Towards the systematic design of multilayer O/W emulsions with tannic acid as an interfacial antioxidant.

This work discusses the possibility of designing multilayer oil-in-water emulsions to introduce the maximum possible amount of an antioxidant at the droplet interfaces for the optimal protection of a linseed oil core against oxidation, using a systematic three-step colloidal procedure. An antioxidant (here Tannic Acid - TA) is chosen and its interactions with a primary emulsifier (here Bovine Serum Albumin - BSA) and several polysaccharides are first examined in solution using turbidity measurements. As a second step, LbL deposition on solid surfaces is used to determine which of the polysaccharides to combine with BSA and tannic acid in a multilayer system to ensure maximum presence of tannic acid in the films. From UV-vis and polarization modulation infrared reflection-absorption (PM-IRRAS) spectroscopic measurements it is suggested that the best components to use in a multilayer emulsion droplet, together with BSA and TA, are chitosan and pectin. BSA, chitosan and pectin are subsequently used for the formation of three-layer linseed oil emulsions, and tannic acid is introduced into any of the three layers as an antioxidant. The effect of the exact placement of tannic acid on the oxidative stabilization of linseed oil is assessed by monitoring the fluorescence of Nile red, dissolved in the oil droplets, under the attack of radicals generated in the aqueous phase of the emulsion. From the results it appears that the three-stage procedure presented here can serve to identify successful combinations of interfacial components of multilayer emulsions. It is also concluded that the exact interfacial placement of the antioxidant plays an important role in the oxidative stabilization of the valuable oil core.

Binding of lanthanide salts to zwitterionic phospholipid micelles.

As the use of lanthanide salts in biophysical systems increases and the separation of lanthanides from nuclear and other wastes with extraction processes has become an important technological challenge, a deeper understanding of the behavior of lanthanides at lipid interfaces is urgently needed. In this work the interaction of lanthanide salts with zwitterionic phospholipids is probed using aqueous micelles of the surfactant dodecyl phosphocholine (DPC), which are useful membrane-mimetic model systems, widely used for the solubilization of membrane proteins in aqueous solutions. Because more than one species exists in lanthanide salt solutions, even at the pH value of 4 used in this experiment, the major goal of this investigation is to examine which species are actually binding to the micelles. Using static and time-dependent europium fluorescence, strong indications are obtained that both the Eu3+ cation and its 1:1 chloride, nitrate, or sulfate complexes bind to the micelles, whereas the europium species do not appear to interact strongly with DPC molecules below the cmc. From isothermal titration calorimetry (ITC) measurements it is found that the lanthanide interaction with DPC micelles increases to the right of the lanthanide series and is - surprisingly - endothermic, underlying the important role of hydration effects in the interaction. The anion of the lanthanide salt strongly influences the thermodynamics: perchlorate and sulfate salts give extraordinary results, switching the interaction to exothermic. A multi-level phenomenological electrostatic model of the europium fluorescence lifetimes strongly suggests that in the case of nitrate salts both Ln3+ and LnNO32+ ions bind to the micelles. Overall a detailed molecular picture of the complexity of lanthanide-lipid interactions at interfaces is emerging from these experiments and the associated modelling effort.

The influence of lanthanide-(III)-nitrates on adsorbed monolayers of dodecylphosphorylcholine at the air-water interface.

Adsorption layers at the air-water interface formed from the soluble zwitterionic surfactant dodecylphosphorylcholine (DPC) serving as a soluble model substance for phospho-lipids were characterized with respect to their equilibrium and dynamic surface properties. To clarify the effect of ionic interactions with electrolyte present in the bulk phase, surface rheological properties upon addition of Ce(NO3)3 and Yb(NO3)3 were determined. In order to account for the surface activity of the nitrate ion, comparative measurements using NaNO3 were carried out additionally. Further experimental information on the bulk hydration characteristics of the aqueous solutions was obtained using dielectric relaxation spectroscopy (DRS). A possible mechanism causing the deviating surface dilatational modulus E in terms of ion specificity is suggested.

