Dissolution Rate Coefficient Research Articles

A two-stage mechanistic reduced-order model of pharmaceutical tablet dissolution: Population balance modeling and tablet wetting functions

We propose a two-stage reduced-order model (ROM) of pharmaceutical tablet dissolution that is comprised of (i) a mechanistic dissolution function of the active pharmaceutical ingredient (API) and (ii) a tablet wetting function. The former is derived from a population balance model, using a high-resolution finite volume algorithm for a given API crystal size distribution and dissolution rate coefficient. The latter is obtained from the mechanistic understanding of water penetration inside a porous tablet, and it estimates the rate at which the API is exposed to the buffer solution for a given formulation and the dimensions of the tablet, contact angle, and surface tension between the solid and liquid phases, liquid viscosity, and mean effective capillary radius of the pore solid structure. In turn, the two-stage model is mechanistic in nature and one-way coupled by means of convolution in time to capture the start time of the API dissolution process as water uptake, swelling, and disintegration take place. The two-stage model correlates dissolution profiles with critical process parameters (CPPs), critical material attributes (CMAs), and other crucial critical quality attributes (CQAs). We demonstrate the model’s versatility and effectiveness in predicting the dissolution profiles of diverse pharmaceutical formulations. Specifically, we formulate and fabricate acetaminophen and lomustine solid tablets using different API content and size distributions, characterize their dissolution behavior, and estimate capillary radius as a function of tablet porosity. The estimations generated by the proposed models consistently match the experimental data across all cases investigated in this study.

Assessing the effect of moderately increasing medium viscosity on the intrinsic dissolution rate and diffusion coefficient of ibuprofen particles

This study investigated the effects of moderately increasing medium viscosity through two viscosity enhancing agents (VEA) (HPMC and sucrose) on the intrinsic dissolution of a BCS class II drug (ibuprofen). The differences in intrinsic dissolution were characterized through the estimation of a diffusion coefficient (D) of ibuprofen in different media with the Levich equation. Increasing viscosity decreased the intrinsic dissolution rate of ibuprofen with both VEAs. The calculated D from intrinsic dissolution data overestimated the experimental value, but the results suggest that intrinsic dissolution could be used as a tool to estimate a relative change in D in different viscosities. 

Analytical models for predicting the behavior of the Fukushima fuel debris during laboratory tests and long-term storage

The results of the laboratory-based aging experiments with synthesized fuel debris (simulated corium–concrete compound) are discussed. The tests included dynamic (i.e. with periodic solution renewal) experiments with monoliths of the synthesized fuel debris. At the time scale of the experiments, at a temperature of 25 °C, no evolution in the composition of the leachate debris was observed. At the elevated temperature regime, T = 50, 90 and 120 °C and pH ranges from acid to alkaline condition (pH=4, pH=7, and pH=9), the major chemistry changes that occurs during the leaching were the increase of Si, Al and Ca, as structural elements of the concrete fraction of the debris, while the major elements of the melted fuel matrix, U and Zr, were found in the leachate in trace quantities. The release of Si, Al and Ca, was proved to be non-congruent (Ca and Al releases much faster compared to Si) that corresponds to the basic concept of multioxide silicate dissolution. Such system behavior as well as a number of the chemical interactions between the elements limits the classical chemical thermodynamic and kinetic methods and forced us to use a simplified, semi-empirical, effective approach. The component release rates measured in batch tests were fitted to a proposed diffusion and kinetics model which accounts also for the various pH and temperature conditions. Long-term prediction of the glassy phase dissolution and surface enrichment with a poorly dissolved fuel fraction, (U, Zr)O2, is based on a first-order kinetic model inherited the significant properties of the more detailed lab-scale model. To evaluate the growth kinetics of (U, Zr)O2 layer on debris surface, dissolution rate coefficients from the aging laboratory experiments were used. These coefficients calculated for high-temperature (T = 50 and 90 °C) surface reactions, were extrapolated to low-temperature debris storage conditions via the Arrhenius equation. It was shown, that the aging of debris in a water environment at room temperature and neutral pH under no-flow storage conditions results in the density accumulation of the (U, Zr)O2 product up to several g/m2. At the alkaline condition (pH=9) these values increase about five times. The further rise in temperature results in further intensification of the silica leaching process that may dramatically increase the accumulation amount of (U, Zr)O2.

