Vapor phase adsorption of water on hydroxyapatite

Penetration of SDF and AgF from the infected dentine towards the unaffected tooth structure

Background

The use of SEM-EDS line scan analysis to evaluate the movement of ions from dental materials towards the tooth structure and the concept of ion movement is well established. This analysis technique was used to determine the ion movement of two commercially available silver- and fluoride-containing products.

Methods

This study aimed to compare the elemental analysis of primary molar teeth treated with silver diamine fluoride (SDF) and water-based silver fluoride (AgF) and to analyse the penetration of SDF and AgF from the infected dentine towards the healthy dentine. The teeth were cleaned from debris and contaminants off the roots and stored until use. A total of 15 primary molars with large active cavitated lesions, not extending into the pulp (specimens), were divided into three test groups: silver diamine fluoride (SDF) (n = 5), water-based silver fluoride (AgF) (n = 5), and deionised water (W) (n = 5) as the control group. The teeth were sectioned, embedded, and received SEM-EDS line scans. The line scan had a total length of 82.65 μm. The visible end of the infected dentine and the start of the more affected dentine were chosen as the starting point to ensure that the infected caries' line distribution towards the affected dentine's transition area was as standardized as possible. Therefore, the infected dentine length of the scan was 22.80 μm (8 scan points of 2.85 μm apart), and the affected dentine, including the healthy dentine, was 59.8 μm (21 scan points). The SEM-EDS line scan from each specimen determined the average fluoride, iodide, and silver weight percentage for that specimen.

Results

The 15 sample SEM-EDS line scans were used to determine the average ion movement in wt%. The Kruskall-Wallis test and Tukey's HSD test were completed at a p < 0.05. SDF and AgF presented no significant fluoride movement in terms of the weight percentage. There was, however, significantly more fluoride movement from infected caries to the healthy dentine with SDF and AgF (p = 0.0010053) compared to the control specimens treated with deionised water. There was no significant difference between SDF and AgF for the movement of the iodide (p = 0.5953) and silver (p = 0.3708) from infected caries to the healthy dentine.

Conclusion

SDF and AgF easily penetrated through infected caries and affected tooth structure to the healthy dentine for the line scan of 82.65 μm. There was no significant difference between SDF and AgF for the movement of ions within the infected dentine nor in the affected/healthy dentine.

Fabrication, Physical-Chemical and Biological Characterization of Retinol-Loaded Poly(vinyl Alcohol) Electrospun Fiber Mats for Wound Healing Applications

Nowadays, there exists a huge interest in producing innovative, high-performance, biofunctional, and cost-efficient electrospun biomaterials based on the association of biocompatible polymers with bioactive molecules. Such materials are well-known to be promising candidates for three-dimensional biomimetic systems for wound healing applications because they can mimic the native skin microenvironment; however, many open questions such as the interaction mechanism between the skin and the wound dressing material remain unclear. Recently, several biomolecules were intended for use in combination with poly(vinyl alcohol) (PVA) fiber mats to improve their biological response; nevertheless, retinol, an important biomolecule, has not been combined yet with PVA to produce tailored and biofunctional fiber mats. Based on the abovementioned concept, the present work reported the fabrication of retinol-loaded PVA electrospun fiber mats (RPFM) with a variable content of retinol (0 ≤ Ret ≤ 25 wt.%), and their physical-chemical and biological characterization. SEM results showed that fiber mats exhibited diameters distribution ranging from 150 to 225 nm and their mechanical properties were affected with the increasing of retinol concentrations. In addition, fiber mats were able to release up to 87% of the retinol depending on both the time and the initial content of retinol. The cell culture results using primary mesenchymal stem cell cultures proved the biocompatibility of RPFM as confirmed by their effects on cytotoxicity (low level) and proliferation (high rate) in a dose-dependent manner. Moreover, the wound healing assay suggested that the optimal RPFM with retinol content of 6.25 wt.% (RPFM-1) enhanced the cell migratory activity without altering its morphology. Accordingly, it is demonstrated that the fabricated RPFM with retinol content below the threshold 0 ≤ Ret ≤ 6.25 wt.% would be an appropriate system for skin regenerative application.

