On Physical Adsorption - American Chemical Society

On Physical Adsorption XV. Evidence of Marked Asymmetry in the Distribution of Adsorptive Potentials of Certain Solid Surfaces SYDNEY ROSS, J. P. OLIVIER, and J . J . HINCHEN Department of Chemistry, Rensselaer Polytechnic Institute, Troy, Ν. Y.

The interpretation of adsorption isotherms in terms

Downloaded by UNIV OF MONTANA on December 13, 2014 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0033.ch033

of a surface displaying a symmetrical (Gaussian) distribution of adsorptive potentials to the adsorb ate has been successfully applied to a number of systems; but marked exceptions have been found, particularly with crystals that were selected as likely to have completely homotattic surfaces. These

exceptional

adsorption

isotherms

are

readily interpreted, however, as the result of a sum of two, or occasionally three, distinctly differ ent Gaussian distributions, presumably deriving from the same number of surface constituents. Among these constituents the expected homotattic substrate, which is associated with a particular crystal face, can always be identified.

A

previous paper of this series (13) describes the analysis of adsorption iso therms for heterogeneous surfaces in terms of a Gaussian distribution of adsorp tive potentials. The distribution of adsorptive potential energies may not, however, have a form that is symmetrical about a mean; it could possess enough asymmetry so that the adsorption isotherm could not be described by a model that assumed a Gaussian distribution. Imagine, for example, an otherwise uniform surface that is contaminated by a small amount of a nonvolatile impurity of higher adsorptive potential. The adsorption isotherms of a gas on this substrate would show a small "knee" at the low-pressure end of the isotherm, thereby indicating the presence of the impurity. The shape of the isotherm would be the same whether we consider the impurity as a coating on a part of the surface of the major constituent or as a mechanically separate ingredient of the mixture. We could, therefore, account for such an isotherm by assigning separately to each of the surfaces its own values of the adsorption parameters V^, γ, and K'. By doing so we are actually describing sub strates of non-Gaussian adsorptive energy distributions in terms of a sum of Gaussian distributions. 317

In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.


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Although not described quite in this way, a number of adsorbents have been reported to be effectively mixtures of two surfaces. The most clear-cut example is a slightly oxidized graphite surface for which the adsorption isotherm of water vapor shows a pronounced knee whose height varies with the degree of oxidation of the surface; reducing the surface with hydrogen eliminates the knee (7, 10). The great dissimilarity of the adsorptive potentials of the two surfaces for the polar water molecule makes it obvious to the eye that two distinct adsorbing surfaces are present. Molybdenum disulfide, most samples of which have a partially oxidized surface, is another example (I, 2). The adsorption isotherms of argon and nitro gen on boron nitride show a similar knee, though to a much less marked extent; it is, in fact, visible only when the low pressure portion is plotted on an expanded scale as was done by Winkler (16). The presence of a trace of boron oxide in the sample was detected by wet analysis; the prominence of the initial concavity of the isotherm to the pressure axis was found to relate to the amount of oxide contaminant in the boron nitride. The surface of a solid is rarely smooth but is interrupted by cracks, crevices, capillaries, cavities, corners, and edges. Even on a molecular scale roughness is frequently introduced by lattice disorder or spiral dislocations. The utmost effort to obtain completely homotattic substrates has not yet succeeded in avoiding residual inhomogeneities. When graphite has a chemically pure adsorbing sur face, the inhomogeneity has the form of a symmetrical distribution about a mean (13); the mean is representative of the basal plane of the graphite lattice. This type of random distribution presumably arises from the factors that disturb the geometrical smoothness of the surface. For oxidized graphite, molybdenum disul fide, and boron nitride the asymmetry of the distribution curve has been traced to the simultaneous presence of two chemically different adsorbing surfaces, each pre sumably with its own random distribution of adsorptive energies. In general, we may consider each chemically distinct adsorbing surface that is present in a mass of adsorbent to have its individual random distribution of adsorptive energies; this applies whether the chemical difference rises from different surface planes of the

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o.04h

PRESSURE

Figure 1.

(mm)

Adsorption isotherm of argon on cadmium bromide at 77.5° Κ

Two lower curves are theoretical isotherms for two surface constituents Sum of two theoretical curves Ο Experimental points In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

ROSS ET AL.

