Solubility Gradient Research Articles

Synthesis of functionally graded CO 3 apatite as surface biodegradable crystals

Apatite, which will have a degradable surface and a relatively stable core, was synthesized at 80±1°C and pH 7.4±0.2 using a specially generated gradient carbonate supply system. X-ray diffraction analysis showed that the (3 0 0) reflection peak shifted to the higher angle direction due to the substitution of CO 2- 3 ions into the PO 3- 4 positions and was broader than that of homogeneous hydroxyapatite. Scanning electron microscopy indicated that the apatite was composed of small coagulated ellipsoidal crystals differing from needle-like crystals with hexagonal section of hydroxyapatite. Electron spectroscopy for chemical analysis showed a negative gradient of carbonate concentration with depth in the crystals. The apparent solubility in 0.5 mol l -1 acetate buffer solution (37°C, pH 4.0) was much higher than that of homogeneous hydroxyapatite. The residual sample showed slim needle-like crystals. These results suggested that graded carbonate-containing apatite, namely CO 3 apatite with solubility gradient in its crystal structure, was formed by this process and may be useful from the view point of surface biodegradable materials as bone representatives.

Channeling instability of upwelling melt in the mantle

We present results of a theoretical study aimed at understanding melt extraction from the upper mantle. Specifically, we address mechanisms for focusing of porous flow of melt into conduits beneath mid‐ocean ridges in order to explain the observation that most oceanic residual peridotites are not in equilibrium with mid‐ocean ridge basalt. The existence of such conduits might also explain geological features, termed replacive dunites, that are observed in exposed mantle sections. We show here, by linear analysis, that flow in a chemically reactive porous media is unstable in the presence of a solubility gradient, such as induced by adiabatic ascent of melt underneath mid‐ocean ridges. The initially homogeneous flow becomes focused in time to produce elongated high‐porosity fingers that act as conduits for transport of fast flowing melt. This instability arises due to a positive feedback mechanism in which a region of slightly higher than average porosity causes increased influx of unsaturated flow, leading to increased dissolution which further reduces the Porosity. Even in the presence of matrix compaction and chemical diffusion the instability is demonstrated to be robust. Our analysis also indicates the existence of growing, traveling waves which transport and amplify porosity and concentration perturbations.

A mechanism for preferential H2O leakage from fluid inclusions in quartz, based on TEM observations

Preferential leakage of H2O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H2O-CO2-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H2O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphine twin boundaries). These defects allow preferential loss of H2O from H2O-CO2-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H2O must have occurred in these rocks.

Fluid-flow processes and diagenesis in sedimentary basins

Abstract Flow of fluids in sedimentary basins causes transport of heat and dissolved mass and is therefore potentially important in relation to diagenetic reactions. Rather large fluid fluxes are required, however, for this type of transport to be significant in terms of dissolution and precipitation of minerals. Before diagenetic processes are attributed to pore water flow, semi-quantitative calculation of flow rates and duration of flow should be attempted. Heat flow in sedimentary basins is normally dominated by conduction, except on a local scale. The upwards pore water flux due to compaction is on average smaller than the subsidence rate, and the thermal anomalies caused by compaction are moderate in modern basins. Dissolution and precipitation of minerals which are in equilibrium with the pore water occur when the flow is oblique relative to the isotherms. Due to the low solubility/temperature gradient of the common silicate and carbonate minerals, very large fluxes of pore water are required to transport significant volumes of mass in solution. The greatest potential for transporting solids and creating secondary porosity is during meteoric water flow, because the flow rate then may be several orders of magnitude higher than what is typical for compaction-driven flow. Quartz overgrowth corresponding to 3% of the rock volume requires a total flow of 10 8 cm 3 cm −2 if the silica is introduced by vertical compaction-driven flow from a source outside the sandstone. Pore water flow precipitating quartz due to cooling will dissolve carbonate cement at a much higher rate. An external import of silica through the flow of water would be expected to cement up the most permeable pathways, such as fractures and well-sorted permeable sand beds. In the case of carbonate cement the solubility gradient is negative and upwards flowing and cooling pore water would cause dissolution rather than precipitation. Also in the case of carbonate rocks, large-scale mass transfer such as dolomitization is easier to explain as occurring at relatively shallow depths rather than during deeper burial. Ore minerals like galena have a steep solubility/temperature gradient, but concentrated precipitation requires rapid cooling of hot water. Such conditions are most likely to be met when hot fluids are cooled near the surface.