Composites Containing Nanohydroxyapatites and a Stable TEMPO Radical: Preparation and Characterization Using Spectrophotometry, EPR and 1H MAS NMR

Hydroxyapatite is the main constituent of mammalian hard tissues. Basic applications of synthetic hydroxyapatites include bone and dental implantology and drug delivery systems. The study of hydroxyapatite surface properties could give greater insight into the processes of bone mineralization and degradation. Nitroxide radicals are stable radicals that exhibit anticancer and antioxidative properties and are often used as spin probes to study the dynamics of complex biological systems. In this work, we attempted to adsorb the stable 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) on two hydroxyapatites (HAs) differing in specific surface area and the degree of hydration. The adsorption was carried out from cyclohexane, 1-chlorobutane and water. The solutions after adsorption were studied spectrophotometrically, while the obtained composites were characterized via NMR and EPR spectroscopy. The results show that it is possible to reproducibly obtain fairly stable composites, where the main factors influencing the adsorbed amount of the radical are solvent polarity and specific surface area of hydroxyapatite. The Langmuir isotherm was determined to be the most suitable adsorption model. The analysis of EPR and NMR spectra allowed us to determine the distribution of the TEMPO molecules on the hydroxyapatite surface, as well as a probable adsorption mechanism. The HA/TEMPO composites could potentially be used to study certain properties of hydroxyapatite surfaces with EPR spectroscopy. They could also be used as fillers after hard tissue surgery, as well as metal-free MRI contrasts.

Binding mechanism and binding free energy of amino acids and citrate to hydroxyapatite surfaces as a function of crystallographic facet, pH, and electrolytes

Hydroxyapatite (HAP) is the major mineral phase in bone and teeth. The interaction of individual amino acids and citrate ions with different crystallographic HAP surfaces has remained uncertain for decades, creating a knowledge gap to rationally design interactions with peptides, proteins, and drugs. In this contribution, we quantify the binding mechanisms and binding free energies of the 20 end-capped natural amino acids and citrate ions on the basal (001) and prismatic (010)/(020) planes of hydroxyapatite at pH values of 7 and 5 for the first time at the molecular scale. We utilized over 1500 steered molecular dynamics simulations with highly accurate potentials that reproduce surface and hydration energies of (hkl) hydroxyapatite surfaces at different pH values. Charged residues demonstrate a much higher affinity to HAP than charge-neutral species due to the formation of superficial ion pairs and ease of penetration into layers of water molecules on the mineral surface. Binding free energies range from 0 to -60 kJ/mol and were determined with ∼ 10% uncertainty. The highest affinity was found for citrate, followed by Asp(-) and Glu(-), and followed after a gap by Arg(+), Lys(+), as well as by His(+) at pH 5. The (hkl)-specific area density of calcium ions, the protonation state of phosphate ions, and subsurface directional order of the ions in HAP lead to surface-specific binding patterns. Amino acids without ionic side groups exhibit weak binding, between -3 and 0 kJ/mol, due to difficulties to penetrate the first layer of water molecules on the apatite surfaces. We explain recognition processes that remained elusive in experiments, in prior simulations, discuss agreement with available data, and reconcile conflicting interpretations. The findings can serve as useful input for the design of peptides, proteins, and drug molecules for the modification of bone and teeth-related materials, as well as control of apatite mineralization.

