Langmuir 1989, 5 , 150-157
150
Topotactic Li+ Insertion to X-Mn02in the Aqueous Phase Kenta Ooi,*9t Yoshitaka Miyai,t Shunsaku Katoh,t Hiroshi Maeda,* and Mitsuo Abes Government Industrial Research Institute, Shikoku, 2-3-3 Hananomiya-cho, Takamatsu 761, Japan, Department of Chemistry, Faculty of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan, and Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152, Japan Received June 13, 1988. I n Final Form: September 9, 1988 The topotactic insertion reaction of lithium ions with X-Mn02was investigated in LiCl + LiOH solutions. The insertion of lithium ions involved a consumption of a nearly equivalent amount of hydroxyl ions in the aqueous phase, as well as an evolution of about a 90% equivalent amount of oxygen gas. Chemical analyses of the Li+-inserted samples showed that this insertion involved a reduction of Mn(1V) to Mn(II1). The major reaction of Li+ insertion could be represented from these results not by an ion exchange but by a new type of redox mechanism: MnOz + xLiOH Li,Mn02 + (x/2)H20+ ( x / 4 ) 0 2 ,where x is the number of moles of inserted lithium per mole of manganese. An X-ray diffraction analysis showed that the insertion of lithium ions proceeded topotactically, involving an increase in the lattice constant (ao) of a spinel structure. A new phase, having a slightly larger a. value than that of X-Mn02,was observed for samples with Li/Mn ratios between 0.08 and 0.2. The a. value of the new p h e increased monotonously with an increase in the Li/Mn ratio in the range Li/Mn > 0.2. The electrode potential vs log C L plot ~ for the Li+-inserted sample (Li/Mn = 0.32) gave a straight line with a near-Nernstian slope (0.060 V/log Cv) at pH 8.3; the electrode potentials were nearly constant, independent of the metal ion concentration in the case of sodium or potassium ions. The electrode potential in a 0.1 M LiCl(1 M = 1mol dm-s) solution decreased with an increase in the Li/Mn ratio of the sample. The potential response to the Li/Mn ratio could be well correlated with the change in the crystal phase of the sample.
-
Introduction Ion-sieve manganese oxides show excellent selectivities for certain ions or groups of ions.1-8 They are prepared by a crystallization of manganese hydroxide containing metal ions, followed by a topotactic extraction of metal ions with acid. Their absorptive properties depend on the kind of metal ions introduced into the manganese-hydroxide precursor. When lithium ions are used as an introducing ion, the ion-sieve manganese oxide obtained shows a high selectivity for lithium ion^.'^^^^ In pH titration studies remarkably high affinities for lithium ions among alkali metal ions have been shown in the range above pH 3. While being prepared, its lithium-selective property is closely related to the formation of such materials as LiMn204during thermal crystallization. LiMn204 has a spinel structure with lithium at the tetrahedral sites and manganese(II1) and manganese(1V) at the octahedal sites of a cubic closed-packed (ccp) oxygen f r a m e ~ o r k . ~An acid treatment of LiMnz04 causes the removal of nearly all of the lithium while maintaining a spinel structure. Shen et al. have proposed a H+/Li+ ion-exchange mechanism for the topotactic extraction of lithium from LiMnz04(LiMn204+ H+ HMnZO4+ Li+h5 On the basis of this mechanism, it has been shown that the ion-sieve manganese oxide has protonated tetrahedral sites located on the ccp oxygen framework. The inserted protons can be selectively ion-exchanged with the lithium ions in a surrounding solution. Pure LiMnz04 has been obtained through a different preparation procedure, e.g., by a solid-phase reaction of a mixture of MnzO, and Li2C03a t relatively high temperature. An acid treatment of this material also yields a spinel-type manganese oxide (h-Mn0z).1(t12Hunter has proposed a surface disproportionation mechanism, instead of one involving an ion exchange, for a topotactic extraction of lithium from the LiMn204with acid (2LiMn204+ 4H+
-
'Government Industrial Research Institute, Shikoku. Nagoya University. *Tokyo Institute of Technology.
