- Email: [email protected]
Characterization of a gel-derived silica intended for use as a reference material
Colloids and Surfaces A: Physicochemical and Engineering Aspects 187– 188 (2001) 123– 130 www.elsevier.nl/locate/colsurfa
Characterization of a gel-derived silica intended for use as a reference material Martin A. Thomas a,*, Nichola J. Coleman b,1 b
a Quantachrome Corporation, 1900 Corporate Dr., Boynton Beach, FL 33426, USA Department of Materials, Imperial College of Science Technology and Medicine, London, SW 7 2AZ, UK
Abstract A new candidate surface area and pore volume reference material is proposed. A mesoporous gel-derived silica monolith has been investigated by nitrogen sorption and mercury intrusion porosimetry. BET specific surface area (165.5 m2 g − 1), specific pore volume (0.986 cm3 g − 1) and mean pore radius (11.9 nm) have been monitored for up to 12 months by nitrogen sorption. No significant changes occurred following either storage or repeated usage. The pore volume measured by mercury intrusion is in excellent agreement despite significant compression. The pore structure remained unaltered despite multiple mercury intrusion cycles up to high pressure. This remarkable stability and durability confers properties that compare favorably with those of commercially available reference materials. The primary advantage of the gel-silica monolith is the elimination of errors associated with powder sampling. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Reference material; BET surface area; Pore volume; Mercury porosimetry; Gel-derived silica
1. Introduction Nitrogen gas sorption and mercury intrusion porosimetry are widely used methods for the determination of surface area, pore volume and pore size of porous solids, i.e. the so-called textural properties of catalysts, ceramics, biomedical and other performance materials [1,2]. There exists a * Corresponding author. Tel.: + 1-561-7314999; fax: + 1561-7329888. E-mail address: [email protected] (M.A. Thomas). 1 Present address: Centre for Contaminated Land Remediation, Department of Earth and Environmental Science, University of Greenwich, London, SE10 9LS UK.
demand, therefore, for materials of predictable and stable pore structure for use as reference materials. Whilst there are a number of BET surface area standards available there are only two pore size standards for gas sorption known to the authors; BAM’s2 PM 103 and PM-104 both of which are mesoporous alumina powders. It is well known to those practiced in the art that powdered materials present a number of challenges in obtaining accurate results. Despite the ready availability of rifflers accurate sub-sampling is, sadly, rarely undertaken. Sample handling, transfer and outgassing can further degrade 2
See Appendix A.
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 6 2 9 - X
124
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
anticipated measurement reproducibility, not to mention the problem of powder elutriation (loss of fine particles from the sample cell under the influence of vacuum and evolution of water vapor). In contrast, a monolithic, mesoporous, thermally stable silica offers the benefits of ease of handling and proven batch-reproducibility, and represents a class of materials so far not offered as a widely available reference material. The durability and stability of the material during gas adsorption has been established. The work reported herein suggests that these desirable properties can also be extended to include characterization by high-pressure mercury intrusion, and consequently suggests the use of this material as a reference material for mercury porosimeters. Any such candidate material should of course undergo round robin testing amongst several competent laboratories to check the reproducibility and ruggedness of the manufacturing process prior to adoption as a reference material.
2. Review of currently available reference materials Twenty or so powdered materials are currently available from the four major, internationally recognized standards authorities, BAM (Germany), IRRM (European Community), LGC (UK) and NIST (USA) (see Appendix A), NIST having recently retired a further three. All are powders and cover a wide range of specific surface area values, although 14 do have certified values of less than 10 m2 g − 1. Most are alumina, plus four silica/quartz, two carbons, two silicon nitrides, plus one each of titania and tungsten. Two are also certified for pore size (BAM), but two more (NIST) are specifically NOT certified for pore size, nor isotherm shape. Whilst the responsible bodies indicate that such certified reference materials (CRM’s) are intended for calibrating and checking measurement instruments, they do not, in fact, represent an extensive range of test standards. Whilst the specific surface area range covered (0.0686– 258 m2 g − 1) seems at first to be admirably broad, note that gas sorption analyzers do not in fact measure the stated spe-
cific quantity (area per unit mass). Rather, the total surface area of sample presented to the instrument for analysis is measured. It is usual therefore, that the mass of sample analyzed is relatively large for low specific surface area material, and commensurately small for high surface area materials. The desired surface area range to be investigated can therefore be approximated by simply adjusting the amount of sample analyzed. This may or may not be appropriate depending on the available precision and accuracy of weighing. Furthermore, it should be recognized that there exists in all samples some degree of inhomogeneity and that taking sub-samples from the original amount of reference material can lead to a greater degree of uncertainty in the measured value than anticipated. Unless, that is, the sample be properly, i.e. representatively, split. NIST suggest ‘coning and quartering’ to be sufficient, but the authors of this paper recommend rotary (spin) riffling when the amount of sample to be taken is small with respect to the certified amount, one eighth or less for example. Unfortunately, many users of gas sorption equipment do not have the inclination to do so, nor have access to suitable equipment. It would be advantageous, therefore, to use a material free of sampling errors by virtue of analyzing the entire quantity supplied. Clearly even a carefully prepared quantity of powdered material does not satisfy this requirement due to losses on transfer from container to cell and so on. Meyer et al. [3] correctly teach that the specific property of a porous reference material relies on the adsorbate employed for the characterization. With the exception of two low surface area standards having values referenced to krypton adsorption (at 77.4 K), all such materials have been certified using nitrogen adsorption at the boiling point of liquid nitrogen (approximately 77.4 K). Only very recently [4], standard materials for mercury porosimetry have been offered by one of the recognized bodies (BAM). In common with the gas sorption counterparts, these materials are not monolithic. They have been certified for single use only, unlike the proposed monolith in this work.
