Interaction of Liquids at Solid Substrates

9 Chemistry at Organic-Inorganic Interfaces

Downloaded by SUNY STONY BROOK on October 25, 2014 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0087.ch009

M I C H A E L B E R T O L U C C I , F E R N J A N T Z E F , and D A V I D L . C H A M B E R L A I N , JR. Stanford Research Institute, Menlo Park, Calif.

The single reflection

ATR

infrared spectra of a series of

synthetic and natural calcium phosphates were examined for changes resulting The asymmetric cm.

from adsorption Ρ—O

-

of organic

stretching

molecules.

frequency (v ) at 1027 3

underwent a high frequency shift of up to 9 cm.

-1

-1

as

a result of adsorption of citric acid, tetracycline, and certain other polar molecules. quency of the Ρ—Ο—C

Similarly, the Ρ—Ο

stretching fre

group in trialkyl phosphates was

shifted to higher frequencies by

hydrogen bonding

with

water and phenol. The relationship between the high fre quency

shift and

the

properties

of

the

solid phase is

discussed. A tentative explanation is offered for the shift in the

lattice

absorption

frequency

as a result of

surface

modification.

T n f r a r e d spectrophotometry *·*

has b e e n a p p l i e d v e r y effectively t o the

s t u d y of a d s o r b e d m o l e c u l e s a n d the changes w h i c h o c c u r i n the

s p e c t r u m of the adsorbate as a result of i n t e r a c t i o n w i t h the adsorbent. E x c e l l e n t b i b l i o g r a p h i e s o n this subject are p r e s e n t e d b y E i s c h e n s a n d P l i s k i n (4)

and Little

(9).

I n a d d i t i o n to p r o v i d i n g a n i n s i g h t i n t o changes that o c c u r i n the a d s o r b e d phase, i n f r a r e d s p e c t r a of o r g a n i c - i n o r g a n i c systems s h o u l d also p r o v i d e some i n d i c a t i o n of changes o c c u r r i n g i n the adsorbent phase. F u r t h e r , s u c h changes s h o u l d also b e r e l a t e d to a d s o r p t i o n a n d a d h e s i o n m e c h a n i s m s . V e r y little a t t e n t i o n has b e e n p a i d to s p e c t r a l changes i n adsorbents a n d , as p o i n t e d out b y A m b e r g ( J ) , s u c h changes w h e n they occur have usually been neglected.

C e r t a i n l y , there are u n d o u b t e d l y

m a n y cases i n w h i c h s p e c t r a l changes are o b s c u r e d because of b r o a d intense a b s o r p t i o n b a n d s

s u c h as S i — Ο

and Ρ—Ο

i n silicates a n d

phosphates. 124 In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

9.

BERTOLUCCi

ET

125

Organic-Inorganic Interfaces

AL.

T h e object of this w o r k w a s to s t u d y the i n t e r a c t i o n of

adsorbed

o r g a n i c molecules w i t h a t o o t h - m i n e r a l ( h y d r o x y a p a t i t e ) adsorbent. e s p e c i a l interest w a s the extent to w h i c h the response

Of

of the m i n e r a l

phase c o u l d b e o b s e r v e d b y i n f r a r e d spectroscopy. Results Model Compound Study.

Homogeneous

systems of t r i e t h y l

phos


p h a t e ( T E P ) w i t h p h e n o l a n d w i t h w a t e r w e r e chosen as m o d e l s for h e t erogeneous systems. S p e c t r a of these systems p r o v i d e d a basis for expect i n g changes i n the s p e c t r u m of s o l i d adsorbent phases.

Table I includes

the d a t a for a 1 : 1 m i x t u r e of T E P a n d w a t e r a n d a 5 % T E P - 5 % s o l u t i o n i n benzene.

phenol

T h e Ο — Η s t r e t c h i n g frequencies of b o t h w a t e r a n d

p h e n o l w e r e s h i f t e d to l o w e r frequencies. i n p h e n o l a n d the P = 0

L i k e w i s e , the C — Ο s t r e t c h i n g

s t r e t c h i n g i n T E P w e r e s h i f t e d to l o w e r f r e

quencies, as expected (4,9).

