Adsorption Studies on Bone Mineral - ACS Publications

Adsorption Studies on Bone Mineral Heats of Adsorption of Nitrogen and Argon at — 195° C. JAMES M. HOLMES Caneton University, Ottawa 1, Canada RALPH A. BEEBE Amherst College, Amherst, Mass.

We have measured isotherms and calorimetric heats of adsorption of nitrogen and argon ad Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0033.ch030

sorbed on bone mineral. The surface of the bone mineral was treated by evacuation at 450° C. and also by covering with approximately a monolayer of methanol or water.

The heats of adsorption of

nitrogen on the bare surface start at 5.5 kcal. per mole and the values fall off to about 2.0 kcal. at the monolayer.

On the chemically treated sur

faces considerably lower heats of adsorption were found.

These effects are explained on the basis

of the attraction of the polarizable nitrogen mole cules to a surface of varying polarity, depending on the method of treatment.

The data obtained

agree well with results reported by Beebe and Emmett on heats of adsorption of nitrogen on bone mineral measured from retention times in chro matography.

η ecause of the importance of the surface chemistry of bone mineral in physiologi^ cal systems, we have undertaken a series of gas adsorption studies on hydroxyapatite in the form of anorganic bone. In a recent publication from this labora tory (4) results of calorimetric studies of the adsorption of water and methanol vapors on bone mineral and on synthetic hydroxyapatite were reported. The adsorption potential for nitrogen on dehydrated hydroxyapatite, whether from bone or from synthetic sources, was rather profoundly altered by the addition to the surface of chemisorbed methanol or water prior to the adsorption of nitrogen at —195° C . This effect was reflected in the specific surface areas, in the B E T C values, and in the resultant values of Ε — E (net heats of adsorption) as shown in Table I of the above paper. τ

L

In the work cited above, as well as in other work (8, 9, 11), the B E T V values as determined by nitrogen adsorption are diminished by the presence of m

291

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

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ADVANCES IN CHEMISTRY SERIES

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0033.ch030

chemisorbed layers. As a result, the question arises whether this apparent diminution in surface area should be attributed to blocking of very narrow pores or to a less condensed packing of the nitrogen molecules on the methanol-covered or water-covered surfaces. To improve our understanding of these phenomena and possibly to answer the specific question indicated, we have undertaken a calorimetric study of the heats of adsorption of nitrogen on the bone mineral both in its dehydrated state and in the state resulting from modification by chemisorbed layers of water or methanol and we have measured heats of adsorption of argon on some of these surfaces. Further interest in the calorimetric data has arisen from a chromatographic study on dehydrated and on water-covered bone mineral surfaces from which heats of adsorption have been derived (3). This appears to be the first report of a comparison of heats of adsorption data by the two methods on the same adsorption system. Experimental Materials. B O N E M I N E R A L . Calorimetric measurements have been made on two samples of anorganic bone mineral obtained from Armour and Co. and designated as Ossar Femur Head lot 34 and lot 33-43, the latter being a mixture of two lots. Both samples were 20/40 mesh and had specific surface areas varying from 95 to 105 sq. meters per gram based on the Weight of the dehydrated material. The properties of lot 34 and the preparation of the Ossar samples have been described by Dry and Beebe (4). Lot 33-43 has been used in an investigation, the results of which are to be published later from this laboratory, dealing with pore size distributions as well as the effect of outgassing temperature on weight loss and specific surface area changes. For reasons discussed elsewhere (4) we have adopted a procedure for dehydration which consists of degassing the samples in vacuo for at least 15 hours at 4 5 0 ° . ADSORBATES. Nitrogen, argon, and helium used for dead space measurements were research grade gases obtained from Matheson Co. and further purified (5). Reagent grade methanol and distilled water were purified and deaerated by bulb to bulb distillation in vacuo. These liquids were stored in flasks equipped with a cold finger so they could be further deaerated before use and both gave constant vapor pressures which agreed with the accepted values. P R E P A R A T I O N O F W A T E R - C O V E R E D A N D M E T H A N O L - C O V E R E D SURFACES.