*
-
3MnO2 + 2Li+ + Mn2++ 2HZ0).l0 On the basis of this mechanism, X-MnOZhas been shown to have a structure that is related to spinel but that has vacant tetrahedral sites instead of protonated tetrahedral sites. Structure refinements of the spinel-related manganese oxides by neutron diffraction technique have shown that the [MnZ]O4 framework of the Li[Mnz]04spinel remains intact during both lithium insertion and e ~ t r a c t i 0 n . l ~The electrochemical insertion of lithium ions in X-Mn02 has been studied in the organic phase from a standpoint of the development of a second battery material.14J6 Because of its uniform character, X-MnOZis suitable as a model compound for a fundamental study of the "ionsieve" property in the aqueous phase. We have studied the absorptive properties of A-MnO, in the aqueous phase.16-18 It shows a remarkably high selectivity for (1) Vol'khin, V. V.; Leont'eva, G. V.; Onolin, S. A. Izu. Akad. Nauk. SSSR, Neorg. Mater. 1973, 9, 1041. (2) Leont'eva, G. V.; Vol'khin, V. V. Zh. Prikl. Khim. 1971,44, 2615. (3) Shen, X. M.; Wang, X. T. Huaxue Xuebao 1981,39, 711. (4) Tsuji, M.; Abe, M. Solu. Extr. Ion Exch. 1984, 2, 253. (5) Shen, X. M.; Clearfield, A. J. Solid State Chem. 1986, 64, 270. (6) Ooi, K.; Miyai, Y.; Katoh, S. Sep. Sci. Technol. 1986, 21, 755. (7) Ooi, K.; Miyai, Y.; Katoh, S. Sep. Sci. Technol. 1987, 22, 1779. (8) Miyai, Y.; Ooi, K.; Katoh, S. Sep. Sci. Technol. 1988, 23, 179. (9) Wickham, D. G.; Croft, W. J. J . Phys. Chem. Solids 1968, 7, 351. (10) Hunter, J. C. J. Solid State Chem. 1981, 39, 142. (11) Mosbah, A.; Verbaere, A.; Tournoux, M. Mater. Res. Bull. 1983,
18, 1375. (12) Goodenough, J. B.; Thackeray, M. M.; David, W. I. F.; Bruce, P. G. Rev. Chim. Miner. 1984,21, 435. (13) David, W. I. F.; Thakeray, M. M.; de Picciotto, L. A.; Goodenough, J. B. J . Solid State Chem. 1987, 67, 316. (14) Hunter, J. C.; Tudron, F. B. Manganese Dioxide Electrode Theory and Practice for Electrochemical Applications; Schumm, B., Jr.,
Middau, R. L., Grotheer, M. P., Hunter, J. C., Eds.;The Electrochemical Society, Inc.: Penningbon, NJ, 1985; p 444. (15) Thackeray, M. M.; Johnson, P. J.; de Picciotto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984,19, 179. (16) Ooi, K.; Miyai, Y.; Katoh, S. S o h . Extr. Ion Exch. 1987,5, 561. (17) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Bull. Chem. SOC. Jpn. 1988, 61, 407.
0743-74~3/89/2405-0l50$01.50/0 0 1989 American Chemical Society
Topotactic Li+ Insertion to X-Mn02 lithium ions in a range above pH 2.5. The distribution coefficients of metal ions at pH 4 are in the order Na+ C K+ C Rb+ C Cs+ 0.1. The areas of these DTA peaks and the weight loss around 570 O C are plotted against the Li/Mn mole ratio in Figure 9. The weight loss around 570 OC decreased linearly with an increase in the Li/Mn ratio. The decrease in the weight loss could be well fitted to the theoretical line derived from the following reaction: Li,Mn02 5700~-Li,Mnzx04, + [(l - 2x)/2]Mn203 + [(I - 2~)/4102(gas)(9) where the weight loss (%) = 9.2(1 - 2%) (10) In the above equation, x denotes the Li/Mn mole ratio.
Ooi et al.
