Study of the retarding mechanism of linear sodium polyphosphates on α-calcium sulfate hemihydrate

Linear sodium polyphosphates possessing different average chain lengths were characterized and studied as retarder for α-calcium sulfate hemihydrate. Their effect on the hydration kinetics of aqueous α-CaSO₄·0.5H₂O pastes was quantified by heat flow calorimetry, mini slump test and determination of time-dependent degree of hydration. Already at very low dosages (0.025% bwob) polyphosphates strongly retard hydration of hemihydrate. The retarding effect decreases with increasing chain length. Interaction between polyphosphates and calcium ions and Ca^2 + concentration dependent formation of insoluble precipitates were studied via pH and turbidity measurements. Additionally, the influence of polyphosphates on individual hydration steps (binder dissolution and gypsum precipitation) was determined. It was found that the working mechanism of polyphosphates mainly relies on a strong decrease of binder dissolution rate while delayed gypsum precipitation from the solution and inhibition of dihydrate crystal growth by adsorption of polyphosphate play a minor role.     

Calcium oxalate crystallization on concrete heterogeneities

The precipitation of calcium oxalate monohydrate (COM) as a waterproofing material for concrete constructions is a promising approach. The kinetics of spontaneous precipitation of the calcium oxalate from supersaturated solutions in batch reactors was investigated. The precipitation took place in supersaturated solutions of equal concentrated solutions of sodium oxalate and calcium nitrate and was monitored when COM precipitated from the solutions as a function of time. The experiments were done both in unseeded and in solutions seeded with COM crystals and also with concrete grains (Portland cement). COM formed exclusively in all cases. From the analysis of the kinetics data, it was concluded that the prevalent mechanism for COM precipitation was surface diffusion. Satisfactory fit of the data to the polynuclear model yielded values for the surface energy ca. 30 mJ m⁻². The relatively low values obtained suggested mainly heterogeneous nucleation. The rates of COM precipitation increased with increasing amounts of inoculating COM seed crystals suggesting significant contribution of seeded growth in the overall process. Inoculation with concrete grains resulted in lower rates possibly due to the release of inhibiting species in the supersaturated solutions from the partially dissolving substrate.     

Study of surface effects in the formation of lead chloride on lead electrodes in aqueous HCl by electrochemical methods and scanning electron microscopy

The formation of lead chloride on lead electrodes in aqueous HCl solutions was investigated using scanning electron microscopy (S.E.M.). Micrographs revealed the presence of two distinct surface layers of PbCl₂ on the electrode during anodic polarization; a basal layer of relatively small crystals and an upper layer of larger crystals. Passivation was found to result from blockage of the electrode by the larger crystals. A mechanistic scheme for the nucleation and growth of PbCl₂ on the electrode surface, involving precipitation of dissolved Pb²⁺ complex species is proposed. This nucleation and growth process was found to be either “progressive” or “instantaneous” in nature depending upon the conditions under which the surface layer was formed.The reactivation phenomenon observed subsequent to passivation, under rotating disc electrode conditions, was found to involve a recrystallization process on the electrode surface. Steady-state currents in the reactivation region were found to be limited by the rate of diffusion of soluble Pb²⁺ species away from the electrode surface. This provided a method of determining the concentration of soluble Pb²⁺ species, c^s_Pb²⁺ in equilibrium with the electrode surface. A plot of c^s_Pb²⁺vs Cl^? concentration generated the solubility curve for PbCl₂ in HCl solutions.     

Crystal habit modification studies with ammonium and potassium dihydrogen phosphate

Face rates of growth of ammonium and potassium dihydrogen phosphate crystals have been measured for crystallisation from pure solutions and from solutions that contained known amounts of foreign ions. The level of supersaturation and the solution pH both effect the crystal habit. Low supersaturation at normal pH (3.8) tends to give tapered crystals, and at a slightly higher pH (?5), the linear growth rates of both principal axes are considerably increased. The presence of foreign ions such as Cr³⁺, Fe³⁺ and Al³⁺ in low concentrations (? 1 × 10^?3 g ion/1 or ? 50 ppm) also causes a tapering habit. A mechanism of habit modification based on the presence of aquo ions at the crystal face is suggested.     