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Adsorptive Potentials of Solid Surfaces

same crystal, surface contamination, oxide, hydroxide, or other surface compounds, or even gross admixture of other chemical substances. Cadmium Bromide The adsorption isotherm of argon at 7 7 . 5 ° Κ on cadmium bromide is shown in Figure 1. This adsorption isotherm shows characteristics that are indicative of


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Figure 2.

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Adsorption isotherm of argon on sodium bromide at 77.5° Κ

Two lower curves are theoretical isotherms for two surface constituents Sum of two theoretical curves Ο Experimental points both low and high degrees of heterogeneity. The latter is indicated by the con cave shape of the isotherm with respect to the pressure axis and the former by the sigmoidal shape at higher pressures. No one of the model adsorption isotherms, such as those shown in Figure 2 of (13), possesses both those features. Using the tables of model isotherms (8, 14) and a process of trial and error, the experimental isotherm shown in Figure 1 can be described quantitatively by a dual distribution of adsorptive energies. We have no information concerning the surface electric field of cadmium bromide and therefore used the tables calculated for 2 a /RT β = 6.5, where a //? = a/2b, and a and b are the van der Waals constants for argon gas. The two model isotherms ultimately obtained are re ferred to as type 1 and type 2 in Figure 1; their sum provides a satisfactory match with the experimental points, which are indicated by circles. The adsorp tion parameters for each of the two surfaces are reported in Table I; the numerical values would be slightly different were the magnitude of the surface field known and taken into account. The two surface constituents are: first, a relatively heterogeneous surface (γ = 3) with an average adsorptive energy, U' (calculated), of 1.76 kcal. per mole, which is present as 59% of the total surface; second, an essentially homotattic surface with an adsorptive energy, U' (calculated), of 1.65 kcal. per mole, present as 41% of the total surface. id

[ά

i(1

kl

The adsorbent was chosen purposefully as one likely to give a near-homotattic surface, because of the hexagonal layer-lattice structure of the crystal; the surface constituent of low heterogeneity is therefore identified with the basal plane of In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.


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cadmium bromide, consisting of close-packed bromine atoms. The cadmium bromide was prepared by dehydration of the tetrahydrate and the second surface constituent could perhaps be cadmium hydroxide, formed by surface hydrolysis. To test this supposition, a sample of finely divided cadmium bromide was prepared under anhydrous conditions, by subliming the material in a stream of dry argon and collecting the particles on a microporous filter. Care was taken to preserve the sample from contact with anything but argon. Detailed information about the method of preparation will be published elsewhere. The adsorption isotherm of argon at 7 7 . 5 ° Κ on this substrate then showed only the characteristic—i.e., the same value of K'—of the type 2 surface; almost all evidence of type 1 was lacking. This result confirms the truth of our analysis of the surface into two distinct dis tributions, one of which, the more heterogeneous, is probably the result of surface hydrolysis. Sodium Bromide Cube crystals of sodium bromide have the same lattice plane—i.e., 1100| —on all faces. Such crystals have often been favorite candidates for the prepara tion of a completely homotattic adsorbent, and the assumption has always been made that the heterogeneity of the resulting surface is negligible. No definite criteria have hitherto been available to check this assumption. In Figure 2 we report the argon adsorption at 7 7 . 5 ° Κ on a specimen of sodium bromide, prepared according to the directions given by Fisher and McMillan (6). The shape of the isotherm is itself sufficient in the light of the present reasoning to indicate a dual distribution of adsorptive energies. A quantitative description of the isotherm in these terms has been obtained by a process of trial and error using 2a /RT β = 6.5, and is shown in Figure 2; the adsorption parameters are reported in Table I. The surface has two constituents, one of high and the other of low degree of heterogeneity, as was the case with the cadmium bromide described previously. Of the total surface, 75% consists of a near-homotattic constituent (γ = 200), which presumably derives from the {lOOJ planes of the crystal; this portion of the surface has a relatively low average argon adsorptive energy U' (calculated) of 1.49 kcal. per mole. The remaining 25% of the surface has a wide distribution (γ z= 3) of adsorptive energies, with the average U' (calculated) at 1.59 kcal. per mole. Once again, chemisorbed water may well be the source of the second sur face constituent. id

Table I.

