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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

3 1 1.05-1 1-31 10561 June 30, 1970 10 275

Progress Report

on HEATS OF ADSORPTION OF AMMONIA

AND CARBON DIOXIDE ON TOOTH COMPONENTS

by

William V. Loebenstein

Chemist, Dental Research Section, National Bureau of Standards,

Washington, D. C. 20234

This investigation is part of the dental research program conducted

by the National Bureau of Standards in cooperation with the Council

on Dental Research of the American Dental Association; the National

Institute for Dental Research; the Dental Research Division of the U. S. Army Medical Research and Development Command; the Dental

Sciences Division of the School of Aerospace Medicine, USAF; and the

Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government, t and review. For this reason, the whole or in part, is not authori; Bureau of Standards, Washingtor

the Report has been specifically |

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

5 accounting documents intended objected to additional evaluation

listing of this Report, either in

Office of the Director, National

/ the Government agency for which

pies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
m^rf’
HEATS OF ADSORPTION OP AMMONIA

AND CARBON DIOXIDE ON TOOIH COMPONENTS

William V. Loebensteln

The study demonstrates how heats of adsorption and area of coverage

can be used as criteria for comparing the ultimate strength of chemically

reactive groups for Incorporation in potential dental adhesives. Carbon

dioxide and ammonia were chosen as the reactive gases because they typified

acidic and basic groups, respectively, as possible C(^ndidates for bonding

linkages in an adhesive. The presence of collagen exerted a profound

influence on the BET area depending upon the gas used when compared with

nitrogen. Arrmonia exhibited a higher surface area as well as a higher heat of

adsorption on dentin than did carbon dioxide. It would therefore qualify as

a better adhesive under the conditions of comparison. The surface area of

dentin available to ammonia, however, was no greater than the surface area

of anorganic dentin. It appears, therefore, that the extent of swelling

of the collagen in dentin was restricted by the pore boundaries of the

apatite mineral. The average heat of adsorption of ammonia on anorganic

dentin based on the anorganic whole is estimated to be at least as large

as it is for whole dentin. Thus, it would appear that the presence of

collagen did not contribute to the adhesive strength of ammonia.
f

mi.
/

HEATS OF ADSORPTION OF AMMONIA

AND CARBON DIOXIDE ON TOOTH COMPONENTS

William V. Loebenstein

I-Introduct ion

A study has been made of the heats of adsorption of two

chemically reactive gases on the surfaces of tooth components.

The purpose was to provide a criterion for the selection of

chemical structures to be Incorporated in potential adhesives.

The choice of ammonia and carbon dioxide was made to typify

relatively simple molecules possessing alkaline and acid forming groups, respectively.

[1,2]

Some investigators have resorted to thermal desorption techniques for determining the strength of chemisorbed

bonds. Such Methods would be Impractical in the present application where the integrity of the collagenous component

must be preserved^ It would be destroyed if subjected to the

extreme temperatures that would be indicated by such methods.

The possibility of measuring the adsorption of the reactive

gases at ambient temperatures was considered and had, in fact,

been done for CO^ on hydroxyapatite and NH^ on other solids

at temperatures of zero degrees and higher. The inability to

cleaning the surface without going to temperatures high enough

to destroy the sample was, again, a reason for not choosing

this direction. Furthermore, a relatively small fraction of
the surface would be involved with forces strong enough to

be measured and therefore would not be as representative of

the sites involved in adhesion for a potential dental adhesive

as would be the case where the less active sites, as well,

were included. Accordingly, it was decided to measure the

isotherms in the vicinity of the respective boiling points of

the gases employed. This would have the additional advantage

of permitting a comparison of the specific surface available

to each gas with that for nitrogen under comparable conditions

Finally, sufficient resolution of several adsorption isotherms

over a 20-degree range in temperature could be expected to

permit isosteric heats of adsorption to be readily obtainable.

It would be extremely useful to be able to correlate the

relative strength of adhesive forces with the different compon

ents of tooth structure. The contribution of collagen to the

adsorption studies can best be understood by a comparison of

the adsorptive properties of collagen-containing tooth components with specimens from which the organic matter had been

selectively extracted (l.e., anorganic).

