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Precipitation of calcium phosphates from electrolyte solutions. VII. The influence of di- and tricarboxylic acids
C o l l o i d s a n d S u r f a c e s , 1 i ( 1 9 8 4 ) 5~
,~8 Elsevier S c i e n c e Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
55
PRECIPITATION OF CALCIUM PHOSPHATES FROM ELECTROLYTE SOLUTIONS. VII.' THE INFLUENCE OF DI- AND qrRICAIIBOXYLIC A C I D S _ •- • . . -- ~ ~ : , . ~ -. ~. ~ . "
-.
•
-
.
I~]ERKA B R E ~ E V I ( ~ *, A I ~ A S E N D I J A R E V I ~ * * a n d H E L G A F O R F - D I o M I L H O F E R * • L a b o r a t o r y f o r P r e c i p i t a t i o n P r o e e u e ~ " R u d j e r B o ] k o v l ~ " I n s t i t u t e , P.O. B o x I 0 1 6 , 4 1 0 0 1 Z a g r e b (Yugosl~vfa) . . " ~ • *Faculty of Technology, Tuzla (Yugoslavia)
( R e c e i v e d 11 J u l y 1 9 8 3 ; a c c e p t e d in final f o r m 2 J a n u a r y 1 9 8 4 ) ABSTRAGT T h e i n f l u e n c e o f a series o f di- and trtcarbox3rlic acid a n i o n s ( C A A ) o n t h e metastabilits, o f a m o r p h o u s c a l c i u m p h o s p h a t e ( A C P ) (I) a n d o n t h e m o r p h o l o g y o f c a l c i u m hyd r o g e n p h o s p h a t e d i h s , d r a t e ( D C P D ) ( I t ) a t 2 9 8 K a n d Initial p H i 7.4 was investigated. [n s y s t e m l, p r e c i p i t a t i o n w-,s i n i t i a t e d f r o m solutions, o f equttmolar ( 3 - 10 -z tool d m - l ) c o n c e n t r a t i o n s o f calcium c h l o r i d e e n d s o d i u m p h o s p h a t e . Matte, s u c e l n t ¢ , m a l e i e a n d fumart¢ acid a n i o n s h a d n o significant e f f e c t o n t h e m e t a s t a b t l l t y p e r i o d w h i t e t h e el'f e e t l v e n e ~ o f t h e o t h e r G A A increased in t h e s t t c c e ~ i o n : t r a n s a c o n i t i c < tartaric < triearballylie < citric < d i h y d r o x y t a r t a r i c acid anion. I n s y s t e m II. p r e c i p i t a t i o n was Initiated a t initial r e a c t a n t c o n c e n t r a t i o n s o f C a = 5 • 10 -I tool d m "~ a n d P C 4 a I ' 10-z m o l d m -3. T h e resulting solid p h a s e was a n int e r c r y s t a l l i n e m i x t u r e o f c a l c i u m d e f i c i e n t a p a t t t a s ( D A ) a n d DCPD. H a b i t m o d i f i c a t i o n o f D C P D was i n d u c e d b y t h e f o l l o w i n g C A A : tartaric < tricarballyilc < dihyd~0.xy~ tartaric < t r a n s a c o n i t i c < citric < malie acid a n i o n . All t h e s e a n i o n s i n h i b i t e d c r y s t a l g r o w t h in the d i r e c t i o n p e r p e n d i c u l a r t o t h e ( 0 0 1 ) plane, p r o m o t i n g t h e g r o w t h o f needles instead o f platelets. S u c e i n l e , f u m a r | e a n d m a l e l c acid a n i o n s did n o t a f f e c t t h e s h a p e o f D C P D crysials.
INTROD UCTION •T h e p r e c i p i t a t i o n of calcium phosphates is o f g r e a t i n t e r e s t i n i n d u s t r i a l , agricultural and biomedical research. A few examples of industrial application are: the preparation of toothpaste, where calcium hydrogen phos, phate dihydrate (DCPD) crystals are used as an abrasive; production of fertilizers; removal of inorganic phosphate from industrial waste waters, etc. Calcium phosphates are also the main inorganic constituents of human and animal endo- and exoskeletons, teeth and pathological mineral deposits, such as urinary Calculi, ~; ~t-~ .... ~ ~ It has'~been shown [1~4] • that in neutral and alkaline pH range, pre: cipitation of calcium phosphates proceeds via the formation of a metastable amorphous precursor (amorphous calcium phosphate, ACP), The' metasta-
0166-6622/84/~03.00
O 1 9 8 4 Elsevier S c i e n c e Publishers B . V .
bility t i m e o f f~is p r e c u r s o r is v e r y sensitive to t h e c o n d i t i o n s o f precipi t a t i o n ; it c o u ] ~ b e p r o l o n g e d b y d e c r e a s i n g t h e r e a c t a n t c o n c e n t ~ t i o n s [ 5 ] , increasing t h e ionic s t r e n g t h [G, 7 ] , o r a d d i n g s o m e foreign s u b s t a n c e s [6, 7 ] a n d ions [ 7 ] . T h e final c o m p o s i t i o n o f t h e p r e c i p i t a t e is es tab lish ed by s e c o n d a r y p r e c i p i t a t i o n o f a c r y s t a l l i n e p h a s e [2, 3] a n d s u b s e q u e n t e q u i l i b r a t i o n processes [ 3 ] . I f p r e c i p i t a t i o n is i n i t i a t e d at relatively high c a l c i u m a n d p h o s p h a t e co~lcentrations ( s u c h as prevail in h u m a n urines) intercryst~lline m i x t u r e s o f c a l c i u m d e f i c i e n t a p a t i t e s ( D A ) a n d D C P D result [ 8 ] . I t has b e e n s h o w n [9] t h a t t h e m o r p h o l o g y o f D C P D crystals p r e c i p i t a t e d u n d e r t h e s e c o n d i t i o m c a n be significantly a~tered b y t h e a d d i t i o n o f c i t r a t e ions. P r e l i m i n a r y investigations also s h o w [5] t h a t c i t r a t e ions m a y d e l a y t h e c o n v e r s i o n o f ACP. In o r d e r t o ge t a b e t t e r u n d e r s t a n d i n g o f th ese effects, t h e i n h i b i t o r y a c t i v i t y o f a series o f di- a n d t r i c a r b o x y l i c acid a n i o n s ( C A A ) has b e e n t e s t e d . T h e results o f this investigation are r e p o r t e d in t h e p r e s e n t paper. METHODS S t o c k sol ut i o ns w e r e p r e p a r e d b y dissolving t h e r e q u i r e d a m o u n t s o f a n a l y t i c a l g r a d e che~nicals in trldtstilled w a t e r . S o l u t i o n s u s e d fo r sample p r e p a r a t i o n w e r e d i l u t e d f r o m s t o c k s o l u t i o n s a n d aged fo r 2 4 h before usage. S a m p l e s w e r e p r e p a r e d by a d d i n g c a l c i u m c h l o r i d e s o l u t i o n s t o e q u a l v o l u m e s o f s o d i u m p h o s p h a t e so lu tio n s w h i c h w e r e p r e a d j u s t e d to pH 7.4 [8]o T h e r e s ul t i ng s y s t e m s w e r e t h o r o u g h l y m i x e d an d k e p t at 2 9 8 K w i t h o u t f u r t h e r stirring. T h e respective c a r b o x y l i c acid was a d d e d t o t h e p h o s p h a t e s o l u t i o n p r i o r t o p H a d j u s t m e n t . S a m p l e s u s e d as c o n trols c o n t a i n e d t h e s a m e c o n c e n t r a t i o n s o f c a l c i u m c h l o r i d e a n d s o d i u m pl~osphate b u t n o c a r b o x y l i c acid was added. Changes o f p H a n d relative t u r b i d i t y w e r e s i m u Y t m m o u s l y r e c o r d e d as a f u n c t i o n o f t i m e [ 2 ] . A R a d i o m e t e r m o d e l 26 p H m e t e r in c o n n e c t i o n w i t h a H e w l e t t - P a c k a r d Moseley 7 1 0 0 BM r e c o r d e r an d a Zeiss rec o r d i n g t u r b i d i m e t e r ( S p e k o l ) w e r e used. T u r b i d i t i e s w e r e m e a s u r e d a t 6 4 0 n m 8gainst a s u s p e n s i o n o f latex LS-54-2 (Rv00 = 0 . 0 4 0 c m -! a t 5 4 6 nm)o A set o f e x p e r i m e n t s was c o n d u c t e d a t p H 7 . 2 0 ± 0 . 0 2 b y m e a n s o f a R a d i o m e t e r pH - s t a t de vi c e , i n t h e s e e x p e r i m e n t s t h e c a r h o x y l i c avid was a d d e d t o t h e p r e c i p i t a t i o n s y s t e m e i t h e r p r i o r t o o r a f t e r s a m p l e prepa r a t i o n . T h e r e a c t i o n slurry was stirred at a c o n s t a n t r a t e t h r o u g h o u t t h e e x p e r i m e n t a n d t h e c o n s u m p t i o n o f s o d i u m ~hydroxide w a s a u t o m a t i c a l l y recorded. T h e m o r p h o l o g y o f t h e p r e c i p i t a t e s w a s o b s e r v e d u n d e r an i n v e r t e d m i c r o s c o p e (E. Leitz, Wetzlar), an O r t h o p l a n p h o t o g r a p l f i c m i c r o s c o p e (E, Leitz, Wetzlar) a n d an e | e c t r o n m i c r o s c o p e ( E ] m i s c o p [, Siemens). T h e sign o f t h e particle c h a r g e a n d t h e e l e c t r o p h o r e t i c m o b i l i t y w e r e determined by microelectrophoresis.
