Indian Journal of chemistryVol. 31A. April 1992. pp. 276-278

Fluoridation of OH-apatite on interactionwith sodium monofluoro phosphate

G CMaiti

Physical Research Wing, Projects & Development India Ltd.Sindri 828 122, India

Received 8 July 1991; revised 15 October 1991;rerevised and accepted 15 January 1992

Ol-l-apatite when reacted with Na2P03F in solid statetransforms into a mixed apatite Ca,(OH).-xFx(P04lJ.The Ca-phosphate frame work of the apatite lattice re-mains unaffected during the fluoridation process. Theevolved HF molecule on hydrolysis of Na2P03F acts asfluoridating agent in the transformation of Ol-I-apatiteinto F-apatite lattice.

The resistance of dental enamel which consists.mainJy of Of-I-apatite can be improved against ca-ries lesion by surface fluoridation 1,2. Several fluori-dating agents have been considered for topical ap-plication''". Sodium monofluoro phosphate(Na2P03F abbreviated MFP), has also been used ascariostic inhibitor+".

In general, the formation of fluoro-apatite ormixed apatite is the desired goal of the fluoride en-amel interaction. The nature of solid state interac-tion between Ca5(OH)(P0-lh and Na2P03F duringtopical application is still disputed'"!". Obviously,the sluggishness of the fluoridation reaction at bio-logically relevant temperatures and the low depthpenetration of the F- into the dental enamel, whichis virtually Ol-I-apatite, represent serious obstaclesto elucidating the nature of interaction. An alterna-tive approach would be to speed up the reactionkinetically by using elevated or even rather hightemperature and to bring it to near completion.

In the present communication we report the re-sults of a study aimed at elucidating the nature of thefluoridation of Ol-l-apatitc by MFP using XRD andinfrared (IR) absorption spectroscopy as main toolsof investigation. IR spectroscopy is particularly suit-ed because incipient fluoridation of the Ol-l-apatitecan readily be detected through va - H mode usingthe OH - as a local probe which sensitively reacts tothe presence of F-. In Oll-apatite structure, OH-ions form one-dimensional chains parallel to thec-axis and they are replaceable by similar anionicspecies, specially by F- ions 11.12.

ExperimentalApatite powder samples were prepared by solid

state. reaction between Ca[HP041·2H20 andCa[CH3COOlz (Merck chemicals). The chemicalswere thoroughly mixed as acetone slurries by me-chanical stirring for 1 hr. The mixture was dried,peletised and heated in an open platinum crucible to1000°C for 10 hr in a stream of decarbonated moistair(PH,Q = 4 torr). The samples were cooled slowly,crushed and powdered in an agate ball mill. Theheating and homogenisation cycles were repeatedthree times in moist air in order to complete thereaction. X-ray diffraction pattern indicated that theproduct consisted of a well-crystallized single phaseof Ol-I-apatite. The purity of the samples was alsoevident from the IR spectroscopic measurements.

Polycrystalline hydroxyapatite, CasOH(PO 4 bpowder was thoroughly mixed dry with 5 wt %, 10wt % and 25 wt % Na2P03F, corresponding to 15;30 and 75 mol % respectively, in an agate ball millfor 1 hr. The samples were designated as A, Band Crespectively. The Na2P03F sample was supplied byMis Deiersdorf Ag, Hamburg. The mechanical mix-tures were heated separately in an open Pt cruciblefor 50 hr at different temperatures. After each heattreatment, the samples were analyzed by IR spec-troscopic and X-ray diffraction methods.

Infrared and Xrray analysisThe IR spectra were recorded on a Perkin-Elmer

225 spectrophotometer for which samples (2 mg)were admixed with KBr (300 mg) in an agate mortarand pelletized in an evacuated 13 mm ~ die. The ap-atite samples were dried for at least 10 hr at 110°Cprior to preparing the KBr pellets. The crystal phasecompositions of the samples and the reaction pro-ducts were determined by taking X-ray diffractionphotographs using a double-radius (229-2 mm)Guinier Camera and monochromatic CuKa 1 radia-tion and Ag as an internal standard. () values werecorrected by comparision with Ag diffraction lines.The unit cell parameters were determined by leastsquares refinement from corrected () values using asuitable computer program.

