Langmuir 1988,4, 953-958 position on metal oxides are dependent upon the molecular structure of the carboxylic acids as well as the surface.
Conclusions The aliphatic C1 to C3 carboxylic acids adsorbed both molecularly and dissociatively on T i 0 2 (anatase) powder at room temperature. While adsorption of the molecular species was reversible, the carboxylates desorbed via two pathways: recombination with surface hydroxyl species at 400 K and decomposition at higher temperatures. The decomposition of carboxylates exhibited two pathways. A net unimolecular dehydration reaction was common to each of the carboxylates studied. Formate decomposed exclusively via unimolecular dehydration. A bimolecular ketonization reaction was dominant for acetate and pro-
953
pionate decomposition. Bimolecular ketonization was suggested to proceed via interactions between surface carboxylates. The selectivities of carboxylate decomposition were dependent upon both the characteristics of the surface and the thermal stabilities of these species.
Acknowledgment. We are grateful to the National Science Foundation (Grant CBT 8311912) and to E.I. du Pont de Nemours & Co., Inc., for support of this work. We also wish to thank J. S. Boaventura and R. H. Staley for assistance in obtaining the infrared spectra and W. E. Farneth for his help in interpreting the TPD data. Registry No. HCOOH, 64-18-6; CH,COOH, 64-19-7; C2H,COOH, 79-09-4; TiOz,1317-70-0;formate, 71-47-6; acetate, 71-50-1; propionate, 72-03-7.
Adsorption on Hydroxyapatite: Role of Hydrogen Bonding and Interphase Coupling D.N. Misra American Dental Association Health Foundation, Paffenbarger Research Center, National Bureau of Standards, Gaithersburg, Maryland 20899 Received July 29, 1987. I n Final Form: March 22, 1988 The adsorptive properties of a number of compounds possessing hydroxyl and/or carboxylate groups were studied on hydroxyapatite (with hydrated surface) from different solvents at room temperature. Generally, it was found that the compounds were adsorbed reversibly from hydrogen-bonding and irreversibly from non-hydrogen-bondingsolvents. A reasonable interpretation is that the appropriate functional groups of the adsorbate molecules are hydrogen-bonded to the apatite surface. In all cases the reversible adsorption may be represented by the Langmuir plots, and it is deduced that the orientation of the hydrocarbon moieties (including benzene rings) of an adsorbate molecule is parallel to the substrate if it is geometrically possible. The irreversible isotherms are characterized by a total adsorption from dilute solutions below a threshold concentration and a constant adsorption above the threshold. The hydrocarbon moieties are oriented away from the surface for irreversibly adsorbed molecules. All compounds were commercially available except for ferric methacrylate, which was synthesized and may act as an interphase coupling agent between a dental resin and hydroxyapatite. No significant gain was shown, however, in diametral tensile strength of a polymer filled with synthetic hydroxyapatite coated with irreversibly adsorbed ferric methacrylate as compared to that of the polymer filled with the clean apatite.
Introduction I t is important to know the chemical and structural characteristics of the compounds that affect their adsorption on hydroxyapatite. This has a bearing on the bonding of prosthetic polymer composites to bone and teeth and may help explain the growth and dissolution of bone in the presence of certain chemicals. Hydroxyapatite, Calo(P04)6(OH)2,is the structural prototype of the bone or tooth mineral. The bone mineral, however, is a complex and variable s ~ b s t a n c e l -and ~ is not stoichiometric with respect to hydroxyapatite. This is the reason that for precise and well-defined physicochemical studies a pure synthetic hydroxyapatite is substituted for bone mineral. These controlled studies do not, however, necessarily represent a phenomenon as it may occur on the bone mineral."" (1) Woodward, H. G. Health Phys. 1962,8, 513. (2)Armstrong, W. D.; Singer, L. Clin. Orthop. 1965, 38, 179. (3)Pellegrino, E. D.; Biltz, R. M. Calcif. Tissue Res. 1972, 10, 128. (4)Misra, D. N.In Methods of Calcified Tissue Preparation; Dickson,
