Journal of Colloid And Interface Science (v.318, #1)
Editorial Board (OFC).
A new specifically designed calixarene for the synthesis of functionalized, nanometric and subnanometric Pd, Pt and Ru nanoparticles by Vincent Huc; Katrin Pelzer (1-4).
A new thioester functionalized calixarene derivative is used for the synthesis of metallic Pd, Pt and Ru nanoparticles, exhibiting several interesting features such as stability and remarkable surface functionalization. Crystalline particles of very small dimensions and good dispersion have been obtained.A new calixarene derivative is used for the synthesis of nanometric and subnanometric Pd, Pt and Ru nanoparticles. These nanoparticles are easily purified and exhibit interesting surface functionalities.
Keywords: Calixarenes; Nanoparticles; Surface functionalization;
Immobilization of trivalent actinides by sorption onto quartz and incorporation into siliceous bulk: Investigations by TRLFS by S. Stumpf; Th. Stumpf; J. Lützenkirchen; C. Walther; Th. Fanghänel (5-14).
The adsorption of Cm(III) on quartz is studied by time resolved laser fluorescence spectroscopy (TRLFS) in the pH range from 3.75 to 9.45. The raw spectra are deconvoluted into three single components. The first one has a peak maximum at 593.8 nm and can be attributed to the Cm(III) aquo ion with an emission lifetime of 68 ± 3 μs . The second one corresponds to an adsorbed species and has a peak maximum at 601.4 nm and an emission lifetime of 123 ± 10 μs . The peak maximum of the third component is shifted to higher wavelength (603.6 nm) while the lifetime remains constant. Additionally, the adsorption of Am(III) on quartz is investigated in batch experiments. Based on the spectroscopic data a sorption mechanism is suggested. In addition, the obtained Am uptake data and the Cm-TRLFS data are modeled simultaneously using a single site Basic Stern model in combination with the charge distribution concept of Pauling. The finally suggested model consists of two bidentate surface complexes where the second one is the product of hydrolysis of the first sorption species. In a separate set of experiments the influence of silicic acid at different concentrations on the Cm(III) speciation in a quartz system is investigated by TRLFS. In suspension silicic acid at low concentration ( 3.5 × 10 −4 mol / L ) has no influence on the Cm(III) speciation. At high concentration ( 3.5 × 10 −2 mol / L ) the Cm(III) speciation is definitely influenced. The results at higher concentration indicate the formation of Cm(III)/silicic acid complexes and the incorporation of Cm(III) into siliceous bulk. This is confirmed by measurements at a quartz single crystal surface. Moreover, these measurements indicate the formation of quartz/Cm(III)/silicic acid ternary complexes at the mineral surface.
Keywords: Curium; Quartz; Silicic acid; Surface complexation; Sorption; Ternary complexes; TRLFS; Single crystals; Adsorption model;
The adsorption of cetyltrimethylammonium bromide and propanol mixtures with regard to wettability of polytetrafluoroethylene by Anna Zdziennicka; Bronisław Jańczuk (15-22).
Measurements of contact angles (θ) of aqueous solutions of cetyltrimethylammonium bromide (CTAB) and propanol mixtures at constant CTAB concentration equal to 1 × 10 −5 , 1 × 10 −4 , 6 × 10 −4 and 1 × 10 −3 M on polytetrafluoroethylene (PTFE) were carried out. The obtained results indicate that the wettability of PTFE by aqueous solutions of these mixtures depends on their composition and concentration. They also indicate that, contrary to Zisman, there is no linear relationship between cos θ and the surface tension ( γ LV ), but a linear relationship exists between the adhesional ( γ LV cos θ ) and surface tension of aqueous solutions of CTAB and propanol mixtures. Curve γ LV cos θ vs γ LV has a slope equal −1 suggesting that adsorption of CTAB and propanol mixtures and the orientation of their molecules at aqueous solution–air and PTFE–aqueous solution interfaces is the same. Extrapolating this curve to the value of γ LV cos θ corresponding to θ = 0 , the value of the critical tension of PTFE wetting equal 23.4 mN/m was determined. This value was higher than that obtained from contact angles of n-alkanes on PTFE surface (20.24 mN/m). The difference between the critical surface tension values of wetting probably resulted from the fact that at cos θ = 1 the PTFE–aqueous solution of CTAB and propanol mixture interface tension was not equal to zero. This tension was determined on the basis of the measured contact angles and Young equation. It appeared that the values of PTFE–aqueous solution of the CTAB and propanol mixtures interface tension can be satisfactorily determined by modified Szyszkowski equation only for solutions in which probably CTAB and propanol molecules are present in monomeric form. However, it appeared that using the equation of Miller et al., in which the possibility of aggregation of propanol molecules in the interface layer is taken into account, it is possible to describe the PTFE–solution interfacial tension for all systems studied in the same way as by the Young equation. On the basis of linear dependence between the adhesional and surface tension it was established that the work of adhesion of aqueous solution of CTAB and propanol mixtures does not depend on its composition and concentration, and the average value of this work was equal to 46.85 mJ/m2, which was similar to that obtained for adhesion of aqueous solutions of two cationic surfactants mixtures to PTFE surface.
