Microviscosity in modified Nafion membranes measured by fluorescence probes

Sodium or quaternary ammonium modified Nafion membranes in forms of Nafion-Na+,Nafion-NMe4+,Nafion-NEt4+ and Nafion-NBu4+ have been prepared by neutralizing Nafion-H+ membrane with NaOH or tetraalkylammonium hydroxide aqueous solution.Intramolecular excimer formation and fluorescence polarization methods using l,3-di(1-pyrenyl)propane and acridine orange as probes respectively have been employed for measurements of the microviscosity within those modified Nafion membranes.Results show that the probes are located in the fluorocarbon/water interface in the cluster of the membranes and the microviscosities around the probe molecules are in the range of ca.120-1200 cp and increase in the order of Nafion-Na+相似文献   

Regulation of transport in artificial membranes by environmental hydrogen-ion concentration

Transport phenomena in biological membranes and in biocatalysis (immobilized enzymes) are influenced by the structure of the matrix. This structure is not invariable but may depend on parameters of adjacent solutions. Weakly acidic ion-exchange membranes are useful tools for modelling ionic influences on such charged matrices. The pH dependence of tracer permeabilities of [³H] water and [¹⁴C] non-electrolytes was determined on polymethacrylic acid/polyhydroxyethylmethacrylate membranes as a function of their cross-linking and hydrophobicity. At lower buffer concentrations, the non-electrolyte permeabilities increase between 20 and 500% if the pH is raised from 6.5 to 7.5. The relative increase depends on membrane properties and the molecular weights of the permeants. Water permeabilities were measured between pH 5 and 8, the whole interval of pH controllability of the ion-exchange membranes used. The results are similar to those for the [¹⁴C] non-electrolytes. This control of permeability experimentally established by pH-variation is consistent with assumptions about the influence of the kind of counter ions (H⁺, Me⁺, Me²⁺) on the degree of dissociation of carboxyl groups within the membrane. Thus the stage of dissociation is responsible for the structure and transport properties of weak ion exchangers.     

Probing lipids in biological membranes using SERS

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Mechanisms behind concentration profiles illustrated by charge and concentration distributions around ions in double layers

The mechanisms behind the behaviour of concentration and charge density profiles in diffuse electric double layers are investigated quantitatively for 1:1 and 2:2 electrolytes. This is done by analysing various contributions to the mean force that acts on each ion. The forces are obtained from the calculated ionic charge and concentration distributions around individual ions at various positions in the double layer. These distributions are presented graphically which allows an immediate visual illustration of the mechanisms in action. Some features studied are charge inversion in double layers for divalent aqueous electrolytes, overcompensation of surface charge due to large amounts of physisorbed counterions, ion size effects in the double layer structure and various mechanisms that cause deviations from the predictions of the Poisson–Boltzmann approximation. A major objective of the paper is to present the results in a visual form and explain aspects of modern double layer theory in a simple manner.     

Counterion-mediated pattern formation in membranes containing anionic lipids

Most lipid components of cell membranes are either neutral, like cholesterol, or zwitterionic, like phosphatidylcholine and sphingomyelin. Very few lipids, such as sphingosine, are cationic at physiological pH. These generally interact only transiently with the lipid bilayer, and their synthetic analogs are often designed to destabilize the membrane for drug or DNA delivery. However, anionic lipids are common in both eukaryotic and prokaryotic cell membranes. The net charge per anionic phospholipid ranges from − 1 for the most abundant anionic lipids such as phosphatidylserine, to near − 7 for phosphatidylinositol 3,4,5 trisphosphate, although the effective charge depends on many environmental factors. Anionic phospholipids and other negatively charged lipids such as lipopolysaccharides are not randomly distributed in the lipid bilayer, but are highly restricted to specific leaflets of the bilayer and to regions near transmembrane proteins or other organized structures within the plane of the membrane. This review highlights some recent evidence that counterions, in the form of monovalent or divalent metal ions, polyamines, or cationic protein domains, have a large influence on the lateral distribution of anionic lipids within the membrane, and that lateral demixing of anionic lipids has effects on membrane curvature and protein function that are important for biological control.     