T w o a d d i t i o n a l shifts w e r e o b s e r v e d i n o u r

work. T h e Ρ—Ο and C—Ο

s t r e t c h i n g i n the Ρ — Ο — C H

ported (2)

2

at 973 a n d 1033 c m . " , r e s p e c t i v e l y .

g r o u p is r e

5

E a c h b a n d was shifted,

1

either b y w a t e r or p h e n o l , b y 2 to 3 c m . " . 1

O n the basis of the d a t a of H a r d y a n d c o - w o r k e r s (6) a n d of F l e t c h e r a n d c o - w o r k e r s ( 5 ) , i t is a s s u m e d t h a t the structure of a 1 : 1 c o m p l e x of t r i e t h y l p h o s p h a t e a n d w a t e r is one i n w h i c h h y d r o g e n b o n d i n g

occurs

o n the o x y g e n a t o m of the p h o s p h o r y l g r o u p ( P = 0 ). T h e Ο — Η s t r e t c h i n g f r e q u e n c y of the p r o t o n d o n o r a n d the P = 0

s t r e t c h i n g f r e q u e n c y of

the p r o t o n acceptor are s h i f t e d to l o w e r frequencies as expected.

I n this

w o r k the effect of h y d r o g e n b o n d i n g was also o b s e r v e d i n the C — Ο — Ρ b o n d s of the p h o s p h a t e ester groups.

The C—Ο

w a s s h i f t e d t o w a r d l o w e r frequencies

(2),

973 c m .

stretch at 1033 c m . "

w h i l e the Ρ — Ο

1

stretch at

was s h i f t e d , b o t h b y w a t e r a n d p h e n o l , to h i g h e r frequencies.

- 1

T h i s h i g h f r e q u e n c y shift is r e a d i l y e x p l a i n e d o n the basis of p o l a r i z a b i l i t y of the ether o x y g e n a t o m , w h i c h m a y share its electrons w i t h p h o s p h o r u s as the P = 0

l i n k is p o l a r i z e d b y h y d r o g e n b o n d i n g :

Ο : -» H

: Ο—Η

+

—Ρ—Ο—C H 2

5

: Ο—Η

±5 — 4 — Ο — C H 2

5

±5 — Ρ = 0 — C H +

2

5

T h i s e x p l a n a t i o n is s i m i l a r to that d i s c u s s e d b y B e l l a m y (3) inductive a n d mesomeric

for

effects i n a c i d s , a c i d c h l o r i d e s , esters,

the and

amides. T h e s e d a t a c o n f i r m B e l l a m y a n d B e e c h e r s assignment ( 2 ) 1033 c m .

P—O—C H 2

b a n d to C — Ο

1

5

a n d the 973 c m . "

1

b a n d to Ρ — Ο

linkage.

In Interaction of Liquids at Solid Substrates; Alexander, A.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

of

the

i n the

126

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

Table I. Infrared Spectra of Hydrogen-Bonded T r i e t h y l Phosphate Infrared Absorption Maxima ( c m . ) -1

TEP-Phenol TEP-Water ΤEP Phenol 1:1 Mixture Water 1:1 Mixture

Assignment


Ο — Η stretch in phenol P = 0 stretch C — 0 stretch

1271

C — Ο stretch

1033°

Ρ — Ο stretch

in p _ o _ c H 2

973

3309

3279

30

1258

Combined doublet at 1250 & 1205

Both the P = 0 and C — Ο stretching modes were shifted

α

1031

Small but definite shift

976

Small but definite shift to a higher frequency

5

Ο — Η stretch in water

C — Ο stretch in P — O — C H 2

Peak broadened & shift not measurable

Broad about 3450

P = 0 stretch

11

1272

1261

1033

1031

Small but definite shift

973

976

3; small but definite shift to a higher frequency

5

Ρ—Ο stretch in P — O — C H , 2

1

Shift

Reference 2. C r y s t a l l i n e Phosphate Studies.

triethyl phosphate,

a series of

infrared spectrophotometry.

