NATIONAL BUREAU OF STANDARDS REPORT

10 609

Progress Report

on ADSORPTION OF WATER ON TOOTH

COMPONENTS AND RELATED MATERIALS

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-11-31 10189 September 14, 1971 10 609

Progress Report

on ADSORPTION OF WATER ON TOOTH

COMPONENTS AND RELATED MATERIALS

By

W. V. Loebenstein

Dental Research Section

National Bureau of Standards

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. E and review. For this reason, the whole or in part, is not authoriz Bureau of Standards, Washington

the Report has been specifically p

! accounting documents intended Approved for public release by the ubjected to additional evaluation Director of the National Institute of "f"g of this "eport' ei,her in c j j

Office of the Director, National standards and Technology (NIST) the Government agency for which on October 9, 2015. pies for its own use - <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

ADSORPTION OF WATER ON TOOTH COMPONENTS

AND RELATED MATERIALS

William V. Loebenstein

National Bureau of Standards

Washington, D.C. 2023^-

INTRODUCTION

The adsorption of water vapor especially in the study

of biological systems has attracted considerable attention

in recent years. Hagmassy, Brunauer, and Mikhail have

submitted that nitrogen is of only limited value in characterizing the surface properties of hardened Portland cement pastes,

silica gels, clay minerals, soils, etc., as compared with water

vapor adsorption. They stated, M Not only are the smallest

pores left out of consideratiortnby nitrogen adsorption, but there

is definite evidence that nitrogen cannot penetrate into many

large pores " This is consistent with some of the observations regarding the surface characteristics of dentin: The

surface area, measured by the conventional nitrogen adsorption,

gave rise to values of about ten square meters per gram. However, upon extraction of the collagen, the anorganic dentin exhibited an area available to nitrogen of more than a ten-fold

increase. J What is more significant, however, is that untreated dentin when measured by a polar molecule such as ammonia

o

exhibited a surface area of 138 m /g which was more in agreement

with the nitrogen value w for anorganic dentin.
If water vapor adsorption could be performed reliably, exped't'ously, and with a sufficiently simple experimental apparatus, its use would be commonplace. This paper utilizes an extremely simple apparatus to study the adsorptive characteristics

of water vapor by tooth components and related dental materials.

It shows how surface areas obtained ft y this means compare with

the results obtained using other adsorbates. Furthermore, it delineates the complete adsorption-desorption hysteresis loop for

each material studied. In certain selected instances scanning^*^

curves are also represented.
EXPERIMENTAL * Both volumetric and gravimetric methods have been used in the

past in adsorption studies. Gravimetric is preferred in water

vapor adsorption because water is a liquid at room temperatures

where the measurement of volume of vapor becomes unwieldy in any

appreciable quantity because of its low vapor pressure. Gravimetric

adsorption has the further advantage of being a direct measure of the

amount adsorbed and not subject to cumulative errors which are ever

present in the volumetric procedure where the incremental additional

amount adsorbed at each point is added to that already there.

The course of the adsorption process can be followed continuously if the balance is an integral part of the adsorption apparatus

and several different balance schemes have been used successfully

in this connection. Commercial equipment is even available for

those who prefer it. It is not the purpose of this paper to discuss the pro’s and con’s of the many methods which have been used

over the years with various degrees of success. Nevertheless, their

advantages and disadvantages were considered very carefully before

deciding on the circulating carrier gas method used in this work.

The schematic outline shown in Table 1 (without making any

claims as to its completeness) serves to illustrate the various

methods reviewed . * 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.

3
The present apparatus (Fig, 1.) uses air as the carrier gas.

In its circulatory path, it passes from the pump through the

saturator^ containing one of more than a dozen different saturated salt solutions ^ 9 ^9 ^3 ^ ^ which fixes the relative humidity.

From here the gas-vapor mixture passes upward through the sample

tube where equilibration takes place; then back to the pump to

complete the cycle. A "Sigma Motor” pump with "tygon" tubing

assures that the circulating vapor contacts no metal, valves, or

foreign lubricants and its total volume is kept desirably small.