In

this work we are essentially concerned with three types of bone mineral surface: ( 1 ) "bare surface," the Ossar sample which has been dehydrated or outgassed by evacuation at 4 5 0 ° for at least 15 hours; (2) "methanol-covered surface," the bare surface to which has been added approximately a monolayer of chemisorbed methanol, and (3) "water-covered surface," which has approximately a monolayer of water chemisorbed on the bare surface. The latter surface is very close in adsorption potential for nitrogen to the surface obtained by outgassing the Ossar samples at 0 ° . Several methods have been used in an attempt to cover the bone mineral surface with a monolayer or more of methanol or water. The most generally used technique consisted of equilibrating the bone mineral in the calorimeter at 0 ° with saturated vapor of either of the two liquids at this temperature. Then the calorimeter was pumped out at 0 ° for several hours until no residual pressure could be detected. From the isotherms reported by Dry and Beebe (4) we then determined the relative pressure which would produce 1.2 to 1.4 layers of methanol or water on the bone mineral and added this amount of the adsorbate to the calorimeter. Other techniques involved no addition of methanol after pumping out at 0 ° and also pumping out only to the required relative pressure. Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

HOLMES AND BEEBE

Adsorption on Bone Mineral

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60 r

Apparatus. The gas-handling and vacuum systems used were standard volumetric equipment (5). Liquid nitrogen was used as a constant temperature bath around the calorimeter and the pilot sample; its temperature was checked by means of an oxygen vapor pressure thermometer. The type of calorimeter and the method of calculating the heats of adsorption from the experimental data were essentially the same as described in previous papers (I, 4, 10). Two calorimeters of the same design were used, one employing a filler made of copper as described by Dry and Beebe (4) and the other a filler of aluminum. (A drawing and brief description of this calorimeter will be supplied on request addressed to the authors at Amherst College.) In one run for nitrogen adsorption on the bare surface we employed a liquid nitrogen trap to prevent contamination of the sample in the calorimeter by condensed mercury. Data from all runs on the various calorimeters and samples checked within the accuracy of the experiments. In all of our experiments a pilot sample was placed in the system very close to the calorimeter, so that it received the same heat treatment as the sample in the calorimeter . This pilot sample was used to obtain more complete isotherms and to establish a final weight of the adsorbent. Weight losses on evacuation at 4 5 0 ° for the samples used averaged 6.1 to 6.3% of the initial weight. A more detailed investigation of the weight loss on evacuation will be reported in a subsequent publication. Remits and Discussion Nitrogen Isotherms. Sample isotherms for nitrogen adsorption at —195° for the variously treated bone mineral surfaces are shown in Figure 1. Curves a, b, and c refer to measurements on the bare, water-covered, and methanol-covered surfaces, respectively. These isotherms are plotted from data measured on the pilot sample and the volumes adsorbed are calculated on the basis of the weight of the sample degassed at 4 5 0 ° . This value was obtained by sealing off the pilot Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.


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ADVANCES I N CHEMISTRY SERIES

""θ

0.2

0.4

0.6

0.8

v/v Figure 2.

•

1.0

1.2

1.4

1.6

m

Differential heats of adsorption for nitrogen on bone mineral at-195° C. Ο,Φ, Q ,θ,0,& Bare surface, runs 1 to 6 Water-covered surface A Methanol-covered surface

sample in vacuo after degassing at 4 5 0 ° at the end of the runs, weighing bulb plus sample, removing the sample, and weighing the parts. Small differences between the isotherms obtained in the pilot sample and in the calorimeter are due to more efficient evacuation of pilot sample 0.4 gram) than the larger sample in the calorimeter 4.5 grams). The significance of our calorimetric data is not seriously affected by these differences, since we calculated the fraction of the surface covered by determining the B E T monolayer volume from the iso therm measured in the calorimeter. We show here only one set of isotherm data, in order to show the similarity of these isotherms to those determined by Dry and Beebe (4), although the tech nique of preparing the modified surface was somewhat different in the two investi gations. The methanol-covered sample shown in Figure 1 was prepared by adding more methanol after the sample was pumped at 0 ° to produce about 1.4 layers. The isotherm data gave good straight-line B E T plots. In Table I are listed the Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

HOLMES AND BEEBE Table I.


BET Parameters for Adsorption of N on Bone Mineral at — 1 9 5 ° 2

Sample

No. of Runs

V, Cc. S.T.P./G.

Specific Surface, Sq. M./G.

Bare surface Water-covered Methanol-covered

6 3 5

22.5 19.6 16.55

97.9 84.0 68.0

m


295

BET C Value

Net Heat of Ads., CaL/Mole.