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
3. Experimental
3.1. Sample preparation Cylindrical (2.5 mm thick× 5.6 mm diameter) gel-derived silica monoliths were prepared by the acid-catalyzed hydrolysis and condensation of an alkoxysilane precursor according to Hench [5]. Briefly, an alkoxy/water/alcohol solution undergoes rapid gelation at low pH. The alcohol acts as a cosolvent since most alkoxides are not water-soluble. Silanol (Si– OH) groups are formed by the hydrolysis reaction followed by the formation of the gel structure as clusters of siloxane. (SiOSi) bonded particles are formed by the subsequent condensation reaction. The aqueous medium becomes viscous, and is eventually solidified by ‘a coherent network of particles that, by capillary action, retains the liquid’ [6]. The gel is aged, during which period it shrinks (including shrinkage of the liquid-filled pores) and undergoes ‘Ostwald ripening’ wherein small voids are eliminated, and stiffening. Subsequent drying is performed carefully to prevent cracking. The resulting monolith is stabilized to reduce the number of surface silanol groups thereby inhibiting rehydroxylation. However, at temperatures below 500°C the resulting surface siloxane groups (ring structures) remain reactive towards rehydroxylation due to the inherent strain in the three-membered ring [7]. Structural relaxation occurs at higher temperatures (due to the formation of four-membered rings [8], with further elimination of hydroxyl groups. Sintering is expected to begin about 800°C; the greater the degree of structure and the larger the pore size the slower the sintering rate. Samples used in this study were stabilized at 900°C and as such are expected to have a largely hydrophilic surface. Densification does not occur until the glass transition point, 1100°C.
3.2. Textural characterization 3.2.1. Gas sorption The mean surface area, pore volume and pore radius values have been determined in accordance with ISO 9277:1995 E [9] which is based
125
on a 1994 IUPAC report [10]. Samples (approximately 0.044 g each) were degassed under vacuum (B 1 Pa) at 100°C for 12 h. Nitrogen adsorptive (99.999%) was used at approximately 77.4 K (Autosorb®-6B, Quantachrome Corporation, Florida, USA). Specific surface areas were evaluated by the BET method [11]. The cross sectional area of an adsorbed nitrogen molecule was taken to be 0.162 nm2 [12]. Estimates of specific pore volume were obtained from the amount of nitrogen adsorbed by the sample in the range 0.994B P/Po B 0.999, assuming a condensed nitrogen density of 0.808 g cm − 3. The mean pore radius, r, was calculated according to Eq. (1) r=
2Vp SBET
(1)
where Vp is the specific pore volume and SBET is the specific surface area.
3.2.1.1. Durability and stability. Five monoliths were outgassed and analyzed as described in the previous section a total of ten times each to establish durability, that is, the performance of the monolith as a function of repeated usage. The stability of the textural features, as a function of time, was assessed by monitoring the specific surface area and the specific pore volume, as above, at monthly intervals over a period of 1 year. Four monoliths, which had been stored in polyethylene bags in a screw-top polypropylene jar over self-indicating silica gel at room temperature, were selected at random each month. The measured specific surface area and specific pore volumes recorded as a function of usage (number of isotherms) and storage time (over a period of 12 months) are shown in Table 1. In both studies, the Wald–Wolfowitz method was employed to test for trends in the data. This non-parametric (distribution-free) method analyzes the signs of the residuals (differences between a single experimental value and the population median); a random sequence of signs indicating that there is no statistically evident trend [13]. The detailed statistical method and results are given elsewhere [14].
126
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
Table 1 Textural characteristics of gel-derived mesoporous silica by nitrogen adsorptiona
BET surface area, A (m2 g−1) Specific pore volume, V (ml g−1) Pore radius, r (nm) Pore radius, r (A, )
Mean
SD
SD (%)
95%
165.5 0.986 11.92 119.2
3.0 0.039 0.0052 5.2
1.8 3.9 4.4 4.4
1.5 0.020 0.27 2.7
a
SD is the standard deviation in measurement units, SD (%) is the standard deviation expressed as a percentage of the mean value, and 95% is the 95% confidence interval for the mean value. Mean pore radii are calculated using the cylindrical pore relationship r =2V/A.
3.2.2. Mercury porosimetry Mercury intrusion analyses were performed using an automated porosimeter (PoreMaster® 60, Quantachrome Corporation, Florida, USA) employing the method of continuous pressurization (scanning). This method, rather than step-wise changes in pressure, was used so that the effect of partial intrusion on the resulting extrusion curves could be easily investigated. Furthermore, critical intrusion pressures were always approached from ‘below’ without overshoot (and therefore eliminate possible pressure-oscillation around a target value). The effects of rapid, step-wise pressure changes, with corresponding waiting (so-called equilibration) periods, and of rapid scanning on this material are the subjects of future studies. Samples were contained in glass sample cells (penetrometers) whose total intrusion capacity was approximately 0.6 cm3. The cells were filled with mercury (triply distilled) after an initial evacuation to below 2.7 Pa (20 mmHg) for no less than 2 min. High-pressure generation was by means of a computer-controlled hydraulic oil piston. The intrusion of mercury into, and extrusion from, pores was measured by a sensitive capacitance circuit. In addition to multiple complete intrusion/ extrusion cycles on multiple silica monoliths, partial intrusion/extrusion runs were obtained on a single monolith. Pore sizes were calculated using the usual pressure –pore size relationship proposed by [15], given below:
r=
2k cos q P
(2)
where k is the surface tension of liquid mercury, q the contact angle between the mercury and the surface of the solid, and P is the applied pressure required to force the mercury into the pore of radius r.
4. Results The nitrogen isotherm (Fig. 1) is of type IV [16] with hysteresis of type H1 [17], formerly denoted type A by de Boer [18], indicating the presence of cylindrical mesopores of narrow distribution. The textural properties of the monolith derived from the gas sorption data are presented in Table 1. The mean C value of 105, typical of a non-microporous oxide, means that these data can be said to meet the requirement of a successful application of the BET method [1].
Fig. 1. Adsorption – desorption isotherm of nitrogen at 77.4 K on gel-derived silica monolith. , Adsorption; , desorption.
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
127
truded at approximately 135 Mpa (19 600). After about 160 MPa (23 200 psi) there is no further intrusion. On lowering the pressure, the monoliths exhibit a significant amount of hysteresis. Extrusion does not begin until about 87 MPa (13 000 psi) and then only slightly. An abrupt change in extruded volume occurs at approximately 36–31 MPa (5200–4500 psi, Fig. 2(b)), and the volume of mercury remaining intruded falls to the linear portion of the curve. For partially intruded runs, the hysteresis remained wide even when a relatively small portion of the total pore volume had been intruded. The textural properties of this gel-derived silica derived from mercury intrusion porosimetry are presented in Table 2.