Results and discussionThere are three spectral regions of interest: (i) the

region of 0 - H stretching vibration at 3500-3600ern - I, (ii) the region of P - ° and P - F stretchingvibrations at 900-1300 cm - 1, (iii) the region ofP- ° and P- F deformation modes at 400-800ern - I which also comprises the °-H deformationvibration at 631 cm - 1. There is no IR-spectroscopic

indication of a solid state reaction at room tempera-ture upon milling together of OH-apatite and MFPcomparable to the reaction between OH-apatite andNH~F, as spectral bands remain nearly unaffectedupto I500e (ref. 13). However, if the intimate me-chanical mixture of OH-apatite and MFP is heatedin dry air for an extended period of time, typically50 hr at above 2000e, characteristic changes occurin the IR spectra.

Fig. 1 depicts the changes in the region of theo - H stretching vibration of sample B containing10 wt %, corresponding to 30 mol % MFP. Afterheating to 2500e a small but distinct bandappearsat 3540 cm - I which suggests incorporation of F-ion in OH- chain of apatite lattice along c-axis".Upon further heating, this band increases in intens-ity while the sharp 3573 ern -I band, indicative ofpure OH-apatite, decreases. Eventually, at 8000e,the band predominates at about 3535 ern -I with asmall remainder of the 3573 em - 1band. This is thetypical IR spectrum of a partially fluorinated OH-apatite ealO(OHI-xFx)z(P04)6 with x_0.511,14.15.Thus, the degree of fluoridation attained at 8000e,as detected by IR spectroscopy, seems to be quitehigh.

Fig. 2 shows the IR spectrum of sample B in theregion of the P- 0, P- F stretching and P- 0,o - H deformation vibrations. The most instructiveregion lies between 600 and 800 em - I,because it isthe range of 0 - H deformation mode of pure OH-apatite at 631 em -I which is clearly separated fromthe v4 modes of the P04 tetrahedra at 601 and 575em - I. The OR deformation band decreases in in-tensity with increase in F- ions substitution; at thesame time, a new band at 745 em - 1 appears in in-

15o'C

ZQt- soO·c·0..

Q:0III

&oo·cdl<t

I , • , , • , I I ! I ,

4000 ~500 ;3000 C7I!.-J

Fig. l-IR spectra in the 0 - H stretching region of sample B (10wt %) and MFP, heated to different temperatures.

NOTES 277

frared spectra II. 12. Fig. 2 shows that the intensity ofthe IR band at 631 em -] is diminished markedly at5000e, and it almost disappears at 800°e. A newband at 745 cm - 1 becomes distinct above 500°e.The IR spectral change can be assigned to the par-tial fluoridation of Ofl-apatite lattice by thc substi-tution of F- ions in the OH-anionic chain along thec-axis. Unfortunately, the three medium strongbands of Na2P03F at 715,730 and 755 cm-I (Fig.2) fall in the same spectral region. However, as seenfrom Fig. 2 these bands rapidly disappear upon heat-ing, which marks the breakdown of Na2P03F mole-cules. Similar spectral changes have also been no-ticed in the case of sample e, where the original0- H deformation band at 631 cm -I disappearscompletely at 6000e (Fig. 2). The results can be ex-plained in terms of nearly complete fluoridation ofOH-apatite lattice. The band at 745 cm-I has beenassigned to the presence of a few residual OH - ionsstill present in the midst of F- ions in the anionchain of apatite lattice, which are very difficult to re-place further due to their strong hydrogen bondingwith neighbouring F- ions 11.12.

Further noteworthy features of Fig. 2 are thesmall bands or shoulders at 528 and 895 cm-Iwhich appear in the spectra of the samples heatedbetween 250° and 600°e. They may be assigned tothe v4 mode of a (HP04)2-, thereby suggesting the

,,,,,,\

\

'1" ,I l ";t' v : -, -»

" '••' \ ,;,' 6 ooc. (C)I , I

I \ II \ II \,, \,, "

"j 1 :f' ::;Q~~)I \ ,

,~: \ 1 I

/ .. ::~:I ••I,

I,I

••• I

\_,

zot-o,Q:oI/)<D<{

RT (8)

,,\\\,

\ ,.. ,\ ,\ ,, '\ ,, '\ I\ ,, ,, I

'.