The dissolution kinetics of natural gypsum: a case study of Eocene facies in the north-eastern suburbs of Paris

The dissolution kinetics of different varieties of natural gypsum samples with different porosity and content of insoluble impurities are experimentally investigated under unsaturated conditions. The main goal of this work is to verify whether and how the petrophysical and petrographic nature of gypsum influence its dissolution rate. Gypsum samples were taken from Priabonian (Ludian) and Lutetian formations located in the north-eastern suburbs of Paris, where the development of sinkholes due to gypsum dissolution is a common phenomenon. Experiments using the rotating disk technique allow us to determine the kinetic rate model parameters of each sample in pure water following the empirical rate expression derived from mixed kinetic theory. The kinetic order n shows a dispersion around a mean value \(n=1.2\) and k, the pure dissolution rate coefficient varies according to the facies and the experimental conditions (k \(\approx \,1\times 10^{-6}\) to 8\(\times 10^{-6}\) mmol/\(\hbox {cm}^2/\hbox {s}\)). These results are adjusted according to the specific roughness and texture of each sample, for the results to be more representative of in situ conditions. Batch experiments are also performed to evaluate the extremely low dissolution rates when the solution is close to equilibrium. The results reveal a double kinetics: far from equilibrium, the dissolution rates show that the linear behavior and the kinetic parameters are relatively close with the values found using the rotating disk method. For concentrations greater than 0.94 ± 0.02 of the equilibrium concentration \(C_\mathrm{{ref}}\), the dissolution rates show a linear behavior but with greater slope depending on the texture of the mineral. No changes of the dissolution rates were observed on pure gypsum crystals when non soluble solid impurities are added. However, a small degree of uncertainty around the value of \(C_\mathrm{{ref}}\) show a significant effect on the parameters of the second kinetics.

Production of fast-dissolving low-density powders for improved lung deposition by spray drying of a nanosuspension

We combined high-energy wet media milling and spray drying to engineer dry powders for inhalation consisting of geometrically large, low-density particles with superior aerodynamic properties and fast dissolution. Peclet number proved to be a useful instrument to guide choice of the additives and process conditions for generating low-density powders by spray drying. Composite dry powders consisted of milled and stabilized budesonide nanoparticles, leucine or albumin as matrix formers, and ammonium carbonate as a pore former. Powders of different composition showed fairly large and comparable geometric particle sizes (de,50 > 4.4 µm) with effective densities strongly depending on the present matrix former. Powders with lowest density reached an aerosol performance of up to 60%, which is well above most commercial, carrier-based products. It was also demonstrated that the nanomilling step was indispensable to yield such good aerosol performance. Dissolution of aerodynamically classified particle fractions showed a very fast onset and was largely completed within 30 min irrespective of the formulation and the impactor stage. Mathematical kinetic modeling was used to deduce the API dissolution rate coefficient from the results obtained using a modified USP 2 apparatus. Dissolution rate was found to be determined by the properties of the API nanoparticles rather than those of the composite particles. The employment of industrially established, solely water-based processes allows introducing the presented approach as a platform technology for the development of well-performing pulmonary formulations.

An experimental investigation into the effect of pore size distribution on the acid-rock reaction in carbonate acidizing

