Surface modification of calcium hydroxyapatite with hexyl and decyl phosphates

COLLOIDS AND ELSEVIER

Colloids and Surfaces A: Physicochemicaland Engineering Aspects 125 (1997) 53 62

A

SURFACES

Surface modification of calcium hydroxyapatite with hexyl and decyl phosphates Hidekazu Tanaka, Akemi Yasukawa, Kazuhiko Kandori, Tatsuo Ishikawa * School of Chemistry, Osaka Universityof Education, 4-698-1 Asahigaoka, Kashiwara-shi, Osaka-fu 582, Japan Received 10 June 1996; accepted 28 August 1996

Abstract

The surface of synthetic calcium hydroxyapatite Calo(PO4)6(OH)z (CaHAP), modified by monohexyl phosphate (HP) and monodecyl phosphate (DP) in acetone at 25°C, has been examined by various methods. The X-ray diffraction (XRD) patterns of the materials modified with HP at less than 0.10moldm -3 or DP at less than 0.15 tool dm -3 were almost the same as that of the unmodified material. The particle morphology was not changed by the modification. The number of alkyl groups of the modified materials was about two groups per nm 2. These alkyl groups were removed by outgassing above 300°C. After this treatment the modified materials exhibited a larger number of surface P-OH groups and a higher negative electrophoretic mobility compared with the unmodified material. The materials with alkyl groups adsorbed much less H20 and CO2 than the unmodified material. © 1997 Elsevier Science B.V.

Keywords: Surface modification; Calcium hydroxyapatite; Alkyl phosphate; IR spectrum; Adsorption of H20 1. Introduction

Calcium hydroxyapatite Cal0(PO4)6(OH)2 ( C a H A P ) is a component of the hard tissues of animals. Synthetic C a H A P is used as an adsorbent for chromatography to separate biomaterials and as a raw material for artificial teeth and bones [1,2]. The interaction between C a H A P surfaces and biomaterials is an important factor in these applications. This interaction relates to the surface structure and properties of CaHAP, such as surface functional groups, surface charge, hydrophobicity and porosity. To control the affinity of this material to the other substances, modification of the surface structure is required. We have found that the P - O H groups on the surface of CaHAP, which act as adsorption sites for various molecules such * Corresponding author. Fax. +81-729-78-3394. 0927-7757/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0927-7757 (96) 03876-9

as H 2 0 and CH3OH , are hydrophilic [3]. Because the number of these surface P - O H groups determines various surface properties, the regulation of the P - O H number should lead to the surface modification of CaHAP. Wilson et al. [4] have modified C a H A P with polyacrylate ions and proposed a modification mechanism whereby the polyacrylate ions bind to the C a H A P surface by displacing the phosphate and Ca 2+ ions. Lebugle and co-workers [5-8] have modified C a H A P with hydroxyethylmethacrylate phosphate and dodecanolphosphate by a coprecipitation method to improve the dispersion of the C a H A P particles in polymers and stated that the surface O H - ions of the C a H A P crystal are replaced by the organic phosphates. Despite these studies, the surface structure and properties of the modified materials have not been satisfactorily characterized. The hydrophilic surface of C a H A P is expected to

54

H. Tanaka et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 125 (1997) 53-62

become hydrophobic as a result of modification by alkyl groups. In a previous study, we tried to modify the CaHAP surface using ethyl phosphate [9]. However, because the modification was done employing a mixture of water and acetone as the solvent, the bulk structure was altered by dissolution and recrystallization of CaHAP due to the high acidity of ethyl phosphate. Therefore, a nonaqueous solution of phosphate with longer alkyl groups may be better for modifying the CaHAP surface without changing the bulk structure. The purpose of this study was to modify the surface structure and properties of CaHAP with alkyl phosphates. We treated the synthetic CaHAP particles with acetone solutions of monohexyl and monodecyl phosphates under various conditions. The modified CaHAPs were characterized by FTIR spectroscopy, thermal analysis, adsorption of H20 and CO/, etc. A mechanism of the surface modification is proposed based on the results obtained.