Multiple experiments are simultaneously performed where not only

the samples are independent, but the humidities are different, as

well. The equipment, moreover, possesses a "fail-safe" attribute

in the event of a power outage or pump malfunction. Once a day

the sample tubes are capped at both ends with their respective

ground glass joints and removed for weighing. Several days may

be required in some instances for a single point, depending upon

the nature of the sample and the closeness to saturation or dehydration on the humidity scale. An analytical balance or micro

balance can be used depending upon the size of the sample.

Both the "Gram-Atic" and "Micro Gram-Atic" balances (Mettler

Instrument Co.) were used in this work.

Where the changes in weight observed from one relative humidity

to the next were especially small, it would be necessary to make

a correction for the difference in density between dry air and

humid air which occupied the dead space of the sample tube. This

correction £ which was added to the apparent weight at each relative humidity may be determined from the relationship:
£ = (0. 014 ) (R.H. ) (V ) milligrams.

Here, is the dead space volume (ml) of the sample tube and

R.H. is the relative humidity expressed as a decimal.

The cross-sectional area of the water molecule used in this

work was 12.5 square angstroms as given by McClellan and Harnsberger^^. The BET "free surface" isotherm equation was used for

determining surface area within the applicable range of humidities.

Only the adsorption boundary curve of the hysteresis loop was used

Bo1 for these calculations . Zero humidity was accomplished by substituting for the saturator a vertical cylindrical tube containing

P 0 r supported on glass wool. The lower end of the tube was fitted

2 5

with a trap to catch the liquid phosphoric acid which resulted from

the uptake of water. At least a week was required before a sample

could be considered dry as evident by no additional loss of weight.

The order in which the points were obtained for an entire adsorption-desorption isotherm hysteresis loop generally consisted of

desorbing from a 100-percent humidity environment in a stepwise

manner until zero humidity was attained. Then adsorption was performed incrementally using each saturator, in turn, to obtain successively higher humidities until 100-percent humidity was again

approached. If only the surface area were required, it would have

been sufficient to take the sample to dryness initially before

obtaining the adsorption points and then adsorbing only to the uppe

limit of the BET range of relative humidity. If more detail was

desired such as the development of a scanning curve, this would be

obtained on the second time around. The hysteresis loop would be

retraced until at some point along the way the direction would be

changed (either from desorption to adsorption or vice versa) and

the new direction pursued in a stepwise fashion as before.
RESULTS

Figures 2, 3* 5, 6, and 7 show the complete adsorptiondesorption hysteresis loops for collagen, dentin, enamel, anorganic whole teeth, _ polymethyl methacrylate (denture base material), and investment coinpound, respectively. The adsorption isotherm which is the lower curve in each case is the basis for determination of surface area. It is over a pdtftion of this curve that

PK?

the familiar free-surface BET equation is applicable. The weight

of water adsorbed at relative humidity __x is, of course, obtained

by subtracting from the weight of the sample, after obtaining steady

state (and correcting for dead space if need be), the weight of the

dehydrated sample. This is then divided by 18 times the dehydrated

sample weight to convert it to JM, the number of moles of water

adsorbed per gram of dry sample. The isotherms (Fig T s 2 through 7)

were obtained by plotting against x. The BET equation:

x/[N( 1-x) ] = [(C-l)/(Nm

C)]-x + l/(N

m

C) . . . (1)

limited to the range 0.08 to 0.33 in x was used to obtain the

surface area available to the water molecule. Surface area is

proportional to the parameter N

m

which, in turn, is evaluated

from the slope and intercept of the straight line which results

from plotting the quantity x/[N(l-x)j against x. The reciprocal

of the sum of the slope plus intercept gives directly. The

factor 7.529 x 10^ when multiplied by N (moles/g) results in

surface area (m2

/g) when the adsorbate is water vapor at room

temperature as it is in this work.
The surface areas obtained by following these steps are shown

in the top row of Table 2 compared with results for other adsorbates

on the same materials. It is significant that large increases were

found for surface areas available to the water molecule as compared

with the nitrogen molecule for dentin as was found for collagen.