Growth inhibition in calcium sulfate crystal using a copolymer in oil fields: theoretical study and experimental evaluations

A poly(itaconic acid-co-sodium vinylsulphonate) (PIASVS) was theoretically studied and experimentally evaluated as an inhibitory agent against the growth of calcium sulfate (CaSO₄) crystals, in both non-saline and saline solutions. Density functional theory revealed that the CaSO₄ crystal precipitation could be precluded through the effective pairing of Ca²⁺ and SO₄^2? ions by carboxylic group polymer heads and that, moreover, the Na⁺ cations of the sulphonate polymer heads could be easily replaced by Ca²⁺. With PIASVS concentration of 50 ppm, lower than what is required in oil recovery processes, the polymer inhibited 33% in CaSO₄ crystals growth in non-saline solution, but the salt increased the inhibitory performance of PIASVS up to 54%. Thermogravimetric analysis, scanning electron microscopy and X-ray diffractometry techniques showed that PIASVS changed the CaSO₄ crystal morphology from a bassanite phase in non-saline solution to a bassanite/gypsum mix. The crystal morphology observations along with the conductivity measurements confirmed the pairing of ions from dissolved CaSO₄ by NaCl and PIASVS. Dynamic light scattering revealed that, the PIASVS cluster size increased in non-saline solution but decreased in saline solutions, suggesting that NaCl increases the PIASVS solubility in aqueous solution. The performance of PIASVS as anti-scaling agent was found to be suitable for the conditions found in the Mexican oil reservoirs.     

An RNA‐Cleaving Catalytic DNA Accelerated by Freezing

The EtNa DNAzyme was isolated during the isopropanol precipitation step of an in vitro selection effort. Although inactive with the intended cofactor, its RNA cleavage activity was observed under a few conditions. With Na⁺, EtNa was highly active in ～50 % ethanol, whereas in water, it was highly active with Ca²⁺. In this work, we showed that the EtNa DNAzyme was accelerated by freezing in water in the presence of Na⁺. The apparent K_d value reached 6.2 mm Na⁺ under the frozen condition, over 20 times tighter than that in water at room temperature. With 10 mm Na⁺, EtNa had a cleavage rate of 0.12 h^?1 after freezing at ?20 °C. This effect was unique to EtNa, as all other tested DNAzymes were inhibited by freezing except for the Na⁺‐specific NaA43. Freezing also inhibited EtNa if Ca²⁺ was used. We attributed this to the concentrations of EtNa and Na⁺ in the micropockets between ice crystals, but divalent metals might misfold DNA. Overall, we have systematically studied the effect of freezing on the RNA‐cleavage activity of DNAzymes. The DNAzyme sequence and the metal ion species are both crucial to determine the effect of freezing.     

Assessment of commercial hydrate inhibitors using the 3‐in‐1 method

Phase equilibria, kinetics, and morphology studies of gas hydrates require separate pieces of equipment and experimentation times in the order of days. Recently, we designed a reactor that allows for tight control of the crystallization temperature. Coupled with a novel method, this reactor can screen the crystal morphology, phase equilibria, and apparent kinetics of gas hydrates. Compared to traditional multi‐trial methods, the main advantage of this method is that only a single experiment, completed in the order of hours, is required to assess: (a) the change in hydrate growth velocity with respect to temperature, (b) the HLV equilibrium temperature at the experimental pressure, and (c) the change in crystal morphology with respect to driving force. Using this 3‐in‐1 method, methane hydrate growth and dissociation was studied in the presence of four commercial inhibitors. Phase equilibria, kinetics, and morphology were obtained for all hydrate systems with inhibitors. The standard uncertainty for the HLV equilibrium temperature was 0.05 K and for pressure 0.005 MPa. The apparent rates of growth were measured for all systems (standard uncertainty was 0.008 mm · s^?1) and the difference between the inhibited systems and the pure system was very clear. Crystal habits varied considerably among inhibitors and radically with respect to the uninhibited system. Overall, we present an innovative technology to assess the morphology, kinetics, and thermodynamics of hydrate forming systems with a single apparatus. Furthermore, with little time investment, small sample sizes can be used to obtain replicates with minimum temperature and pressure uncertainties.     

