Surface free energies of hydroxyapatite, fluorapatite and calcium fluoride

Mutrrials Chemistry

and Physics,

SURFACE

FREE

CALCIUM

FLUORIDE

ENERGIES

H.P.

H.J. BUSSCHER,

17 (1987)

OF

553-558

553

FLUORAPATITE

HYDROXYAPATITE,

DE JONG AND

AND

J. ARENDS

Laboratory for Materia Technica, Antonius Deusinglaan 1, 9713 AV Groningen (The Netherlands) Received January 30,

accepted March 12, 1987

1987;

ABSTRACT

Surface free energies of hydroxyapatite, fluorapatite and fluoride have been determined from contact angle calcium and compared with available literature data. measurements Surface free energies Y of the above materials against vacuum are all rather high is the range between 72 and 95 mJ.m-2, of the materials in the whereas surface free energies Y presence of saturated vapor areSFairly low between 28 and 48 mJ.mv2. It is discussed that this difference is due to the from the liquid presence of an adsorbed film originating droplets employed in the measuring procedure. Literature data on the surface free energies of the above materials obtained by independent techniques and theories show an extremely large variation but nevertheless confirm the results on hydroxyapatite and fluorapatite.

INTRODUCTION Surface calcium

interaction growth

free

energies

fluoride [2]

of

biological

of hydroxyapatite, fluorapatite and to

many

tissues

phenomena with

as

e.g.

the

bone

[l] or crystal and are therefore extremely important to determine.

The determination energetic,

relate

ionic

of the

surface

surfaces

free

energy of these highly by classical methods, as summarized

e.g.

by Wu C31 is troublesome. Contact angle measurements, a technique often employed to estimate the surface free energy of low

energetic

in this

respect.

0254-0584/87/$3.50

surfaces 141 have been discarded The main

reason

for not employing

for a long time this

approach

0 ElsevierSequoia/Printed inThe Netherlands

on these surfaces has been the fact that adsorption of liquid or molecules

vapor region

from

droplet or the three phase boundary

yields a large reduction in the surface free energies of materials.

these

the

spreading

This

pressure

reduction,

known

as' the

equilibrium

is frequently neglected in surface free

"e

energy determinations from contact angles [3]. Recently surface

free

contact

angle

caLculationa

be

simultaneously determined from

provided

measurements,

the

proper liquids and

techniques are used [6].

the

it

appropriate

seemed

surface

such

crystals

can

energies

Therefore determine

showed [5] that spreading pressures as well as

we

as

free

energy

to

of

apply this method to

highly

energetic, ionic and calcium fluorapatite

hydroxyapatite,

fluoride.

EXPERIMENTAL Advancing the

sessile

mixtures

type contact angles have been measured at 25°C by drop

and

hydroxyapatite fluoride planes

water, on

II water/n-propanol single

crystalline (Zillerthal), fluorapatite (Durango) and calcium

the

Both

the

basal

as well as the prismatic

apatites have been used, whereas in the case of

fluoride only the < 1 1 1 > planes were measured. Prior

the

carefully and

with

cc-bromonaphthalene

(Harshaw). of

calcium to

technique

contact

angle

polished

in

measurement

the

surfaces

have

been

0.05 pm A1203 slurry in distilled water

after thorough rinsing in water allowed to dry overnight at

25°C in a dust free surrouding. The

surface

measured polar

Yy

tensions

Y1 of the liquids employed have been

with a tensiometer. Subsequently the dispersion Y: and components of Y1 have been calculated from measured

contact angles on paraffin by

cos

8 = -1 + z.(Y; . Yf)S

.

Y-i

assuming that for paraffin Ys = Yz = 25.5 mJ.m-' [73.

(1)

surface

The

energies

free

Y

S

the

of

and their

solids

and polar Yg components have been calculated by YZ square fitting the measured contact angle data to the

dispersion least

geometric mean equation [S]

cos

8

=

-1 + 2.(Yt . Yf)% * Y-t + 2.(YE

.

P4

Y1)

.

Y

-1 1

-

lr,.

Y;l (2)

in

which

is the equilibrium spreading pressure, defined as

xe

[91: “e

=

Ys - Ysv

where

is

Ys

the

solid surface free energy against vacuum and

surface free energy in the presence of is sv saturated vapor. Surface free energies Ys and Y,, have both been the

Y

solid

calculated from the experimental data accounting for

separately

neglecting respectively the spreading pressure term in eqn.

and (2).

RESULTS AND DISCUSSION In

Table

spreading

I surface free energies Ys as well as Ysv and the

pressures

Ile are

summarized,

as

calculated

from

measured contact angles. Whereas the

the

surface

free energies Ys are all fairly high,

surface free energies Ysv are all reasonably low due to the

presence

interesting have

adsorbed

of

to

note

vapor in

molecules

on

the surface. It is

this respect that although Ys and Ysv

been separately calculated, their difference approximately

equals

the calculated spreading pressure i(,, in accordance with

thermodynamic definitions. The reduction in surface free energy observed during contact angle

measurements

mixed

water/n-propanol

surfaces [5].

should be attributed to the adsorption of a film, as recently discussed for polymer

556

-2 I. Surface free energies and spreading pressures [mJ.m ]

Table for

(PAP) and calcium (HAP), fluorapatite (CAF). f denotes the standard deviation obtained for 7

hydroxyapatite

fluoride

independent series of contact angle measurements.