.r " MI=PI

I,,I,,

/,,,I,

I I

", ,, ,, ,"~

J500 1000

Fig. 2-lnfrared spectra in the region of (P- O. P- F) stretchingand OH deformation modes at different temperatures C

for sample B. sample C and MFP.

278 INDIAN J CHEM, SEe. A, APRIL 1992

formation of an acid phosphate as an intermediatereaction product. In order to test this point, pureNa2POJF was heated in moist air. Above 250°C, hy-drolysis indeed occurred which was confirmed byXRD analysis. This is also confirmed by Fig. 2showing the IR spectra of sample C containing 75mol % of MFP. The formation of the acid phosphateis obviously enhanced compared to that in sampleB. It also demonstrates that at 600°C no unreactedOH-apatite remains in the case of sample C.

The IR spectral changes can be explained, at leastat the elevated temperatures used in this study, interms of the formation of an acid phosphate as anintermediate during the reaction of MFP with OH-apatite. The hydrolysis of Na2P03F under ambientcondition via the interaction of adsorbed water mol-cules causes the evolution of MF molecules. Thefluoridation of Ol-l-apatite can be related to the in-teraction of evolved HF molecules.

NaZP03F + H20 - Na2HP04H20 + HF

Ca5(P04hOH + xHF-Ca(P04hOHI-xFx + xH20

The evolved HF is reactive enough to cause replace-ment of OH- ion by F- ions in the anionic chain.Consequently, a mixed apatite lattice is built up.XRD results also indicate the formation ofNazHP04 at the intermediate stage of heating of themechanical mixture (Fig. 3). However, at and above600°C, XRD patterns indicate that Na2HPO~ istransformed into more stable Na4P207• However,the main residual product in the case of sample B isa mixed apatite (F, 0 - H apatite) and it is nearly apure F-apatite in the case of sample C. The forma-tion of mixed apatite and fluoroapatite in the case ofsample B and sample C respectively has been con-

Scmpje C 01 BOO°c

"~50 30

26Fig. 3- XRD patterns of sample C at different temperatures.

firmed by the measured lattice parameter values.The lattice parameter values in the case of the sam-ple Bare: ao = 9.391 A(4); Co = 6.878 A(3) whereasin the case of sample C ao = 9.375 A (3);Co = 6.876 A (2). The pure fluoroapatite lattice haslattice parameter values ao = 9.368 A, Co = 6.879 A,which are nearly similar to the measured values forthe sample C. However, the fluoridation remains in-complete in the case of sample B, which is still lowerin the case of sample A as the amount of Na2P03F isinsufficient. The results show that fluoridation ofOH-apatite in bulk can occur only above 500°C.But as the hydrolysis of Na2P03F is the first step inthis solid state reaction liberating HF, the surface le-vel fluoridation even at lower temperature may takeplace with the reactive HF molecules.

Another point worth mentioning is that the rigidframework of the apatite structure appears to be leftunperturbed by the fluoridation reaction. This is insharp contrast to the fluoridation reaction occurringbetween OH-apatite and NH4F reported in our ear-lier communication" where Ca2+ ions are removedfrom the structure to form a CaF2 overlayer.

According to XRD and infrared spectroscopicevidences, the interaction between Na2P03F andCas(OH)(P04h does not occur favourably below200°C as NaZP03F is comparatively stable com-pared to other fluoridating agents. The direct substi-tution of PO,f2 - into Ol-l-apatite lattice in the placeof HPO~ - seems to be unfavourable, as crystallineOH-apatite does not contain any noticeable amountof HPO~- ions it its lattice. So the fluoridating canoccur only via the hydrolysis of Na2P03F, whichseems to occur readily only above 200°C. Bulk flu-oridation has been noticed above 500°C in the pres-ence of excess NazPQ3F.

15

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