AgI Particles Research Articles

Reversibility of Structural Rearrangements in Bovine Serum Albumin during Homomolecular Exchange from AgI Particles

The reversibility of the homomolecular exchange of bovine serum albumin (BSA) from AgI particles was studied by differential scanning calorimetry, the binding of 8-anilino-1-naphthaIene-sulfonic acid, and circular dichroism spectroscopy. The structure of BSA in solution before adsorption, in the adsorbed state, and in solution after exchange was analyzed. To investigate the influence of electrostatics and surface crowding effects, the experiments were performed at pH 4 and 7, two conditions of charge density at the AgI surface, and three degrees of surface coverage. BSA adsorbed on hydrophobic AgI particles adopts a perturbed state. After release from the surface, the protein aggregates through surface-induced hydrophobic patches as indicated by a decrease in hydrophobicity together with an increase in beta -sheet content. This surface-induced aggregation is more pronounced at low pH and when the ratio of adsorbed/dissolved BSA increases but is independent of the AgI surface charge: A lower BSA concentration allows more time for the adsorbed molecules to relax at the sorbent surface.: At pH 4, the native BSA is less stable than at pH 7. Both these conditions promote structural changes upon adsorption, which results in released BSA molecules having a different conformation than that of the protein before adsorption.

Optical Properties of Gold−Silver Iodide Nanoparticle Pair Structures

Nanoparticle pair structures composed of metal (Au) and semiconductor (AgI) segments with radii ca. 16 nm have been prepared in porous anodic aluminum oxide via electrochemical template synthesis. The metal and semiconductor members can be prepared such that they are in contact or separated in the porous host. UV/visible spectra of composites in which the Au and AgI segments are in contact show a red-shifted and broadened Au particle plasmon resonance band and the absence of the AgI exciton peak normally observed at λ = 425 nm. A blue shift in the Au particle plasmon band and the reappearance of the AgI exciton peak occur as the average interparticle spacing is increased. The changes in plasmon resonance peak position are consistent with an increased probability of Au and AgI intermixing as the spacing parameter d12 is decreased. The absence of the AgI exciton peak in the case of contacting Au and AgI particles is consistent with a large increase in the effective static dielectric constant upon phase inte...

Thiol-Derivatized AgI Nanoparticles: Synthesis, Characterization, and Optical Properties

Thiol-passivated AgI nanoparticles of 7−15 nm were synthesized in large quantities by changing the initial molar ratio of iodide to 1-thioglycerol, and thus their competitive reaction rates with silver ions was adjusted, leading to a precise control of the final particle size. Powder X-ray diffraction and transmission electron microscopy showed that these particles crystallized as a mixture of β- and γ-AgI, with the content of the γ-phase increasing from 26% for 7 nm to 60% for 15 nm. Elemental analysis, thermogravimetric analysis, and Fourier transform infrared spectroscopy indicated that thiolate anions are attached to the AgI surface through mercapto groups, and furthermore, the silver thiolate molecular complex exists as a result of the competitive reactions. The percentage of the initial thiolate used for capping AgI particles increased linearly with the increase in [I-]/[1-thioglycerol], showing that almost pure thiol-covered AgI particles of 15 nm can be obtained. Optical spectroscopic studies in d...

Isolating and identifying atmospheric ice-nucleating aerosols: a new technique

Laboratory studies examined two key aspects of the performance of a continuous-flow diffusion chamber (CFD) instrument that detects ice nuclei (IN) concentrations in air samples: separating IN from non-IN, and collecting IN aerosols to determine chemical composition. In the first study, submicron AgI IN particles were mixed in a sample stream with submicron non-IN salt particles, and the sample stream was processed in the CFD at −19°C and 23% supersaturation with respect to ice. Examination of the residual particles from crystals nucleated in the CFD confirmed that only AgI particles served as IN in the mixed stream. The second study applied this technique to separate and analyze IN and non-IN particles in a natural air sample. Energy-dispersive X-ray analyses (EDS) of the elemental composition of selected particles from the IN and non-IN fractions in ambient air showed chemical differences: Si and Ca were present in both, but S, Fe and K were also detected in the non-IN fraction.

Ionic conductivity of small-grain polycrystals of silver iodide

Small particles of AgI with various average size (140–680 nm) have been prepared by a gas-evaporation method. The powder X-ray diffraction patterns show that they are mixture of γ- and β-phases, and the mass fraction of the latter is roughly estimated at 20–25%. The room temperature conductivity of a polycrystalline pellet of small AgI particles shows enhancement by decreasing particle size, and the conductivity of 1.4×10 −4 Ω −1 cm −1 has been observed in 140 nm-grain AgI polycrystal. The overall activation energy for conduction varies from 0.38 to 0.28 eV, depending on the average size of the particles.

Direct observation of chemical reactions in silica‐coated gold and silver nanoparticles

The direct observation of chemical reactions of single particles in real time has been achieved using silver nano‐particles covered in a 10 nm silica shell (see cover). Transmission electron micrographs reveal how molecular iodine diffuses through the shell and reacts with Ag, giving AgI. A single AgI particle (mid‐gray) is shown to form on the surface of the silica while the Ag core shrinks. magnified image

Mechanism of formation of silver halide ultrafine particles in reverse micellar systems.

