Surface modification of calcium fluoro and hydroxyapatite by 1-octylphosphonic dichloride

Applied Surface Science 257 (2011) 9002–9007

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Surface modification of calcium fluoro and hydroxyapatite by 1-octylphosphonic dichloride Abdallah Aissa, Hassen Agougui, Mongi Debbabi ∗ Laboratoire de Physico-Chimie des Matériaux, Faculté des Sciences de Monastir, 5019 Monastir, Tunisia

a r t i c l e

i n f o

Article history: Received 27 November 2010 Received in revised form 13 May 2011 Accepted 18 May 2011 Available online 26 May 2011 Keywords: Hydroxyapatite Fluorapatite Surface reactivity 1-Octylphosphonic dichloride

a b s t r a c t The reactivity of the surface of calcium hydroxyapatite (CaHAp) and fluorapatite (CaFAp) was tested and compared by grafting the 1-octylphosphonic dichloride (C8 H17 OPCl2 ) using a molar ratio x = 2 or 4, x = n(organic)/n(apatite). Successful synthesis was confirmed by different characterisation techniques such as X-ray powder diffraction patterns, IR spectroscopy, MAS-NMR (1 H and 31 P) and chemical analysis. The difference between their specific surface area (SSA: 57.46 for HAp and 12.09 m2 /g for FAp), the percentage of carbon measured after treatment with (C8 H17 OPCl2 ) and the intensities of IR bands attributed to the grafted moiety suggests that the surface of hydroxyapatite is more reactive than that of fluorapatite. The 31 P CP-MAS-NMR spectra of treated fluorapatite show a significant change in isotropic signal due to the protonation and deprotonation of superficial phosphate group. This can be explained by the difference in the nature of inorganic material. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The apatites are present in abundance in the natural medium and are the main constituent of the teeth and bones. They are exploited mainly as source of phosphorus for the industry of fertilizers [1,2]. In addition, they have other applications such as cement, dental implant, ion exchange, chromatography and catalysis [3–5]. In several studies of the surface reactions of apatite [6–8], it has been suggested that the surface holds active sites including calcium hydroxide groups ( CaOH) and phosphate groups ( POH). The hydroxyl group present on the surface of apatite seems to be the cause of the grafting organic molecules. Therefore, several studies concerning the grafting of macromolecules (polymers, amino acids, phosphonates) on the surface apatite were performed [9–11]. The interaction between the apatite surface and organic molecules relates to various surface properties, e.g., surface functional groups, acidity and basicity, surface charge, hydrophilicity, and porosity. Tanaka et al. [12,13] studied also the modification of the surface of the hydroxyapatite following the action of the pyrophosphoric acid and alkylphosphate; hexyl and decyl phosphate in organic solvent. They showed that the apatitic surface can be modified without affecting of their crystalline structure and particle morphology. D’Andrea and Fadeev [14] modified the surface of two different hydroxyapatites by the action of the n-alkyl and n-fluoroalkylphosphonic acid RP(O)(OH)2 (R = n-C8 H17 , n-C18 H37 ,

∗ Corresponding author. Tel.: +216 98 439 692. E-mail address: [email protected] (M. Debbabi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.087

and n-C8 F17 -(CH2 )2 ). This study showed that the concentration of RP(O)(OH)2 used has a big effect on the modification of the hydroxyapatite surface. The creation of (Ps–O–P) bond between Ps–O of the apatitic surface and the RP(O)(OH)2 groups has also been confirmed in this study. The aim of the present study is to compare the surface reactivity of CaHAp and CaFAp by grafting the 1-octylphosphonic dichloride. In the text the 1-octylphosphonic dichloride was designed by OPO. The formation of new hybrid organic–inorganic compound was discussed on the basis of the obtained results.

2. Experimental The synthesis of hydroxy- and fluorapatites CaHAp [15] and CaFAp [16] was carried out by a double decomposition method. A solution containing the metallic cation Ca(NO3 )2 ·4H2 O was added drop wise to a solution of diammonium phosphate + NH4 F (for CaFAp), maintained at boiling temperature, under nitrogen stream. The pH value of the mixture is maintained approximately to 11 by regular additions of small amounts of ammoniac solution. The suspension was filtered after 4 h, after that washing was carried out and the recovered solid product was dried at 100 ◦ C for 24 h. The functionalized hydroxyapatites were obtained by addition of a quantity of organic reagent 1-Octylphosphonic dichloride (C8 H17 POCl2 (OPO), corresponding to a molar ratio n(phosphonate)/n(apatite) = 2 or 4 to a suspension of 0.5 g of CaHAp or CaFAp in 50 mL of dichloromethane and vigorous stirring. The suspension was allowed to stir for 48 h at 25 ◦ C. The