Dependence of Ni Corrosion with a (Li0.62K0.38)2CO3 Coating on the Temperature and Atmosphere

Abstract The corrosion of Ni coated with alkali carbonates was measured in a CO2/O2 atmosphere at 873—1173 K. The corrosion of Ni coated with carbonates generally obeyed a parabolic rate law under a 67% CO2–33% O2 atmosphere. The parabolic rate constant decreased with an increase in the temperature with a (Li0.62K0.38)2CO3 coating under a 67% CO2–33% O2 atmosphere. This was explained by the solubility of NiO that was formed on Ni metal during corrosion. Hot corrosion was observed several times with (Li0.62K0.38)2CO3 under low CO2 pressure at 1073 K. This hot corrosion could be explained by the dissolution/reprecipitation mechanism of the NiO scale, due to the solubility gradient in the carbonate film.

Growth of protein single crystals in a periodically changing solubility gradient: description of the method and first results

A new method for the crystallization of proteins is presented and thermodynamic reasons for its working are given. By periodically combining phases of crystallization and dissolution, the surface of the protein crystals will be “cleaned” continuously of polluting material. The periodically changing solubility gradient is established by changing the temperature. The location of crystallization (sink) is separated spatially from a source of protein material.

Diagnostic and therapeutic problems in prostatitis. Therapeutic position of ofloxacin.

Lower tract localisation studies have been the gold standard in diagnosing various kinds of prostatitis. Four clinical categories are recognised: acute and chronic bacterial prostatitis, nonbacterial prostatitis, and prostatodynia. In acute bacterial prostatitis Gram-negative bacteria are the most common pathogens isolated. The roles of Gram-positive bacteria in chronic bacterial prostatitis and of Chlamydia trachomatis in nonbacterial prostatitis are contentious. Treatment of these various forms of prostatitis has been a challenge to the clinician. The lack of penetration of various drugs into the prostatic tissue, because of poor lipid solubility, ionisation, protein binding, and unfavourable pH gradients from the plasma to the prostatic fluid, may be the main reasons for poor results. The minimum inhibitory concentration (MIC) of antibacterial drugs used, and the concentration of drugs actually obtained in the prostate, combined with the influence of pH and inoculum size, and the effect of prostatic fluid and prostatic extracts on MIC are important factors in determining at least the theoretical efficacy of various drugs in the treatment of prostatitis. The new fluoroquinolone ofloxacin, the first quinolone to be approved for the treatment of chronic bacterial prostatis, has excellent penetration into human prostate and high in vitro activity in human urine and prostatic tissue. From a theoretical standpoint, ofloxacin should be ideal in the treatment of chronic bacterial prostatis. Comparative studies have shown its superiority to carbenicillin. Several noncomparative studies have also been reported. The initial results are promising, but further investigations are needed.

Transport Considerations in the Hot Corrosion of Ni by Molten Alkali Carbonates

The hot corrosion of Ni induced by molten alkali carbonates has been studied to elucidate the mechanism of attack and in particular to determine whether the Rapp‐Goto theory of fluxing is applicable to this system. Two principal rate‐determining processes were considered in some detail: transport of oxidant from the gas phase through the corrosion products, including the molten salt phase, to the site of active oxidation, and dissolution/reprecipitation of oxide within the corrosion product layer owing to a solubility gradient. To help evaluate these processes, we measured the solubility of in various carbonate salts and we experimentally characterized the scales with respect to average porosity and tortuosity. The kinetics of hot corrosion were measured by TGA under conditions expected to affect oxidant transport and/or fluxing. Experimental rates of oxidation were then compared with calculated maximum rates according to the two rate‐determining processes to evaluate the feasibility of rate control by one or the other process. It was concluded that fluxing was very likely to be rate controlling in the early stages of hot corrosion, but later, when the actual hot corrosion rate had increased considerably, oxidant transport was controlling. The distribution of salt within the pores of the scale changes with time, and this is a key issue in determining the rate of reaction.

