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Langmuir 2008, 24, 2772-2778
New Nanomaterials for Consolidating Stone Maria J. Mosquera,*,† Desiree´ M. de los Santos,† Antonio Montes,† and Lucila Valdez-Castro‡ Departamentos de Quı´mica-Fı´sica and de Fı´sica de la Materia Condensada, Facultad de Ciencias, Campus UniVersitario Rı´o San Pedro, UniVersidad de Ca´ diz, 11510 Puerto Real, Ca´ diz, Spain ReceiVed NoVember 22, 2007 A novel sol-gel synthesis, in which a surfactant acts to make the pore size of the gel network more coarse and uniform, is shown to provide an effective alternative for the consolidation of stone. The new mesoporous silica avoids the main inconvenience of current commercial consolidants, which is their tendency to crack inside the pores of the stone. Since the cracking of xerogels is a well-known drawback of the sol-gel process, the synthesis presented here can be extended to other applications. Finally, preliminary studies of the effectiveness of the novel surfactant-templated sol in consolidating a typical biocalcareous stone are also discussed.
Introduction The sol-gel process has been found to be successful in applications for the conservation of stonework. Commercial products containing alkoxysilanes, such as tetraethoxysilane (TEOS), are commonly used for the consolidation of stone. These products polymerize within the pores of stone by means of a classic sol-gel process, thereby strengthening the material. Their advantages are well-known: the low viscosity of TEOS allows it to penetrate deeply into porous stone; after polymerization, which occurs upon contact with environmental moisture, a stable gel with a silicon-oxygen backbone is formed.1 In spite of their extensive use and indisputable benefits, TEOSbased consolidants suffer some practical drawbacks. Most of these inconveniences are a consequence of the formation of a dense microporous gel network inside the stone, which is the typical gel obtained from TEOS sols. As reported in previous studies,2-5 these consolidating materials tend to form brittle gels susceptible to cracking. Cracking is generated by the high capillary pressures supported by the gel network during drying. The capillary tension value is given by the Young-Laplace equation. Equation 1 shows this expression which corresponds to cylindrical pores, where γ is the liquid-vapor tension, θ is the contact angle of the liquid to the pore wall, and rp is the pore radius.
Pmax )
2γLV cos θ rp
(1)
To address these problems, several trials have been carried out in recent years to obtain mesoporous consolidating gels. For * To whom correspondence should be addressed. E-mail: mariajesus.
[email protected]. Phone: (34)956016331. Fax: (34)956016471. † Departamento de Quı´mica-Fı´sica. ‡ Departamento de Fı´sica de la Materia Condensada. (1) Wheeler, G. S.; Fleming S. A.; Ebersole, S. In Proceedings of the 7th International Congress on Deterioration and ConserVation of Stone; Delgado, J., Henriques, F., Jeremias, F., Eds.; Lisbon, 1992. (2) Lewin, S. Z.; Wheeler, G. E. Proceedings of the 5th International Congress on Deterioration and ConserVation of Stone; Presses Polytechniques Romades: Lausanne, Switzerland, 1985. (3) Scherer G. W.; Wheeler G. E. In Proceedings of the 4th International Symposium on the ConserVation of Monuments in the Mediterranean; Moropoulou, A., Zezza, F., Kollias E., Papachristodoulou, I., Eds.; Rhodes. (4) Mosquera, M. J.; Pozo, J.; Esquivias, L.; Rivas, T.; Silva, B. J. Non-Cryst. Solids 2002, 311, 185. (5) Mosquera, M. J.; Pozo, J.; Esquivias, L. J. Sol-Gel Sci. Technol. 2003, 26, 1227.