This study presents the correlation analysis between pore size distribution and diffusion coefficient based on the experimental results to evaluate the effect of pore size distribution on the acid-rock reaction in carbonate acidizing. The mercury injection capillary pressure (MICP) was used to analyze the pore size distribution of four limestone samples. Also, the acid-rock reaction experiments were conducted to measure diffusion coefficients using rotating disk apparatus (RDA). In the experimental results, micropore dominated rock samples have lower dissolution rate and diffusion coefficient than macropore dominated rock samples. It is obviously revealed that the dissolution rate and diffusion coefficient for limestone samples with similar mineral compositions are quite different due to various pore size distribution. In the result of correlation analysis, it is revealed that neither porosity nor permeability exhibits a strong correlation with diffusion coefficient compared with pore size distribution. Therefore, it can imply that the petro-physical properties such as porosity and permeability have a lower relevance to the acid-rock reaction kinetics than pore size distribution. To investigate the effect of pore size distribution on matrix acidizing, optimum injection rates were derived by the experimental results. As a result, the optimum injection rates increase with the mean and median pore size, and the micropore dominated rock samples show lower optimum injection rate than the macropore dominated rock samples. Although the pore size distribution shows a strong correlations with matrix acidizing, the optimum injection rate has been estimated without ample considerations of pore size distribution. Therefore, the effect of pore size distribution on diffusion coefficient should be considered to design the optimum injection conditions in carbonate acidizing job.

Isothermal Kinetic Mechanism of Coke Dissolving in Hot Metal

The carburization of molten iron is close to saturation in the blast furnace process, while that in the flash ironmaking process is uncertain because there is no pressure from solid charge and no carburization reactions occurring between the deadman and hot metal. Some experiments were conducted to reveal the kinetic mechanism of coke dissolving in carbon-iron melts. Reduced iron powder, electrolytic iron as well as chemical pure graphite were used as experiment materials. With high-purity argon injected as the protective gas, the specimens were heated up to 1873 K in a tubular resistance furnace to study the isothermal mechanism. The results show that the composition of the ferrous sample affects the dissolution rate. When the FeO content of the iron-bearing material rises from 0% to 4.76%, the apparent dissolution rate constant, kt, falls from 7.98 × 10−6 m/s to 5.48 × 10−6 m/s. There are some differences amongst the dissolution rate coefficients of different cokes despite interacting with similar carbon-iron melts, with coke 1 of 7.98 × 10−6 m/s, coke 2 of 5.17 × 10−6 m/s, and coke 3 of 3.77 × 10−6 m/s. Besides, this index decreases with the increase of the dissolution time and solely depends on the procedure of the mass transfer. A negative correlation is demonstrated between kt and the sulfur content in the iron bath as well. The content of silicon dioxide in the coke has a significant influence on kt. Additionally, the dissolution rate coefficient increases with the increase of the graphitization degree of coke.

Kinetics and Mechanisms of Dolime Dissolution in Steelmaking Slag

The kinetics of dolime dissolution containing 20 and 50 pct of magnesium oxide in steelmaking slags has been studied in a high-temperature experiment. The dissolution tests were carried out under static conditions at temperatures of between 1573 K and 1873 K (1300 °C to 1600 °C). Cylindrical samples of dolime were introduced into pre-melted slag at predetermined time intervals. The changes in mass and geometrical parameters of the samples, as well as CaO and MgO concentrations in the melt, were determined to calculate mass transfer and diffusion coefficients. The results revealed that the rate-limiting step in the process of dolime dissolution is diffusion. The increase in MgO content in the initial sample caused a drop in dolime dissolution rate and diffusion coefficients. The prediction of MgO solubility in steel slag, as well as the effect of slag basicity, was carried out using the thermodynamic software FactSage 7.1. In addition, the activation energy of diffusion has been calculated using the dependence of the logarithm of diffusion coefficients on the inverse temperature.

Surface-controlled dissolution rates: a case study of nanoceria in carboxylic acid solutions.

Nanoparticle dissolution in local milieu can affect their ecotoxicity and therapeutic applications. For example, carboxylic acid release from plant roots can solubilize nanoceria in the rhizosphere, affecting cerium uptake in plants. Nanoparticle dispersions were dialyzed against ten carboxylic acid solutions for up to 30 weeks; the membrane passed cerium-ligand complexes but not nanoceria. Dispersion and solution samples were analyzed for cerium by inductively coupled plasma mass spectrometry (ICP-MS). Particle size and shape distributions were measured by transmission electron microscopy (TEM). Nanoceria dissolved in all carboxylic acid solutions, leading to cascades of progressively smaller nanoparticles and producing soluble products. The dissolution rate was proportional to nanoparticle surface area. Values of the apparent dissolution rate coefficients varied with the ligand. Both nanoceria size and shape distributions were altered by the dissolution process. Density functional theory (DFT) estimates for some possible Ce(IV) products showed that their dissolution was thermodynamically favored. However, dissolution rate coefficients did not generally correlate with energy of formation values. The surface-controlled dissolution model provides a quantitative measure for nanoparticle dissolution rates: further studies of dissolution cascades should lead to improved understanding of mechanisms and processes at nanoparticle surfaces.