A. Aissa et al. / Applied Surface Science 257 (2011) 9002–9007

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Table 1 Chemical composition (±0.02) of fluoro and hydroxyapatite before and after reaction with 1-octylphosphonic dichloride. Samples

%Ca

%P

%C

SSA (m2 /g)

Grafting densities (molecule/nm2 )

Molecules number C8 H17 P(O)O2 /g (10−18 )

CaHAp =2

37.12 30.68

17.76 19.21

0.38 1.97

57.46 –

– 2.15

– 123.60

=4

28.19

20.84

2.85

3.11

178.75

39.84

18.51

0.17

=2

32.40

19.37

0.96

–

4.99

60.22

=4

30.14

20.19

1.28

–

6.64

80.36

CaFAp n(OPO) n(CaFAp) n(OPO) n(CaFAp)

obtained mixture was filtered, washed three times with 60 mL of dichloromethane, and finally dried at 100 ◦ C for 12 h [17].

3. Characterization techniques The calcium and phosphorus contents were obtained by ICPOES on a Horiba Jobin Yvon Model Activa. The chemical analysis of the carbon has been determined according to the Anne method [18]. The IR spectra were recorded on a Perkin Elmer FT-IR system spectrophotometer as KBr pellets in the 4000–400 cm−1 region. Solid-state 31 P MAS-NMR spectra were recorded at 121.5 MHz on a Bruker spectrometer Advanced 300 (rotor 4 mm, spinning rate 2–12 KHz). X-ray powder diffractograms were obtained at room temperature on a PANalytical X’Pert PRO MPD equipped with copper anticathode tube. The specific surface areas (SSA) measurements were performed by BET-method (adsorptive gas N2 , carrier gas He, heating temperature 150 ◦ C) using sorptometer EMS-53 and KELVIN 1040/1042 (Costech International).

4. Results and discussions 4.1. Elemental analysis The results of the chemical analysis, specific surface area (SSA), grafting densities and the deduced number of alkylphosphonate molecules grafted on the surface of apatite are summarized in Table 1. The Grafting densities (molecule/nm2 ) were calculated using the formula [14]: =

1 6 × 105 (%C) × SSA 1200nc − MW (%C)

where MW is the molecular weight of the grafted group C8 H17 P(O)O2 (Mw = 192 g/mol), nc the number of carbon atoms in the grafted molecule (n = 8), %C the weight carbon percentage given by chemical analysis and SSA the specific surface area of the inorganic support (m2 /g) determined by BET (N2 adsorption). For all modified phases the percentages of total carbon measured increase with the increasing of organic moiety in the starting solution. These obtained values are distinctly superior for CaHAp compared to the CaFAp. This difference is originally due to the nature of apatitic surface. The CaFAp with Ca/P = 1.66 and SSA = 12.09 m2 /g is more stochiometric and has a less porous surface than CaHAp with Ca/P = 1.62 and SSA = 57.46 m2 /g. For all modified samples, the phosphorus measured increases gradually when 1-octylphosphonic dichloride content increases. This is in agreement with obtaining new organic–inorganic hybrid compounds. Fig. 1 shows the higher reactivity of the hydroxyapatite surface deduced from the number of grafted molecules C8 H17 PO(O)2 .

– 12.09

–

–

4.2. IR spectroscopy In all IR spectra of the solids prepared (Fig. 2(a) and (b)), the vibration bands of PO4 3− groups of the apatite structure are detected: (s ) (963 cm−1 ), (ıs ) (477 cm−1 ), (as ) (1040–1095 cm−1 ) and (ıas ) (568–604 cm−1 ). Moreover, characteristic bands of hydroxyl ions are observed towards 3572 cm−1 (s ) and 630 cm−1 (L ) for CaHAp. The two broad bands located at 1450 and 1630 cm−1 were assigned respectively to the carbonate ions and water adsorbed on the surface. The intense bands around 2300 cm−1 was attributed to CO2 molecules adsorbed from atmosphere [19]. After reaction with 1-octylphosphonic dichloride, several bands attributed to the phosphonate group are observed essentially the band at 2850 cm−1 which is due to CH vibration. Other vibration bands were observed in the frequency range of 740–900 cm−1 which are attributed to the vibration P–O–H bonds. The band at 1314 cm−1 was assigned to the vibrations of P O group [20]. All these bands are mainly observed in the case of the CaHAp and their intensities increase with increasing the molar ratio n(OPO)/n(CaHAp). The HPO4 − band observed at 874 cm−1 for pure CaHAp was modified for treated phases. This modification can be due to the protonation or deprotonation of the HPO4 groups. 4.3. X-ray diffraction XRD patterns of the reacted CaHAp and CaFAp compared to the starting apatites reveal that the apatite structure is not modified (see Fig. 3(a) and (b)). However, for CaHAp treated of OPO with a molar ratio n(POP)/n(apatite) = 4, three additional peaks were