Keywords: Polytetrafluoroethylene; Cetyltrimethylammonium bromide; Wettability; Surface and interfacial tension;
Controlling the magnetic filaments length by tuning the particle interactions by F. Martínez-Pedrero; M. Tirado-Miranda; A. Schmitt; J. Callejas-Fernández (23-28).
Initially stable samples of monodisperse superparamagnetic particles were aggregated in the presence of an external magnetic field and different amounts of electrolyte. The aggregation process was monitored using dynamic light scattering (DLS). When the magnetic field was turned off, a significant change of the effective diffusion coefficient was observed at all electrolyte concentrations. This jump was interpreted in terms of filament break-up and additional rotational diffusive modes. Therefore, the length of the magnetic filaments (MF) was determined from the measured average diffusion coefficients applying an adequate theoretical approach. The results prove that the MFs disassemble completely at low electrolyte concentrations. At intermediate amounts of electrolyte added, a partial cluster break-up is observed. Only at high salt concentrations, the chains withstand the absence of the magnetic field. The results show that average filament size can be predicted and controlled by tuning the relative strength of the magnetic and electric interactions.
Keywords: Superparamagnetic particles; Aggregation; Light scattering; DLS; Magnetic filaments; Diffusion coefficients; Rupture;
Synthesis and photoluminescence of well-dispersible anatase TiO2 nanoparticles by Jiaoxing Xu; Liping Li; Youjun Yan; Hui Wang; Xuxu Wang; Xianzhi Fu; Guangshe Li (29-34).
High-purity anatase TiO2 nanoparticles were prepared using a low-temperature sol–gel route. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetric analysis, UV–vis spectroscopy, and photoluminescence. It is shown that the as-prepared sample crystallized in a pure anatase phase with an average crystallite size of about 7 nm, and the surfaces were highly hydrated. These nanoparticles were stabilized as a water suspension via the cooperation of DLVO force and surface hydration force. These suspensions showed characteristic band-gap emission at 397 ± 1.5 nm , which is a little red-shifted compared with the band-gap energy of indirect electronic transition measured in the UV–vis absorption spectrum. These observations were explained by the light-induced relaxation of polar water molecules in the surface hydration layer.Surface hydration effect on the stern layers formation process.
Keywords: Anatase; Zeta potentials; Surface hydration; Photoluminescence;
Insight into hydroxides-activated coals: Chemical or physical activation? by J. Alcañiz-Monge; M.J. Illán-Gómez (35-41).
The objective of this paper is to get an insight into the chemical activation mechanism using KOH and NaOH as activated agents. Three coals have been selected as carbon precursors. It was found that KOH and NaOH develop a similar narrow microporosity, independently of the coal rank, whereas only KOH generates supermicroporosity. Temperature-programmed desorption experiments, carried out with impregnated anthracite, show differences on the gas evolved during the activated carbon preparation using the two activating agents. Thus, whereas hydrogen profiles are quite similar for both activated agents, the CO and H2O profiles are different. It is remarkable the high amount of H2O evolved at the maximum treatment temperature for both activating agents. The results obtained allow concluding that the chemical activation is due to a combination of different process driving the development of material porosity.H2 and H2O evolution on the TPD experiment over AK sample.