example, Scherer et al.6 added different colloidal oxide particles to a TEOS solution, and the current authors7,8 mixed colloidal fumed silica particles and TEOS under ultrasonic agitation. The pores formed between these relatively large particles are also large. The main inconvenience of this procedure is that the addition of the colloid prevents the formation of a transparent gel. The colloid-polymer composite exhibits notable whiteness, which can modify the color of the consolidated stone. Recently, Miliani et al.9 synthesized particle-modified silica consolidants with a range of colloidal particles. In this way the consolidant color can be tailored by changing the oxide particles used as filler. The drawback of this procedure is the need to synthesize specific starting sols with a particle oxide that perfectly matches the color of the stone in each case. The discovery of surfactant-templated synthesis in the early 1990s10,11 permitted ordered mesoporous silica gels to be obtained, with promising applications in fields as different as electronics and medicine. The surfactant, added in a concentration above its critical micellar concentration (cmc), acts as the structure-directing agent during polymerization. These surfactant aggregates are later removed by calcination or extraction procedures. The main advantage of these new materials, called molecular sieves, is their ordered mesopores with uniform size, which is determined by the size of the surfactant aggregate. This kind of synthesis prevents the cracking of the gel during the drying phase for two reasons:12,13 (1) the surfactant creates a coarsening of the gel network that reduces the capillary pressure; (2) the decrease of surface tension provided by the surfactant also reduces the capillary pressure. Therefore, the starting point of our investigation was the desire to obtain a crack-free xerogel by using a surfactant (6) Escalante, M. R.; Valenza, J.; Scherer, G. W. In Proceedings of the 9th International Congress on Deterioration and ConserVation of Stone; Fassina, V., Ed., Venice, 2000. (7) Mosquera, M. J.; Bejarano, M.; de la Rosa-Fox, N.; Esquivias, L. Langmuir 2003, 19, 951. (8) Mosquera, M. J.; de los Santos, D. M; Montes, A. Proceedings of the Materials Research Society Symposium; Materials Research Society: Boston, 2005. (9) Miliani, C.; Velo-Simpson, M. L.; Scherer, G. W. J. Cultural Heritage 2007, 8, 1. (10) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartulli, J. C. Nature 1992, 359, 710. (11) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; Mc Cullen, S. B.; Higgins, J. B.; Schenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (12) Scherer, G. W. J. Non-Cryst. Solids 1989, 107, 135. (13) Scherer, G. W. J. Am. Ceram. Soc. 1990, 73, 3.
10.1021/la703652y CCC: $40.75 © 2008 American Chemical Society Published on Web 02/01/2008
New Nanomaterials for Consolidating Stone
as a template for the pores of the gel. However, we used a substantially lower surfactant content than that used in the classic templating routes. The present work involves an attempt to design a synthesis according to the following special features of the consolidation process: The sol must have low viscosity to penetrate into the stone walls of a building. It must then gel in situ in a few hours. Finally, the surfactant must be spontaneously removed. In accordance with these requirements, the synthesis is designed in the following way. (1) The surfactant used is a primary amine. As Tanev and Pinnavaia14 reported, the silica interacts with the template by hydrogen bonding. Therefore, the removal of surfactant in a neutral pathway must prove to be easier14,15 than that corresponding to the classic route involving electrostatic interactions between cationic surfactants and anionic inorganic species. In fact, the neutral templating route enabled removal of the surfactant by a series of methods other than calcination, which is the typical procedure used when the template is ionic. According to the literature,15 the neutral surfactant can be removed by solvent extraction, by heating at lower temperatures than the calcination temperature, in vacuo, and even by simple air drying. (2) A relatively small amount of the surfactant is addeds below its cmcssince a higher concentration promotes the instantaneous gelation of TEOS. Therefore, we rely on the selfassembly being driven by surfactant molecules, although micelles are not created, to avoid the instantaneous sol-gel transition. (3) Different ethanol contents are used in the starting sol to delay gelation. As Brinker and Scherer earlier reported,16 increasing ethanol concentrations may delay the hydrolysis phase by reducing the catalytic activity under acidic or basic conditions. In addition, the ethanol also delays the sol-gel transition by reversing the hydrolysis reaction (re-esterification) or even by reversing the condensation reactions in which siloxane bonds are created.16-18 Experimental Section Synthesis. Gels were prepared from a starting sol containing TEOS in the presence of an amine primary surfactant (n-octylamine). The silica sol was prepared by refluxing TEOS, ethanol, water, and 0.07 N hydrochloric acid at 60 °C for 90 min. Next, n-octylamine was added to the mixture under vigorous stirring. The mole ratios of the mixture were 1 TEOS:5 H2O:0.004 HCl:0.003 n-octylamine. Four sols with different mole ratios of TEOS to ethanol (1:22, 1:11, 1:7, and 1:4) were prepared and stirred at ambient temperature for 10 min. The surfactant-templated materials are denoted by their TEOS:ethanol molar ratio, i.e., 1:22. After completion of the synthesis, sols were cast in cylindrical molds. The dimensions of the wet xerogels were 3.15 cm diameter and 4.61 cm length. Dried xerogels were obtained by simple exposure of the sols to laboratory conditions (relative humidity of 60% and temperature of 20 °C) until a constant weight was reached. To simulate the solvent evaporation throughout the rock, it was limited by pinholes. For comparison, a popular commercial consolidant, Tegovakon V100 (hereafter named TV), supplied by GoldschmidtDegussa, was also allowed to gel under the same conditions. Tegovakon consists of partially prepolymerized TEOS, dibutyltin dilaurate (DBTL) catalyst, and a small amount ethanol. (14) Tanev, P. T.; Pinnavia, T. J. Science 1995, 267, 865. (15) Prado, A. G. S.; Airoldi, C. J. Mater. Chem. 2002, 12, 3823. (16) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990. (17) Gupta, R.; Mozumdar, S.; Chaudhury, N. K. Biosens. Bioelectron. 2005, 21, 549. (18) Zarraga, R.; Alvarez-Gasca, D. E.; Cervantes, J. Silicon Chem. 2002, 1, 397.