2. Experimental CaHAP particles were synthesized by the wet method [10]. 0.405 mol of Ca (OH)2 were dissolved in 20 dm 3 deionized distilled water free from CO2 in a sealed Teflon vessel. To the solution were added 0.267 tool of H3PO 4 and the precipitates obtained were aged at 100°C for 48 h. The molar ratio of Ca/P in the initial solution was 1.52, which was less than the stoichiometric ratio of CaHAP (1.67). The resulting CaHAP particles were washed thoroughly with deionized distilled water and dried in an air oven at 70°C for 1 day. The obtained material exhibited a Ca/P ratio of 1.60 and was a calcium-deficient CaHAP. The surface modification was carried out by treating the samples in acetone solutions containing different amounts of monohexyl or monodecyl phosphates at 25°C for 60min. The samples treated were thoroughly washed with acetone by ultrasonication and centrifugation and were vacuum-dried at room temperature for 1 day. The acetone used was dried with molecular sieves (3 A) and distilled. The monohexyl and monodecyl phosphates were synthesized as previously reported

[11]. The purity of the phosphates was confirmed by elemental analysis and FTIR. The materials thus modified were characterized by the following conventional techniques. Powder X-ray diffraction (XRD) patterns were taken on a Rigaku diffractometer (Tokyo, Japan) using a Ni-filtered Cu K~ radiation (30 kV and 15 mA). Morphology of the particles was observed by a Jeol transmission electron microscope (Tokyo, Japan). Carbon contents of the modified samples were determined using a Yanagimoto CHN analyzer (Tokyo, Japan). Ca 2+ and PO43- contents were determined using a Seiko inductively coupled plasma (ICP) spectrometer (Tokyo, Japan) and the molybdenum blue method, respectively, by first dissolving in an HC1 solution. Differential thermal analysis (DTA) and thermal gravimetry (TG) curves were traced on a Seiko thermo-analyzer (Tokyo, Japan) in an air stream and an N2 stream at a heating rate of 5°C min-1. IR transmission spectra in vacuo were measured on a Digilab Fourier transform infrared (FTIR) spectrometer (Cambridge, MA, USA) using a vacuum cell. For the IR measurements, 30mg of samples was pressed into a disk of 10 mm diameter. Adsorption isotherms of N 2 and CO2 were measured using a volumetric apparatus at the boiling point of N 2 and 25°C, respectively. The adsorption isotherm of H 2 0 w a s determined gravimetrically at 25°C. Before the adsorption measurements, the samples were treated by outgassing under 10 -3 Pa at various temperatures from 100 to 400°C for 2h. Electrophoretic mobility of the particles was measured in an aqueous solution including 1.0 X l 0 - 4 mol dm -3 KC1 using a Pen-Kern electrophoresis equipment (Bedford Hills, USA). The pH of the suspension was adjusted by adding HC1 or KOH solutions.

3. Results and discussion

3.1. Structure of modred CaHAP Fig. 1 shows the XRD patterns of the CaHAP modified at different concentrations of DP. As seen from this figure, the materials treated with DP concentrations below 0.10moldm -3 show only the peaks characteristic of CaHAP (patterns

1t. Tanaka et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 125 (1997) 53-62

55

e,,

;

;0

!

1'5

20 20, degree

Fig. 1. XRD patterns of materials modified with DP at different concentrations (mol dm 3): a, 0; b, 0.10; c, 0.15. a and b) and the crystallinity is essentially not changed by the modification. However, the material treated with 0.15 mol dm -3 DP shows three additional peaks at 20=2.95 ° (d=2.99 nm), 5.60 ° (1.58 nm) and 8.30 ° (1.06 nm) as seen in pattern c. In the modification with HP, such additional peaks did not appear when C a H A P was treated at an H P concentration of less than 0.10 mol dm -3. As reported previously, the C a H A P modified with ethyl phosphate in a mixture of acetone and water shows X R D peaks at 2 0 = 6.3 °, 25.4 ° and 25.6 ° in addition to the peaks due to CaHAP, indicating that the C a H A P structure is modified by a layered structure [9]. Therefore, three additional peaks in pattern c would be regarded as the peaks due to a layered structure, of which the detailed structure is reported in our other paper [12]. The transmission electron microscopy ( T E M ) observation revealed that the particle morphology is not influenced by the modification; the mean particle width of the modified particles is nearly equal to that of the unmodified particles (Table 1). The BET N2 specific surface area is also not changed by the modification (Table 1). It is inferred, therefore, that at a low concentrations of less than 0.10 mol dm 3 H P or less than 0.15 tool d m - 3 DP only the particle surface is modified. Because this study was intended to modify only the surface

