
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Pressure , MPa
268
0
∆V , mm .g 3
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Fig. 3. Water intrusion and extrusion cycles obtained at room temperature for the hydrophobized MCM-41 type materials. The applied pressure of the liquid is plotted as a function of the volume variation of the system. The values of pore volumes which can be deduced from the volume variations at the end of the intrusion steps are systematically smaller than the initial pore volumes determined by nitrogen sorption, mainly because of the presence of high amounts of grafted phase from which water is excluded.
pressure can not be described by means of Eq. (1). Therefore, the extrusion process has to be governed by another mechanism:the nucleation of a vapor bubble (cavitation) is necessary to generate two liquid/vapor menisci in a pore initially filled with liquid water only. The extrusion is experimentally observed when the probability for nucleation of the vapor phase (in each pore) becomes significant over the time of the experiment. Since the nucleation rate usually
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4
obeys an Arrhenius law, we expect the extrusion pressure to vary much more rapidly with the pore radius Rp than the R−1 p scaling law corresponding to Eq. (1). We show in a more detailed and theoretical analysis [28] that the nucleation process of a vapor phase in a cylindrical capillary can quantitatively account for the values of the mean extrusion pressures reported here for the samples MTS-1, MTS-2 and MTS-3. We can conclude that, in a model cylindrical pore, intrusion and extrusion are not governed by the same mechanism. The description of the withdrawal of water in these materials using the Eq. (1) and assuming a receding contact angle θ r is therefore not adequate. Finally, from the tendency extrapolated from the experimental data of extrusion (dotted line in Fig. 4) we can define a critical pore size Rc for the extrusion of water to be observed at a given pressure PL ≥ Patm for some materials presenting the same hydrophobic properties as the MCM-41 type supports under study:this critical pore size stands approximately between around 4 nm.
5 6
Mean pore radius , nm Fig. 4. Logarithmic representation of the mean intrusion pressure PINT (䊉) and the mean extrusion pressure PEXT (䉬) as a function of the mean pore radius for the hydrophobized MCM-41 type materials. The horizontal dotted line represents the atmospheric pressure Patm . The symbol (䊊) represents the extrapolation of the extrusion tendency to the mean pore radius of the support MTS-4. (Mean pore radii of the grafted supports were determined by means of Broekhoff–De Boer theory from nitrogen sorption data).
3.2. Silica gels and CPG The water intrusion and extrusion curves related to these three supports are reported in Fig. 5. For the silica gel FK, stable intrusion and extrusion cycles were obtained according to the procedure described above. For the silica gel HP, only half of the first intruded water volume was recovered after extrusion under the same experimental procedure. As
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Norm. ∆V , a.u. Fig. 6. Comparison between the water intrusion curves obtained by simulation of the intrusion of water according to the Laplace–Washburn equation into a 2D-connected network (-䊊-) and a non-interconnected medium (—). The same pore size distribution was applied in both cases and chosen arbitrary centered at 4 nm assuming a gaussian distribution (σ = 0.6 nm). The advancing contact angle θ a was set to 115◦ .
HP
30
(1)
20 10 0
Pressure , MPa
Pressure , MPa
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269
(2) -400
-300
-200
-100
0
∆V , mm .g 3
-1
Fig. 5. Water intrusion and extrusion cycles obtained at room temperature for (up) the hydrophobized silica gel FK and Controlled Pore Glass support and (down) the hydrophobized silica gel HP.