Langmuir, Vol. 24, No. 6, 2008 2773 Experimental Procedures. As the consolidant must diffuse into the rock pores, the sols were subjected, immediately after their synthesis, to a rheology study using a concentric cylinder viscosimeter (model DV-II+ with UL/Y adapter) from Brookfield. Experiments were performed at 25 °C by the recirculated water from a thermostatic bath. A shear stress versus shear rate flow curve was generated. For comparison, the rheological properties of the Tegovakon V100 were also evaluated. The structure of the powdered xerogels was characterized by X-ray diffraction (Bruker D8 Advance diffractometer equipped with Cu tube X-ray and using Cu KR radiation) and by nitrogen physisorption (Sorptomatic 1990 from Fisons Instruments). The degree of amine residue was evaluated by means of Fourier transform infrared spectrophotometry (FTIR). The spectra were recorded in KBr pellets using a Vertex 70 spectrophotometer from Bruker (4 cm-1 resolution) in the region from 4000 to 400 cm-1. To investigate surfactant removal as a function of time, FTIR spectra were obtained 2, 6, and 8 months after the synthesis. Finally, a preliminary study of the effectiveness of the surfactanttemplated materials for consolidation was carried out for a variety of “El Puerto de Santa Marı´a biocalcareous stone”, the most popular monumental building rock employed in Spain’s southwest region. For example, the cathedral of Seville was built with this stone. The stone consists of quartz (∼48%) and calcite (∼52%) and has a porosity of around 15% calculated by mercury intrusion porosimetry (MIP). The consolidation was performed by immersing 4 cm cubes to a depth of around 1 cm in closed flasks for 24 h. Next, the specimens were transferred to open vials until constant weight. For comparison, other stone specimens were consolidated, under the same conditions, with TV. Samples corresponding to untreated stone and stone treated with each of the consolidants were characterized by the procedures described below. To characterize the chemical compatibility achieved between consolidants and the stone specimens, FTIR spectra were recorded by using the procedure described earlier. Changes in the pore structure of the stone after consolidation were checked by MIP. It is carried out on rock specimens with a volume of around 1 cm3 by using a Pascal porosimeter from Fisons Instruments. Pressure was varied between 0.1 kPa and 400 MPa, which permitted a wide range of pore radius dimensions, from 58 000 to 1.9 nm, to be characterized. A JEOL JSMT-300 scanning electron microscope was used to visualize changes in the morphology of the stone after consolidation. Internal fragments of treated stone specimens and their untreated counterpart were coated with a thin conducting layer of gold. In addition, two properties, compressive strength and vapor permeability, that play a key role in the consolidation suitability, were investigated. Compressive strength was measured by using a Shimadzu autograph-AGI with a maximum force capacity of 50 kN. Ten 1 × 1 × 2 cm prisms for each specimen were measured. Water vapor permeability was evaluated using an automatic setup developed in our laboratory. The test is performed as follows by a protocol established in a previous paper:19 Three specimens for each sample studied are cut from 4 cm stone cubes into 1 cm thick slabs. They are placed as a cover on a cup without an upper side. A moisturesaturated ambient condition is maintained in the cup, whereas the relative humidity outside is around 20%. The cup mass is recorded continuously by means of computer software. Vapor diffusion coefficients were calculated from steady flow data.
Results and Discussion Sol-Gel Characterization. All the sols exhibited a Newtonian behavior at the shear range evaluated. Therefore, the viscosity was calculated as the slope of the shear rate vs shear stress curve. In all the cases, the linear regression coefficients were above 0.99. The viscosity values obtained from sols containing 1:22, 1:11, 1:7, and 1:4 TEOS:ethanol mole ratios were 2.23, 2.29, (19) Mosquera, M. J.; Benı´tez, D.; Perry, S. H. Cem. Concr. Res. 2002, 32, 1883.
