Piotr Warszyński

ROLE OF ELECTROSTATIC INTERACTIONS IN PARTICLE ADSORPTION

Abstract The role of the electrostatic double-layer interactions in adsorption of colloid particles at solid/liquid interface was reviewed. The phenomenological formulation of the governing PB equation was presented with the expressions for the pressure tensor enabling one to calculate forces, torques and interaction energies between particles in electrolyte solutions. Then, the limiting analytical results for an isolated double-layer (both spherical and planar) were discussed in relation to the effective surface potential concept. The range of validity of the approximate expression connecting the surface potential and the effective surface potential with surface charge for various electrolytes was estimated. The results for double-layer systems were next presented including the case of two planar double-layers and two dissimilar spherical particles. Limiting solutions for short and long distances as well as for low potentials (linear HHF model) were discussed. The approximate models for calculating interactions of spheres, i.e., the extended Derjaguin summation method and the linear superposition approach (LSA) were also introduced. The results stemming from these models were compared with the exact numerical solution obtained in bispherical coordinate system. Possibilities of describing interactions of nonspherical particles (e.g., spheroids) in terms of the Derjaguin and the equivalent sphere methods were pointed out. In further part of the review the role of these electrostatic interactions in adsorption of colloid particles was discussed. Theoretical predictions were presented enabling a quantitative determination of both the initial adsorption flux for low surface coverages and the surface blocking effects for larger surface coverages. Possibility of bilayer adsorption for dilute electrolytes was mentioned. The theoretical results concerning both the adsorption kinetics and structure formation were then confronted with experimental evidences obtained in the well-defined systems, e.g., the impinging-jet cells and the packed-bed columns of monodisperse spherical particles. The experiments proved that the initial adsorption flux was considerably increased in dilute electrolytes whereas the monolayer coverages were considerably decreased due to lateral interactions among particles. It was then concluded that the good agreement between experimental and theoretical data confirmed the thesis of an essential role of the electrostatic interactions in adsorption phenomena of colloid particles.

Structure and ordering in localized adsorption of particles

Abstract Localized, sequential adsorption of colloid particles interacting via screened Coulomb potential was analyzed theoretically and experimentally. The two-dimensional (2D) pair correlation functions were simulated by using the Monte Carlo technique for various surface concentrations θ and for various screening length parameters κα, characterizing the “softness” of the particle-particle interaction potential. For κα ⪢ 1 the hard-disk limiting behavior was confirmed (maximum surface concentration θmx = 55%). The θmx for soft disks, which is generally much smaller than the above value, was also determined as a function of the κα parameter. A distinctive tendency toward a short-range ordering, analogous to the 3D results obtaining previously for colloid suspensions, was found for surface concentration close to the θmx value. Theoretical predictions were tested by applying the direct experimental method based on microscope observations of particle adsorption. A monodisperse polystyrene suspension was used (particle size 0.90 μm) and the adsorbing surface was made of mica sheets. Experimental results proved to be in good agreement with Monte Carlo simulations illustrating well the tendency towards structurization (2D quasi-liquid phase formation) for surface concentrations close to the predicted θmx values.

Photocatalytic Activity of Titanium Dioxide Modified by Silver Nanoparticles

Photocatalytic activity of Ag/TiO(2) composites obtained by photoreduction treatment (PRT) was investigated. The composite materials, containing various ratio of silver nanoparticles (0.6-3.7 wt %) were obtained by depositing silver on the Evonic-Degussa P25 titania surface. Selected samples whose color varied between light rose and purple brown were examined by SEM, TEM, XPS, DRS, and BET techniques. Flat band potential was determined using Roy method. TEM analysis showed spherically shaped silver nanoparticles of the diameter 4-12 nm. The XPS measurements revealed that silver particles were obtained mainly in metallic form. DRS spectra and photovoltage measurements showed that silver nanoparticles modified the P25 spectral properties but they changed neither the band gap nor the location of flat band potential. The photocatalytic activity of Ag/P25 composite was compared to the photocatalytic activity of pure P25 in the photooxidation reaction of an important potable water contaminant humic acid (HA) and two model compounds, oxalic acid (OxA) and formic acid (FA). The photodecomposition reaction was investigated in a batch reactor containing aqueous suspension of a photocatalyst illuminated by either UV or artificial sunlight (halogen lamp). The tests proved that a small amount of silver nanoparticles deposited on the titania surface triggers the increase in photocatalytic activity; this increase depends, however, on the decomposed substance.