G. R., Ed.; Elsevier: New York, 1984; p 435.
The adsorption studies were carried out on synthetic hydroxyapatite for a number of solutes from different solvents a t room temperature and without any special treatment of the substrate. The apatite is likely to have a t least a monolayer of physically adsorbed water under these ambient conditions.12-16 The surface-active groups of a potential coupling agent must not interact too strongly with the substrate lest they sequester calcium or phosphate ions or dissolve the apatite, thus destroying any anchoring on the surface. On the other hand, if the interaction is too ( 5 ) Fleisch, H. Clin. Orthop. 1964,32, 170. (6) Elliott. J. G. Calcif. Tissue Res. 1969, 3, 293. (7)Termine, J. D. C l h . Orthop. 1972,85, 207. (8) Meyer, J. L.; Eick, J. D.; Nancollas, G. H.; Johnson, L. N. Calcif. Tissue Res. 1972, 10, 91. (9) Posner, A. S. Fed. Proc. Fed. Am. SOC.E r p . Biol. 1973,32, 1933.
(10)Glimcher, M. J.; Bonar, L. C.; Grynpas, M. D.; Landis, W. R.; Roufoase, A. H. J. Cryst. Growth 1981,53, 100. (11)Beth, F.; Blumenthal, N. C.; Posner, A. S. J. Cryst. Growth 1981, 53, 63. (12)Dry, M. E.; Beebe, R. A. J. Phys. Chem. 1960, 64, 1300. (13)Loebenstein, W.V. J . Dent. Res. 1973, 52, 271. (14)Zahradnik, R.T.;Moreno, E. C. Arch. Oral Biol. 1975,20, 317. (15) Rootare, H.M.; Craig, R. G. J. Dent. Res. 1978, 56, 1437. (16) Misra, D. N. Calcif. Tissue Int. 1986, 38, 333.
This article not subject to U.S.Copyright. Published 1988 by the American Chemical Society
954 Langmuir, Vol. 4, No. 4, 1988
Misra Table I. Adsorbates and Solvents
~
adsorbatea L-ascorbic acid L-ascorbic acid 6-palmitate 2-furoic acid methyl benzoate phenyl benzoate triphenylacetic acid 4-(dimethylaminojsalicylic acid ferric methacrylatef ~~~
~
purity gold label (ACSj 97% recryst (ethanol) 99% 99% 99070 recryst (ethanol) 9970
mu, OC 193 (dec) 115-118 134 liq 68-70 270-273 146-147 90-95
ethanole rev (95%) rev (95% ) rev (95%) no ads (99.8%) no ads (99.8%) rev (99.8%) rev (95%) not stable
solventb,c,d CHICll not sol not sol not sol no ads no ads not sol irr irr
cyclohexane not sol not sol not sol rev rev not sol not sol
a All
compounds were obtained from Aldrich Chemical Company, Milwaukee, WI, except for ferric methacrylate, which was synthesized. *From which adsorption isotherms were determined at room temperature. The solutions were filtered if necessary. The columns give the nature of adsorption, whether reversible or irreversible. Sources: ethanol (Publiker Industries Inc., Philadelphia, PA, 190 proof; The Warner-Graham Co., Cockeysville,MD, 200 proof, U.S.P. grade), dichloromethane (Fisher certified reagent grade), cyclohexane (Mallinckrodt, Inc., Paris, KY, A.R. grade). dThe concentrations were determined in descending order: in ethanol at 278, 275,276,290,286,224 (or 245),and 342 nm; in CHzClzat 290,286, 348,and 380 nm; and in cyclohexane at 290 and 284 nm. eThe number in parentheses represents the purity (95% = 190 proof; 99.5% = 200 proof'). fThis compound is not stable in ethanol or acetone, as a precipitate appears in filtered solution after a day. Methacrylic acid contained 0.1% hydroquinone as stabilizer and was obtained from Rohm & Haas Co., Philadelphia, PA.