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Figure 2. Nitrogen adsorption isotherms for the silica xerogels under study. Figure 1. Upper image: Wet gels obtained immediately after the gelation. The mold used for the gels can be seen on the right. Lower image: Xerogels obtained after drying in laboratory conditions (a) from the commercial consolidant Tegovakon V100 and (b) from the 1:7 sol prepared in our laboratory.
2.59, and 3.13 mPa‚s, respectively. As expected, the reduction in ethanol content promotes a slight increase in the viscosity value. Similarly, the viscosity of the TV sol is 2.77 mPa‚s. The gel times of the synthesized sols, except the 1:4 sol that gelled immediately, range from 8 to 24 h. Although with Tegovakon the gelation time was slighter longer (from 24 to 48 h), these times are sufficient to ensure penetration into the stone. At this point in the discussion, we emphasize that the gelation times and the structure of the gels could be different when the sols are applied inside the stone materials. For example, the sols contain a small amount of acid that might easily be consumed by reaction with calcareous rocks during the absorption of the consolidants. Therefore, some of these products might prove to be unsuitable when they are applied to a particular type of stone. After drying under laboratory conditions, transparent and crackfree xerogels were obtained from all the sols synthesized in our laboratory, whereas the gel obtained from TV sol showed significant cracking (see Figure 1). This shows that the synthesis proposed is an effective procedure for avoiding cracks during drying. The volume reduction for the materials under study was 97%, 92%, 90%, and 89% for 1:22, 1:11, 1:7, and 1:4 sols, respectively. These results perfectly match those obtained by Scherer et al.20 As reported in the cited paper, the gels containing more ethanol contract more during drying because they have a low initial density, and thus their bulk modulus is lower. The X-ray patterns of all silicas synthesized in our laboratory corresponded to amorphous materials without any detectable ordered structure. As could be expected, the Tegovakon pattern was also amorphous. It can be concluded that, in this case, the surfactant does not promote an ordered pore structure, which is typical of many materials synthesized via a surfactant template. The low surfactant concentration used, below the cmc, avoids the formation of micelles, which are responsible for the ordered pore structure. Nitrogen adsorption isotherms for the samples under study are shown in Figure 2. To characterize the role played by the surfactant on the textural properties, a 1:11 xerogel sample prepared without surfactant (denoted as N-S) was also investigated. The xerogel obtained from the commercial consolidant exhibited adsorption at low relative pressures (below 0.1), which (20) Scherer, G. W.; Haereid, S.; Nilson, E.; Einarsrud, M. A. J. Non-Cryst. Solids 1996, 202, 42.
then levels off; this is indicative of adsorption in micropores (type I isotherm). The silica synthesized without surfactant showed a similar type I profile. When the surfactant was used in the synthesis, we observed some significant differences in the curves obtained. First, the isotherms contain linear to step-shaped uptakes at partial pressure between 0.1 and 0.6, and this is even more notable for the 1:7 specimen. As inferred from the results of recent studies,21,22 the nitrogen condensation for surfactant-templated materials consisting of cylindrical uniform pores usually takes place at low relative pressures (below typical values, which are above p/p0 > 0.4). Therefore, these profiles actually correspond to a material consisting of a uniform mesopore network. Second, the 1:11 and 1:22 specimens exhibit a parallel and almost horizontal hysteresis loop at high pressures, which is classified as type H4, whereas the 1:7 specimen shows a type H3 loop. Type H3 and H4 loops have previously been reported22 for surfactant-assembled materials that exhibited a secondary disordered porosity consisting of larger mesopores. Therefore, this would suggest that this kind of material could be constituted by two different kinds of porosity: uniform size mesopores generated by the surfactant templating, which are classified as primary porosity, and other larger size and heterogeneous pores, for instance, interparticulate mesopores and macropores, which are referred to as secondary porosity. We investigated the presence of the two kinds of porosity in our materials by using the high-resolution Rs plot method,23,24 in which hydroxylated silica gel25,26 was used as a reference nonporous adsorbent. For all surfactant-assembled silicas under study, the Rs plots exhibited three linear segments: (1) for 0.1 < Rs < 0.8, which corresponds to micropore adsorption; (2) for 0.8 < Rs < 1.7, capillary condensation in uniform primary mesopores; (3) for 1.7 < Rs < 3.3, capillary condensation in secondary pores. Therefore, the intercept of segment 1 was used to assess the micropore volume, and the slope of such a linear segment was used to calculate the total surface area (Stotal). Further, we used the slope of segment 3 for calculating the surface area of the secondary porosity (Ssecondary). The intercept of linear segment 3 allowed us to calculate the volume of primary mesopores (Vprimary). The volume of secondary porosity was calculated by subtracting the primary and micropore volumes from the total pore volume. (21) Mercier, L.; Pinnavia, T. J. EnViron. Sci. Technol. 1998, 32, 2749. (22) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169. (23) Kruk, M.; Jaroniec, M.; Ryoo, R.; Kim, J. M. Microporous Mater. 1997, 12, 93. (24) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (25) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410. (26) Qiao, S. Z.; Bhatia, S. K.; Zhao, X. S. Microporous Mesoporous Mater. 2003, 65, 287.