O n the basis of

c a l c i u m phosphates

the results w i t h was

examined

by

P e r t i n e n t p r o p e r t i e s of these m a t e r i a l s are

s u m m a r i z e d i n T a b l e I I , a n d t h e i r s p e c t r a l characteristics are s h o w n i n Table III.

N o n e of the s y n t h e t i c h y d r o x y a p a t i t e s

[Caio(P0 )6(OH) ] 4

2

h a d t h e s t o i c h i o m e t r i c C a / P r a t i o of 1.667, a l t h o u g h t h e y s h o w e d

the

apatite lattice structure. A t y p i c a l infrared transmission spectrum

(be

t w e e n 1500 a n d 700 c m . " ) of a d r y p o w d e r s y n t h e t i c h y d r o x y a p a t i t e is 1

s h o w n i n F i g u r e 1. T h e intense a b s o r p t i o n of the Ρ — Ο this r e g i o n .

The

use

of

s t r u c t u r e obscures a l l d e t a i l i n

single a t t e n u a t e d t o t a l reflection

technique,

c o u p l e d w i t h scale e x p a n s i o n , r e s u l t e d i n the s p e c t r u m s h o w n i n F i g u r e 2. T h e m a x i m a i n the a s y m m e t r i c Ρ — Ο " s t r e t c h i n g b a n d is r e a d i l y m e a s u r e d .


9.

ET AL.

BERTOLUCCi

Table II.

127

Organic-Inorganic Interfaces Properties of Calcium Phosphates X-ray Pattern

Specific Surface meter J gram

a


Sample Hydroxyapatite—synthetic Sample No. 1 Sample No. 2 Sample No. 3 Sample No. 4 Natural (contains fluoride) Fluoroapatite—synthetic Hydroxyapatite—commercial BiogelHTP Kerr-McGee 6

Calcium Phosphate Baker A.R. (Tricalcium Phosphate) Synthetic

Ca/P

2

Hydroxyapatite Hydroxyapatite Hydroxyapatite Amorphous

180 104 152 120

1.62 1.64 1.64

0.8

1.667

6

50

Fluoroapatite

1.53 1.57 (1.66 )

62 27.4

Hydroxyapatite

1.53 1.54

36 97

— 0

c

Hydroxyapatite Whitlockite OCa (P0 ) ] 3

4

2

° Measured by the flow method of nitrogen adsorption. Crystal from Gerlos Zillerthalen, Austrian Alps, courtesy Stanford University Geology Department; Ca/P ratio is assumed. Data supplied by the manufacturer. h

c

Table III.

Major Absorption Band of Synthetic and Commercial Phosphates from 5000 to 400 c m . " 1

Material Hydroxyapatite—synthetic Sample No. 1 Sample No. 2 Sample No. 3 Sample No. 4

Sodium Chloride Region, cm. 1

1091 1096 1093 1092

1027 1026 1028 1024

960 960 960 960

Potassium Bromide Region, cm.' 1

625 absent

592 592

Apatite—natural

1093

1042

Fluoroapatite—synthetic

1097

1030

961 963

Hydroxyapatite—commercial Biogel H T P Kerr-McGee

1086 1092

1020 1027

960 961

621 622

592 591

Calcium Phosphate Baker A.R. (Tricalcium Phosphate) Synthetic No. 2

1090 1100

1027 1028

960 absent

623

592

T h e i n f r a r e d spectra of these m a t e r i a l s w e r e t a k e n b e f o r e a n d after t h e y w e r e exposed t o solutions of a v a r i e t y o f adsorbates.