Mechanical denaturation of high polymers in solutions. XXXIX. Flow-induced crystallization of high molecular weight syndiotactic-rich poly(vinyl alcohol)

The crystallization of syndiotactic-rich poly(vinyl alcohol) (s-PVA) with a molecular weight of 6.73 × 10⁵ from aqueous solutions was carried out under Couette flow with or without seed crystals. The crystallization of s-PVA from aqueous solutions occurred below the surface of the solutions even in the presence of seed placed in the solution; whereas, in the case of aqueous s-PVA solutions containing 0.02% octyl alcohol (antifoaming reagent) only the longitudinal growth of seed occurred. The lengths of the fibers obtained were 2.5 to 3.0 cm.     

Effect of magnetic field on deposition and adhesion of calcium carbonate particles on different substrates

Magnetic field effects on CaCO₃ adhesion to substrates were evaluated statistically as for the number of crystals, the occupied surface area per crystal and the crystal size distributions. CaCO₃ was precipitated from Na2CO₃ and CaCl₂ solutions, which prior to precipitation were or were not exposed to a magnetic field under flowing conditions. Glass, copper and stainless steel plates were used as substrates. The precipitation process occurred at 30°C, and in the case of glass plate experiments were also conducted at 60°C. The CaCO₃ deposits were photographed using an optical polarizing microscope equipped with a camera and then the images were analyzed statistically. For each system studied 14 plates were used. It was found that in all systems the magnetic field (MF) influenced the number of crystals deposited and their size distribution. The changes were statistically significant (t-Student's test). The nature of the substrate surface was found to have a significant effect on the amount of CaCO3 deposited. However, the average amount of calcium carbonate deposited on the glass surface at 30°C determined from 14 analyzed plates was only slightly less than that from MF-treated solutions (0.689 and 0.748 mg/cm², respectively), and it was practically the same at 60°C (0.539 and 0.534 mg/cm²). At this higher temperature the deposition of calcium carbonate was reduced relative to that at 30°C. From the results obtained it may be concluded that the MF affects both nucleation and crystal growth of CaCO₃.     

The role of hydrothermal pathways in the evolution of the morphology of ZnO crystals

Understanding growth mechanisms usually leads to the successful preparation of a targeted microstructure. However, the large number of parameters that influence the shape and the size of nanostructures often make it difficult to predict the outcome. We investigated the growth of wurtzite-type ZnO by closely following such a multistage process. We associated the diverse morphology of the precipitated crystals, prepared under the same hydrothermal conditions, to the differences in the transient crystallization processes during the precipitation. By altering the pH of the suspension after the precipitation, we gain control over the resulting morphology of the ZnO and show how to grow identical crystallites with different shapes and crystal sizes. Here we report, for the first time, on how ZnO platelets grow in a certain Zn²⁺/OH⁻ ratio along the basal and one of the prismatic directions to form sphalerites with very high surface area. We also offer an explanation as to how sphere-like hierarchical structures composed of plate- or rod-like ZnO crystals form through self-assembly driven processes.     

The Influence of Shear Stress on Crystallization in an Ultrasound Levitator

Industrial precipitation processes often use chemical agents to influence crystal morphology and size distribution. This experimental study deals with the investigation of physical parameters including an alternative method to affect crystal growth, thus, avoiding the presence of additives as intrinsic impurities. The influence of shear stress acting on growing crystals within a droplet is investigated in an ultrasound levitator. An ultrasound levitator enables the suspension of a single droplet against gravity and the study of containerless precipitation with specific mechanical forces acting on crystals. The levitator is used as a three‐phase reactor with precipitation from the gas and liquid, and as a reactor for precipitation from two different solutions. Calcium carbonate is used as a model system. The variation of temperature and the amount of applied shear stress leads to different amounts of calcium carbonate morphologies. An increase in the shear stress results in more rounded or spherical crystals. The intensity of the shear stress also influences the particle size distributions of the precipitated crystals, i.e., with increasing shear stress, particle size distributions are shifted to smaller sizes.     