Keywords: Activated carbon; Chemical activation; Thermal analysis; Microporosity;
The study of controlling pore size on electrospun carbon nanofibers for hydrogen adsorption by Ji Sun Im; Soo-Jin Park; Tae Jin Kim; Young Ho Kim; Young-Seak Lee (42-49).
Polyacrylonitrile (PAN)-based carbon nanofibers (CNFs) were prepared by using electrospinning method and heat treatment to get the media for hydrogen adsorption storage. Potassium hydroxide and zinc chloride activations were conducted to increase specific surface area and pore volume of CNFs. To investigate the relation between pore structure and the capacity of hydrogen adsorption, textural properties of activated CNFs were studied with micropore size distribution, specific surface area, and total pore volume by using BET (Brunauer–Emmett–Teller) surface analyzer apparatus and the capacity of hydrogen adsorption was evaluated by PCT (pressure–composition–temperature) hydrogen adsorption analyzer apparatus with volumetric method. The surface morphology of activated CNFs was observed by SEM (scanning electron microscope) images to investigate the surface change through activation. Even though specific surface area and total pore volume were important factors for increasing the capacity of hydrogen adsorption, the pore volume which has pore width (0.6–0.7 nm) was a much more effective factor than specific surface area and pore volume in PAN-based electrospun activated CNFs.HK pore volume of CNFs: (a) as-received, ZC-W2, ZC-W4, and ZC-W6 (ZnCl2 treated samples); (b) as-received, PH-M4, PH-M6, and PH-M8 (KOH treated samples).
Keywords: Hydrogen adsorption; Carbon fiber; Electrospinning; Chemical activation; Physical activation;
Synthesis and characterization of silicalite-1/carbon-graphite membranes by James O. Titiloye; Islam Hussain (50-58).
Silicalite-1/carbon-graphite composite membranes have been prepared using a standard hydrothermal synthesis method and characterized by XRD, SEM, TGA, BET and permeation experiments. Single gas permeation fluxes and binary mixtures separation and selectivity data are reported for methane, ethane and propane using the composite membranes. Carbon-graphite oxidized for 4 h prior to membrane preparation had the most promising separation properties. The permeation fluxes for the binary mixtures reflect that of the single component flux ratios. At 20 °C the membranes show high separation selectivity toward lighter component in binary mixtures. Single gas permeances for methane and ethane were found to decrease with increasing temperatures while that of propane fluctuates.Separation through a silicalite-1 layer showing a meso/microporous crack.
Keywords: Silicalite-1; Carbon-graphite; Membranes; Permeation; Separation selectivity;
l-Tryptophan transport through a hydrophobic liquid membrane using AOT micelles: Dynamics of the process as revealed by small angle X-ray scattering by Roberto Rinaldi; Pedro L.O. Volpe; Iris L. Torriani (59-67).
Hydrophobic liquid membranes have a high technological potential in many fields of separation science. The dynamics of these systems is very complex and still not fully understood. In this work we studied the effect of the incorporation of cationic and anionic l-tryptophan at pH 1.8 and 10.0, respectively, in Aerosol-OT reverse micelles performing small angle X-ray scattering experiments. The use of a synchrotron radiation source allowed efficient in situ data acquisition. Several insights on l-tryptophan transport dynamics through hydrophobic membranes containing AOT could be obtained from these SAXS experiments, such as amino acid site localization and changes in the reverse micelle sizes.
Keywords: Hydrophobic liquid membrane; Small-angle X-ray scattering; Reverse micelles; Amino acid transport; Aerosol-OT; l-Tryptophan;
Three-dimensional Monte Carlo simulations of internal aggregate structures in a colloidal dispersion composed of rod-like particles with magnetic moment normal to the particle axis by Akira Satoh (68-81).
