Efficient method for in situ agitation of liquids directly inside NMR spectrometer.

The objective of the method is to allow agitation and fast homogenization of liquid systems in NMR tubes, directly inside the NMR spectrometer. The setup makes it possible to record spectra of samples that are macroscopically not stable, as dispersions of large particles. It makes also possible to fasten the homogeneization of liquid during a reaction or a phase transition. In the present paper, the method has been evaluated using homogeneous liquid extraction (HLLE). This configuration can also be used to introduce gases in different systems to perform various types of experiments. The set up consists in a Teflon tube inserted in the NMR tube bringing gas that yields agitation by bubbling. The gas flow is tuned using an electronically operated valve connected to gas line and to the NMR console. The method details how to reach proper homogenization without any perturbation, as liquid leaks.•An easy method for agitation of liquids inside NMR spectrometers.•The set up can be used for the insertion of gases in the NMR tube inside the spectrometer.•The method allows the study of the mixing of biphasic systems by NMR techniques.

Structure-Property-Function Relationships of Iron Oxide Multicore Nanoflowers in Magnetic Hyperthermia and Photothermia.

Magnetite and maghemite multicore nanoflowers (NFs) synthesized using the modified polyol-mediated routes are to date among the most effective nanoheaters in magnetic hyperthermia (MHT). Recently, magnetite NFs have also shown high photothermal (PT) performances in the most desired second near-infrared (NIR-II) biological window, making them attractive in the field of nanoparticle-activated thermal therapies. However, what makes magnetic NFs efficient heating agents in both modalities still remains an open question. In this work, we investigate the role of many parameters of the polyol synthesis on the final NFs' size, shape, chemical composition, number of cores, and crystallinity. These nanofeatures are later correlated to the magnetic, optical, and electronic properties of the NFs as well as their collective macroscopic thermal properties in MHT and PT to find relationships between their structure, properties, and function. We evidence the critical role of iron(III) and heating ramps on the elaboration of well-defined NFs with a high number of multicores. While MHT efficiency is found to be proportional to the average number of magnetic cores within the assemblies, the optical responses of the NFs and their collective photothermal properties depend directly on the mean volume of the NFs (as supported by optical cross sections numerical simulations) and strongly on the structural disorder in the NFs, rather than the stoichiometry. The concentration of defects in the nanostructures, evaluated by photoluminescence and Urbach energy (EU), evidence a switch in the optical behavior for a limit value of EU = 0.4 eV where a discontinuous transition from high to poor PT efficiency is also observed.

Connecting Rheological Properties and Molecular Dynamics of Egg-Tempera Paints based on Egg Yolk.

Egg-tempera painting is a pictorial technique widely used in the Middle Ages, although poorly studied in its physico-chemical aspects until now. Here we show how NMR relaxometry and rheology can be combined to probe egg-tempera paints and shed new light on their structure and behavior. Based on recipes of the 15th century, model formulations with egg yolk and green earth have been reproduced to characterize the physicochemical properties of this paint at the mesoscopic and macroscopic scales. The rheological measurements highlight a synergetic effect between green earth and egg yolk, induced by the interactions between them and the structural organisation of the system. 1 H NMR relaxometry emphasizes the presence and the structure of a network formed by the yolk and the pigment.

Ion influence on surface water dynamics and proton exchange at protein surfaces - A unified model for transverse and longitudinal NMR relaxation dispersion.

In all biologically relevant media, proteins interact in the presence of surrounding ions, and such interactions are water-mediated. Water molecules play a crucial role in the restructuring of proteins in solution and indeed in their biological activity. Surface water dynamics and proton exchange at protein surfaces is investigated here using NMR relaxometry, for two well-known globular proteins, lysozyme and bovine serum albumin, with particular attention to the role of surface ions. We present a unified model of surface water dynamics and proton exchange, accounting simultaneously for the observed longitudinal and transverse relaxation rates. The most notable effect of salt (0.1 M) concerns the slow surface water dynamics, related to rare water molecules embedded in energy wells on the protein surface. This response is protein-specific. On the other hand, the proton exchange time between labile protein-protons and water-protons at the protein surface seems to be very similar for the two proteins and is insensitive to the addition of salts at the concentration studied.