5. Discussion
Fig. 2. (a) Mercury intrusion –extrusion on gel-derived silica monolith. , Intrusion to 85.9 MPa; , to 89.6 MPa; , to 98.2 MPa; ", to 171.7 MPa; , extrusion from 85.9 MPa; , from 89.6 MPa; , from 98.2 MPa; 2, from 171.7 MPa, – – – , extrapolation of linear compression region. Arrows added for clarity to indicate , intrusion and ¡, extrusion. (b) Mercury extrusion from previously intruded monolith (detail). , Previously intruded to 85.9 MPa; , to 89.6 MPa; , to 98.2 MPa; 2, to 171.7 MPa; —, mean intrusion curve (for reference).
The specific pore volume has been confirmed by mercury intrusion porosimetry with little entrapment of intruded mercury when the system is returned to atmospheric pressure (Fig. 2(a and b)). The intrusion curve initially exhibits a reproducible, linear volume change as a function of applied pressure (approximately 2.34×10 − 2 ml g − 1 MPa − 1), before the onset of a steep rise at 80 MPa (11 600 psi). The pore volume is 99% in-
Statistical analysis indicates that, based on nitrogen sorption data, this material compares favorably with commercially available standard reference materials for surface area and pore size analysis. The stability investigation indicated that both the specific surface area and the specific pore volume remain statistically unchanged during repeated usage, i.e. there was no observable trend (neither up nor down) in the data. Surface area values lay in the range − 1.25 to + 1.65 m2 g − 1 from the median value and pore volumes lay in Table 2 Textural characteristics of gel-derived mesoporous silica by mercury intrusiona
Pore volume, V (ml g−1) Area140, A (m2 g−1) Radius140, r (nm) Radius140, r (A, ) Area180, A (m2 g−1) Radius180, r (nm) Radius180, r (A, ) a
Mean
SD
SD (%)
0.971 217.7 8.92 89.2 166.8 11.64 116.4
0.037 5.1 0.35 3.5 3.9 0.45 4.5
3.8 2.3 3.9 3.9 2.3 3.9 3.9
Area140 and area180 are the specific surface areas calculated using advancing (intrusion) contact angles of 140 and 180°, respectively. Radius140 and radius180 are the mean pore radii calculated using the cylindrical pore relationship r = 2V/A and contact angles of 140 and 180°, respectively.
128
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
Fig. 3. Mercury porosimetric curve on a powdered material (drug compound). , Intrusion; , extrusion; – – – extrapolation of logarithmic compaction region. Arrows added for clarity to indicate , intrusion and ¡, extrusion.
the range − 0.0027 to + 0.0056 ml g − 1 from the median value. The durability investigation yielded similar results. That the measured textural properties were not altered by repeated thermal cycling is indicative of a tough or resilient material. Fracturing of the material might have been expected to increase surface area slightly. Pore volume might have been reduced, or at least the isotherm would have assumed an upturn at highest relative pressure values. Furthermore, loss of structural integrity on a macroscopic scale would have been immediately apparent due to the monolithic nature of the sample; none was observed. The mercury porosimetry data is clearly affected by isostatic compression of the monolith at pressures up to about 80 MPa (11 600 psi). This reversible, i.e. elastic, behavior can be differentiated from the compaction process initially undergone by a powdered material (Fig. 3). Compaction usually conforms to some logarithmic volume change as a function of applied pressure, which can be clearly seen in the example given, up to the point at which mercury intrudes into the remaining voids. Furthermore, compaction is essentially irreversible as demonstrated by the extrusion data for the powder, there being little or no extrusion of mercury at pressures corresponding to the compaction region. Compression of the monolith causes constriction of the pores until the silica structure stiffens sufficiently
to form a rigid structure, at least to a point at which the mercury can intrude into the shrunken pores. Therefore, when a normal contact angle of 140° is used to calculate pore size, the pore radii appear significantly smaller than expected from nitrogen sorption data. Furthermore, the surface area calculated from the intrusion data, according to the method of Rootare et al. [19], greatly exceeds the BET value. Interestingly, when the unusual contact angle of 180° is used, the surface area value is in remarkable agreement with the BET value. Whether this happy coincidence is a direct consequence of the compression mechanism is worthy of future study. Degree of penetration into the pore structure does have some effect on the extrusion curve. For those ‘partial intrusion’ runs, some pores obviously remain unfilled and can expand during the pressure-letdown during the extrusion phase giving a more rounded appearance to the extrusion ‘knee’ (Fig. 2(b)). This also gives rise to a higher, average extrusion pressure than for the completely intruded run, which therefore exhibits the widest hysteresis. The reversible and reproducible nature of the compression is consistent with the resilience indicated by the nitrogen sorption durability data. The linear portion of the intrusion curve yields an elastic modulus of 604×106 N m − 2 for the monolith. Unlike monoliths stabilized at lower temperatures, which undergo some inelastic deformation [20], the monoliths studied here exhibit a high degree of reversibility and repeatability, even under repeated usage. The total pore volume is properly measured as the total volume change during intrusion, since the compression merely substitutes for intrusion into a perfectly rigid porous structure. Not only are the gas sorption and mercury porosimetery data in excellent agreement (0.986 and 0.971 g ml − 1, respectively), they agree with a simple calculation based on geometric volume and skeletal density. Assuming perfectly cylindrical geometry, each monolith has an envelope volume of 0.0616 ml. The density of gel-derived silica calcined at 900°C has been reported previously [20,21] as 2.30 g ml − 1. Taking this value the pore volume of the average disk (0.0436 g) is 0.9775 ml g − 1. Although one cannot accurately measure the pore size of an uncom-
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
pressed sample by mercury porosimetry, the resulting pore size of a compressed sample can be determined. Pore size can be computed from any non-wetting contact angle (90° B q 5 180°) desired, for example see Fig. 4, albeit incorrectly. However, for the proposed purpose, pore size calculations are actually unnecessary, since it is the intrusion pressure that is characteristic and is what the porosimeter records. Therefore, use of these monoliths in calibration and monitoring of mercury porosimeters remains viable, since both compression and critical intrusion pressure, are highly reproducible (see Table 2 and Fig. 2(a)). The observed hysteresis in the mercury porosimetry data, in the absence of significant entrapment, can be attributed to a reduction in contact angle between intrusion (an advancing meniscus) and extrusion (a receding meniscus), [22,23]. Both intrusion and extrusion curves can be made to yield identical modal pore diameters by appropriate adjustment of contact angles. For example, Liu et al. [24] chose to select a smaller contact angle for the receding meniscus (rather than a larger one for the advancing meniscus) in their work on cement paste. For the silica monolith, an extrusion contact angle of approximately 106° is required to match an intrusion angle of 140, and 111° for a corresponding 180° intrusion angle (the ratios of the cosines being the
129
same 2.78 in both cases). Significant network effects can be discounted due to the high degree of reversibility, the parallel nature of intrusion and extrusion curves, and the H1 type hysteresis of the nitrogen sorption isotherm.
