Diana Bernin

The gamma distribution model for pulsed-field gradient NMR studies of molecular-weight distributions of polymers

Self-diffusion in polymer solutions studied with pulsed-field gradient nuclear magnetic resonance (PFG NMR) is typically based either on a single self-diffusion coefficient, or a log-normal distribution of self-diffusion coefficients, or in some cases mixtures of these. Experimental data on polyethylene glycol (PEG) solutions and simulations were used to compare a model based on a gamma distribution of self-diffusion coefficients to more established models such as the single exponential, the stretched exponential, and the log-normal distribution model with regard to performance and consistency. Even though the gamma distribution is very similar to the log-normal distribution, its NMR signal attenuation can be written in a closed form and therefore opens up for increased computational speed. Estimates of the mean self-diffusion coefficient, the spread, and the polydispersity index that were obtained using the gamma model were in excellent agreement with estimates obtained using the log-normal model. Furthermore, we demonstrate that the gamma distribution is by far superior to the log-normal, and comparable to the two other models, in terms of computational speed. This effect is particularly striking for multi-component signal attenuation. Additionally, the gamma distribution as well as the log-normal distribution incorporates explicitly a physically plausible model for polydispersity and spread, in contrast to the single exponential and the stretched exponential. Therefore, the gamma distribution model should be preferred in many experimental situations.

Fluorescence recovery after photobleaching in material and life sciences: Putting theory into practice

Fluorescence recovery after photobleaching (FRAP) is a versatile tool for determining diffusion and interaction/binding properties in biological and material sciences. An understanding of the mechanisms controlling the diffusion requires a deep understanding of structure-interaction-diffusion relationships. In cell biology, for instance, this applies to the movement of proteins and lipids in the plasma membrane, cytoplasm and nucleus. In industrial applications related to pharmaceutics, foods, textiles, hygiene products and cosmetics, the diffusion of solutes and solvent molecules contributes strongly to the properties and functionality of the final product. All these systems are heterogeneous, and accurate quantification of the mass transport processes at the local level is therefore essential to the understanding of the properties of soft (bio)materials. FRAP is a commonly used fluorescence microscopy-based technique to determine local molecular transport at the micrometer scale. A brief high-intensity laser pulse is locally applied to the sample, causing substantial photobleaching of the fluorescent molecules within the illuminated area. This causes a local concentration gradient of fluorescent molecules, leading to diffusional influx of intact fluorophores from the local surroundings into the bleached area. Quantitative information on the molecular transport can be extracted from the time evolution of the fluorescence recovery in the bleached area using a suitable model. A multitude of FRAP models has been developed over the years, each based on specific assumptions. This makes it challenging for the non-specialist to decide which model is best suited for a particular application. Furthermore, there are many subtleties in performing accurate FRAP experiments. For these reasons, this review aims to provide an extensive tutorial covering the essential theoretical and practical aspects so as to enable accurate quantitative FRAP experiments for molecular transport measurements in soft (bio)materials.

Surface effects on the structure and mobility of the ionic liquid C6C1ImTFSI in silica gels