Formation of biocompatible nanocapsules with emulsion core and pegylated shell by polyelectrolyte multilayer adsorption.

The aim of this work was to develop a novel method of preparation of loaded nanosize capsules based on liquid core encapsulation by biocompatible polyelectrolyte (PE) multilayer adsorption, with or without pegylated outermost layer. Using AOT (docusate sodium salt) as emulsifier, we obtained cores, stabilized by an AOT/PLL (poly-L-lysine hydrobromide) surface complex. These positively charged cores were encapsulated by layer-by-layer adsorption of polyelectrolytes, biocompatible polyanion PGA (poly-L-glutamic acid sodium salt), and biocompatible polycation PLL. We used the saturation method for formation of consecutive layers, and we determined the optimal conditions concerning concentration of surfactant and polyelectrolytes to form stable shells. The average size of the obtained capsules was 60 nm. Pegylated external layer were prepared using PGA-g-PEG (PGA grafted by PEG poly(ethylene glycol)). The capsules were stable for at least a period of 3 months. These nanocapsules were biocompatible when tested for cytotoxicity in a cellular coculture assay and demonstrated no or very low nonspecific binding to peripheral blood mononuclear cells when tested by flow cytometry. In order to study drug effects on leukemia cells, beta-carotene and vitamin A have been encapsulated as model drugs.

Layer-by-layer deposition of polyelectrolytes. Dipping versus spraying.

We studied the properties of polyelectrolyte multilayer films prepared using the technique of polyelectrolyte deposition from solution (dipping) or supplying the solutions to the surface by spraying. The quality of films obtained by those two techniques was compared to find out whether the well-established dipping procedure can be replaced with the spraying technique. Neutron and X-ray reflectometric studies were performed on the samples of interest. We found that multilayers prepared by dipping are thicker, denser and less rough than films having the same number of layers, i.e., having the same number of deposition cycles, obtained by spraying.

Enhanced deposition of particles under attractive double-layer forces

Abstract The kinetics of Brownian particle deposition from flowing suspensions in the vicinity of a stagnation point (rotating disk, spherical, cylindrical, and impinging jet collectors) was studied theoretically and experimentally. On the basis of the numerical solutions of governing transport equations a considerable enhancement of particle deposition was predicted for particle and collectors bearing opposite surface charge (zeta potentials). The initial deposition rate of particles can be increased by many times when decreasing the ionic strength of the suspension to the value 10 −5 –10 −6 M . This effect, which we shall refer to as the reverse salt effect, is completely opposite to what one commonly observed for equally charged particle and collector surfaces (as well as in coagulation kinetic studies). These theoretical predictions were confirmed quantitatively by the direct microscope observation method using mica collector (modified by chemisorption of silano compounds) and a monodisperse suspension of submicrometer latex particles (negatively charged). The experiments also furnished a direct evidence of the coupling between macroscopic hydrodynamic flow and the microscopic colloid forces and proved that the asymmetric double-layer dynamics cannot be properly described by the constant surface charge boundary conditions.

Novel approach to long sustained multilayer nanocapsules: influence of surfactant head groups and polyelectrolyte layer number on the release of hydrophobic compounds