weak, water or other fluids may displace the coupling agent. A coupling agent possessing a hydrogen-bonding moiety could effectively interact with the adsorbed water on the apatite substrate. The agent will be easily displaced from the surface by water or any other hydrogen-bonding solvent if it does not possess hydrophobic moieties concomitantly with its hydrogen-bonding groups. The stability of the bonding between a coupling agent and the substrate may be enhanced if the molecular linkages of the agent have the proper geometry to interact via its hydrogen-bonding groups with the surface while exposing its hydrophobic or polymerizable moieties toward the solution. These factors had been considered in individual c a s e ~ . l ~ - ~ l The reversibility or irreversibility of adsorption of solutes possessing hydroxyl and/or carboxylates groups onto hydroxyapatite was attributed to hydrogen- or non-hydrogen-bonding characteristics of the solvent^.^*^^ In this report this aspect was explored for a wide variety of compounds (Table I). There was no integrated rationale for the selection of the adsorbates; it was somewhat arbitrary. Ascorbic acid was chosen because it possesses multiple hydroxyl groups, and its palmitate ester was selected to determine the role of a long hydrocarbon chain on adsorption characteristics of the acid. Furoic acid, 4-(dimethy1amino)salicylicacid, and triphenylacetic acid possess carboxylate groups. These two acids possess the capability for chelation, and the latter has a strong hydrophobic moiety in the three benzene rings. Methyl and phenyl esters of benzoic acid and ferric methacrylate cannot contribute toward hydrogen bonding but can definitely accept a hydrogen bond from an appropriate solvent or substrate. Ferric methacrylate also possesses polymerizable groups and can act as a interphase coupling agent between a prosthetic resin and bone mineral. The role of hydrogen bonding in the adsorption process was inferred by determining the isotherms of the adsorbates a t room temperature on synthetic hydroxapatite from different but limited number of solvents. The effectiveness of ferric methacrylates as an interphase cou(17) Misra, D. N.; Bowen, R. L. J. Colloid. Interface Sci. 1977, 61, 4. (18) Misra, D. N.; Bowen, R. L. J . Phys. Chem. 1977,81, 842. (19)Misra, D.N.; Bowen, R. L.; Antonucci, J. M.; Cuthrell, W. F. J . Colloid Interface Sci. 1980, 77, 143. (20) Misra, D. N. J. Dent. Res. 1985, 64, 1405. (21)Misra, D.N.J. Dent. Res. 1986, 65, 706. (22) Misra, D.N.; Bowen, R. L. Biomaterials 1981, 2, 28. (23)Misra, D.N. In Adsorption o n and Surface Chemistry of Hydroxyapatite; Misra, D. N., Ed.; Plenum: New York, 1984;p 105. (24)Misra, D.N.;Bowen, R. L. Colloids Surf. 1987, 26, 101.
pling agent was determined by comparing the diametral tensile strength of a dental resin filled with synthetic hydroxyapatite coated with the compound to that of one filled with clean apatite.
Materials and Methods Hydroxyapatite. The synthetic apatite was tribasic calcium phosphate (Fisher certified, with a chemical formula given as approximately Calo(P04)6(OH)z).It was purified by repeatedly washing with boiling distilled water before use. Its physical and chemical analyses have been reported.25 It has a surface area (BET,N,) of 41 m2/g. The amount of physically adsorbed water (1.58%, about 1.5 monolayers) on the apatite was determined by evacuating (at 100 Pa) the weighed samples at 105 "C for several hours and reweighing after dry air was introduced into the vessel a t room temperature. The source and purity of all other chemicals are given in Table I. The solvents were used as is without any additional drying. Preparation a n d Characterization of Ferric Methacrylate. To a given amount of methacrylic acid neutralized with aqueous sodium hydroxide solution, appropriate molal amounts of Fe(N03)3-9H20in aqueous solution were added. The precipitate was filtered, washed with distilled water and dried at 45 "C and reduced pressure. T h e yield was almost quantitative in accord with the formula iron(metha~ry1ate)~. The dried solid was dissolved in dichloromethane, the solution was filtered, and the solvent was evaporated. The semicrystalline solid was brown and had a melting range 9C-95 "C. The spectrophotometric analysis of Fe complexed with l,l0-phenanthrolineZ6agreed with the calculated value (-99%) within the experimental error. The IR spectrum of the compound is similar to that of sodium methacrylate and has C-0 at 1241 cm-', C=O a t 1558 cm-', and C = C a t 1635 cm-'. Reversible Adsorption and Desorption. The apatite samples (LOO0 g each) were shaken with a series of standard solutions (10 mL each) of an adsorbate (Table I) at room temperature (23.0 & 0.5 "C) for approximately 30 min, a period observed to be sufficient for attainment of equilibrium in each case. The slurry was filtered through a medium-pore fritted disk by application of a light suction for 5 s. The concentration of the adsorbate in the filtrate was determined, after proper dilutions and/or use of spacers, from ita absorbance at a suitable wavelength on a double-beam spectrophotometer (Varian DMS80) and a standard concentration vs absorbance plot. The adsorbed amount, A (mol/g), was determined by the relation A = VAC/ W , where V
(25)Misra, D.N.; Bowen, R. L.; Wallace, B. M. J. Colloid Interface Sci. 1975, 51, 36. (26)Willard, H. H.;Furman, N. H.;Bricker, C. E. Elements of Quantitatiue Analysis; Van Nostrand: New York, 1956;pp 496-497. (27)Afifi, A, A.; Azen, S. P. Statistical Analysis. A Computer Oriented Approach; Academic: New York, 1972; pp 108-113, 146-151.