Otolith growth in fishes

Abstract 1. 1. The sagittal otoliths of teleosts rest in a groove in the base of the otic cleft resulting in restriction of growth along part of the ventral edge of the otolith. 2. 2. The rate of crystalline deposition of the otolith is low at the sulcus, increasingly higher along the sulcul surface rising to a maximum at the growing edge of the otolith. 3. 3. It is proposed that the gradient in growth of crystalline deposition between sulcus and otolith edge is maintained by a pH gradient resulting from high to low levels of metabolic activity. 4. 4. Decreasing crystal widths along sulcus to edge surface are in accordance with those predicted by the Ostwald-Lussac law of crystal size changes along solubility gradients. 5. 5. The metabolic basis driving the pH gradient makes it probable that check rings in the sulcul part of the otolith are caused by surges in metabolic activity, not age.

Hot corrosion of materials

Abstract

Optimization of Controlled Drug Release Through Micropelletization

AbstractMicropelletization technique using crosslinked gelatin matrix was chosen to evaluate its utility in controlled release medications. Trimethoprim, which has a very high solubility gradient in gastric pH, was selected in this investigation. Micropellets were formed by the modified spray congealing technique. The effects of exposure of the crosslinking agents to the gelatin matrix of the micropellets on the effectivity as the controlled release drug delivery system were investigated. The total product yield, content uniformity and the reproducibilty of the sucessive batches were decidedly superior to either the pure drug or the non-crosslinked ones. Particle size distribution was observed to vary depending on the content of the crosslinked gelatin in the micropellets. Scanning electron micrographs confirmed the porous surface topography of the micropellets. The drug release characteristics was suggested as the diffusion controlled first order dissolution process.

Copolymer fractionation by gradient high-performance liquid chromatography

The size distribution and composition of statistical copolymers has been investigated by size-exclusion chromatography (SEC) and off-line gradient high-performance liquid chromatography of the SEC fractions. The copolymers were poly(styrene—acrylonitrile) (SAN) and poly(styrene—methyl methacrylate) (SMMA) model mixtures. SAN copolymers were separated with a suitable gradient of increasing dissolution power on normal-phase columns as well as on reversed-phase columns. In contrast, SMMA copolymers were eluted from a reversed-phase column by a solubility gradient as broad bands, whereas the same gradient gave excellent resolution on a normal-phase column. The elution characteristics of SMMA samples roughly parallelled their solubility characteristics in the system investigated but there was an extra contribution to retention on active columns.

The nature of early caries lesions in enamel.

Since 1935, various mechanisms have been suggested for the formation of subsurface lesions and, in particular, the surface layer covering enamel lesions. The relatively intact mineral-rich and porous surface layer is most likely caused by kinetic events. The suggested mineral-rich outer layer in sound enamel, the organic matrix, the pellicle, or a non-uniform ion distribution have all been shown to be non-essential for surface layer formation; they may, however, influence the rate of surface layer formation. Models based on outer surface protection by adsorbed agents, the dissolution-precipitation mechanism, and combinations of these two models, as well as models based on porosity or solubility gradients, are discussed in this paper together with their advantages and disadvantages. Most models have not explained some important recent experimental observations on initial in vivo caries lesion formation: e.g., initial enamel lesions formed in vivo do not have a surface layer initially but develop this mineral-rich layer later on; and the fact that the F- level in the solid sound enamel is not determining the subsurface lesion formation. Furthermore, the observations that in vitro fluoride ions in the liquid at very low levels (approximately equal to 0.02 ppm) determine surface layer formation are difficult to explain. A new kinetic model for subsurface lesion formation is described, in which inhibitors such as F- or proteins play an important role. The model predicts that if lesions depth and demineralization period are denoted by df and t, lesion progress can be described by: dfp = alpha t + c, where alpha and c are constants with 1 less than or equal to p less than or equal to 3, depending on the lesion formation conditions. If lesion progress is entirely diffusion-controlled, p = 3, corresponding to low inhibitor concentrations; if the inhibitor content is so high that the progress is controlled by processes at the crystallite surface, p = 1. A kinetic mechanism for surface layer formation in vivo is proposed, based on the assumption that F- is a main inhibitor in the plaque-covered acidic in vivo situation. The inhibiting fluoride, adsorbed onto the crystallite surfaces at OH- vacancies, originates from the so-called fluoride in the liquid phase (FL) between the enamel crystallites. Under acidic conditions (plaque), we have, due to an influx of fluoride from the saliva or plaque as FL, an aqueous phase in the enamel supersaturated with respect to the mineral for a small distance (x*) only.(ABSTRACT TRUNCATED AT 400 WORDS)