Bis(hydroxylamino)triazines: High Selectivity and Hydrolytic Stability of Hydroxylamine-Based Ligands for Uranyl Compared to Vanadium(V) and Iron(III).

The development of ligands with high selectivity and affinity for uranium is critical in the extraction of uranium from human body, radioactive waste, and seawater. A scientific challenge is the improvement of the selectivity of chelators for uranium over other heavy metals, including iron and vanadium. Flat ligands with hard donor atoms that satisfy the geometric and electronic requirements of the UVIO22+ exhibit high selectivity for the uranyl moiety. The bis(hydroxylamino)(triazine) ligand, 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H2bihyat), a strong binder for hard metal ions (FeIII, TiIV, VV, and MoVI), reacted with [UVIO2(NO3)2(H2O)2]·4H2O in aqueous solution and resulted in the isolation of the complexes [UVIO2(bihyat)(H2O)], [UVIO2(bihyat)2]2-, and {[UVIO2(bihyat)(µ-OH)]}22-. These three species are in equilibrium in aqueous solution, and their abundance varies with the concentration of H2bihyat and the pH. Reaction of H2bihyat with [UVIO2(NO3)2(H2O)2]·4H2O in CH3CN gave the trinuclear complex [UVI3O6(bihyat)2(µ-bihyat)2]2-, which is the major species in organic solvents. The dynamics between the UVIO22+ and the free ligand H2bihyat in aqueous and dimethyl sulfoxide solutions; the metal binding ability of the H2bihyat over pyridine-2,6-dicarboxylic acid (H2dipic) or glutarimidedioxime for UVIO22+, and the selectivity of the H2bihyat to bind UVIO22+ in comparison to VVO43- and FeIII in either UVIO22+/VVO43- or UVIO22+/FeIII solutions were examined by NMR and UV-vis spectroscopies. The results revealed that H2bihyat is a superior ligand for UVIO22+ with high selectivity compared to FeIII and VVO43-, which increases at higher pHs. Thus, this type of ligand might find applications in the extraction of uranium from the sea and its removal from the environment and the human body.

Investigations of the Hofmeister series and other specific ion effects using lipid model systems.

From the ion point-of-view specific ion effects (SIE) arise as an interplay of ionic size and shape and charge distribution. However in aqueous systems SIE invariably involve water, and at surfaces they involve both interacting surface groups and local fields emanating from the surface. In this review we highlight the fundamental importance of ionic size and hydration on SIE, properties which encompass all types of interacting forces and ion-pairing phenomena and make the Hofmeister or lyotropic series of ions pertinent to a broad range of systems and phenomena. On the other hand ionic hydrophobicity and complexation capacity also determine ionic behavior in a variety of contexts. Over the years we have carried out carefully designed experiments on a few selected soft matter model systems, most involving zwitterionic phospholipids, to assess the importance of fundamental ionic and interfacial properties on ion specific effects. By tuning down direct Coulomb interactions, working with different interfacial geometries, and carefully tuning ion-lipid headgroup interactions it is possible to assess the importance of different parameters contributing to ion specific behavior. We argue that the majority of specific ion effects involving relatively simple soft matter systems can be at least qualitatively understood and demystified.

Semi-Interpenetrating Polymer Networks with Predefined Architecture for Metal Ion Fluorescence Monitoring.