of CaHAP, the materials treated with 0.02 and 0 . 0 7 5 m o l d m 3 H P and with 0.02 and 0.10 mol d m - 3 DP were investigated in detail. To ascertain the degree of modification, the carbon contents of the modified samples were determined. The populations of alkyl groups per unit area of the particle surface, evaluated from the carbon content and the BET N z specific surface area, are listed in Table 1. As seen in this table, the number of alkyl groups per nm 2 (abbreviated as n,) increases with increasing concentrations of H P or DP and na is roughly the same for the modifications with H P and DP. The na value required to completely cover the particle surface amounts 5 groups per nm 2 as estimated from the cross-sectional area (0.2 nm 2) of an n-alkyl group by assuming that the alkyl chains orient perpendicularly to the surface in closest packing. The na values given in Table 1 corresponds to 20-40% of five groups per nm 2, which means that the surface is not fully occupied by alkyl groups. The thermal stability of the surface alkyl groups on C a H A P as examined by outgassing the modified materials at 100 400°C for 2 h. Fig. 2 shows na as a function of outgassing temperature for the samples treated with 0 . 0 7 5 m o l d m 3 H P and 0.10 m o l d m -3 DP. U p o n raising the outgassing temperature, na decreases and is extremely low

H. Tanaka et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 125 (1997) 53-62

56

Table 1 Specific surface areas, particle sizes and chemical compositions of unmodified calcium hydroxyapatite and samples modified with hexyl and decyl phosphates Sample

SN m~g- 1

Particle width nm

Ca/P

C wt%

Surface alkyl groups per nm z

Mass loss wt% 200 300°C

700-800°C

Original

105

16

1.60

0

0

--

0.21

0.020 mol dm 3 HP 0.075 mol dm 3 HP

101 101

15 15

1.54 1.52

1.8 2.2

1.5 1.8

2.7 3.5

0.72 0.79

0.020 mol dm -3 DP 0.100 mol dm -3 DP

100 103

15 15

1.52 1.47

2.2 4.2

1.1 2.2

2.2 5.1

0.58 0.80

t'q

1.5 ' .......

~D

--

a

~N.....,...%.....,. i""-....,

o

~

0.5

'

"

~

"

•

'

'

.

.

,

,

.

.

.

~

....~

b

. ....,,........

b~ e

o

loo

2OO

3OO

4OO ~

outgassing temperature, oC Fig. 2. Number of alkyl groups (n~) of materials modified with 0.075 mol dm -3 HP ([2) and 0.10 mol dm -3 DP (©) vs. outgassing temperature.

above 300°C, showing that most of the alkyl groups are removed by outgassing above 300°C. Note that the materials modified with H P and DP exhibit the same na values and the same thermal stability, suggesting that the modification mechanism is analogous for both the phosphates.

3.2. Thermal analysis of modified CaHAP The state of alkyl groups was investigated by thermal the T G and DTA curves of without the modification in

on the C a H A P surface analysis. Fig. 3 depicts the materials with and an air stream. The T G

'....................................................

I

"....,....,.. ..... "'",......,.