Dissolutive flow in nanochannels: transition between plug-like and Poiseuille-like

Dynamic properties of dissolutive flow in nanochannels were investigated by molecular dynamics simulations. It turned out that the liquid flow pattern changes greatly after the dissolution effect taken into consideration. Liquid inside the channel has a plug-like velocity profile when the dissolubility is low, whereas a Poiseuille-like flow was observed as the dissolubility increases. By introducing the dissolution term to molecular kinetic theory, we explained the physical mechanisms of velocity transition. During which a modified dimensionless Galilei number was proposed to describe the effect of main forces. The results showed that in pressure-driven flow, when the dissolubility is low, the dominant dissipation is the viscous dissipation and the theoretical model of insolubility is acceptable. However, as the dissolubility increases, the dissolving dissipation takes priority, which results in the velocity profiles becoming Poiseuille-like. In addition, we analyzed the evolution of fluid density, number of dissolved solid particles and concentration distribution of solute. The liquid density varying from layered oscillation to uniform distribution was obtained, which can be described by a critical number. The analysis of solute concentration helps to establish the scaling relation among the dissolution rate, convection velocity, and diffusion coefficient. These findings not only help to understand the physical mechanisms of dissolutive flow but also help to control and optimize the flow patterns in dissoluble channels.

An experimental study on acid-rock reaction kinetics using dolomite in carbonate acidizing

This study presents experimental results on the reaction kinetics of dolomite with fresh and spent acid in carbonate acidizing. Spent acid is defined as a partially reacted acid involving reaction products, such as calcium and magnesium ions, during an acid-rock reaction. A newly designed apparatus was developed to quantify the effect of spent acid on the acid-rock reaction kinetics at reservoir conditions. A total of 4 runs with 36 experiments at same conditions using the apparatus were repeatedly carried out to investigate the reaction kinetics of dolomite rock under the various concentrations of fresh acid and spent acid. The reaction results showed the dolomite disk was more dissolved in the spent acid than the fresh acid. Moreover, it was observed that the kinematic viscosity was higher, and the pH was lower in the spent acid, when compared with the fresh acid. From the results, it was revealed that the dissolution rate and diffusion coefficient for spent acid were higher than those of fresh acid due to the higher kinematic viscosity and the lower pH of spent acid. This means that the chemical reaction becomes more vigorous due to the long contact time and high acidity of the spent acid at the surface of the dolomite disk. These results suggest that the uncommon ion effect by the impurities, such as ferric oxide and aluminum oxide in clay, which is not related to the reaction of HCl and dolomite, can promote the reactions. A conventional approach using fresh acid could result in significant errors in determining the injection parameters, such as injection rate, volume, and pumping schedule in a carbonate acidizing job, because of the underestimated the dissolution rate and diffusion coefficient. Therefore, the effect of spent acid on acid-rock reaction kinetics should be essentially considered to design the injection conditions in carbonate acidizing.

Defining level A IVIVC dissolution specifications based on individual in vitro dissolution profiles of a controlled release formulation

Regulatory guidelines recommend that, when a level A IVIVC is established, dissolution specification should be established using averaged data and the maximum difference between AUC and Cmax between the reference and test formulations cannot be greater than 20%. However, averaging data assumes a loss of information and may reflect a bias in the results. The objective of the current work is to present a new approach to establish dissolution specifications using a new methodology (individual approach) instead of average data (classical approach). Different scenarios were established based on the relationship between in vitro-in vivo dissolution rate coefficient using a level A IVIVC of a controlled release formulation. Then, in order to compare this new approach with the classical one, six additional batches were simulated. For each batch, 1000 simulations of a dissolution assay were run. Cmax ratios between the reference formulation and each batch were calculated showing that the individual approach was more sensitive and able to detect differences between the reference and the batch formulation compared to the classical approach. Additionally, the new methodology displays wider dissolution specification limits than the classical approach, ensuring that any tablet from the new batch would generate in vivo profiles which its AUC or Cmax ratio will be out of the 0.8–1.25 range, taking into account the in vitro and in vivo variability of the new batches developed.