We have treated a suspension composed of ferromagnetic rod-like particles with a magnetic moment normal to the particle axis in order to investigate aggregation phenomena of such a suspension by means of cluster-moving Monte Carlo simulations. In the present study, we have considered a three-dimensional mono-dispersed model system composed of such rod-like particles. Internal structures of self-assembled clusters have been discussed quantitatively in terms of radial distribution, pair correlation, orientational pair correlation functions, number distributions of clusters, and order functions. The main results obtained here are summarized as follows. Rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction more significantly with magnetic particle–particle interactions. In such raft-like clusters, the direction of each particle axis has a tendency to incline in parallel formation, but is not so parallel as in a two-dimensional dispersion. As the volumetric fraction increases, longer raft-like clusters are formed, but such raft-like clusters do not aggregate further to form thicker clusters, which is in significantly contrast with a dispersion of spherical particles, where thicker chain-like clusters are observed under certain conditions. For the case of strong magnetic particle–particle interactions, sufficiently long raft-like clusters are formed along the magnetic field direction, even if the influence of an external magnetic field is of the same order of that of the thermal energy. However, rod-like particles in such clusters do not necessarily incline in significantly parallel formation along a certain direction. Self-assembled tube-like clusters are formed when magnetic particle–particle interactions are much more dominant than the rotational Brownian motion under circumstances of rod-like particles inclining in a certain direction.Long raft-like clusters are formed in the magnetic field direction (y-axis direction) for a suspension composed of magnetic rod-like particles with a magnetic moment normal to the particle axis. Also, tube-like clusters are observed under certain conditions.
Keywords: Ferromagnetic colloidal dispersion; Aggregation phenomena; Rod-like particle; Raft-like cluster; Radial distribution function; Orientational pair correlation function;
Facile fabrication of large area of aggregated gold nanorods film for efficient surface-enhanced Raman scattering by Yuling Wang; Shaojun Guo; Hongjun Chen; Erkang Wang (82-87).
An effective and facile method for fabrication of large area of aggregated gold nanorods (AuNRs) film was proposed by self-assembly of AuNRs at a toluene/water interface for the first time. It was found that large area of aggregated AuNRs film could be formed at the interface of toluene and water due to the interfacial tension between the two phases. The obtained large area of aggregated AuNRs film exhibits strong surface-enhanced Raman scattering (SERS) activity with 4-aminothiophenol (4-ATP) and 2-aminothiophenol (2-ATP) as the probe molecules based on the strong electromagnetic coupling effect between the very adjacent AuNRs. Enhancement factors (EF) were used to estimate the SERS activity of the aggregated AuNRs film, which is obtained to be 1.7 × 10 5 for 7a vibration of 4-ATP. SERS intensity is compared with AuNRs deposited directly on glass, indicating high SERS activity and reproducibility of the aggregated AuNRs film. In addition, SERS activity has also been successfully demonstrated for dye molecule (Rhodamin 6G (R6G)) and biological small molecule (adenine) on the aggregated AuNRs film, showing great potential of the aggregated AuNRs film as a convenient and powerful SERS substrate for biological tags and biological molecular detection.Effective and facile fabrication of large area of aggregated gold nanorods film was proposed, which exhibits strong SERS activity.
Keywords: SERS; Au nanorods; Self-assembly;
Influence of ligand structure on the stability and oxidation of copper nanoparticles by Petri Kanninen; Christoffer Johans; Juha Merta; Kyösti Kontturi (88-95).
The stability and oxidation of copper nanoparticles stabilized with various ligands have been studied. Lauric acid-capped copper nanoparticles were prepared by a modified Brust–Schiffrin method. Then, ligand exchange with an excess of different capping agents was performed. Oxidation and stability were studied by UV–vis, XRD, and TEM. Alkanethiols and oleic acid were found to improve air stability. The oxidation resistance of thiol-capped copper nanoparticles was found to increase with the chain length of the thiol. However, excess thiol caused etching of the particles under nitrogen. With oleic acid no etching was observed under nitrogen. After oxidation, no traces of the ligand-exchanged particles were found, suggesting their dissolution due to excess ligand. Oleic acid protected the particles against oxidation better than the tested thiols at large excess (ligand–copper ratio 20:1).Lauric acid capped copper nanoparticles are ligand-exchanged with oleic acid and thiols. Oxidation of the nanoparticles is studied by following the Cu(O) plasmon absorbance.
Keywords: Copper nanoparticles; Oxidation; Ligand exchange; Etching; Alkanethiols; Oleic acid; Lauric acid;
Anisotropic self-organization of hybrid silica based xerogels containing bridged positively charged 1,4-diazoniabicycle[2.2.2]octane chloride group by Leliz T. Arenas; Adriana C. Pinheiro; Juliana D. Ferreira; Paolo R. Livotto; Vitor P. Pereira; Márcia R. Gallas; Yoshitaka Gushikem; Tania M.H. Costa; Edilson V. Benvenutti (96-102).