Multiscale Water Dynamics on Protein Surfaces: Protein-Specific Response to Surface Ions.

Proteins function in crowded aqueous environments, interacting with a diverse range of compounds, and among them, dissolved ions. These interactions are water-mediated. In the present study, we combine field-dependent NMR relaxation (NMRD) and theory to probe water dynamics on the surface of proteins in concentrated aqueous solutions of hen egg-white lysozyme (LZM) and bovine serum albumin (BSA). The experiments reveal that the presence of salts (NaCl or NaI) leads to an opposite ion-specific response for the two proteins: an addition of salt to LZM solutions increases water relaxation rates with respect to the salt-free case, while for BSA solutions, a decrease is observed. The magnitude of the change depends on the ion identity. The developed model accounts for the non-Lorentzian shape of the NMRD profiles and reproduces the experimental data over four decades in Larmor frequency (10 kHz to 110 MHz). It is applicable up to high protein concentrations. The model incorporates the observed ion-specific effects via changes in the protein surface roughness, represented by the surface fractal dimension, and the accompanying changes in the surface water residence times. The response is protein-specific, linked to geometrical aspects of the individual protein surfaces, and goes beyond protein-independent Hofmeister-style ordering of ions.

Green earth pigments dispersions: Water dynamics at the interfaces.

HYPOTHESIS: The objective is to elucidate the multiscale dynamics of water within natural mixtures of minerals, green earth pigments that are mainly composed of phyllosilicates containing large amount of iron. In particular, the interaction of water with the different kinds of surfaces has to be probed. One issue is to examine the influence of surface type, basal or edge, on the dispersion quality. EXPERIMENT: The study was carried out using 1H variable field NMR relaxometry on various green earth pigment dispersions and concentrations. To analyse the data, a new analytical model was developed for natural phyllosilicates containing large amount of paramagnetic centres. FINDING: The proposed theoretical framework is able to fit the experimental data for various samples using few parameters. It allows to determining water diffusion and residence times in complex phyllosilicate dispersions. Furthermore, it makes it possible to differentiate the contribution of the basal and edge surfaces and their respective surface area in interaction with water. Moreover, NMR relaxation profile reveals to be highly sensitive to the structural aspect of the phyllosilicates and to the accessibility of water to iron, hence allowing to discriminate clearly between two very similar phyllosilicates (glauconite and celadonite) that are difficult to distinguish by standard structural methods.

Green Earth pigments aqueous dispersions: NMR relaxation rates dataset.

The data presented here are related to the research paper entitled "Green Earth pigments dispersions: water dynamics at the interfaces". The nuclear magnetic resonance (NMR) relaxometry data are provided for various aqueous Green Earth (GE) pigments dispersions with volume fraction spanning approximately from 0.1 to 0.5. For two of them (Cyprus GE and Bohemian GE), the NMR relaxation profiles from 10 kHz to 30 MHz (1H frequency) is given for several temperatures spanning from 293 to 318K. In addition, the X-ray diffraction pattern is provided for France GE (Kremer pigments) for the identification of the main mineral component. The nitrogen gas isotherms are provided for Cyprus GE and Bohemian GE.

Specificity of Counterion Binding to a Conjugated Polyelectrolyte: A Combined Molecular Dynamics and NOESY Investigation.

Poly(thiophen-3-ylacetic acid) (PTAA) is a representative of conjugated polyelectrolytes which are used in many optoelectronics devices. The performance of these devices is affected by the polymer conformation, which, among others, depends on the nature of the counterion. In this study, the binding of tetrabutylammonium counterions (TBA+) on PTAA was determined using a combination of nuclear Overhauser effect spectroscopy (NOESY) and molecular dynamics (MD) simulation. It was found that TBA+ ions specifically bind on the hydrophobic main chain of PTAA, while, according to MD simulations, alkali counterions predominantly bind in the vicinity of negatively charged carboxylic groups located on side chains. The MD trajectories were used to compute the relaxation matrices and the NOESY spectra. With the help of these latter calculations, the changes of intensities in experimental NOESY spectra upon binding of TBA+ ions to PTAA were interpreted.