154
-
e
0
0
100
300
500
0.2 0.3 L i / M n ratio
0.1
0
0
0.4
Figure 9. DTA peak areas (top) and weight loss (bottom) as a function of the Li/Mn mole ratio. Top: 0,exothermic peak around 280 "C; A, s u m of endothermic peaks around 520 and 570 "C; 0,endothermic peak around 520 O C . Bottom: weight loss between 400 and 600 "C. The dotted line corresponds to the weight loss calculated by using eq 9 and 10.
Temperature/ "C Figure 7. DTA curves. Symbols are the same as those in Table I.
Phase 1
~
Phase 2 0% (
I8
#
,
Ht
ehert conductor
I \
~
Phase 3
~
,\
\
tit+
,- 4 + P Soli phase
H 2 0 , Li' OH-
Electrotyte
Figure 10. Phase scheme of a X-Mn02 homogeneous-phase system in contact with an inert conductor and electrolyte.
100
300
500
Temperature/ "C Figure 8. TG curves. Symbols are the same as those in Table I. Equation 9 indicates that the LiMnzOl part does not change upon heating above 570 O C , while the Mn02 part transforms to Mn203upon heating. The agreement between the theoretical line and the experimental results supporta a redox mechanism which involves the reduction of Mn(1V) by Li+ insertion. The area of the exothermic peak decreased steeply with an increase in the Li/Mn ratio, due to an inhibitory effect of the inserted Li+ ions on the transformation from a spinel type to /3-Mn02. The peak area became zero at a Li/Mn value (0.29) considerably smaller than the maximum Li/Mn ratio (0.42). This suggests a delocalized distribution of Li+ ions in the solid; such a delocalization of Li+ ions may cause an inhibition of the thermal transformation even a t lower Li/Mn ratios than 0.42. The X-ray diffraction pattern of a sample (Li/Mn = 0.29) heat-treated at 350 O C showed new diffraction peaks at d = 0.485 and 0.456 nm in addition to the original peaks at dill = 0.472
nm. These new peaks may be attributed to the thermally decomposed product of Lb.5Mnz04. Electrode Potential. There have been many reports concerning the electrode potential of manganese oxide in the aqueous phase.21v22i2&-40 The electrochemical process in the present case is complex compared with those for /3-Mn02 and y-MnOz, since X-Mn02 involves the Li+ insertion reaction in addition to the general electrochemical reaction involving the insertion of protons (MnOz + H+ + e- MnOOH). The insertion reaction of Li+ ions in manganese oxide has been studied extensively in the or-
-
(26) Korver, M. P.; Johnson, R. S.; Cahoon, N. C. J. Electrochem. SOC. 1960,107, 587. (27) Ferrell, D. T.; Voeburgh, W. C. J. Electrochem. SOC.1961,98,334.
(28) Convington, A. K.; Talkdar, P. K.; Thirsk, H. R. Electrochem. Technol. 1967,5, 523. (29) Kozawa, A.; Powers, R. A. Electrochem. Technol. 1967, 5, 535. (30) Kozawa, A.; Powers, R. A. J. Electrochem. SOC.1966,113, 870. (31) Era, A.; Takehara, 2.;Yoshizawa, S. Electrochim. Acta 1968,13, 207. (32) Caudle, J.; Summer, K. G.; Tye, F. L. J. Chem. SOC.,Faraday Trans. 1973,69, 885. (33) Tari, I.; Hirai, T. Electrochim. Acta 1981,26, 1657. (34) Tari, I.; Hirai, T. Electrochim. Acta 1982, 27, 149. (35) Tye, F. L. Electrochim. Acta 1976,21, 415. (36) Maskell, W. C.; Shaw, J. E. A.; Tye, F. L. J. Power Sources 1982, 8, 113. (37) Maskell, W. C.; Shaw, J. E. A,; Tye, F. L. Electrochim. Acta 1983, 28, 225. (38) Maskell, W. C.; Shaw, J. E. A.; Tye, F. L. Electrochim. Acta 1983, 28, 231. (39) Tye, F. L. Electrochim. Acta 1985, 30, 17. (40) Atlung, S.; Jacobsen, T. Electrochim. Acta 1981, 26, 1447.