[ά

Adsorption Parameters of Argon Adsorbed at 77.5° K. U' {Calcd.), Vfi, Cc./G.

y

K'

Typel Type 2

Dual Surface of Cadmium Bromide 0.37 3 20.6 0.26 oo 42.1

Typel

0.080

Kcal./Mole

1.76 1.65

Dual Surface of Sodium Bromide Type 2

0.24

3

200

61.6

120

1.59

1.49

A verification of the interpretation of the surface of sodium bromide, when prepared under usual conditions, as a dual distribution is again provided by a new preparation under anhydrous conditions of a sample of the sublimed crystals, which when used as an adsorbent shows evidence of type 2 surface only. The In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

ROSS ET AL.


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adsorption of argon at Κ by these crystals is shown in Figure The whole isotherm is described by the sames values of K' and γ that are reported in Table I for the type 2 surface of sodium bromide. The surface area of the new preparation is much greater than that of the sample reported in Table I.


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Figure 3. Adsorption isotherm of argon on anhydrous preparation of sodium bromide at 77.5° Κ Theoretical isotherm corresponding exactly to that labeled type 2 in Figure 2 Ο Experimental points Other Examples from Previously Reported Data The shape of the isotherm in Figure 2 resembles all those reported for the alkali halides as adsorbents, as well as other inorganic salts that were prepared in attempts to procure near-homotattic substrates. Examples are the sodium chloride of Orr (9) and those of Ross and coworkers (11, 12, 15), the potassium chloride of Clark (3), the calcium fluorides of Edelhoch and Taylor (5) and Ross and Winkler (15), and the sodium bromide of Fisher and McMillan (6). A part of the surface (type 2) of each of the adsorbents was sufficiently homotattic to allow two-dimensional phase transitions to manifest themselves on lowering the tem perature of adsorption, as near-discontinuities in the isotherms; if these portions of the surface had any other than a very narrow range of adsorptive potentials, the phase transitions would occur over too wide a range of pressure to be identified as such. On the type 1 part of these surfaces, phase transitions cannot be identified because of the great heterogeneity of this part of the substrate. Ross and Boyd (11) prepared crystals of sodium chloride in which both the {100} and the {111} surfaces were developed; the adsorption isotherm of ethane at Κ gave evidence of two type 2 —i.e., near-homotattic—surfaces, as well as showing the initial knee, which is evidence of a type 1 surface. This isotherm is shown in Figure 4, which also includes for comparison an ethane isotherm meas ured at the same temperature for a sodium chloride adsorbent that had only J100 j faces developed; the pressure characteristic of the phase transition of ethane on the homotattic {100} surface of sodium chloride at 901 .° Κ shows on both iso therms as the location of a discontinuity at ρ = 45 . X 10 mm.; the homotattic {ill} portion of the surface is responsible for the convex shape (with respect to the pressure axis) of the isotherm beyond the "knee," though the rise is not suffi-

901 .°

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ADVANCES IN CHEMISTRY SERIES

ciently pronounced to be described as another discontinuity. The evidence is almost enough to determine relative magnitudes of the surface fields and the adsorptive potentials of the {HI} and (100} crystal faces. The absence of a phase change on the { H I ] faces is probably genuine; though enough hetero geneity in the characteristic distribution associated with those faces could, even if a phase change were to occur, dissolve the evidence. Supposing the two crystal faces to have been equally well developed, however, as indeed was indicated by the appearance of the crystals in the microscope, the two-dimensional phase change of ethane on the {100} and its absence on the {1H| faces show that the former substrate has the weaker surface field. Furthermore, the relative pressures at which the two crystal faces make themselves manifest in the adsorption isotherm— i.e., lower pressure for { ] than for J100l-are evidence of a smaller adsorptive potential for the {100} faces.

PRESSURE

Figure 4.

(microns)

Adsorption isotherm of ethane on sodium chlo ride at 90.Γ Κ

A. Adsorbent as prepared displayed both [100 ) and [111] faces. Data of Ross and Boyd (11) B. Adsorbent as prepared displayed only {100} faces. Data of Ross and Winkler (15) Surface Electric Field The surface field of a near-homotattic surface can be estimated from the observed value of the two-dimensional critical temperature T . According to de Boer (4), a

c

where a and T refer to the observed two-dimensional van der Waals interaction constant and the observed two-dimensional critical temperature, respectively; the same symbols with the superscript id refer to the ideal values of these quantities; and a

e

λ = a —a

id

=


(2)

ROSS ET AL.