A consideration equally as important as the strength of

an adhesive bond is the concentration or number of such bonds

per unit area of tooth material. Inasmuch as adsorption

measurements afford a means of determining surface area accessible to the particular adsorbate used, they provide a measure

of the maximum number of adhesive links at the molecular level
line ultimate strength of an adhesive should be the resultant of

the bond strength per unit linkage summed over the number of such

bonds per unit cross section.
II-Materials *

Caries-free human teeth were ground in a diamond mortar until the particles passed a 120 mesh sieve. The enamel was not separated from the dentin. The mixture was repeatedly extracted with

ethylened iamine ^ ^ ^ and washed with water until no trace of organic

matter remained. This constituted the anorganic whole tooth samples

used in this work. The dentin samples were prepared as described ^5*1

in an earlier publication. Water-pumped nitrogen, helium, and carbon dioxide were obtained from Air Products Corp. The ammonia was

supplied by J.T. Baker Chemical Co. The nitrogen purification train

consisted of heated copper turnings for removal of oxygen followed

by "Ascarlte" and "Anhydrone". The carbon dioxide was merely dried

over "Anhydrone", as was the helium. Drying of the ammonia was

accomplished by passing the gas through a column of pelletized KOH.

* Certain commercial materials and equipment are identified in this paper to

specify adequately the experimental procedure. In no Instance does such

identification imply recommendation or endorsement by the National Bureau of

Standards or that the material or equipment identified is necessarily the best

available for the purpose.
Ill-Experimental

A simplified surface area apparatus was used for the adsorption measurements, although conventional volumetric gas-adsorp

tion equipment would have sufficed as long as the sample tube was

equipped with an entrance and exit at the t<)p and bottom for scrub

blng with an inert gas. The cryostat used to maintain the sample

at the desired temperature is shown in Fig. 1. It was similar to

one first described by Scott and Mellors^'^ It consisted of a

^-'inch copper bar bent into an Inverted U. Copper plates were

brazed on to one leg to form radial fins, yet sufficiently compact

to allow that leg to be lowered into a dewar flask containing liquid nitrogen. Attached to the other leg was a cylinder fashioned

out of copper plate open top and bottom so it could be raised to

enclose the sample tube and slotted to enhance circulation, i/hen

in place it, in turn, was contained in a dewar flask filled with

a liquid which would not freeze at the temperature of the experiment. Trichlorethylene or toluene were the liquids used for this

purpose. The top of the inverted copper U bar was covered with a thick layer of polystyrene foam for heat Insulation. Heat was

supplied on demand through a hand-wound glass-inclosed immersion

heater at about 2f^ watts. The temperature sensing probes were

thermistors attached as plug-in elements for ease of substitution. Fenwal thermistors KB22J1 and KB23J1 were used for the

ammonia adsorption experiments while JD12J4 and JD21J1 were used

for carbon dioxide. (The high sensitivity of the th’Hemistors restricted the temperature range of each.)
The thermoregulator shown in Fig. 2 was constructed of

parts readily obtainable. The secondary windings of the transformer, T^, besides providing the source of e.m.f., comprised

the the two inductive arms of the AC bridge. Stability of the

amplifier was accomplished by negative current feedback brought

about by the unbypassed cathode resistors. Decoupling was obtained by the cathode -follower stage. The thermo-regulatorampllfler was so sensitive, that even a 5000 ohm carbon resistor could be used In place of the thermistor In an emergency

(with somewhat more sluggish respons^ by using the highest setting of potentiometer, P^. However, It Is Important that resistor and potentiometers and P^ (by composition or compensation) have virtually no temperature coefficient of resistance In order for the bridge to Ignore changes In room temperature. The apparatus, using thermistors, held the temperature

within ±1/4 degree eUen though a glass well was used to Isolate

the thermistor from direct contact with the liquid coolant.

Agitation of the liquid was provided by bubbling dry air

into the bottom of the dewar. Temperatures were measured by

a chromel-alumel thermocouple connected to a Rubicon Precision

potentiometer (Cat. no. 274^;) and were monitored by a Honeywell chart recorder.