Lungmuir, Vol. 5, No. l, 1989 155
Topotuctic Li+ Insertion to X-Mn02 ganic phase from a viewpoint of the development of Li/ Mn02 battery materia1.14J6>4143On the basis of analyses of homogeneous-phase reactions for MnOz/Mn00Hn~37~40 and Prussian blue/Everitt's salt systems,44we can consider the phase scheme for a X-MnO2system in contact with an inert metal and electrolyte, in which the H+ and Li+ transport reaction is active (Figure 10). The thermodynamic equilibrium conditions are given as GHS
= aHl
(12)
= Gem
(13)
where ai is the electrochemical potential of species i and superscripts s, 1, and m signify the phases of the solid, liquid, and metal, respectively. The electrode potential can be derived from these equilibrium conditions (Appendix A), using eq 11 and 13 as follows:
E = (RT/F)[(u," + uH'+J)/RT+ In U H
+ (e In Z / d N , ) + (8 In z/dNH)] (14)
Equivalently, from eq 12 and 13
E = (RT/F)[(uem + uL/@)/RT + In uLi + (a In Z / d N , )
+ 0.059 log uLi(at 25 OC)
(17)
Since the Li+ and proton contents of the solid phase vary with the pH or Li+ concentration in the solution, the prerequisite for eq 16 is fulfilled only under limited conditions. The condition that both Li/Mn ratio and pH remain constant for different Li+ concentrations in the aqueous phase could be obtained for a sample with the Li/Mn ratio of 0.32 in a NH,C1-NH3 buffer solution at pH 8.3. A plot of the electrode potential vs the logarithm of the Li+ ion concentration (log CLi)shows a straight line with a slope of 0.060 (V/log CLi)(Figure 11); this value is close to the theoretical value expected from eq 17 and is consistent with the following redox reaction:
+ xLi+ + xe
-
0.3
1-
0.5
0.3 -3
-4
-2
-1
0
Log CJ r n ~ l a d r n - ~
Figure 11. Electrode potential at pH 8.3 as a function of the metal ion concentration. Sample, 300 mg; solution, 1M NH C1 + NH3 (aq) buffer solution containing metal chloride, 10 cm4 0 LiCl in X-Mn02;0 , NaCl in X-Mn02;@, KCl in h-Mn02;A,LiCl in @-Mn02;4, KC1 in @-Mn02.
+ (e In Z/dNLi)]
In the above, uemis the chemical potential of an electron in the metal phase, is the standard chemical potential of species i in the liquid phase, and Ni and ai are the number of species i in the solid phase and the activity in solution, respectively. Z represents the partition function for the degrees of freedom associated with the distributions of electrons, Li+ ions, and protons; it varies with the Li/Mn and H/Mn ratios of the solid. Equations 14 and 15 indicate that the electrode potential can be described in terms of the activity of either the protons or the lithium ions. When the last two terms on the right-hand side of eq 15 are independent of uLi,we have E (V vs SCE) = const + ( R T / F ) In U L ~ (16)
Mn02
1
0.6
-3'
(15)
= const
:,
(11)
UL? = UL/ GB ,
0.5
Li,Mn02
(18)
In the case of Na+ or K+ ions, however, the electrode po(41) Ikeda, H.; Saito, T.; Tamura, H. Proceedings of the Manganese Dioxide Symposium. Vol. I; Kozawa, A., Brodd, R. J., Eds.; I.C. Sample Office: Cleveland, OH, 1976; p 384. (42) Ohzuku, T.; Kogo, Y.; Hirai, T. The 2nd Battery Material Symposium; Kordesch, K. V., Kozawa, A., Eda.; IBA: Cleveland, OH, 1985; p 391. (43) Nardi, J. C. in ref 14, p 419. (44) Ohzuku, T.;Sawai, K.; Hirai, T. J. Electrochem. SOC.1985,132, 2828.