Internalization of Fluoride in Hydroxyapatite Nanoparticles

Hydroxyapatite (HAP) is a cost-effective material to remove excess levels of fluoride from water. Historically, HAP has been considered a fluoride adsorbent in the environmental engineering community. This paper substantiates an uptake paradigm that has recently gained disparate support: assimilation of fluoride to bulk apatite lattice sites in addition to surface lattice sites. Pellets of HAP nanoparticles (NPs) were packed into a fixed-bed media filter to treat solutions containing 30 mg-F/L (1.58 mM) at pH 8, yielding an uptake of 15.97 ± 0.03 mg-F/g-HAP after 864 h. Solid-state ¹⁹F and ¹³C magic-angle spinning nuclear magnetic resonance spectroscopy demonstrated that all removed fluoride is apatitic. A transmission electron microscopy analysis of particle size distribution, morphology, and crystal habit resulted in the development of a model to quantify adsorption and total fluoride capacity. Low- and high-estimate median adsorption capacities were 2.40 and 6.90 mg-F/g-HAP, respectively. Discrepancies between experimental uptake and adsorption capacity indicate the range of F^- that internalizes to satisfy conservation of mass. The model was developed to demonstrate that F^- internalization in HAP NPs occurs under environmentally relevant conditions and as a tool to understand the extent of F^- internalization in HAP NPs of any kind.

Nanoparticle Size Effect on Water Vapour Adsorption by Hydroxyapatite

Handling and properties of nanoparticles strongly depend on processes that take place on their surface. Specific surface area and adsorption capacity strongly increase as the nanoparticle size decreases. A crucial factor is adsorption of water from ambient atmosphere. Considering the ever-growing number of hydroxyapatite nanoparticles applications, we decided to investigate how the size of nanoparticles and the changes in relative air humidity affect adsorption of water on their surface. Hydroxyapatite nanoparticles of two sizes: 10 and 40 nm, were tested. It was found that the nanoparticle size has a strong effect on the kinetics and efficiency of water adsorption. For the same value of water activity, the quantity of water adsorbed on the surface of 10 nm nano-hydroxyapatite was five times greater than that adsorbed on the 40 nm. Based on the adsorption isotherm fitting method, it was found that a multilayer physical adsorption mechanism was active. The number of adsorbed water layers at constant humidity strongly depends on particles size and reaches even 23 layers for the 10 nm particles. The amount of water adsorbed on these particles was surprisingly high, comparable to the amount of water absorbed by the commonly used moisture-sorbent silica gel.

Hydroxyapatite-based sorbents: elaboration, characterization and application for the removal of catechol from the aqueous phase

Hydroxyapatite (HAP) is highly considered as good sorbent for the removal of metals from the aqueous phase. However, soluble metals co-exist with organic pollutants in wastewaters. But little work has been devoted to investigate the reactivity of HAP for the removal of organic compounds. The main objective of this work is to study the reactivity of HAP-based sorbents for the removal of catechol as a model organic pollutant from an aqueous solution. Thus, HAP sorbents were firstly synthesized using calcium carbonate and potassium dihydrogen phosphate under moderate conditions (25-80°C, atmospheric pressure). A zinc-doped HAP was also used as sorbent, which was obtained from the contact of HAP with an aqueous solution of zinc nitrate. All the sorbents were characterized by different standard physico-chemical techniques. The sorption of catechol was carried out in a batch reactor under stirring at room temperature and pressure. Zinc-doped HAP sorbent was found to be more reactive than non-doped HAP sorbents for the fixation of catechol. The highest sorption capacity was of 15 mg of C per gram of zinc-doped HAP sorbent. The results obtained suggest the reaction scheme of HAP sorbents with metals and organic pollutants when HAP sorbents were used for the treatment of complex wastewaters.

Graphene oxide papers with high water adsorption capacity for air dehumidification

Graphene oxide (GO) has shown a high potential to adsorb and store water molecules due to the oxygen-containing functional groups on its hydrophilic surface. In this study, we characterized the water absorbing properties of graphene oxide in the form of papers. We fabricated three kinds of graphene oxide papers, two with rich oxygen functional groups and one with partial chemical reduction, to vary the oxygen/carbon ratio and found that the paper with high oxygen content has higher moisture adsorption capability. For the GO paper with reduction, the overall moisture absorbance was reduced. However, the absorbance at high humidity was significantly improved due to direct formation of multilayer water vapor in the system, which derived from the weak interaction between the adsorbent and the adsorbate. To demonstrate one application of GO papers as a desiccant, we tested grape fruits with and without GO paper. The fruits with a GO paper exhibited longer-term preservation with delayed mold gathering because of desiccation effect from the paper. Our results suggest that GO will find numerous practical applications as a desiccant and is a promising material for moisture desiccation and food preservation.