""'...d" e

5wt% I

I

I

I

2OO

40O

600

800

•,... ,, ' ' ,,. e" I

1000

temperature, oC Fig. 3. T G - D T A curves of materials unmodified and modified with HP and DP. The solid lines are TG curves of materials unmodified (a) and modified with 0.02mol dm 3 HP (b), 0.075moldm -3 HP (c), 0 . 0 2 m o l d m -3 DP (d) and 0 . 1 0 m o l d m 3 DP (e). The dotted lines are DTA curves of materials unmodified (a') and modified with 0.02 mol dm-3 HP (b'), 0 . 0 7 5 m o l d m -3 HP (c'), 0 . 0 2 m o l d m -3 DP (d') and 0.10 mol dm -3 DP (e').

curve a of the unmodified C a H A P shows a monotonous mass loss from r o o m temperature to about 1000°C, which is caused by release of adsorbed

H. Tanaka et al. / Colloids SurJhces A: Physicochem. Eng. Aspects 125 (1997) 53 62

and/or bound H 2 0 [13]. T G curves, b, c, d and e of the HP- and DP-modified materials show a mass loss at 200-300°C while the DP-modified materials exhibit a mass loss at a somewhat higher temperature compared to the HP-modified ones. These mass losses accompany a DTA exothermic peak with a small shoulder as found in curves b', c', d' and e'. As shown in Table 1 the mass loss at 200-300°C increases with an increase in the carbon contents of the modified samples. Therefore, these mass losses and exothermic peaks can be ascribed to combustion of the surface alkyl groups of the modified materials. To confirm this assignment we measured T G and DTA curves in an N2 stream. The DTA curves thus obtained showed no exothermic peak, while the T G curves showed a mass loss at the same temperature as the curves taken in an air stream. In general, desorption or removal of substances is an endothermic process. Accordingly, the fact that release of alkyl groups without combustion shows no endothermic peak may be because enthalpy change by this process is too small to be detected by the present equipment with a sensitivity of 3 gV. Indeed, no DTA endothermic peak was detected in removal of alkyl groups from alkoxylated silica in an N2 stream using the same equipment. It has been reported that thermal decomposition of surface alkyl groups of silica in the absence of 02 takes place by a mechanism whereby the SiO-C bond is broken and a Si OH bond is generated yielding alkene [14]. Analogously to this, thermal decomposition of surface alkyl groups on CaHAP in a n N 2 stream would proceed in the following reactions (for instance the HP-modified material) P-O-C6H13 (surface)-*P-O-H (surface) + C H z = C H - C 4 H 9 (gas)

(1)

This mechanism is also supported by F T I R measurements as described later. As seen in Fig. 3, the area of the exothermic peak of the DP-modified materials was less than that of the HP-modified materials; this cannot be interpreted as being due to the difference of carbon number between hexyl and decyl groups. Furthermore, comparing DTA curves d' and e', the sample with a higher carbon content shows a smaller exothermic peak. These contradictions in the DTA curves of the

57

DP-modified materials may be due to complex thermal decomposition, though the detailed mechanism remains unexplained for the moment. It has been proposed that the mass loss at 600-800°C is due to reaction of HPO 2- ions in nonstoichiometric CaHAP 2HPO4z --*PzO4- + H 2 0 ( 2 0 0 - 6 0 0 ° C )

(2)

P20 4- + 2 O H - ~ 2 P O 3 + H z O ( 6 0 0 - 8 0 0 ° C ) (3) [10,15,16] Because reaction (2) occurs in a wide temperature range, the mass loss is not clearly recognized in Fig. 3. Table 1 lists the mass loss at 700-800°C estimated by magnifying the T G curves of Fig. 3. The mass loss increases with an increase in the carbon number of alkyl groups, and the HP- and DP-modified materials show similar mass loss. Moreover, the mass loss of the modified samples occurs at a lower temperature than that of unmodified samples; therefore, we should consider the thermal decomposition mechanisms to be different. The T G and DTA results suggest that the surface modification of CaHAP and the thermal decomposition on the surface of the modified CaHAP take place by reactions (4), (5), (6) and (7) shown in Scheme 1. R and R' in reactions (4) and (5) represent alkyl groups. Reaction (4) is the reaction of the CaHAP surface with HP or DP; the alkyl phosphates dissociate by giving H + ion to neighboring HPO ] ion and the H2PO4 ions formed coordinate with C a 2+ ion. Nakatsuka et al. [17] have modified CaCO3 particles by alkyl dihydrogenphosphates in aqueous systems and propose the formation of ROPO3CaHCO3 as a modification mechanism. This may be a similar mechanism to reaction (4), although in reaction (4) the ionexchange of alkyl phosphate ions with phosphate ions is not considered. The surface phosphate ions are presumed to increase with reaction (4). Nevertheless, the Ca/P molar ratios, obtained using the amount of phosphate ions determined by the molybdenum blue method, were not changed by the modification. This is because the alkyl phosphate ions can not be assayed by this method. Hence, more precise Ca/P ratios were evaluated using the total amount of phosphate ions; this value was obtained by adding the amount of alkyl phosphate ions estimated from the carbon