Even enamel showed a ratio of about two to one in this regard. In

each case where the discrepancy is large it can be attributed to

or protein-type material

the presence of collagen^as one of the components. In contrast,

anorganic whole teeth showed little change from one adsorbate to

another

n
DISCUSSION

When a greater amount of adsorbate is observed during desorption at the same relative pressure as was found during adsorption at constant temperature, the phenomenon is termed "adsorption-desorption hysteresis". It is observed with other adsorbates besides water, although water hysteresis is frequently

quite pronounced. Even with water in some instances hysteresis

may be absent. Its causes are only partially understood. A different mechanism may very well be controlling at low relative

pressures than at intermediate or high values In the latter

case, capillary condensation within the pore structure is the

te-f

reason most frequently cited by investigators 1

. The miriad of

shapes and sizes of pore openings could easily account for many

types of hysteresis loops. Everett classified four types of

hysteresis independent of the shape of the isotherm. Type A

shows the loop confined to the mid-portion of the relative pressure range. In type the loop extends all the way to saturation,

but is absent in the lower range of pressures. Type C has the

loop extending over the entire range of pressures; while type D

appears to be intermediate between types B and C.

Relating to the present investigation, it appears that denture

base material. Fig. ($), might represent type A or B. An interesting aspect of this adsorbent is that the adsorption isotherm, itself, has no inflection point and is always convex to the pressure

axis. This is characteristic of type J3_ isotherms (Brunauer's classification) in which the heat of adsorption is less than the

heat of liqufaction of water. In other words, the methyl methacry-
late polymer would appear to be hydrophobic.

Enamel, Fig. (?) , would probably qualify as a type B hysteresis. The same would seem to be true for anorganic whole teeth.

Fig {S') 3 although the choice between types B and _D is probably a matter of degree.

ft's (7^ The Durallium investment compound^is an interesting case that

deserves special comment. The initial desorption from saturation

produced a curve which fell below the subsequent adsorption curve

originating from zero humidity! A second desorption, however, using

the same sample resulted in a higher curve as it should have been.

The initial abnormal behavior indicated some irreversible change

taking place in the adsorbent. Its history 1 J disclosed that it

possessed a vitreous (or glass-like) phase as a result of its hightemperature preparation. It was therefore quite possible that

over the period of weeks required for a complete desorption-adsorption cycle, the condensed phase of adsorbed water had actually dissolved some of the vitreous material. Upon continuing the desorption, the water vapor which was removed left behind salts as a precipitate, together with irregularities in the surface of the

glass-like phase. Both of these processes would have resulted In

increasing the surface area of the sample which would manifest

itself by disclosing a higher adsorption curve. This phenomenon

of more than doubling the surface area of newly formed glass as

a result of water washing has been known for many years

Once reacted with water the sample, of course, became stabilized.

The hysteresis loops for rat-tail collagen and for dentin.

Figures (z) and (3 ) , are excellent examples of type C. It is an

1
interesting observation that swelling is also known to occur with

these two systems during water adsorption. Other systems which

have been reported where swelling occurs such as ammonia on silk

fibroin^

3

^; water on casein^^; water on gelatine

.

water on

Pi / collagen 1*^1 ; and polar gases on certain clays (to mention a few) all share in common the fact that they exhibit type __C hysteresis. It is significant that when the collagen is removed from a tooth, the type of adsorption hystereses obtained with water vapor

changes from type C to B. Correspondingly, appreciable dimensional

changes as a function of relative humidity are also drastically

[37l reduced virtually to zero as found b y fDi

c

ksemi a nd Barton, . Other

materials which have been found to exhibit dimensional changes with

fjg

hum‘dity L * 9 have demonstrated the existence of dimensional hysteresis, as well. There is every reason to believe that dentin would

behave in a similar manner. If so, the method of drying a tooth

prior to the application of an adhesive or direct filling resin,

etc., would merit critical reexamination.

This discussion of adsorption hysteresis has so far been confined to the boundary curves which comprised the outer envelope of

the isotherm plot. It is possible by means of an infinite number

of different paths to reach any point within the loop. Such paths

[fl

are commonly referred to as scanning curves"’". They are obtained

by changing direction (adsorption to desorption or vice-versa)

before reaching either end of the isotherm. Qne such scanning

curve is shown for collagen. Pig. (Z) 3 where the process was changed

from desorption to adsorption before the sample was completely dried.