57
l l t s p e c t r a w e r e t a k e n f r o m K B r pellets. P r e c i p i t a t e s w e r e w a s h e d w i t h supernatants o f the respective control systems. C h e m i c a l a n d i n s t r u m e n t a l analysis o f t h e p r e c i p i t a t e s w e r e c a r r i e d o u t as p r e v i o u s l y d e s c r i b e d [ 8 ] . R E SU L'rs T h e i n f l u e n c e o f succiniv, malic, tartaric, d i h y d r o x y t a r t a r i c , f u m a r i c , m a l e i c , t r a n s a c o n i t i c , citric a n d tricarballylic acid a n i o n s o n t h e precipit a t i o n o f c a l c i u m p h o s p h a t e s was investigated, T ab le 1 s h o w s t h e i r f o r m u l a s w i t h t h e c o r r e s p o n d i n g association c o n s t a n t s a n d C a - c o m p l e x e q u i l i b r i u m c o n s t a n t s [10]. T h e c o n c e n t r a t i o n s o f t h e additives varied f r o m 1 • 10 -4 t o 1 • 10 -3 tool d m -~. All investigations have b e e n c a r r i e d o u t o n t w o d i f f e r e n t p r e c i p i t a t i o n systems. Model system 1 In t h e first m o d e l s y s t e m , p r e c i p i t a t i o n was i n i t i a t e d b y m i x i n g s o l u t i o n s o f e q u i m o l a r c o n c e n t r a t i o n s ( 6 - 10 -3 tool d m -3) o f c a l c i u m c h l o r i d e a n d s o d i u m p h o s p h a t e . T h e initial p r e c i p i t a t e in such a s y s t e m is a m o r p h o u s c a l c i u m p h o s p h a t e w h i c h p r e c e d e s t h e f o r m a t i o n o f c r y s t a l l i n e oetacalc i u m p h o s p h a t e (OCP) a n d c a l c i u m d e f i c i e n t a p a t | t e s [2, 8 ] . P r e c i p i t a t i o n passes t h r o u g h several steps, t h e c h a n g e s being r e f l e c t e d in t h e respective p H a n d t u r b i d i t y vs. t i m e curves (Fig. 1). T h e first step c o r r e s p o n d s t o t h e f o r m a t i o n o f ACP a n d is e v i d e n c e d b y a p H d r o p f r o m 7.4 t o ca. 7.2 a n d a rise in t h e relative t u r b i d i t y . A f t e r t h a t a stage o f relative stability o f AC P sets in w i t h insignificant c h a n g e s in p H mid t u r b i d i t y . T h e o n s e t o f s e c o n d a r y p r e c i p i t a t i o n o f a c r y s t a l l i n e p r e c i p i t a t e (OCP) [2, 3, 5] is r e f l e c t e d by a s u b s e q u e n t fall o f p H a n d a rise in t h e relative t u r b i d i t y . T h e t i m e elapsed b e t w e e n t h e f o , , . , a t i o n o f A C P a n d t h e o n s e t o f s e c o n d a r y p r e c i p i t a t i o n ( m e t a s t a b f l i t y t i m e o f ACP, tin) was d e t e r m i n e d f r o m t h e t u r b i d i t y curves [5, 6 ] a n d t a k e n as a m e a s u r e f o r t h e stabilizing e f f e c t o f C A A . As it is s h o w n in Fig. 1, tm is significantly l o n g e r in t h e p r e s e n c e o f c i t r a t e ions ( 1 . 10 -4 tool d m -~, s y s t e m 3) t h a n in t h e c o n t r o l ( s y s t e m 1). In o r d e r t o p r o v e t h a t this e f f e c t was n o t c a u s e d solely b y t h e reduct i o n o f t h e c o n c e n t r a t i o n o f free c a l c i u m ions ,.',xe t o c a l c i u m c i t r a t e c o m plex f o r m a t i o n , s y s t e m 2 was p r e p a r e d . In tbis s y s t e m t h e c a l c i u m conc e n t r a t i o n was r e d u c e d b y an a m o u n t e q u i v a l e n t t o t h e a m o u n t o f c i t r a t e a d d e d in s y s t e m 3, w h i l e t h e p h o s p h a t e c o n c e n t r a t i o n r e m a i n e d t h e same. A c o m p a r i s o n o f t h e r e s ul t i ng c u r v e t o c u r v e 3 u n d o u b t e d l y s h o w s t h a t t h e e f f e c t o f c i t r a t e ions o n ACP s t ab ility w a s g r e a t e r t h a n c o u l d be attributed to complexing. In T a b l e 2 t h e e f f e c t o f d i f f e r e n t initial c o n c e n t r a t i o n s (1 • 10 -4, 5 - 10 -4 a n d 1 - 10 -5 m o l d m -3) o f all investigated C A A o n tm is s h o w n . T h e re-
58
TA~LEI' L i s t o f c a r b o x y l i c a c i d s . A s s o c i a t i o r constants o f a c l d s a n d s t a n t s , I - . 0. 2 9 8 K [ 1 0 ] Carboxylic acid Succinic
H,C--COOH
Ca-eomi)Zex, q . i l i b ~ - l u m
con"
Association constants
C~-complex equilibrium cortstsnts ~ . ~.. ,
K s = 10,.,~
K , •- 1 0 T M
K,., = 10""'*
K "= 1 0 " * (Ca** + HL,- *ffiC a H L ' )
X I : lOS;13t
K s -~ 101.oo
K s . * =: 104.zol
K = 10 .'s,
!
H~C . - - C O O H Malie
OH I
HC--COOH
(Ca,'+
HI," : * ( Y s I t L * )
I
H I C--CGOH Tartaric
•OH I
HO--COOH
! =, 0.2 K C l K t ~ 104-t*
K. " 10. "'1"
Ks.,
K s •.10 *'ej
TM
1 0 z'sz
I
HC--COOH
K ffi 1 0 ' ' ' l ( C a ~ * + H L " ffi C a H L +)
I OH
Dihydroxytartaric
1 I" 0 . 2 K C I
OH I HO-- C--CO OH
no literature data
I
HO--C--COOH I OH
HC--COOH
Fumarlc
tl H(3OC---CH Malele
HG--cooH !
Tricarballylic
K s = 10*.s*
K,.,
"= t & "'I
K...
H.C--COOH
K I =~ l O S - .
HC--COOH
X"I =- I 0 s ' 4 *
Jt~e "~ I ~.a.t:t
HL~COO H !
Ks " 1 0 T M
"ffi
K,,,=
I0"" K , - 10 ''i2 (i = 0 . 1 6 )
t & "'3
I
HIC-~OOH
K . 3 TM l & ' S "
1 Transaconltic
:"0.16
HffiC. - - C O O H !
HC---COOH |
HOOC-CH
no literature
data
59
TABLE
(continued)
1
C~hoxyllc •
•,
acid ,.
-: .
Citric
.
:~ . / _ ~ : : /
-,
-
.
.
.
-
, : :,=
,.,
H,C--COOH
"~."
, .Ass0daflon
,".
..... -r:..Cs~omplex
. o0rtstants
~:.
" K,
H O " - CI - - C O O H
~ -,
~10
;:;.e
K%=" =".
10 4'7:''' . .
HaC .-:~COOH
" .
, .... . . , . :
""=. . . 1. .0. . ~ ' j * - " .
'X
:
equilibrium
eoMtant8
:.
- ' : ( C a. '
... -
.-7"'..
:
' + H A L .-
CaH,IG
_
K , , , "= 10 ~'°s~ . . . .
•
:
-
+) -
K = 102"6. (Ca=++ K
HL
C a H L 0)
z° =
= 10 "*~" (Ca"+
L'"
= CaL-)
K = 10 "'~* (Ca ~*+ 2 H L z - = Ca(HL)~ z-) K
= I0
*'as
( C a ' + + 2 L ' " -. C a ( L ) , ' - )
72
7.O
6C
..~
.
(c
m---.
cle4r
5olut|0n
2(]
0
2~ '
'
TI~E x 1021s
'
;2
.
.
96
'
.
.
.
Wfg,. 1. C h a n g e s o f p H a n d relative t u z b f d i t y o f t h e p r e o l p | t a t t o n s y s t e m s a t 2 9 8 K as a f u n c t f o n .of: time.: Z e r o - t i m e t o t a l , re.s e t a n t . c o n e e n t r a f l © n s : ( 1 ) p O . = C a = 3 - 1 0 - ~
mo|dm-'~;
(2)
PO4 = 3 . . 1 0 - ~ m o | d m - ~ , C a = 2 . 9 . 107;rnoldm-~;
(3)
PO.=Ca=
3 - 10 ~'" t o o | d m ~', c i t r a t e :="1 .::10-4 nsol d m - ] j tm is t h e t|nt~ o f m e t a s t a b U | t y o f ACP. .
"
. :
I
,
k
d ]
.
:
I
__~ r
"
I ,"
~ 1 q
"
.
2
.
--
~
~
~
~
;
P L a r"
:"
"
"
:' ' --
:
~
"
"'
' b
:i
60
.~-"-~ :-" .: ~ ~ ~'-,
TABLE2
T h e e f f e c t of: c a r b o x y l i c acid anions o n t he lifetime o f ACP (tin). Z e r o - t i m e p h o s p h a t e and calcium c o n c e n t r a t i o n s : Ca = PO 4 f f i 3 - 1 0 -] too] d m -]. tm values f o r t h e c o n t r o l systems:
tm =•58
rain
(PO 4 = 3 • 10 -t mol drn -],
Ca = 2.9 - 10 -] me| dm-~);
rain ( P O , = 3 - 10 - ] h t o l d m -], C a f f i 2 . 5 - l O - ' m o l d m - " ) 3 • 10-" mo t d i n - ' , Ca = 2.0 - 10-" tool d m - ] ) Carboxylie acid
Concentration (too[ d m "z)
SuccinCt
1-10"* ft.10"*
58 ± 4 68 * 7
Malie
1.10"* 5 , 1 0 "~'
58 • 3 61 ± 1
Ta~a~c
1 - 1 0 -4 &.10" 1 . 1 0 "3
57 i 2 77 i 6 112, 6
D i h y d r o x y t a r t arlc
1-10"* 5 - 1 0 .4 1 , 1 0 -]
71 t 6 174 t 12 116 h
Fumarie
1 . 1 0 -4
60 i 12
Maleic
1-10-" 5. I0". s
59 z 3 68 i 4
1-10-* 5 , J.O "4 1-10-"
59 ± 4 8 3 ~" 5 125 • 9
Trlcarballylic
Citric
1 , 1 0 .4
5. 10" " 1.10"= 'Pransaeonit [e
and~ t m f f i f 9 0 m i "
tm " 67
(PU,"
tm (rain)
84
-
i 6
135 i 8 " 260 i 11
1 * 1 0 -~'
68 ± 1
5- I 0 "~' 1 - I 0 -z
74 ± 2 97 "~ 5
suits should be compared to tm values of the controls in which the calcium concentration was reduced by the respective amount. All CAA-containing systems showing tm values greater than the corresponding control systems prolong the period of ACP metastability not only because of complex formation but als0 because of the interaction of the foreign substances with the calcium phosphate precipitate, It isshown that malic, succinic, fumaric and maleic acid anions don0t i n f l u e n c e tm, ror they d o so Only s l i g h t l y . W i t h t h e a d d i t i o n o f t h e o t h e r e x a m i n e d c a r b o x y l i c a c i d s , f m in"
• -
-
'
61'
creases:in the following .~CCeSsion::t)'ansaconitic <. tartaric :< tricarballyl.iC ~ < citric < , d i h y d r o x y ~ c acid, ;~A,~m i g h t be expected,'the~ metdu~tabllity period :: o f :;ACP/pKrtlcles~increased ,:'With': inclea~ing ? CA~k. cdncentrati0n;" I n order :to: find o u t at~Whtcl~st~e o f the: p~dVipitation process the ~i n teractioit i between c A / k : rand A C P t a k e s place, s e v e r a l pH-stat~ ;experiments w e r e performed.-The carbox~rlic a c i d was=added -prior t o or.' a t some t i m e a f t e r t h e , f o r m a t i o n ~of the a m o q ) h c u s ~precipitate:= The r e s u l t s 0btained with citric and ' d i h y d r o x y ~ c " a c i d axe p r e s e n t e d in Fig. : 2; . Curve 'A represents the .sysl;em w i t h o u t additives; The first upward surge in NaOH consumption (due to ACP formation) and the following inflection corre spond to tm, while the second steep p a r t signifiesprecipitation o f the erys•
2Z
i
f
22
B
,A
~0
(/) q~ 70 z
addition of dihydr0xytortark: / acid
t2
add~ti0~ of :itri¢ acld
0
E
I~
0
-I
I
I
I
lZ
24
36
i,8
TIME x 10-Zls Fig. 2, C o n s u m p t i o n o f N a O H as a f u n c t i o n o f t i m e i n p | l - s t a t e x p e r i m e n t s ; 2 9 8 K . Z e r o - t i m e t o t a l r e a c t a n t c o n c e n t r a t i o n s : P O , = Ca = 3 • 10 -= m , ) l d m - ~ ; a d d i t i v e s ( A ) - - ; ( B ) 1 • 1 0 - * t o o | d m -~ c i t r i c a c M a d d e d 1 2 0 s a f t e r s a m p l e p r e p a r a t i o n ; ( C ) 1 - 1 0 - * t o o ! d m -3 c i t r i c s c i J a d d e d p r i o r t o s a m p | e p r e p a r a t i o n ; ( D ) 5 - 1 0 - " t o o l d m -~ d i h y d r o x y t a r t a r f c acid a d d e d 1 2 0 s a f t e r s a m p l e p r e p a r a t i o n ; ( E ) 5 • 1 0 - " t o o l d m -= d i h y droxytarterfe acld added p r i o r t o sample preparation.