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Table 1. Textural Parameters Obtained from the Isotherms and the BET and rs Plot Methods for the Silica Materials under Study sample
SBET (m2/g)
Stotal (m2/g)
Ssecondary (m2/g)
1:22 1:11 1:7 1:4 N-S TV
755 968 1087 602 424 216
716 953 1496 618
38 56 44 20
254
Sexternal (m2/g)
Vpore (cm3/g)
Vmicropore (cm3/g)
Vprimary (cm3/g)
Vsecondary (cm3/g)
0.04 0.05 0.09 0.06 0.21 0.10
0.49 0.60 0.80 0.30
0.05 0.13 0.03 0.00
6.4 48.9
0.58 0.78 0.92 0.33 0.23 0.18
In the cases of the silica prepared without surfactant and the TV, which are typical microporous materials, the Rs plots exhibited only two segments: (1) for 0.1 < Rs < 1, which corresponds to micropore adsorption; (2) for 1 < Rs < 2, capillary condensation in larger pores. Thus, the slope of segment 1 is used to calculate the total surface area of the material, whereas segment 2, in which micropore adsorption is completed, is used to calculate the external (macropore and mesopore) surface area (slope) and micropore volume (intercept). The textural data obtained are shown in Table 1. As can be appreciated in the Rs plots supplied in the Supporting Information, the number of data points recorded at low pressures is small. However, the S total data measured as the slope of the first segment corresponding to low pressure is quite accurate, except for the no-surfactant material, since the regression coefficients obtained are above 0.99 and the number of data points used for fitting is more than 10. In the case of the no-surfactant material, the data obtained in the low-pressure range are not sufficient for its accurate determination; thus, the total surface data obtained are not included in Table 1. First, it should be noted that the total surface area obtained is in good agreement with the calculated BET surface area. This confirms that there is no condensation in mesopores in the Rs interval selected as segment 1, and therefore, the approach developed is correct. Regarding the results obtained, we observed that the surfactant plays a key role in the structure of the silica. All the materials synthesized in the presence of surfactant show a high percentage of mesopores (primary porosity), and this percentage rises to around 80% of the total pore volume. In contrast, the silicas synthesized without surfactant exhibited a high micropore volume (91% for N-S silica and 55% for TV). Due to the high proportion of mesopores in the materials under study, the pore size distribution was obtained from desorption data by using the Barrett-Joyner-Halenda (BJH) method.27 Since the hysteresis loops of the isotherms indicate the possibility of pore constrictions, the pore size distribution was also calculated using the adsorption branch, and the results obtained are similar to those obtained using the desorption branch. As could be
Figure 3. BJH pore size distribution for the silica xerogels under study.