The development of new synthetic approaches for the preparation of efficient 3D luminescent chemosensors for transition metal ions receives considerable attention nowadays, owing to the key role of the latter as elements in biological systems and their harmful environmental effects when present in aquatic media. In this work, we describe an easy and versatile synthetic methodology that leads to the generation of nonconjugated 3D luminescent semi-interpenetrating amphiphilic networks (semi-IPN) with structure-defined characteristics. More precisely, the synthesis involves the encapsulation of well-defined poly(9-anthrylmethyl methacrylate) (pAnMMA) (hydrophobic, luminescent) linear polymer chains within a covalent poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) hydrophilic polymer network, derived via the 1,2-bis-(2-iodoethoxy)ethane (BIEE)-induced crosslinking process of well-defined pDMAEMA linear chains. Characterization of their fluorescence properties demonstrated that these materials act as strong blue emitters when exposed to UV irradiation. This, combined with the presence of the metal-binding tertiary amino functionalities of the pDMAEMA segments, allowed for their applicability as sorbents and fluorescence chemosensors for transition metal ions (Fe3+, Cu2+) in solution via a chelation-enhanced fluorescence-quenching effect promoted within the semi-IPN network architecture. Ethylenediaminetetraacetic acid (EDTA)-induced metal ion desorption and thus material recyclability has been also demonstrated.

Amphiphilic Polymer Conetworks Based on End-Linked "Core-First" Star Block Copolymers: Structure Formation with Long-Range Order.

Amphiphilic polymer conetworks are cross-linked polymers that swell both in water and in organic solvents and can phase separate on the nanoscale in the bulk or in selective solvents. To date, however, this phase separation has only been reported with short-range order, characterized by disordered morphologies. We now report the first example of amphiphilic polymer conetworks, based on end-linked "core-first" star block copolymers, that form a lamellar phase with long-range order. These mesoscopically ordered systems can be produced in a simple fashion and exhibit significantly improved mechanical properties.

Helix formation by alanine-based peptides in pure water and electrolyte solutions: insights from molecular dynamics simulations.

Specific ion effects on oligopeptide conformations in solution are attracting strong research attention, because of their impact on the protein-folding problem and on several important biological-biotechnological applications. In this work, we have addressed specific effects of electrolytes on the tendency of oligopeptides toward formation and propagation of helical segments. We have used replica-exchange molecular dynamics (REMD) simulations to study the conformations of two short hydrophobic peptides [Ace-(AAQAA)3-Nme (AQ), and Ace-A8-Nme (A8)] in pure water and in aqueous solutions of sodium chloride (NaCl) and sodium iodide (NaI) with concentrations of 1 and 3 M. The average helicities of the AQ peptide have been analyzed to yield Lifson-Roig (LR) parameters for helix nucleation and helix propagation. The salt dependence of these parameters suggests that electrolytes tend to stabilize the helical conformations of short peptides by enhancing the helix nucleation parameter. The helical conformations of longer oligopeptides are destabilized in the presence of salts, however, because the helix propagation parameters are reduced by electrolytes. On top of this general trend, we observe a significant specific salt effect in these simulations. The hydrophobic iodide ion in NaI solutions has a high affinity for the peptide backbone, which reflects itself in an enhanced helix nucleation and a reduced helix propagation parameter with respect to pure water or NaCl solutions. The present work thus explains the computational evidence that electrolytes tend to stabilize the compact conformations of short peptides and destabilize them for longer peptides, and it also sheds additional light on the specific salt effects on compact peptide conformations.

Organized Silica Films Generated by Evaporation-Induced Self-Assembly as Hosts for Iron Oxide Nanoparticles.

In this work, we prepared oriented mesoporous thin films of silica on various solid substrates using the pluronic block copolymer P123 as a template. We attempted to insert guest iron oxide (FexOy) nanoparticles into these films by two different methods: (a) by co-precipitation-where iron precursors are introduced in the synthesis sol before deposition of the silica film-and subsequent oxide production during the film calcination step; (b) by preparing and calcining the silica films first then impregnating them with the iron precursor, obtaining the iron oxide nanoparticles by a second calcination step. We have examined the structural effects of the guest nanoparticles on the silica film structures using grazing incidence X-ray scattering (GISAXS), high-resolution transmission electron spectroscopy (HRTEM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and Raman microscopy. Formation of nanoparticles by co-precipitation may induce substantial changes in the film structure leading, in our adopted process, to the appearance of lamellar ordering in the calcination stage. On the contrary, impregnation-based approaches perturb the film structures much more weakly, but are also less efficient in filling the pores with nanoparticles.