Electrical characterization of hydroxyapatite-based bioceramics

This paper studies the AC conductivity and permittivity of hydroxyapatite (HA)-based ceramics from 0.1 Hz-1 MHz at temperatures from room temperature to 1000 degrees C. HA-based ceramics were prepared either as dense ceramics or in porous form with interconnected porosity and were sintered in either air or water vapour. Samples were thermally cycled to examine the influence of water desorption on AC conductivity and permittivity. Surface-bound water was thought to contribute to conductivity for both dense and porous materials at temperatures below 200 degrees C. At temperatures below 700 degrees C the permittivity and AC conductivity of HA was also influenced by the degree of dehydration and thermal history. At higher temperatures (700-1000 degrees C), bulk ionic conduction was dominant and activation energies were of the order of approximately 2 eV, indicating that hydroxyl ions are responsible for conductivity.

Electrospun PVA/HAp nanocomposite nanofibers: biomimetics of mineralized hard tissues at a lower level of complexity

Based on the biomimetic approaches the present work describes a straightforward technique to mimic not only the architecture (the morphology) but also the chemistry (the composition) of the lowest level of the hierarchical organization of bone. This technique uses an electrospinning (ES) process with polyvinyl alcohol (PVA) and hydroxyapatite (HAp) nanoparticles. To determine morphology, crystalline structures and thermal properties of the resulting electrospun fibers with the pure PVA and PVA/HAp nanocomposite (NC) before electrospinning various techniques were employed, including transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), scanning electron microscopy (SEM), x-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition, FT-IR spectroscopy was carried out to analyze the complex structural changes upon undergoing electrospinning as well as interactions between HAp and PVA. The morphological and crystallographic investigations revealed that the rod-like HAp nanoparticles exhibit a nanoporous morphology and are embedded within the electrospun fibers. A large number of HAp nanorods are preferentially oriented parallel to the longitudinal direction of the electrospun PVA fibers, which closely resemble the naturally mineralized hard tissues of bones. Due to abundant OH groups present in PVA and HAp nanorods, they strongly interact via hydrogen bonding within the electrospun PVA/HAp NC fibers, which results in improved thermal properties. The unique physiochemical features of the electrospun PVA/HAp NC nanofibers prepared by the ES process will open up a wide variety of future applications related to hard tissue replacement and regeneration (bone and dentin), not limited to coating implants.

Dissolution and re-crystallization processes in multiphase silicon stabilized tricalcium phosphate

Ultrasonically accelerated dissolution of multiphase silicon stabilized tricalcium phosphate powders in water or Earle's balanced salt solution transforms the powders into needle-like calcium deficient apatite crystals with the c-axis (001) oriented along the needle. Ion exchange with the solution occurs primarily in the first hours of immersion. The transformation is driven by an interaction between the crystal surface and adsorbed water leading to the growth of crystallites which have the most stable surface configuration. First principles calculations of the surface energies of various hydroxyapatite surfaces with and without adsorbed water shows that depending on the ion concentrations in the fluid that determine the chemical potential of tricalcium phosphate, either Ca-rich (010) or stoichiometric (001) layers are the dominant surfaces. The higher the chemical potential, the more elongated in the (001) direction the crystallites become to minimize the total surface energy. The loss of a calcium Ca(2+) compensated by the addition of two H(+) is strongly favoured energetically on the (001) and Ca-rich (010) surfaces. A high concentration of excess Si at grain boundaries may be partly responsible for the rapid transformation of multiphase Si-TCP.