Objective comparison of concentration profiles in solids by means of mathematical statistics

To compare concentration profiles in solids in an objective manner, a multi-dimensional generalization of the comparison of two means by Student'st-test is proposed. The approach is based on piecewise modelling the profiles using rougha priori information, estimating the model parameters and their covariance matrix and comparing the estimated parameters by use of multidimensional test variables. The piecewise modelling is treated in the most important cases in such a way that the partial functions are straight lines or cubic polynoms (splines) with continuity at all knots in each case. It enables us to compare whole profiles as well as the most relevant parts of them. Two ways of estimating the covariance matrix as a generalization of the variance estimation are discussed. The approach is useful for any kind of line profiles if rough information on the profile type is available. As an example SIMS depth-profiles of aluminum obtained from thin-layer systems after different technological heat treatments are considered. These profiles are modeled by three straight lines.     

Nitrogen concentration profiles in oxy-nitrided high-speed steel

Nuclear microanalysis has been applied for the determination of in-depth concentration profiles of nitrogen in oxy-nitrided high-speed steel. The concentration profiles were deduced from measurements of the nitrogen content, determined by means of the¹⁴N(d, α)¹²C reaction, for the set of initially identical samples after the removal of surface layers of sequentially increasing thicknesses. Strong blocking of the nitrogen diffusion due to the presence of oxygen has been observed.     

Organization and dynamics of NBD-labeled lipids in membranes analyzed by fluorescence recovery after photobleaching

Lateral diffusion of membrane constituents plays an important role in membrane organization and represents a central theme in current models describing the structure and function of biological membranes. Fluorescence recovery after photobleaching (FRAP) is a widely used approach that provides information regarding dynamic properties and spatial distribution of membrane constituents. On the basis of the unique concentration-dependent fluorescence emission properties of a fluorescently labeled cholesterol analogue modified at the tail region, 25-[N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol (25-NBD-cholesterol), we have previously shown that it exhibits local organization even at very low concentrations in membranes. In this paper, we address aspects regarding the molecular size and dynamics of such an organized assembly of 25-NBD-cholesterol by monitoring its lateral diffusion characteristics using FRAP. To obtain a comprehensive understanding of the organization and dynamics of 25-NBD-cholesterol in the membrane, we compare its diffusion properties to that of a fluorescent phospholipid analogue 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-(1,3-benzoxadiazol-4-yl)) (NBD-PE). Our results indicate significant differences in the membrane dynamics of these NBD-labeled lipids. Importantly, on the basis of a novel wavelength-selective FRAP approach, our results show that the organization of 25-NBD-cholesterol is heterogeneous, with the presence of fast- and slow-diffusing species which could correspond to predominant populations of monomers and dimers of 25-NBD-cholesterol. The potential application of the wavelength-selective FRAP approach to monitor the organization and dynamics of molecules in membranes therefore represents an exciting possibility.     

Influence of galvanostatic polarisation on evolution of ion concentration profiles in ion-selective membranes,studied by means of laser ablation inductively coupled plasma mass spectrometry

Ion concentration profiles and their evolution in galvanostatically polarized solvent polymeric poly(vinyl chloride) based Pb²⁺-selective membranes were visualized using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).Galvanostatic polarisation of the membrane exerts different effects depending on the lipophilic salt cation present in the membrane. For the membrane with transferable Na⁺ cations parallel effects of spontaneous extraction and forced Pb²⁺ ions incorporation were observed. However, for the membrane with non-transferable organic cations, galvanostatically forced evolution of Pb²⁺ concentration profiles dominates over spontaneous extraction and it can be described by linear diffusion equations related to constant flux across the membrane/solution interface.     