6. Conclusions The mercury intrusion data supports the proposal [14,25] that a gel-derived silica monolith possesses the necessary reproducibility, stability and durability to be adopted as a new reference material, though this should be confirmed by an interlaboratory comparison study using standardized methods. Its physical form (a cylinder) permits a simple, independent estimate of pore volume, which is impossible for a powdered material. Since the nitrogen isotherm forms a plateau above P/Po = 0.97 two distinct advantages arise, at least in terms of a candidate reference material. Firstly, this behavior allows the measurement of total pore volume at a relative pressure that is considerably lower than the saturated equilibrium vapor pressure, Po, thus reducing the risk of condensation during data acquisition at the top of the isotherm. Secondly, this behavior permits the determination of total pore volume within a range of relative pressure values — a much more achievable, reproducible and therefore desirable condition. Furthermore, the evidence indicates that the same material can be used as a pore volume reference for mercury intrusion. Its ability to withstand multiple intrusions permits a more rapid checking of porosimeters by virtue of the fact that the evacuation and filling step can be eliminated for subsequent tests. This is of particular benefit in the reduction of waste mercury.
Acknowledgements Fig. 4. Pore size distribution of monolith calculated from mercury intrusion data. , Cumulative pore volume (intrusion) and —, pore size distribution calculated using a contact angle of 140°; – – –, pore size distribution calculated using a contact angle of 180°.
N.J. Coleman gratefully acknowledges the support of the EPSRC (UK). Thanks to Dr M. Bellantone (Imperial College) and M. Tucker (Quantachrome Ltd.) for their assistance and ad-
130
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
vice. Thanks to Dr K.W. Powers (University of Florida) for invaluable discussion.
Appendix A. Internationally recognized standards organizations Bundesanstalt fur Materialforschung und – pru¨ fung (BAM), (Federal Institute for Materials Research and Testing). Bundesanstalt fur Materialforschung und -pru¨ fung, Division I.1 Inorganic Chemical Analysis, Reference Materials, Zweiggela¨ nde Adlershof, Rudower Chaussee 5, D-12489 Berlin, Germany. Tel.: +49-30-63925830/5827; fax: + 49-3063925972; ftp://ftp.bam.de/G3/a – i/rmgesune.pdf. Institute for Reference Materials and Measurements (IRMM). I.R.R.M., Retieseweg, B-2440 Geel, Belgium. Tel.: + 32-14-571211; fax: + 32-14-584273; http:// www.irmm.jrc.be/rm/physical.pdf. Laboratory of the Government Chemist (LGC). LGC’s Office of Reference Materials, LGC, Queens Road, Teddington, Middlesex, TW11 0LY, UK. Tel: + 44-20-89437000; fax: + 44-2089432767; [email protected]; http://www.lgc.co.uk/ products/rm/pspsur.pdf National Institute of Standards and Technology (NIST). Standard Reference Materials Program, National Institute of Standards and Technology, 100 Bureau Drive, Room 204, Building 202, Gaithersburg, MD 20899-2322, USA. Tel.: + 1-301-9756776; fax: + 1-301-9483730; [email protected]; http://ois.nist.gov/srmcatalog/tables/view – table. cfm?table=301-4.htm.
References [1] J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, second ed., Academic Press, London, 1982. [2] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London, 1999. [3] K. Meyer, P. Klobes, B. RohlKuhn, Cryst. Res. Technol. 32 (1) (1997) 175 –185. .
[4] P. Klobes, B. Rohl-Kuhn, K. Meyer, Certified reference materials (CRM’s) for mercury porosimetry method, The Second International TRI/Princeton Workshop, Princeton, NJ, USA, June 19 – 21, 2000. [5] L.L. Hench, Sol – Gel Silica: Processing, Properties and Technology Transfer, Noyes Publications, New York, 1998. [6] R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. [7] C.J. Brinker, R.J. Kirkpatrick, D.R. Tallant, B.C. Bunker, B.J. Markey, J. Non-Cryst. Sci. 99 (1988) 418. [8] L.L. Hench, S.H. Wang, Phase Transitions 24 – 26 (1990) 785. [9] International Organization for Standardization (ISO) 9277: 1995 (E), Determination of the Specific Surface Area of Solids by Gas Adsorption using the BET method, Geneva, Switzerland. [10] J. Rouquerol, D. Avnir, D.H. Everett, C. Fairbridge, M. Haynes, N. Pericone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, Guidelines for the characterization of porous solids, Stud. Surf. Sci. Catal. 87 (1994) 1. [11] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [12] A.L. McClellan, H.F. Harnsberger, J. Colloid Interf. Sci. 23 (1976) 577. [13] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, second ed., Ellis Horwood, Chichester, 1988. [14] N.J. Coleman, L.L. Hench, Ceram. Int. 26 (2) (2000) 179 – 186. [15] E.W. Washburn, Proc. Natl. Acad. Sci. USA 7 (1921) 115. [16] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem. Soc. 62 (1940) 1723. [17] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Rouqurol, J.R. Rouqurol, T. Siemiewska, Reporting physisorption data for gas/solid systems, Pure Appl. Chem. 57 (4) (1985) 603. [18] A. Author, in: J.H. de Boer, D.H. Everett, F.S. Stone (Eds.), The Structure and Properties of Porous Materials, Butterworths, London, 1958, p. 195. [19] H.M. Rootare, C.F. Prenzlow, J. Phys. Chem. 71 (1967) 2733. [20] K.W. Powers, The development and characterization of sol gel substrates for chemical and optical applications, Ph.D. dissertation, Materials Science and Engineering Department, University of Florida, Gainesville, 1998. [21] S. Wallace, Porous silica gel monoliths: structural evolution and interactions with water, Ph.D. dissertation, University of Florida, Gainesville, 1991. [22] S. Lowell, J.E. Shields, J. Colloid Interf. Sci. 80 (1) (1981) 192 – 196. [23] S. Lowell, J.E. Shields, Powder Surface Area and Porosity, third ed., Academic Press, London, 1982. [24] Z. Liu, D. Winslow, Cement Concrete Res. 25 (4) (1995) 769 – 778. [25] N.J. Coleman, L.L. Hench, Ceram. Int. 26 (2) (2000) 171 – 178.
