Experimental and theoretical study of adsorption of synthesized amino acid core derived surfactants at an air/water interface.

The adsorption characteristics of amino acid surfactants, synthesized as substances with different volumes and hydrophilic head properties, have been previously described experimentally, without robust theoretical explanation. A theoretical model enabling the characterization of the adsorption behavior and physicochemical properties of this type of biodegradable surfactants, based on molecular structure, would be beneficial for assessment of their usefulness in colloids and interface science in comparison with typical surface-active substances. In this paper, the adsorption behaviour of synthesized amino acid surfactants at the liquid/gas interface was analyzed experimentally (by surface tension measurements using two independent techniques) and theoretically by means of an elaborate model, considering the volume of the surfactant hydrophilic "head" and its ionization degree. It was shown that the adsorption behavior of the synthesized compounds can be successfully described by the proposed model, including the Helfand-Frisch-Lebowitz isotherm based on the equation of state of 2D hard disk-like particles, with molecular properties of surfactant particles obtained using molecular dynamics simulations (MDS). Model parameters allow for direct comparison of physicochemical properties of synthesized amino acid surfactants with other ionic and non-ionic surface-active substances. Furthermore, it was revealed that intermolecular hydrogen bonds allow the formation of surfactant dimers with high surface activity.

Nanocomposite multifunctional polyelectrolyte thin films with copper nanoparticles as the antimicrobial coatings.

The work presents the formation and physicochemical characterization of polyelectrolyte-copper nanocomposite coatings using: poly(diallyldimethylammonium chloride) (PDADMAC) as a polycation, poly(sodium 4-styrenesulfonate)(PSS) as a polyanion and negatively charged copper nanoparticles (CuNPs) to obtain biocompatible surfaces with an antibacterial functionality. The mass and thickness of composite films were investigated by the quartz crystal microbalance with dissipation monitoring (QCM-D) and the ellipsometry whereas, the structure and morphology of coatings were examined using scanning electron microscopy (SEM). The increase of the UV-Vis absorption confirmed the formation of the consecutive layers of the film. Antibacterial activity of the coatings was tested on a representative Gram-positive bacteria strain, Staphylococcus aureus. The microbiological tests were performed and bacteria visualized using fluorescent staining and microscopic technique. It was demonstrated that nanostructured films had antibacterial properties, which makes polyelectrolyte multilayer films containing copper an interesting material in biomedical applications area, e.g., for the prevention of microbial deposition on surfaces.

Interfacial and structural characteristics of polyelectrolyte multilayers used as cushions for supported lipid bilayers.

The surface properties of polyelectrolyte multilayers (PEMs) obtained via sequential adsorption of oppositely charged polyions from their solutions and used as cushions for supported lipid bilayers were investigated. Five types of polyelectrolytes were used: cationic polyethyleneimine (PEI), poly(diallyldimethylammonium)chloride (PDADMAC), and poly-l-lysine hydrobromide (PLL); and anionic polysodium 4-styrenesulfonate (PSS) and poly-l-glutamic acid sodium (PGA). The wettability and surface free energy of the PEMs were determined by contact angle measurements using sessile drop analysis. Electrokinetic characterisation of the studied films was performed by streaming potential measurements of selected multilayers and the structure of the polyelectrolyte multilayer was characterized by synchrotron X-ray reflectometry. The examined physicochemical properties of the PEMs were correlated with the kinetics of the formation of supported lipid bilayers atop the PEM cushion.

Gadolinium alginate nanogels for theranostic applications.

Synthesis of theranostic nanoparticles, which combine both therapeutic and diagnostic capabilities in one platform can be considered as a step forward personalized medicine, since it allows tracing the delivery of the drug to targeted organ. Thus, the aim of this work was to prepare gadolinium alginate gel nanoparticles (gadolinum nanogels - GdNG) by the reverse microemulsions and physical crosslinking method as the vehicles able to carry hydrophilic drugs and to be traced by the Magnetic Resonance Imaging (MRI). The average size of synthesized nanoparticles was about 110nm and the batch concentration was 1010 particles/ml. The morphology of nanogeles was visualized by Cryo-Scanning Electron Microscopy. Surface of nanogels particles was modified by the Layer-by-Layer (LbL) technique using natural polyelectrolytes. The cytotoxicity of non-modified and LbL modified nanogels was evaluated by the cellular viability quantification and cell death assessments using MTT and LDH biochemical tests, respectively. We encapsulated the model compound - fluorescent dye (Rhodamine b) in nanogels networks and proved the possibility of GdNG visualization by MRI.