58

H. Tanaka et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 125 (1997) 53-62

O OH RO\ /OH ~ P ( o + HO/P

-o ."'~ $" ~

_

%o

HO /OH " /~P o ~ No-

RON /OH /P'~ ,oo "~ I;" Ca2+

Ca2+

HO,N, /OH

RO OH HO\ /OH \ o / 200~300oc

HO\

/

(4)

OH -t- CH2=R'

o//P\o: ..o/'XXo --- o//P\o:, o/%o

HO-- O\ / 300-600°C-O\ / O \ /O- 2H20 o,,~F ~ \OH + HO/p~o , o//P\o/p% +

(5)

- 0 \ /OH

-o N / o N / o-

-o\ /o-

o~ \o /

o//PXo -

..~p

pNN, + 4OH-70o-80o°c ~ '%0

+

(6)

-o\ /o-

p

-o/

4- 2H20

(7)

~o

Scheme1. content to that of the phosphate ions determined by the molybdenum blue method. The values are given in Table 1 and decrease with modification of CaHAP; i.e. the total number of alkyl phosphate and phosphate ions increases with modification of CaHAP, implying that the alkyl phosphate ions attach to the CaHAP surface without exchange with the surface phosphate ions. Upon outgassing of the modified samples at 200-300°C, H2PO4 ions are generated by release of alkyl groups from RHPO4 ions via reaction (5). The H2PO4 ions formed react with neighboring H2PO4 ions to yield P2062- ions and H20 molecules at 300-600°C (reaction 6). Furthermore, these P2O2- ions are hydrolyzed evolving H20 at 700-800°C (reaction 7). The mass loss during reaction (7) is estimated to be twice that of the unmodified material during reaction (3). As seen in Table 1, the mass loss at 700-800°C of the modified materials is 3-4 times

that of the unmodified material. This may be attributed to the increase of surface HzPO 4 ions due to the modification as speculated from reactions (4) and (5). However, there is no experimental evidence for the formation of P2062- ions in reaction (6) for the moment. Further experiments are required to verify this. 3.3. In vacuo I R spectra of modified CaHAP

For the surface characterization of the modified materials we took IR spectra of the modified materials in vacuo. The spectra of the materials untreated and treated with 0.10 mol dm-3 DP are shown in Fig. 4. Spectrum a of the unmodified CaHAP outgassed at 300°C possesses a strong sharp band at 3572 cm-1 due to O H - ions in the crystal sites, and a weak band at 3655 cm-1. The latter weak band consists of the bands at 3680,

H. Tanaka et al. /Colloids Surfaces A. Physicochem. Eng. Aspects 125 (1997) 53-62

59

ee-~

4000

3500

3000

3750

3700

3650

3600

w a v e n u m b e r , c m -1 Fig. 4. IR spectra in vacuo of unmodified materials outgassed at 300°C (a and a') and materials modified with 0.10 mol dm -3 DP and outgassed at 200°C (b and b') and 300°C (c and c').