200

Molecule C8H17P(O)O2/gx10-18

n(OPO) n(CaHAp) n(OPO) n(CaHAp)

180 160 140 120 100 80 60 40 20 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

n(Phosphonate) n(apatite) Fig. 1. Number of grafted molecules on the apatite surface as function of n(phosphate)/n(apatite) molar ratio; (· · ·CaFAp; —CaHAp).

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A. Aissa et al. / Applied Surface Science 257 (2011) 9002–9007

a

b

c

c

P=O

b

Absorbance (a. u)

Absorbance (a. u)

P-O-H

C-H

b

C-H

a

4000

3500

3000

2000

2500

1500

1000

a

4000

500

3500

3000

2500

2000

1500

1000

500

cm -1

cm -1

Fig. 2. (a): IR spectra of CaHAp: ungrafted (a); n(OPO)/n(CaHAp) = 2 (b) and n(OPO)/n(CaHAp) = 4 (c). (b): IR spectra of CaFAp: ungrafted (a); n(OPO)/n(CaFAp) = 2 (b) and n(OPO)/n(CaFAp) = 4 (c).

a

b

* CaHPO4

Intensity (a. u)

Intensity (a. u)

c

* *

c

b

b

a

10

a

10

20

30

40

50

60

70

80

90

2θ (°)

20

30

40

50

60

70

80

90

2θ (°) Fig. 3. (a): X-ray powder diffraction pattern of CaHAp before and after surface modification by grafting OPO: ungrafted (a); n(OPO)/n(CaHAp) = 2 (b) and n(OPO)/n(CaHAp) = 4 (c). (b): X-ray powder diffraction pattern of CaFAp before and after surface modification by grafting OPO: ungrafted (a); n(OPO)/n(CaFAp) = 2 (b) and n(OPO)/n(CaFAp) = 4 (c).

A. Aissa et al. / Applied Surface Science 257 (2011) 9002–9007

Table 2 Crystal sizes (Dhkl ) evaluated from the width at half maximum intensity of the (0 0 2) and (3 1 0) reflections of pure and grafted apatite.

*

*

19% CaHPO4

Intensity (a. u)

9005

81% CaHAp

Sample

ˇ1/2 (0 0 2) (◦ )

˚ D0 0 2 (A)

ˇ1/2 (3 1 0) (◦ )

˚ D3 1 0 (A)

CaHAp CaHAp-(OPO)2 CaHAp-(OPO)4 CaFAp CaFAp-(OPO)2 CaFAp-(OPO)4

0.190 0.194 0.209 0.141 0.160 0.168

430 420 390 579 510 485

0.422 0.450 0.499 0.214 0.225 0.230

200 188 169 396 376 368

(002) (310)

(100)

*

using the Debye–Scherrer equation [23] and are reported in Table 2:

CaHAp-(OPO)4

*

D= 10

20

30

40

50

60

70

K ˇ1/2 cos 

where  is the wavelength,  is the diffraction angle, K is a fixed constant equal to 0.9 for apatite crystallites and ˇ1/2 line is the width at half maximum of a given reflection. The low decrease of the values of D0 0 2 and D3 1 0 with increasing amount of phosphonate, shows that the crystallite size is slightly affected by the presence of grafted moieties in the (a, b) plane and along the c direction.