Coarse-Grained Models of Aqueous Solutions of Polyelectrolytes: Significance of Explicit Charges.

The structure of polyelectrolytes is highly sensitive to small changes in interactions between their monomers. In particular, interactions mediated by counterions play a significant role and are affected by both specific molecular effects and generic concentration effects. The ability of coarse-grained models to reproduce the structural properties of an atomic model is thus a challenging task. Our present study compares the ability of different kinds of coarse-grained models: (i) to reproduce the structure of an atomistic model of a polyelectrolyte (the sodium polyacrylate) and (ii) to reproduce the variations of this structure with the number of monomers and with the concentration of different species. We show that adequate scalings of the gyration radius of the polymer Rg with the number of monomers N and with the box size Lbox are only obtained, first, if the monomer charges and the counterions are explicitly described and, second, if an attractive Lennard-Jones contribution is added to the interaction between distant monomers. Also, we show that implicit ion models are relevant only to the high electrostatic screening regime.

Ion-specificity and surface water dynamics in protein solutions.

Ion-specific effects at the protein surface are investigated here in light of the changes they infer to surface water dynamics, as observed by 1H NMR relaxation (at 20 MHz). Two well-known proteins, hen egg-white lysozyme (LZM) and bovine serum albumin (BSA), show qualitatively opposite trends in the transverse relaxation rate, R2(1H), along a series of different monovalent salt anions in the solution. Presence of salt ions increases R2(1H) in the case of lysozyme and diminishes it in the case of BSA. The effect magnifies for larger and more polarizable ions. The same contrasting effect between the two proteins is observed for protein-solvent proton exchange. This hints at subtle effects ion-binding might have on the accessibility of water surface sites on the protein. We suggest that the combination of the density of surface charge residues and surface roughness, at the atomic scale, dictates the response to the presence of salt ions and is proper to each protein. Further, a dramatic increase in R2(1H) is found to correlate closely with the formation of protein aggregates. The same ordering of salts in their ability to aggregate lysozyme, as seen previously by cloud point measurements, is reproduced here by R2(1H). 1H NMR relaxation data is supplemented by 35Cl and 14N NMR relaxation for selected salt ions to probe the ion-binding itself.

Thermoelectricity and Thermodiffusion in Magnetic Nanofluids: Entropic Analysis.

An analytical model describing the thermoelectric potential production in magnetic nanofluids (dispersions of magnetic and charged colloidal particles in liquid media) is presented. The two major entropy sources, the thermogalvanic and thermodiffusion processes are considered. The thermodiffusion term is described in terms of three physical parameters; the diffusion coefficient, the Eastman entropy of transfer and the electrophoretic charge number of colloidal particles, which all depend on the particle concentration and the applied magnetic field strength and direction. The results are combined with well-known formulation of thermoelectric potential in thermogalvanic cells and compared to the recent observation of Seebeck coefficient enhancement/diminution in magnetic nanofluids in polar media.

Spectral induced polarization of Na-montmorillonite dispersions.

Montmorillonite (Mt) clays have a high specific surface area and surface charge, which confer them remarkable adsorption properties. Nevertheless, their electrochemical and aggregation behavior are not completely elucidated because of the complexity of their microstructural and interfacial properties. In this work, the conductive and dispersive properties of Na-Mt suspensions of weight fractions 0.5-5.2% were investigated for the first time using the spectral induced polarization method. A four-electrode system was used to reduce errors introduced by electrode polarization and contact resistances. Complex conductivity spectra in the low-frequency range of 0.1Hz to 45kHz were successfully described using a triple layer model of the basal surface of Mt and a complex conductivity model that considers conduction of the diffuse layer and polarization of the Stern layer. Aggregate size distributions were inferred from inverted relaxation time distributions. We found that the negative and permanent surface charge of the basal plane of Na-Mt controls its quadrature (imaginary) conductivity, which is not very sensitive to pH and salinity (NaCl) in the 100Hz to 45kHz frequency range. For lower frequencies, the sudden increase of the quadrature conductivity at the highest salinities was explained by considering coagulation of Na-Mt particles.

Influence of counterions on the conformation of conjugated polyelectrolytes: the case of poly(thiophen-3-ylacetic acid).