3668 and 3655 cm -1 as shown in the expanded spectrum a'. These three bands have been assigned to the stretching modes of surface P-OH groups of CaHAP by H-D exchange and adsorption of various molecules and ions [ 13]. The surface P-OH groups are generated by protonation of the surface PO]- ions of CaHAP to balance the surface charge. Spectrum b of the modified material outgassed at 200°C has a very weak surface P-OH band at 3655 cm-1, CH bands at 2960, 2930 and 2855 cm-1, and a very broad band due to strongly adsorbed H20 centered at 3200 cm -1. The CH bands almost disappear upon outgassing above 300°C (spectrum c). This proves that alkyl groups of the modified material are removed at 300°C, well in accordance with the change in the carbon content with outgassing temperature shown in Fig. 2. The surface P-OH bands of spectrum b' are extremely weak because of the perturbation of

the surface P-OH groups by strongly adsorbed H20 and/or surface alkyl groups. It is noteworthy that the surface P-OH bands of spectrum c' of the modified material outgassed at 300°C are stronger than those of spectrum a' of the unmodified material outgassed at the same temperature, and that the wavenumbers of the bands for the modified material differ from those of unmodified CaHAP. These results indicate that the modification increases and number of surface P-OH groups. This seems reasonable taking into account the mechanisms involved in the surface modification (reaction 4) and the release of alkyl groups (reaction 5).

3.4. Surface characteristics of modified CaHAP Because the modified particles were found to have surface alkyl groups as described above, the

60

H. Tanaka et a L / Colloids Surfaces A: Physicochem. Eng. Aspects 125 (1997) 53 62

surface hydrophobicity of the modified materials was evaluated from the preferential dispersion of the particles in water and hexane. The particles were dispersed in a mixture of water and hexane by shaking rigorously and were allowed to stand for the phase separation. The modified particles dispersed in hexane phase while the unmodified particles dispersed in water phase, showing that the particles are made hydrophobic by the modification. Furthermore, to certify their hydrophobicity we measured the adsorption of H 2 0 o n the modified materials. Fig. 5 displays the H 2 0 adsorption isotherms on the unmodified and the DP-modified material outgassed at various temperatures from 100 to 400°C. It is obvious from the isotherms shown in Figs. 5(b) and (d) that the amount of adsorbed H20 is decreased by the

modification, especially for the samples outgassed at 100°C as shown by the square symbols. We have reported that CaHAP adsorbs H 2 0 irreversibly via a strong hydration to surface Ca 2+ ions, and that the adsorbed H 2 0 is desorbed by evacuation above 300°C [13]. As stated before, the samples outgassed at 100°C have surface alkyl groups, so that these groups interfere with H20 adsorption, in good agreement with the result of the preferred dispersion. However, the modified materials outgassed above 300°C having no surface alkyl group adsorb less H 2 0 compared with the unmodified materials, as can be seen from the isotherms shown by the circular and triangular symbols in Fig. 5. This can be interpreted by assuming that the surface Ca 2+ ions which are the strong adsorption sites for H 2 0 a r e covered by

40

40 0

(a)

30

c~ 30

20

20

10

10

(c) % %

t'N

E

o

E

i

I

I

I

0.2

0.4

0.6

0.8

.,,¢'x(7~~ ' d ' ^ 0 O 0 X ( ~ ' : ' - O° o o O v - n O u _ ~ O O D [] ~ O O uu

011

0

0.2

p/pO

d

i

I

I

0.4

0.6

0.8

8 &

,.Q

e.t..\

^ c~

6

n

o

/ g *°* [][]°

D

0

o

(d)

o

0 &

~oAO&OA

n

n

0 #,0 A

[]

c ~ o o o ° [] ~00¢0(~0¢

¢ ¢

¢

0

[] D

0

0

[]

r

0

0

i

I

I

I

0.02

0.04

0.06

0.08

0.1

D

mmma~:rno°

0 0

0.02

Q

9

,

,

0.04

0.06

0.08

0.1

p/pO Fig. 5.H2•ads•rpti•nis•therms•nmateria•sunm•dified((a)and(b))andm•difiedwith•.••m••dm temperatures: [], 100°C; ~ , 200°C; ©, 300°C; A, 400°C.