Correct Ions ^ were made for the non-ideal behavior of

the adsorbates at their respective Isotherm temperatures and

equlllbrum pressures.
IV-Pre treatment

In any adsorption work, seemingly insignificant variations

in the preparation of the surface can have a devastating effect

upon the reproducibility of the measurement. Accordingly, it

was necessary to establish a procedure for pretreatment of the

sample. Beebe ^ has pointed out in the adsorption of methanol

on hydroxyapatite that the adsorption on a freshly outgassed

surface was always greater than subsequent adsorptions in which

the sample was pumped free of the adsorbing gas at room temperature prior to repeating the run. The former represented chemisorption as well as physisorption, while the latter only physisorptlon. This observation was equally valid for the gases

used in the present investigation.

For the anorganic whole teeth, the sample was purged of

impurities by heating overnight in a stream of purified helium

at 110 C. The heating was accomplished by a thermostatically

controlled sleeve furnace. Upon cooling to room temperature,

the helium was replaced with the gas to be adsorbed. This was

then removed by evacuating the sample for If. minutes while still

at room temperature. The sample was then cooled to the temperature of the experiment before the adsorbing gas ^ introduced

.

When dentin was used as the adsorbent, two separate sets

of experiments were performed. In one set the purging in helium

w'as accomplished at 50 C because o:p4lie possibility that higher

temperatures might cause denaturation of the collagen.
The remaining steps in the pretreatment were the same as those

described for the anorganic whole teeth. In the second set of experiments, the temperature of purging was brought back up to 110 C. It was

reasoned that the desirability of having the same maximum temperature

as the anorganic whole teeth was of compelling Importance in case

denaturation of the collagen was either absent or minimal.

It is significant that following the second set of experiments , it

was possible to reproduce the improved low temperature adsorptive

characteristics which followed the 50 C pretreatment providing the sample

was first equilibrated with room air (50 percent relative humidity)

then again pretreated at 50 C. While this does not preclude denaturation,

it indicates that the adsorptive characteristics were reversible.

The order of experiments consisted of the first and last isotherms

carried out at the same temperature: -33

0

(the boiling point) for

NH^ and -78,4 C (the sublimation point) for CO

2

to insure that no

poisoning of the surface had taken place during the intervening runs

.

The other Isotherms were displaced by five and ten degrees on each side

of these fixed points. Except for the initial and final runs, the order

in which the experiments were performed was randomized.
1

V-Results and Discussion

In all sets of experiments, families of adsorption isotherms were obtained corresponding to the five different temperatures as typified by the curves for NH^ on dentin pretreated

at 50 C as shown in Fig. 3. They were type II (sigmoid) in

each case although in the higher temperature isotherms the

inflection points were usually beyond atmospheric pressure -

the upper limit attainable with the apparatus.

Surface areas were calculated using the experimental

points between 0.1 and 0.3 relative pressure of the corresponding

boiling point and sublimation point Isotherms in the usual BET

linear transformation:

x/[v(l-x)] = [{/C-l)/{v^C)]x + l/(v^C) - - - - (1)

where X = relative pressure; v = volume of gas adsorbed

(ml/g); while v^^ and are the characteristic parameters representing,

respectively, the gas volume needed to cover a monolayer and a function of the

heat of adsorption which will be discussed later.

The resultant surface areas are shown in Table 1 using

the molecular cross sections as indicated for each gas.

Values of surface areas available to adsorbed at liquid

nitrogen temperatures are shown in the bottom row.

The values for anorganic whole teeth are in substantial

agreement for each of the three adsorbents. This becomes more

apparent when it is realized that C 02 ^ because of its larger

size, is denied access to pores readily accessible to NH^ and
even to N,

It has lonf: been known that the surface of dentin available to

is of the order of 10 m^/gl this is consistent with the value of

7.5 and 6.5 obtained in this work (depending upon the conditions of

pretreatment ) . The surprising result is that the same sample when measured

by NH^ disclosed an area of I60 m /g, far greater than could be accounted

for based on differences in molecular size. Indeed, it agrees well with

2

the value that gave (149 m /g) when the collagen had been selectively

removed from the dentin.