Langmuir, Vol. 4 , No. 4, 1988 955
Adsorption on Hydroxyapatite
i14
4
12
, Ln
$ N -
0
c X
0
I
1
I
1
4
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8
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20 24 28 32 36 Concentration, mmolI L
12
16
1
40
1
121 40
1
44
0
Figure 2. Adsorption isotherm of L-ascorbic acid 6-palmitate
I
0
10
20
30
40
50
60
70
80
90 100
Concentration, rnrnollL
on synthetic hydroxyapatite from ethanol (95%)at 23 "C: ( 0 ) isotherm; (A) adsorbed amount after desorption with excess Langmuir plot. The straight line is obtained by linear ethanol; (0) regression.
Figure 1. Adsorption isotherm of L-ascorbic acid on syntk.jtic
-17
hydroxyapatite from ethanol (95%)at 23 OC: ( 0 )isotherm; (A) adsorbed amount after desorption with excess ethanol; (0) Langmuir plot. The straight line is obtained by linear regression. (L) is the volume of solution in contact with the weight, W (g), of adsorbent, and AC (mol/L) is the difference of the initial and equilibrium concentration of the solution. The duplicate or triplicate adsorption values were reproducible within a range 6 8 % . The control experiments showed that solutions of ascorbic acid and its palmitate ester in ethanol (95%)are stable for at least a day, and there is no discernable change in the amounts adsorbed for a day after 30 min. The solutions of all other adsorbates are stable indefinitely. The adsorbed solutes were desorbed by repeatedly washing the hydroxyapatite with excess pure solvent (100-125 mL, total volume) after the equilibria were reached. The amount of the desorbed solute was determined from the total volume of the filtrate and its concentration. Irreversible Adsorption and Desorption. The adsorption of solutes (Table I) 4-(dimethylamino)salicylicacid or ferric methacrylate was similarly determined on hydroxyapatite from dichloromethaneat room temperature (23 f 0.5 "C). The solutions of Fe(meth), (meth represents methacrylate) are not stable in ethanol or acetone, but the solutions in CHzClz(filtered after a day to remove cloudiness) were stable. The irreversibly adsorbed solute could not be desorbed by washing with excess (100-125 mL) dichloromethane. Composite Preparation and Strength. To determine the interphase coupling interaction of ferric methacrylate between apatite (pure apaptite as control) and methacrylate monomers, the specimens were prepared by mixing two pastes that contained either amine or peroxide. The composition of the monomeric mixture was BIS-GMA (69.95% by weight, Freeman Chem.), triethylene glycol dimethacrylate (29.95%, Startomer Co.), and butylated hydroxytoluene (Oslo%,Eastman Chem., as added stabilizer). The monomers may also have contained unspecified stabilizers as received. The amine was NJ-dimethyl-p-toluidine (0.27% based on stabilized monomer, Aldrich analyzed),and the peroxide was benzoyl peroxide (1.08%,Aldrich analyzed). The monomer-to-filler ratio was 1.2 by weight in all cases. The specimens for diametral strength measurements were molded in a stainless steel die (diameter 6.1 mm, thickness 3.2 mm) as described in The American Dental Association Specification No. 9 (Guide,1972-1973). They were stored in air or water at 37 "C for 28 days before their strengths were measured on an Instron testing instrument at a cross-head speed of 0.5 cm/min. The examination of the fractured specimens and the diametral tensile strength vs time curve showed that the failures were brittle under given experimental conditions,since there was no evidence of deformation, and the curves fell off precipitously after maximum strength. The strenghts could be compared with each other because the mode of failure was brittle in each case.