236 105 23

830 710 480

B E T parameters for the variously treated samples. Between run 1 and run 4 of Figure 1, the bone mineral sample had been heated to 4 5 0 ° in vacuo 12 times, treated with methanol twice, and with water once. In view of the complex nature of the adsorbent, it seems noteworthy that the isotherms from runs 1 and 4 are so nearly coincident. Heats of Adsorption of Nitrogen at - 1 9 5 ° . The results of the calorimetric work for nitrogen on bone mineral at — 1 9 5 ° are represented in Figure 2, where the differential heats as measured for successive small increments are plotted against the coverage expressed as V/V . The V values were determined as described above by B E T treatment of the nitrogen isotherms. Curves a, b, and c of Figure 2 represent, respectively, the heat for the bare, water-covered, and methanol-covered surface. m

m

There is a considerable spread of the experimental points of curve a, especially in the regions of lowest ( 0 . 0 1 to 0 . 0 3 V/V ) and intermediate coverage (0.2 to 0.5 V/V ). Because of our interest in comparing the present results with the chromatographic data discussed below, it was deemed desirable to obtain calorimetric data for the smallest feasible initial increments of nitrogen. This resulted in a higher percentage error in the measurements as reflected in the spread in the heat values for the initial increments of different runs. The differential heat values of Figure 2 for the region V/V — 0.05 to 0.4, unlike those for coverage V < 0.2 or V > 0.5, do not represent equilibrium conditions. The resulting thermal drift, discussed in some detail below, gives rise to an uncertainty in the heat values reflected in the very considerable spread of points in the intermediate region (V = 0.2 to 0 . 5 ) . The experimental points shown for curve a of Figure 2 are based on the observed heat evolution at the end of 5 minutes after admission of a given increment. This thermal drift was absent for the treated surfaces, and hence all points on curves b and c represent measurements under equilibrium conditions. m

m

m

m

m

m

H E A T D A T A ON BARE SURFACE.

From Figure 2 it is seen that the heats of

adsorption for nitrogen fall off from an initially high value of about 5.4 kcal. per mole at the lowest coverage studied to less than 1.7 kcal. at the estimated completion of the monolayer. The initial high values are doubtlessly attributable to the effect of the strongly polarizing ionic surface of the bone mineral upon the easily polarizable nitrogen molecules. Energetic heterogeneity in physical adsorption as related to experimental heat data on chabazite surfaces has been discussed in detail in a recent publication by Kington and Macleod ( 7 ) . It seems probable that the considerations put forth by Kington and Macleod are pertinent to the present system and that much of the high energy of adsorption is attributable to the quadripole interaction with the polar or ionic surface. The heats of adsorption of nitrogen on the bare surface remain above 3 kcal. per mole until about 8 0 % of the surface is covered. The expected drop in the heat curve at the completion of the monolayer is also evident. Although the heat measurements above the monolayer are more difficult and less precise, the heat values remain well above the heat of vaporization as the second layer begins to Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

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ADVANCES IN CHEMISTRY SERIES

fill. We take this as evidence for orientation effects of the polar surface extending into the second adsorbed layer of nitrogen.


H E A T D A T A O N T R E A T E D SURFACES.

In contrast to the data of curve a of

Figure 2, the initial differential heat values of curves h and c are far lower when nitrogen is physisorbed on top of an approximate monolayer of chemisorbed water or methanol. This observation is, of course, consistent with the lower B E T C values calculated from the isotherms and reported earlier in this paper. In the case of the water-covered surface, there is evidence for a plateau in curve b in the second half of the monolayer of nitrogen. A similar plateau was found in heat values for nitrogen on two other similar but not identical watercovered bone mineral surfaces. In one set of experiments we measured heats of adsorption for nitrogen at —195° on a bone mineral surface subjected to a long outgassing at room temperature but never heated above that temperature. The resulting surface, while probably having essentially a monolayer of chemisorbed water, would probably contain some carbonate and oxalate in higher percentage than would be found in the bone mineral surface after being outgassed at 4 5 0 ° . The differential heat-coverage curve for nitrogen on this material outgassed at room temperature ran some 200 cal. per mole lower than curve b of Figure 2, but had the same general form. In the adsorption of nitrogen on top of the methanol-covered surface we see from curve c that the heat values run even lower than in curve b. It has been suggested by Dry and Beebe (4) that the chemisorbed monolayer of methanol is probably oriented with the methyl groups away from the surface, thus presenting a paraffin-like surface to the nitrogen adsorbate. It was hoped that the methanolcovered surface might present an energetically homogeneous surface to the nitrogen which might result in virtually constant differential heats for successive increment at low coverage. From curve c of Figure 1, it is apparent that this is not the case, since the heat for the initial increment in particular is about 600 cal. per mole higher than that for successive increments. Moreover in experiments not represented in Figure 2, it was found that the initial heat value for nitrogen on the methanol-covered surface was very sensitive to the method of adding the estimated monolayer of methanol. For instance, if the method of Dry and Beebe was followed, the initial heat value for nitrogen was 3.2 kcal. per mole. Thus we were not successful in preparing a truly paraffin-like surface, but there were always "holes" in the methanol layer through which the electrostatic attraction of the underlying bone mineral could make itself felt. THERMAL

D R I F T OR S L O W H E A T

EVOLUTION.

