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Langmuir 1991, 7, 2422-2424
Adsorption of Low Molecular Weight Poly(acry1ic acid) on Hydroxyapatite: Role of Molecular Association and Apatite Dissolution D.N. Misra American Dental Association Health Foundation, Paffenbarger Research Center, Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received October 3, 1990. I n Final Form: June 23, 1991 Adsorption of low molecular weight poly(acry1ic acid) from aqueous solution on hydroxyapatite shows that the isotherm first rises, reaches a maximum, then decreases,and is irreversible. This is qualitatively explained on the basis of the increasing self-associationof polymeric molecules with concentration and the inability of associated molecules to adsorb as their hydrogen-bonding capability is used up. As surface effects decrease with the increase in initial concentration of the acid, the Ca to P ratio in solution rises and eventually reaches the experimental ratio in the apatite. The initial part of the isotherm satisfies the Langmuir plot. The ratio of the geometrical area of the molecule to the area derived from the Langmuir plot is about the same as that obtained by DiMarzio on the basis of theoretical considerations.
Introduction Adsorption phenomenon for molecular compounds on hydrated surfaces, e.g., hydroxyapatite, very much depends upon the interplay of hydrogen bonding between solute, solvent, and Not only the shape and nature of the isotherm but also ita reversibility depends upon this interplay. These considerations do not generally apply if the solvent is water and particularly if the adsorbate is an ion. In the latter event the charge-determining ions"" and specifically the hydrogen ion concentration of the solutionl2-lB play important roles. In any case, the possibility of a reaction with adsorbent" must be ruled out and complexation on the surface and/or in solution should be examined. It was considered interesting to study the adsorption of low molecular weight poly(acry1ic acid) from aqueous solution onto synthetic hydroxyapatite (with hydrated to delineate the role of some of the above factors. In order to explore the interaction or role of hydrogen, calcium, and phosphate ions in the adsorption process, their concentrations were also monitored in the filtrate. As hydroxyapatite is the structural prototype of bone or tooth mineral, this study may also help elucidate (1) Misra, D. N.; Bowen, R. L. Biomaterials 1981, 2, 28. (2) Misra, D. N. In Adsorption on and Surface Chemistry of Hydroxyapatite; Misra, D. N., Ed.; Plenum: New York, 1984; p 105. (3) Misra, D. N. In Methods of Calcified Tissue Preparation; Dickson, C. R., Ed.; Elsevier: New York, 19&1; pp 435,442. (4) Misra, D. N. J. Dent. Res. 1985, 64, 1405. (5) Misra, D. N. J. Dent. Res. 1986,65, 706. (6) Misra, D. N. Langmuir 1988, 4, 953. (7) Misra, D. N. J. Dent. Res. 1989, 68, 42. (8) Misra, D. N. J. Colloid Interface Sci. 1990,135, 363. (9) Somasundaran, P. J . Colloid Interface Sci. 1968,27,659. (10) Saleeb, F. Z.; DeBruyn, P. L. J. Electroanah Chem. Interfacial Electrochem. 1972,37,99. (11) Bell, L. C.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1973, 42, 250. (12) Bell, L. C.; Posner, A. M.; Quirk, J. P. Nature 1972,239, 515. (13) Chander, S.;Fuerstenau, D. W. J. Colloid Interface Sci. 1979,70,
506. (14) Chander, S.; Fueratenau, D. W. Colloids Surf. 1982,4, 101. (15)Mishra, R. K.; Chander,S.; Fueratenau,D. W. ColloidsSurf.1980, 1, 105. (16) Lin, J.; Raghaven, S.; Fuerstenau, D. W. Colloids Surf. 1981,3,
357.
