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The Journal of Physical Chemistry, 76, 18, pp. 2584-2585, 1972-12-01
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Adsorption isotherm and surface area determination below the triple
point
Litvan, G. G.
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[Reprinted from the Journal of Physical Chemistry, 76,2584 (1972).]
Copyright 1972, by t h e American Chemical Society and reprinted by permission of t h e copyright owner.
Adsorption Isotherm and Surface Area Determination below the Triple Point
Building Materials Section, Division of Bzcildiw Research, National Research Council o/ Canada, Ottawa, Canada (Received March 1.9, 1972)
Pz~blication costs assisted bu A'ational Research Council of Canada
Attention is drawn to the consequences of the finding that adsorbates are in a liquidlike state a t temperatures below their bulk freezing point. If the relative pressure is based on the saturation vapor pressure of the under- cooled liquid, differences between the Nz and Kr surface areas and those between the BET and Dubinin- Raduskevich areas become minimal. On application of inappropriate relative pressure values in constructing isotherms, apparent supersaturation may be indicated.
Adsorptive properties of systems are commonly characterized b y isotherms where t h e experimentally determined amount of adsorbed vapor is plotted against thc relative pressure, p/pO (p is t h e vapor pressure (vp) of t h e adsorbate and p o l the v p of t h e substance in t h e bulk statc a t t h e temperature of t h e experiment). A t temperatures, T, above the triple point, Tt,, p O is taken as t h a t of the bulk liquid, pol, and a t T
<
Ttr t h a t of t h e solid pO,.I n calorimetric1-j and dilatometric5.6 experiments no evidence can be found for "freezing" in the first two adsorbed layers a t any T, and recently i t has bcen shou-n7 t h a t even a t higher coverages the adsorbate is in a liquidlike state.
T h e direct consequence of this finding is t h a t i n estimating the surface area by the B E T method t h c value of pol has t o b e substituted in the equation even if T
<
Ti,. T h e failure to follow this practice, sug-gested b y the first proponents of the I<r methodls is the reason for t h e discrepancies observed between t h e N2 and I i r surface arcas, when the experimcntal tem- perature is 77OIi, 39" below the Tt, of krypton. This is proved b y t h e discovery t h a t t h e surface area values obtained b y t h e two methods can b e brought into alignment \\-hen pO, is substituted \\-it11 adjustable (1) H. P. R. Frederikse, Physica, 15, 860 (1949).
(2) J. A. Morrison and L. E. Drain, J . Chem. Phys., 19, 1063 (1951). (3) J. A. Morrison, L. E. Drain, and J. S. Dugdale, Can. J . Chem.,
30, 890 (1952).
(4) K. S. Dennis, E. L. Pace, nnd Ch. S. Baughman, J . Amer. Chem. SOC., 7 5 , 3267 (1953).
(5) G. G. Litvan, Can. J . Chem., 44, 2617 (1966).
(6) G. G. Litvan and R. McIntosh, ibid., 41, 3095 (1963).
(7) E. W. Sidebottom and G. G. Litvan, Trans. Paradau Soc., 6 7 ,
2726 (1971).
(8) R. A. Beebe, J. B. Beckwith, and J. M. Honig, J . Amer. Chem. SOC., 67, 1554 (1945).
parameters pol9 and QIO the value of which was identical with pol calculated by extrapolation. The authors of ref 9 and 10 take the view, hoxvever, that the close agreement between Q and pol is fortuitous.
The purpose of this note is to show that recognizing the liquidlike state of the adsorbate a t T
<
T t , provides theoretical support for the practice of applying pol in the krypton surface area calculation and implies that similar practice has to be followed in the case of other adsorbates, e.y., argon, xenon, and methane at liquid nitrogen temperature.Furthermore, if experimeritally determined adsorp- tion data are plotted in terms of inappropriate p O value, (such as pO,) distortion of the curve results.' For example, in the case of ltrypton at 77"K, pO, = 1.75
Torr while pol = 2.63 Torr, thus when p/pO, . . = 1 the
value of p/pOl is only 0.66. When the pressure in the cell exceeds this value, condensation on the walls of the cell restores it to pO,. The obtained isotherm, if plotted as a function of p/pO,, crosses the ordinate at unity relative pressures instead of approaching i t
any way to the adsorbent since it is not yet saturated. I n the sudy of xenon adsorption on porous silver a t 77.4"I<,13 p/pO, rose temporarily to a value of approx- imately 1.08 but, if the values are recalculated the true relative pressure was apparently only -0.02 8
(applying p O = 2.06 X Torr, obtained by extrap- '- olation14). Supersaturation is clearly an artefact as
shown not only by the above consideration but also by the reasonably good superimposition of the Ar, Iir, and X e isotherms when they were plotted in terms of p/pOl. I t is very probable that the differences between the B E T and Dubinin-Raduslievich areas will become minimal when the proper pol values are used. Diffi- culties are encountered, hoxvever, in determining ac- curate pol values which, in some cases, are SO0 below
Tt,
(xenon at 77OK).Acknowleclyment. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.
- -
asymptotically. This was reported10 in 1965 and
(9) J. M. Haynes, J . Phys. Chem., 6 6 , 182 (1962). in a study of the adsorption On
(lo) P. Chenebault and A. Schurenkamper, ibid., 69, 2300 (1965). gases On and surfaces'11 It (11) R . H. van Dongen, J. H. Kaspersma, and J. H . DeBoer, Surface be emphasized that the phenomenon considered here sci., 28, 237 (1971).
occurs below T t , and is distinct from that discussed (12) S. Brunauer, J. Skalny, and E. E. Bodor, J . Colloid Interface by Brunauer and coxvorkers. l 2 Sci., 30, 546 (1969).
(13) J. P. Hobson, J . Phys. Chem., 73, 2720 (1969).
If On the is p may exceed (i4) ~LandolbBSrnstein Zahlenwerte und Funktionen," part Za, pO, temporarily, but this phenomenon is not related in ~ p r ~ ~ ~ r - v e r l a g , Berlin, 1960, p 4.