I
3 t
-
0.50
0.1
0.2
0.3
0.4
LilMn ratio
Figure 12. Electrode potential response to the Li/Mn ratio of Li+-inserted sample. Sample, 300 mg; solution, 0.1 M LiC1, 10 cm3; -., eq 19;--, eq 20. tential was nearly constant, independent of the metal ion concentration. These results indicate that a redox-type insertion reaction occurs for Li+ ions alone and does not proceed for the other alkali metal ions. The Li+-selective response of X-Mn02 may be due to an ion-sieve effect of the vacant tetrahedral sites of the ccp oxygen framework; the vacant tetrahedral sites may be too narrow for Na+ or K+ ions to enter. The exclusions of Na+ and K+ ions from the vacant sites have also been observed in a nonaqueous cell of X-MnO,; the discharge of X-Mn02 in an electrolyte containing sodium or potassium salts instead of lithium salts gave a very poor discharge perf~rmance.'~ The Li+-selectiveresponse was unique for X-Mn02 and was not observed in the @-MnO2system (Figure 11). Potential Response to Li/Mn Ratio. The electrode potential appearing in a topotactic electrochemical reaction of metal oxide strongly depends on the crystal and electronic natures of the material. The electrode potentials measured in 0.1 M LiCl solutions for samples with different Li/Mn ratios are shown in Figure 12. According to the redox model expressed in eq 7 , the plot of the potential vs Li/Mn ratio corresponds to the discharge curve
Ooi et al.
156 Langmuir, Vol. 5, No. 1, 1989 of X-Mn02 by the insertion of Li+ ions. The potential decreased with the Li/Mn ratio up to 0.08, then became nearly constant between 0.08 and 0.18, and again decreased continuously above 0.18. On the basis of the criteria for a homogeneous- or heterogeneous-phase electrochemical r e a c t i ~ nthe , ~ above ~ ~ ~ ~result shows that the discharge of X-Mn02 by Li+ insertion proceeds through homogeneous-phase (Li/Mn C 0.08), heterogeneous-phase (0.08 C Li/Mn C 0.18), and homogeneous-phase (Li/Mn > 0.18) reactions. The electrochemical behavior was well correlated with a change in the crystal phase of the solid. This indicates that the electronic structures of Li+-inserted X-Mn02 strongly depend on their crystallographic nature. The potential response to the Li/Mn ratio in the homogeneous-phase regions (Li/Mn < 0.08 and > 0.18) was analyzed under the assumption of an independent mobility of the electrons and Li+ ions in the solid phase (the effect of protons was ignored because of negligible proton contents in the solid). Two different models were considered by analogy with the models proposed for the discharge behavior of electrolytic manganese o ~ i d e . ~ One ~ , ~ con" sidered the free mobility of all the inserted Li+ ions (one solid-solution model). The other considered the free mobility of the Li+ ions in the range above the mid-insertion point (two solid-solution model); this assumes the presence of stable species of Lio,5Mn204.The electrode potentials are given by the following two equations a t a constant activity of Li+ ions in solution at 25 "C (Appendix B). For a one solid-solution model
E = const - 0.059 log [x2/(1 - x)(0.42 - x ) ]
Li+ insertion. Therefore, the Li+ insertion in X-Mn02can proceed without destruction of the spinel structure. Appendix A Derivation of Eq 14 and 15. The thermodynamic equilibrium conditions for proton and Li+ ions at the phase boundary solid/electrolyte can be obtained from eq 11and 12 as UH'
+ FP = UH' + FJ/1
(AI)
+ FP = uL) + FJ/1
(A2)
and uLi8
Here ui and # are the chemical and electric potentials, respectively. Equations A1 and A2 give
F(P - J/1) = UH'
- UHs
= ULil - UL?
(A3)
The thermodynamic equilibrium condition for electrons at the metal/solid phase boundary (eq 13) is given by uern- FJP" = u,S - F P
(A41
where ui is the chemical potential and superscripts m, s, and 1 signify the phases of metal, solid, and electrolyte, respectively. Equation A4 gives
F('"
-
P) = u , -~u,S
(A5)
The electrode potential is given by
E = $P- $1
=P-P+P-J/1
(19)
(A6)
for a two solid-solution model E = const - 0.059 log [ x ( x - 0.21)/(1 - x)(0.42 - x ) ] (20)
From eq A3, A5, and A6
In the above x represents the Li/Mn ratio of the solid. Equation 19 well describes the potential behavior in the range x C 0.08, while eq 20 gives a much better fit to the experimental data than eq 19 in the range x > 0.2. This also supports our conclusion that the modes of Li+ insertion are different below and above the mid-insertion point.