It was found

that c i t r i c a c i d , t e t r a c y c l i n e (as t h e H C 1 s a l t ) , a n d c e r t a i n other o r g a n i c


128

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

m o l e c u l e s cause a shift i n the a s y m m e t r i c Ρ — Ο " s t r e t c h i n g f r e q u e n c y at 1027 to 1028 c m . " . 1

T a b l e I V i n c l u d e s the p e r t i n e n t d a t a for the effect of selected a d sorbates u p o n s y n t h e t i c h y d r o x y a p a t i t e . T h e adsorbates w e r e chosen for t h e i r m o l e c u l a r structures or for t h e i r i m p o r t a n c e to d e n t a l p r o b l e m s . T h e frequencies r e p o r t e d i n T a b l e I V are a c c u r a t e a n d r e p r o d u c i b l e to ± 2 cm." . 1

T h e r e f o r e , o n l y the values of shifts of 9 c m . "

and 5 cm."

1

1

for c i t r i c a c i d

for o x y t e t r a c y c l i n e H C 1 are of significance for

synthetic


h y d r o x y a p a t i t e S a m p l e N o . 1. B y w a y of contrast, t a r t a r i c a c i d c a u s e d n o shift i n the 1027 peak of S a m p l e N o . 1, w h i c h h a d a specific surface of o n l y 180 m e t e r / g r a m , a l t h o u g h it d i d cause a shift for S a m p l e N o . 2 2

w h i c h h a d a specific surface of o n l y 104 m e t e r / g r a m . 2

Further, Sample

N o . 3, w i t h a specific surface of 152 m e t e r / g r a m w a s not affected 2

by

either c i t r i c a c i d or t e t r a c y c l i n e · H C 1 . T h e d a t a i n T a b l e I V for the three samples of h y d r o x y a p a t i t e s u m m a r i z e the range for shifts i n these samples of h y d r o x y a p a t i t e . Shifts have not b e e n o b s e r v e d i n c o m m e r c i a l l y a v a i l a b l e h y d r o x y a p a t i t e s , one s a m p l e of n a t u r a l a p a t i t e , or c a l c i u m phosphates.

L i k e w i s e , c a l c i u m phosphates

p r e p a r e d b y m e t h o d s w h i c h y i e l d h i g h specific surface h a v e also not r e s p o n d e d i n this m a n n e r to adsorbates. T h e a s y m m e t r i c s t r e t c h i n g f r e q u e n c y of the Ρ — Ο " i o n is the o n l y v i b r a t i o n of the P 0

4

3

" i o n i n the 5000 to 400 c m . "

1

region

noticeably

affected b y a d s o r b e d molecules. F*7*

11 \

\

1500 Figure 1.

;

I

1000

500

Transmission spectrum of synthetic hydroxyapatite



9.

BERTOLUcci E T A L .

Organic-Inorganic Interfaces

9

Figure 2.

^ WAVELENGTH

129

10 microns

Single reflection ATR spectra of synthetic hy droxyapatite

Absorbed citric acid Plain material, pH adjusted with H CI to pH 4.08

Discussion T h e h i g h f r e q u e n c y shift i n the a s y m m e t r i c Ρ — Ο " s t r e t c h i n g fre q u e n c y c a u s e d b y a d s o r p t i o n o n h y d r o x y a p a t i t e appears to b e a p e r t u r b a t i o n of l a t t i c e b o n d s as a result of surface changes. T h e m i n i m u m specific surface necessary to cause a l a t t i c e shift b y a p a r t i c u l a r adsorbate has not b e e n ascertained.

T h e difference

i n sensitivity between

different

p r e p a r a t i o n s of h y d r o x y a p a t i t e is s h o w n i n T a b l e I V . T h e s e differences are best e x p l a i n e d , at present, b y differences i n surface groups r e s u l t i n g f r o m m i n o r differences i n w a s h i n g p r o c e d u r e .