Anisotropic self-organized hybrid silica based xerogels were obtained. The ordered structure was imposed by the double charged 1,4-diazoniabicycle[2.2.2]octane chloride group bonded in a bridged way. This was confirmed by the presence of well defined X-ray diffraction peaks corresponding to an interplanar distance with the same length estimated for the organic bridged groups. The material was characterized by elemental analysis using CHN technique and the chloride ion was analyzed by a potentiometric titration. 13C and 29Si CP MAS solid state NMR spectroscopy and thermogravimetric analysis were also performed. The material that can be obtained in the form of powders and transparent monoliths or films, is thermally stable up to 260 °C and the samples with high organic content presented birefringence properties.
Keywords: Nanostructured material; Organofunctionalyzed silica; Anisotropy; Sol–gel; Dabco;
Characterization of Langmuir–Blodgett organoclay films using X-ray reflectivity and atomic force microscopy by Jaseung Koo; Seongchan Park; Sushil Satija; Aleksey Tikhonov; Jonathan C. Sokolov; Miriam H. Rafailovich; Tadanori Koga (103-109).
Monolayers of organoclay platelets were formed at the air/water interface using the Langmuir technique and were then investigated either by in situ or lifted onto Si wafers and studied ex situ, using X-ray reflectivity (XR) methods. The XR data showed that the surfactant molecules on the clay platelets formed a dense, self-assembled monolayer where the molecules were tilted at an angle of 35 ° ± 6 ° from the normal to the dry clay surface. The surfactant layers only covered a fraction of the clay platelet surface area, where the fractional surface coverage for the three clays studied (C6A, C15A, and C20A) was found to be 0.90, 0.86, and 0.73, respectively. These values were significantly higher than those estimated from the cation exchange capacity (CEC) values. Rather than being uniformly distributed, the surfactant was clustered in patchy regions, indicating that the surface of the clay platelets had both polar and non-polar segments. This heterogeneity confirmed the hypothesis which was previously invoked to explain the distribution of the clay platelets in melt mixed homopolymer and polymer blend nanocomposites.Using the LB method, monolayer films of various commercial montmorillonite clays were formed at the air/water interface, and X-ray reflectivity measurements were performed both at the air/water interface and after lifting the films off the surface onto silicon wafers. These results were further compared with atomic force microscopy results.
Keywords: Organoclay; Langmuir–Blodgett technique; X-ray reflectivity; Cation exchange capacity;
One-pot synthesis of ZnS/polymer composites in supercritical CO2–ethanol solution and their applications in degradation of dyes by Yun Xie; Chengliang Zhang; Shiding Miao; Zhimin Liu; Kunlun Ding; Zhenjiang Miao; Guimin An; Zhenzhong Yang (110-115).
A facile method to decorate the polymeric hollow spheres with ZnS nanoparticles has been presented. In this method, the precursors, Zn(Ac)2⋅H2O and CH3CSNH2, were first adsorbed by the polymer substrate in supercritical CO2–ethanol solution at 35 °C. Followed by heating the mixture at 100 °C for 2 h, ZnS/polymer composites were obtained. The as-produced ZnS/polymer composites were characterized by means of IR spectra, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy. It was demonstrated that crystalline ZnS nanoparticles with size of 3–5 nm were uniformly decorated on the polymer spheres under suitable conditions. The resultant ZnS/polymer composites exhibited high efficiency for degrading eosin B, methyl orange and methylene blue under UV light irradiation.ZnS nanoparticles were successfully decorated on hollow polymer spheres via a supercritical route. The resultant ZnS/polymer composites exhibited high efficiency for degrading methyl orange, methylene blue and eosin B.
Keywords: Polymeric hollow spheres; ZnS; Hybrid; Photocatalysis;
Effect of organic solvents on J aggregation of pseudoisocyanine dye at mica/water interfaces: Morphological transition from three-dimension to two-dimension by Hiroshi Yao; Yoshinobu Morita; Keisaku Kimura (116-123).