Evidence for Very Narrow

Pores. For the initial increments of curve a for nitrogen on the bare surface of bone mineral at —195° (V/V < 0.05), the observed time-temperature curves were normal, exhibiting no evidence for any slow heat evolution, and the same was true for increments at V/V > 0.4, as well as for all points represented in curves b and c. However, for increments of curve a in the region V/V = 0.05 to 0.4, heat continued to be liberated for some time after the initial rapid thermal process. For different nitrogen increments within this range of coverage the heat produced in this slow process was from 5 to 12% in excess of that instantaneously evolved. The slow process was observed over a period of 20 to 30 minutes. In one extreme case it was still not complete after 45 minutes, which was about the maximum practicable period for observation. A similar phenomenon was reported by Beebe and Dowden (2) for nitrogen and several other gases on chromic oxide and by Kington and Aston (6) for the m

m

m



HOLMES AND BEEBE

297


nitrogen-titanium dioxide system. In the former publication the observations were discussed in detail and possible mechanisms were suggested. In the experiments of Beebe and Dowden as well as in the present experiments, the slow evolution of heat could not be accounted for on the basis of slow adsorption of nitrogen from the gas phase, since the magnitude of the pressure drop after the first minute following admission of a given increment indicated that only a negligibly small amount of nitrogen was disappearing from the gas phase during the period of slow heat evolution. From our experimental observations we may conclude that the initial increments of curve a of Figure 1 represent adsorption of nitrogen on sites of the bare surface of the bone mineral which are readily accessible and possess a high adsorption potential, resulting in the rapid evolution of high heats of adsorption of the order of 5.0 to 5.5 kcal. per mole. Successive increments of nitrogen may now be instantaneously adsorbed on readily accessible sites of lower adsorption potential, but this nitrogen may then be slowly transferred to less accessible sites of high potential. This transfer may well be due to a slow migration into very narrow pores. After the narrow pores are filled, the thermal drift would disappear and further increments of nitrogen would release the heats of adsorption on the readily accessible but low energy sites corresponding to V/V > 0.4. m

It is especially noteworthy that the thermal drift effect was not observed for nitrogen adsorption on top of a water-covered or methanol-covered surface. From this observation we may conclude that the pores or crevices, present in the bare bone mineral used for curve a are now filled by water or methanol, thus precluding any nitrogen adsorption in these difficultly accessible areas. We may conclude further that these pores are very narrow ones, because they are filled by no more than an approximate monolayer of chemisorbed water or methanol. This would suggest that the pores might be no greater than 8 to 10 A. across or perhaps 2 to 3 molecular diameters. Such pores might be in fact grain boundaries between the submicroscopic crystallites of hydroxy apatite. y

In light of the present evidence it would seem that the decreased values for V caused by the presence of chemisorbed layers of water or methanol, as initially observed by Dry and Beebe and confirmed by the present work, can best be explained on the basis of the blocking, to nitrogen adsorption, of the very narrow, pores by the chemisorbed monolayers rather than by the alternative suggestion of a less condensed packing of the nitrogen molecules adsorbed on top of water or methanol. In a subsequent paper we intend to present evidence, based on gas adsorption studies, to shed further light on the structure of bone mineral. In constructing a plausible model, we shall make use of this evidence for very narrow pores or grain boundaries as well as of the adsorption-desorption hysteresis to evaluate the pore size distribution in the region of coarser pore structure. m

COMPARISON

WITH

HEATS

O F ADSORPTION

DETERMINED

FROM

GAS-SOLID

CHROMATOGRAPHY. Beebe and Emmett (3) have recently reported heat values for nitrogen adsorption on bone mineral as determined from the retention times of nitrogen pulses in a helium stream at several temperatures. These authors report 5.6 and 5.8 kcal. per mole for pulses containing, respectively, 2.3 and 0.02 ml. of nitrogen over a bone mineral column of 380-ml. nitrogen monolayer capacity, prepared in such a way as to correspond to the bare surface used for curve a of Figure 1 of this work. From a comparison with Figure 1 we note a close agreement between the heat values obtained from gas-solid chromatography and Copeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.


298


1.0 '

' 0.5

' 1.0

1

v/v Figure 3.