6q
t a l l i n e phase. Because o f c o n s t a n t stirring in t h i s e x p e r i m e n t , t i n , w a s sig-. n i f i c a n t l y shorter than in the comparable experiment ~without stirring (curve 1 in F i g . 1). N o significant change in tm w a s observed w h e n citric or d i h y d r o x y t a r t a r i c avid was a d d e d s h o r t l y a f t e r t h e o n s e t o f precipitat i o n (curves B a n d D) o r a t a n y t i m e a t t e r t h a t ( t h e b r o k e n parts o t curves B a n d D s h o w t h e c o n s u m p t i o n o1~ 2;..OH d u e t o n e u t r a l i z a t i o n o f t h e additives). W h e n t h e additives were a d d e d t o t h e r e a c t a n t solutions, i.e. b e f o r e sample p r e p a r a t i o n (curves C a n d E), t h e o n s e t o f p r e c i p i t a t i o n was d e l a y e d a n d tm was significantly longer. A p p a r e n t l y C A A c o u l d stabilize ACP o n l y w h e n p r e s e n t in statu nascendi, e.g. in t h e very b e g i n n i n g o f precipitate f o r m a t i o n . To facilitate i n t e r p r e t a t i o n , the influence o f c i t r a t e o n t h e surface charge o f p r e c i p i t a t e particles a n d t h e i r I R spectra was also d e t e r m i n e d . T h e electrophore~ic m o b i l i t y o f ACP particles in systems c o n t a i n i n g e q u i m o l a r c o n c e n t r a t i o n s o f t h e precipitating c o m p o n e n t s at p H 7.2 was close t o zero. In t h e p r e s e n c e o f citrate ions t h e surface charge c h a n g e d t o negative and t h e e l e c t r o p h o r e t i c m o b i l i t y was o f t h e o r d e r o f 10 L a m 2 V -= s -= b u t d e c r e a s e d with t i m e as a c o n s e q u e n c e o f particl,~ g r o w t h . H t spectra o f ACF precipitates obt~Ened in t h e pre~ence o f citrate ions ( 1 . 10 -4 tool 6ii
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FREOUENCY Icrn t Fig. 3. I R s p e c t r a of" a m o r p h o u s calc[urri p h ~ s p h s t e s obLalnec~ f r o m p r e c i p i t a t e s : ( A ) i n t h e presence o f 1 • 10 -~ m o l d r n "z c i t r ~ t e ; ( B ) w i t h o u t c i t r a t e .
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dm•3, - Fig. 3) s h o w peaks at 1600~/1620 cm~' and 1 4 0 5 cm~', c o r r e s p o n d i n g to the symmetrical and antisymmetric~ ~ b r a t i o n s o f the C O O - g r o u p . s i m i l a r p e a k s were O b t a i n e d f r o m ~ m i x t u r e s o f p o w d e r e d l s o d i u m citrate a n d calcium p h o s p h a t e b u t n o t f r o m t h e c o n t r o l s ( s y s t e m s p r e p a r e d witho u t additives). " " Model system
II
•
T h e s e c o n d p r e c i p i t a t i o n system was p r e p a r e d at a relatively high supersaturation, i.e. ~a t total c o n c e n t i a t i o n s Ca ffi 5 • 10 -2 m o l d m -3 p~d PO4 -1 • 10 -2 tool d m -3. U n d e r t h e s e c o n d i t i o n s t h e f o r m a t i o n o f intercrystalline m i x t u r e s o f DCPD a n d DA was observed [ 8 ] , Precipitation started imm e d i a t e l y a f t e r m i x i n g o f t h e reactants, a n d t h e r e was c o n s e q u e n t l y a rapid d r o p in pH ( f r o m 7.4 t o ca. 6). During aging o f t h e system for 24 h t h e p H d r o p p e d f u r t h e r t o ca. 5.2. U s i n g this m o d e l system, t h e influence o f t h e series o f CAA o n t h e morp h o l o g y o f DCPD crystals was studied. In Fig. 4 t h e c o r r e s p o n d i n g changes in crystal s h a p e are r e p r e s e n t e d . I t is s h o w n t h a t all effective CAA i n h i b i t e d crystal g r o w t h in t h e same d i r e c t i o n as citrate ions, i.e. in t h e d i r e c t i o n p e r p e n d i c u l a r to t h e ( 0 0 1 ) p l a n e [ 9 ] . With increasing C A A c o n c e n t r a t i o n in t h e system, an increasing t e n d e n c y t o w a r d s t h e f o r m a t i o n o f needleshaped crystals b e c a m e apparent. H a b i t m o d i f i c a t i o n o c c u r r e d even w h e n t h e additive was a d d e d t o t h e s y s t e m after t h e p r e c i p i t a t e h a d already b e e n f o r m e d , T h e i n t e n s i t y o f t h e e f f e c t was analogous w i t h t h e succession: malic :> citric :> transaconittc :> d i h y d r o x y t a r t a r l c :> tricarballylic :> tartaric acid ions. Succinic, fumaric and maleic acid d i d n o t offect t h e shape o f DCPD crystals. D I S C U S S I O N
T w o aspects o f t h e i n t e r a c t i o n o f s o m e di- and tricarboxyl~v acid ions with calcium p h o s p h a t e s w e r e investigated: (1) stabilization o f ACP (Figs. 1, 2 and Table 2) and (2) t h e i n f l u e n c e o n t h e m o r p h o l o g y o f DCPD crystals (Fig. 4). Stabilization of ACP
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T h e e x p e r i m e n t a l results s h o w (Table 2) t h a t all e x a m i n e d organic acids can b e d i v i d e d i n t o t h r e e groups d e p e n d i n g o n t h e i r i n f l u e n c e o n tin: ( I ) malic, succinic, maleic and fumaric acid w i t h n o significant effect, (2) tartaric a n d tricarballylic acid w i t h ~the e f f e c t being d e t e c t a b l e a t higher C A A c o n c e n t r a t i o n s , a n d (3) transaconitic, citric a n d d i h y d r o x y t a r t ~ r i c acid, s h o w i n g a stabilizing e f f e c t a t all e x a m i n e d c o n c e n t r a t i o n s . T o explain this succession o n e mu.st t a k e t h e situation in .the solution .
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F i g . 4 . M i e r o g r 3 p h s o f p r e c i p i t a t e s i s o l a t e d 72 h a f t e r s a m p l e p r e p a r a t i o n . Initial pH 7.4, temperature 2 9 8 K , z e r o - t i m e t o t a l r e a c t a n t c o n c e n t r a t i o n s : i=O4 = 1 . 10-= t o o l d m - J , C a = 6 , ] 0 - * m o l d m - ) . A d d i t i v e s : ( a ) - - ; ( l ~ - g ) 5 . 1 0 -~ m o l d m - ] l a r t a r i c , t r l earballylic, dlhydroxytartartc, transaeonitfc, citric and maiic acid, r e s p e c t i v e l y ; t r a n s a e o n i t i e ~cid: ( h ) 8 • t O - " t o o l d m -3, ( i ) 1 • 1.0 -~ m o i d m -~.
into consideration. Therefore w e u s e d t h e stability constants listed in Table 1 to ca/culate distribution curves w h i c h s h o w t h e relative a m o u n t s o f different ionic species as a f u h c t i o n o f p H . A n e x a m p l e is given in Fig. 6, It ha~ b e e n s h o w n that in the neutral pH region, citric aci~l exists predominantly it) the form o f the negatively charged C a - c o m p l e x ( C a L - ~ 98%)
65
[9] w h i l e tricarballylic acid a p p e a r s in t h e f o r m o f t h e n e g a t i v e l y c h a r g e d C a - c o m p l e x ( C a L - ~ 15%) a n d ligand ( L ~ - ~ 8 0 ~ ) (Fig. 5). Also in t h e case o f t a r t a r i c acid t h e negative ligands (L2-: a n d H L , ) a n d t h e n e g a t i v e l y c h a r g e d iC a ~ : o m p l e x ( C a L l - ) a p p e a r in significant a m o u n t . C a r b o x y l i c acids w i t h n o stabilizing e f f e c t h a v e b e e n f o u n d t o a p p e a r in t h e f o r m o f t h e n e u t r a l c o m p l e x (CuL °) a n d t h e n e g a t i v e l y c h a r g e d ligand~ L2-). C o r r e l a t i n g t h e s e findings w i t h t h e d a t a listed in T a b l e 2, w e c o n c l u d e t h a t t h e i n t e r a c t i o n a bi l i t y o f t h e C A A w i t h t h e solid p h a s e d e p e n d s o n (1) t h e c o n c e n t r a t i o n o f t h e negatively c h a r g e d C a - c o m p l e x in s o l u t i o n a n d (2) t h e n t u n b e r o f f u n c t i o n a l g r o u p s ( - - C O O H a n d - - O H ) . A p p a r e n t l y t h e larger n u m b e r o f f u n c t i o n a l g r o u p s o f negatively c h a r g e d species increases t h e p o l a r a t t r a c t i o n b e t w e e n t h e a d s o r b a t e a n d t h e positive sites a t t h e p r e c i p i t a t e / s o l u t i o n interface.