Kinetics and dissolution of intratracheally administered nickel oxide nanomaterials in rats

BackgroundThe toxicokinetics of nanomaterials are an important factor in toxicity, which may be affected by slow clearance and/or distribution in the body.MethodsFour types of nickel oxide (NiO) nanoparticles were single-administered intratracheally to male F344 rats at three doses of 0.67–6.0 mg/kg body weight. The rats were sacrificed under anesthesia and the lung, thoracic lymph nodes, bronchoalveolar lavage fluid, liver, and other organs were sampled for Ni burden measurement 3, 28, and 91 days post-administration; Ni excretion was measured 6 and 24 h after administration. Solubility of NiO nanoparticles was determined using artificial lysosomal fluid, artificial interstitial fluid, hydrogen peroxide solution, pure water, and saline. In addition, macrophage migration to trachea and phagosome-lysosome-fusion rate constants were estimated using pulmonary clearance and dissolution rate constants.ResultsThe wire-like NiO nanoparticles were 100% dissolved by 24 h when mixed with artificial lysosomal fluid (dissolution rate coefficient: 0.18/h); spherical NiO nanoparticles were 12% and 35% dissolved after 216 h when mixed with artificial lysosomal fluid (1.4 × 10−3 and 4.9 × 10−3/h). The largest irregular-shaped NiO nanoparticles hardly dissolved in any solution, including artificial lysosomal fluid (7.8 × 10−5/h). Pulmonary clearance rate constants, estimated using a one-compartment model, were much higher for the NiO nanoparticles with a wire-shape (0.069–0.078/day) than for the spherical and irregular-shaped NiO nanoparticles (0–0.012/day). Pulmonary clearance rate constants of the largest irregular-shaped NiO nanoparticles showed an inverse correlation with dose. Translocation of NiO from the lungs to the thoracic lymph nodes increased in a time- and dose-dependent manner for three spherical and irregular-shaped NiO nanoparticles, but not for the wire-like NiO nanoparticles. Thirty-five percent of the wire-like NiO nanoparticles were excreted in the first 24 h after administration; excretion was 0.33–3.6% in that time frame for the spherical and irregular-shaped NiO nanoparticles.ConclusionThese findings suggest that nanomaterial solubility differences can result in variations in their pulmonary clearance. Nanoparticles with moderate lysosomal solubility may induce persistent pulmonary inflammation.

Insights into the dissolution mechanisms of detergent agglomerates: An approach to assess dissolution heterogeneity

In this work, the dissolution mechanisms of detergent agglomerates with different binders were investigated in aqueous solution. The dissolution processes of detergent agglomerates were online monitored by using in situ UV–VIS spectrophotometer and electric conductivity probe. Dissolution profiles were correlated by Weibull model to evaluate the time-dependent dissolution rate coefficient and to classify the type of dissolution rate function kt(t). The Kullback-Leibler information distance dK-L was proposed to assess the degree of dissolution heterogeneity. The results indicate that the sodium linear alkylbenzene sulfonates (NaLAS) and sodium carbonates (Na2CO3) in detergent agglomerates have different dissolution behaviors, and their dissolution rates are influenced by the type and content of binders. Moreover, detergent agglomerates using semi-solid NaLAS paste or liquid linear alkylbenzene sulfonic acid (HLAS) as binders in granulation processes follow different dissolution mechanisms in water.