a consequence, “reduced” hysteresis cycles were registered for the subsequent runs. Nevertheless, if the system (sample HP + water) was left overnight at Patm , extrusion effectively occurred and a further intrusion curve similar to the initial one was obtained. Such a low kinetic of extrusion was only observed for this sample. Finally, only one intrusion could be recorded on the CPG sample, as extrusion did not happen even after a storage of several days at Patm (as for sample MTS-4). In Fig. 7, the mean intrusion and extrusion pressure for these three supports are reported as a function of the mean pore radii in logarithmic scales, and compared to the data of the MTS support series. 3.3. Intrusion mechanism in interconnected pores During the intrusion of a non-wetting liquid in an interconnected porous network presenting a given pore size distribution, well-known pore blocking effects are expected. Their influence on the macroscopic intrusion curves (P versus V) can easily be determined by simulating injection experiments in model networks. An example is reported in Fig. 6. A porous network with a given gaussian pore size distribution was generated by means of a 2-D square-lattice network (each connection is linked to 4 pores). Then, an algorithm was applied to simulate the injection curve:for a given value of the liquid pressure PL , all the pores already connected to external liquid phase and with a pore radius such as Rp ≥ −2γ cos θ a /PL are invaded by the liquid. In Fig. 6, the result is compared to the injection in a net-
work presenting the same pore size distribution but where pore-blocking effects are absent. The effect of the connectivity is a “smoothing” of the intrusion curve at the beginning of the injection. If we now compare the experimental intrusion curves for the MCM-41 type materials (Fig. 3) and for the particular case of the silica gel FK, we can note the same trend at the beginning of the intrusion for the connected gel, which is not the case for MCM-41s, see Fig. 5. The existence of pore-blocking effects in the disordered and connected texture of the silica gel could account for this experimental result. The important point is that this basic injection model indicates that the mean intrusion pressure in the connected medium is practically not affected by network effects. The mean intrusion pressure can be obtained from Eq. (1), taking into account the mean pore radius of the sample. Actually, this is the reason why Fadeev et al. could validate the use of the Laplace–Washburn equation for the description of the mean intrusion pressure of water in a series of hydrophobized silica gels [20,21] by checking its linear dependency as a function of the inverse mean pore radius (R−1 p ). Some of their results [21] have been reported in Fig. 7 (symbols (+) and line A). In this figure, it appears that the values of the mean intrusion pressures in the silica gels FK and HP, and in the CPG support are close to the values measured in the MCM-41 type materials and therefore in quite good agreement with the previous R−1 p scaling law. Thus, in spite of very different pore geometry, the mean intrusion pressure in those hydrophobic media is governed by the same mechanism. Only the pre-factor of the R−1 p scaling law is different, due to slight differences in the surface chemistry.
4. Extrusion mechanism in interconnected pores The value of the mean extrusion pressure for sample FK is in very good agreement with the data of Fadeev et al [21].
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Pressure, MPa
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100
A
10
B
from ref. [21]
1 0,1 0,01 1
from ref. [21]
Patm 2
3
4
5 6
Mean pore radius , nm Fig. 7. Logarithmic representation of the mean intrusion and extrusion pressures as a function of the mean pore radii (similar plotting as Fig. 5):mean intrusion pressure (䊉) and mean extrusion pressure (䉬) for the MTS supports, respectively (䊐) and (䊊) for the silica gel FK, () and (⊕) for the silica gel HP (first intrusion) and () for the intrusion in the CPG support. The symbols (+) and (×) represent respectively the mean intrusion and extrusion pressure values extracted from the exhaustive study published by Fadeev et al in Fig. 3 of reference [21]. These data were obtained on silica gels chemically modified by means of octyldimethylchlorosilane.
See symbol (䊊) and line B in Fig. 7. Thus, the mean extrusion pressure in this series of silica gels obeys a R−1 p scaling law, in the same way as the mean intrusion pressure. The physics of the extrusion process in these connected media is therefore completely different from the one observed in the MCM-41 type supports. Actually, the extrusion of water from an interconnected pore network can occur if both following requirements are fulfilled: (1) the possibility for the vapor phase to form during the pressure release and (2) the possibility for the resulting menisci to propagate into the pore network from pore to pore to empty the porous medium, for values of the applied liquid pressure PL ≥ Patm . Note that in these supports, the first condition (nucleation of the vapor phase) is absolutely not necessary in all the pores of the network, contrary to the case of the non-interconnected MCM-41 supports. In fact, according to this interpretation, only a few preferential nucleation sites are required for the vapor phase to appear. Then, the extrusion can proceed exclusively by propagation of menisci if the second condition is fulfilled. Consequently, to interpret properly the extrusion mechanism in interconnected pores, one has to consider these two phenomena:nucleation and propagation. Nucleation is most probable to take place in a favourable environment such as the smallest pores and/or the most hydrophobic sites. Anyway, large pores for which extrusion by means of nucleation is impossible at PL ≥ Patm (in the time scale of the experiment), can be emptied by propagation of menisci from surrounding pores at PL ≥ Patm . The withdrawal condition (Patm ≥ −2␥ cos θ r /Rp ) being satisfied for all the pores, complete extrusion could then occur by menisci propagation from the initial nucleation sites. Of course, pore-blocking effects are also expected during the propagation, as detailed previously for the intrusion process.
