Wenju Wu

Determination of interfacial tension from crystallization and dissolution data: a comparison with other methods

Methods for the determination of interfacial tension between a solid and a liquid are reviewed including solubility/particle size, crystallization and dissolution kinetics. The use of solubility as a function of particle size, originally put forward by Ostwald and later corrected by Freundlich, may be unjustified for determining interfacial tension at solid-liquid interfaces. The interfacial tension values between solutions and sparingly soluble minerals such as hydroxyapatite, fluorapatite, brushite, octacalcium phosphate, calcium oxalate monohydrate, barium sulfate, calcium sulfate, calcite, and divalent metal fluorides are discussed. A comparison of these results is made with contact angle or wetting measurements. The interfacial tension values obtained from constant composition reaction kinetics are of the same order of magnitude as those determined using a contact angle method involving thin layer wicking techniques.

Biomineralization mechanisms: a kinetics and interfacial energy approach

Abstract The calcium phosphates and oxalates are among the most frequently encountered biomineral phases and numerous kinetics studies have been made of their crystallization and dissolution in supersaturated and undersaturated solutions, respectively. These have focused mainly on parameters such as solution composition, ionic strength, pH, temperature, and solid surface characteristics. There is considerable interest in extending such studies to solutions more closely simulating the biological milieu. The constant composition method is especially useful for investigating the mechanisms of these reactions, and in the present work, the interfacial tensions between water and each of these surfaces have been calculated from measured contact angles using surface tension component theory. Values for the calcium phosphate phases such as dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), hydroxyapatite (HAP), and fluorapatite (FAP) may be compared with data calculated from dissolution kinetics experiments invoking different reaction mechanisms. Agreement between the directly measured interfacial energies and those calculated from the kinetics experiments provides valuable corroborative information about individual growth and dissolution mechanisms. For the calcium phosphates, the much smaller interfacial tensions of OCP and DCPD in contact with water as compared with those of HAP and FAP support the suggestion that the former phases are precursors in HAP and FAP biomineralization. The ability of a surface to nucleate mineral phases is closely related to the magnitude of the interfacial energies. Constant composition studies have also shown that HAP is an effective nucleator of calcium oxalate monohydrate, both of which are frequently observed in renal stones.

Synthesis and In vitro characterization of a tissue-Selective fullerene: vectoring C60(OH)16AMBP to mineralized bone

Abstract A tissue-vectored bisphosphonate fullerene, C60(OH)16AMBP [4,4-bisphosphono-2-(polyhydroxyl-1,2-dihydro-1,2-methanofullerene[60]-61-carboxamido)butyric acid], designed to target bone tissue has been synthesized and evaluated in vitro. An amide bisphosphonate addend, in conjunction with multiple hydroxyl groups, confers a strong affinity for the calcium phosphate mineral hydroxyapatite of bone. Constant composition crystal growth studies indicate that C60(OH)16AMBP reduces hydroxyapatite mineralization by 50% at a concentration of 1 μM, following a non-Langmuirian mechanism. Parallel studies with C60(OH)30 also indicate an affinity for hydroxyapatite, but at a reduced level (28% crystal growth rate reduction at 1 μM) compared with C60(OH)16AMBP. This study is the first to demonstrate that a fullerene-based material can be successfully targeted to a selected tissue as a step toward the development of such materials for medical purposes, in general.

A new understanding of the relationship between solubility and particle size

Most discussions of the relationships between crystal solubility and particle size have hitherto been concerned with vapor condensation and have led to the prediction that the vapor pressure increases with curvature. Here, thermodynamic arguments are presented to show that such relationships, describing crystal solubility as a function of particle size, originally put forward by Ostwald and later corrected by Freundlich, may be unjustified for determining interfacial tension at solid–liquid interfaces. The Kelvin or Gibbs–Thomson equations are valid for liquid–vapor systems, but not for solid–liquid interfaces. Recent experimental observations have demonstrated that interfacial tension data obtained by the solubility–size approach are unreasonable. This leads to the conclusion that “Ostwald ripening” may not be due to a higher solubility of smaller crystals, but rather to a net negative interfacial tension between solid and solution.

Measurements of the kinetic constants of protein adsorption onto silica particles

Abstract The early events pertaining to protein (human serum albumin: HSA) adsorption and desorption onto silica particles were studied employing real time, in situ measurements. The experimental method involved continuous measurements of the outflowing concentration of HSA with a fluorimeter, based on the natural fluorescence of the protein molecules. The adsorption (desorption) took place inside a well-stirred compartment, where particles were brought into contact by injection into a stream of a protein solution of known concentration. Intense mixing and sufficient protein supply rate allowed the process to take place solely under kinetic control. The acquired data were interpreted, according to a kinetic model, in terms of protein binding rates. From the latter, the kinetic association ( k a ) and dissociation ( k d ) constants were determined. To avoid the influence of steric hindrance, only data points obtained within the first 0.5 s of the initialization of the experiments were used. The experiments were performed at different protein concentrations, ranging from 7.25 nM to 14.5 μM. The real kinetic constants were determined by extrapolating the data obtained to zero protein concentration. Protein concentration effects were found to be pronounced in the determination of the kinetic association constant, producing values underestimated by as much as 30-fold for a 14.5 μM concentration. The concentration effect on the kinetic dissociation constant was not very significant: it was only of the order of a factor 2. The ratio of favorable to unfavorable protein orientations, also known as von Smoluchowski’s factor ( f ), was found to be 0.064 for the system of silica and HSA. For HSA adsorbing onto silica particles, the following values were found: k a =4.529×10 6 l mol −1 s −1 ; k d =0.21 s −1 . To convert from stirred to stationary conditions, both kinetic constants should be reduced by a factor 62.5, decreasing von Smoluchowski’s f factor to 0.001.