Water dynamics in ionomer membranes by field-cycling NMR relaxometry

The dynamic behavior of water within two types of ionomer membranes, Nafion and sulfonated polyimide, has been investigated by field-cycling nuclear magnetic relaxation. This technique, applied to materials prepared at different hydration levels, allows the proton motion on a time scale of microseconds to be probed. The NMR longitudinal relaxation rate R(1) measured over three decades of Larmor angular frequencies omega is particularly sensitive to the host-water interactions and thus well-suited to study fluid dynamics in restricted geometries. In the polyimide membranes, we have observed a strong dispersion of R(1)(omega) following closely a 1/square root omega law in a low-frequency range (correlation times from 0.1 to 10 micros). This is indicative of a strong interaction of water with "interfacial" hydrophilic groups of the polymeric matrix (wetting situation). Variations of the relaxation rates with water uptake reveal a two-step hydration process: solvation and formation of disconnected aqueous clusters near polar groups, followed by the formation of a continuous hydrogen bond network. On the contrary, in the Nafion we observed weak variations of R(1)(omega) at low frequencies. This is typical of a nonwetting behavior. At early hydration stages, R(1)(omega) evolves logarithmically, suggesting a confined bidimensional diffusion of protons in the microsecond time range. Such an evolution is lost at higher swelling where a plateau related to three-dimensional diffusion is observed.     

Multivariate comparison of concentration profiles in materials analysis

Hierarchical cluster analysis (HCA) and principal components analysis (PCA) were applied to find groups between similar depth-profiles in thin-layers investigated by Rutherford backscattering spectrometry (RBS).HCA yields in one run an objective hierarchy of similarity for several profiles. Among the similarity coefficients examined the linear measure, the Euclidean distance and the exponential measure respond with different sensitivity to overall shifts in direction of the concentration axis, whereas the correlation measure relates to parallelism of the profiles.For agglomerative HCA with Euclidean distance, a lowest significant linkage level has been defined by use of Fisher'sF-test. For divisive HCA based also on Euclidean distance, the maximum of a separating function marks the most separating cluster step. The outcomes of both proposals agree for the data set investigated.PCA is useful for verifying the results of HCA and yields additional information about the data structure. In the actual example quite different positions of features (concentrations at definite depths) in the space of the two first principal components hint at peculiarities during the metallurgical coating process.     

Determination of concentration profiles from diffusion experiments in liquid monotectic alloys by AAS

To study the transport behaviour in monotectic melts diffusion experiments have been carried out in the liquid homogeneous phase of aluminium-indium alloys. The penetration curves from mass transport experiments in capillaries with a ratio length/diameter >25 have been measured by Atomic Absorption Spectroscopy (AAS). The optimized quantitative measurement procedure for Al and In, the working parameters in the concentration range 0–50 mg/l and the observed interferences are described. By exact adjusting of sample and standard solutions, the concentrations of both elements have been determined with relative standard deviations less than 1%.     

Determining concentration depth profiles in fluorinated networks by means of electric force microscopy

By means of electric force microscopy, composition depth profiles were measured with nanometric resolution for a series of fluorinated networks. By mapping the dielectric permittivity along a line going from the surface to the bulk, we were able to experimentally access to the fluorine concentration profile. Obtained data show composition gradient lengths ranging from 30 nm to 80 nm in the near surface area for samples containing from 0.5 to 5 wt. % F, respectively. In contrast, no gradients of concentration were detected in bulk. This method has several advantages over other techniques because it allows profiling directly on a sectional cut of the sample. By combining the obtained results with x-ray photoelectron spectroscopy measurements, we were also able to quantify F/C ratio as a function of depth with nanoscale resolution.     

Polydispersity effects on diffusion in polymers: Concentration profiles of d-polystyrene measured by forward recoil spectrometry

Forward recoil spectrometry is shown to be a useful technique for measuring diffusion of d-polymer chains in h-polymer melts. Concentration profiles of a deuterated diffusing species may be determined with a depth resolution of 80 nm and a sensitivity of 0.1 vol % d-polymer in h-polymer. Consequently diffusion coefficients as small as 10^?16 cm²/s can be readily measured. If polymer chains diffuse by a reptation mechanism, the concentration profile ø(x) of diffusing polydisperse polymer should be quite different from ø_m(x), the Fickian solution, which one obtains for monodisperse polymer. This idea was tested by measuring ø(x) of polydisperse d-polystyrene (d-PS) diffusing into h-PS. The results are in excellent agreement with the ø(x) predicted from the reptation model and the experimentally determined molecular weight distribution.