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Fig. 5. The distrihuti©n of tricarb~lylie species as a function of pH if the initial concentrations of trtcarb~d|yl[c acid and calcium ions are 1 . 1 0 - ' tool dm -3 and 3- 10 -s t o o | d m -J, r e s p e e t t v e l y .
A c c o r d i n g t o t h e pH-stat e x p e r i m e n t s (Fig. 2) C A A stabilize ACP if p r e s e n t in t h e initial stages o f p r e c i p i t a t e f o r m a t i o n . I R s p e c t r a 3) s h o w t h a t t h e a d d i t i v e is n o t o n l y surface a d s o r b e d (as p r o v e n b y trophoresls) b u t also o c c l u d e d w i t h i n t h e particles, T h e s e findings
only (Fig. elecmay
66
be h e l p f u l in e x p l a i n i n g t h e m e c h a n i s m o f t h e stabilizing a c t i o n o f C A A on ACP. I t has been suggested b y us [4, 5] t h a t A C P s p h e r u l e s are f o ~ n e d by p r e v a l e n t l y h o m o g e n e o u s n u c l e a t i o n a n d s u b s e q u e n t aggregation o f t h e p r i m a r y particles. I f ACP f o r m a t i o n p r o c e e d s in t h e p r e s e n c e o f additives t h e s e m a y b e a d s o r b e d a t t h e s ur f a c e o f t h e p r i m a r y p articles a n d o c c l u d e d d u r i n g s u b s e q u e n t aggregation i n t o t h e s p h e m l e s . O n c e f o ~ e d tlle s p h e • l e s g r o w f o r s o m e t i m e unt i l s e c o n d a r y p r e c i p i t a t i o n o f a c r y s t a l l i n e p h a s e a b r n p t l y c ha nges t h e c o n d i t i o n s in t h e s u r r o u n d i n g s o l u t i o n . X-ray stu d ies have s h o w n [11, 12] t h a t c r y s t a l l i z a t i o n is a discontinuovls p ro cess p r i o r to w h i c h ACP exists in a m e t a s t a b l e state. T h e m e t a s t a b i l i t y p e r i o d o f ACP, tin, c o r r e s p o n d s t o this p e r i o d p r e c e d i n g c r y s t a l l i z a t i o n . Si nc e t h e first f o r m e d c r y s t a l l i n e p r e c i p i t a t e is usually f o u n d in i n t i m a t e c o n t a c t w i t h t he ACP spherules [ 2 ] , c learly t h e p r e c u r s o r serves as a h e t e r o g e n e o u s n u c l e a t o r o f this phase. We m a y t h e n c o n s i d e r tm as an i n d u c t i o n p e r i o d for h e t e r o g e n e o u s n u c l e a t i o n , i t has b e e n f o u n d t h a t this i n d u c tion p e r i o d is i n d e p e n d e n t o f t h e s ur f ace a r e a o f ACP available t o n u c l e a t e [ 1 3 ] . This, t o g e t h e r w i t h t h e finding t h a t CAA have n o e f f e c t w h e n a d d e d after t h e Onset o f p r e c i p i t a t i o n (Fig. 2), s h o w s t h a t t h e rate o f n u c l e a t i o n of t h e crystalline p r e c i p i t a t e is n o t c o n t r o l l e d b y a process at th e A C P / s o l u t i o n interface. We m a y t h e n assume t h a t b e f o r e s e c o n d a r y p r e c i p i t a t i o n sets in (i.e. d u r i n g t h e p e r i o d o f m e t a s t a b f l i t y ) , ACP s p h e ~ l c s are subj e c t to internal r e a r r a n g e m e n t s ( d e h y d r a t i o n , h y d r o l y s i s , crystal o r d e r i n g ) wh ich m a k e t h e m a suitable n u c l e a t o r fo r OCP. A p p a r e n t l y tm d e p e n d s on t h e rate o f t he s e processes w h i c h m a y be significantly c h a n g e d b y t h e p r e s e n c e o f t h e foreign ions w i t h i n t h e a m o r p h o u s particles. This h y p o t h esis is s u p p o r t e d by t h e findings o f M o n t e l e t al. [12] w h o s h o w e d t h a t in situ c o n v e r s i o n o f ACP ( w i t h o u t a n y c h a n g e o f t h e s o l i d / s o l u t i o n ratio) p r o c e e d s as a result o f at least t w o processes, i.e. a c o n t i n u o u s process i n t e r p r e t e d as h y d r o l y s i s a n d a d i s c o n t i n u o u s o n e crystallization. lnfluetzce on the m o r p h o l o g y o f DCPD crystals
T h e i n f l u e n c e o f t h e series o f di- a nd t r i c a r b o x y l i c acid iQns o n th e m o r p h o l o g y o f DCPD crystals m a ni f e s t s as a c h a n g e in cry s tal s h a p e (Fig. 4). H a b i t m o d i f i c a t i o n was i n d u c e d b y p r e f e r e n t i a l a d s o r p t i o n o f negatively c h a r g e d c a r b o x y l i c acid ions or t h e respective C a - c o m p l e x e s o n t h e ( 0 0 1 ) crystal plane [ 9 ] . T h e succession o f t h e C A A with regard t o effectiveness is d i f f e r e n t t h a n in t h e previous case (stabilization o f ACP) a n d c a n n o t be e x p l a i n e d b y c o m p l e x f o r m a t i o n a n d th e n u m b e r o f f u n c t i o n a l groups. We m u s t t h u s l o o k f o r s o m e specificity o f t h e i n t e r a c t i o n . T h e p h e n o m e n o n o f h a b i t m o d i f i c a t i o n b y a n o rg an ic m a t e r i a l c a n be e x p l a i n e d in several w a y s [ 1 4 | . R e c e n t t h e o r i e s a t t r i b u t e a n i m p o r t a n c e t o t h e c o n f i g u r a t i o n o f t h e p o l a r g r o u p s o f t h e a d s o r b e d species. W h e t s t o n e and c o - w o r k e r s [15, 16] s t u d i e d t h e i n f l u e n c e o f a large n u m b e r o f o r g a n i c
67
a c i d m o l e c u l e s o n t h e m o r p h o l o g y o f NH4NO3, IKNO3 a n d NaNO3, a n d concluded that crystal habit modification o~ginates only w h e n a polar g r o u p fits t h e ion a r r a n g e m e n t s l o f t h e sub3trate. T h e effectiveness o f additives as" h a b i t m o d i f i e r s ~of h e x a n e d i o i e acid [ 1 7 ] a n d c a l c i u m s u l p h a t e [ 1 8 ] have b e e n a c c o r d i n g l y e x p l a i n e d . I n o r d e r t o e x p l a i n th~ i n t e n s i t y o f t h e e f f e c t o f t h e individual c a r b o x y l i c acid a n i o n o n D C P D it is n e c e s s a r y t o k e e p an a c c o u n t o f t h e g e o m e t r i c a l a r r a n g e m e n t a n d n u m b e r o f t h e i r p o l a r groups, I t seems t h a t t h e H O C O C H ( O H ) ( C O O H ) C H - - g r o u p is t h e m o s t effectiv e o n e (malic a n d c i t r i c acid). M u c h less e f f e c t i v e are c a r b o x y H e acids w i t h a d d i t i o n a l - - O H g r o u p s att a c h e d t o t h e n e i g h b o u r i n g (3 a t o m s (tartaric a n d d i h y d r o x y t a r t a r i c acid). T h e n u m b e r o f c a r b o x y l i c g r o u p s plays a role t o o . D i c a r b o x y l i c acids, like m a l e i c , f u m a r i c a n d succinic acid, d i d n o t a d s o r b at t h e D C P D cry stal su rfa c e w h i l e t r i c a r b o x y l i c acids w i t h v e r y similar s t r u c t u r e (tricarballylic a n d t r a n s a c o n i t i c acid) did. We c o u l d n o t f/nd a s a t i s f a c t o r y c o r r e l a t i o n b e t w e e n t h e Ca 2+ spacings in t h e c r y s t a l l o g r a p h i c ( 0 0 1 ) p l a n e o f t h e D C P D cry s tal ( c a l c u l a t e d f r o m t h e c r y s t a l s t r u c t u r e given in Ref. [ 1 9 ] a n d t h e a n i o n spacings o f t h e effective C A A groups. I t s h o u l d , h o w e v e r , b e t a k e n i n t o a c c o u n t t h a t ads o r p t i o n o c c u r s at t h e c r y s t a l / s o l u t i o n in terface w h e r e ionic a r r a n g e m e n t s m a y be d i f f e r e n t t h a n in t h e c r ys t a l lattice. I t m a y t h e r e f o r e b e futile t o s p e c u l a t e a b o u t possible g e o m e t r i c fits o f t h e m o l e c u l a r s t r u c t u r e o f C A A w i t h t h e ions in t h e substrate. ACKNOWLEDGEMENTS T h e a u t h o r s wish t o t h a n k Dr. B. Koji~-Prodi~ for X-ray d i f f r a c t i o n w o r k a n d a c k n o w l e d g e t h e financial s u p p o r t o f t h e S e l f m a n a g e m e n t C o u n c i l for S c i e n t i f i c R e s e a r c h o f S,R. C r o a t i a (SIZ V) an d o f t h e N a t i o n a l I n s t i t u t e s o f H e a l t h , B e t h e s d a , Md ( p r o j e c t No. 02-092-N). REFERENCES
I
E.D. Eanes, I.H. Gillessen and A.S. Posner, in H.S. Pefser (Ed.), Crystal Growth, Pergamon Pte~, Oxford, 1967, p. 373. 2 Lj. Bre~evtd and H. F//redi*Mtlhofer, Calcif. Tigsue Res., 10 (1979) 82. 3 R. Despotovi~, N. Filipovi~-Vtneekovt¢~r~Id H. F//redi-Mtlhofer, Caleif. Tissue Res., 1 8 ( 1 9 7 6 ) 13.
4 H. l~'redi-Mtlhofar, LJ. Bre~evid and B. Purgart~, Faraday Discuss. Chem. Soc., 61 (1976) 184.
5 L|. Bre~evi,Sand H. Ft~'gedi-Mllhofer, in I.W. Mullin (Ed.), IndusttqJd C~ystalllzation, Plenum, New York, 1 9 7 6 , p. 9 7 7 . 6 H. F~redi-Mllhofer, L|. Bre~evl~, E. Oljlea, B. Purgar~, Z. Gass and G. Perovi~, in A.L. Smith (~d.), Particle Growth In Suspension, Academic Press, London, 7
N e w York, 1 9 7 3 , p . 1 0 9 . I.D. T e n n i n e , R . A . Peckauslms and A.S. Posner, Arch. Bloehem. B i o p h y f . , 1 4 0 (1970) 318.