It occurred at a relative humidity of 0.04. The scanning curve

/ O
resulting from continuing the adsorption beyond this point remained

with n the loop distinct from either boundary curve until they converged at (or near) saturation. This is a characteristic behavior

®0j

shared by all primary scanning curves.

Where the hysteresis loop extends throughout the entire range

of the adsorption isotherm and is sufficiently pronounced as is the

case with dentin, an excellent opportunity is afforded to explore

the behavior of scanning curves in detail. This aspect of the investigation has evolved to such an extent that it deserves to be

treated as an entity in its own right.

One important aspect of the hysteresis studies, however, was

the fact that the hysteresis loops could be retraced on subsequent

cycles. There were no indications of degeneration as have been

reported by others Good reproducibility was also observed

from sample to sample for the same material.
CONCLUSIONS

An apparatus, simple to construct and based on a recirculating

system, has been used with multiple samples for obtaining water

vapor adsorption isotherms gravimetrically . While having ruggedness

as well as fail-safe properties, it possesses accuracy and reproducibility . The complete adsorption-desorption isotherms were obtained

for collagen, dentin, anorganic whole teeth, polymethyl methacrylate,

and a commercial investment compound.

Surface areas available to the water molecule were determined

from BET plots based on data obtained from the appropriate portion

of the respective adsorption boundary curve. Complete drying with

was a prerequisite for reliable surface area evaluation. Collagen and collagen-containing materials showed the greatest increases

in surface area available to the water molecule compared with nitrogen. These areas (measured by water vapor adsorption) were in

reasonably good agreement with previously obtained values on the

same samples using NH^ as the adsorbate.

Well defined and rep^g^ucible hysteresis loops were found and

characterized according to type. The presence of collagen In appreciable quantities in teeth appeared to be responsible for the persistence of the hysteresis loop down to the lowest humidities. No

degeneration of the hysteresis loops was found for those cases

(collagen and dentin) where the experiment was repeated. The anomalous initial hysteresis behavior of investment compound could

satisfactorily be explained based on an interaction with adsorbed

water. Primary scanning curves were obtained in selected instances

and (after in', tiation) remained distinct from either boundary curve.
REFERENCES

[1]

Hagmassy, /Jr.|, J.j; Brunauer, S.; and Mikhail, R.Sh.: Pore

Structure Analysis by Water Vapor Adsorption - Jour of Colloi d

and Interface Sci—

29: 485-491 ( 1969 ) — /yyVA

[2] Brauer, G.M. : Measurement of the Surface Area and the Heats

of Wetting of Dentin Powders - ADHESIVE RESTORATIVE DENTAL MATERIALS - II Public Health Service (hb no. l4$4), 2o3-21^'Tl96FT

"

[3] Loebenstein, W.V. : Surface Studies of Natural and Synthetic

Bone Mineral and Teeth - Ibid

,

213-223 (1965)

[4]

Loebenstein, W.V.Heats of Adsorption of Ammonia and Carbon

Dioxide on Tooth Components - (Paper presented at the 1969 IADR

meeting - to be published)

[3] Rao, K.S.: Hysteresis in Sorption II - Jour Phys Chem

4f: 506-512 (1941)

[6]

Razouk, R.I., and Salem, A.S.: The Adsorption of Water

Vapor on Glass Surfaces - Jour Phys and Colloid Chem c 2: 1208-1227

( 1948 )

’ “ ^

[7] Kanagy, J.R.

Adsorption of Water Vapor by Untanned Hide and

Various Leathers at 100°F - Jour of Research-NBS 38 : 119-128 (1947)

aN)*

[8] McBain, J.W., and Bakr, A.M.: A New Sorption Balance - Jour Amer .Chem Soc 48: 69 O-695 ( 1926 ) /w»v»

[9] Berlin, E.; Anderson, B.A.; and Pallansch, M.J.: Water Vapor

Sorption Properties of Various Dried Milks and Wheys - Jour of

Dairy Sci 51: 1339-1344 ( 1968 ) /WW

[10] Wiig, E.O., and Juhola . A . J.