Morphological and spectroscopic properties of pseudoisocyanine (PIC) J aggregates produced at mica/solution interfaces have been characterized by absorption/fluorescence spectroscopy, fluorescence microscopy, and atomic force microscopy. Addition of organic solvents (1-propanol (PrOH) or 1,4-dioxane (Dox)) into aqueous solutions of the PIC dye induced a transition of the morphology of the interfacial J aggregates. The characteristic feature of this transition is the thickness (or height) change of the aggregate domain layers from three-dimensions to two-dimensions: The domain area of the J aggregates was dependent on the amount of the organic cosolvent, while the domain thickness was dependent on the type of the cosolvent. In pure aqueous solution, the J aggregates at the mica/water interface had a three-dimensional structure with the height of ∼3 nm (multilayer structure). In mixed solvents of PrOH/water or Dox/water (5 or 10 vol%), the interfacial aggregates became a bilayer or monolayer structure, respectively, assuming that PIC molecules are adsorbed on their molecular plane perpendicular to the mica surface. Meanwhile, optical properties (band width and peak position) of the J band were invariant upon addition of the organic cosolvents, suggesting that molecular packing in the J aggregates is essentially unchanged. These results revealed that spectroscopic properties of the interfacial PIC J aggregates were determined only by the lateral (two-dimensional) interaction within the adsorbed monolayer of PIC molecules on mica, and interlayer interaction in the multilayered J aggregate was consequently small.Addition of organic solvents into aqueous solutions of a pseudoisocyanine (PIC) dye induced a three-dimensional to two-dimensional morphological transition of interfacial J aggregates formed at the mica/water interface.
Keywords: J aggregates; Solid/liquid interface; Morphology; Atomic force microscopy; Fluorescence microscopy;
Homogeneous and heterogeneous binary colloidal clusters formed by evaporation-induced self-assembly inside droplets by Young-Sang Cho; Gi-Ra Yi; Shin-Hyun Kim; Mark T. Elsesser; Dana R. Breed; Seung-Man Yang (124-133).
In this paper, we report the preparation of binary clusters of colloidal particles with different sizes or species into complex structures using oil-in-water emulsion droplets as confining geometries. First, polystyrene or silica particles with bimodal size distribution were packed densely by evaporation-induced self-assembly inside oil-in-water emulsion droplets. The configurations of larger particles inside the droplets minimize the second moment of the particle locations for the ratio of large to small particle sizes less than 3. Also, the configurations of bimodal clusters were predicted by using a surface evolver simulation, and the simulation predictions were compared with the experimental results. In addition, heterogeneous colloidal clusters were produced by emulsifying the binary mixture suspension of polystyrene and silica particles in aqueous medium followed by evaporating the oil phase. A density gradient centrifugation was applied to fractionate the asymmetric binary dimers comprised of PS and silica microspheres.We present the fabrication homogeneous or heterogeneous clusters. Surface evolver simulation was performed to predict the structures of the clusters. Density gradient centrifugation was applied to fractionate hetero-dimers.
Keywords: Colloidal clusters; Emulsions; Surface evolver; Binary dimers;
Wicking in twisted yarns by Tao Liu; Ka-fai Choi; Yi Li (134-139).
This paper investigated vertical wicking in twisted yarns. A mathematical model was developed based on a capillary penetration mechanism. By using a macroscopic force balance approach, the wicking time was derived as a function of the capillary rise of the liquid. Packing of fibers in the yarn was assumed to be uniform. In order to validate our model, a series of experiments was conducted on polyester yarns. The results showed a good agreement between the experimental data and the theoretical predictions. The influence of twist level of the yarn on the capillary flow was also investigated.Wicking behavior of a liquid in a twisted yarn can be described by the theory developed in this paper.
Keywords: Wicking; Twisted yarn; Capillary penetration mechanism;
Reagentless functionalization of gold nanoparticles via a 3 + 2 Huisgen cycloaddition by Walrati Limapichat; Amit Basu (140-144).
A mild method for functionalization of gold nanoparticles is reported. The reactions of azide functionalized nanoparticles with propynoic acid derivatives provide triazole functionalized nanoparticles under very mild reaction conditions. Characterization of the nanoparticle-bound triazoles using 1H and 13C NMR spectroscopy indicates that both the 1,4 and 1,5 triazole regioisomers are formed on the nanoparticle surface.
Keywords: Click; Triazole; Azide; Alkyne; Regiochemistry; Monolayer; NMR;