68
8 9 10 11 12 13 14 lb 16 IV t8 19
if. F~'redl-Milhofer, B. Purgart~, LJ. B r e ~ e ~ s n d N. Pavko~I3, Cs/c[/', T l s ~ e Res., 8 ( 1 9 7 1 ) 142. ~ LJ. Bre~evtd a n d H . F6"redl'Mllhofer. Caiclf. Tissue InL, 28 ( 1 9 7 2 ) 131, L. G. Sillen, Stability C~ns*mnts o f Metal--Ion Complexes, T h e Chemfcal S o c i e t y , L o n d o n , 1964. EA). E a n e s a n d A~q. Posner, T r a ~ . N.Y. Aead. 8c|., Set. II, 28 (1965) 233. G. Montel, G . Bonel, J.C. ifeughebaest, J.C. q~romhe and C. R e y , J. Cryst. G r o w t h , 53 (1981) V4. J.L. Meyer and E.D. Fanes, Calclf. Tissue Res., 25 (1978) 59; 9 0 9 . A.G. Walton, T h e F o r m a t i o n and Properties o f Precipitates, lnterscienee, N e w York, L o n d o n , S | d n e y , 1967, p. 173. K.N. Davies and J. Whetstone, J. Chem. 8oe., 61 (19'/6) 153. J. Whetstone, Jo Chem. Soc., (1956) 4 8 4 1 . A.E. Nielsen, Faraday Discuss. Chem. So0,, 61 (1976) 163. M.T. McCall and M.E. Tadros, Colloids Surfaces, 1 (1980) 161. N.A. Curzy and D°W. Jones, J. Chem. Soc. Az (1971) 3"/25.
,~8 Elsevier S c i e n c e Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
55
PRECIPITATION OF CALCIUM PHOSPHATES FROM ELECTROLYTE SOLUTIONS. VII.' THE INFLUENCE OF DI- AND qrRICAIIBOXYLIC A C I D S _ •- • . . -- ~ ~ : , . ~ -. ~. ~ . "
-.
•
-
.
I~]ERKA B R E ~ E V I ( ~ *, A I ~ A S E N D I J A R E V I ~ * * a n d H E L G A F O R F - D I o M I L H O F E R * • L a b o r a t o r y f o r P r e c i p i t a t i o n P r o e e u e ~ " R u d j e r B o ] k o v l ~ " I n s t i t u t e , P.O. B o x I 0 1 6 , 4 1 0 0 1 Z a g r e b (Yugosl~vfa) . . " ~ • *Faculty of Technology, Tuzla (Yugoslavia)
( R e c e i v e d 11 J u l y 1 9 8 3 ; a c c e p t e d in final f o r m 2 J a n u a r y 1 9 8 4 ) ABSTRAGT T h e i n f l u e n c e o f a series o f di- and trtcarbox3rlic acid a n i o n s ( C A A ) o n t h e metastabilits, o f a m o r p h o u s c a l c i u m p h o s p h a t e ( A C P ) (I) a n d o n t h e m o r p h o l o g y o f c a l c i u m hyd r o g e n p h o s p h a t e d i h s , d r a t e ( D C P D ) ( I t ) a t 2 9 8 K a n d Initial p H i 7.4 was investigated. [n s y s t e m l, p r e c i p i t a t i o n w-,s i n i t i a t e d f r o m solutions, o f equttmolar ( 3 - 10 -z tool d m - l ) c o n c e n t r a t i o n s o f calcium c h l o r i d e e n d s o d i u m p h o s p h a t e . Matte, s u c e l n t ¢ , m a l e i e a n d fumart¢ acid a n i o n s h a d n o significant e f f e c t o n t h e m e t a s t a b t l l t y p e r i o d w h i t e t h e el'f e e t l v e n e ~ o f t h e o t h e r G A A increased in t h e s t t c c e ~ i o n : t r a n s a c o n i t i c < tartaric < triearballylie < citric < d i h y d r o x y t a r t a r i c acid anion. I n s y s t e m II. p r e c i p i t a t i o n was Initiated a t initial r e a c t a n t c o n c e n t r a t i o n s o f C a = 5 • 10 -I tool d m "~ a n d P C 4 a I ' 10-z m o l d m -3. T h e resulting solid p h a s e was a n int e r c r y s t a l l i n e m i x t u r e o f c a l c i u m d e f i c i e n t a p a t t t a s ( D A ) a n d DCPD. H a b i t m o d i f i c a t i o n o f D C P D was i n d u c e d b y t h e f o l l o w i n g C A A : tartaric < tricarballyilc < dihyd~0.xy~ tartaric < t r a n s a c o n i t i c < citric < malie acid a n i o n . All t h e s e a n i o n s i n h i b i t e d c r y s t a l g r o w t h in the d i r e c t i o n p e r p e n d i c u l a r t o t h e ( 0 0 1 ) plane, p r o m o t i n g t h e g r o w t h o f needles instead o f platelets. S u c e i n l e , f u m a r | e a n d m a l e l c acid a n i o n s did n o t a f f e c t t h e s h a p e o f D C P D crysials.
INTROD UCTION •T h e p r e c i p i t a t i o n of calcium phosphates is o f g r e a t i n t e r e s t i n i n d u s t r i a l , agricultural and biomedical research. A few examples of industrial application are: the preparation of toothpaste, where calcium hydrogen phos, phate dihydrate (DCPD) crystals are used as an abrasive; production of fertilizers; removal of inorganic phosphate from industrial waste waters, etc. Calcium phosphates are also the main inorganic constituents of human and animal endo- and exoskeletons, teeth and pathological mineral deposits, such as urinary Calculi, ~; ~t-~ .... ~ ~ It has'~been shown [1~4] • that in neutral and alkaline pH range, pre: cipitation of calcium phosphates proceeds via the formation of a metastable amorphous precursor (amorphous calcium phosphate, ACP), The' metasta-
0166-6622/84/~03.00
O 1 9 8 4 Elsevier S c i e n c e Publishers B . V .
bility t i m e o f f~is p r e c u r s o r is v e r y sensitive to t h e c o n d i t i o n s o f precipi t a t i o n ; it c o u ] ~ b e p r o l o n g e d b y d e c r e a s i n g t h e r e a c t a n t c o n c e n t ~ t i o n s [ 5 ] , increasing t h e ionic s t r e n g t h [G, 7 ] , o r a d d i n g s o m e foreign s u b s t a n c e s [6, 7 ] a n d ions [ 7 ] . T h e final c o m p o s i t i o n o f t h e p r e c i p i t a t e is es tab lish ed by s e c o n d a r y p r e c i p i t a t i o n o f a c r y s t a l l i n e p h a s e [2, 3] a n d s u b s e q u e n t e q u i l i b r a t i o n processes [ 3 ] . I f p r e c i p i t a t i o n is i n i t i a t e d at relatively high c a l c i u m a n d p h o s p h a t e co~lcentrations ( s u c h as prevail in h u m a n urines) intercryst~lline m i x t u r e s o f c a l c i u m d e f i c i e n t a p a t i t e s ( D A ) a n d D C P D result [ 8 ] . I t has b e e n s h o w n [9] t h a t t h e m o r p h o l o g y o f D C P D crystals p r e c i p i t a t e d u n d e r t h e s e c o n d i t i o m c a n be significantly a~tered b y t h e a d d i t i o n o f c i t r a t e ions. P r e l i m i n a r y investigations also s h o w [5] t h a t c i t r a t e ions m a y d e l a y t h e c o n v e r s i o n o f ACP. In o r d e r t o ge t a b e t t e r u n d e r s t a n d i n g o f th ese effects, t h e i n h i b i t o r y a c t i v i t y o f a series o f di- a n d t r i c a r b o x y l i c acid a n i o n s ( C A A ) has b e e n t e s t e d . T h e results o f this investigation are r e p o r t e d in t h e p r e s e n t paper. METHODS S t o c k sol ut i o ns w e r e p r e p a r e d b y dissolving t h e r e q u i r e d a m o u n t s o f a n a l y t i c a l g r a d e che~nicals in trldtstilled w a t e r . S o l u t i o n s u s e d fo r sample p r e p a r a t i o n w e r e d i l u t e d f r o m s t o c k s o l u t i o n s a n d aged fo r 2 4 h before usage. S a m p l e s w e r e p r e p a r e d by a d d i n g c a l c i u m c h l o r i d e s o l u t i o n s t o e q u a l v o l u m e s o f s o d i u m p h o s p h a t e so lu tio n s w h i c h w e r e p r e a d j u s t e d to pH 7.4 [8]o T h e r e s ul t i ng s y s t e m s w e r e t h o r o u g h l y m i x e d an d k e p t at 2 9 8 K w i t h o u t f u r t h e r stirring. T h e respective c a r b o x y l i c acid was a d d e d t o t h e p h o s p h a t e s o l u t i o n p r i o r t o p H a d j u s t m e n t . S a m p l e s u s e d as c o n trols c o n t a i n e d t h e s a m e c o n c e n t r a t i o n s o f c a l c i u m c h l o r i d e a n d s o d i u m pl~osphate b u t n o c a r b o x y l i c acid was added. Changes o f p H a n d relative t u r b i d i t y w e r e s i m u Y t m m o u s l y r e c o r d e d as a f u n c t i o n o f t i m e [ 2 ] . A R a d i o m e t e r m o d e l 26 p H m e t e r in c o n n e c t i o n w i t h a H e w l e t t - P a c k a r d Moseley 7 1 0 0 BM r e c o r d e r an d a Zeiss rec o r d i n g t u r b i d i m e t e r ( S p e k o l ) w e r e used. T u r b i d i t i e s w e r e m e a s u r e d a t 6 4 0 n m 8gainst a s u s p e n s i o n o f latex LS-54-2 (Rv00 = 0 . 0 4 0 c m -! a t 5 4 6 nm)o A set o f e x p e r i m e n t s was c o n d u c t e d a t p H 7 . 2 0 ± 0 . 0 2 b y m e a n s o f a R a d i o m e t e r pH - s t a t de vi c e , i n t h e s e e x p e r i m e n t s t h e c a r h o x y l i c avid was a d d e d t o t h e p r e c i p i t a t i o n s y s t e m e i t h e r p r i o r t o o r a f t e r s a m p l e prepa r a t i o n . T h e r e a c t i o n slurry was stirred at a c o n s t a n t r a t e t h r o u g h o u t t h e e x p e r i m e n t a n d t h e c o n s u m p t i o n o f s o d i u m ~hydroxide w a s a u t o m a t i c a l l y recorded. T h e m o r p h o l o g y o f t h e p r e c i p i t a t e s w a s o b s e r v e d u n d e r an i n v e r t e d m i c r o s c o p e (E. Leitz, Wetzlar), an O r t h o p l a n p h o t o g r a p l f i c m i c r o s c o p e (E, Leitz, Wetzlar) a n d an e | e c t r o n m i c r o s c o p e ( E ] m i s c o p [, Siemens). T h e sign o f t h e particle c h a r g e a n d t h e e l e c t r o p h o r e t i c m o b i l i t y w e r e determined by microelectrophoresis.