The Adsorption of Water Vapor

on Activated Charcoal - Jour Amer Chem Soc 71: 561-568 (1949)

/vy#

[11]

Kantro, D.L.; Brunauer, S.; and Weise, C.H.: Development of

Surface in the Hydration of Calcium Silicates - ADVANCES I N CHEMISTRY

SERIES 33: 199-219 (1961)

'

/vyA

[12]

Carpenter, F.G.: Water Vapor Adsorption on Chars of Different

BET Areas - Unpublished NBS Report #4918 , 22-34 (1956)
[13] Tomes, L.A.; Hunt, C.M.; and Blaine, R.L.: Some Factors

Affecting the Surface Area of Hydrated Portland Cement as Determined by Water-Vapor and Nitrogen Adsorption - Jour of Rese a rchNBS 59; 357-364 (1957)

/VV\A'

[14] Bichowsky, F.R.v., and Storch, H. : An Improved Form of

Gas-Washing Bottle - Jour Amer Chem Soc 37: 2695-2696 (1915)

[15] O’Brien, F.E.M.: The Control of Humidity by Saturated Salt

Solutions - Rev of Sci Instr 25: 73-76 (1948) " 1

' "

1 a /w*

[16] Carr, D.S., and Harris, B.L.: Solutions for Maintaining

Constant Relative Humidity - Ind and ni Eng mi m iiwiChem 4l: 20l4-201t (1949) .\AA- [17] Wexler, A., and Hasegawa, S.: Relative Humidity - Temperature Relationships of some Saturated Salt Solutions in the Temperature Range 0° to 50°C - Jour of Research-NBS 53: 19-26 ( 1974

)

~ 111 1,1 1 TT " rt/W

[181 Hedlin, C.P., and Trof imenkoff , F.N.: Relative Humidities

over Saturated Solutions of Nine Salts in the Temperature Range

0 to 90 °F - HUMIDITY AND MOISTURE , Edited by Wexler, A., 3: ^19-^20

Re inhold PubT florp.; N.Y."Ti9W"' -

[19] McClellan, A. I., and Harnsberger, H.F.: Cross-Sectional

Areas of Molecules Adsorbed on Solid Surfaces - Jour of Colloid

and Interface Scl 23: 577-599 ( 1967 )

" /vw

[20] Lippens, B.C.; Linsen, B.G.; and DeBoer, J.H. : Studies on

Pore Systems in Catalysts I. - The Adsorption of Nitrogen; Apparatus

and Calculations - ~Jour of Catalysis 3: 32-37 (1964) 1 v 'WK‘ /W*

[21]

Brunauer, S.; Emmett, P.H.; and Teller, E.: Adsorption of

Gases in Multimolecular Layers - Jour Amer Chem So c 60: 309-319

(1938)

[22] Calculated from the data of: Bull, H.B.: Adsorption of

Water Vapor by Proteins - Jour Amer Chem Soc 66: l499-l c 07 (1944) ' —

/WVN,

[23] Recalculated from the data of: Kanagy, J.R.: Influence of

the Temperature on the Adsorption of Water Vapor by Collagen and

Leather - Jour of Amer Leather Chemist Ass’n 4

P

: 12-14 (19 P

1 0) r i »i
[24] Zettlemoyer, A.C.j Schweitzer, E.D.; and Walker, W.C.: The

Internal Surface Area of Hide - Ibid 4l: 253-262 (1946)

[ 23 ] Zettlemoyer, A.C.; Chand, A.; and Gamble, E.: Sorption by

Organic Substances I. Krypton and Nitrogen on Polyethylene, Nylon

and Collagen - Jour Amer Chem Soc £2: 2732-2717 (1930)

[26] Ventkateswaran, A.: Sorption of Aqueous and Nonaqueous

Media by Wood and Cellulose - Chem Rev fO: 6 19-637 ( 1970 )

[27] Mikhail, R.Sh.; Brunauer, S.; and Bodor, E.E.