The addition of simple salt to a solution of conjugated polyelectrolyte can lead to substantial changes in its optical properties caused by the conformational change of the polymer chain. The effect of the addition of alkali metal and tetraalkylammonium chlorides to solutions of lithium salt of poly(thiophen-3-ylacetic acid) is investigated by NMR. The fractions of free alkali metal counterions are in agreement with predictions of the cylindrical Poisson-Boltzmann cell model. On the other hand, the fractions of free tetraalkylammonium counterions deviate from the prediction of this model and diminish with increasing size of these counterions. This trend is consistent with observed ultraviolet-visible absorption spectra and measured self-diffusion coefficients of the polyion in mixtures with tetraalkylammonium salts. A transition to more constricted conformation of the polyion chain becomes more pronounced with the lengthening of alkyl groups in the added tetraalkylammonium cation. Taking into account the obtained fractions of free counterions, existing thermodynamic data are reanalysed in order to determine thermodynamic parameters for binding of different counterions to the polyion. This analysis shows that standard enthalpies of binding of alkali metal counterions are distinctively different, which is most probably related to differences in hydration shells of counterions. On the other hand, such an analysis fails in the case of tetraalkylammonium chlorides where obviously more complex changes take place.

Ion association in aqueous and non-aqueous solutions probed by diffusion and electrophoretic NMR.

The results of diffusion and electrophoretic NMR (eNMR) measurements are reported for a series of tetramethylammonium (TMA) electrolytes (with sulphate, fluoride, acetate, chloride, bromide, nitrate, iodide and perchlorate as anions) in deuterated solvents such as water, dimethylsulphoxide (DMSO), acetonitrile, methanol and ethanol. In addition, similar data are presented for aqueous solutions of tetraalkylammonium salts with increasing alkyl chain length. The combination of diffusion NMR and eNMR yields the effective charge for the TMA cation. Relative to the nominal charge of znom = 1 of TMA, the effective charge in the different solvents is found to be progressively smaller in the order water > DMSO > methanol > acetonitrile > ethanol. A part of this observed trend is ascribed to regular ion-ion interactions incorporated in the Onsager limiting law. Indeed, in solvents with high dielectric constants such as water, DMSO and methanol, the Onsager limiting law describes well the observations for all tetraalkylammonium ions. For ethanol and acetonitrile, there is a significant difference between the experimental data and the expected limiting-law behavior that is attributed to ion association (ion pairing) not taken into consideration by the Onsager limiting law.

Aqueous solutions of tetraalkylammonium halides: ion hydration, dynamics and ion-ion interactions in light of steric effects.

Molecular simulations have allowed us to probe the atomic details of aqueous solutions of tetramethylammonium (TMA) and tetrabutylammonium (TBA) bromide, across a wide range of concentrations (0.5 to 3-4 molal). We highlight the space-filling (TMA(+)) versus penetrable (TBA(+)) nature of these polyatomic cations and its consequence for ion hydration, ion dynamics and ion-ion interactions. A well-established hydration is seen for both TMA(+) and TBA(+) throughout the concentration range studied. A clear penetration of water molecules, as well as counterions, between the hydrocarbon arms of TBA(+), which remain in an extended configuration, is seen. Global rotation of individual TBA(+) points towards isolated rather than aggregated ions (from dilute up to 1 m concentration). Only for highly concentrated solutions, in which inter-penetration of adjacent TBA(+)s cannot be avoided, does the rotational time increase dramatically. From both structural and dynamic data we conclude that there is absence of hydrophobicity-driven cation-cation aggregation in both TMABr and TBABr solutions studied. The link between these real systems and the theoretical predictions for spherical hydrophobic solutes of varying size does not seem straightforward.

Salicylate isomer-specific effect on the micellization of dodecyltrimethylammonium chloride: large effects from small changes.