Linseed oil based nanocapsules as delivery system for hydrophobic quantum dots.

In the present work, the CdSe/ZnS hydrophobic quantum dots were embedded within the polyelectrolyte nanocapsules. The core of the capsules, which consists of a mixture of the linseed oil with chloroform, was prepared using the spontaneous emulsification technique. The obtained emulsions were stabilized with lecithin and encapsulated using the layer-by-layer (LbL) adsorption of polyelectrolytes. The pair of biocompatible polyelectrolytes was used: the cationic poly-l-lysine hydrobromide (PLL) together with the anionic poly-d-glutamic acid sodium salt. The saturation LbL method, which is based on the stepwise formation of consecutive layers on the initial emulsion without the intermediate rinsing step, was applied to form the capsule shells. Their growth was evidenced by the capsule size and electrophoretic mobility measurements. The emulsion and the capsules were deposited on a mica surface and the deposit topology was examined by the means of atomic force microscopy (AFM). The presence of quantum dots within the oil cores was confirmed by recording the fluorescent spectra of the samples containing CdSe/ZnS. In order to evaluate cytotoxicity of the capsules, their influence on the viability of mouse embryonic fibroblasts was examined using the MTT test, followed by optical-microscope observation of morphology of the cells after hematoxylin-eosin staining.

Natural oil nanoemulsions as cores for layer-by-layer encapsulation.

In this study, emulsions of three different natural oils were prepared using spontaneous emulsification technique. The effect of three emulsifiers, AOT, lecithin and cholesterol on emulsion properties was studied. Their influence on interfacial tension at oil/water interface was evaluated by the pendant drop shape analysis method. Then, the mean droplet size, zeta potential and stability of emulsions were investigated in relation with the type of oil, surfactant, oil-to-ethanol ratio and surfactant concentration. We found that in the case of linseed oil, fine emulsion droplets are formed without any surfactant due to its low oil/water interfacial tension. A hydrophobic dye (Coumarin 6) was encapsulated within oil cores and its presence was confirmed by fluorescence spectroscopy and microscopy. The obtained emulsions can be used alone or as the cores for layer-by-layer encapsulation, which was demonstrated by enclosing droplets within first layer of synthetic polycation poly(allyamine hydrochloride) (PAH).

Polyelectrolyte multilayer capsules with quantum dots for biomedical applications.

The aim of this work was to encapsulate the CdTe quantum dots within the nanocapsules that were prepared by the layer-by-layer adsorption of polyelectrolytes. Two different polyelectrolyte pairs were used as components of the shell: synthetic polycation poly(allyamine hydrochloride) (PAH), together with anionic poly(sodium styrene sulfonate) (PSS), and biocompatible cationic poly-L-lysine hydrobromide in a pair with biocompatible anionic poly-D-glutamic acid sodium salt (PGA). The saturation method was used for formation of consecutive layers on the initial CdTe-polyelectrolyte complex. A growth of the polyelectrolyte shell was followed with the electrophoretic mobility and light scattering measurements, in order to determine the zeta potential and the size of capsules, respectively. The fluorescent spectra of the quantum dots, which are embedded within the capsules, were characterized with spectrofluorimeter. Later on, they were deposited on a negatively charged mica surface and studied by the means of atomic force microscopy (AFM). In order to estimate the cytotoxicity of capsules, their influence on the B-lymphoblastoid cell line proliferation and on unspecific binding to the P-blood mononuclear cells was examined using the flow cytometry.

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.

Characteristics of model polyelectrolyte multilayer films containing laponite clay nanoparticles.

Polyelectrolyte films structure formed by the "layer-by-layer" (LbL) technique can be enriched by addition of charged nanoparticles like carbon nanotubes and silver or hydroxyapatite nanoparticles, which can improve properties of the polyelectrolyte films or modify their functionality. In our paper we examined the formation and properties of model polyelectrolyte multilayers containing a synthetic layered silicate, Laponite. The Laponite nanoparticles were incorporated into multilayer films, which were formed from weak, branched polycation PEI and strong polyanion PSS. Since charge of PEI is pH-dependent, we build up multilayer films in two deposition conditions: pH = 6 when PEI was strongly charged and pH = 10.5 when charge density of PEI was low. Thicknesses of the films constructed with various numbers of Laponite layers were measured by single wavelength ellipsometry. We also determined the differences in permeability for selected electroactive molecules using cyclic voltamperometry. Properties of the films containing clay nanoparticles were compared with model polyelectrolyte multilayer films PEI/PSS formed at the same conditions. We found that Laponite nanoparticles strongly influenced PEI/PSS multilayer film properties. Replacement of PSS by Laponite eliminated the oscillations of the film thickness in the case when PEI was weakly charged. PSS layer adsorbed on top of PEI/Laponite bilayers increased the thickness of multilayer films and improved their barrier properties so synergistic effects between these properties for polyelectrolytes and Laponite nanoparticles could be observed.