Investigation of

a Complete Pore Structure Analysis - Jour of Colloid and Interface

Sci 26: 45-53 (1968)

'WvV

[28]

Everett, D.H.Adsorption Hysteresis - THE SOLID GAS INTERFACE

Chap. 38 , Edited by Flood, E.A., Marcel Dekker, Inc., N . Y

[29]

Brunauer, S.s THE ADSORPTION OF CASES AND VAPORS. I- PHYSICAL ADSORPTION, Chap. Ill, Princeton Univ. Press., Princeton,

N.J. (1945)

[30]

Unpublished report of Mabie, C.P., Research Assoc., American

Dental Association, National Bureau of Standards, Washington, D.C.

[31

Marshall, P.A., and Moore, W.J.: Sorption of Ammonia by

Silk Fibroin - Jour ~ Amer Chem Soc £4: 4779-^783 (1932)

[32j Berlin, E.; Anderson, B.A.; and Pallansch, M.J.: Contraction

of Dried Casein Particle Surfaces Effected by Sequential H^O Vapor

Sorption and Desorption Cycles - Jour of Colloid and Interface Sci

33: 312-316 (1970)

/Will

[ 33] Ghosh, J.C., and Gyani, B.P.: Adsorption of Water Vapour

on Gelatine - Jour of Indian Chem Soc 30: 730-742 (1933)

/V\AN

[ 34] Green, R.W.

The Adsorption of Water Vapour on Collagen

and Elastin _ Trans of the Royal Soc of New Zealand 77: 24-46 (1948)
[35] Mooney, R.W.

;

Keenan, A.G.j and Wood, L.A.: Adsorption of

Water Vapor by Montmorillonite . I. Heat of Desorption and Application of BET Theory - Jour Amer Chem Soc 74: 1367-1374 (1972) 1 11 " 1

[36] Barrer, R.M., and Reay, J.S.S.: SURFACE ACTIVITY, Vol. 2,

p-79j> Edited by Schulman, J.H., Butterworth, London"TT997

)

[371 Barton, J.A., Jr., and Dickson, G. : Thermal and Hygroscopic

Dimensional Ghanges of Tooth Structure - Talk delivered at the 48th

I ADR Mtg. New York* N.Y. (1970)

[ 38

]

Haines, R.S., and McIntosh, R # : Length Changes of Activated

Carbon Rods Caused b£ Adsorption of Vapors - Jour Chem Phys It:

28-38 (1947)

" Av"

[39

]

Quinn, H.W., and McIntosh, R # : The Hysteresis Loop in

Adsorption Isotherms on Porous Vycor Glass and Associated Dimensional Changes of the Adsorbent, II. - Can Jour of Chem 38: 74^-7 F 6

( 1957)

" " ***

[40]

Rao, K.S., and Das, B.: Varietal Differences in Gelatin,

Egg Albumin, and Casein in Relation to Sorption - Desorption Hysteresis with Water - Jour Phys Chem 72: 1223-1230 ( 1968 )

ri /vH
WATER

VAPOR

ADSORPTION

METHODS

Table

1

Classification

of

Water

Vapor

Adsorption

Methods
CD rS

-P

PiO

Pi<D A

-PO

Pio

m m-i w w g

+> O i-H Pi 3

-P w

w (1)

id

CD -P P -P

rd £

&T5 w W 0) Q) P -P in rd (d

• ftrQ

<n g P

i—I o o

u w

C nd O

^ C

<D U '—

<D 3

W U

rd Q) &0

« e

rd Pi

(D <U P -P

<: <d£

E-t W t5 CQ CD rQP

CN O

m

<D TS

i

i rd 8

Eh

o

(6/iouiuj) N

PCI

ATI

VF
Figure

3.

Sorption

isotherms

for

water

on

human

dentin

at

23

°C.

Light

circles

are

adsorption

points
Figure

4.

Sorption

isotherms

for

water

on

human

enamel

at

23

°C.

Light

circles

are

adsorption

points

(6/|ouj) +

oi xN

C?

ft* l
(6/ |Otu) 0! x H
i

u

>

i-<

mL

C

(6/iouj) +01 x N

lji i

ka

»niT Y
<r> o

00

o

% o

CP

O

LO

9 o

9 o

ro

6

OJ

o

o

o

6

RELATIVE

HUMIDITY


1