expected, all the materials prepared in our laboratory exhibited a narrow pore size distribution centered at ∼3.5 nm (Figure 3), which is a clear indication of a uniform pore size network. It is obvious that these uniform mesopores, which correspond to primary porosity, are created by the surfactant molecules. This is corroborated by the BJH distribution of the silica synthesized under the same conditions, but without surfactant. In this case, no pores are detected in the mesoporous range (Figure 3). Concerning the effects on the silica structure produced by varying the ethanol concentration, we observed a clear modification of the pore volume at different ethanol molar ratios. Specifically, the primary volume is clearly modified (see Table 1), whereas the pore size remains unaltered. The 1:7 specimen shows the highest pore volume (0.92 cm3/g), whereas the 1:4 sample has the lowest value (0.30 cm3/g). In the case of the 1:4 sol, which contains the lowest ethanol concentration, we think that the surfactant-TEOS interaction is partially frustrated by the instantaneous alkoxysilane gelation. Therefore, its primary volume, which is created by the surfactant-TEOS interaction, is notably lower than those obtained from the sol with the highest ethanol content. For the other sols (1:22, 1:11, 1:7), the tendency is inverted since increasing the ethanol concentration reduces the gel porosity. As discussed in the literature,16,17 protic solvents such as ethanol at acidic pH can form a hydrogen bond with SiO- species, making them less nucleophilic. Thus, the interaction with the surfactant molecule, which is responsible for the primary pore formation, is more difficult for the sols with high ethanol concentration. Concerning the micropore volume, we assume this is determined by the sol-gel transition and thus is independent of the presence of the surfactant, as corroborated by the similar values obtained for sols prepared with different ethanol contents. The presence of residual surfactant in the silicas under study was investigated by FTIR at several time intervals. Since the spectra of all the silicas prepared in the presence of surfactant were similar, Figure 4 shows, as an example, the spectra of the 1:7 specimen at the recorded times. As could be expected, the peaks ranging between 3600-3400 and 1180-1080 cm-1, which are typically representative of the silica materials,15 were recorded at different times. After 2 months (t1), we can observe a peak at 1412 cm-1 which is attributed to amine C-N stretching,15 another peak at 3082 cm-1, assigned to N-H stretching,28 and a weak peak at 2922 cm-1, which is attributed to amine C-H stretching.15 These bands confirm the residual amine presence after air drying for 2 months. To confirm that C-H stretching band cannot be attributed to ethanol residues, we also show in Figure 4 a spectrum corresponding to a 1:7 gel, but synthesized without the n-octylamine. As can clearly be appreciated, this gel does not present the cited band at 2922 cm-1. After 6 months, the C-H and N-H stretching peaks have (27) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (28) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A: Theory and Applications in Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1997.
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Figure 4. FTIR spectra for the silica xerogel with a 1:7 TEOS: ethanol molar ratio. The time elapsed after the synthesis was 2 months for t1, 6 months for t2, and 8 months for t3. The xerogel with a 1:7 TEOS:ethanol molar ratio prepared without octylamine is named No-S.
disappeared, confirming that the surfactant is removed. These results suggest that the surfactant removal by air drying is effective but requires a long time, around 6 months. From these findings, we can assume that the nonionic surfactant interacts with TEOS by hydrogen bonding according to the assembly mechanism proposed in the literature.14,29 However, the syntheses proposed in this paper do not promote an ordered pore structure, as is clearly shown by XRD; this is typical of most materials synthesized via a surfactant template. After the sol-gel transition, we depend on simple air drying for the partial removal of the surfactant; this enables mesoporous silica with a tailored uniform pore size structure to be obtained. Concerning the role of the surfactant to prevent cracking, we think that the octylamine acts in two different ways: first, by coarsening the pore structure, which reduces the capillary pressure during drying and, second, by reducing the surface tension of the solvent. As the ethanol evaporates sooner than the water, the solvent is rich in water toward the end of the drying, when cracking is most likely. We measured the effect of the octylamine in water by the drop weight technique, and the surface tension was notably reduced (from 71.66 to 33.92 mN/m at 26 °C). Since the reduction of the surface tension promotes reduction in the capillary pressure, according to the Young-Laplace equation (eq 1), we think that the surfactant also prevents cracking by reducing the surface tension of the solvent. Effectiveness of Consolidation. All the sols under study penetrated to the top surface of the 4 cm cubes of the stone tested in only a few hours. As expected from the similar viscosity values, the infiltration rate of surfactant-templated sols was close to that corresponding to the commercial sol. The FTIR spectra of untreated and consolidating stone specimens are shown in Figure 5. As similar spectra are obtained (29) Ayral, A.; Balzer, C.; Dabadie, T.; Guizard, C.; Julbe, A. Catal. Today 1995, 25, 219.
Mosquera et al.