2θ (°) Fig. 4. Phases identification of CaHAp-(OPO)4 using X’Pert High Score software.

observed at 2 = 26.52◦ (d = 3.359 nm), 26.75◦ (3.32 nm) and 30.30◦ (2.94 nm). These peaks are assigned to monetite phase (CaHPO4 ), according to reference code 01-075-1520 of the ICDD-PDF22003 database. The semi-quantitative analysis using the software X’Pert High Score [21] gives 81% (CaHAp) and 19% (monetite). The identification of these phases is illustrated in Fig. 4. The new phase was obtained after a partial dissolution of apatite due to the presence of HCl formed during the grafting process [22]. For pure and reacted fluoroapatite a highly crystalline apatite structure was obtained with no secondary phase, in agreement with chemical analysis showing its higher stochiometry. The influence of the grafting rate on the evolution of the crystallite size D can be evaluated from the line broadening of the (3 1 0) and (0 0 2) reflections in the perpendicular and parallel directions to the c lattice axis, respectively. The values of D were calculated

a

4.4. Solid state NMR spectroscopy 4.4.1. 31 P CP-MAS-NMR The 31 P CP/MAS-NMR spectra for untreated apatites show an intense isotropic signal around 2.8 ppm characteristic of PO4 group in apatitic phase [6], whereas the 31 P NMR of 1-octylphosphonic dichloride shows the presence of an isotropic signal at 52.85 ppm [24]. The chemical shifts ıiso of CaHAp and CaFAp before and after reaction with 1-octylphosphonic dichloride are reported in Table 3. The treatment of both different apatites leads to the apparition of a large signal in the range 25–40 ppm (Fig. 5(a) and (b)). The signal was attributed to the modification of the ( CaOH) active site on the apatitic surface by formation of an organometallic bond Ca–O–Porg [25]. The simulation of 31 P NMR spectra using

b

31

P(Ca-O-P)

31

P(Ca-O-P)

POxH c

Intensity (a. u)

Intensity (a. u)

c

POx

P-O-Porg b

b

a a 50

50

40

30

20

10

0

-10

-20

-30

-40

-50

40

30

20

10

0

ppm

-10

-20

-30

-40

-50

ppm Fig. 5. (a): 31 P CP/MAS-NMR spectra of CaHAp before and after surface modification by grafting OPO: ungrafted (a); n(OPO)/n(CaFAp) = 2 (b) and n(OPO)/n(CaFAp) = 4 (c). (b): 31 P CP/MAS-NMR spectra of CaFAp before and after surface modification by grafting OPO: ungrafted (a); n(OPO)/n(CaHAp) = 2 (b) and n(OPO)/n(CaHAp) = 4 (c).

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A. Aissa et al. / Applied Surface Science 257 (2011) 9002–9007

Cl

Ca

O + HCl

O P

Ca

OH

O OH

Ca

Ca10(PO4)6X2

Cl

C8H17

P

OH

C8H17

Ca

P

OH

P

+

P

O Cl

OH

Ca

Ca10(PO4)6X2

Ca Ca P

OH

Ca

Ca10(PO4)6X2

OH OH

Ca

O

C8H17 O + 2 HCl

OH

Scheme 1. Mechanism proposed of grafting OPO on the apatitic surface.

Table 3 Chemical shift ıiso of CaHAp and CaFAp before and after reaction with 1octylphosphonic dichloride. Spectra

ıiso ± 0.1 (ppm)

OPO CaHAp

31

52.85 2.87

n(OPO) n(CaHAp) n(OPO) n(CaHAp)

31 31

P–1 H

0.91; 2.88; 4.41; 30.63

32.01

=4

31

P–1 H

0.98; 2.83; 4.24; 29.25

37.58

31

P

=2

31

P–1 H

−6.31; 0.30; 2.72; 4.48;

26.34

=4

31

P–1 H

0.47; 2.90; 5.38; 31.17

27.00

2.87

a

– –

=2

CaFAp n(OPO) n(CaFAp) n(OPO) n(CaFAp)

P P

%(Ca–O–Porg )

1

H(CH3)

1

H(-CH2)

–

1

15

1

H(H2O)

10

5

0

H(OH-)

-5

-10

-15

ppm

b 1

H(CH3)

H(HPO42-)

1

1

H(-CH2-)

Intensity (a. u)

Domfit program shows that the largest signal characterizing the Ca–O–Porg bond is decomposed into two different signals ca. 26 and 32 ppm (Fig. 6). This result suggests that the organic phosphorus can be bound to one or two calcium atoms of apatitic surface. Scheme 1 shows the mechanism of the reaction between the apatitic surface and the 1-octylphosphonic dichloride (OPO). The preference to Ca–OH site during the formation of new hybrid compound apatite-phosphonate may be related to two different causes. Firstly, to the difference between the number of active sites ( Ca–OH and P–OH) mentioned by several authors [7,8,26]. In their works, the authors have calculated the number of each type using the faces (1 0 0). These faces are predominant for the crystallites of apatitic materials. In most of these studies the number of Ca–OH sites is greater than the P–OH sites. On the

Intensity (a. u)

Sample

1

H(H2O)

15

Fig. 6.