(17) Misra, D. N. Calcif. Tissue Int. 1991, 48,362. (18)Dry, M. E.; Beebe, R. A. J. Phys. Chem. 1960,64, 1300. (19) Rootare, H. M.; Craig,.R. G. J. Dent. Res. 1978, 56, 1437. (20) Misra, D. N. Calcif. Tassue Int. 1986, 38, 333. (21) Loebenstein, W. V. J. Dent. Res. 1973,52, 271.
certain facets of the mechanism of adhesion of polyacrylate or ionomeric cementa with bone or
Materials and Methods2s Hydroxyapatite. Tribasic calcium phosphate [Fishercertified, C-127,with a chemical formula given as approximately Calo(OH)z(PO&]was repeatedly washed with boiling distilled water before use; the physical and chemical details of its characterizationhave been rep0rted.s It had a surfacearea (BET, Nz) of 41.0 m2/g, and Ca/P = 1.57 0.03. The amount of physically adsorbed water on apatite was 1.58%) which is equivalentto about 1.5monolayers. X-ray analysis showed that dicalcium phosphate dihydrate is probably less than 1% of the weight of apatite. Poly(acry1ic acid). A solution of the polymer (65 wt %, average molecular weight 2000) was obtained from Aldrich ChemicalCo., Milwaukee,WI. An appropriateamount of solution was diluted with distilled water to give an acid concentration of about 2 g/100 mL of solution. The exact concentration was determinedby taking a given volume of solution and evaporating off the water at 105 O C at reduced pressure. The concentration of poly(acry1ic acid) in solution was determined from its absorbance at 203 nm using quartz spacers on a double beam spectrophotometer (Varian, DMS 80). The absorbance vs concentration plots followed an identical Beer's law curve when the stock solution that was used was dilutedwith distilled water, water saturated with the apatite, and water containing 50 mmol/L of either CaClz or HaPo,. Control experiments with washed apatite samples containing known amounts of the adsorbate gave the amounts of carbon by combustionchromatographic analysis (Leco CS-244)within the experimental limits. Adsorption and Desorption Measurements. The apatite samples (LOO0 g each) were shaken with a series of standard solutions (10mL each) of poly(acry1icacid) at room temperature (23.0 0.5 "C) for 4 days, a period observed to be sufficient for attainment of equilibrium. The slurry was filtered through a medium-pore fritted disk by application of a light suction for 5 s. The adsorbed amount was determinedfrom the concentration
*
(22) Robb, I. D.; Sharplea, M. J. Colloid Interface Sei. 1982,89,301. (23) Wilson, A. D.; Prosser, H. J.; Powis, D. M. J.Dent. Res. 1983,62, 590. (24) Belton, D.; Stupp, S.I. Macromolecules 1983, 16, 1143. (25) Certain commercial materiels and equipment are identified in
this paper to specify the experimental procedure. In no instance does such identification implyrecommendationor endorsementby the National Institute of Standards and Technology or the ADA Health Foundation or that the material or equipment identifiedis necessarily the best available for the purpose. (26) Misra, D. N.; Bowen, R. L.; Wallace, B. M. J. Colloid Interface Sci. 1975, 51, 36.
0743-7463/91/2407-2422$02.50/0 0 1991 American Chemical Society
Letters
Langmuir, Vol. 7, No. 11, 1991 2423
12 -
Table I. Adsorption of Poly(acry1ic acid) from Aqueous Solution on Hydroxyapatite and Ca and P in the Solution
10 -
initial concn, equil concn, adsorpmg/100mL mg/100mL tion, Ca,d P,d (pHPb (PHP mg/g mmol/L mmol/L Ca/P*
S6-
4-
'! b
0
100200300400Mo8007008009001oO0 Concentralion, mg / lOOmL
Figure 1. Adsorption isotherm8i of poly(acry1icacid) on synthetic hydroxyapatite from water at 23 O C : curve A, experimental isotherm,a composite of four separateruns; curve B, constructed isotherm by assuming that only unassociated molecules (decreasing with concentration) are adsorbed and that it follows a Langmuir plot. of the acid in the filtrate. The filtrate was also used for Ca (by Arsenazo I11 methodn), P (hy vanado-molybdate complex methodB),and pH determinatio3s (TableI). In order to minimize the surface effects on the Ca/P ratio, Ca and P were determined in slurries containing 0.1 g of 8.patite samples. Control experiments showed that Ca and P rrnalyses are not affected by the presence of the polymer. The ad sorption valuesdonein triplicate were reproducible within a range of 5-12%. The desorptionof the adsorbs& was determined by repeatedly washing the apatite with distilled water ( 1 W 1 2 5 mL, total volume) after the equilibrium vraa reached. The amount of the desorbed solute was determined from the total volume of the filtrate and its concentration. The desorption characteristics of the solute were also determined by monitoring its adsorption by equilibrating three apatite samples (1g each) for 2 days with a standard solution (1 g/100 mL 10 mL each) and then diluting each solution with 5,10, and 20 inL of distilled water and further equilibrating for 2 days.
