Colloidal pH Research Articles

Degradation of polyvinylpyrrolidone-coated iron oxide nanoparticles through ozonation: Steric to electrosteric repulsion and electrostatic interactions

Surface degradation followed by aggregation of polyvinylpyrrolidone-coated iron oxide nanoparticles (PVP-IONPs) during ozonation make it possible to remove the toxic NPs from aqueous medium and reduce their environmental hazards. This study investigated the effect of concentration of NPs (50–150 mg/L), colloid pH (4−10), and ozonation time (10–30 min) on aggregates formation. The starting and ozonated NPs were characterized using DLS, XRD, VSM, UV-Vis, FTIR, XPS, SEM, and TEM. The optimal conditions (107.7 mg/L, pH 6.07, and 30 min ozonation) were determined by response surface method (RSM) to achieve maximum values of z-average (3.2 µm) and zeta potential (−6.64 mV). The steric repulsion changed to electrosteric and electrostatic interactions (van der Waals attraction) due to the degradation of the PVP coating, in which the clusters grew to form micrometer-sized aggregates. Irreversibility of NPs aggregation under optimal conditions of ozonation was confirmed by sonication of suspension; although the loose agglomerates formed at early periods were prone to breaking during sonication. A 77% reduction in COD of colloidal PVP-IONPs after ozonation under optimal conditions confirmed the significant degradation of PVP with ozone.

Direct and Water-Mediated Adsorption of Stabilizers on SERS-Active Colloidal Bimetallic Plasmonic Nanomaterials: Insight into Citrate-AuAg Interactions from DFT Calculations.

In previous studies, AuAg colloidal nanostar formulations were developed with the two-fold aim of producing optimized surface-enhanced Raman spectroscopy (SERS) substrates and investigating the nature of the capping process itself. Findings demonstrated that the nanoparticle metals are alloyed and neutral, and capping by stabilizers occurs via chemisorption. This study utilizes citrate as the model stabilizer and investigates the mechanistic aspects of its interaction with mono- (Au20) and bimetallic (Au19Ag) surfaces by density functional theory (DFT) calculations. Citrate was modeled according to the colloid's pH and surrounded by a water and sodium first solvation shell. A population of stable cluster-citrate structures was obtained, and energies were refined at the uB3LYP//LANL2TZ(f)/cc-pVTZ level of theory. Solvation was accounted for both explicitly and implicitly by the application of the continuum model SMD. Results indicate that both direct binding and binding by water proxy through the charge-transfer complex formation are thermodynamically favorable. Water participation in citrate adsorption is supported by the adsorption behavior observed experimentally and the comparison between experimental and DFT-simulated IR spectra. Vibrational mode analysis suggests the possible presence of water within a crystal in dried nanostar residues. All ΔGads(aq) indicate a weak chemisorptive process, leading to the hypothesis that citrate could be displaced by analytes during SERS measurements.

Influence of temperature and particle concentration on the pH of complex nanocolloids

The pH of colloids is an important electrokinetic property which determines phase stability. We report the effect of temperature and nanoparticle concentration on pH of different nanocolloids of nanomaterials of varied morphologies and sizes. Measurements over a temperature range show that the pH of nanocolloids is a strong function of temperature and the concentration of the dispersed phase. Charge transport mechanisms leading to changes in the effective proton population are discussed. The mannerism in which the electric double layer (EDL) at the particle-fluid interface affects the pH of nanocolloids is presented by appealing to the DLVO theory of electrokinetics dispersion.

CHARACTERIZATION OF PHOTOCHEMICALLY INITIATED GOLD NANOPARTICLES SYNTHESIZED VIA INCOHERENT ULTRAVIOLET RADIATION

A modified Turkevich pathway of synthesizing water suspended gold nanoparticles using incoherent ultraviolet radiation as photochemical reaction initiator produced broader absorbance band corresponding to localised surface plasmonresonsnce around 530 nm with average particle size separated into two distinct distributions from 5 nm to 100 nm depending on ultraviolet wavelength. Presence and dynamics of nanoparticle growth was observed with photon correlation spectroscopy where aged colloids produced submicron sized agglomerated particles. Post-synthesis colloidal pH shows gradual degradation of particle stability.