Characterization of a gel-derived silica intended for use as a reference material Martin A. Thomas a,*, Nichola J. Coleman b,1 b
a Quantachrome Corporation, 1900 Corporate Dr., Boynton Beach, FL 33426, USA Department of Materials, Imperial College of Science Technology and Medicine, London, SW 7 2AZ, UK
Abstract A new candidate surface area and pore volume reference material is proposed. A mesoporous gel-derived silica monolith has been investigated by nitrogen sorption and mercury intrusion porosimetry. BET specific surface area (165.5 m2 g − 1), specific pore volume (0.986 cm3 g − 1) and mean pore radius (11.9 nm) have been monitored for up to 12 months by nitrogen sorption. No significant changes occurred following either storage or repeated usage. The pore volume measured by mercury intrusion is in excellent agreement despite significant compression. The pore structure remained unaltered despite multiple mercury intrusion cycles up to high pressure. This remarkable stability and durability confers properties that compare favorably with those of commercially available reference materials. The primary advantage of the gel-silica monolith is the elimination of errors associated with powder sampling. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Reference material; BET surface area; Pore volume; Mercury porosimetry; Gel-derived silica
1. Introduction Nitrogen gas sorption and mercury intrusion porosimetry are widely used methods for the determination of surface area, pore volume and pore size of porous solids, i.e. the so-called textural properties of catalysts, ceramics, biomedical and other performance materials [1,2]. There exists a * Corresponding author. Tel.: + 1-561-7314999; fax: + 1561-7329888. E-mail address: [email protected] (M.A. Thomas). 1 Present address: Centre for Contaminated Land Remediation, Department of Earth and Environmental Science, University of Greenwich, London, SE10 9LS UK.
demand, therefore, for materials of predictable and stable pore structure for use as reference materials. Whilst there are a number of BET surface area standards available there are only two pore size standards for gas sorption known to the authors; BAM’s2 PM 103 and PM-104 both of which are mesoporous alumina powders. It is well known to those practiced in the art that powdered materials present a number of challenges in obtaining accurate results. Despite the ready availability of rifflers accurate sub-sampling is, sadly, rarely undertaken. Sample handling, transfer and outgassing can further degrade 2
See Appendix A.
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 6 2 9 - X
124
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
anticipated measurement reproducibility, not to mention the problem of powder elutriation (loss of fine particles from the sample cell under the influence of vacuum and evolution of water vapor). In contrast, a monolithic, mesoporous, thermally stable silica offers the benefits of ease of handling and proven batch-reproducibility, and represents a class of materials so far not offered as a widely available reference material. The durability and stability of the material during gas adsorption has been established. The work reported herein suggests that these desirable properties can also be extended to include characterization by high-pressure mercury intrusion, and consequently suggests the use of this material as a reference material for mercury porosimeters. Any such candidate material should of course undergo round robin testing amongst several competent laboratories to check the reproducibility and ruggedness of the manufacturing process prior to adoption as a reference material.
2. Review of currently available reference materials Twenty or so powdered materials are currently available from the four major, internationally recognized standards authorities, BAM (Germany), IRRM (European Community), LGC (UK) and NIST (USA) (see Appendix A), NIST having recently retired a further three. All are powders and cover a wide range of specific surface area values, although 14 do have certified values of less than 10 m2 g − 1. Most are alumina, plus four silica/quartz, two carbons, two silicon nitrides, plus one each of titania and tungsten. Two are also certified for pore size (BAM), but two more (NIST) are specifically NOT certified for pore size, nor isotherm shape. Whilst the responsible bodies indicate that such certified reference materials (CRM’s) are intended for calibrating and checking measurement instruments, they do not, in fact, represent an extensive range of test standards. Whilst the specific surface area range covered (0.0686– 258 m2 g − 1) seems at first to be admirably broad, note that gas sorption analyzers do not in fact measure the stated spe-
cific quantity (area per unit mass). Rather, the total surface area of sample presented to the instrument for analysis is measured. It is usual therefore, that the mass of sample analyzed is relatively large for low specific surface area material, and commensurately small for high surface area materials. The desired surface area range to be investigated can therefore be approximated by simply adjusting the amount of sample analyzed. This may or may not be appropriate depending on the available precision and accuracy of weighing. Furthermore, it should be recognized that there exists in all samples some degree of inhomogeneity and that taking sub-samples from the original amount of reference material can lead to a greater degree of uncertainty in the measured value than anticipated. Unless, that is, the sample be properly, i.e. representatively, split. NIST suggest ‘coning and quartering’ to be sufficient, but the authors of this paper recommend rotary (spin) riffling when the amount of sample to be taken is small with respect to the certified amount, one eighth or less for example. Unfortunately, many users of gas sorption equipment do not have the inclination to do so, nor have access to suitable equipment. It would be advantageous, therefore, to use a material free of sampling errors by virtue of analyzing the entire quantity supplied. Clearly even a carefully prepared quantity of powdered material does not satisfy this requirement due to losses on transfer from container to cell and so on. Meyer et al. [3] correctly teach that the specific property of a porous reference material relies on the adsorbate employed for the characterization. With the exception of two low surface area standards having values referenced to krypton adsorption (at 77.4 K), all such materials have been certified using nitrogen adsorption at the boiling point of liquid nitrogen (approximately 77.4 K). Only very recently [4], standard materials for mercury porosimetry have been offered by one of the recognized bodies (BAM). In common with the gas sorption counterparts, these materials are not monolithic. They have been certified for single use only, unlike the proposed monolith in this work.
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
3. Experimental
3.1. Sample preparation Cylindrical (2.5 mm thick× 5.6 mm diameter) gel-derived silica monoliths were prepared by the acid-catalyzed hydrolysis and condensation of an alkoxysilane precursor according to Hench [5]. Briefly, an alkoxy/water/alcohol solution undergoes rapid gelation at low pH. The alcohol acts as a cosolvent since most alkoxides are not water-soluble. Silanol (Si– OH) groups are formed by the hydrolysis reaction followed by the formation of the gel structure as clusters of siloxane. (SiOSi) bonded particles are formed by the subsequent condensation reaction. The aqueous medium becomes viscous, and is eventually solidified by ‘a coherent network of particles that, by capillary action, retains the liquid’ [6]. The gel is aged, during which period it shrinks (including shrinkage of the liquid-filled pores) and undergoes ‘Ostwald ripening’ wherein small voids are eliminated, and stiffening. Subsequent drying is performed carefully to prevent cracking. The resulting monolith is stabilized to reduce the number of surface silanol groups thereby inhibiting rehydroxylation. However, at temperatures below 500°C the resulting surface siloxane groups (ring structures) remain reactive towards rehydroxylation due to the inherent strain in the three-membered ring [7]. Structural relaxation occurs at higher temperatures (due to the formation of four-membered rings [8], with further elimination of hydroxyl groups. Sintering is expected to begin about 800°C; the greater the degree of structure and the larger the pore size the slower the sintering rate. Samples used in this study were stabilized at 900°C and as such are expected to have a largely hydrophilic surface. Densification does not occur until the glass transition point, 1100°C.