57
l l t s p e c t r a w e r e t a k e n f r o m K B r pellets. P r e c i p i t a t e s w e r e w a s h e d w i t h supernatants o f the respective control systems. C h e m i c a l a n d i n s t r u m e n t a l analysis o f t h e p r e c i p i t a t e s w e r e c a r r i e d o u t as p r e v i o u s l y d e s c r i b e d [ 8 ] . R E SU L'rs T h e i n f l u e n c e o f succiniv, malic, tartaric, d i h y d r o x y t a r t a r i c , f u m a r i c , m a l e i c , t r a n s a c o n i t i c , citric a n d tricarballylic acid a n i o n s o n t h e precipit a t i o n o f c a l c i u m p h o s p h a t e s was investigated, T ab le 1 s h o w s t h e i r f o r m u l a s w i t h t h e c o r r e s p o n d i n g association c o n s t a n t s a n d C a - c o m p l e x e q u i l i b r i u m c o n s t a n t s [10]. T h e c o n c e n t r a t i o n s o f t h e additives varied f r o m 1 • 10 -4 t o 1 • 10 -3 tool d m -~. All investigations have b e e n c a r r i e d o u t o n t w o d i f f e r e n t p r e c i p i t a t i o n systems. Model system 1 In t h e first m o d e l s y s t e m , p r e c i p i t a t i o n was i n i t i a t e d b y m i x i n g s o l u t i o n s o f e q u i m o l a r c o n c e n t r a t i o n s ( 6 - 10 -3 tool d m -3) o f c a l c i u m c h l o r i d e a n d s o d i u m p h o s p h a t e . T h e initial p r e c i p i t a t e in such a s y s t e m is a m o r p h o u s c a l c i u m p h o s p h a t e w h i c h p r e c e d e s t h e f o r m a t i o n o f c r y s t a l l i n e oetacalc i u m p h o s p h a t e (OCP) a n d c a l c i u m d e f i c i e n t a p a t | t e s [2, 8 ] . P r e c i p i t a t i o n passes t h r o u g h several steps, t h e c h a n g e s being r e f l e c t e d in t h e respective p H a n d t u r b i d i t y vs. t i m e curves (Fig. 1). T h e first step c o r r e s p o n d s t o t h e f o r m a t i o n o f ACP a n d is e v i d e n c e d b y a p H d r o p f r o m 7.4 t o ca. 7.2 a n d a rise in t h e relative t u r b i d i t y . A f t e r t h a t a stage o f relative stability o f AC P sets in w i t h insignificant c h a n g e s in p H mid t u r b i d i t y . T h e o n s e t o f s e c o n d a r y p r e c i p i t a t i o n o f a c r y s t a l l i n e p r e c i p i t a t e (OCP) [2, 3, 5] is r e f l e c t e d by a s u b s e q u e n t fall o f p H a n d a rise in t h e relative t u r b i d i t y . T h e t i m e elapsed b e t w e e n t h e f o , , . , a t i o n o f A C P a n d t h e o n s e t o f s e c o n d a r y p r e c i p i t a t i o n ( m e t a s t a b f l i t y t i m e o f ACP, tin) was d e t e r m i n e d f r o m t h e t u r b i d i t y curves [5, 6 ] a n d t a k e n as a m e a s u r e f o r t h e stabilizing e f f e c t o f C A A . As it is s h o w n in Fig. 1, tm is significantly l o n g e r in t h e p r e s e n c e o f c i t r a t e ions ( 1 . 10 -4 tool d m -~, s y s t e m 3) t h a n in t h e c o n t r o l ( s y s t e m 1). In o r d e r t o p r o v e t h a t this e f f e c t was n o t c a u s e d solely b y t h e reduct i o n o f t h e c o n c e n t r a t i o n o f free c a l c i u m ions ,.',xe t o c a l c i u m c i t r a t e c o m plex f o r m a t i o n , s y s t e m 2 was p r e p a r e d . In tbis s y s t e m t h e c a l c i u m conc e n t r a t i o n was r e d u c e d b y an a m o u n t e q u i v a l e n t t o t h e a m o u n t o f c i t r a t e a d d e d in s y s t e m 3, w h i l e t h e p h o s p h a t e c o n c e n t r a t i o n r e m a i n e d t h e same. A c o m p a r i s o n o f t h e r e s ul t i ng c u r v e t o c u r v e 3 u n d o u b t e d l y s h o w s t h a t t h e e f f e c t o f c i t r a t e ions o n ACP s t ab ility w a s g r e a t e r t h a n c o u l d be attributed to complexing. In T a b l e 2 t h e e f f e c t o f d i f f e r e n t initial c o n c e n t r a t i o n s (1 • 10 -4, 5 - 10 -4 a n d 1 - 10 -5 m o l d m -3) o f all investigated C A A o n tm is s h o w n . T h e re-
58
TA~LEI' L i s t o f c a r b o x y l i c a c i d s . A s s o c i a t i o r constants o f a c l d s a n d s t a n t s , I - . 0. 2 9 8 K [ 1 0 ] Carboxylic acid Succinic
H,C--COOH
Ca-eomi)Zex, q . i l i b ~ - l u m
con"
Association constants
C~-complex equilibrium cortstsnts ~ . ~.. ,
K s = 10,.,~
K , •- 1 0 T M
K,., = 10""'*
K "= 1 0 " * (Ca** + HL,- *ffiC a H L ' )
X I : lOS;13t
K s -~ 101.oo
K s . * =: 104.zol
K = 10 .'s,
!
H~C . - - C O O H Malie
OH I
HC--COOH
(Ca,'+
HI," : * ( Y s I t L * )
I
H I C--CGOH Tartaric
•OH I
HO--COOH
! =, 0.2 K C l K t ~ 104-t*
K. " 10. "'1"
Ks.,
K s •.10 *'ej
TM
1 0 z'sz
I
HC--COOH
K ffi 1 0 ' ' ' l ( C a ~ * + H L " ffi C a H L +)
I OH
Dihydroxytartaric
1 I" 0 . 2 K C I
OH I HO-- C--CO OH
no literature data
I
HO--C--COOH I OH
HC--COOH
Fumarlc
tl H(3OC---CH Malele
HG--cooH !
Tricarballylic
K s = 10*.s*
K,.,
"= t & "'I
K...
H.C--COOH
K I =~ l O S - .
HC--COOH
X"I =- I 0 s ' 4 *
Jt~e "~ I ~.a.t:t
HL~COO H !
Ks " 1 0 T M
"ffi
K,,,=
I0"" K , - 10 ''i2 (i = 0 . 1 6 )
t & "'3
I
HIC-~OOH
K . 3 TM l & ' S "
1 Transaconltic
:"0.16
HffiC. - - C O O H !
HC---COOH |
HOOC-CH
no literature
data
59
TABLE
(continued)
1
C~hoxyllc •
•,
acid ,.
-: .
Citric
.
:~ . / _ ~ : : /
-,
-
.
.
.
-
, : :,=
,.,
H,C--COOH
"~."
, .Ass0daflon
,".
..... -r:..Cs~omplex
. o0rtstants
~:.
" K,
H O " - CI - - C O O H
~ -,
~10
;:;.e
K%=" =".
10 4'7:''' . .
HaC .-:~COOH
" .
, .... . . , . :
""=. . . 1. .0. . ~ ' j * - " .
'X
:
equilibrium
eoMtant8
:.
- ' : ( C a. '
... -
.-7"'..
:
' + H A L .-
CaH,IG
_
K , , , "= 10 ~'°s~ . . . .
•
:
-
+) -
K = 102"6. (Ca=++ K
HL
C a H L 0)
z° =
= 10 "*~" (Ca"+
L'"
= CaL-)
K = 10 "'~* (Ca ~*+ 2 H L z - = Ca(HL)~ z-) K
= I0
*'as
( C a ' + + 2 L ' " -. C a ( L ) , ' - )
72
7.O
6C
..~
.
(c
m---.
cle4r
5olut|0n
2(]
0
2~ '
'
TI~E x 1021s
'
;2
.
.
96
'
.
.
.
Wfg,. 1. C h a n g e s o f p H a n d relative t u z b f d i t y o f t h e p r e o l p | t a t t o n s y s t e m s a t 2 9 8 K as a f u n c t f o n .of: time.: Z e r o - t i m e t o t a l , re.s e t a n t . c o n e e n t r a f l © n s : ( 1 ) p O . = C a = 3 - 1 0 - ~
mo|dm-'~;
(2)
PO4 = 3 . . 1 0 - ~ m o | d m - ~ , C a = 2 . 9 . 107;rnoldm-~;
(3)
PO.=Ca=
3 - 10 ~'" t o o | d m ~', c i t r a t e :="1 .::10-4 nsol d m - ] j tm is t h e t|nt~ o f m e t a s t a b U | t y o f ACP. .
"
. :
I
,
k
d ]
.
:
I
__~ r
"
I ,"
~ 1 q
"
.
2
.