Results
All adsorption isotherms were determined from solutions on synthetic hydroxyapatite at room temperature (23 f
0
10
30 40 50 60 70 Concentration, rnrnollL
20
90
80
100
Figure 3. Adsorption isotherm of 2-furoic acid on synthetic hydroxyapatite from ethanol (95%)at 23 "C: ( 0 )isotherm; (A) adsorbed amount after desorption with excess ethanol; (0) Langmuir plot. The straight line is obtained by linear regression. 8
1
/
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A
l
I
30 40 50 60 70 Concentration, rnrnollL
l
80
l
lo
90 100
Figure 4. Adsorption isotherm of methyl benzoate on synthetic
hydroxyapatite from cyclohexane at 23 O C : ( 0 )isotherm; (A) adsorbed amount after desorption with excess cyclohexane; (0) Langmuir plot. The straight line is obtained by linear regression.
0.5 "C). The isotherms of L-ascorbic acid (Figure l),Lascorbic acid 6-palmitate (Figure 2), and 2-furoic acid (Figure 3) were determined from ethanol (95%). The isotherms of methyl benzoate (Figure 4) and phenyl benzoate (Figure 5) were determined from cyclohexane. The isotherm of triphenylacetic acid (Figure 6) was determined from ethanol (99.5%). The isotherms of 4-(dimethylamino)salycilic acid were determined from two solvents, ethanol (95%, Figure 7) and dichloromethane (Figure 8). The isotherms of ferric methacrylate was determined from dichloromethane (Figure 8). The adsorbed amounts of
956 Langmuir, Vol. 4, No. 4, 1988 8-
1
1
Misra
I
1
7m -
z
12
L--@
m
-I
-
X CH Fe (0c C d C n . , ,
8 F
0 FERRIC METHACRYLATE
1
0
1
2
1
1
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1
l
6
l
8
l
l
10
I
l
I
12
l
1
14
l
16
I
I
18
Concentration, mmollL
Figure 8. Adsorption isotherms of 4-(dimethy1amino)salycilic acid ( 0 )and ferric methacrylate (A)on synthetic hydroxyapatite from dichloromethaneat 23 "C; (0or A) adsorbed amount after desorption with excess dichloromethane. 0
10
20
30
40
50
70
60
80
90 100
Concentration, mmollL Figure 5. Adsorption isotherm of phenyl benzoate on synthetic hydroxyapatite from cyclohexane at 23 O C : ( 0 )isotherm; (A) adsorbed amount after desorption with excess cyclohexane; (0)
Langmuir plot. The straight line is obtained by linear regression.
Table 11. Diametral Tensile Strengths of Polymer Filled with Hydroxyapatite mean tensile
composite" untreated apatite (control)
32
Fe(meth)3-coatedc apatite
28 m
2 E -
121
,
A
24
-
20
2 N
$ X
ci
s 0 -
strength f stand dev,
no. of sampled dry, 8 wet, 9 dry, 8 wet, 7
MPa 24.3 f 3.2 23.8 f 2.8 25.8 f 2.5 26.9 f 2.7
" Monomer-to-hydroxyapatite ratio was 1.2 by weight. *Dry samples were stored in air in closed bottles and wet ones under water at 37 O C for 28 days before being tested. "mount of the adsorbate was 17.8 x mol/g (equivalent to monomolecular coverage). amount does slowly change with time. Probably, the following reactions were responsible for the change: Fe(meth)3 + 3H20
- -+ + Fe(OH),
3H(meth)
20Fe(meth)3 + 3Calo(P04)6(OH)z 18FePO4J 30Ca(meth)z + 2Fe(OH)3 0
4
8
12
16
20
24
28
32
36
GFe(meth),
Concentration, mmollL
Figure 6. Adsorption isotherm of triphenylacetic acid on synthetic hydroxyapatite from ethanol (99.5%)at 23 O C : ( 0 )isotherm; (A)adsorbed amount after desorption with excess ethanol; (0) Langmuir plot. The straight line is obtained by linear regression.