:
:
Results
A
:
The complete adsorption j sotherm, a composite of four separate runs, of poly(acry1icacid) on synthetic hydroxyapatite from aqueous solution at 23 "C is presented in Figure 1 (see also Table I), rmd the isotherm from dilute solutions is shown in Figure 2. Table I also presents the pH values of the initial and equilibrated solutions and the concentrations of Ca and P in the filtrates. The desorption experiments showed that the adsorbate is neither removed nor affected either by repeated washing with excess water or by subsequent dilution of a slurry. The adsorption is, therefore!, irreversible.
1001'0 (3.48) 2001'O (3.34) 400l1O (3.25) 6OO1lo (3.19) 800i10(3.16) 10001lo(3.14) 6000.1'~ (3.19) 1O000.1'~ (3.14) 20000.1'~ (3.08)
58.1 (4.75) 125.0 (4.70) 299.0 (4.53) 511.8 (4.43) 736.0 (4.36) 960.0 (4.30) - (4.37)
4.19 7.50 10.10 8.82 6.40 4.00
-
5.25 6.75 9.70 13.10 16.40 20.00 14.60
10.00 12.22 14.90 17.39 19.52 21.89 9.36
0.52 0.56 0.66 0.75 0.84 0.91 1.56
- (4.23)
-
21.00
13.4u
1.57
- (4.03)
-
33.40
21.31
1.57
The number represents initial concentration, superscript gives volume of solution (in mL), and subscript gives amount of apatite (in g). b Concentration in mmol/L may be obtained by dividing by 72, the molecular weight of the repeating monomeric unit. Equilibration time is 4 days. The desorption experimentado not showany removal of adsorbate. d The amount of released Ca or P in (mol/g) may be obtained by multiplying the first six values by 0.01 and the last three values by 0.1. e The ratio in distilled water is 0.50 for slurry density 1g/10 mL. f The amount of dicalcium phosphate dihydrate in the used apatite is the same as before.
7-
6-
. P
5-
1
4.
CI
:
0
10
20
30
40
50
Bo
70
Bo
W
100
110
120
Concentfallon, mg I lWmL
Figure 2. Adsorption isotherm of poly(acry1icacid) on synthetic hydroxyapatite from low-concentration aqueous solutions at 23 " C curve A, isotherm (e),Langmuirplot (0);curve B is a straight line obtained by linear regression (intercept = 114.95 g/L; slope = 41.20 gig).
formation of "supermolecular structures" and spatial networks between the molecular aggregates in concentrated solutions. It is possible to explain, a t least qualitatively, the maximum in adsorption in the present case on the basis of self-association of poly(acry1ic acid), which increases Discussion with concentration. The hydrogen-bonding capability of poly(acry1ic acid), which is an effective mechanism for a The adsorption isotherm cd poly(acry1icacid) has three to get adsorbed on hydrated apatite solute significant features: (i) it has a maximum (Figure l),(ii) surface, is therefore used up by self-association as the it is Langmuiran in shape a t lower concentrations (Figure concentration increases, and the associated molecules do 2), and (iii) it is irreversible. Analysis of nitrogen not tend to adsorb. These considerations may not apply adsorption does not show iiny significant microporous for higher molecular weight polymers and no adsorption structure of the adsorbent. With similar adsorbents using maximum may be observed. No adsorption maximum a sodium salt of poly(acry1ic acid) having much higher average molecular weights, other i n v e s t i g a t o r did ~ ~ not ~ ~ ~ ~ was observed for the sodium salts of higher molecular weight polyacrylic a ~ i d . The ~ ~ self-association l ~ ~ of polyobserve any maximum in the isotherms. Adsorption (acrylic acid) in aqueous solution was not determined by maxima have been observed for the isotherms of other any other method. polymers29 and have been explained on the basis of The existence of the maximum may be easily demon(27) Vogel, G. L.; Chow,L. C.; Brown, W. E. Caries Res. 1983,17,23. strated by constructing the isotherm on the basis that the (28) Brabson, J. S.; Dunn, R. L.; Epps, E. Z.;Hoffman,W. M.; Jacob, equilibrium concentration of unassociated polymer (C)is K.D.J . Assoc. Off. Anal. Chem. 1958,41, 517. related to its total concentration (CT) a t equilibrium as C (29) Lipatov, Yu. S.; Sergeeva, L. M. Adsorption of Polymers; Wiley: New York, 1974; pp 132-134. = CT(1 - ( Y ~ T )where , a is the degree of association