--
~
~
~
~
;
P L a r"
:"
"
"
:' ' --
:
~
"
"'
' b
:i
60
.~-"-~ :-" .: ~ ~ ~'-,
TABLE2
T h e e f f e c t of: c a r b o x y l i c acid anions o n t he lifetime o f ACP (tin). Z e r o - t i m e p h o s p h a t e and calcium c o n c e n t r a t i o n s : Ca = PO 4 f f i 3 - 1 0 -] too] d m -]. tm values f o r t h e c o n t r o l systems:
tm =•58
rain
(PO 4 = 3 • 10 -t mol drn -],
Ca = 2.9 - 10 -] me| dm-~);
rain ( P O , = 3 - 10 - ] h t o l d m -], C a f f i 2 . 5 - l O - ' m o l d m - " ) 3 • 10-" mo t d i n - ' , Ca = 2.0 - 10-" tool d m - ] ) Carboxylie acid
Concentration (too[ d m "z)
SuccinCt
1-10"* ft.10"*
58 ± 4 68 * 7
Malie
1.10"* 5 , 1 0 "~'
58 • 3 61 ± 1
Ta~a~c
1 - 1 0 -4 &.10" 1 . 1 0 "3
57 i 2 77 i 6 112, 6
D i h y d r o x y t a r t arlc
1-10"* 5 - 1 0 .4 1 , 1 0 -]
71 t 6 174 t 12 116 h
Fumarie
1 . 1 0 -4
60 i 12
Maleic
1-10-" 5. I0". s
59 z 3 68 i 4
1-10-* 5 , J.O "4 1-10-"
59 ± 4 8 3 ~" 5 125 • 9
Trlcarballylic
Citric
1 , 1 0 .4
5. 10" " 1.10"= 'Pransaeonit [e
and~ t m f f i f 9 0 m i "
tm " 67
(PU,"
tm (rain)
84
-
i 6
135 i 8 " 260 i 11
1 * 1 0 -~'
68 ± 1
5- I 0 "~' 1 - I 0 -z
74 ± 2 97 "~ 5
suits should be compared to tm values of the controls in which the calcium concentration was reduced by the respective amount. All CAA-containing systems showing tm values greater than the corresponding control systems prolong the period of ACP metastability not only because of complex formation but als0 because of the interaction of the foreign substances with the calcium phosphate precipitate, It isshown that malic, succinic, fumaric and maleic acid anions don0t i n f l u e n c e tm, ror they d o so Only s l i g h t l y . W i t h t h e a d d i t i o n o f t h e o t h e r e x a m i n e d c a r b o x y l i c a c i d s , f m in"
• -
-
'
61'
creases:in the following .~CCeSsion::t)'ansaconitic <. tartaric :< tricarballyl.iC ~ < citric < , d i h y d r o x y ~ c acid, ;~A,~m i g h t be expected,'the~ metdu~tabllity period :: o f :;ACP/pKrtlcles~increased ,:'With': inclea~ing ? CA~k. cdncentrati0n;" I n order :to: find o u t at~Whtcl~st~e o f the: p~dVipitation process the ~i n teractioit i between c A / k : rand A C P t a k e s place, s e v e r a l pH-stat~ ;experiments w e r e performed.-The carbox~rlic a c i d was=added -prior t o or.' a t some t i m e a f t e r t h e , f o r m a t i o n ~of the a m o q ) h c u s ~precipitate:= The r e s u l t s 0btained with citric and ' d i h y d r o x y ~ c " a c i d axe p r e s e n t e d in Fig. : 2; . Curve 'A represents the .sysl;em w i t h o u t additives; The first upward surge in NaOH consumption (due to ACP formation) and the following inflection corre spond to tm, while the second steep p a r t signifiesprecipitation o f the erys•
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TIME x 10-Zls Fig. 2, C o n s u m p t i o n o f N a O H as a f u n c t i o n o f t i m e i n p | l - s t a t e x p e r i m e n t s ; 2 9 8 K . Z e r o - t i m e t o t a l r e a c t a n t c o n c e n t r a t i o n s : P O , = Ca = 3 • 10 -= m , ) l d m - ~ ; a d d i t i v e s ( A ) - - ; ( B ) 1 • 1 0 - * t o o | d m -~ c i t r i c a c M a d d e d 1 2 0 s a f t e r s a m p l e p r e p a r a t i o n ; ( C ) 1 - 1 0 - * t o o ! d m -3 c i t r i c s c i J a d d e d p r i o r t o s a m p | e p r e p a r a t i o n ; ( D ) 5 - 1 0 - " t o o l d m -~ d i h y d r o x y t a r t a r f c acid a d d e d 1 2 0 s a f t e r s a m p l e p r e p a r a t i o n ; ( E ) 5 • 1 0 - " t o o l d m -= d i h y droxytarterfe acld added p r i o r t o sample preparation.
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t a l l i n e phase. Because o f c o n s t a n t stirring in t h i s e x p e r i m e n t , t i n , w a s sig-. n i f i c a n t l y shorter than in the comparable experiment ~without stirring (curve 1 in F i g . 1). N o significant change in tm w a s observed w h e n citric or d i h y d r o x y t a r t a r i c avid was a d d e d s h o r t l y a f t e r t h e o n s e t o f precipitat i o n (curves B a n d D) o r a t a n y t i m e a t t e r t h a t ( t h e b r o k e n parts o t curves B a n d D s h o w t h e c o n s u m p t i o n o1~ 2;..OH d u e t o n e u t r a l i z a t i o n o f t h e additives). W h e n t h e additives were a d d e d t o t h e r e a c t a n t solutions, i.e. b e f o r e sample p r e p a r a t i o n (curves C a n d E), t h e o n s e t o f p r e c i p i t a t i o n was d e l a y e d a n d tm was significantly longer. A p p a r e n t l y C A A c o u l d stabilize ACP o n l y w h e n p r e s e n t in statu nascendi, e.g. in t h e very b e g i n n i n g o f precipitate f o r m a t i o n . To facilitate i n t e r p r e t a t i o n , the influence o f c i t r a t e o n t h e surface charge o f p r e c i p i t a t e particles a n d t h e i r I R spectra was also d e t e r m i n e d . T h e electrophore~ic m o b i l i t y o f ACP particles in systems c o n t a i n i n g e q u i m o l a r c o n c e n t r a t i o n s o f t h e precipitating c o m p o n e n t s at p H 7.2 was close t o zero. In t h e p r e s e n c e o f citrate ions t h e surface charge c h a n g e d t o negative and t h e e l e c t r o p h o r e t i c m o b i l i t y was o f t h e o r d e r o f 10 L a m 2 V -= s -= b u t d e c r e a s e d with t i m e as a c o n s e q u e n c e o f particl,~ g r o w t h . H t spectra o f ACF precipitates obt~Ened in t h e pre~ence o f citrate ions ( 1 . 10 -4 tool 6ii
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FREOUENCY Icrn t Fig. 3. I R s p e c t r a of" a m o r p h o u s calc[urri p h ~ s p h s t e s obLalnec~ f r o m p r e c i p i t a t e s : ( A ) i n t h e presence o f 1 • 10 -~ m o l d r n "z c i t r ~ t e ; ( B ) w i t h o u t c i t r a t e .
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dm•3, - Fig. 3) s h o w peaks at 1600~/1620 cm~' and 1 4 0 5 cm~', c o r r e s p o n d i n g to the symmetrical and antisymmetric~ ~ b r a t i o n s o f the C O O - g r o u p . s i m i l a r p e a k s were O b t a i n e d f r o m ~ m i x t u r e s o f p o w d e r e d l s o d i u m citrate a n d calcium p h o s p h a t e b u t n o t f r o m t h e c o n t r o l s ( s y s t e m s p r e p a r e d witho u t additives). " " Model system
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T h e s e c o n d p r e c i p i t a t i o n system was p r e p a r e d at a relatively high supersaturation, i.e. ~a t total c o n c e n t i a t i o n s Ca ffi 5 • 10 -2 m o l d m -3 p~d PO4 -1 • 10 -2 tool d m -3. U n d e r t h e s e c o n d i t i o n s t h e f o r m a t i o n o f intercrystalline m i x t u r e s o f DCPD a n d DA was observed [ 8 ] , Precipitation started imm e d i a t e l y a f t e r m i x i n g o f t h e reactants, a n d t h e r e was c o n s e q u e n t l y a rapid d r o p in pH ( f r o m 7.4 t o ca. 6). During aging o f t h e system for 24 h t h e p H d r o p p e d f u r t h e r t o ca. 5.2. U s i n g this m o d e l system, t h e influence o f t h e series o f CAA o n t h e morp h o l o g y o f DCPD crystals was studied. In Fig. 4 t h e c o r r e s p o n d i n g changes in crystal s h a p e are r e p r e s e n t e d . I t is s h o w n t h a t all effective CAA i n h i b i t e d crystal g r o w t h in t h e same d i r e c t i o n as citrate ions, i.e. in t h e d i r e c t i o n p e r p e n d i c u l a r to t h e ( 0 0 1 ) p l a n e [ 9 ] . With increasing C A A c o n c e n t r a t i o n in t h e system, an increasing t e n d e n c y t o w a r d s t h e f o r m a t i o n o f needleshaped crystals b e c a m e apparent. H a b i t m o d i f i c a t i o n o c c u r r e d even w h e n t h e additive was a d d e d t o t h e s y s t e m after t h e p r e c i p i t a t e h a d already b e e n f o r m e d , T h e i n t e n s i t y o f t h e e f f e c t was analogous w i t h t h e succession: malic :> citric :> transaconittc :> d i h y d r o x y t a r t a r l c :> tricarballylic :> tartaric acid ions. Succinic, fumaric and maleic acid d i d n o t offect t h e shape o f DCPD crystals. D I S C U S S I O N
T w o aspects o f t h e i n t e r a c t i o n o f s o m e di- and tricarboxyl~v acid ions with calcium p h o s p h a t e s w e r e investigated: (1) stabilization o f ACP (Figs. 1, 2 and Table 2) and (2) t h e i n f l u e n c e o n t h e m o r p h o l o g y o f DCPD crystals (Fig. 4). Stabilization of ACP
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T h e e x p e r i m e n t a l results s h o w (Table 2) t h a t all e x a m i n e d organic acids can b e d i v i d e d i n t o t h r e e groups d e p e n d i n g o n t h e i r i n f l u e n c e o n tin: ( I ) malic, succinic, maleic and fumaric acid w i t h n o significant effect, (2) tartaric a n d tricarballylic acid w i t h ~the e f f e c t being d e t e c t a b l e a t higher C A A c o n c e n t r a t i o n s , a n d (3) transaconitic, citric a n d d i h y d r o x y t a r t ~ r i c acid, s h o w i n g a stabilizing e f f e c t a t all e x a m i n e d c o n c e n t r a t i o n s . T o explain this succession o n e mu.st t a k e t h e situation in .the solution .