SL2B aptamer and folic acid dual-targeted DNA nanostructures for synergic biological and chemotherapy to combat colorectal cancer

The influence of chemical structure on NAPL mixture nonideality evolution, rate-limited dissolution, and contaminant mass flux was examined. The variability of measured and UNIFAC modeled NAPL activity coefficients as a function of mole fraction was compared for two NAPL mixtures containing structurally-different contaminants of concern including toluene (TOL) or trichloroethene (TCE) within a hexadecane (HEXDEC) matrix. The results showed that dissolution from the NAPL mixtures transitioned from ideality for mole fractions > 0.05 to nonideality as mole fractions decreased. In particular, the TCE generally exhibited more ideal dissolution behavior except at lower mole fractions, and may indicate greater structural/polarity similarity between the two compounds. Raoult's Law and UNIFAC generally under-predicted the batch experiment results for TOL:HEXDEC mixtures especially for mole fractions ≤ 0.05. The dissolution rate coefficients were similar for both TOL and TCE over all mole fractions tested. Mass flux reduction (MFR) analysis showed that more efficient removal behavior occurred for TOL and TCE with larger mole fractions compared to the lower initial mole fraction mixtures (i.e. < 0.2). However, compared to TOL, TCE generally exhibited more efficient removal behavior over all mole fractions tested and may have been the result of structural and molecular property differences between the compounds. Activity coefficient variability as a function of mole fraction was quantified through regression analysis and incorporated into dissolution modeling analyses for the dynamic flushing experiments. TOL elution concentrations were modeled (predicted) reasonable well using ideal and equilibrium assumptions, but the TCE elution concentrations could not be predicted using the ideal model. Rather, the dissolution modeling demonstrated that TCE elution was better described by the nonideal model whereby NAPL-phase activity coefficient varied as a function of COC mole fraction. For dynamic column flushing experiments, dissolution rate kinetics can vary significantly with changes in NAPL volume and surface area. However, under conditions whereby NAPL volume and area are not significantly altered during dissolution, mixture nonideality effects may have a greater relative control on dissolution (elution) and MFR behavior compared to kinetic rate limitations.

Chemical structure influence on NAPL mixture nonideality evolution, rate-limited dissolution, and contaminant mass flux

The influence of chemical structure on NAPL mixture nonideality evolution, rate-limited dissolution, and contaminant mass flux was examined. The variability of measured and UNIFAC modeled NAPL activity coefficients as a function of mole fraction was compared for two NAPL mixtures containing structurally-different contaminants of concern including toluene (TOL) or trichloroethene (TCE) within a hexadecane (HEXDEC) matrix. The results showed that dissolution from the NAPL mixtures transitioned from ideality for mole fractions >0.05 to nonideality as mole fractions decreased. In particular, the TCE generally exhibited more ideal dissolution behavior except at lower mole fractions, and may indicate greater structural/polarity similarity between the two compounds. Raoult's Law and UNIFAC generally under-predicted the batch experiment results for TOL:HEXDEC mixtures especially for mole fractions ≤0.05. The dissolution rate coefficients were similar for both TOL and TCE over all mole fractions tested. Mass flux reduction (MFR) analysis showed that more efficient removal behavior occurred for TOL and TCE with larger mole fractions compared to the lower initial mole fraction mixtures (i.e. <0.2). However, compared to TOL, TCE generally exhibited more efficient removal behavior over all mole fractions tested and may have been the result of structural and molecular property differences between the compounds. Activity coefficient variability as a function of mole fraction was quantified through regression analysis and incorporated into dissolution modeling analyses for the dynamic flushing experiments. TOL elution concentrations were modeled (predicted) reasonable well using ideal and equilibrium assumptions, but the TCE elution concentrations could not be predicted using the ideal model. Rather, the dissolution modeling demonstrated that TCE elution was better described by the nonideal model whereby NAPL-phase activity coefficient varied as a function of COC mole fraction. For dynamic column flushing experiments, dissolution rate kinetics can vary significantly with changes in NAPL volume and surface area. However, under conditions whereby NAPL volume and area are not significantly altered during dissolution, mixture nonideality effects may have a greater relative control on dissolution (elution) and MFR behavior compared to kinetic rate limitations.