Influence of pH on the structure of multilayer films composed of strong and weak polyelectrolytes.

The sequential adsorption of oppositely charged polyelectrolytes called the "layer by layer" technique is a method for formation of ultrathin films with controlled thickness and interfacial properties. Composition of polyelectrolyte solutions, pH, and electrolyte concentration are important parameters governing formation of multilayer films. Since pH is the factor controlling charge of weak polyelectrolytes, the structure of multilayers should be sensitive to its value. In this paper we focused on formation of PE multilayer films composed from weak and strong polyelectrolytes. We used weak, branched polycation polyethyleneimine (PEI, 70 kDa) and strong polyanion poly-4-styrenesulfonate (PSS, 70 kDa) to form films by the layer-by-layer technique on the surface of silicon wafers under two deposition conditions: pH = 6 when PEI was strongly charged and pH = 10.5 when the charge density of PEI was low. Thicknesses of films were measured by single wavelength ellipsometry, and the results were confronted with ones concerning mass of the adsorbed films obtained by quartz crystal microbalance. We found that, depending on pH of the solutions, combination of weakly and strongly charged polyelectrolytes gave either linear or nonmonotonic increase of film thickness with a number of deposited PE layers. We observed a good correlation between multilayer film thickness and adsorbed mass. The atomic force microscopy images of surface topography of PEI/PSS films demonstrated large differences between films deposited at pH = 6 and 10.5. Additionally the cyclic voltamperometry was used to determine the differences in permeability of films formed at various pH conditions.

Probing of polyelectrolyte monolayers by zeta potential and wettability measurements.

Detection of the very first step of polyelectrolyte adsorption onto a solid support is of great importance for understanding mechanisms of solid surface modification. It was shown that streaming potential and contact angle measurements can be successfully used for polyelectrolyte (PE) adsorption characterization in a broad range of surface coverage. Cationic polyallylamine hydrochloride (PAH) was used for the formation of the layer. The electrokinetic characteristics of the substrate covered by the PAH layer were compared with contact angles measured under wet (captive air bubble/substrate in water) and dry (sessile water droplet/dried substrate) conditions. It has been demonstrated that contact angle values determined under both conditions are in good agreement. The observed rapid increase in the contact angle from zero for the bare mica surface to the value close to one characteristic of the PAH monolayer appears in the same PAH coverage range as zeta potential value changes due to adsorption. These results show that wettability can be as sensitive to the presence of small amounts of adsorbed species as electrokinetic measurements.

Structure of poly(acrylic acid) in electrolyte solutions determined from simulations and viscosity measurements.

In this work, the structure of poly(acrylic acid) (PAA) molecules in electrolyte solutions obtained from molecular dynamic simulations was compared with experimental data derived from dynamic light scattering (PCS), dynamic viscosity, and electrophoretic measurements. Simulations and measurements were carried out for polymer having a molecular weight of 12 kD for various ionic strengths of the supporting electrolyte (NaCl). The effect of the ionization degree of the polymer, regulated by the change in the pH of the solution in the range 4-9 units, was also studied systematically. It was predicted from theoretical simulations that, for low electrolyte concentration (10(-3) M) and pH = 9 (full nominal ionization of PAA), the molecule assumed the shape of a flexible rod having the effective length L(ef) = 21 nm, compared to the contour length L(ext) = 41 nm predicted for a fully extended polymer chain. For an electrolyte concentration of 0.15 M, it was predicted that L(ef) = 10.5 nm. For a lower ionization degree, a significant folding of the molecule was predicted, which assumed the shape of a sphere having the radius of 2 nm. These theoretical predictions were compared with PCS experimental measurements of the diffusion coefficient of the molecule, which allowed one to calculate its hydrodynamic radius R(H). It was found that R(H) varied between 6.6 nm for low ionic strength (pH = 9) and 5.8 nm for higher ionic strength (pH = 4). The R(H) values for pH = 9 were in a good agreement with theoretical predictions of particle shape, approximated by prolate spheroids, bent to various forms. On the other hand, a significant deviation from the theoretical shape predictions occurring at pH = 4 was interpreted in terms of the chain hydration effect neglected in simulations. To obtain additional shape information, the dynamic viscosity of polyelectrolyte solutions was measured using a capillary viscometer. It was found that, after considering the correction for hydration, the experimental results were in a good agreement with the Brenner's viscosity theory for prolate spheroid suspensions. The effective lengths derived from viscosity measurements using this theory were in good agreement with values predicted from the molecular dynamic simulations.