Figure 5. FTIR spectra for untreated and consolidating stone samples under study. The spectra at the same scale for untreated and 1:7consolidated stone in the region 2000-400 cm-1 are shown in the inset.
from all the materials prepared in the presence of surfactant, the results for the 1:7-consolidated stone are shown as an example. For comparison, the spectrum of TV-consolidated stone is also presented. As described in the Experimental Section, the stone consists of quartz and calcite. Therefore, the stone spectra show the typical doublet assigned to the quartz crystal with peaks in the 777 and 795 cm-1 regions.30 The broad peak (1550-1350 cm-1) centered at 1445 cm-1 and a sharp peak at 879 cm-1 are indicative of CO32- asymmetric stretching from calcite.31 Bands attributed to silicates and carbonates are also observed. The particular peak at 1074 cm-1 is assigned to Si-O-Si asymmetric stretching.32 The symmetric stretching peak (800 cm-1) is overlapped by the quartz doublet. The band at 455 cm-1 is attributed to O-Si-O deformation.33 Moreover, the bands ranging from 3000 to 1790 cm-1 can be assigned to organic components of the marine fossils present in the stone.27 Finally, the broad envelope spanning 36503250 cm-1 observed in the untreated stone spectrum is assigned to the hydrogen-bonded silanol groups with absorbed molecular water at the stone external surface.32 All these bands are maintained unaltered after stone consolidation. However, some changes observed in the spectra after consolidation are associated with the peaks found in the 3000-1790 cm-1 region of the spectrum of untreated stone. In the consolidated stones, the 30001790 cm-1 peaks, attributed to the organic components of the stone,28 are weaker, whereas the band attributed to carbonate vibration (1445 cm-1) is sharper. This suggests that the consolidation modifies the carbonate region. Figure 6 shows a comparison of micrographs of the untreated biocalcareous stone and the stone specimens treated with TV and 1:7 material. The treatment with the other xerogels (1:11 and 1:22) produced similar morphologies. The untreated stone reveals (30) Ainsworth, S. M. J. ASTM Int. 2005, 2, 1498. (31) Ahn, D. J.; Berman, A.; Charych, D. J. Phys. Chem. 1996, 100, 12455. (32) Ou, D. L.; Seddon, A. B. J. Non-Cryst. Solids 1997, 210, 187. (33) Clark, T., Jr.; Ruiz, J. D.; Fan, H.; Brinker, C. J.; Swanson, B. I.; Parikh, A. N. Chem. Mater. 2000, 12, 3879.
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Figure 6. Scanning electron microscopy micrographs of the biocalcareous stone tested: (a) untreated; (b) consolidated with TV; (c) consolidated with 1:7 material. Table 2. Properties of the Untreated Stone and of Its Consolidated Counterpartsa
a
sample
porosity (%)
surface area (m2/g)
strength (MPa)
vapor diffusivity (cm2/s)
untreated stone 1:7 stone 1:11 stone 1:22 stone TV stone
15.06 (1.81) 8.32 (1.12) 8.78 (0.74) 12.89 (1.26) 11.88 (0.89)
0.34 (0.12) 1.61 (0.11) 1.87 (0.78) 5.25 (0.18) 0.72 (0.25)
31.13 (6.66) 41.43 (5.48) 45.26 (7.05) 37.52 (6.65) 30.48 (7.98)
3.81 × 10-7 (0.48 × 10-7) 3.21 × 10-7 (0.56 × 10-7) 2.91 × 10-7 (0.36 × 10-7) 3.40 × 10-7 (0.92 × 10-7) 2.54 × 10-7 (0.89 × 10-7)
Data correspond to average values, and standard deviations are included in parentheses.
the presence of quartz and calcite crystals, typical of this rock (Figure 6a). The TV-consolidated stone shows the formation of a dense gel coating (pores are not observed) inside the stone, severely affected by cracking (Figure 6b). In contrast, the 1:7consolidated stone presents a continuous and crack-free mesoporous coating, consisting of uniform spheres (Figure 6c). Minerals of the rock are also observed in the right of micrograph c. Changes in the stone porosity after consolidation were investigated by mercury intrusion porosimetry. The porosity values are given in Table 2, and the pore size distributions are shown in Figure 7. The biocalcareous stone presents two clearly different pore sizes. Of the total pore volume, 62% corresponds to intergranulate pores with an average size of around 48 µm, and 38% comprises smaller pores with an average radius of around 1 µm. After consolidation, porosity is reduced in all the cases (Table 2). Comparing the pore size distribution, we observe that all the consolidants reduce the macropore size whereas the micropore size is either increased or maintained. This demonstrates that the intergranulate pores are partially sealed, reducing their size. The total porosity of stone consolidated with TV and
Figure 7. Pore size distribution for the untreated and consolidated stone samples under study.