Crystallisation of pentaerythritol ii. growth inhibiting impurities

The growth rate of pentaerythritol crystals from carefully purified aqueous solution has been measured in a fluidised bed. Very high growth rates were obtained of nearly the same order of magnitude as the estimated diffusion control limit of 10^?6 m/s at 30°C and 0.025 mass fraction supersaturation. The growth rate was also measured in cells containing static purified solution to which specific impurities had been added. The only impurity which had significant effect in low concentrations was formaldehyde and it was found that 10 parts/10⁶ reduced the growth rate by over two orders of magnitude. It was found that all the samples of commercial pentaerythritol analysed contained ≤20 parts/10⁶ which on solution was more than enough to cause very serious growth inhibition. Chemical scavengering of this trace of formaldehyde is essential if reasonable growth rates are to be achieved in industrial crystallisers.     

The crystallization of magnesium hydroxide

The thermodynamic association constant for the formation of the ion-pair MgOH⁺ in solutions of magnesium hydroxide has been studied by potentiometric and conductometric methods.The growth of magnesium hydroxide seed crystals in supersaturated solutions over a range of concentrations of magnesium and hydroxide ions and in the presence of phosphonate additives has been investigated by a precision conductance method. After a rapid initial surge of growth, an equation first order with respect to supersaturation can be used to explain the experimental data. A surface diffusion process is proposed as the controlling step in the crystallization process.     

Electrochemical characterization of the protective film formed by the unsymmetrical Schiff's base on the mild steel surface in acid media

The corrosion inhibition efficiency of a newly synthesized Schiff's base for the corrosion of mild steel was studied in 1.0 M HCl and 0.5 M H₂SO₄ solutions. The results of weight loss measurements, electrochemical impedance and potentiodynamic polarization measurements consistently demonstrated that the Schiff's base synthesized is a good corrosion inhibitor with an inhibitory efficiency of approximately 92% at an optimum inhibitor concentration of 600 mg/L. The inhibition in both of the corrosive media was observed to be a mixed type. The potential of zero charge (PZC) at the metal–solution interface was determined for both the inhibited and uninhibited solutions to provide the mechanism of inhibition. The inhibitor formed a film on the metal surface through chloride or sulfate bridges depending upon the medium. The temperature dependence of the corrosion rate was also studied in the temperature range from 27 to 50 °C. The value of the activation energy (E_a) calculated showed that the inhibition film formation on the metal surface occurred through chemisorption. The thermodynamic parameters such as the adsorption equilibrium constant (K_ads) and the free energy of adsorption (ΔG_ads) were calculated and discussed. Several adsorption isotherms were tested and the experimental data fit well with the Langmuir adsorption isotherm.     

An improved method for evaluating detergent builders for water hardness control

Commercial detergent additives to control water hardness may act through sequestration, crystal growth inhibition, precipitation, or ion exchange. These builders lower the free hardness (Ca⁺⁺, Mg⁺⁺ concentration by different mechanisms. A full factorially designed experiment has been developed to evaluate builders functioning by the sequestration or crystal growth inhibition of calcium carbonate or magnesium hydroxide. The builder’s performance is determined by its ability to prevent precipitation while in the presence of carbonate and hardness ions. The tests are based on incubation followed by filtration and determination of calcium and magnesium in the filtrate by Inductively Coupled Plasma (ICP). Variables in the design include builder concentration, temperature, pH and time. Regression equations and response surfaces for tripolyphosphate and several polyacrylates and phosphonates are included.     

Corrosion and scale processes and their inhibition in simulated cooling water systems by monosaccharides derivatives: Part I: EIS study

A study of the ordinary mild steel/cooling water interface, with and without inhibitor, was carried out using electrochemical impedance spectroscopy (EIS). EIS spectra in uninhibited medium reveal that a layer of corrosion and scale products is formed naturally and evolves with the immersion time. Monosaccharides derivatives were used as corrosion inhibitors. Most of them exhibit good behaviour against corrosion of ordinary steel in simulated water systems. Their inhibiting efficiency increases with concentration rise and depends on molecular structure especially -OH groups' position on two adjacent carbons and their number. In addition, the effect of MoO₄²⁻ on the corrosion inhibition in the presence of the best inhibitor was also studied by EIS. The addition of MoO₄²⁻ enhanced the inhibition efficiency.     