Intergrowth of calcium phosphates: an interfacial energy approach

A factor which is usually ignored in discussions of the induced crystallization of one phase by the surface of another is the surface free energy of the nucleus/substratum interface. Interfacial energies of hydroxyapatite (HAP), octacalcium phosphate (OCP) and fluorapatite (FAP) microcrystals against aqueous solutions, measured using a thin-layer wicking technique, were 9.0, 4.3 and 18.5 mJ m−2, respectively. The calculated low interfacial energy, 0.93 mJ m−2, between OCP and HAP provides strong support for the suggestion that OCP is the first forming phase that induces HAP crystallization in calcium phosphate precipitation reactions. Using the constant composition kinetics method, the nucleation and growth of OCP on titanium oxide surfaces were investigated. The interfacial energy calculated from the nucleation and growth data compared favorably with that obtained by thin layer wicking. Extending the interfacial energy approach to polymeric substrata such as poly(methyl methacrylate) (PMMA), and the radiofrequency glow discharge treated PMMA demonstrates, in predicting the ability of surfaces to induce mineral nucleation, the importance of the Lewis base parameter.

Application of the extended DLVO theory—the stability of alatrofloxacin mesylate solutions

Electrophoretic mobility and contact angle measurements have been made on alatrofloxacin mesylate and its formulations which were protected from or exposed to light, and its degradation product compound (F). In aqueous solution, the light-protected alatrofloxacin mesylate had a zeta-potential of +19 mV, a negligible electron-acceptor (γi+) surface tension parameter and an electron-donor surface tension parameter γi−=32.5 mJ m−2, which was higher than that of water. This caused the particles to be very hydrophilic and to form very stable suspensions in aqueous solution due, mainly, to a net Lewis acid–base (polar) repulsion. After the suspensions were exposed to light, the zeta-potential of the degradation product increassed to +37.8 mV, but the electron-donor surface tension parameter decreased to γi−=8 mJ m−2, making the molecules or particles very hydrophobic and causing them to flocculate. The energies of attraction in the latter case were mainly hydrophobic (90%) with about 10% resulting from van der Waals forces.

Kinetics and Surface Energy Approaches to the Crystallization of Synthetic and Biological Calcium Phosphates

Constant composition methods have been used to investigate the mechanisms of crystal growth and dissolution of synthetic and biological calcium phosphates. Interfacial tensions between water and each of these surfaces were calculated from measured contact angles using surface tension component theory. The data, 4.5 x 10−3, 8.8 x 10−3 and 10.4 x 10−3 J m−2 for human dentin, human enamel and hydroxyapatite, respectively, compared well with the data calculated from dissolution kinetics experiments and provided information concerning the growth and dissolution mechanisms. The ability of a surface to nucleate other phases is closely related to the magnitude of the interfacial energies.

Kinetics and energetics of plasma protein adsorption onto mineral microparticles

The early events pertaining to protein adsorption and desorption onto various mineral particles were studied employing real time, in situ measurements. The experimental method involved continuous measurements of the outflowing protein concentration with a fluorescence spectrometer, based on the natural fluorescence of the proteins. The acquired data were interpreted, according to a kinetic model, in terms of protein binding rates. From the latter, the kinetic association (k/sub a/) and dissociation (k/sub d/) constants were determined. To avoid the influence of steric hindrance, only data points obtained within the first 0.5 s from the initialization of the experiments were used. Protein concentration effects were found to be pronounced in the determination of the kinetic association constant and less significant in the case of the kinetic dissociation constant. The energy of the non-covalent bonds that are formed between the protein and the surface during adsorption was calculated from the kinetic data. Finally, use of the extended DLVO (XDLVO) theory allowed the calculation of the type of non-covalent bonds, as well as the ratio of favorable to unfavorable protein orientations upon adsorption, also known as von Smoluchowskis factor, f.

The Influence of Additives and Impurities on Crystallization Kinetics

Expressions describing the influence of additives and impurities on the kinetics of mineralization and demineralization have been derived from an interfacial tension point of view. In aqueous solution, the Lifshitz-van der Waals interfacial tension component changes very little, but the Lewis base (or electron-donicity) surface tension parameter varies markedly as a function of additive and impurity concentrations. The inhibiting effects of simple cations on crystal growth in solution may result from the increase in interfacial tension accompanying their adsorption on the surfaces. In the case of polymers or macromolecules, the kinetics of crystallization will not only depend upon the substrata but also the surface properties of the additives. Nucleation of calcium phosphate phases was observed only on surfaces having low solid/solution interfacial tension and relatively high electron-donicity of the solid surfaces. Such properties were found for human serum albumin immobilized on polymer solid surfaces of poly(methyl methacrylate) (PMMA) and poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), and following radiofrequency glow discharge (RFGD) treatment which mimicked the adsorption of impurity OH− ions on the surfaces.