Nanoemulsion-templated long sustained polyelectrolyte (PE) nanocapsules (average size < 200 nm) loaded with Oil Red O and two cyanine-type photosensitizers: IR-786 and IR-780 were successfully fabricated by using the layer-by-layer (LbL) technique. All nanoproducts were subjected to in vitro release characteristics and analysis of selected control parameters, i.e., type of surfactant head group, characteristic release time and surfactant–polyelectrolyte interactions. Their properties were characterized by means of dynamic light scattering (DLS) compared with scanning electron microscopy (SEM) and atomic force microscopy (AFM). In our studies for construction of oil-in-water nanoemulsion templates we selected three cationic surfactants with different nature of hydrophilic head groups, i.e., double-headed (or so-called dicephalic-type) N,N-bis[3,3′(trimethylammonio)propyl]dodecanamide dimethylsulfate (C12(TAPAMS)2), bulky saccharide-derived 2-(dodecyldimethylammonio)ethylglucoheptonamide bromide (D2GHA-12) and a classic dodecyltrimethylammonium bromide (DTABr) for comparison. The polyelectrolytes were the following: polyanion of poly(sodium 4-styrenesulfonate) (PSS) and polycation of poly(diallyldimethylammonium chloride) (PDADMAC). The in vitro release profile features, studied spectrophotometrically, were interpreted in the framework of diffusion-controlled processes and stability of the first interfacial PE–surfactant complex. Accordingly, the multicharge and bulky structure of the surfactant are found to be the most desirable factors for fabrication of long sustained and stable nanocapsules encapsulating a hydrophobic active substance.

Synthesis and antimicrobial activity of monodisperse copper nanoparticles.

Metallic monodisperse copper nanoparticles at a relatively high concentration (300 ppm CuNPs) have been synthesized by the reduction of copper salt with hydrazine in the aqueous SDS solution. The average particles size and the distribution size were characterized by Dynamic Light Scattering (DLS), Nanosight-Nanoparticle Tracking Analysis (NTA). The morphology and structure of nanoparticles were investigated using Scanning Electron Microscopy (SEM). The chemical composition of the copper nanoparticles was determined by X-ray Photoelectron Spectroscopy (XPS). Monodisperse copper nanoparticles with average diameter 50 nm were received. UV/vis absorption spectra confirmed the formation of the nanoparticles with the characteristic peak 550 nm. The antimicrobial studies showed that the copper nanoparticles had high activity against Gram-positive bacteria, standard and clinical strains, including methicillin-resistant Staphylococcus aureus, comparable to silver nanoparticles and some antibiotics. They also exhibited antifungal activity against Candida species.

Biocompatible long-sustained release oil-core polyelectrolyte nanocarriers: From controlling physical state and stability to biological impact

It has been generally expected that the most applicable drug delivery system (DDS) should be biodegradable, biocompatible and with incidental adverse effects. Among many micellar aggregates and their mediated polymeric systems, polyelectrolyte oil-core nanocarriers have been found to successfully encapsulate hydrophobic drugs in order to target cells and avoid drug degradation and toxicity as well as to improve drug efficacy, its stability, and better intracellular penetration. This paper reviews recent developments in the formation of polyelectrolyte oil-core nanocarriers by subsequent multilayer adsorption at micellar structures, their imaging, physical state and stability, drug encapsulation and applications, in vitro release profiles and in vitro biological evaluation (cellular uptake and internalization, biocompatibility). We summarize the recent results concerning polyelectrolyte/surfactant interactions at interfaces, fundamental to understand the mechanisms of formation of stable polyelectrolyte layered structures on liquid cores. The fabrication of emulsion droplets stabilized by synergetic surfactant/polyelectrolyte complexes, properties, and potential applications of each type of polyelectrolyte oil-core nanocarriers, including stealth nanocapsules with pegylated shell, are discussed and evaluated.

Encapsulation of liquid cores by layer-by-layer adsorption of polyelectrolytes.

The aim of this work was to develop the method of preparation of loaded, submicron nanocapsules based on the liquid core encapsulation by polyelectrolyte (PE) multilayer adsorption. The procedure of PE adsorption on the emulsion droplets requires a specific selection of surfactants, which have good properties as emulsifiers and provide a stable surface charge for sequential adsorption of PE without losing stability of emulsion. Using AOT as emulsifier this study obtained droplets, stabilized by AOT/PDADMAC surface complexes. These positively charged liquid cores were then modified by sequential adsorption of polyelectrolytes to obtain nanocapsules with the average size of 200 nm, with various combinations of polyelectrolytes (PDADMAC, CHIT, PAH, PSS, ALG). This study demonstrated the formation of consecutive layers of PE shells by measuring zeta potential of capsules after adsorption of each layer. It visualized the cores by dissolving fluorescent dye Coumarine6 in oil phase and multilayer shells by using FITC labelled polycation.