We report on how the dynamical and structural properties of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C6C1ImTFSI) change upon different degrees of confinement in silica gels. The apparent diffusion coefficients of the individual ions are measured by (1)H and (19)F pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy, while the intermolecular interactions in the ionogels are elucidated by Raman spectroscopy. In addition, the local structure of the ionic liquid at the silica interface is probed by solid-state NMR spectroscopy. Importantly, we extend this study to a wider range of ionic liquid-to-silica molar ratios (x) than has been investigated previously, from very low (high degree of confinement) to very high (liquid-like gels) ionic liquid contents. Diffusion NMR measurements indicate that a solvation shell, with a significantly lower mobility than the bulk ionic liquid, forms at the silica interface. Additionally, the diffusion of the C6C1Im(+) and TFSI(-) ions decreases more rapidly below an observed molar ratio threshold (x < 1), with the intrinsic difference in the self-diffusion coefficient between the cation and anion becoming less pronounced. For ionic liquid molar ratio of x < 1, Raman spectroscopy reveals a different conformational equilibrium for the TFSI(-) anions compared to the bulk ionic liquid, with an increased population of the cisoid isomers with respect to the transoid. Concomitantly, at these high degrees of confinement the TFSI(-) anion experiences stronger ion-ion interactions as indicated by the evolution of the TFSI(-) characteristic vibrational mode at ∼740 cm(-1). Furthermore, solid-state 2D (29)Si{(1)H} HETCOR NMR measurements establish the interactions of the ionic liquid species with the silica surface, where the presence of adsorbed water results in weaker interactions between (29)Si surface moieties and the hydrophobic alkyl protons of the cationic C6C1Im(+) molecules.

Probe diffusion in phase-separated bicontinuous biopolymer gels.

Probe diffusion was determined in phase separated bicontinuous gels prepared by acid-induced gelation of the whey protein isolate-gellan gum system. The topological characterization of the phase-separated gel systems is achieved by confocal microscopy and the diffusion measurements are performed using pulsed field gradient (PFG) NMR and fluorescence recovery after photo-bleaching (FRAP). These two techniques gave complementary information about the mass transport at different time- and length scales, PFG NMR provided global diffusion rates in the gel systems, while FRAP enabled the measurements of diffusion in different phases of the phase-separated gels. The results revealed that the phase-separated gel with the largest characteristic wavelength had the fastest diffusion coefficient, while the gel with smaller microstructures had a slower probe diffusion rate. By using the diffusion data obtained by FRAP and the structural data from confocal microscopy, modelling through the lattice-Boltzmann framework was carried out to simulate the global diffusion and verify the validity of the experimental measurements. With this approach it was found that discrepancies between the two experimental techniques can be rationalized in terms of probe distribution between the different phases of the system. The combination of different techniques allowed the determination of diffusion in a phase-separated biopolymer gel and gave a clearer picture of this complex system. We also illustrate the difficulties that can arise if precautions are not taken to understand the system-probe interactions.

Pixel-based analysis of FRAP data with a general initial bleaching profile

In Jonasson et al. (2008) , we presented a new pixel‐based maximum likelihood framework for the estimation of diffusion coefficients from data on fluorescence recovery after photobleaching (FRAP) with confocal laser scanning microscopy (CLSM). The main method there, called the Gaussian profile method below, is based on the assumption that the initial intensity profile after photobleaching is approximately Gaussian. In the present paper, we introduce a method, called the Monotone profile method, where the maximum likelihood framework is extended to a general initial bleaching profile only assuming that the profile is a non‐decreasing function of the distance to the bleaching centre. The statistical distribution of the image noise is further assumed to be Poisson instead of normal, which should be a more realistic description of the noise in the detector. The new Monotone profile method and the Gaussian profile method are applied to FRAP data on swelling of super absorbent polymers (SAP) in water with a Fluorescein probe. The initial bleaching profile is close to a step function at low degrees of swelling and close to a Gaussian profile at high degrees of swelling. The results obtained from the analysis of the FRAP data are corroborated with NMR diffusometry analysis of SAP with a polyethylene glycol probe having size similar to the Fluorescein. The comparison of the Gaussian and Monotone profile methods is also performed by use of simulated data. It is found that the new Monotone profile method is accurate for all types of initial profiles studied, but it suffers from being computationally slow. The fast Gaussian profile method is sufficiently accurate for most of the profiles studied, but underestimates the diffusion coefficient for profiles close to a step function. We also provide a diagnostic plot, which indicates whether the Gaussian profile method is acceptable or not.