Specific interactions of sodium salts with alanine dipeptide and tetrapeptide in water: insights from molecular dynamics.

We examine computationally the dipeptide and tetrapeptide of alanine in pure water and solutions of sodium chloride (NaCl) and iodide (NaI), with salt concentrations up to 3 M. Enhanced sampling of the configuration space is achieved by the replica exchange method. In agreement with other works, we observe preferential sodium interactions with the peptide carbonyl groups, which are enhanced in the NaI solutions due to the increased affinity of the less hydrophilic iodide anion for the peptide methyl side-chains and terminal blocking groups. These interactions have been associated with a decrease in the helicities of more complex peptides. In our simulations, both salts have a small effect on the dipeptide, but consistently stabilize the intramolecular hydrogen-bonding interactions and "α-helical" conformations of the tetrapeptide. This behavior, and an analysis of the intermolecular interaction energies show that ion-peptide interactions, or changes in the peptide hydration due to salts, are not sufficient determining factors of the peptide conformational preferences. Additional simulations suggest that the observed stabilizing effect is not due to the employed force-field, and that it is maintained in short peptides but is reversed in longer peptides. Thus, the peptide conformational preferences are determined by an interplay of energetic and entropic factors, arising from the peptide sequence and length and the composition of the solution.

Influence of nanoreactor environment and substrate location on the activity of horseradish peroxidase in olive oil based water-in-oil microemulsions.

Oxidative enzymatic reactions using horseradish peroxidase (HRP) were carried out in water-in-oil (w/o) microemulsions composed of olive oil/lecithin/1-propanol/water, a model biomimetic system. The substrates used (gallic acid, octyl gallate and 2,2'-azino-bis[3-ethylbenzo-thiazoline-6-sulfonic acid] (ABTS)) have different hydrophobicities and possible locations in the microemulsion system. HRP reactivity with reference to substrate hydrophobicity and structural characteristics of the microemulsions is discussed. The nature of the enzyme microenvironments was examined using dynamic light scattering (DLS), differential scanning calorimetry (DSC) and diffusion NMR (DOSY) methodologies while the location of various enzymatic substrates in the microemulsion phase was assessed by solubility measurements and by taking pressure-area isotherms of mixed monolayers of the substrates with dipalmitoyl-phosphatidylcholine (DPPC), which is a major constituent of lecithin. In contrast to the bulk aqueous phase, in the severely restricted environment of the polar domains of the microemulsion HRP reacted faster with octyl gallate, a substrate that is solubilized at the lipid interfaces. HRP was deactivated in the olive oil microemulsions within a few hours, a phenomenon that has also been observed in other microemulsion systems.

Stabilization of lead sulfide nanoparticles by polyamines in aqueous solutions. A structural study of the dispersions.

Lead sulfide (PbS) nanoparticles have been synthesized in aqueous solutions by a reaction between inorganic lead salts and sodium sulfide and stabilized using the cationic polyelectrolytes branched poly(ethylenimine) (PEI), poly(allylamine hydrochloride) (PAH), and poly(diallyldimethylammonium chloride) (PDDA). The structures of the polyamine-stabilized nanoparticle dispersions were examined in detail using UV-vis spectroscopy, small-angle X-ray scattering (SAXS), static and dynamic electrophoretic mobility measurements, and transmission electron microscopy (TEM). Considerable differences were found between the stabilizing efficiencies of these polyelectrolytes, which cannot be attributed to their charge densities or their persistence lengths. Small monodisperse nanoparticles of PbS with a tight stabilizing shell were consistently found only when PEI was used as a stabilizer even at high pH values, although its charge density is then very low. The excellence of PEI as a stabilizer is mainly due to the extensive branching of the chains and the presence of uncharged secondary and tertiary amine groups, which apparently serve as good anchoring points at the nanoparticle surfaces. None of the polyelectrolytes examined here provide long-term protection of the nanoparticles toward oxidation by air, showing that a need for more complex multipurpose stabilizers exists for aqueous PbS dispersions.