material

Y,”

YE

HAP, basal plane

3814

51f20

89f24

39fll

48f14

HAP, prismatic plane

33fl

47f 9

80f 9

30f 3

45f 5

PAP, basal plane

34fl

38f 6

72f 7

28f 3

40f 5

FAP, prismatic plane

33fl

62.t 9

952 8

48f 7

48.t 3

CAF, plane

34fl

49f 4

83i 4

36f 4

45f 5

The

main

actually water

reason for employing water/n-propanol mixtures is

the

fact

that

n-propanol

and

on

a wide variety of solid substrates

exhibit

similar spreading pressures. By

ellipsometric determination of adsorption isotherms of water and n-propanol

[lOI

for 45

this

yielding

fluoride,

fact

was

spreading

also

established for calcium

pressures Ee of 48 and 53 mJ.m -2

water and n-propanol respectively, comparing well with xe = -2 derived mJ.m from contact angle measurements reported in

this study. Stasczuk

et

al.

1111

discussed

the

reduction of quartz, another highly energetic

surface free energy material

due to the

presence of adsorbed water films and showed that the presence of 2

statistical

dispersion study

monolayers

surface

free

of

water

energy

yields a reduction of the

from 76 to 25 mJ.m-'. In this

we observe a similar reduction of the surface free energy

of ionic crystals (see Table I). Very energy

few

data

exist in the literature on the surface free

of HAP, FAP and CAF. In Table II the few data that could

be collected are summarized. Some

comments

dislocation

core

should

be made on the data in Table II. The

data on hydroxyapatite

are

obtained in vacuum

and might be interpreted as Ys values, whereas in the solubility on HAP adsorbed water layers are undoubtedly experiments present, indicating that results from these methods should be

557

Table

for

values

Literature

II.

the surface free energies Y

[mJ.m -2] of HAP, FAP and CAF2. reference

Y

material

plane

method

HAP

not specified

dislocation

HAP

not specified

solubility data

47f 2

1 21

FAP

basal

slow cleavage

95f25

[I31

FAP

prismatic

slow cleavage

480f30

Cl31

CAF2



slow cleavage

140-510

[14,151

theoretical

540-1019

[16]

1121

100

core data



CAF2

identified

as

Ysv

hydroxyapatite

values.

from

conclusion

that

the

correspond

with

available

theoretically

A

Table

comparison

I

and

results

objections

Table

from be

the results for thus

yields the

contact angle measurements

literature, can

of II

despite

the fact that

raised against employing the

contact angle method on ionic crystals. Derivation

of

surface

free

energies

from

slow cleavage

experiments as done for fluorapatite and calcium fluoride may in principle

only

cleavage

be

planes

carried

and

out

for the real crystallographic

therefore not for the prismatic planes of

fluorapatites.

The result presented in Table II for the prisplane of fluorapatite from Aning et al. [13] is therefore

matic

probably authors plane

erroneously [13].

for

with

those

that

also

fluorapatite presented for

measurements conclusion

high,

Discarding

predominantly literature data.

and comparing the results from Table I in

Table II again yields the conclusion

fluorapatite

are can

as also correctly stated by the the literature data on the prismatic

in be

the

results of the contact angle

correspondence drawn

because

of

with

the

literature. No

on the results for calcium fluoride, the

large

variation

in

reported

CONCLUSION It

can

objections,

be

stated

contact

that,

angle

despite

measurements

possible

theoretical

on ionic crystals can

yield valuable information on their surface free energy and that the

data

obtained are in correspondence with the few available

literature data on this subject.

REFERENCES 1

C.A. van Blitterswijk, J.J. Grote, W. Kuypers, C.J.G. Blokvan Hoek and W.Th. Daems, Biomaterials, 6 (1985) 243.

2

J. Christoffersen and Growth, 49 (1980) 29.

3

s. wu, 1982.

Polymer

4. D.K. Owens 1741.

M.R.

Christoffersen,

J.

Crystal

Interface and Adhesion, Marcel Dekker Inc.

and R.C. Wendt, J. Applied Pal. Sci., 13 (1969)

J. 5. H.J. Busscher, G.A.M. Kip, A. van Silfhout and J. Arends, Colloid Interface Sci., 114 (1986) 307. 6. H.J. Busscher, A.W.J. van Pelt, H.P. de Jong and J. Arends, J. Colloid Interface Sci., 95 (1983) 23. 7

F.M. Fowkes, Industrial and Engineering Chemistry, 56 (1964) 12.

8

S. Wu, J. Adhesion, 5 (1973) 39.

9

to the and D.A. Haydon, An introduction R. Aveyard principles of surface chemistry, Cambridge University Press, 1973.

10

G.A.M. Kip, unpublished data.

11

P. Staszczuk, B. Janszuk Phys., 12 (1985) 469.

12

J. Arends 186.

13

M. Aning. D.D. Welch and B.S.H. Royce, Physics Letters, 37A (1971) 253.

14

V.D. Kusnetsov and P.P. Teterin, Surf. Eng. Sol., (1957) 39.

15

J.J. Gilman, J. Appl. Phys., 31 (1960) 2208.

16

B. Klobe, Handbuch 1933, p. 764..

and

and

E. Chibowski, Mater. Chem.

W.L. Jongebloed, Caries Research, 11 (1977)

der Physlk, Verlag van Julius Springer