The available experimental results obtained for the three samples under study and other results extracted from [21], all of which are gathered in Fig. 7, indicate that the extrusion for all the silica gels (except sample HP) obey to a R−1 p scaling law. This means that the mean extrusion pressure can also be described in these materials by means of Eq. (1), taking into account the mean pore radius of the pore size distribution, and assuming the existence of a receding contact angle θ r . Thus, the important point is that the pressure hysteresis observed between intrusion and extrusion can be described by means of wetting hysteresis (θ = θa − θr ) as concluded earlier [20,21]. Actually, the existence of an initial stage in the extrusion process, i.e. the local formation of the vapor phase by nucleation, can not be underscored by such experiments because (1) the resulting volume variation is not significant enough to be detected, and because (2) the whole extrusion process is governed by the propagation of the menisci in the pore network. In other words, it is highly probable that the menisci generated by nucleation in the most favourable sites are unable to propagate as long as the applied pressure is to high to allow their motion in the surrounding pores. The macroscopic extrusion mechanism in these interconnected pore networks is then completely different from the one observed in the non-interconnected MCM-41 type materials. Following the previous arguments, the presence of nucleation sites is required in the first stage of the extrusion. As deduced from the tendency extrapolated from the MCM-41 data, a critical pore size Rc (around 4 nm) is expected for nucleation to take place. If the pore size distribution of a connected sample does not include any pore of radius Rp ≤ Rc then this initial stage of nucleation becomes highly improbable and, as a consequence, extrusion could be impossible. Now considering the case of the CPG support, we note that extrusion does not occur in this material. Indeed, its hydrophobic properties are quite similar to the other samples under study. Anyway, its pore size distribution is narrow (as revealed by nitrogen sorption and by the low dispersion of the water intrusion pressure) and centred around 5.6 nm. Therefore, the absence of small pores in which nucleation could be favoured is one possible explanation to account for this lack of extrusion. Finally, the specific case of sample HP, with a low kinetic of emptying at atmospheric pressure, has not been reported before to our knowledge. The previous arguments fail to describe completely this experimental observation, which suggest the contribution of an additional mechanism to the extrusion process. In the disordered structure of a silica gel, some breaks in the continuous liquid paths can occur during the extrusion due to connectivity effects, resulting in the entrapment of part of the confined liquid. This mechanism has been clearly identified in mercury intrusion porosimetry [13]. The presence of a residual amount of trapped water being thermodynamically unfavourable at Patm , pore drying by means of a vapor phase transport to the external liquid phase is a possible pathway to account for this result.
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5. Conclusion
Acknowledgements
The technique of water porosimetry applied to model micelle-templated materials of the MCM-41 type provided a unique opportunity to investigate the behavior of a non-wetting liquid confined in lyophobic mesopores of model geometry. We could then compared the hysteretic properties of the intrusion/extrusion phenomena in the ideal pore geometry of the MCM-41 type supports and in interconnected pore networks (silica gel and Controlled-Pore-Glass supports). For all these materials, the mean intrusion pressure was shown to be inversely proportional to the mean pore radius, which confirms the validity of the Laplace–Washburn equation down to extremely small pore radius (1.3 nm), regardless of the mesoporous texture. In model MCM-41 supports, the extrusion can not be described as the withdrawal of menisci with a receding contact angle according to the Laplace–Washburn equation. We show that the extrusion has to be governed by another mechanism:the nucleation of the vapor phase. For this geometry, the argument of wetting hysteresis is not relevant to explain the entire experimental pressure hysteresis in our water intrusion/extrusion experiments, which had not been clearly pointed out before. The mean extrusion pressure in these materials does not obey the Laplace–Washburn equation. It decreases much more rapidly with increasing pore radius than the R−1 p scaling law corresponding to this equation. During the pressure release, the pressure threshold required for extrusion appears to be related to the capacity for a stable vapor phase to form into the metastable confined liquid. From this series of model pore geometry, we defined a critical pore size Rc (around 4 nm):nucleation is impossible for pores of radius inferior to Rc . In interconnected media, the extrusion process is different. If the pore size distribution is large enough, the extrusion is governed by Eq. (1), in the same way as the intrusion. This is due to the fact that nucleation may occur in the smallest pores, then followed by propagation. Our experimental results on the silica gel FK are in good agreement with other data reported earlier [21]. For an interconnected medium presenting a low pore size distribution, if the mean pore size is too large, then nucleation is impossible, as observed for the CPG sample. Finally, whereas the intrusion can always by described by means of the Laplace–Washburn equation (applied to mean values of the intrusion pressure and the pore radius), nucleation effects are predominant for the extrusion process. Slow kinetic effects have also been reported, suggesting that vapor phase transports in the porous medium are expectable during the extrusion. All these results confirm that the phenomenon of forced-intrusion and extrusion of a non-wetting liquid such as water in mesoporous materials can be considered and treated as a confined phase transition.
We are indebted to T. Martin, C. Biolley, A. Galarneau, D. Brunel, F. Di Renzo, F. Fajula (Laboratoire des Matériaux Catalytiques et Catalyse en Chimie Organique, Montpellier, France) for providing us with most of the materials and for fruitful discussions as well as R. Denoyel (Centre de Thermodynamique et Microcalorimétrie, Marseille, France). B. Lefevre and A. Saugey are supported by the French Ministry of Defense (DGA).
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