Numerical modelling of multicomponent LNAPL dissolution kinetics at residual saturation in a saturated subsurface system

Characterization of aquifers contaminated by petroleum hydrocarbons is limited by the use of dissolution mass transfer correlations developed for single compounds without considering the effects of the mass transfer limitations in presence of other components. A one-dimensional implicit numerical model is developed for the coupled mass transfer and transport processes and the results are analysed using existing mass transfer correlations for better understanding of the single and multicomponent dissolution processes. The mass transfer coefficient in the multicomponent system is found to be more nonlinear and extending with changing slope, unlike the exponential reduction for single compound system. During the initial phase, the dissolution rate of a soluble compound is very high due to the high concentration gradient, and as dissolution progresses, its effective solubility decreases with change in mole fraction. At higher pore volumes, the mole fractions of lower solubility fractions increase which can result in higher effective solubility. The kinetics of interphase mass transfer by dissolution and sorption is favoured by coupled biodegradation. However, mass transfer limitation for more soluble compounds under the conditions of low residual saturation and low mole fraction is observed to be due to low dissolution rate coefficient rather than decreasing concentration gradient.

Wet creep of hardened hydraulic cements — Example of gypsum plaster and implication for hydrated Portland cement

Gypsum plaster exhibits a dramatic creep when placed in a very humid environment. We have combined mechanical tests of wet bending creep of set plaster and holographic interferometry measurements of dissolution rate and diffusion coefficient to look for the origin of this wet creep. Both these experiments have been performed in the absence and presence of various known anti-creep admixtures. It appears that the creep rate and dissolution rate are strongly correlated. This correlation has allowed to propose surface-driven pressure solution creep as mechanism of wet creep of gypsum plaster, i.e., the sequence dissolution in the grain boundary water/diffusion/precipitation at the grain surface. An order of magnitude analysis shows that this dissolution–diffusion–recrystallization series can also contribute to the creep of hydrated Portland cement.

Effect of different water conditions on dissolution of nanosilver

This study evaluates the time-dependent dissolution of nanosilver (nAg) in common electrolytes and natural waters. nAg was synthesized via Tollens' method using sodium citrate as stabilizer; its morphology, UV-Vis spectrum, and particle size were characterized. The dissolved silver was monitored over time using filtration, centrifugation, and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Our results indicated that nanoparticle aggregation, Cl(-) presence, and natural organic compounds could affect the dissolution behavior of nAg. The dissolution of nAg was highly dependent on Cl(-) concentration. Excessive Cl(-) enhanced nanoparticle dissolution, whereas natural organic compound inhibited the dissolution. The dissolution data fitted well with the first-order kinetic model, and the dissolution rate coefficients were calculated using the first-order equation. This study showed the dissolution of nAg under various water conditions. The obtained results may be helpful in predicting nAg behavior in relevant environmental aquatic systems.

Numerical modelling and experimental investigation of drug release from layered silicone matrix systems

Medical devices and polymeric matrix systems that release drugs or other bioactive compounds are of interest for a variety of applications. The release of the drug can be dependent on a number of factors such as the solubility, diffusivity, dissolution rate and distribution of the solid drug in the matrix. Achieving the goal of an optimal release profile can be challenging when relying solely on traditional experimental work. Accurate modelling complementing experimentation is therefore desirable. Numerical modelling is increasingly becoming an integral part of research and development due to the significant advances in computer simulation technology.This work focuses on numerical modelling and investigation of multi-layered silicone matrix systems. A numerical model that can be used to model multi-layered systems was constructed and validated by comparison with experimental data. The model could account for the limited dissolution rate and effect of the drug distribution on the release profiles. Parametric study showed how different factors affect the characteristics of drug release. Multi-layered medical silicone matrices were prepared in special moulds, where the quantity of drug in each layer could be varied, and release was investigated with Franz-diffusion cell setup. Data for long-term release was fitted to the model and the full depletion of the system predicted. The numerical model constructed for this study, whose input parameters are the diffusion, effective dissolution rate and dimensional solubility coefficients, does not require any type of steady-state approximation. These results indicate that numerical model can be used as a design tool for development of controlled release systems such as drug-loaded medical devices.