In attempting to reconcile these observations with an hypothesis of

what is actually happening, one might postulate that collagen inhibits

access of nitrogen to the smaller pores of the dentin. This would

2 satisfactorily explain the low (7.5 ni /g) value for on dentin and the

2

large value (149 m /g) for N2 on anorganic dentin. It would also explain

the essential agreement among all these adsorbates on anorganic whole

teeth. The present paper does not Include adsorption on collagen alone

[ 12] although Marshall and Moore found appreciable adsorption of NH^ on

another protein, silk fibroin, accompanied by swelling. If NH^ caused

3

the collagen to expand, this might unblock the pores allowing the NH

2

free access to the entire surface of the mineral component (I60 m /g)

.

Thi^

would also be consistent with experiments now taking place (but as yet

unpublished) on the adsorption of water vapor on dentin where again the

collagen would swell to allow the water to reach the mineral pores for

2 an area of about I50 m /g.

The abo'Ve picture does not account for the intermediate behavior of

CO2 on dentin. If the collagen does not allow access of CO^ to any but the

larger pores, one would expect a smaller area than for because
//

CO^ has a larger cross section and would be denied some access to

pores available to N^, yet the area available to it was much greater,

(50 m /g). The role played by CO^ is, however, complicated according to

Spencer, et al^ These authors point out that although the CO^

molecules have no permanent dipole, they are highly polarizable and, in

addition, have a permanent electric quadrapole which is quite strong.

Tliese factors are bount/ to exert a significant Influence on the adsorption

characteristics such as that observed here.

No appreciable adsorption of on collagen takes place at liquid

4 151

temperatures, ’ although adsorption by water at room temperature is

equivalent to about ^00 m^/g^^^ again accompanied by swelling.

It would appear that the actual adsorption of NH^ and H^O on

dentin (which by weight is about 1/3 collagen) is less than would be

anticipated in the light of these considerations unless the mineral

component restricts the degree of swelling of the collagen to such

an extent that full advantage of its adsorptive capacity cannot be

realized.

n? 1 It has recently been reported by Hagymassy, Brunauer, and Mlkliall

from other considerations that perhaps H^O rather than N^ should be used as

the standard adsorbate for
surface area and pore size deterimiat Ion, especially in biological systems.

The difference between the surface areas available to

NH for the 50 C and 110 C pretreatment temperatures for den3 three percent

tin Is probably attributable to the/d if ference in the amounts

of residual water present in each case.

Insofar as the present work is concerned, the NH^ molecule

(at l 60 m^/g for dentin) indicates far more points of attachment than does CO^.

From each of the fam1 ly of curves such as that shown In

Fig. 3 it was possible to apply the Clausius -Clapeyron relationship :

> (Inwi p)

•v

( 2 )

which defines the Isosterlc Heat of Adsorption, in terms

of the pressure, absolute temperature and gas constant while

the amount adsorbed, v, is held constant. This was accomplished

by plotting In p against l/T for various ordinates of the

families of isotherms such as that shown in Fig. 3 according

to the integrated linear form of Eq. (2):

In p = - (Qgrp/R )

( l/T ) + const. ( 3 )

ST

which is valid providing the change in Q, over the temperature
Interval used is negligible. This assumption was reinforced

by good linearit.v obtained from the plots of Eq . ( 3 ).

Fig. 4 compares the resultant isosteric heat* as a function of the amount of gas adsorbed for each of the systems

studied. The value of v on each abscissa was calculated

m

from Eq. (l) and represents the point at which a monolayer

was completed. Values corresponding to the smallest amounts

of

adsorbed could not be calculated becauseNthe poor resolution

of the families of isotherms in this region. Typical curves

are represented by both NH^ and CO^ adsorption on anorganic

whole teeth and by NH^ on dentin pretreated at 110 C. In

these systems the isosteric heat fell off rapidly until the

monolayer was formed. Beyond here it became almost flat.

Somewhat less common but often observed the investigations of others is the shape of the curves for both NH^ and

CO^ on dentin pretreated at 30 C. Definite maxima were found

in both instances. It occured in each case beyond the amount

adsorbed that was required to fill a monolayer. It is extremely likely that at coverages below the resolution of these

isotherms^ the heats of adsorption were much higher. Since the

temperature of pretreatment is the only condition which was

different, the existence of the maximum (at least in these

experiments) appears to be related to the residual gases which

were less completely removed at the lower pretreatment temperadsorbed

ature and this would suggest /Water. Despite their difference

in shape, the isosteric heats of adsorption at higher coverages
/

for NH^ on both preparations of dentin was reasonably constant

at about 6 kcal/mol.