Adsorption from solutions on synthetic hydroxyapatite: nonaqueous vs. aqueous solvents

It is useful in restorative dentistry and etiology of bone diseases to know that certain general rules may be derived from a priori considerations of the adsorption of chemicals to synthetic hydroxyapatite (which is the structural prototype of tooth and bone mineral). This article systematically synthesizes the extensive studies on the subject by the author, and rationalizes and extends these rules. Though not a review article, the work of some researchers may not have found an adequate representation here, because they did not determine, in the case of nonaqueous solvents, the reversibility of adsorption; or, in the case of aqueous solvents, the concentration of all constituent ions of the system, thus missing the occurrence of processes other than adsorption. The adsorption of solutes from nonaqueous solvents on hydroxyapatite is mainly regulated by the interplay of hydrogen bonding between adsorbate, adsorbent, and solvent. It does not involve any significant role of ionic nature of the apatite surface, because it is masked by the chemisorbed and physisorbed water. This interplay of hydrogen bonding in conjunction with chemical and structural characteristics of the adsorbed molecules controls their reversibility and the orientation of the adsorbates on the surface. In general, the process may be considered as true adsorption, because no other material exchange process is involved. The "adsorption" of solutes from aqueous solutions generally involves ion-exchange with the apatite surface, and may be affected by the concentrations of potential determining ions (Ca(2+), phosphates, H(+), etc.) in the solution. The solute in aqueous solution may be removed by two other mechanisms, either by formation of a surface complex or by chemically reacting with calcium or phosphate ions to form a new phase that precipitates out of solution. Therefore, the concentration of calcium and phosphate ions in solution should also be monitored to elucidate the mechanism of the process. Adsorption of polymers seems to be determined primarily by their multiple hydrogen bonding, their relative molar mass, and their ability to self-associate. The hydrogen ion concentration plays a decisive role in each one of the above mechanisms.

Adsorption and orientation of tetracycline on hydroxyapatite

Adsorption of tetracycline from separate solutions of ethanol, p-dioxane, and chloroform onto synthetic hydroxyapatite (containing about 1.5 monolayer of physisorbed water) was studied in order to understand its interaction with bone and teeth. The adsorption isotherms of tetracycline are reversible and Langmuirian from ethanol and p-dioxane and are almost identical. The isotherm is irreversible from chloroform, and a constant amount of adsorbate is removed from the solutions above a certain concentration. The irreversibly adsorbed compound is completely desorbed by prolonged repeated washing with ethanol. An analysis of the reversible isotherms showed that at maximum coverage the ring or polycyclic structure of the molecule stands perpendicular to the surface with appropriate hydroxyl groups and keto-oxygens hydrogen bonded to the surface. However, the adsorption from chloroform is irreversible and at maximum adsorption is about one and half times larger than that from either ethanol or p-dioxane. The process of adsorption does not affect the chemical integrity of tetracycline.

Temperature dependence of water transport into the mineralized matrix of freeze-dried human dentine

Ten dentine sections cut perpendicular to the dentinal tubules from human mature non-carious third molars, were freeze-dried and then rehydrated by immersion in water at four temperatures, 10, 25, 40 and 70 degrees C. The uptake of water by the sections was assessed as a function of rehydration time. The data were analysed using a theoretical model in which the uptake is ascribed to two processes. The first is capillary suction of water into the dentinal tubules, the second is the diffusion of water from the two faces of the section into the mineralized matrix. By fitting theory to experiment the diffusion coefficient of water in intertubular dentine at the different temperatures was found. Using the Arrhenius relation the activation energy of diffusion of water in intertubular dentine was calculated as Ea = 29.5 +/- 2.2 kJ/mol, which is of the order of the strength of a hydrogen bond. Transport of water in the mineralized matrix during rehydration is thus most likely a hopping of water molecules along the surfaces of the collagen and/or mineral, each jump involving the breaking of one hydrogen bond.