Specific effects of the sodium salts of m- and p-hydroxybenzoates (m-HB and p-HB) on the aggregation process of dodecyltrimethylammonium chloride have been investigated by isothermal titration calorimetry, electrical conductivity, and (1)H NMR and compared with already reported data for the sodium salt of o-hydroxybenzoate (o-HB). For p-HB, it has been found that the aggregate is only formed by spherical micelles at all p-HB concentrations. On the other side, the situation is more complex for o-HB, where two distinct states of aggregation can be involved, depending on the concentration of o-HB. At high salt concentration, rodlike micelles are formed, whereas at lower concentration spherical aggregates are predominant. The transition from the cylinder to the sphere increases the mobility of the surfactant because the core of the rodlike micelles is more closely packed due to the expulsion of water from the interior of the aggregate. m-HB exhibits an intermediate behavior between these two extreme situations. The effect of the position of hydrophilic substituents on the aromatic ring on the insertion of the hydroxybenzoate anion in the micellar aggregate has been discussed.

Nanoporous photocathode and photoanode made by multilayer assembly of quantum dots.

We perform in this paper a kinetic study of the photoelectrochemical responses of nanoporous thin photoactive films. The films were fabricated by by a layer-by-layer assembly of positively charged polyelectrolytes (poly-l-Lysine, pLys) and negatively charged semiconductor nanoparticles (NPs) on a carboxylic acid terminated alkanethiol-modified gold electrode. Two types of NPs were used to build uniform films: cadmium selenide (CdSe) and cadmium selenide/cadmium sulfide core/shell (CdSe@CdS). Large photocathodic and photoanodic currents were recorded for CdSe and CdSe@CdS sensitized films, respectively. A theoretical model of the photocurrent responses was developed to analyze the kinetics of photoinduced processes and coupled reactions, showing that the multilayer films behave as homogeneous nanoporous semiconducting photoelectrodes.

What tunes the structural anisotropy of magnetic fluids under a magnetic field?

In the present study, the structure of monophasic ionic magnetic fluids under a static magnetic field is explored. In these aqueous electrostatically stabilized ferrofluids, we vary both the isotropic interparticle interactions and the anisotropic dipolar magnetic interaction by tuning the ionic strength and the size of the nanoparticles. Small angle neutron scattering measurements carried out on nanoparticles dispersed in light water exhibit miscellaneous 2D nuclear patterns under a magnetic field with various q-dependent anisotropies. In this nondeuterated solvent where the magnetic scattering is negligible, this anisotropy originates from an anisotropy of the structure of the dispersions. Both the low q region and the peak of the structure factor can be anisotropic. On the scale of the interparticle distance, the structure is better defined in the direction perpendicular to the field. In the thermodynamic limit (q-->0), the model previously described in ref 10 matches the data without any fitting parameters: the interparticle interaction is more repulsive in the direction parallel to the magnetic field. At low q, the amplitude of the anisotropy of the pattern is governed by the ratio of two interaction parameters: the reduced parameter of the anisotropic magnetic dipolar interaction, gamma/Phi, over the isotropic interaction parameter, , in zero field, which is proportional to the second virial coefficient.

Liquid-liquid phase-transfer of magnetic nanoparticles in organic solvents.

We report a novel route for the preparation of well-defined colloidal dispersions of magnetic nanoparticles stabilized by steric repulsion in organic solvents. The usual methods standardly lead to the surfaction of multiparticle aggregates, incompatible with our long-term aim of studying and modeling the influence of magnetic dipolar interactions in colloidal dispersions which are free of aggregates, all other interactions being perfectly defined. A new and reproducible method based on a surfactant-mediated liquid-liquid phase transfer of individually dispersed gamma-Fe(2)O(3) nanoparticles from an aqueous colloidal dispersion to an organic phase is developed. The choice of the reagent and the preparation techniques is discussed. Among several solvent/surfactant pairs, the cyclohexane/dimethyldidodecylammonium bromide (DDAB) system is found to fulfill the colloidal stability criterion: aggregation does not appear, even upon aging. A complete transfer of isolated particles is observed above a threshold in DDAB concentration. The nanoparticle surface is then fully covered with adsorbed DDAB molecules, each surfactant head occupying a surface of 0.57+/-0.05 nm(2). The volume fraction of the cyclohexane-based organosols is easily tunable up to a volume fraction of 12% by modifying the volume ratio of the organic and of the aqueous phases during the liquid-liquid phase transfer.