On comparing the Isosteric heats of NH^ and CO2 for

corresponding surfaces at the completion of a monolayer^ it

Is seen that the values for NH^ are greater as would be expected based upon their known heats of liqulfaction and sublimation, respectively.

The heat of adsorption over the first layer is, of course,

dependent upon the distribution function. The present method

can be employed to obtain these heats at successively smaller

fractions of the surface by going to higher temperatures. It

has been shown, for example, I- that the isosteric heat of adsorption of CO^ on synthetic hydroxyapatite when measured at

zero degrees, dropped from 13. ^ to about 9 kcal/mol when measured up to 1/4 of a monolayer.

The ultimate strength of an adhesive might be considered

as proportional to the Integral, U, where

A

0

( 4 )

and A is the surface area. However, the average isosteric

heat, ^3^. over the entire surface would be defined as

(5-32, = ( 1 /A) St

0
so by combining equations (4) and (5)j It is seen that

U = A QST ( 6 )

Unfortunately, the present results do not extend to low

enough fractions of the surface to evaluate Q^rp with any

degree of precision. However, an Independent estimate is

available from the c parameter evaluated from the BET equation. Brunauer defined as " - - the average heat

of adsorption on the less active part of the adsorbing surface." in terms of the heat of liquefaction E according to

1j Eq. (7):

E^ = Ej^ - RT In c (7)

Values of E^ computed in this manner are also shown in

Table 1. Assuming, therefore, that E^'^Qgrp and using

the surface areas of Table 1, it can be estimated from Eq . (6)

that NH^ (U=1136) at -33 C would be about five times as strong

an adhesive for dentin pretreated at 50 C as would CO^ (U=215) at

-78 C.

It is appreciated that this study is of a preliminary

nature and is intended merely to point the direction. If

these results are substantiated by additional experiments, (see Sec.

VII) , it would show that the presence of collagen was not especially
Estimates

/4

beneficial either to NH^ as an adhesive or to CO^. Estimates

can be made of U-values from Eq. (6) of 523 and 230 kcal’mol

2 -1

m *g for NH^ and CO^, respectively, adsorbed on anorganic whole

teeth. Wliole teeth, however, include enamel which has a much lower surface area than dentin. If the values of U obtained for anorganic whole

teeth were extrapolated to anorganic dentin, assuming the same heats

of adsorption corresponding adhesive strengths would probably be Increased by

a factor sufficient to nullify any apparent advantage of dentin over

anorganic whole teeth. Thus, the surface area expected for anorganic

dentin when measui^ed by would be 1^9 x 67/64 = I56 m *g and

U = 156 X 7.8 = 1217 as compared with a U-value of II36 for dentin.

itself.
/

VICone 1 US ions

Subject to the limitations of the method, gas adsorption

can be used as a tool in the search for better dental adhesives

It provides quantitative information relating to the strength

per unit adhesive linkage as well as the maximum concentration

of such linkages. Ammonia appeared to score better than carbon dioxide on both of these counts under the conditions selected for these experiments. The presence of collagen, while

not complet.ely understood, seemed to contribute no profound

influence upon the theoret'eal ultimate strength of the bonding

Among the limitations of the method are:

(1) The possibility that the relat've adsorptive properties of the gases would be seriously modified when chemicall','

combined as reactive appendages in a practical adhesive.

(2) The fact that there may be no gas representative

of some chemical groups whose adhesive properties may be of

interest

VII-Suggest ions for Future Work

Reinforcement of some of the points brought out in this

paper could be attained by:

(1) The adsorption of methylamine on dentin.