Controlled production of ZnO nanoparticles from zinc glycerolate in a sol-gel silica matrix.

The controlled production of ZnO nanoparticles within an amorphous silica matrix is achieved using a new methodology consisting of four stages. First, precursor zinc glycerolate nanoparticles are produced within reversed micelles of glycerol in heptane stabilized by the surfactant Aerosol-OT (bis-ethylhexyl sodium sulfosuccinate, AOT). The surface of these nanoparticles is then modified by exchanging AOT with bis-trimethoxysilyl-ethane (BTME). The surface-modified nanoparticles are copolymerized with tetramethoxysilane (TMOS) to provide a composite silica material, in which the nanoparticles are apparently dissolved, producing a uniform distribution of zinc in the silica matrix. Finally, the conversion of zinc to ZnO is achieved by heating the material at 700 degrees C, leading to a uniform dispersion of very small (<10 nm) ZnO particles within the amorphous matrix. The fluorescence spectrum of the ZnO particles within the matrix is blue-shifted, as expected from the strong quantum confinement achieved. The properties of the system at all stages in this synthetic process are monitored using TEM, XRD, fluorescence and FT-IR spectroscopy. Glycerol forms complexes with many metal ions, so the present procedure may be generalized to provide uniform distributions of metal ions and subsequently metal oxide nanoparticles in amorphous silica.

Formation mechanism of nanotubes comprising layers of PbS nanoparticles in polymer-surfactant solutions.

The crystallization of PbS in aqueous solutions containing the surfactant sodium dodecyl sulfate (SDS) and hydrophilic polymers resulted in a novel type of metastable nanotubes, the walls of which consist of layers of ordered PbS nanoparticles, apparently separated by layers of surfactant molecules. Information on the mechanism of formation of these structures was obtained by focusing on the roles of the polymer, and of the insoluble lead dodecyl sulfate (Pb(DS)2) present in the system. TEM investigations of the early stages of crystallization revealed the coexistence of PbS and Pb(DS)2 precipitates, the latter being surprisingly important for nanotube formation, and allowed to follow the evolution of layered structures from combination of the two types of crystals. Six different hydrophilic polymers have been used, which interact with SDS with varying strengths. Surprisingly, and in contrast to previous hypotheses, layered nanostructures were observed in all polymer solutions, regardless of the strength of polymer-surfactant interactions. This indicates that, although the presence of a polymer is necessary, polymer-SDS interactions are not a driving force for the formation of the layered structures and nanotubes. On the contrary, the interactions between the polymer chains and the growing particles appear to be of the utmost importance. Results presented here can be interpreted in terms of two alternative mechanisms for layered nanostructure and nanotube formation.

NMR investigation of the interaction of vanadate with carbasilatranes in aqueous solutions.

Reaction of vanadate with carbasilatranes [methoxy{N,N',N' '-2,2',3-[bis(1-methylethanolato)(propyl)]amino}silane (1), methoxy{N,N',N' '-2,2',3-[bis(1-ethanolethanolato)(propyl)]amino}silane (2), and {N,N',N' '-2,2',2-[bis(ethanolato)(glycolpropyl ether)]amino}silane (3)] in aqueous solution results in the formation of vanadosilicates and five-coordinated chelate vanadium(V) complexes as evidenced by 51V, 1H, and 13C NMR spectroscopy. Chiral carbasilatrane S,S-1 was characterized in the solid state by X-ray diffraction, revealing a trigonal bipyramidal geometry around the metal ion, with one unidentate methoxy group and one atrane nitrogen atom at the axial positions and one carbon and two atrane oxygen atoms at the equatorial plane of the bipyramid. Crystal data (Mo Kalpha; 100(2) K) are as follows: orthorhombic space group P2(1)2(1)2(1); a = 8.8751(6), b = 9.7031(7), c = 14.2263(12) A; Z = 4. The complexation of vanadium either with 1 or 2 is stereoselective yielding approximately 94% of the complex containing ligand in the S,R-configuration. The lower ability of the S,S- and R,R-diastereoisomers of 1 and 2 to ligate vanadate was attributed to stereochemical factors, dictating a square pyramidal geometry for the chelated complexes. A dynamic process between the vanadium chelate complexes and the respective carbasilatranes was evaluated by 2D {1H} EXSY NMR spectroscopy. These spectra show that the vanadate complexes with the open carbasilatranes exchange more slowly with the free ligand compared to the respective alcohol aminate complexes.