The chemical potential in the solution phase can be written as
Conclusion The topotactic insertion of lithium ions in X - M n 0 2 could be represented by a new type of redox mechanism in the aqueous phase involving the evolution of oxygen gas and the reduction of Mn(1V) to Mn(II1). The electrode potential measurement suggests the independent mobility of Li+ ions and electrons in X-Mn02. Since a mixed Mn site valence produces a good electronic conductor of the spinel,12the Li+ insertion process represented by eq 7 can be divided into the following two steps: (1)insertion of lithium ions into tetrahedral vacant sites of the ccp oxygen framework of X-Mn02,accompanying a reduction of M n (IV) to Mn(II1) Mn02 + xLi+ + xe -., Li,Mn02
u? = -RT(d In Z/dNi)
and (2) a migration of excess positive charge (-xe) to the surface of X-Mn02 powder followed by the oxidation of hydroxyl ions in the aqueous phase xOH-(s) -., (x/2)H20(s) + (x/4)02(s) + xe In the above equation, (s) represents the surface of a particle. According to this model, it is not necessary for the sites for OH- oxidation to be neighboring the sites for (45) Kozawa, A.; Powers, R. A. J.Chem. Educ. 1972, 49, 587.
E =
(l/F)(Ue"
- U:
= (l/F)(ue" - ues
u/ =
+ UH' - UH')
+ U L / - UL?)
+ RT In ai
(-47)
(AB)
where uil,o and ai are the standard chemical potential and the activity of species i, respectively. The chemical potential in the solid phase can be written (-49)
where 2 is a partition function of the solid. From eq A7, A8, and A9, the electrode potential can be written as
E = ( R T / F ) [ ( u e r+n uH1*O)/RT + In a~
+ (8 In Z / d N e ) + (a In z/aNH)] (14)
E = (RT/F)(uem+ U ~ ~ ) / +R InT aLi+ (a In Z / a N e ) + (a In Z/dNLi)] (15) Appendix B Derivation of Eq 19 and 20. Equation 15 gives the electrode potential a t a constant activity of Li+ ions in solution: E = const + ( R T / F ) [ ( aIn Z / a N e ) + (a In Z/dNLi)] (Bl) When an independent mobility of electrons, Li+ ions, and protons is assumed, the partition function can be represented by the product of the partition function for each species: Z = ZeZLiZH (B2) When all sites for species i are assumed to have the same
Langrnuir 1989,5, 157-160 energy
up,the partition
function Zi can be written
as40
Zi = [NiJ/Ni!(Nit - N,)!]exp(-Niup/RT) (B3) where Nitand Ni are the total possible number of sites for species i and its number in the solid phase, respectively. The chemical potential of species i in the solid phase can be written by using the Stirring approximation (In N ! = N l n N - N ) as ut = -RT(d In Zi/dNi) = u;v0 RT In [Ni/(Ni,- Ni)]
+
(B4)
By substituting eq B2 and B4 into B1, we can obtain E = const - ( R T / F ) In [ N e / ( N e t Ne)]( R T / F ) In [NLi/(NLit- NLJI (B5)
In a model of the movement of electrons among manganese sites and the movement of Li+ ions among vacant tetrahedral sites, Nut is equal to 0.42Ne,. By introducing this relation and the condition of electroneutrality (Ne = Nu) into eq B5, we obtain
157
E = const - ( R T / F ) In [N2/(Net - NJ(0.42Net - N e ) ] (B6) = const - 0.059 log [x2/(1 - x)(0.42 - x ) ] (at 25 "C) (19) where x is the Li/Mn ratio (Ne/Net). In a model of two solid solutions, in which the sites for Li+ ions are different below and above the mid-insertion point, the total number of possible sites for Li+ ions (Nu() and the number of movable Li+ ions (NL[)can be calculated for the system above the mid-insertion point: N~it'= 0.5NLit; NL[ = N L -~0.5NLit (B7) When we assume a movement of electrons among manganese sites, the electrode potential in the range 0.21 < x < 0.42 can be calculated from eq B5 as E = const 0.059 log [ X ( X - 0.21)/(1 - ~)(0.42- x ) ] (25 "C) (20) Registry No. Li, 7439-93-2; MnOz, 1313-13-9.