The mechanism of formation of silver halide ultrafine particles in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane reverse micellar systems has been studied. The particle formation process was followed by a change in the UV-visible absorption spectra. The effects of the concentrations of reactants and water content of reverse micellar solution on the size of formed particles were investigated. The size of AgCl particles decreased with increasing water content or reactant concentration, whereas the size of AgBr and AgI particles increased. A possible interpretation is the dissolution of AgCl, which is caused by some impurities in the AOT. Particle coagulation processes of AgBr and AgI particles are found to be dominated by the concentration of micellar droplets containing two or more particles.

Time-Resolved Optical Study on the Charge Carrier Dynamics in a TiO2/AgI Sandwich Colloid

Preparation of a sandwich colloid involving TiO{sub 2} and AgI (hexagonal) particles in alkaline aqueous solution is described. Time-resolved optical absorption experiments are reported that involve pulsed laser excitation at 355 nm (TiO{sub 2} bandgap) and at 410 nm (AgI bandgap). Charge carrier dynamics was probed directly by transient absorption measurements in the near-infrared and by reaction with methyl viologen (MV{sup 2+}). Reduction of MV{sup 2+} was observed to occur over a period of 100-200 {mu}s. MV{sup +} so produced reduced AgI to silver. By contrast, MV{sup 2+} reduction upon bandgap excitation of neat alkaline TiO{sub 2} colloid was complete within 1 {mu}s. These observations signal a prolonged electron-hole separation in the case of the AgI/TiO{sub 2} sol and indicate the formation of sandwich particles. 33 refs., 5 figs.

Increasing of the solubility of the AgI nuclei in rain drops induced by the complex formation with dominant ions

Rainwater has a complex chemical composition. Investigations showed that such cations as Ca 2+, Mg 2+, K +, NH 4 + and anions SO 4 2-, NO 2 −, Cl −, and HCO 3 − are constantly present in rainwater. Among them Cl − and NH 4 + form the stable complex compounds with Ag + -ions and thus can increase the solubility of AgI. The I −-ions having the property of complex formation to Ag + are introduced into the cloud media by crystallizing agents at seeding. The simple calculations of AgI solubility, taking into account the property of Ag + to form the complex compound with Cl −, I − and NH 4 + at same concentrations as in rainwater, showed that the solubility increase did not exceed 50% and in average was of 13%. Particularly, the solubility in one third of the samples increased from 2 to 5%, in the other two third of the samples it increased from 5 to 25%. And only in two cases the solubility increased by 36% and 46%, respectively, was observed. The increase of solubility was due to the presence of NH 4 +-ions. Just this ion was one of the main impurities of the rainwater in the studied region. The contribution of halogen-ions was very negligible in spite of the high values of the stability constants of their complexes. The more the solubility the less the lifetime of AgI particles in raindrops. According to our calculations the particles with diameters of 0.03–0.1 μm which can serve as the ice-forming nuclei may exist not less than 10 minutes in the rainwater containing the ligands.

Nucleation and stabilization of quantized AgI clusters in aqueous solution

Stable, quantized AgI clusters (Q-AgI) have been prepared in aqueous solution using poly(ethyleneimine) (PEI) to inhibit particle growth. The optical absorption spectrum shows an exciton band at 323 nm, the shape and intensity of which improve when the colloid contains excess iodide ion. The cluster can be generated by peptization of fresh AgI sols with PEI. Under illumination, colloidal silver metal is produced and I − is released from the lattice. PEI is photooxidized. The cluster formed is the smallest AgI particle which still displays an exciton absorption band. The nucleation is complete within 200 μs. Solid state titrations of Q-AgBr and Q-PbI 2 with I − and Ag + respectively, yield mixed Q-state colloids with excitonic bands lying between the bands of the pure colloids.

Study of interaction between small particles of AgI and HgI 2

The interactions involving clusters can be characterized as either surface chemisorption reactions or bulk reactions. Chemisorptive reactions involved the formation of surface adsorbate bonds, while bulk reactions are characterized by a more extensive breakage and rearrangement of bonds. The interaction between small particles of AgI and HgI 2 have been investigated using electron diffraction, X-ray diffraction and EDAX. The results of the study indicate the possibility of bulk reactions when clusters of two different materials are brought into contact. The clusters of the product of interaction may contain a stoichiometric compound and some unreacted interactants.

Critical phenomena in particles of mesoscopic size

Anomalous optical properties of small silver particles and some silver halides are discussed as displays of critical phenomena due to particle size decreasing or to their interaction with each other in an ensemble. Special consideration is given to weak localization of conduction electrons in a small metal particle, to plasmon delocalization in ensemble of metal particles, to the effect of zero- dimensional percolation in a small dielectric particle, and to the effect of size on the superionic transition in small AgI particles and thin envelopes.