323


where μ is the dipole induced on the adsorbate by the electric field, F, of the sub strate, and d is the diameter of the adsorbed molecule. The induced dipole is related to the surface field by (3)

μ = Ft where ξ is the polarizability of the adsorbate. Hence


(4)

In spite of the presence of the type 1 part of the surface, one can frequently esti mate the two-dimensional critical temperature of the phase transition, provided that the type 2 part of the surface is uniform enough to make the transition manifest. Thus we can use this procedure, for example, with the supra- and subcritical isotherms of methane, ethane, and xenon adsorbed by the {100^ face of sodium chloride, published by Ross and Clark (12). Using estimates of T from these data in Equation 4 yields a value for the surface field of F = 1.5 ± 0.5 Χ 10 e.s.u. per sq. cm.; the poor precision of the result is due to the difficulty of inter polating the temperature of the critical isotherm by eye. The surface field of the 1100 j face of sodium bromide can be estimated in a similar way, using the data of Fisher and McMillan (6) for the adsorption of methane and krypton. The re sult for the surface field is again between 1 and 2 X 10 e.s.u. per sq. cm. a

c

5

5

A better estimate of the surface field of the { 100} face of sodium bromide can be made from the argon isotherm reported in Figure 2 by assuming that the type 2 part of the surface is actually much closer to homotattic than would be indicated by a γ value of 200; since this value had been assigned on the basis of no surface field, the assumption that it is actually higher is well justified. If we now put γ > 1000 for the type 2 surface, we find on refitting it to the model isotherms that 2a/RTβ = 5.7, rather than the previously assumed ideal ratio of 6.5. Assuming β — β for argon, we calculate the induced dipole to be 0.24 debye and the surface field to be 1.4 X 10 e.s.u. per sq. cm. The same result is derived from the argon adsorption isotherm for the sodium bromide prepared under anhy drous conditions (Figure 3). ίά

5

For potassium chloride, Clark (3) observed phase transitions to take place at temperatures lower than on a sodium chloride substrate. We deduce that the surface field of potassium chloride is greater than that of sodium chloride or sodium bromide. The configuration of ionic surfaces at which both positive and negative ions are present does not lead to the concept of a uniform surface field expressable by a single number, but rather to a field fluctuating from point to point along the sur face. This varying field induces varying dipoles in the adsorbed molecules and the average component of these dipoles normal to the surface is the quantity that we calculate from the observed lowering of a or T . When we use Equation 4 for such a surface, we are calculating the equivalent uniform surface field that, if it were actually present, would cause the observed effects. ia

a

c

i d

Acknowledgment The authors gratefully acknowledge grants in aid of these researches from Esso Research and Engineering Co., Linden N . J., and the donors of The Petro leum Research Fund, administered by the American Chemical Society. In SOLID SURFACES; Copeland, L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

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Literature Cited (1) Ballou, Ε. V., Ross, S., J. Phys. Chem. 57, 653 (1953). (2) Cannon, P., Ibid., 64, 858 (1960). (3) Clark, H., Ph.D. thesis, Rensselaer Polytechnic Institute, 1954. (4) de Boer, J. H., "The Dynamical Character of Adsorption," p. 155, Clarendon Press, Oxford, 1953. (5) Edelhoch, H., Taylor, H . S., J. Phys. Chem. 58, 344 (1954). (6) Fisher, Β. B., McMillan, W. G., J. Chem. Phys. 28, 549, 555, 563 (1958). (7) Millard, B., Caswell, E. G., Leger, Ε. Ε., Mills, D. R., J. Phys. Chem. 59, 976 (1955). (8) Olivier, J. P., Ph.D. thesis, Rensselaer Polytechnic Institute, 1960. (9) Orr, W. J., Proc. Roy. Soc. (London) 173A, 349 (1939). (10) Pierce, C., Smith, R. N., Wiley, J. W., Cordes, H., J. Am. Chem. Soc. 73, 4551 (1951). (11) Ross, S., Boyd, G. E., U. S. At. Energy Comm., MDDC864 (1947). (12) Ross, S., Clark, H., J. Am. Chem. Soc. 76, 4291, 4297 (1954). (13) Ross, S., Olivier, J. P., J. Phys. Chem. 65, 608 (1961). (14) Ross, S., Olivier, J. P., "On Physical Adsorption," Interscience, New York, in prepa ration. (15) Ross, S., Winkler, W., J. Am. Chem. Soc. 76, 2637 (1954). (16) Winkler, W., Ph.D. thesis, Rensselaer Polytechnic Institute, 1955. RECEIVED May 9,

1961.


On Physical Adsorption - American Chemical Society

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