Effect of Fatty Acyl and Cation Content of Cardiolipins on in Vitro Calcification Evangelos Dalas, Panayiotis V. Ioannou, and Petros G. Koutsoukos* Department of Chemistry and Research Institute of Chemical Engineering and Chemical Processes at High Temperatures, P.O. Box 1239, University Campus, GR-26110 Patras, Greece Received May 25, 1988. In Final Form: September 13, 1988 Since cardiolipin, CL, among other acidic phospholipids is known to affect biological calcification by nucleation and growth of hydroxylapatite, HAP, the role of its fatty acyl and cation content was investigated by a sustained supersaturation technique using synthetically prepared saturated cardiolipins. It was found that both its fatty acyl and cation content affected in a complex way the induction as well as the rate of HAP growth. In general, short-chain CLs resulted in short induction periods that strongly depended on their cation content while long-chain CLs, irrespective of their cation content, yielded higher rates of HAP overgrowth. The kinetics of nucleation and growth of HAP were typical of the HAP forming on synthetic HAP seed crystals.
Introduction The mineralization of tissues in mammals, including man, is known as calcification; studies of this process are mainly concerned with the formation of calcium phosphates, the inorganic constituents of bone and teeth. Under physiological conditions, the thermodynamically most stable calcium phosphate phase is hydroxylapatite (Ca5(P04)30H,HAP). In the past 20 years there has been increasing evidence that the formation of HAP in vertebrates is associated with phospholipids (heretofore PLs).14 It is suggested5 that mineralization takes place a t specific sites, controlled by proteins and proteoglycans via a two-stage mechanism. On the basis of experiments both in vitro and in vivo, it has been proposed that a superstructure, consisting of an acidic (1) hnever, J.; Vogel, J. J.;Benson, L. A. J. Dent. Res. 1973,52,1056. (2) Odotuga, A. A.; Prout, R. E. S.;Hoare, R. J. Arch. Oral Biol. 1975, 20, 311. (3) Boskey, A. L. Metab. Bone Dis. Relat. Res. 1978, I , 137. (4) Vogel, J. J.; Boyan-Salyers, B. D.; Cambell, M. M. Metab. Bone Dis. Relat. Res. 1978, 1, 149. (5) Levy, R. J.; Lian,J. B.; Gallop, P. Biochem. Biophys. Res. Commun. 1979, 91, 41.
0743-7463/89/2405-0157$01.50/0
PL (like phosphatidylserine, PS, phosphatidylinositol, PI, cardiolipin, CL, and recently phosphatidic acid, PA), Ca2+, and phosphate ions is first formed, serving as the initiation nucleus for HAP d e p o ~ i t i o n . ~The * ~ second stage involves the rate of growth of the HAP nuclei formed, as well as the orientation of HAP crystals. At this stage, various macromolecules such as proteoglycans and noncollagenous proteins are thought to be i n v o l ~ e d . ~ ~ ~ The role of fatty acyl chains and the cations of the acidic PLs with respect to nucleation and subsequent growth of HAP crystals has not received due attention. The divalent cations may affect nucleation and crystal growth of HAP because of their interactions with both the acidic PLs and the inorganic phosphate. Cardiolipin is a phospholipid found in the inner mitochondrial membrane of the eukaryotic cells, and its function is uncertain.1° CL can form a Ca-CL-PI complex (6) Boskey, A. L.; Posner, A. S.Calcif. Tissue Res. 1977,1977,23,251. (7) Raggio, C. L.; Boskey, A. L.; Boyan, B. D.; Urist, M. Trans. Annu. Meet-Orthop. Res. SOC.1983, 8, 20. (8) Vogel, J. J. J. Colloid Interface Sci. 1986, 111, 152. (9) Blumenthal, N. C.; Posner, A. S.;Silvermann, L. D.; Rosenberg,L. C. Calcif. Tissue Int. 1979, 27, 75.
0 1989 American Chemical Society
