Interactions between bottle-brush polyelectrolyte layers: effects of ionic strength and oppositely charged surfactant

Interactions between cationic bottle-brush polyelectrolyte layers adsorbed on mica across salt and oppositely charged surfactant solutions were investigated with the interferometric surface force apparatus, and the results were compared with what is known for similarly charged linear polyelectrolytes. Ellipsometric measurements demonstrated that the bottle-brush polyelectrolytes, which contain 45 units long poly(ethylene oxide) side chains, are more readily desorbed than linear equivalents when the ionic strength of the solution is increased. It is argued that this is due to the steric repulsion between the poly(ethylene oxide) side chains that reduces the surface affinity. The preadsorbed bottle-brush polyelectrolyte layers were also exposed to sodium dodecyl sulfate (SDS) solutions. It was found that the presence of SDS affected the force profiles less than observed for similarly charged linear polyelectrolytes. This observation was attributed to excluded volume constraints imposed by the poly(ethylene oxide) side chains that reduces the accessibility of the charged polyelectrolyte segments and counteracts formation of large aggregates within the layer.     

Desorption of bottle-brush polyelectrolytes from silica by addition of linear polyelectrolytes studied by QCM-D and reflectometry

The possibility of exchanging adsorbed layers of PEO(45)MEMA:METAC-X brush polyelectrolytes (with two different charge densities, 10 and 75 mol%, denoted by X), with poly(MAPTAC), a highly charged linear polyelectrolyte, was investigated by quartz crystal microbalance with dissipation and reflectometry. The studies were conducted on a silica substrate at pH 10, conditions under which only electrostatic interactions are effective in the adsorption process. Based on the results, it was concluded that PEO(45)MEMA:METAC-10 forms an inhomogeneous layer at the interface through which poly(MAPTAC) chains can easily diffuse to reach the surface. On the other hand, the PEO(45)MEMA:METAC-75 layer was not affected when exposed to a poly(MAPTAC) solution. We argue that the observed effect for PEO(45)MEMA:METAC-75 is due to the formation of a homogeneous protective brush layer, in combination with the small difference in surface affinity between the bottle-brush polyelectrolyte and poly(MAPTAC), together with the difficulty of displacing highly charged polyelectrolyte chains once they are adsorbed on the oppositely charged surface. We also use the combination of QCM-D and reflectometry data to calculate the water content and layer thickness of the adsorbed layers.     

Lubrication properties of bottle-brush polyelectrolytes: an AFM study on the effect of side chain and charge density

The effect of side chain to charge ratio on the frictional properties of adsorbed layers formed by bottle-brush polyelectrolytes with poly(ethylene oxide) side chains has been investigated. The brush polyelectrolytes were preadsorbed from 0.1 mM NaNO(3) solutions onto mica and silica surfaces; the interfacial friction was then measured in polyelectrolyte-free solutions via AFM (with the silica surface acting as the colloidal probe). It was concluded that the decisive factor for achieving favorable lubrication properties is the concentration of nonadsorbing poly(ethylene oxide) side chains in the interfacial region. However, contrary to what may be expected, the results showed that an ideal brush layer structure with the adsorbed polymers adopting comb-like conformation is not necessary for achieving a low coefficient of friction in the asymmetric mica-silica system. In fact, the lowest coefficient of friction (<0.01) under applied pressures as high as 30 MPa was observed for a system with a side chain to charge ratio of 9:1, incapable of forming brush-like layers.     

Structure characterization of cellulose nanofiber hydrogel as functions of concentration and ionic strength

Carboxylated cellulose nanofibers (CNFs), having an average width of 7 nm and thickness of 1.5 nm, were produced by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation method. The fiber cross-sectional dimensions were determined using small-angle X-ray scattering (SAXS), transmission electron microscopy and atomic force microscopy techniques, where the rheological properties under different concentration and ionic strength were also investigated. The formation of hydrogel was evidenced by increasing the CNF concentration or ionic strength of the solvent (water), while the gel structure in ion-induced CNF hydrogels was found to be relatively inhomogeneous. The gelation behavior was closely related to the segmental aggregation of charged CNF, which could be quantitatively characterized by the correlation length (ξ) from the low-angle scattering profile and the scattering invariant (Q) in SAXS.     

Nanocellulose/Carbon Nanoparticles Nanocomposite Film Modified Electrode for Durable and Sensitive Electrochemical Determination of Metoclopramide

A novel electrochemical sensor based on nanocellulose‐carbon nanoparticles (NC‐CNPs) nanocomposite film modified glassy carbon electrode (GCE) is developed for the analysis of metoclopramide (MCP). Atomic force microscopy, scanning electron microscopy and electrochemical impedance spectroscopy were used to characterize the roughness, surface morphology and performance of the deposited modifier film on GCE. SEM image demonstrated that modifier nanoparticles are uniformly deposited on GCE, with an average size of less than 50 nm. The electrochemical behavior of MCP and its oxidation product is studied using linear sweep and cyclic voltammetry over a wide pH range on NC‐CNPs modified glassy carbon electrode. The results revealed that the oxidation of MCP is an irreversible and pH‐dependent process that proceeds in an adsorption‐controlled mechanism and results in the formation of a main oxidation product, which adsorbs on the surface of NC‐CNPs/ GCE. The modified electrode showed a distinctive anodic response towards MCP with a considerable enhancement (49 fold) compared to the bare GCE. Under the optimized conditions, the modified electrode exhibited a wide linear dynamic range of 0.06–2.00 µM with a detection limit of 6 nM for the voltammetric determination of MCP. The prepared modified electrode showed several advantages such as simple preparation method, high stability, reproducibility, and repetitive usability. The modified electrode is successfully applied for the accurate determination of trace amounts of MCP in pharmaceutical and clinical preparations.     

Hydrogen-abstraction reactions of fully hydrogenated silicon fullerene cages with the amino radical: a density functional study

Structural Chemistry - We applied density functional theory calculations to study reactions NH2 ?+ SinHn fullerenes (n?=?4, 6, 8, 10, 20, 24, 30, 36, and 50). The reactions...     

Synthesis of 1,2,3 triazole-linked pyrimidines catalyzed by mg-Al-LDH-immobilized-CuI as a heterogeneous catalyst

A useful and efficient procedure was obtained for the synthesis of 1,2,3-triazole-linked pyrimidines via click reaction of propynylated pyrimidine and aromatic azides in the presence of Mg-Al-LDH-immobilized-CuI with high-to-excellent yields. Moreover, the prepared catalyst was characterized by the FT-IR spectroscopy, XRD, BET, SEM, and ICP techniques. The developed synthetic technique offers numerous advantages such as a clean reaction, easy workup, high reaction yields, and short reaction time.     

Nanofibrillated cellulose: properties reinvestigated

The scientific publications on nanofibrillated cellulose (NFC) were reviewed in the light of recent developments in the field of characterization of NFC, and the evolving understanding of the material. This led to several insights, which challenged few of the established assumptions with regard to e.g. rheological properties of NFC suspensions, and factors affecting tensile strength and barrier properties of NFC films. The realizations may promote the wider application of nanofibrillated celluloses.