Calcium hydroxyapatite adsorption

Kandon. K. et al.. Protein adsorption characteristics of calcium hydroxyapatites modified with pyrophosphoric acids. Colloids Surf. B, 58, 98, 2007. 

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

Wilson, Prosser Powis (1983) studied the adsorption of polyacrylate on hydroxyapatite using infrared and chemical methods. They observed an exchange of ions and concluded that polyacrylate displaced surface phosphate and calcium, and entered the hydroxyapatite structure itself (Figure 5.2). They postulated that an intermediate layer of calcium and aluminium phosphates and polyacrylates must be formed at the cement-... 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

Only a few systematic studies have been carried out on the mechanism of interaction of organic surfactants and macromolecules. Mishra et al. (12) studied the effect of sulfonates (dodecyl), carboxylic acids (oleic and tridecanoic), and amines (dodecyl and dodecyltrimethyl) on the electrophoretic mobility of hydroxyapatite. Vogel et al. (13) studied the release of phosphate and calcium ions during the adsorption of benzene polycarboxylic acids onto apatite. Jurlaanse et al.(14) also observed a similar release of calcium and phosphate ions during the adsorption of polypeptides on dental enamel. Adsorption of polyphosphonate on hydroxyapatite and the associated release of phosphate ions was investigated by Rawls et al. (15). They found that phosphate ions were released into solution in amounts exceeding the quantity of phosphonate adsorbed. 

The adsorption of amino acids on rutile and hydroxyapatite exhibits some characteristics of specific adsorption. The results can be interpreted in terms of electrostatic models of adsorption, however, if reorientation of adsorbed molecules is taken into consideration. The electrokinetic behavior of hydroxyapatite in glutamic acid is complicated because of a chemical reaction, possibly involving calcium ions. The study shows that it is necessary to take into consideration the orientation of adsorbed molecules, particularly for zwitterionic surfactants. 

Humic substances. Analogous to the reactions described above, humic substances (the polymeric pigments from soil (humus) and marine sediments) can be formed by both enzymatic and non-enzymatic browning. High concentrations of free calcium and phosphate ions and supersaturation with respect to hydroxyapatite can sustain in soil, because adsorption of humic acids to mineral surfaces inhibits crystal growth (Inskeep and Silvertooth, 1988). A similar adsorption to tooth mineral in a caries lesion can be anticipated for polycarboxylic polymers from either the Maillard reaction or enzymatic browning. 

Recently, Bartels and Arends113 studied the adsorption of poly(4-vinylpyridinium fluoride) with different hexadecyl group content on hydroxyapatite. Adsorbance decreased as the hexadecyl content, i.e. the charge density, was increased. Desorption experiments showed that the adsorption of this polyelectrolyte in water is essentially irreversible. However, the polymer partially desorbed when excess calcium ions were added. Bartels and Arends concluded that adsorption of poly(4-vinylpyridinum fluoride) occurs as a result of the uptake of fluoride ions by hydroxyapatite which releases phosphate ions into water. They also suggested that this adsorption phenomenon can be interpreted in terms of an ion-exchange mechanism. 

Adsorption This method is the simplest way to immobilize enzymes. Enzymes can be adsorbed physically on a surface-active adsorbent by contacting an aqueous solution of enzyme with an adsorbent. Commonly employed adsorbents are (Zaborsky, 1973) alumina, amon-exchange resins, calcium carbonate, carbon, cation-exchange resins, celluloses, clays, collagen, colloid-ion, conditioned metal, glass plates, diatomaceous earth, and hydroxyapatite. The advantages of adsorption techniques are as follows ... 

Holmes, J. M., and Beebe, R. A. Surface areas by gas adsorption on amorphous calcium phosphate and crystalline hydroxyapatite. Calc. Tiss. Res. 7, 163-174 (1971). 

Rykke M, S0nju T, Skj0rland K, Ralla G Protein adsorption to hydroxyapatite and to calcium fluoride in vitro and amino acid analyses of pellicle formed on normal enamel and on calcium-fluoride-covered enamel in vivo. Acta Odontol Scand 1989 47 245-251. 

Other reports (27-29) have focused on the role of citric acid, as a source of carboxylate anions, during precipitation of calcium phosphates from electrolyte solutions. It has been found that citrate anions inhibit the ciystal growth of calcium phosphates and hinder their transformation into hydroxyapatites. This was attributed to the adsorption of citrate anions into the crystals and the displacement of an equivalent amount of phosphate anions. Interestingly, Rhee and Tanaka (30) found that the presence of a collagen membrane in the medium changed the behavior of citrate anions from being an inhibitor to becoming a promoter of calcification, provided that the molar ratios of calcium to citric acid were between 2 and 12. 

We wish to report some work on the adsorption of acid and alkali-precursor gelatins, the water-soluble products of collagen. The study of both types allowed us to determine whether the differences exhibited by the two kinds in solution are reflected in the adsorbed state, and offers some insight into the state of flexible molecules at interfaces. The use of two dissimilar adsorbents, glass and stainless steel powders, should yield information on segment-surface interactions during the adsorption process. Some effects of added calcium ions were also studied since the Ca-ions of hydroxyapatite in bone and teeth are intimately related to collagen. 

Zhu XD, et al. Bovine serum albumin adsorption on hydroxyapatite and biphasic calcium phosphate and the correlation with zeta potentials and wettability. In Nakamura T, Yamashita K, Neo M, editors. Bioceramics 18, Pts 1 and 2 2006. p. 73—6. 

Zhang et al. used Langmuir monolayers to study the initial stage of nucleation and crystallization of calcium phosphate [160]. The monolayers used in their study consisted of dipalmitoylphosphatidylcholine, arachidic acid, and octadecylamine. The experiments show that the adsorption of calcium ions to the respective mono-layer is a prerequisite for subsequent nucleation. The authors also demonstrate that calcium phosphate forms through a multistage assembly process, in which first an amorphous calcium phosphate dihydrate layer forms, which then recrystallizes to form a crystalline hydroxyapatite layer (Fig. 12). This transformation of an amorphous phase to a crystalline phase provides direct evidence of a multistep crystallization process, which the authors claim is similar to the processes occurring in biomineralization. 

Unlike with calcium carbonate, there are a few computational studies on the interaction of organic molecules with calciuin phosphate. Leeuw and Rabone investigated the adsorption of citric acid to the (0110) and (0001) faces of hydroxyapatite... 

Other ways in which the Ca P ratio could be caused to depart from the ideal include (1) the presence of an additional crystalline compound, octacalcium phosphate, whose unit cell composition is Ca8(HP04)2(P04)4-5H20 and whose Ca P ratio is 1 3-3, giving rise to the possibility that biological mineral may be an intercrystalline mixture (2) substitution of other ions for calcium in the crystal lattice. For example, hydroxyapatites prepared by precipitation from solutions of different pH have different Ca P ratios. Substitution of hydronium ions for calcium ions causes a decrease in the Ca P ratio (3) adsorption of excess phosphate or calcium phosphate complexes on to the crystal surface. 

Interestingly, the composition, phase, morphology, and placement of hydroxyapatite will influence the speed and extent of bone growth. Since the resorption process is siuface-driven by the adsorption of osteoblasts, the ultimate solubility of a ceramic will be directly related to its surface area - i.e., crystal size and density. In addition, careful cmitrol of processing parameters is necessary to prevent thermal decomposition of hydroxyapatite into other soluble calcium phosphate phases (e.g., tricalcium phosphate, Ca3(P04)2, tetracalcium phosphate, Ca4(P04)20, and CaO), which... 

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