1

1

'

2.0 m

Differential heats of adsorption for argon on bone mineral at -195° C. Ο

Bare surface

A

Methanol-covered surface

the calorimetric heat values at low coverage. This suggests that the observed chromatographic retention times are related to relatively high energy sites at low coverage. However, quantitative'comparison of the data obtained by the two methods might be pushed too far because of possible differences in the surfaces used in the two sets of experiments. Moreover, the chromatographic work was of necessity carried out in the temperature range 0 ° to —78°, which is far above —195° used in the calorimetric measurements. Some less carefully executed chromatographic work by Beebe and Emmett (3) on an essentially water-covered surface yielded values in the range 2.5 to 3.0 kcal. per mole, in qualitative agreement with the low coverage calorimetric data for nitrogen on a comparable surface. Adsorption Data for Argon on Bone Mineral at — 1 9 5 ° . In previous sections we have emphasized that the polarizability of the adsorbate on the polar bone mineral surface contributes to high heats of adsorption. For comparison we have made calorimetric measurements of the heat of adsorption of argon at —195° on the bare surface of bone mineral and on a methanol-covered surface. The data for differential heats of adsorption of argon at —195° are shown in Figure 3 and isotherms as measured on the pilot sample are recorded in Figure 4. In the case of argon adsorption there is again a marked difference between its heat of adsorption on a bare and methanol-covered surface, though as might be expected the magnitude of the effect is less than in the case of nitrogen. On the bare surface the highest value of heat of adsorption measured by us was 3.2 kcal. per mole and the heat values fall off to slightly over 2 kcal. at the mono layer (V/V = 1). On the methanol-treated surface the highest initial heat value measured was 2.2 kcal. per mole and the heat values dropped rapidly to a value of 1.9 at V/V — 0.3. In both heat-coverage curves of Figure 3 the heat values approach the heat of vaporization (1.8 kcal. per mole) at the higher coverages. m

m


299

Adsorption on Bone Minerai


HOLMES AND BEEBE

o

I

ι

I

I

I

I

I

1

1

1

1

O.I

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

10

RELATIVE PRESSURE

Figure 4.

Isotherms for argon on bone mineral at —195°

C.

Upper curve. Bare surface Ο Adsorption · Desorption Lower curve. Methanol-covered surface A Adsorption Y Desorption The thermal drift, which was observed in the calorimetric work with nitrogen for intermediate coverages on the initially bare bone mineral, was not in evidence at any coverage with argon as the adsorbate. In Figure 4, isotherms as determined on the pilot sample for the adsorption of argon on bone mineral surfaces are shown for comparison with those of nitrogen. The shapes of the isotherms show the same relative behavior as the nitrogen isoCopeland et al.; SOLID SURFACES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

300


therms in Figure 1. A relatively large desorption hysteresis has been found in the case of argon adsorbed on the bare surface. Evidence of desorption hysteresis is also seen on the methanol-covered surface. We did not carry our data to high relative pressures and are thus observing only a portion of a scanning loop here. B E T plots from isotherms on bare and treated surfaces in the calorimeter and in the pilot sample gave very good straight lines in the relative pressure range 0.05 to 0.35. The values of V/V used in plotting heat data in Figure 3 were calculated using the monolayer volumes as obtained from these B E T plots for the argon data on the calorimeter sample. m

Literature Cited


(1) Amberg, C. H., J. Am. Chem. Soc. 79, 3980 (1957). (2) Beebe, R. Α., Dowden, D. Α., Ibid., 60, 2912 (1938). (3) Beebe, R. Α., Emmett, P. H., J. Phys. Chem. 65, 184 (1961). (4) Dry, M. E., Beebe, R. Α., Ibid., 64, 1300 (1960). (5) Holmes, J. M., Beebe, R. Α., Can. J. Chem. 35, 1542 (1957). (6) Kington, G. L., Aston, J. G., J. Am. Chem. Soc. 73, 1929 (1951). (7) Kington, G. L., Macleod, A. C., Trans Faraday Soc. 55, 1799 (1959). (8) Kiselev, Α. V., Kovaleva, Ν. V., Korolev, A. Ya., Shcherbakova, K. D., Compt. rend. Acad. Sci. U.R.S.S. (Doklady Akad. Νauk S.S.S.R.) 124, 617 (1959). (9) McIver, D. S., Tobin, H. H., J. Phys. Chem. 64, 683 (1960). (10) Millard, B., Beebe, R. Α., Cynarski, J., Ibid., 58, 468 (1954). (11) Stone, F. S., Tiley, P. F., Nature 167, 654 (1951). RECEIVED May 29, 1961. Contribution from Department of Chemistry, Amherst College. Research supported by Grant A-2896 from the National Institutes of Health.


Adsorption Studies on Bone Mineral - ACS Publications

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