Solid dissolution: Effect of mass transport-precipitation coupling

Solubility-limited dissolution models are often used to calculate solid dissolution rates. In these models, the finite solid solubility limits the rate of solid dissolution. However, inhomogeneities such as temperature gradients can produce solubility gradients and, hence, can cause precipitation of solid downstream from the dissolving solid surface. Solubility-limited dissolution models, based on a one-dimensional diffusion-convection equation, are used to investigate the effect of precipitation on solid dissolution rates. Precipitation is found to increase solid dissolution rates. This effect is found to be particularly important at long times, especially if transport of dissolved solid is purely diffusion controlled, i.e. the convective velocity is zero.

The effect of precipitation on the long-term release of radionuclides from used fuel

Solubility-limited dissolution models can be used to determine source terms for the safety assessment of used nuclear fuel disposal in geological vaults. These models usually assume that the solubility of the used-fuel matrix is constant. However, spatial inhomogeneities can lead to solubility gradients and, hence, cause precipitation downstream from a dissolving used-fuel matrix. In this paper, the effect of precipitation on the used-fuel dissolution rate has been investigated. Precipitation of uranium-containing solids has been found to enhance the dissolution rate of used fuel, especially at long times. The enhancement factor has been calculated for various vault conditions.

A comparative study of hydrogen permeabilities and solubilities in some palladium solid solution alloys

Reproducible permeability data have been obtained for the approximately isoelectronic alloys Pd-25 at.% Ag, Pd-5.75 at.% Ce and Pd-8 at.% Y; the previously observed enhanced performance of the Pd-Y alloy has been confirmed. Hydrogen solubility measurements have also been made for the three alloys over the same range of pressures and temperatures as those employed in the permeability studies. Using these data the detailed shapes of the permeability versus temperature plots have been successfully analysed and it has been shown that the hydrogen diffusion coefficients in all three alloys are very similar. The present results show clearly that the enhanced performance of the Pd-Y diffusion membrane is due to the substantially greater hydrogen solubility gradient in this alloy membrane compared with that in Pd-25% Ag.

Flows of 22Na in layer-type composite membranes

The permeability of 22Na across simple and layer-type composite membranes of parlodion containing different amounts of polystyrenesulfonic acid has been measured as a function of external NaC1 concentration and corrected for the effects of aqueous stationary layers present at the membrane-solution interfaces. It was found that the permeability of 22Na in the two opposite directions across the composite membrane was different whereas it was the same across simple membranes. In both cases, the permeability of 22Na increased with increase in the concentration of the external solution. In the presence of a concentration gradient, the composite membranes generated ±58 mV for a tenfold difference in the concentration and gave values for the permeability of 22Na which were different in the two directions across the membrane. Corrections applied to the two permeability values in the two opposite directions due to the presence of the electric potential difference across the membrane gave values which agreed qualitatively with those measured in the absence of the concentration gradient. The asymmetry or rectification in the permeability of 22Na was attributed to the presence of a permanent Na-solubility gradient in the composite membrane which assisted the flow of Na in one direction (high charge density side to low charge density side of the membrane) and opposed it in the opposite direction. Measurement of distribution of 22Na in the various layers of the composite membrane showed that there was accumulation and depletion of 22Na in the membrane phase, i.e., 22Na concentration profile asymmetric, when 22Na was flowing from the high charge density side to the low charge density side and vice versa, respectively. The need to recognize the existence of such a directional quantity (solubility gradient) and its contribution to fluxes measured across the biological membranes which have a layered structure is pointed out.