Attraction of iodide ions by the free water surface, revealed by simulations with a polarizable force field based on Drude oscillators.

Recent theoretical and experimental studies have shown that polarizable anions, such as iodide and bromide, preferentially accumulate close to the surface of electrolyte solutions. This finding is in sharp contrast to the previously prevailing idea that salts are dielectrically excluded from the free water surface and opens up new avenues for research in specific salt effects. In this work, we have verified the ability of a recently introduced polarizable water model, SWM4-DP, to reproduce this behavior, by simulations of a NaI/water slab, corresponding to a 1.2 M solution. The water and ion polarizabilities are modeled by classical Drude oscillator particles. As revealed by the simulations, a double layer is formed close to the free water surface, with the iodide ions located closer to the interface and the sodium ions at a neighboring, interior layer. Near the surface, all solution species acquire an induced dipole moment, that is perpendicular to the surface and points toward the exterior. The double charge layer causes ordering of water at a subsurface region. Simulations with a simpler system of a single iodide ion in a water slab show that the surface position is stabilized by induced charge interactions; in contrast, the charge-dipole interactions between the iodide permanent charge and the water permanent dipole moment favor the bulk position. Thus, the polarizabilities of ion and water are essential for explaining the increased preference of iodide for the air-water interface, in accordance with other studies.

Study of copper sulfide crystallization in PEO-SDS solutions.

The crystallization of copper sulfide in aqueous supersaturated solutions in the presence of the polymer poly(ethylene oxide), PEO, and the surfactant sodium dodecyl sulfate, SDS, was investigated. In these systems, copper sulfide precipitation competes with the reaction between copper cations and dodecyl sulfate anions. The competition of the two reactions may affect the reaction products significantly; therefore it is important to study the properties of the surfactant salt, copper dodecyl sulfate (Cu(DS)2), in detail. The thermodynamic solubility constant of Cu(DS)2 was measured at 8 degrees C and was equal to (2.4 +/- 0.4) x 10(-10) M3. The Krafft point of Cu(DS)2 and its solubility curve (precipitation temperature for a range of concentrations) were also measured. The latter was found to be very close to room temperature. Temperature is thus a very significant parameter in these systems and must be carefully controlled in all experiments. The crystallization of copper sulfide in PEO-SDS solutions was investigated in solutions with compositions above and below the solubility curve. Copper sulfide nanoparticles predominate and are stabilized at temperatures above the solubility curve. Surprisingly, at temperatures below the solubility curve CuxS coexists with Cu(DS)2, which appears in the form of lamellar crystals. The system is further complicated by the presence of at least two different types of copper sulfides corresponding to different oxidation states of copper. Our results suggest that the predominance of Cu(DS)2 at lower temperatures is due to its limited solubility and is modified by the CuI/CuII redox equilibrium in combination with the solution pH.

Speed selection mechanism for propagating fronts in reaction-diffusion systems with multiple fields.

We introduce a speed selection mechanism for front propagation in reaction-diffusion systems with multiple fields. This mechanism applies to pulled and pushed fronts alike, and operates by restricting the fields to large finite intervals in the comoving frames of reference. The unique velocity for which the center of a monotonic solution for a particular field is insensitive to the location of the ends of the finite interval is the velocity that is physically selected for that field, making thus the solution approximately translation invariant. The fronts for the various fields may propagate at different speeds, all of them being determined though through this mechanism. We present analytic results for the case of piecewise parabolic potentials, and numerical results for other cases.