Rootare, Deitz, a n d C a r

p e n t e r ( J O ) discuss h y d r o l y s i s reactions of surface p h o s p h a t e ions a n d t h e


130

INTERACTION

f o r m a t i o n of complexes

OF

LIQUIDS

AT

SOLID

SUBSTRATES

o n t h e surface of h y d r o x y a p a t i t e crystals. T h e

extent of h y d r o l y s i s a n d t h e c o n c e n t r a t i o n of surface complexes have a pronounced

should

effect o n the a d s o r p t i o n of o r g a n i c m o l e c u l e s

by

h y d r o x y a p a t i t e crystals. Table I V .

Effect of Adsorbed Molecules upon the Infrared Spectrum of Synthetic Hydroxyapatites


Sample No. 1 Adsorption Peak (cm.' )

Adsorbate

1

None Citric acid Tartaric acid Sodium citrate Oxytetracycline · HC1 Glycine

Shift (cm.' ) 1

0 9 (high frequency) 2 1 5 (high frequency) 1

1027 1036 1025 1028 1032 1028 Sample No. 2

Adsorbate

Adsorption Peak (cm.' )

Shift (cm.' )

1026 1031 1031 1032 1033 1030 1031

0

1

None Citric acid Tartaric acid Tetracycline · HC1 Oxytetracycline * HC1 Phenol Gelatin

1

il il

(high frequency

Sample No. 3 Adsorption Peak (cm.' )

Adsorbate None Citric acid Tetracycline

1

HC1

Shift (cm.' ) 1

0 3 (high frequency) 0

1029 1032 1029

T h e o b s e r v e d h i g h f r e q u e n c y shifts i n t h e 1026-1028 c m . " b a n d of 1

high-surface hydroxyapatites m a y be explained b y the following physical p i c t u r e of a d s o r p t i o n : a n a d s o r b e d m o l e c u l e , A R , c o n t a i n i n g a n electrophilic group,

A — e . g . , citric acid—coordinates

with

the oxygen

surface — Ρ : Ο : Η g r o u p , g i v i n g t h e structure Ο O—P:0:AR


of a

9.

BERTOLUCCi

ET

AL.

Organic-1norganic Interfaces

131

w h e r e the three n o n c o o r d i n a t e d o x y g e n atoms are i n the lattice of the crystal.

T h e decrease i n electron d e n s i t y a r o u n d the surface o x y g e n is

t r a n s f e r r e d b y i n d u c t i v e effects t h r o u g h the Ρ a t o m to the adjacent l a t t i c e oxygens, r e s u l t i n g i n a n i n c r e a s e d electron d e n s i t y b e t w e e n the Ρ a n d Ο atoms i n the lattice.

:0: " .. δ' Ο—Ρ : Ο : A R


:0: O—P:0:AR

Ο

Η

T h i s results i n a shift to a h i g h e r f r e q u e n c y , i n a m a n n e r analogous to the t r i e t h y l p h o s p h a t e - p h e n o l system. T h i s e x p l a n a t i o n of the o b s e r v e d shift is tentative, p e n d i n g f u r t h e r clarification.

Experimental S y n t h e t i c h y d r o x y a p a t i t e s w e r e p r e p a r e d b y the m e t h o d of H a y e k a n d co-workers ( 7 , 8 ) , m o d i f i e d to p r o d u c e a p r o d u c t of s m a l l p a r t i c l e size. T h e p r o d u c t s w e r e i s o l a t e d b y w a s h i n g the g e l s l u r r y w i t h w a t e r u n t i l free of n i t r a t e i o n . T h e resultant suspensions w e r e t h e n s p r a y - d r i e d w i t h a M i n o r T y p e 53 N i r o A t o m i z e r ( C o p e n h a g e n , D e n m a r k ) . T h e r e s u l t a n t d r y p r o d u c t s w e r e i n the f o r m of porous, s p h e r i c a l p o l y c r y s t a l l i n e agglomerates of 5 to 8 μ average d i a m e t e r . T h e specific surfaces of these p r o d u c t s r a n g e d f r o m 104 to 180 m e t e r / g r a m . 2