31

P CP/NMR-MAS Simulated spectrum of CaHAp-(OPO)2.

10

5

0

ppm

-5

-10

-15

Fig. 7. (a): 1 H CP/MAS-NMR spectra of CaHAp before and after surface modification by grafting OPO. (b): 1 H CP/MAS-NMR spectra of CaFAp before and after surface modification by grafting OPO.

A. Aissa et al. / Applied Surface Science 257 (2011) 9002–9007

other hand, the preference was related to the nature of the atom Ca or P, therefore the low electronegativity of Ca compared to P (Ca: 1.00 and P: 2.19) [27] facilitates the elimination of proton from the Ca–OH site. For all grafted phases the integral calculation of the new signal using Domfit program shows that surface of hydroxyapatite is more reactive than that of fluorapatite (Table 3). These results confirm the conclusions of chemical analysis and IR spectroscopy. In addition, the treatment of fluoroapatite leads to a significant broadening of the isotropic signal originating from the bulk phosphorus groups PO4 3− that presents three new signals overlapped at isotropic chemical shifts 5.5, 0.6 and −6.3 ppm (Fig. 5(b)). These signals are attributed respectively to the –POx, –POxH groups [6] and to the formation of Ping –O–Porg covalent bond [17]. The index x is used for phosphate ions at the surface of fluorapatite when different numbers of oxygen atoms in the phosphate ions are exposed to the mineral surface. This effect can be explained by the protonation of the fluorapatite surface by the HCl molecules formed during grafting. The intensities of these signals are very weak in the case of hydroxyapatite. According to the works of Laghzizil et al. [28], the fluoroapatite is a better conductor than hydroxyapatite materials. This is related to the nature of the mobile species. In fact, F− ion is more electronegatif than OH− and possesses a higher mobility along the c axis of the apatitic structure. 4.4.2. 1 H CP/MAS-NMR The results of 1 H CP/MAS-NMR are given in Fig. 7(a) and (b). For ungrafted CaHAp, the 1 H CP/MAS NMR spectrum exhibits two signals at 0 and 5.3 ppm attributed respectively to hydroxyl group of the hydroxyapatite and water molecules adsorbed on the surface [29]. The 1 H CP/MAS-NMR spectrum of pure fluorapatite shows only the presence of a signal around 5.5 ppm [6]. For treated hydroxyapatite two new signals at 0.80 and 3.44 ppm are obtained. They are respectively associated to the protons of –CH2 – and –CH3 groups of 1-octylphosphonic dichloride (CH3 –(CH2 )7 –PO(Cl)2 ) (Fig. 7(a)). However, in the case of the fluorapatite, the spectrum displays three new signals at 0.84, 2.83 and 7.10 ppm, associated respectively to the protons of –CH2 –, –CH3 and HPO4 2− groups. The signal at 7.10 ppm showing a significant protonation of fluoroapatite surface, in agreement with 31 P CP/MAS-NMR (Fig. 5(b)). 5. Conclusion In this present study, the obtained results can be summarized as follows. The reaction of 1-octylphosphonic dichloride (OPO) with CaHAp and CaFAp is effective and leads to a grafted organic phosphonate moiety at the apatitic surface. According to 31 P CP/MAS-NMR, the new hybrid compound apatite–phosphonate was obtained by the formation of organometallic bond Ca–O–Porg . All characterization techniques show that the surface of hydroxyapatite is more reactive than that of fluoroapatite, this was related to the difference in Ca/P ratio and specific surface area SSA. During the grafting process, a protonation and deprotonation of apatitic surface was observed essentially for CaFAp, which is probably due to its high conductivity. For CaHAp-(OPO)4 a new phase (monetite) as

9007

detected by powder X-ray diffraction due to the partial dissolution of apatitic surface.

Acknowledgments The authors thank the INRAP (Institut Nationale de Recherche et d’Analyses Physico-chimiques), INRST (Institut Nationale de Recherche Scientifique et Technologique), the Company of Phosphate Gafsa and the University of Monastir (Tunisia).

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