~
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Concentration, mmollL
Figure 7. Adsorption isotherm of 4-(dimethylamino)salycilicacid on synthetic hydroxyapatite from ethanol (95%)at 23 O C : (0) isotherm; (A) adsorbed amount after desorption with excess ethanol; (0)Langmuir plot. The straight line is obtained by linear
-
+ Calo(P04)6(OH)z GFePO, + 9Ca(meth)2+ Ca(OH)*
The adsorption characteristics of solutes from three different solvents and their solubilities are presented in Table I. The isotherms shown in Figures 1-7 are typical of the Langmuir-type, and each is fully reversible. The reversibility of adsorption in each case is confirmed by a complete removal of the adsorbate with excess solvent, as is seen in the figures. The isotherms of 4-(dimethylamino)salycilic acid and ferric methacrylate from dichloromethane (Figure 8) are typical of irreversible adsorption: total removal of all adsorbate below a threshold concentration and constant above it. The irreversibility of adsorption in each case is confirmed by nonremoval of the adsorbate with excess solvent. A known amount of the irreversibly adsorbed acid on a dried sample of apatite is, however, completely desorbed by washing with excess ethanol (95%). Diametral tensile strengths of the polymer composites with uncoated or ferric methacrylate coated hydroxyapatite are presented in Table 11. Polymer composites filled with methacrylate-coated apatite had the same tensile strength as the polymer filled with pure apatite. An analysis of variances indicated that the strengths were not statistically different a t a 95% confidence level, whether the composites were kept dry or wet for 28 days at 37 "C.
regression.
Fe(meth), do not show any discernable change for about a day after the equilibration period of 30 min, but the
Discussion The adsorption isotherms of L-ascorbic acid, L-ascorbic acid 6-palmitate, furoic acid, triphenylacetic acid, and
Langmuir, Vol. 4, No. 4, 1988 957
Adsorption on Hydroxyapatite
Table 111. Constants Derived from the Adsorption Isotherms
adsorbate L-ascorbic acid L-ascorbic acid 6-palmitate 2-furoic acid methyl benzoate phenyl benzoate triphenylacetic acid DMASAd DMASA ferric methacrylate
solvent ethanol (95%) ethanol (95%) ethanol (95%) cyclohexane cyclohexane ethanol (99.5%) ethanol (95%) dichloromethane dichloromethane
Langmuir plot constants0 slope X lom3, intercept X g/mol 10-2, g/L 10.51 2.06 29.74 5.46 9.96 6.10 14.14 2.18 14.56 1.54 52.32 3.82 29.27 8.32
maximum adsorption X lo6, mol/g 9.51 3.36 10.04 7.07 6.87 1.91 3.42 13.25 17.80
mol area u , * , nm2 ~ 0.72 (0.7) 2.03 (2.0) 0.68 (0.7) 0.96 (1.0) 0.99 (1.0) 3.56 (2.4) 1.99 (2.1) 0.51 (0.5) 0.38 (0.4)
heat term b, L/mol 51 55 16 65 95 137 35
“Correlation coefficient of linear regression in each case is >0.99. Maximum adsorption = l/slope, and heat term = slope/intercept. * u = S / N M , where S is surface area (41.0 m2/g) of apatite, N is Avogardro’s number, and M is maximum amount of adsorbate. CAreasgiven in parentheses are obtained from molecular models. 4-(Dimethylamino)salicylic acid.