The Hofmeister series effect in adsorption of cationic surfactants--theoretical description and experimental results.

Interfacial properties of cationic surfactants show strong dependence on the type of surfactant counterion or on the type of anion of a salt added to the surfactant solution. In the paper, the models of ionic surfactant adsorption that can take into account ionic specific effects are reviewed. Model of ionic surfactant adsorption based on the assumption that the surfactant ions and counterions undergo nonequivalent adsorption within the Stern layer was selected to describe experimental surface tension isotherms of aqueous solutions of a number of cationic surfactants. The experimental isotherms for: n-alkyl trimethylammonium cationic surfactants, namely: C(16)TABr (CTABr or CTAB), C(16)TACl, C(16)TAHSO(4), C(10)TABr and C(12)TABr as well as decyl- and dodecylpyridinium salts with and without various electrolyte anions as Cl(-), Br(-), F(-), I(-), NO(3)(-), ClO(4)(-) and CH(3)COO(-) were described in terms of the model and a good agreement between the theory and experiment was obtained for a wide range of surfactants and added electrolyte concentrations. A very pronounced Hofmeister effect in dependence of surface tension of cationic surfactants on the type of anion was found. Analysing this dependence in terms of the proposed model of ionic surfactant adsorption, strong correlation between "anion surface activity" (the model parameter accounting for ion penetration into the Stern layer), and the ion polarizability was obtained. That suggests that the mechanism related to the dispersive interaction of polarized ion with electric field at interface is responsible for Hofmeister series effects in surface activity of cationic surfactants. The same mechanism was proposed recently to explain the dependence of surface tension increase with electrolyte concentration on anion and cation type.

Surface properties of mixtures of surface-active sugar derivatives with ionic surfactants: theoretical and experimental investigations.

Surface properties of systems that are mixtures of ionic surfactants and sugar derivatives-anionic surfactant sodium dodecyl sulfate and n-dodecyl-beta-D-maltoside (SDS/DM) and cationic surfactant dodecyltrimethylammonium bromide and n-dodecyl-beta-D-glucoside (DTABr/DG)-were investigated. The experimental results obtained from measurements of surface tension of mixtures with various ratio of ionic to nonionic components were analyzed by two independent theories. First is Motomura theory, derived from the Gibbs-Duhem equation, allowing for indirect evaluation of the composition of mixed monolayers and the Gibbs energies of adsorption, corresponding to mutual interaction between surfactants in mixed adsorbed film. As second theory we used our newly developed theoretical model of adsorption of ionic-nonionic surfactant mixtures. Using this approach, we were able to describe the experimental surface tension isotherms for mixtures of surface-active sugar derivatives and ionic surfactants. We obtained a good agreement with experimental data using the same set of model parameters for a whole range of studied compositions of a given surfactant mixture. The values of surface excess calculated from both theories agreed with each other with a reasonable accuracy. However, the newly developed model of adsorption of ionic-nonionic surfactant mixtures has the advantage of straightforward determination of surface layer composition. By the solution of equations of adsorption, one can obtain directly the values of surface excess of all components (surfactant ions, counterions, and nonionic surfactants molecules), which are present in the investigated system.

Characterization of polyelectrolyte multilayers by the streaming potential method.