C o m m e r c i a l hydroxyapatite sample N o . 1 was B i o g e l H T P from B i o r a d L a b o r a t o r i e s . C o m m e r c i a l S a m p l e N o . 2 w a s o b t a i n e d f r o m the K e r r - M c G e e Corporation. N a t u r a l h y d r o x y a p a t i t e f r o m G e r l o s , Z i l l e r t h a l e n , i n the A l p s , w a s s u p p l i e d b y the S t a n f o r d U n i v e r s i t y D e p a r t m e n t of T h e c a l c i u m phosphate used was Baker A . R. Grade.

Austrian Geology.

I n f r a r e d spectra w e r e o b t a i n e d o n a P e r k i n E l m e r 221 s p e c t r o p h o tometer, u s i n g a C o n n e c t i c u t I n s t r u m e n t C o m p a n y single-reflection a t t e n u a t e d t o t a l reflectance a t t a c h m e n t a n d 5 X scale e x p a n s i o n . T h e s a m p l e w a s p l a c e d as a d r y p o w d e r o n a K R S - 5 p r i s m . T h i s a r r a n g e m e n t p r o v i d e d the v e r y short s a m p l e p a t h l e n g t h r e q u i r e d to o b t a i n r e s o l u t i o n of the b r o a d , intense p h o s p h a t e b a n d . F i g u r e 1 shows the t r a n s m i s s i o n s p e c t r u m of a d r y p o w d e r s a m p l e of h y d r o x y a p a t i t e . F i g u r e 2 shows t h e s i n g l e reflection A T R spectra of u n t r e a t e d h y d r o x y a p a t i t e a n d the m i n e r a l c o n t a i n i n g a d s o r b e d c i t r i c a c i d . A w a v e l e n g t h m a r k e r w a s u s e d for e a c h s p e c t r u m to ensure correct a l i g n m e n t of the t w o spectra. T h e shift, t h o u g h small, was real a n d reproducible.


132

INTERACTION

OF

LIQUIDS

AT

SOLID

SUBSTRATES

Acknowledgments T h i s w o r k w a s s p o n s o r e d b y t h e N a t i o n a l Institute of D e n t a l R e s e a r c h under Contract

P H 43-65-82.

H a r o l d E d i n g c a r r i e d o u t surface

area

measurements, a n d M i l t o n Silverstein contributed verly h e l p f u l discussion.


Literature Cited (1) Amberg, C. H., "The Solid-Gas Interface," Vol. 2, p. 887, E. A. Flood, ed., Marcel Dekker, Inc., New York, 1967. (2) Bellamy, L. J., Beecher, L., J. Chem. Soc. 1953, 730. (3) Bellamy, L. J., "The Infrared Spectra of Complex Molecules," p. 395, 2nd ed., John Wiley and Sons, Inc., New York, 1958. (4) Eischens, R. P., Pliskin, W. Α., "Advances in Catalysis," Vol. X, p. 1, Academic Press, Inc., New York, 1958. (5) Fletcher, J. M., Scargill, D., Woodhead, J. L., J. Chem. Soc. 1961, 1705. (6) Hardy, C. J., Fairhurst, D., McKay, H. A. C., Willson, A. M . , Trans. Faraday Soc. 60(501), 1626 (1964). (7) Hayek, E., Stadlmann, W., Angew. Chem. 67, No. 12, 327 (1955). (8) Hayek, E., Newesely, H., Inorganic Syntheses 7, 63 (1963). (9) Little, L. H., "Infrared Spectra of Adsorbed Species," Academic Press, Inc., London, 1966. (10) Rootare, H. M., Deitz, V. R., Carpenter, F. G., J. Colloid Sci. 17, 193 (1962). RECEIVED November 8,

1967.


Interaction of Liquids at Solid Substrates

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