Magnetic resonance analysis of capillary formation reaction front dynamics in alginate gels

The formation of heterogeneous structures in biopolymer gels is of current interest for biomedical applications and is of fundamental interest to understanding the molecular level origins of structures generated from disordered solutions by reactions. The cation‐mediated physical gelation of alginate by calcium and copper is analyzed using magnetic resonance measurements of spatially resolved molecular dynamics during gel front propagation. Relaxation time and pulse‐field gradient methods are applied to determine the impact of ion front motion on molecular translational dynamics. The formation of capillaries in alginate copper gels is correlated to changes in translational dynamics. Copyright

Chemical shift imaging NMR to track gel formation

In this work we have combined (1)H and (19)F NMR chemical shift images to investigate the dynamic processes of gel formation of a cellulose solution. Chemical shift imaging (CSI) NMR is shown to be a valuable technique for studying phase changes in soft materials. The technique provides spatial position of each chemical component, and by repeatedly recording sample images the dynamic rearrangements in the material can be followed in detail. CSI NMR follows the same principles as magnetic resonance imaging, but can be performed on most of the nowadays commercial NMR probes. Position resolution of the chemical shift gives the opportunity to derive diffusion rate data of individual components during the gel formation process. The results suggest that the method can be used for detailed studies of dynamic processes in multi-component systems and to extract diffusion coefficients for the components investigated.

NMR Studies of Nonionic Surfactants

NMR has contributed greatly to our current understanding of self-association, phase behaviour, and molecular dynamics in aqueous systems of oligo(ethylene oxide) mono-alkyl ether (CmEn) nonionic surfactants, which are extensively used in both basic scientific studies and technical applications as emulsifying agents and detergents. This review provides a comprehensive but concise overview of the various NMR techniques that have been applied to nonionic surfactants. We describe conventional experimental procedures, such as the measurement of quadrupole splittings, nuclear relaxation times, and self-diffusion coefficients to characterize liquid crystalline phases, micelles, and microemulsions, as well as more advanced imaging and diffusion-diffusion 2D correlation approaches to investigate the structure of phase-separated systems and the spatial organization of anisotropic liquid crystalline domains on the micro- to millimetre length scales.

Silicate species of water glass and insights for alkali-activated green cement

Despite that sodium silicate solutions of high pH are commonly used in industrial applications, most investigations are focused on low to medium values of pH. Therefore we have investigated such solutions in a broad modulus range and up to high pH values (∼14) by use of infrared (IR) spectroscopy and silicon nuclear magnetic resonance (29Si-NMR). The results show that the modulus dependent pH value leads to more or less charged species, which affects the configurations of the silicate units. This in turn, influences the alkali-activation process of low CO2 footprint cements, i.e. materials based on industrial waste or by-products.

Multi-Scale Characterization of Lyotropic Liquid Crystals Using 2H and Diffusion MRI with Spatial Resolution in Three Dimensions

The ability of lyotropic liquid crystals to form intricate structures on a range of length scales can be utilized for the synthesis of structurally complex inorganic materials, as well as in devices for controlled drug delivery. Here we employ magnetic resonance imaging (MRI) for non-invasive characterization of nano-, micro-, and millimeter scale structures in liquid crystals. The structure is mirrored in the translational and rotational motion of the water, which we assess by measuring spatially resolved self-diffusion tensors and spectra. Our approach differs from previous works in that the MRI parameters are mapped with spatial resolution in all three dimensions, thus allowing for detailed studies of liquid crystals with complex millimeter-scale morphologies that are stable on the measurement time-scale of 10 hours. The data conveys information on the nanometer-scale structure of the liquid crystalline phase, while the combination of diffusion and data permits an estimate of the orientational distribution of micrometer-scale anisotropic domains. We study lamellar phases consisting of the nonionic surfactant C10E3 in O, and follow their structural equilibration after a temperature jump and the cessation of shear. Our experimental approach may be useful for detailed characterization of liquid crystalline materials with structures on multiple length scales, as well as for studying the mechanisms of phase transitions.