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F i g . 4 . M i e r o g r 3 p h s o f p r e c i p i t a t e s i s o l a t e d 72 h a f t e r s a m p l e p r e p a r a t i o n . Initial pH 7.4, temperature 2 9 8 K , z e r o - t i m e t o t a l r e a c t a n t c o n c e n t r a t i o n s : i=O4 = 1 . 10-= t o o l d m - J , C a = 6 , ] 0 - * m o l d m - ) . A d d i t i v e s : ( a ) - - ; ( l ~ - g ) 5 . 1 0 -~ m o l d m - ] l a r t a r i c , t r l earballylic, dlhydroxytartartc, transaeonitfc, citric and maiic acid, r e s p e c t i v e l y ; t r a n s a e o n i t i e ~cid: ( h ) 8 • t O - " t o o l d m -3, ( i ) 1 • 1.0 -~ m o i d m -~.
into consideration. Therefore w e u s e d t h e stability constants listed in Table 1 to ca/culate distribution curves w h i c h s h o w t h e relative a m o u n t s o f different ionic species as a f u h c t i o n o f p H . A n e x a m p l e is given in Fig. 6, It ha~ b e e n s h o w n that in the neutral pH region, citric aci~l exists predominantly it) the form o f the negatively charged C a - c o m p l e x ( C a L - ~ 98%)
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[9] w h i l e tricarballylic acid a p p e a r s in t h e f o r m o f t h e n e g a t i v e l y c h a r g e d C a - c o m p l e x ( C a L - ~ 15%) a n d ligand ( L ~ - ~ 8 0 ~ ) (Fig. 5). Also in t h e case o f t a r t a r i c acid t h e negative ligands (L2-: a n d H L , ) a n d t h e n e g a t i v e l y c h a r g e d iC a ~ : o m p l e x ( C a L l - ) a p p e a r in significant a m o u n t . C a r b o x y l i c acids w i t h n o stabilizing e f f e c t h a v e b e e n f o u n d t o a p p e a r in t h e f o r m o f t h e n e u t r a l c o m p l e x (CuL °) a n d t h e n e g a t i v e l y c h a r g e d ligand~ L2-). C o r r e l a t i n g t h e s e findings w i t h t h e d a t a listed in T a b l e 2, w e c o n c l u d e t h a t t h e i n t e r a c t i o n a bi l i t y o f t h e C A A w i t h t h e solid p h a s e d e p e n d s o n (1) t h e c o n c e n t r a t i o n o f t h e negatively c h a r g e d C a - c o m p l e x in s o l u t i o n a n d (2) t h e n t u n b e r o f f u n c t i o n a l g r o u p s ( - - C O O H a n d - - O H ) . A p p a r e n t l y t h e larger n u m b e r o f f u n c t i o n a l g r o u p s o f negatively c h a r g e d species increases t h e p o l a r a t t r a c t i o n b e t w e e n t h e a d s o r b a t e a n d t h e positive sites a t t h e p r e c i p i t a t e / s o l u t i o n interface.
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A c c o r d i n g t o t h e pH-stat e x p e r i m e n t s (Fig. 2) C A A stabilize ACP if p r e s e n t in t h e initial stages o f p r e c i p i t a t e f o r m a t i o n . I R s p e c t r a 3) s h o w t h a t t h e a d d i t i v e is n o t o n l y surface a d s o r b e d (as p r o v e n b y trophoresls) b u t also o c c l u d e d w i t h i n t h e particles, T h e s e findings
only (Fig. elecmay
66
be h e l p f u l in e x p l a i n i n g t h e m e c h a n i s m o f t h e stabilizing a c t i o n o f C A A on ACP. I t has been suggested b y us [4, 5] t h a t A C P s p h e r u l e s are f o ~ n e d by p r e v a l e n t l y h o m o g e n e o u s n u c l e a t i o n a n d s u b s e q u e n t aggregation o f t h e p r i m a r y particles. I f ACP f o r m a t i o n p r o c e e d s in t h e p r e s e n c e o f additives t h e s e m a y b e a d s o r b e d a t t h e s ur f a c e o f t h e p r i m a r y p articles a n d o c c l u d e d d u r i n g s u b s e q u e n t aggregation i n t o t h e s p h e m l e s . O n c e f o ~ e d tlle s p h e • l e s g r o w f o r s o m e t i m e unt i l s e c o n d a r y p r e c i p i t a t i o n o f a c r y s t a l l i n e p h a s e a b r n p t l y c ha nges t h e c o n d i t i o n s in t h e s u r r o u n d i n g s o l u t i o n . X-ray stu d ies have s h o w n [11, 12] t h a t c r y s t a l l i z a t i o n is a discontinuovls p ro cess p r i o r to w h i c h ACP exists in a m e t a s t a b l e state. T h e m e t a s t a b i l i t y p e r i o d o f ACP, tin, c o r r e s p o n d s t o this p e r i o d p r e c e d i n g c r y s t a l l i z a t i o n . Si nc e t h e first f o r m e d c r y s t a l l i n e p r e c i p i t a t e is usually f o u n d in i n t i m a t e c o n t a c t w i t h t he ACP spherules [ 2 ] , c learly t h e p r e c u r s o r serves as a h e t e r o g e n e o u s n u c l e a t o r o f this phase. We m a y t h e n c o n s i d e r tm as an i n d u c t i o n p e r i o d for h e t e r o g e n e o u s n u c l e a t i o n , i t has b e e n f o u n d t h a t this i n d u c tion p e r i o d is i n d e p e n d e n t o f t h e s ur f ace a r e a o f ACP available t o n u c l e a t e [ 1 3 ] . This, t o g e t h e r w i t h t h e finding t h a t CAA have n o e f f e c t w h e n a d d e d after t h e Onset o f p r e c i p i t a t i o n (Fig. 2), s h o w s t h a t t h e rate o f n u c l e a t i o n of t h e crystalline p r e c i p i t a t e is n o t c o n t r o l l e d b y a process at th e A C P / s o l u t i o n interface. We m a y t h e n assume t h a t b e f o r e s e c o n d a r y p r e c i p i t a t i o n sets in (i.e. d u r i n g t h e p e r i o d o f m e t a s t a b f l i t y ) , ACP s p h e ~ l c s are subj e c t to internal r e a r r a n g e m e n t s ( d e h y d r a t i o n , h y d r o l y s i s , crystal o r d e r i n g ) wh ich m a k e t h e m a suitable n u c l e a t o r fo r OCP. A p p a r e n t l y tm d e p e n d s on t h e rate o f t he s e processes w h i c h m a y be significantly c h a n g e d b y t h e p r e s e n c e o f t h e foreign ions w i t h i n t h e a m o r p h o u s particles. This h y p o t h esis is s u p p o r t e d by t h e findings o f M o n t e l e t al. [12] w h o s h o w e d t h a t in situ c o n v e r s i o n o f ACP ( w i t h o u t a n y c h a n g e o f t h e s o l i d / s o l u t i o n ratio) p r o c e e d s as a result o f at least t w o processes, i.e. a c o n t i n u o u s process i n t e r p r e t e d as h y d r o l y s i s a n d a d i s c o n t i n u o u s o n e crystallization. lnfluetzce on the m o r p h o l o g y o f DCPD crystals
T h e i n f l u e n c e o f t h e series o f di- a nd t r i c a r b o x y l i c acid iQns o n th e m o r p h o l o g y o f DCPD crystals m a ni f e s t s as a c h a n g e in cry s tal s h a p e (Fig. 4). H a b i t m o d i f i c a t i o n was i n d u c e d b y p r e f e r e n t i a l a d s o r p t i o n o f negatively c h a r g e d c a r b o x y l i c acid ions or t h e respective C a - c o m p l e x e s o n t h e ( 0 0 1 ) crystal plane [ 9 ] . T h e succession o f t h e C A A with regard t o effectiveness is d i f f e r e n t t h a n in t h e previous case (stabilization o f ACP) a n d c a n n o t be e x p l a i n e d b y c o m p l e x f o r m a t i o n a n d th e n u m b e r o f f u n c t i o n a l groups. We m u s t t h u s l o o k f o r s o m e specificity o f t h e i n t e r a c t i o n . T h e p h e n o m e n o n o f h a b i t m o d i f i c a t i o n b y a n o rg an ic m a t e r i a l c a n be e x p l a i n e d in several w a y s [ 1 4 | . R e c e n t t h e o r i e s a t t r i b u t e a n i m p o r t a n c e t o t h e c o n f i g u r a t i o n o f t h e p o l a r g r o u p s o f t h e a d s o r b e d species. W h e t s t o n e and c o - w o r k e r s [15, 16] s t u d i e d t h e i n f l u e n c e o f a large n u m b e r o f o r g a n i c
67
a c i d m o l e c u l e s o n t h e m o r p h o l o g y o f NH4NO3, IKNO3 a n d NaNO3, a n d concluded that crystal habit modification o~ginates only w h e n a polar g r o u p fits t h e ion a r r a n g e m e n t s l o f t h e sub3trate. T h e effectiveness o f additives as" h a b i t m o d i f i e r s ~of h e x a n e d i o i e acid [ 1 7 ] a n d c a l c i u m s u l p h a t e [ 1 8 ] have b e e n a c c o r d i n g l y e x p l a i n e d . I n o r d e r t o e x p l a i n th~ i n t e n s i t y o f t h e e f f e c t o f t h e individual c a r b o x y l i c acid a n i o n o n D C P D it is n e c e s s a r y t o k e e p an a c c o u n t o f t h e g e o m e t r i c a l a r r a n g e m e n t a n d n u m b e r o f t h e i r p o l a r groups, I t seems t h a t t h e H O C O C H ( O H ) ( C O O H ) C H - - g r o u p is t h e m o s t effectiv e o n e (malic a n d c i t r i c acid). M u c h less e f f e c t i v e are c a r b o x y H e acids w i t h a d d i t i o n a l - - O H g r o u p s att a c h e d t o t h e n e i g h b o u r i n g (3 a t o m s (tartaric a n d d i h y d r o x y t a r t a r i c acid). T h e n u m b e r o f c a r b o x y l i c g r o u p s plays a role t o o . D i c a r b o x y l i c acids, like m a l e i c , f u m a r i c a n d succinic acid, d i d n o t a d s o r b at t h e D C P D cry stal su rfa c e w h i l e t r i c a r b o x y l i c acids w i t h v e r y similar s t r u c t u r e (tricarballylic a n d t r a n s a c o n i t i c acid) did. We c o u l d n o t f/nd a s a t i s f a c t o r y c o r r e l a t i o n b e t w e e n t h e Ca 2+ spacings in t h e c r y s t a l l o g r a p h i c ( 0 0 1 ) p l a n e o f t h e D C P D cry s tal ( c a l c u l a t e d f r o m t h e c r y s t a l s t r u c t u r e given in Ref. [ 1 9 ] a n d t h e a n i o n spacings o f t h e effective C A A groups. I t s h o u l d , h o w e v e r , b e t a k e n i n t o a c c o u n t t h a t ads o r p t i o n o c c u r s at t h e c r y s t a l / s o l u t i o n in terface w h e r e ionic a r r a n g e m e n t s m a y be d i f f e r e n t t h a n in t h e c r ys t a l lattice. I t m a y t h e r e f o r e b e futile t o s p e c u l a t e a b o u t possible g e o m e t r i c fits o f t h e m o l e c u l a r s t r u c t u r e o f C A A w i t h t h e ions in t h e substrate. ACKNOWLEDGEMENTS T h e a u t h o r s wish t o t h a n k Dr. B. Koji~-Prodi~ for X-ray d i f f r a c t i o n w o r k a n d a c k n o w l e d g e t h e financial s u p p o r t o f t h e S e l f m a n a g e m e n t C o u n c i l for S c i e n t i f i c R e s e a r c h o f S,R. C r o a t i a (SIZ V) an d o f t h e N a t i o n a l I n s t i t u t e s o f H e a l t h , B e t h e s d a , Md ( p r o j e c t No. 02-092-N). REFERENCES
I
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