(2) The adsorption of ammonia on collagen,

( 3 ) Solvent exchange of swollen collagen and dentin

to attempt to show lerger surface areas accessible to nitrogen

USCOMM-NBS-DC
11[
Table 1

BET Surface Area Accessible to Each Adsorbate

Anorgan i

Whole Teeth

(rtl2/g)

Dent i

(^0 C Prep)(llO C Prep)

(m2/g)

Dentin

(m^/g)

Anorgan i. c Dentin

(m2/g)

NH^(1^.6 a )

002(21.0 A^)

(16.2

67

(E;L=7.8)

48

(E^=4.8)

64

160

(E,=7.l)

60

(E^=4.3)

138

(E.=6.9)

7 . 6 . 149

E

1

values (kcal/mol) were determined from the BET c -parameters
Caption for Figures

Figure 1: Cryostat Used in Gas Adsorption Measurements

Figure 2: Schematic Diagram of Thermistor-Sensed ThermoRegulator ^

2

' lOj.000 ohms; R^^ Rj^, Rp 0-12 megohms; Rg, Rr^: l^^OO ohms; Rg, Rg , 1^200 ohms; R^q, R]_g^

R^r : 0.47 megohms; 220 ohms; ^ 16 ' (W.W. ) l,f00 ohms;

P^: 0.^ megohms; ?

2

: (W.W.

)

7^000 ohms; Pg : (W.W.) ^0 ohms;

C 2

: (Electrolytic) 8 microfarads; Cg, C^^: (Electrolytic)

4 microfarads; Cp, Cg, Cj

,

Cg: 0.1 microfarads; Cg : (Paper)

4 microfarads; C]_q: 0.05 microfarads; : 6h6; V2 , Vg

6sL7; : 6SN7; T^: 6.3 volts (C.T.), 3 amperes. Filament

Transformer; Ry^ ^ 5000 ohm Plate Circuit Relay; Thermistor:

(see text)

Figure 3: Typical Family of Adsorption Isotherms at the

Boiling Point and at Temperatures Displaced Five and Ten Degrees Above and Below it.

Figure 4: Isosteric Heats as Functions of the Amounts of Gas

Adsorbed

.

ro a.

CVI Q.

(T
EQUILI

BRIU O

CO

fi/|iu 03aU0SQV INOOWV

EQUILIBRIUM

PRESSURE

mm

otHg

Bibliography

[ 1] A. C. Zettlemoyer and J. J. Chessick Jour of Phys Chem ^ 1131 (I960)

[2] M. S. Goldstein EXPERIMENTAL MEITODS IN CATALYTIC RESEARCH - Chapter 9, Academic Press, N.Y. and London (1968)

[3] V. R. Deltz, F. G. Carpenter and R. G. Arnold CARBON 2^5 (1964)

[4] S. C. Peckman, F. L. Losee and I. Ettleman Jour Dent Res ^5 194? (1956)

[5] E. F. Huget, G. M. Brauer and W. V. Loebensteln Ibid 291 (1968)

[ 6] W. V. Loebensteln and V. R. Deltz Jour of Res. NBS ;46 51 (1951)

[ 7] R. B. Scott and J. W. Mellors Ibid ^ 243 (1945)

[ 8] W. V. Loebensteln "Calculations and Comparisons of Non-Ideal Gas

Corrections for Use in Gas Adsorption" (to be published)

[9] R. A. Beebe Proc of the First Workshop on Adhesive Restorative

Dental Materials, NIDR - HEW 75 (1961)

[10] A. L. McLellan and H. F. Harnsberger Jour of Colloid and Interface

Scl 23 577 (1967)

[11] G. M. Brauer. Adhesive Restorative Dental Materials-II . Public Health Service

[12] P. A. Marshall and W. J. Moore Jour Am Chem Soc 7^ 4779 (1952)1 Pub. No. 1494,

I p. 203 (1965) . [13] W. B. Spencer, C. H. Amberg and R. A. Beebe Jour Phys Chem 6^ 719 (1958)

[14] A. C. Zettlemoyer, A. Chand and E. Gamble Jour Am Chem Soc 7^ 2752 (1-950)

[15] R. G. Mltton and R. J. Mawhimney Jour Soc Leather Trades’ Chemists

39 172 (1955)

[16] J. R. Kanagy Jour of the Am Leather Chemists Assn ^ 12 (1950)

[17] J. Hagymassy, Jr., S. Brunauer and R. SH. Mikhail Jour of Colloid

and Interface Scl 29_ 485 (1969)

[18] H. Cochrane, P. L. Walker, Jr., W, S. Dlethorn and H. C. Friedman

Ibid ^ 405 (1967)

[19] S. Brunauer THE ADSORPTION OP GASES AM) VAPORS p-158 Princeton Unlv.

Press, Princeton, N. J. (1943)