Langmuir 1990, 6, 122-124
122
Surface Area of Clays Robert S. Murray* and James P. Quirk Waite Agricultural Research Institute, The University of Adelaide, private mail bag no. 1, Glen Osmond, South Australia 5064 Received March 29, 1989 In view of the assumptions employed in obtaining them, the surface areas of clays derived from adsorption and from desorption isotherms of nitrogen are in remarkable agreement. There is, in general, striking accord between the value from the adsorption isotherm, derived by using the BET equation, and the value from analysis of the desorption isotherm, obtained by using the Kelvin equation, micropore analysis, or a simple extrapolation of the pore size data into the micropore region. The latter empirical method, originally proposed by Innes (ref l),seems not only successful in this regard but may also be justified in terms of micropore geometry.
Introduction Measures of the surface area of a porous material may, in some cases, be obtained separately from the adsorption and desorption isotherms of a nonpolar adsorptive such as nitrogen. This applies to systems of mesopores (2-50 nm) and/or macropores (>50 nm), with welldefined shapes, which are free to empty independently so that network effects are absent.293Such materials would normally be expected to exhibit type IV and type IT isotherms, re~pectively.~ Nitrogen adsorption data in the relative pressure range (p/po)0.05-0.3 are commonly used, in conjunction with the BET e q ~ a t i o nto , ~estimate the surface area of porous or finely divided materials. Despite its theoretical shortcomings, this equation seems capable of yielding sensible estimates of surface area when applied to nitrogen sorption by systems in which there are few micropores3 (i.e., C2 nm). Alternatively, methods based on adsorption by a nonporous or macroporous reference material may be used. However, in the absence of microporosity and where the adsorption of the reference material can be described by the BET equation, these methods are equivalent to direct BET treatment of the data. Nitrogen desorption data may also be used to estimate surface area. If the statistical film thickness of the adsorbate on the adsorbent is known as a function of the relative pressure and if a realistic pore model is available, then the Kelvin equation may be applied to the desorption data to derive a pore size distribution. This allows a further estimate of surface area to be made by summing the surface area contributions arising from small, conceptual pore size classes over the entire pore size distribution. Such an analysis of the desorption isotherm of nitrogen is, however, widely held to be unreliable below about p / p o = 0.5;3 this corresponds to a pore radius or slit width of about 2.7 nm. Use of the Kelvin equation in this relative pressure range represents an attempt to describe a "meniscus" with a radius of less than four nitrogen molecules by using macroscopic thermodynamics and bulk condensate properties. In addition, there is the suggestion that tensile failure of the condensate may preempt its e ~ a p o r a t i o n . ~ Calculation of the cumulative surface area from the ~~
~
(1) Innes, W. B. Anal. Chem 1957,29, 1069.
(2) Wall, G C., Brown, R. J. C. J . Colloid Interface Sci. 1981, 82, 141. (3) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosrty, 2nd ed.; Academic Press London, 1982; Chapter 2
0743-7463/90/2406-0122$02.50/0
pore size distribution of some porous materials may include, then, a component of the desorption isotherm for which available theories are less than satisfactory. This is demonstrated by the number of approaches adopted by various workers for extracting, from nitrogen desorption data, the surface area contribution of micropores and the smaller mesopores. Lippens and de Boer4 and Dollimore and Heal5 used the Kelvin equation down to low relative pressures (0.08, CO.01, respectively). Stul and Van Leemput' and Lecloux et al.7 used the Kelvin equation down to p/po = 0.40 (equivalent to a void width of about 2.2 nm) followed by various methods of isotherm analysis appropriate to micropores. It should be noted that in each of these approaches the comparison between the derived, cumulative surface area and that obtained from the BET analysis of the adsorption data is somewhat paradoxical. Hysteresis in nitrogen sorption isotherms is absent below about p / p o = 0.4, and so the value of the cumulative surface area obtained from the desorption isotherm includes a component obtained from the same data as that subjected to BET analysis. In other words, a single set of data is subjected to two entirely different interpretations. Underlying this is the implausible assumption that, a t a given relative pressure, the statistical thickness of the adsorbate film, but not the total amount of adsorbate itself, depends upon whether adsorption or desorption is occurring. In proposing a parallel-plate pore model for the calculation of pore size distribution, Inned used the Kelvin equation down to p/po = 0.30 (equivalent to a void width of about 1.8 nm) and assumed, for smaller voids, the simple, empirical relationship v = ad2 (1) in which a is a constant and u is the total volume within voids which have a width less than d. This gives the surface area A associated with voids narrower than d as A=
2Iddulx = 2Jd2a x
=o
dx = 4ad
Innes' commented that "most of the data in this range appear to fit the expression u = ad2",but it is not clear (4) Lippens, B. C.; de Boer, J. H. J. Catal. 1964,3,44. ( 5 ) Dollimore, D.; Heal, G. R. J. Colloid Interface Sci. 1970, 33, 508. (6) Stul, M. S.; Van Leemput, L. Surf. Technol. 1982, 16, 101. (7) Lecloux, A. J.; Bronckart, J.; " d e , F.; Dodet, C.; Marchot, P.; Pirard, J. P. Colloids Surf. 1986, 19, 359.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 1, 1990 123
Surface Area of Clays
Table I . Specific Surface Area (SSA) and Disposition of Surface Area in Clays and Clay Soils desorption adsorption sample
c(BET) 139 145 200 168 114
Muloorina illite" Willalooka illiteb Fithian illite' Urrbrae B clayd Wyoming bentonitee$
SSA, m2/g
SSA, m2/g
Calcium Clay 144 190 92 145 36
7% SSA in voids of width (nm)