1:22 was slightly higher than the porosity of the stone consolidated with the other two materials. This could be associated with a higher shrinkage of these two gels during the drying phase inside the stone. In the case of 1:22, the high shrinkage is caused by the high ethanol content. For TV, the formation of a dense microporous gel promotes a high capillary pressure, which is responsible for the high shrinkage. A significant feature is the appearance of new pores in the stone consolidated with the materials prepared in our laboratory with size ranging between 1 and 15 nm. We think that these pores actually correspond to the pores of the gel network since this pore size overlaps with those obtained by nitrogen adsorption experiments. In addition, we calculated the surface area of the stone specimens from mercury porosimetry data. The calculation is based on pore volume and radii data using a cylindrical pore model. The surface area values obtained (Table 2) are obviously different from those from the nitrogen adsorption test since the pore size range evaluated is also different. The higher surface area values of the stone consolidated with the materials prepared in the presence of surfactants can be explained by the presence of mesopores in the range of 1-15 nm, which we attributed to the pore network of the gels inside the stone. Although the compressive strength test is not usually employed to determine consolidant effectiveness, and other methods based on the stone failure under tension are usually considered, we used the compressive strength values of the stone as a first approximation to the changes produced in the mechanical resistance of the stone (see Table 2). The consolidation with TV does not provide an increase in the stone strength. However, the samples treated with the consolidants synthesized in our laboratory show a significant increase in their compression strength values. Specifically, 1:7 and 1:11 consolidants exhibit increases in strength of 33% and 45%, respectively. In the case of 1:22 material, the increase in strength was rather less (20%). The variations in strengths are clearly associated with the efficacy of the consolidation in the stone and, subsequently, with the changes in the stone porosity. The strength of the stone consolidated with the commercial product does not increase after the treatment because the stone porosity is only slightly reduced.
2778 Langmuir, Vol. 24, No. 6, 2008
Mosquera et al.
Water vapor permeability testing of the stone before and after the treatments allowed the effect of the consolidants on the breathability of the stone to be evaluated. Using the automatic setup developed in our laboratory, it was possible to record a large quantity of data (>3000) for each test, which assured the adequate characterization of the process kinetics. Since the pore size of the stone is substantially greater than the average distance that a vapor molecule travels between collisions, we assume that the walls of the pore do not contribute to the vapor transport and, thus, the water vapor transport is described by Fick’s law, according the following equation:
A∆C t l
Q)D
(2)
where D is the vapor diffusivity (m2/s), Q is the cup mass decrease (g), l is the specimen thickness (m), A is the specimen area (m2), ∆C is the moisture concentration gradient (g/m3), and t is time (s). Thus, by plotting Ql/A∆C versus time, vapor diffusivity values, which are shown in Table 2, were calculated directly from the slope of a linear regression of data when a steady vapor flow was attained. This occurred when the relative moisture in the climatic chamber was stabilized. The high linear correlation coefficients (r > 0.995) exhibited in all the plots suggest that Fickian diffusion does indeed occur. All the consolidants give reductions ranging from 11% to 33%. Our results are in agreement with changes in porosity. As no consolidant completely blocks the rock pores, the breathability of the stone after consolidation is only slightly reduced. The vapor transport across the pore network of the gels could be Knudsen diffusion since the pores of the network are very small, and consequently, a more complex kinetic model may apply. However, this study is beyond the scope of the present work.
Conclusions We have designed a novel consolidant synthesis in which the sol-gel transition occurs in the presence of a surfactant. This provides an efficient means of avoiding cracking of the gel while it is drying inside the stone. In accordance with the special features of the consolidation process, we designed a special route in which a neutral surfactant was added in a low concentration (below the cmc). To prevent the sol-gel transition from taking place instantaneously, we tried different concentrations of ethanol in the starting sol. From the results obtained, we found that consolidation with the mixtures containing 1:7 and 1:11 TEOS:ethanol molar ratios were more effective. We have demonstrated that these two consolidants fill the rock pores more efficiently and provide a significantly greater increase in the compression strength of the stone than the commercial consolidant tested in this study. In summary, the two consolidants offer improvements over the commercial product. Acknowledgment. We are grateful to the Spanish Government and the European Regional Development Fund (ERDF) for financial support under research projects MAT2004-00801 and MAT2007-60681. We are also grateful to the Consejerı´a de Innovacio´n, Ciencia y Empresa of the Andalusia Government for financial support under research project TEP2092. D.M.d.l.S. is grateful for her predoctoral grant (FPI) associated with the cited project. We also thank Goldschmidt-Degussa for supplying Tegovakon V100. Supporting Information Available: Rs plots obtained from nitrogen adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org. LA703652Y