Adsorption of 4-methacryloxyethyl trimellitate anhydride (4-META) on hydroxyapatite and its role in composite bonding

The adsorption of 4-methacryloxyethyl trimellitate anhydride (4-META) was studied from ethanol and dichloromethane onto synthetic hydroxyapatite (containing about 1.5 monolayers of physisorbed water) in order to study its role in restorative composite bonding to teeth. The adsorption isotherm of 4-META was S-shaped and reversible from ethanol, and followed the Langmuir plot at lower concentrations. The isotherm was irreversible from dichloromethane, and a constant amount of adsorbate was removed from the solutions above a certain concentration. The irreversibly adsorbed compound was completely removed by washing with ethanol. Therefore, the bonding between teeth and the restorative resin containing 4-META as a coupling agent is micromechanical and not chemical in nature. An analysis of isotherms showed that the benzene rings of the adsorbate molecules lie flat on the surface for both solvents. The molecules adsorbed from ethanol rotate about the solvent-modified (esterified) or unmodified carboxylic anhydride moieties with methacrylate hydrocarbon groups which are folded upward. However, the molecules adsorbed from dichloromethane remain fixed to the surface without rotation, and their methacryloxyethyl groups are folded over the rings. The diametral tensile strength of a BIS-GMA polymer with adsorbate-covered apatite was approximately equal to that of the composite filled with untreated apatite.

Quantification of free water in human dental enamel

The amount of free water in 50 samples of air-dried enamel from permanent and deciduous teeth was measured by the Karl Fischer method. Samples included both contemporary and burial specimens. The mean values obtained showed that free water constituted about 1% of enamel mass. However, the range of individual values varied considerably, from 0.56 to 1.48%. The proportion of free water did not seem to depend on the patient's age, the type of tooth, or the relationship between the tooth and its oral environment. On the other hand, the deciduous enamel tested displayed a mean free water content that was three times the mean for the permanent teeth (3.01 vs. 1.00%) and the five burial teeth, a mean content of 1.68%.

Vapor pressure isotherms, composition and density of hyperdense bones of horse, whale and porpoise

Water adsorption studies of three types of hyperdense bone show vapor pressure isotherms and enthalpy to be different from that for pure apatitic mineral. The mineral content of these bones varied from 80 to 98%, much greater than for the better known cortical bone. The high mineral content of all these bones and the variation of mineral content from one type of bone to the next make it possible to evaluate the contribution of the mineral component in bone to water adsorption. Both water content and the fractional shrinkage when dried decrease with increasing mineral content of the bone and increase with the organic component. The total water content of hyperdense bone is much less than the maximum water adsorbed by the equivalent pure mineral powder or by anorganic bone. It was concluded that: (1) Very little water appears to be adsorbed by the mineral in hyperdense bone; (2) It is likely that the mineral crystallites are coated with noncollagenous organic matter; (3) Water is taken up by organic matter both intra- and extrafibrillar; and (4) the enthalpy associated with the adsorption of water by HAP is higher than for hyperdense bone at all corresponding vapor pressures.

Water on apatites

Adsorption of water was studied gravimetrically at 23 degrees C in an open system at several relative humidities on a variety of apatitic calcium phosphates including enamel, deproteinized enamel, and bone mineral. The amount of adsorbed water increases linearly with the surface areas of the synthetic apatites and does not appear very sensitive to calcium to phosphorus ratio of the apatites. The adsorption results correlate very well up to about two monolayers with a conventionally determined isotherm. Higher uptake of water even by "deproteinized" enamel or bone may be due to the presence of pore structure and incompletely removed organic matter.

Adsorption of benzoic acid on pure and cupric ion-modified hydroxyapatite: implications for design of a coupling agent to dental polymer composites

The adsorption isotherms of benzoic acid on synthetic hydroxyapatite (containing about 1.5 monolayers of physisorbed water) were studied from ethanol, dimethylsulfoxide, p-dioxane, methylene chloride, and benzene to discern the role of solvent in the process. The adsorption is reversible from the first three solvents and follows the Langmuir plots. It is irreversible from the last two, and a constant amount of absorbent is removed from solutions above a certain concentration. The isotherms of potassium benzoate on the apatite from ethanol and dimethyl sulfoxide were reversible. The isotherms of the acid on cupric ion-modified apatite surfaces from ethanol and benzene were identical with those obtained on the pure hydroxyapatite. This may demonstrate that any "surface chelation" with the cation may not be a significant factor for adsorption to occur. The adsorptive behavior seems to depend upon the interplay of hydrogen-bonding among the solute, the solvent, and the hydrated apatite surface. The capability of a solvent to hydrogen-bond may determine whether adsorption from it will be reversible or irreversible. Based upon its compatibility with a solvent, the benzene ring is upright or lies flat on the surface. The adsorbed molecules rotate about the center of the carboxylate groups which are hydrogen-bonded to the surface. These factors should be considered in designing or selecting a suitable surface-active moiety for a coupling agent between tooth mineral and a restorative resin.