Polyelectrolyte multilayer adsorption on mica was studied by the streaming potential method in the parallel-plate channel setup. The technique was calibrated by performing model measurements of streaming potential by using monodisperse latex particles. Two types of polyelectrolytes were used in our studies: poly(allylamine) hydrochloride (PAH), of a cationic type, and poly(sodium 4-styrenesulfonate) (PSS) of an anionic type, both having molecular weight of 70,000. The bulk characteristics of polymers were determined by measuring the specific density, diffusion coefficient for various ionic strengths, and zeta potential. These measurements as well as molecular dynamic simulations of chain shape and configurations suggested that the molecules assume an extended, wormlike shape in the bulk. Accordingly, the diffusion coefficient was interpreted in terms of a simple hydrodynamic model pertinent to flexible rods. These data allowed a proper interpretation of polyelectrolyte multilayer adsorption from NaCl solutions of various concentrations or from 10(-3) M Tris buffer. After completing a bilayer, periodic variations in the apparent zeta potential between positive and negative values were observed for multilayers terminated by PAH and PSS, respectively. These limiting zeta potential values correlated quite well with the zeta potential of the polymers in the bulk. The stability of polyelectrolyte films against prolonged washing (reaching 26 h) also was determined using the streaming potential method. It was demonstrated that the PSS layer was considerably more resistant to washing, compared to the PAH layer. It was concluded that the experimental data were consistent with the model postulating particle-like adsorption of polyelectrolytes with little chain interpenetration. It also was concluded that due to high sensitivity, the electrokinetic method applied can be effectively used for quantitative studies of polyelectrolyte adsorption, desorption, and reconformation.

On the adsorption behavior of soluble, surface-chemically pure hemicyanine dyes at the air/water interface.

Equilibrium surface tension (sigma(e)) versus concentration isotherms of surface-chemically pure aqueous solutions of the homologous series of N-n-alkyl-4'-(dimethylamino)-stilbazium bromides ("hemicyanines") were measured at 295 K. The adsorption parameters of saturation adsorption and standard free energy of adsorption of the hemicyanines were determined from the evaluation of the sigma(e) vs c isotherms using a two-state approach to the surface equation of state. The adsorption parameters reveal a very pronounced phenomenon of alternation (even/odd effect), which has so far not been found to a large extent such as this. Thus, the cross-sectional areas of the odd members are almost twice those of the related even members. Surface activity of the odd members is stronger than that of the even members. UV-vis investigations showed that there is no indication of aggregate formation in the adsorption layer. The thermodynamic results give evidence for distinct differences between the surface conformations of the even- and the odd-chain hemicyanine dyes although the reasons for this are not known.

Coupling of hydrodynamic and electric interactions in adsorption of colloidal particles.

The role of the coupling between hydrodynamic and electric interactions in adsorption of colloid particles is reviewed. First the general formulation of the problem of fluid motion connected with the macroscopic suspension flow as well as microscopic particle motion is presented together with the general description of the colloid particle adsorption. Then two limiting cases are considered: motion of uncharged particles and electric interactions of stationary particles. The theoretical consideration are supported by illustrative experimental results. Next the electrokinetic effects in particle adsorption process are considered and the theoretical background for the description of these effects is given. The effect of the electroviscous drag on the particle aggregation and adsorption kinetics is discussed. Also the electrokinetic lift force, which is an example of the non-linear electrokinetic effect, is considered and the possible implications on particle adsorption are examined. In the following part of the review the influence of electro-hydrodynamic coupling on the particle transport to the interface is considered. First the 'inverse salt effect' for the initial adsorption flux when the interface is not covered by adsorbed particles is examined. Then, the electro-hydrodynamic scattering effect which occurs when the interface is partially covered by particles is considered and its experimental evidence concerning the adsorption kinetics and the local structure of adsorbate is considered. Also a novel application of the scattering effect to measure interparticle forces the 'colloid particle collider' is presented and some experimental results obtained using this technique are shown. All results reported in this review suggest strong coupling of the hydrodynamic and electric forces and their pronounced influence on colloid particle adsorption.

Calculations of Double-Layer Electrostatic Interactions for the Sphere/Plane Geometry

A numerical scheme for solving the nonlinear Poisson-Boltzmann equation for the sphere/sphere and plane/sphere geometries has been developed. The method is based on an alternating direction overrelaxation procedure using the Newton-Raphson iteration to solve the nonlinear equation stemming from finite-difference discretization. The novelty of the algorithm consists in using the grid transforming functions that allow a more uniform distribution of mesh points in the vicinity of the particle. The method was used to perform extensive calculations for opposite surface potentials of the interface and the particle immersed in a symmetric electrolyte solution. The electric potential distribution (within and outside the sphere) was calculated, as well as the force and energy of interaction (from the integral of the force over separation distance), for the constant potential, the constant charge, and the mixed cases. The energy profiles calculated for various kappaa were compared with the analytical approximations derived using the Hogg-Healy-Fuerstenau method and the linear superposition approach (LSA). These calculations enabled us to conclude that the LSA can be used as a good estimate of interaction energy for a broad range of kappaa values at distances greater than kappa-1, i.e., for problems pertinent to colloid particle adsorption.