3DP((c)and(d)).Outgassing

H. Tanaka et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 125 (1997) 53-62

the HzPO4 ions generated by reactions (5) and (6). This finding supports, together with the F T I R results (Fig. 4), the idea that the surface of the modified materials does not return to the original state even after removal of the surface alkyl groups. The interaction between C O 2 and CaHAP is an important factor for denaturation of human tooth and bone by CO32- [18] and formation of longlifetime radicals such as CO , CO 2 and CO 3on the calcified tissues [19]. To examine the interaction of the modified C a H A P with CO2, we

61

0 0

"~

0

-1

0 ~' o

-2

O O

"~ .~ ~ -3

O

LX

©

O A

E A I

-4 4

(a)

6

' 8

'

10

A

12

pH

6

4

Fig. 7. Electrophoretic mobility of unmodified and modified particles vs. pH of suspensions: I , unmodified; O, modified with 0 . 1 0 m o l d m 3 DP; ± , modified with 0 . 1 0 m o l d m -3 DP and outgassed at 300°C.

.j

2

E &

30

I

I

I

I

200

400

600

800

1000

0

.8

(b)

2.5

O

O O O 1.5

O O

A

A

A

A

A

0.5 • 0~

0

•

J

i

i

i

20

40

60

80

100

p, ton" Fig. 6. CO2 adsorption isotherms on materials unmodified (open symbols) and modified with 0.10mol d m -3 DP (filled symbols). Outgassing temperatures: A, A , 200°C; ©, 0 , 300°C.

measured the adsorption isotherms of CO2 on the modified materials at 25°C. Fig. 6 compares the adsorption isotherms of CO/ on the unmodified and DP-modified materials outgassed at 200 and 300°C for 2 h. The amount of CO2 adsorbed decreases with the modification and the modified material outgassed at 300°C adsorbs less CO2 than the unmodified material outgassed at the same temperature, though alkyl groups of the modified materials are almost completely removed. These results are the same for the adsorption of H 2 0 . We have reported that CaHAP chemisorbs COz on surface Ca 2 ÷ ions and less acidic surface P - O H groups [20]. Because the adsorption of CO2 at a lower pressure was markedly reduced by the modification with DP as shown in Fig. 6, the strong chemisorption sites appear to decrease upon modification. The results described above demonstrates that the type and population of surface P - O H groups of C a H A P are changed by modification with HP and DP. For further confirmation, the dissociation of these surface P - O H groups of the modified materials in aqueous medium was examined by electrophoresis. In Fig. 7 the electrophoretic mobilities of the particles are plotted against the pH of

62

IL Tanaka et al. / Colloids Surfaces' A: Physicochem. Eng. Aspects 125 (1997) 53-62

the suspensions, showing that the particles with and without the modification are negatively charged and the mobilities o f the particles decrease with an increase in pH. This means that the surface charge originates mainly f r o m the dissociation o f surface P - O H groups [21]. The modified particles outgassed at 300°C (triangular symbols) are clearly more negatively charged than the unmodified particles. F r o m the modification process (reaction 4), the concentration o f surface P - O H groups is supposed to increase with the modification. Thus, the modified particles should be more negatively charged than the unmodified ones. However, the mobilities o f the modified particles having alkyl groups are almost the same as those o f the unmodified particles. This can be explained by assuming that long alkyl groups attached to the particle surface shift the slipping plane to reduce the zeta potential.

4. Conclusions It was possible to modify the surface o f C a H A P by treatment with acetone solutions o f H P o f less than 0 . 1 0 m o l d m -3 and D P o f less than 0.15 mol d m - 3 without altering its bulk structure. The modified C a H A P s exhibited hydrophobicity as confirmed by the preferential dispersion and the adsorption o f H 2 0 . The surface alkyl groups were removed u p o n outgassing above 300°C, producing more surface P - O H groups than in the original C a H A P as shown by F T I R and electrophoresis. A mechanism o f surface modification is proposed based on the results obtained.

Acknowledgment The authors are grateful to Mr. M a s a o F u k u s u m i o f O s a k a Municipal Technical Research Institute for his help with T E M measurements.

This study was supported in part by the N i p p o n Sheet Glass F o u n d a t i o n for Materials Science and Technology and the Grants-in-Aid for Scientific Research (B and C ) of the Ministry o f Education, Science, Sports, and Culture, in Japan.

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