4-(dimethy1amino)salicylic acid on synthetic hydroxyapatite from ethanol solutions and of methyl or phenyl benzoate from cyclohexane solution are reversible. These isotherms may be represented by their Langmuir plots:28 CJm= C J M + l J b M wher C is the equilibrium concentration of the solution, m is the amount of adsorbate per unit amount of adsorbent, M is the saturation or maximum adsorbed amount, and b is the constant related to the apparent heat of adsorption. The Langmuir plots for the isotherms are shown in Figures 1-7, and the constants are presented in Table I11 (columns 3 and 4). The linearity is very good in all cases. The maximum amounts adsorbed are obtained from the slopes of the Langmuir plots ( M = l/slope) and are given in Table I11 (column 5). The constant b is obtained by dividing the slope by the intercept, and its values are given in Table I11 (column 7). The adsorption of 4-(dimethylamino)salycilic acid or ferric methacrylate on hydroxyapatite from dichloroethane is irreversible. The saturation amounts are obtained from the horizontal plateaus (Figure 8) of the isotherms and are given in Table I11 (column 5). The irreversibly adsorbed acid is quantitatively desorbed by repeated washing with ethanol (95%), whereas ferric methacrylate is not fully desorbed by ethanol. Perhaps, the reactions (given in Materials and Methods) complicate the full recovery in the latter case. The saturation amounts could be used to calculate the effective cross sectional areas (a = S / N M Table 111, column 6) of the adsorbate molecules. The areas thus obtained may be compared with the orientations of the molecular models on the surface. In order to limit the scope and number of these orientations, it may be reasonably assumed that the adsorbates which contain hydroxyl and /or carboxylate groups would hydrogen-bond to the hydrated apatite surface. If an adsorbate molecule contains only one or a number of closely spaced groups or atoms capable of hydrogen bonding, it may tend to rotate about that center at room temperature, and its area would be equal to the effective area of a hexagonally close-packed circular disk (=2(31/2)?, where r is the projected radius of the molecule from the center of rotation). In case an adsorbate molecule is anchored to the surface by a number of hydrogen-bonding groups or atoms that are well-separated in space, it may not rotate and its effective area may be equal to the projected area. There is another factor that impinges on the orientation of the adsorbed molecule on the surface, and it is the (28) Adamson, A. W. Physical Chemistry of Surfaces; Interscience: New York, 1960; p 574.
hydrogen-bonding capability of the solvent. This capability may also generally determine the reversibility of adsorption from a solvent: reversible from hydrogenbonding and irreversible from non-hydrogen-bonding solvent (Table I). The molecules possessing organophilic moieties such as hydrocarbon chains or rings may be so adsorbed as to have a maximum interaction of these groups with solvents not capable of hydrogen bonding (or only capable of very weak hydrogen bonding). On the basis of the above stipulations, the effective cross sectional areas of the adsorbate molecules generally agree very well with the areas determined from the maximum adsorbed amounts (Table 111, column 6). The adsorbed molecules of L-ascorbic acid and its palmitate are hydrogen-bonded to the surface by their hydroxyl groups and oxygens and are not able to rotate (areas given in Table 111). In the latter case the palmitate chain also lies stretched on the surface with its carboxylate oxygens hydrogen-bonded to the substrate. Furoic acid molecules lie flat on the surface and rotate around the center of the carboxylate and ring oxygens. If a molecule of methyl or phenyl benzoate were rotating about the carboxylate oxygens with phenyl rings inclined 45O to the surface, it would occupy an area of about 1.0 nm2. The effective area of hydrogen-bonded triphenylacetic acid rotating on the surface corresponds to an area of 2.4 nm2 vs the experimental area of 3.56 nm2. No other dynamical configuration yields an area equivalent to the experimental one. Rotating about the center of two oxygens an adsorbed molecule of triphenylacetic acid occupies an area of about 2.1 nm2 while the phenyl ring is lying flat and an area of about 0.5 nm2 if the ring is standing up on the surface. The multiple hydrogen-bonded ferric methacrylate molecule with hydrocarbon moieties tightly packed over the Fe-0 rib cage occupies an effective surface area of about 0.4 nm2 whether the molecule is fixed or rotates about the center of the cage. Indeed, there is some arbitrariness in determining the effective areas of the adsorbed molecules on the basis of the above postulates, and in some cases more than one configuration could be possible. There is, however, a good correspondence with the experimental areas in all cases except for triphenylacetic acid. The effective areas of many other a d ~ o r b a t e s ~ ’ - ~previously ~~J” determined from their adsorption isotherms are also in accord with the areas estimated on the basis of the foregoing postulates. It would be expected that in addition to hydrogen bonding absorptive forces would also have some components of dispersion and dipole interactions. But, hydrogen bonding should be the determining force since it explains the reversibility or irreversibility of adsorption from different solvents and the surface orientations of various