Letters
2424 Langmuir, Vol. 7, No. 11,1991
(assumed to be 0.001). Assuming that the isotherm follows a Langmuir plot30 -c= - c m M
+-b1M
and only the unassociated molecules are adsorbed (m), the saturation amount (M) of adsorbate may be calculated by using constants (intercept = 114.16 g/L, slope = 42.01 g/g) that are about the same as obtained for the experimental isotherm at lower concentration in Figure 2. The isotherm may, thus, be constructed (curve B, Figure 1) and is a symmetrical semielliptical curve where the maximum adsorption, [bM/ ( b 441, occurs at maximum CT = ‘/2a. The value of CY = 0.001 is within a realistic ballpark figure since the curves obtained for the values of 0.01 and O.OOO1 are not realistic at all. To construct a better isotherm, a more realistic model would also require one to consider an equilibrium between more than two associated molecules and the possibility of the existence of supermolecular structures in concentrated solutions. But, this would be complicated and require an introduction of several arbitrary constants. The calcium or phosphate ions released to the solution monotonically increase with the acid concentration but do not appear to have any relationship with the amount of the adsorbate (Table I). It may, therefore, be considered that this process is mainly related to the dissolution of apatite in the presence of acid. The ratio of the released Ca to P increases with acid concentration,and when enough acid is present, it becomes equal to the experimental ratio in the apatite. These characteristics of the ratio may be explained on the basis of surface effects, keeping in mind that the apatite surface is deficient in ~ a l c i u m . ~ The amount of calcium (A) on the stoichiometric surface may be easily calculated if it is assumed that the 100 face (area = p = 0.648 nm2) of hydroxyapatite is exposed and that three calcium ions per unit cel126*31 are available for any interaction (A = 3S/pN = 31.52 X mol/g, where S is
+
(30) Adamson, A. W.Physical Chemistry of Surfaces; Interscience: New York, 1960, p 574. (31) Kukura, M.; Bell,L. C.; Poener, A. M.; Quirk,J. P. J . Phys. Chem. 1972,76,900.
the surface area of the apatite and N is Avogadro’s number). The Ca to P ratio becomes equal to the experimental ratio as the first unit cells on the surface (~105.07X mol/g) are removed. It is seen that the isotherm for lower concentrations follows the Langmuir plot (eq 1)very well (Figure 2). The area occupied by an adsorbed poly(acry1icacid) molecule, calculated from the saturation amount (M), is 5.6 nm2 (length = 7 nm, breadth = 0.6 nm). Thus, the fraction of surface occupied at the maximum coverage is 4.2/5.6 = 0.75. This fraction (Vr) may also be calculated on the basis of theoretical treatment of the subject by DiMarz ~ o Equation . ~ ~ 6 of ref 21 (noting that CYI = CYZ = l / 2 , 2 = 4) becomes for the maximum coverage
where r is the ratio of length to breadth (=7/0.6 = 11.7) of the polymer molecule. Equation 2 when numerically solved for the value of r (=11.7) yields Vr equal to 0.71. This value is in good agreement with the value obtained above, when the experimental errors and the restraints of DiMarzio’s treatment are considered. This agreement also shows that the adsorbate molecules become uncoiled and lie flat fully extended on the surface at least in low concentration regions. It appears reasonable since in this configurationthe moleculesare able to form the maximum number of hydrogen bonds with the surface and will not be able to desorb, making the adsorption irreversible. This study does not support the mechanism of adhesion of polyacrylate cements to hydroxyapatite by Wilson et aLZ3who proposed that the irreversibility is caused by the polyacrylate ions displacing the phosphate ions on the surface.
Acknowledgment. I thank Dr. E. A. DiMarzio for certain helpful comments regarding his equation. Registry No. Poly(acry1ic acid), 9003-01-4;hydroxyapatite, 1306-06-5. (32) DiMarzio, E. A. J. Chem. Phys. 1962,36, 1563.