Deposition of deep-sea manganese nodules

Manganese at equilibrium in seawater occurs dominantly as Mn 2+ and inorganic complexes at a concentration ratio of about 1:0.72; solubility decreases exponentially with increasing pH or Eh. However, the nodule oxides birnessite and todorokite are at least four orders of magnitude undersaturated relative to the Mn concentrations of seawater, and are metastable relative to hausmannite and manganite. This apparent lack of equilibrium is explicable by the mechanism of precipitation. Surfaces assist Mn precipitation by catalyzing equilibration between dissolved and reactive O 2 and simultaneously also by adsorbing ionic Mn species. The effective Eh at the surface becomes 200–400 mV above that of seawater; the oxidation rate of Mn increases about 10 8 ×, and the activation energies for Mn oxidation decrease ~ 11.5 kcal/mole. Consequently, marine Mn nodules and crusts form by adsorption and catalytic oxidation of Mn 2+ and ferrous ions at nucleating surfaces such as sea-floor silicates, oxyhydroxides, carbonates, phosphates and biogenic debris. The resulting ferromanganese surfaces autocatalyze further growth. In addition, Mn-fixing bacteria may also significantly accelerate accretion rates on these surfaces. Mn which accumulates in submarine sediments may be diagenetically recycled in response to steep solubility gradients causing upward migration from more acidic and reducing horizons toward the sea floor. In contrast, the concentrations of the predominant ferric complexes, Fe(OH) 3 0 and Fe(OH) 4 −, are relatively less sensitive to the Eh's and pH's found in this environment; Fe is therefore not as readily recycled within buried sediments. Consequently, Fe is not so effectively enriched on the sea floor, although it precipitates more readily than Mn because seawater is saturated in amorphous Fe(OH) 3. The metastable, perhaps kinetically-related, Mn oxides of nodules have a characteristic distribution: birnessite predominates in oxidizing environments of low sedimentation rate and todorokite where sedimentation rates and diagenetic Mn mobility are higher. Surface adsorption and cation substitution within the disordered birnessite-todorokite structure account for the high trace element content of Mn nodules.

Precipitation and Purification of Cucumber Mosaic Virus by Polyethylene Glycol (PEG) and Reverse Concentration PEG Gradient Centrifugation

Cucumber mosaic virus (CMV) was purified by repeated precipitations with 8% PEG and 0.2M NaCl, followed by PEG solubility gradient (0∼6% PEG stabilized in 35∼5% sucrose) centrifugation. Virus yields ranged from 500 to 800mg per kilogram of fresh tissue. Purified preparation was highly infectious and contained uniform virus particles. Sedimentation pattern showing a single peak suggests that no aggregation occurred.

The application of polyethylene glycol solubility-concentration gradients in plant virus research

A simple method for determining the solubility of plant viruses and other macromolecules in aqueous solutions of polyethylene glycol (PEG) is described. The method involves centrifugation of PEG-precipitated virus through a reverse concentration gradient of PEG, MW 6000, incorporated and stabilized in a positive sucrose gradient. The concentration of PEG at which the virus particles resolubilize is termed the solubility concentration. The solubility concentration of virus particles for a number of model systems was dependent upon their surface/volume ratio and their surface charge characteristics, the latter in turn depending on the pH and ionic conditions existing within the solubility gradient. Solubility gradient centrifugations were of value both analytically, in providing information for purification procedures and in the characterization of viruses and virus strains, and preparatively, allowing separation of certain morphologically or electrophoretically distinct components. The technique of solubility gradient centrifugation is potentially applicable to other systems having distinct solubility characteristics in an appropriate solvent.