Effect of PgTPhPBr on the electrochemical and corrosion behaviour of 304 stainless steel in H2SO4 solution

Weight-loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were used to study the inhibition of 304 stainless steel corrosion in 1 M H₂SO₄ at 50 °C by propargyltriphenylphosphonium bromide (PgTPhPBr). The inhibiting effects of propyltriphenylphosphonium bromide (PrTPhPBr) and propargyl alcohol (PA) were also studied for the sake of comparison. For the investigated compounds, Tafel extrapolation in the cathodic region gave a corrosion inhibition efficiency of 98% at 1 × 10^–3 M. Adsorption of both PgTPhPBr and PA was found to follow Frumkin's isotherm while adsorption of PrTPhPBr obeys that of Temkin. In the anodic domain, PgTPhPBr acted as a good passivator. The impedance spectra recorded at the corrosion potential (E _cor) revealed that the charge transfer process in the inhibited and uninhibited states controls corrosion of 304 stainless steel.     

A Chemical Precipitation Method to Synthesize Na3CrF6 Crystal with Unique Shapes

Na₃CrF₆ crystals with unique morphology were prepared by a chemical precipitation method. Through XRD and FESEM analysis, the shape characteristics of the Na₃CrF₆ crystals were studied. The results indicated that the terminal pH and concentration of Cr³⁺ and NaF encouraged the growth of spherical, truncated cuboid, concave cubic and cubic shape Na₃CrF₆ crystals. Molecular simulations revealed that shape formation was due to crystal bonding energy.     

Assessing the effect of zinc on the crystallization of calcium carbonate

Scaling, often due to the precipitation of calcium carbonate, is a current problem in economical and technical life. To study this phenomenon, some experimental procedures have been developed. They are based for example on electrochemical precipitation, use of nanofiltration and rapid controlled precipitation (RCP). This latter technique appears as an efficient one because it permits to study the early stage of crystallization. An other technique could be considered as a promising route among all of them because it permits to follow in real time and in situ, the electrocrystallization of calcium carbonate. Some morphometric characteristics of crystals like: size, diameter, shape and also other parameters as the rate of covering surface of the electrode could be quantified. Experiments were carried out with this procedure and RCP test in order to study the effect of zinc on the precipitation of calcium carbonate.

Experiments were carried out with the water of Hamma (locality situated near the city of Constantine) which has a high power of scaling. The process occurs in two steps in control water: spontaneous crystallization followed by growth of crystals. In presence of zinc (0.05, 0.1 and 10 mg/L), we observed a reduction of nucleation and a high inhibition of growth of crystal. At lower concentration, i.e. 10 μg/L, the crystallization still occurred but crystals were smaller than in control water. These results were in good agreement with those obtained previously.

Moreover, the analysis by SEM revealed a change of morphology in presence of zinc and a change of crystalline varieties.     

Preparation and properties of flame-retardant polyphosphate esters: Low-temperature solution polycondensation of 3,3,′5,5′-tetrabromobisphenol AF and aryl phosphorodichloridates

This article describes the syntheses of aromatic polyphosphates from the reaction of various aryl phosphorodichloridates with 3,3′,5,5′-tetrabromobisphenol AF (TBPAF) in a chlorinated hydrocarbon solvent under low-temperature conditions. The new polyphosphates obtained were characterized by infrared, ¹³C and ³¹P nuclear magnetic resonance spectra, elemental analysis, inherent viscosity, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, limiting oxygen index, contact angle, and molar mass measurement. All of the polyphosphates obtained had high yields, and the inherent viscosities were in the range 0.12 - 0.15 dL g⁻¹. All of the polymers start degrading between 210 and 267°C and had 14 - 26% residual mass at 700°C in nitrogen. Polymer E, having a methoxy group in the side chain phenyl ring, showed better thermal stability than the other polymers. The X-ray diffraction patterns revealed that all of the polyphosphates were amorphous. These polyphosphates had glass transition temperatures between 140 and 154°C. Polymers obtained from TBPAF had excellent flame retardency, as indicated by high limiting oxygen index values in the range of 63 - 68. The water contact angles (θ_w) of all of the polyphosphates were in the range of 74 - 87°. The contact angles of polymers A and B were larger than those of other polyphosphates that contain more oxygen (polymers C and E) or bromine atoms (polymer D). © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 59–65, 1997