Misra
958 Langmuir, Vol. 4, No. 4, 1988 molecules. It may be expected that methyl or phenyl benzoate should be irreversibly adsorbed from cyclohexane since the solvent cannot hydrogen-bond. Nevertheless, the reversibility becomes understandable when it is considered that these adsorbates can only weakly hydrogen-bond with the surface and should have equivalent dispersion interactions with the solvent. The heat term is another constant obtained from the Langmuir plots. But, it can be neither directly related to the heat of adsorption nor easily correlated for different adsorbates, since it is a composite term including the heat of solution and the heat and entropy of adsorption. It may, however, be compared for chemically similar compounds adsorbed from the same solvent. It is not surprising that the term is about the same for L-ascorbic acid and its palmitate when both are adsorbed from ethanol (95%) (Table 111, column 7). Of all the adsorbates that were studied, only ferric methacrylate could act as a coupling agent between hydroxyapatite and a prosthetic resin, since it contains methacrylate groups which could copolymerize with the resin monomers. The efficacy of ferric methacrylate as an interphase coupling agent was judged by comparing the diametral tensile strength of a dental resin filled with apatite coated with the compound to that of one filled with the clean apatite (Table 11). The reason why ferric methacrylate is not effective as a coupling agent but ZrO(methacrylate)22o is, as judged from the strengths of the composites, is not evident. It may be that the higher charge density of ferric ion inhibits the polymerization of methacrylate groups. On the basis of the present and previouslg studies, it would be difficult to suggest any general characteristics of the polymerizing groups on the coupling agent that may render it more effective. Perhaps a coupling agent may be more effective if it possesses an amine moiety that could initiate polymerization from the surface itself.20 Certain observations may be made about the surface pretreatment and its possible effect on adsorption. The present studies were carried out on a calcium-deficient hydroxyapatite surface. The reversibility of adsorption and the data obtained in this and previous st~dies"-"@~~~
can be interpreted on the basis of the interplay of the hydrogen bonding between the solute, solvent, and substrate. This implies that the ionic constituent and/or the electrostatic nature of the surface do not play any major role in the adsorption process. Any pretreatment of the surface would probably not affect the adsorption res ~ l t s ' ~ Jbecause ~ J ~ in an ambient atmosphere a layer of physically adsorbed water on the top of a chemisorbed one completely masks any ionic contribution or electrostatic component of the surface. This implies that a molecular adsorption of a solute per unit area of surface should be independent of the ionic nature of any hydrated substrate. This inference is being verified for other adsorbents for some adsorbates. It may be pointed out that even after an irreversibly adsorbed coupling agent effectively polymerizes with the resin, the apatite substrate may be weakened by an acid or other chemicals in the mouth or body. A more durable linkage may be obtained if the chemical bond is reinforced with micromechanical bonding between the substrate and resin where polymeric tags penetrate into the micropores. The apatite surface may be made microporous either by a suitable etching process or by remineralization.
Acknowledgment. This investigation was supported, in part, by USPHS Research Grant DE05129 to the American Dental Association Health Foundation from the National Institutes of Health-National Institute of Dental Research and is part of the dental research program conducted by the National Bureau of Standards in cooperation with the American Dental Association Health Foundation. Registry No. (BIS-GMA)(triethyleneglycol dimethacrylate), 26426-05-1; CHzClz, 75-09-2;ethanol, 64-17-5;cyclohexane, 11082-7; (L)-ascorbic acid, 50-81-7; (L)-ascorbic acid 6-palmitate, 137-66-6;2-furoicacid, 88-14-2;methyl benzoate,93-58-3;phenyl benzoate, 93-99-2; triphenylacetic acid, 595-91-5; 4-(dimethylamino)salicylicacid, 23050-91-1;ferric methacrylate,69399-95-7; hydroxyapatite, 1306-06-5. (29) Misra, D. N.; Johnston, A. D. J.Bioned. Muter. Res. 1987, 21, 1329. (30) Miera, D. N. Proceedings of International Conference on Sur-
factants (New Delhi); Mittal, K. L.; Ed., in press.