3.2. Textural characterization 3.2.1. Gas sorption The mean surface area, pore volume and pore radius values have been determined in accordance with ISO 9277:1995 E [9] which is based
125
on a 1994 IUPAC report [10]. Samples (approximately 0.044 g each) were degassed under vacuum (B 1 Pa) at 100°C for 12 h. Nitrogen adsorptive (99.999%) was used at approximately 77.4 K (Autosorb®-6B, Quantachrome Corporation, Florida, USA). Specific surface areas were evaluated by the BET method [11]. The cross sectional area of an adsorbed nitrogen molecule was taken to be 0.162 nm2 [12]. Estimates of specific pore volume were obtained from the amount of nitrogen adsorbed by the sample in the range 0.994B P/Po B 0.999, assuming a condensed nitrogen density of 0.808 g cm − 3. The mean pore radius, r, was calculated according to Eq. (1) r=
2Vp SBET
(1)
where Vp is the specific pore volume and SBET is the specific surface area.
3.2.1.1. Durability and stability. Five monoliths were outgassed and analyzed as described in the previous section a total of ten times each to establish durability, that is, the performance of the monolith as a function of repeated usage. The stability of the textural features, as a function of time, was assessed by monitoring the specific surface area and the specific pore volume, as above, at monthly intervals over a period of 1 year. Four monoliths, which had been stored in polyethylene bags in a screw-top polypropylene jar over self-indicating silica gel at room temperature, were selected at random each month. The measured specific surface area and specific pore volumes recorded as a function of usage (number of isotherms) and storage time (over a period of 12 months) are shown in Table 1. In both studies, the Wald–Wolfowitz method was employed to test for trends in the data. This non-parametric (distribution-free) method analyzes the signs of the residuals (differences between a single experimental value and the population median); a random sequence of signs indicating that there is no statistically evident trend [13]. The detailed statistical method and results are given elsewhere [14].
126
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
Table 1 Textural characteristics of gel-derived mesoporous silica by nitrogen adsorptiona
BET surface area, A (m2 g−1) Specific pore volume, V (ml g−1) Pore radius, r (nm) Pore radius, r (A, )
Mean
SD
SD (%)
95%
165.5 0.986 11.92 119.2
3.0 0.039 0.0052 5.2
1.8 3.9 4.4 4.4
1.5 0.020 0.27 2.7
a
SD is the standard deviation in measurement units, SD (%) is the standard deviation expressed as a percentage of the mean value, and 95% is the 95% confidence interval for the mean value. Mean pore radii are calculated using the cylindrical pore relationship r =2V/A.
3.2.2. Mercury porosimetry Mercury intrusion analyses were performed using an automated porosimeter (PoreMaster® 60, Quantachrome Corporation, Florida, USA) employing the method of continuous pressurization (scanning). This method, rather than step-wise changes in pressure, was used so that the effect of partial intrusion on the resulting extrusion curves could be easily investigated. Furthermore, critical intrusion pressures were always approached from ‘below’ without overshoot (and therefore eliminate possible pressure-oscillation around a target value). The effects of rapid, step-wise pressure changes, with corresponding waiting (so-called equilibration) periods, and of rapid scanning on this material are the subjects of future studies. Samples were contained in glass sample cells (penetrometers) whose total intrusion capacity was approximately 0.6 cm3. The cells were filled with mercury (triply distilled) after an initial evacuation to below 2.7 Pa (20 mmHg) for no less than 2 min. High-pressure generation was by means of a computer-controlled hydraulic oil piston. The intrusion of mercury into, and extrusion from, pores was measured by a sensitive capacitance circuit. In addition to multiple complete intrusion/ extrusion cycles on multiple silica monoliths, partial intrusion/extrusion runs were obtained on a single monolith. Pore sizes were calculated using the usual pressure –pore size relationship proposed by [15], given below:
r=
2k cos q P
(2)
where k is the surface tension of liquid mercury, q the contact angle between the mercury and the surface of the solid, and P is the applied pressure required to force the mercury into the pore of radius r.
4. Results The nitrogen isotherm (Fig. 1) is of type IV [16] with hysteresis of type H1 [17], formerly denoted type A by de Boer [18], indicating the presence of cylindrical mesopores of narrow distribution. The textural properties of the monolith derived from the gas sorption data are presented in Table 1. The mean C value of 105, typical of a non-microporous oxide, means that these data can be said to meet the requirement of a successful application of the BET method [1].
Fig. 1. Adsorption – desorption isotherm of nitrogen at 77.4 K on gel-derived silica monolith. , Adsorption; , desorption.
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
127
truded at approximately 135 Mpa (19 600). After about 160 MPa (23 200 psi) there is no further intrusion. On lowering the pressure, the monoliths exhibit a significant amount of hysteresis. Extrusion does not begin until about 87 MPa (13 000 psi) and then only slightly. An abrupt change in extruded volume occurs at approximately 36–31 MPa (5200–4500 psi, Fig. 2(b)), and the volume of mercury remaining intruded falls to the linear portion of the curve. For partially intruded runs, the hysteresis remained wide even when a relatively small portion of the total pore volume had been intruded. The textural properties of this gel-derived silica derived from mercury intrusion porosimetry are presented in Table 2.
5. Discussion
Fig. 2. (a) Mercury intrusion –extrusion on gel-derived silica monolith. , Intrusion to 85.9 MPa; , to 89.6 MPa; , to 98.2 MPa; ", to 171.7 MPa; , extrusion from 85.9 MPa; , from 89.6 MPa; , from 98.2 MPa; 2, from 171.7 MPa, – – – , extrapolation of linear compression region. Arrows added for clarity to indicate , intrusion and ¡, extrusion. (b) Mercury extrusion from previously intruded monolith (detail). , Previously intruded to 85.9 MPa; , to 89.6 MPa; , to 98.2 MPa; 2, to 171.7 MPa; —, mean intrusion curve (for reference).