Semiconductor photophysics. 5. Charge carrier trapping in ultrasmall silver iodide particles and kinetics of formation of silver atom clusters

Reactions of reducing species from acetonitrile media with silver iodide particles ({approximately}25-{angstrom} diameter) have been investigated by pulse radiolysis techniques. Injection of electrons into these ultrasmall particles leads to transient bleaching of the adsorption of AgI at wavelengths close to the onset of absorption ({approximately}400 nm) with the concomitant reduction of AgI to metallic silver. The reduction of Ag{sup +} ions and formation of silver atoms and/or dimeric Ag{sub 2} molecules on three different size AgI particles ({approximately}100, 35, and {approximately}25 {angstrom}) have also been examined by picosecond laser spectroscopy.

Observations of Silver Iodide Plumes over the Grand Mesa of Colorado

Abstract A series of wintertime airborne tracing experiments was examined to determine some characteristics of the plumes of silver iodide smoke released either from the ground or from an aircraft over the Grand Mesa of Colorado. The plumes were identified in nearly every experiment by detecting the airborne AgI particles and often also by observing resulting ice particle plumes in essentially the same airspace. The lateral and vertical plume positions of Wound-released AgI from eight sites were determined for several wind, cloudiness and stability conditions. The instantaneous ground-released plume had a median spreading angle of 15° and meandered within a median angle of 38°. The median plume height above the crest exceeded 500 m. The lateral spreading rates of aircraft-released AgI were estimated at over 2 m s−1 for cloudy conditions and less in clear conditions. The implications for future cloud seeding strategies are discussed.

Electron-transfer reactions of extremely small AgI colloids

Small colloidal AgI particles (particle diameter (20–50 Å) have been prepared in water and acetonitrile, and optical effects due to size quantization have been observed. Electron transfer reactions involving electron donors and electron acceptors with AgI have been studied by pulse radiolysis techniques. Both reduction and oxidation of the colloids led to transient bleaching of semiconductor absorption. The recovery of the bleaching has been attributed to corrosion processes. Electrons injected into AgI colloids produce metallic silver and hydrogen. Hydrogen evolution is catalyzed by metallic silver formation.

Ice‐Nucleation Activity of AgI Particles on the Surface of Water and into the Volume

AbstractThe ice‐nucleation activity of AgI particles is investigated. The critical supercoolings are determined for water drops with particles situated on the drop surface and suspended into the drop volume. The ice‐nucleation activity of the AgI particles decreases with the gradually penetration of the particles into the volume. A large alteration (about 10 °C) for the mean freezing temperature is established. An attempt is made to explain this phenomenon with the lowering of the nucleation work on the three‐phase particle/water/water vapour contact.

On the use of emulsions for studying salt nucleation and ice heterogeneous nucleation by agI particles

On the use of emulsions for studying salt nucleation and ice heterogeneous nucleation by agI particles

On the use of emulsions for studying salt nucleation and ice heterogeneous nucleation by AgI particles

The nucleation of salts of highly concentrated undercooled droplets of aqueous saline solutions has been studied by differential scanning calorimetry of their emulsions. Super-saturation has been found at ∼ 200°C for a solution whose concentration is approximately 1 mol of salt per 2 mol of water. In addition, water droplets seeded with AgI particles have also been studied by using emulsions. With these emulsions, the properties of a heterogeneous mechanism can be easily investigated. The influence of the cooling rate on the nucleation temperature of ice was determined, as well as the influence of precooling on the isothermal crystallization of ice. The results reported in this work are compared with those obtained on emulsions of pure water.

Ice Nucleation by Aerosol Particles. Theory of Condensation-Freezing Nucleation

Abstract A probabillstic theory of heterogeneous condensation-freezing nucleation is developed for monodisperse aerosol particles. An assumption is made that the physical process is heterogeneous condensation nucleation followed by heterogeneous freezing nucleation. The theory is expanded to a polydisperse system. Computed results based on the theory show a striking resemblance particularly in shape with experimental data. Using the theory and an empirical expression for deposition nucleation, the ice nucleation probability of pure AgI particles is reconstructed on the supersaturation-supercooling plane. Importance of proper environmental conditions free from transient supersaturation for condensation-freezing nuclei measurement and necessity of condensation-freezing nucleation modeling are pointed out.

Properties of Pyrotechnic Nucleants Used in Grossversuch IV

Abstract The characteristics of the pyrotechnic nucleant Silverspare, containing only 2% by weight of silver iodide and produced in the USSR are investigated and compared with other widely used materials. It was found that it had a high ice nucleation effectiveness of 2.1013g−1 of AgI at −10°C. The rate of nucleation is high so that ice crystal production practically ceases within two minutes after burn. Silver-spare aerosol has a pronounced bimodal size distribution with AgI particles in the small diameter mode at d = 0.025 μm and KCI particles constituting the mode at d = 0.13 μm. The large amount of hygroscopic material present, leads to a large fraction of active CCN in the smoke. It is shown that the contact nucleation mechanism is too slow to explain the high nucleation rate of J = 8.4 × 10−3 s−1 for Silverspare and that the fast-acting deposition (sorption) and/or condensation-freezing mechanisms are dominant. As expected, the effectiveness per gram of PbI2 of the lead iodides Alfaplom and S-50 is ...