Adsorption of zirconyl salts and their acids on hydroxyapatite: use of the salts as coupling agents to dental polymer composites

Zirconyl methacrylate (I) and zirconyl-2-ethylhexanoate (II) were synthesized, and their adsorption isotherms from solutions onto synthetic hydroxyapatite were studied. The isotherms of methacrylic and 2-ethylhexanoic acids were also determined from the same solvents. The adsorption of I was irreversible from methylene chloride, and that of II was irreversible from cyclohexane. The adsorption in both cases was constant from solutions above a certain concentration, and exhaustive below this threshold concentration. Both compounds rendered the dried apatite powder extremely hydrophobic; however, the adsorbate was slowly washed off by excess water. The configuration of the adsorbate molecules, deduced from the maximum adsorption and other adsorption characteristics of the two compounds, indicated that: (i) in both cases the adsorbate may be held to the surface by concerted hydrogen bonding of the carboxylate and zirconyl oxygen atoms; and (ii) the hydrocarbon moieties in both adsorbates expose themselves toward the solution, thereby making the dried surface hydrophobic. The adsorptive behavior of the respective acids was similar to that of the salts. Polymer, filled with synthetic hydroxyapatite covered with irreversibly adsorbed I, had a diametral tensile strength about 50% greater than that of the polymer filled with untreated apatite. The strength of the composite was not affected by treatment of the apatite with II or with the acids.

Surface area and pore size analysis for human enamel and dentine by water vapour sorption

The water sorption apparatus described allowed water sorption isotherms to be plotted on several samples simultaneously, with the added possibility of recording their nuclear magnetic resonance spectra for any points on the isotherm. Water sorption showed that human dentine had a saturated water content (removable in vacuo at room temperature) of approx. 10 per cent w/w (21 per cent v/v), probably in micropores or associated with the organic phase, and a specific surface of approx. 150 m2/g. A similarly measured water content for outer enamel was approx. 0.9 per cent w/w (2.7 per cent v/v), with a specific surface of 5.5 m2/g. For whole enamel, the slightly larger values of 1.15 per cent w/w (3.4 per cent v/v) and 8.7 m2/g were obtained. Results are shown to depend upon the conditions of measurement. Pore-size distribution analysis of enamel and dentine by water sorption, in this study and in investigations by others, is shown to be subject to many uncertainties and of doubtful value. Some uncertainty is also believed to apply to surface area measurements.

Structure and porosity of human cervical enamel studied by polarizing microscopy and transmission electron microscopy

Dehydration by alcohols and concentrated solutions of highly soluble salts caused refractive changes in enamel which were enhanced in the cervical regions. Electron microscopy showed crystals in cervical enamel to vary from normal orientation, size and habit to marked disorientation and reduced size. Consequent increases in surface area and inter-crystalline volume could account for the exaggerated optical behaviour. The changes in optical properties of enamel occurred only when the osmotic pressures of the imbibing solutions exceeded 500 atm. It is proposed that the internal pore system may take up water freely but exclude large hydrated molecules and ions which thereby create a negative pressure on the water contained in these pores. Once this negative pressure exceeds the tensile strength of the water-pore system, molecules may be torn from their attachment sites, and possibly from each other, so that a sudden evacuation of the pores is accompanied by a simultaneous reduction in refractive index. It is likely that such sudden evacuation would alter the potential of the pore surfaces to react with small molecules and ions subsequently invading the tissue.