The specific pore volume has been confirmed by mercury intrusion porosimetry with little entrapment of intruded mercury when the system is returned to atmospheric pressure (Fig. 2(a and b)). The intrusion curve initially exhibits a reproducible, linear volume change as a function of applied pressure (approximately 2.34×10 − 2 ml g − 1 MPa − 1), before the onset of a steep rise at 80 MPa (11 600 psi). The pore volume is 99% in-
Statistical analysis indicates that, based on nitrogen sorption data, this material compares favorably with commercially available standard reference materials for surface area and pore size analysis. The stability investigation indicated that both the specific surface area and the specific pore volume remain statistically unchanged during repeated usage, i.e. there was no observable trend (neither up nor down) in the data. Surface area values lay in the range − 1.25 to + 1.65 m2 g − 1 from the median value and pore volumes lay in Table 2 Textural characteristics of gel-derived mesoporous silica by mercury intrusiona
Pore volume, V (ml g−1) Area140, A (m2 g−1) Radius140, r (nm) Radius140, r (A, ) Area180, A (m2 g−1) Radius180, r (nm) Radius180, r (A, ) a
Mean
SD
SD (%)
0.971 217.7 8.92 89.2 166.8 11.64 116.4
0.037 5.1 0.35 3.5 3.9 0.45 4.5
3.8 2.3 3.9 3.9 2.3 3.9 3.9
Area140 and area180 are the specific surface areas calculated using advancing (intrusion) contact angles of 140 and 180°, respectively. Radius140 and radius180 are the mean pore radii calculated using the cylindrical pore relationship r = 2V/A and contact angles of 140 and 180°, respectively.
128
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
Fig. 3. Mercury porosimetric curve on a powdered material (drug compound). , Intrusion; , extrusion; – – – extrapolation of logarithmic compaction region. Arrows added for clarity to indicate , intrusion and ¡, extrusion.
the range − 0.0027 to + 0.0056 ml g − 1 from the median value. The durability investigation yielded similar results. That the measured textural properties were not altered by repeated thermal cycling is indicative of a tough or resilient material. Fracturing of the material might have been expected to increase surface area slightly. Pore volume might have been reduced, or at least the isotherm would have assumed an upturn at highest relative pressure values. Furthermore, loss of structural integrity on a macroscopic scale would have been immediately apparent due to the monolithic nature of the sample; none was observed. The mercury porosimetry data is clearly affected by isostatic compression of the monolith at pressures up to about 80 MPa (11 600 psi). This reversible, i.e. elastic, behavior can be differentiated from the compaction process initially undergone by a powdered material (Fig. 3). Compaction usually conforms to some logarithmic volume change as a function of applied pressure, which can be clearly seen in the example given, up to the point at which mercury intrudes into the remaining voids. Furthermore, compaction is essentially irreversible as demonstrated by the extrusion data for the powder, there being little or no extrusion of mercury at pressures corresponding to the compaction region. Compression of the monolith causes constriction of the pores until the silica structure stiffens sufficiently
to form a rigid structure, at least to a point at which the mercury can intrude into the shrunken pores. Therefore, when a normal contact angle of 140° is used to calculate pore size, the pore radii appear significantly smaller than expected from nitrogen sorption data. Furthermore, the surface area calculated from the intrusion data, according to the method of Rootare et al. [19], greatly exceeds the BET value. Interestingly, when the unusual contact angle of 180° is used, the surface area value is in remarkable agreement with the BET value. Whether this happy coincidence is a direct consequence of the compression mechanism is worthy of future study. Degree of penetration into the pore structure does have some effect on the extrusion curve. For those ‘partial intrusion’ runs, some pores obviously remain unfilled and can expand during the pressure-letdown during the extrusion phase giving a more rounded appearance to the extrusion ‘knee’ (Fig. 2(b)). This also gives rise to a higher, average extrusion pressure than for the completely intruded run, which therefore exhibits the widest hysteresis. The reversible and reproducible nature of the compression is consistent with the resilience indicated by the nitrogen sorption durability data. The linear portion of the intrusion curve yields an elastic modulus of 604×106 N m − 2 for the monolith. Unlike monoliths stabilized at lower temperatures, which undergo some inelastic deformation [20], the monoliths studied here exhibit a high degree of reversibility and repeatability, even under repeated usage. The total pore volume is properly measured as the total volume change during intrusion, since the compression merely substitutes for intrusion into a perfectly rigid porous structure. Not only are the gas sorption and mercury porosimetery data in excellent agreement (0.986 and 0.971 g ml − 1, respectively), they agree with a simple calculation based on geometric volume and skeletal density. Assuming perfectly cylindrical geometry, each monolith has an envelope volume of 0.0616 ml. The density of gel-derived silica calcined at 900°C has been reported previously [20,21] as 2.30 g ml − 1. Taking this value the pore volume of the average disk (0.0436 g) is 0.9775 ml g − 1. Although one cannot accurately measure the pore size of an uncom-
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
pressed sample by mercury porosimetry, the resulting pore size of a compressed sample can be determined. Pore size can be computed from any non-wetting contact angle (90° B q 5 180°) desired, for example see Fig. 4, albeit incorrectly. However, for the proposed purpose, pore size calculations are actually unnecessary, since it is the intrusion pressure that is characteristic and is what the porosimeter records. Therefore, use of these monoliths in calibration and monitoring of mercury porosimeters remains viable, since both compression and critical intrusion pressure, are highly reproducible (see Table 2 and Fig. 2(a)). The observed hysteresis in the mercury porosimetry data, in the absence of significant entrapment, can be attributed to a reduction in contact angle between intrusion (an advancing meniscus) and extrusion (a receding meniscus), [22,23]. Both intrusion and extrusion curves can be made to yield identical modal pore diameters by appropriate adjustment of contact angles. For example, Liu et al. [24] chose to select a smaller contact angle for the receding meniscus (rather than a larger one for the advancing meniscus) in their work on cement paste. For the silica monolith, an extrusion contact angle of approximately 106° is required to match an intrusion angle of 140, and 111° for a corresponding 180° intrusion angle (the ratios of the cosines being the
129
same 2.78 in both cases). Significant network effects can be discounted due to the high degree of reversibility, the parallel nature of intrusion and extrusion curves, and the H1 type hysteresis of the nitrogen sorption isotherm.
6. Conclusions The mercury intrusion data supports the proposal [14,25] that a gel-derived silica monolith possesses the necessary reproducibility, stability and durability to be adopted as a new reference material, though this should be confirmed by an interlaboratory comparison study using standardized methods. Its physical form (a cylinder) permits a simple, independent estimate of pore volume, which is impossible for a powdered material. Since the nitrogen isotherm forms a plateau above P/Po = 0.97 two distinct advantages arise, at least in terms of a candidate reference material. Firstly, this behavior allows the measurement of total pore volume at a relative pressure that is considerably lower than the saturated equilibrium vapor pressure, Po, thus reducing the risk of condensation during data acquisition at the top of the isotherm. Secondly, this behavior permits the determination of total pore volume within a range of relative pressure values — a much more achievable, reproducible and therefore desirable condition. Furthermore, the evidence indicates that the same material can be used as a pore volume reference for mercury intrusion. Its ability to withstand multiple intrusions permits a more rapid checking of porosimeters by virtue of the fact that the evacuation and filling step can be eliminated for subsequent tests. This is of particular benefit in the reduction of waste mercury.
Acknowledgements Fig. 4. Pore size distribution of monolith calculated from mercury intrusion data. , Cumulative pore volume (intrusion) and —, pore size distribution calculated using a contact angle of 140°; – – –, pore size distribution calculated using a contact angle of 180°.
N.J. Coleman gratefully acknowledges the support of the EPSRC (UK). Thanks to Dr M. Bellantone (Imperial College) and M. Tucker (Quantachrome Ltd.) for their assistance and ad-
130
M.A. Thomas, N.J. Coleman / Colloids and Surfaces A: Physicochem. Eng. Aspects 187–188 (2001) 123–130
vice. Thanks to Dr K.W. Powers (University of Florida) for invaluable discussion.
Appendix A. Internationally recognized standards organizations Bundesanstalt fur Materialforschung und – pru¨ fung (BAM), (Federal Institute for Materials Research and Testing). Bundesanstalt fur Materialforschung und -pru¨ fung, Division I.1 Inorganic Chemical Analysis, Reference Materials, Zweiggela¨ nde Adlershof, Rudower Chaussee 5, D-12489 Berlin, Germany. Tel.: +49-30-63925830/5827; fax: + 49-3063925972; ftp://ftp.bam.de/G3/a – i/rmgesune.pdf. Institute for Reference Materials and Measurements (IRMM). I.R.R.M., Retieseweg, B-2440 Geel, Belgium. Tel.: + 32-14-571211; fax: + 32-14-584273; http:// www.irmm.jrc.be/rm/physical.pdf. Laboratory of the Government Chemist (LGC). LGC’s Office of Reference Materials, LGC, Queens Road, Teddington, Middlesex, TW11 0LY, UK. Tel: + 44-20-89437000; fax: + 44-2089432767; [email protected]; http://www.lgc.co.uk/ products/rm/pspsur.pdf National Institute of Standards and Technology (NIST). Standard Reference Materials Program, National Institute of Standards and Technology, 100 Bureau Drive, Room 204, Building 202, Gaithersburg, MD 20899-2322, USA. Tel.: + 1-301-9756776; fax: + 1-301-9483730; [email protected]; http://ois.nist.gov/srmcatalog/tables/view – table. cfm?table=301-4.htm.
References [1] J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, second ed., Academic Press, London, 1982. [2] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London, 1999. [3] K. Meyer, P. Klobes, B. RohlKuhn, Cryst. Res. Technol. 32 (1) (1997) 175 –185. .
[4] P. Klobes, B. Rohl-Kuhn, K. Meyer, Certified reference materials (CRM’s) for mercury porosimetry method, The Second International TRI/Princeton Workshop, Princeton, NJ, USA, June 19 – 21, 2000. [5] L.L. Hench, Sol – Gel Silica: Processing, Properties and Technology Transfer, Noyes Publications, New York, 1998. [6] R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. [7] C.J. Brinker, R.J. Kirkpatrick, D.R. Tallant, B.C. Bunker, B.J. Markey, J. Non-Cryst. Sci. 99 (1988) 418. [8] L.L. Hench, S.H. Wang, Phase Transitions 24 – 26 (1990) 785. [9] International Organization for Standardization (ISO) 9277: 1995 (E), Determination of the Specific Surface Area of Solids by Gas Adsorption using the BET method, Geneva, Switzerland. [10] J. Rouquerol, D. Avnir, D.H. Everett, C. Fairbridge, M. Haynes, N. Pericone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, Guidelines for the characterization of porous solids, Stud. Surf. Sci. Catal. 87 (1994) 1. [11] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [12] A.L. McClellan, H.F. Harnsberger, J. Colloid Interf. Sci. 23 (1976) 577. [13] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, second ed., Ellis Horwood, Chichester, 1988. [14] N.J. Coleman, L.L. Hench, Ceram. Int. 26 (2) (2000) 179 – 186. [15] E.W. Washburn, Proc. Natl. Acad. Sci. USA 7 (1921) 115. [16] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J. Am. Chem. Soc. 62 (1940) 1723. [17] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Rouqurol, J.R. Rouqurol, T. Siemiewska, Reporting physisorption data for gas/solid systems, Pure Appl. Chem. 57 (4) (1985) 603. [18] A. Author, in: J.H. de Boer, D.H. Everett, F.S. Stone (Eds.), The Structure and Properties of Porous Materials, Butterworths, London, 1958, p. 195. [19] H.M. Rootare, C.F. Prenzlow, J. Phys. Chem. 71 (1967) 2733. [20] K.W. Powers, The development and characterization of sol gel substrates for chemical and optical applications, Ph.D. dissertation, Materials Science and Engineering Department, University of Florida, Gainesville, 1998. [21] S. Wallace, Porous silica gel monoliths: structural evolution and interactions with water, Ph.D. dissertation, University of Florida, Gainesville, 1991. [22] S. Lowell, J.E. Shields, J. Colloid Interf. Sci. 80 (1) (1981) 192 – 196. [23] S. Lowell, J.E. Shields, Powder Surface Area and Porosity, third ed., Academic Press, London, 1982. [24] Z. Liu, D. Winslow, Cement Concrete Res. 25 (4) (1995) 769 – 778. [25] N.J. Coleman, L.L. Hench, Ceram. Int. 26 (2) (2000) 171 – 178.