Similar Surface Charge Densities Research Articles

Oxidized cellulose nanofibers from sugarcane bagasse obtained by microfluidization: Morphology and rheological behavior

It is advantageous to understand the relationship between cellulose fiber morphology and the rheological behavior of its dispersions so that their application can be optimized. The goal of this study was to produce sugarcane bagasse-sourced cellulose dispersions with different numbers of high-pressure homogenization cycles. Microfluidization produced cellulose nanofibers (between 5 and 80 nm in diameter) with similar surface charge densities and crystallinities (measured on the resulting films). Oscillatory rheology showed that TEMPO-oxidized cellulose dispersions exhibited gel-like behavior. However, not only did the samples with more microfluidization cycles present a lower storage modulus, but the sample with 100 cycles completely lost the gel-like characteristic, presenting a viscous fluid rheological behavior. Thixotropy loop tests revealed the influence of nanofiber length on the dispersion's structure, as evidenced by the decrease in the hysteresis value along with fiber breakage. Therefore, our findings demonstrate that the rheological properties of the dispersion can be tuned according to the length of the nanofibers, allowing for targeted applications.

Effect of Fibril Length on the Ice Recrystallization Inhibition Activity of Nanocelluloses

Ice recrystallization inhibition (IRI) activity has been recently discovered on nanocelluloses, which enables potential applications in several fields. This study focused on the effect of fibril length on the IRI activity of nanocelluloses. The 2, 2, 6, 6-tetramethylpiperidine-1-oxyl oxidized cellulose nanofibrils (TEMPO-CNFs) with similar surface charge densities (SCDs) and fibril widths, but with different fibril lengths were prepared by sonication treatment. The IRI activity of nanocelluloses was enhanced in TEMPO-CNFs receiving sonication less than 30 min because of the reduction of fibril aggregation. Further increase of sonication time to 60 and 120 min led to a decreased IRI activity due to the reduction of fibril lengths. Our results indicate longer nanocelluloses are more IRI active. The IRI activity was not correlated with the viscosity or gelling properties of nanocellulose dispersions. Research findings are useful in producing nanocelluloses with enhanced IRI activity and in understanding the underlying IRI mechanism.

Towards supported bolaamphiphile membranes for water filtration: Roles of lipid and substrate

Supported biomimetic membranes hold potential for applications such as biosensors and water purification by filtration. The current paper reports on the preparation of a supported bolaamphiphile membrane on two polymeric nanofiltration membranes: NF-270 made of polyamide with carboxylic surface charges and NTR-7450 made of sulfonated polyethersulfone with sulfonic surface charges. The results, obtained using a proprietary microfluidic device, confirmed that the supported membrane coverage on the polymeric membranes was governed by the double-layer interactions, in agreement with previous studies on silica, mica, and gold. Remarkably, the formation of the biomimetic membrane was more favorable on the sulfonated polyethersulfone than on the polyamide surface, although both surfaces exhibit a similar surface charge density. It is suggested that the higher dissociation constant of sulfonic groups was the main reason for the higher coverage of NTR-7450. Finally, spinach aquaporins, which are trans-membrane proteins that facilitate the water transport, were incorporated into a supported membrane on NTR-7450. The incorporated aquaporin resulted in enhanced pressure-driven water transport through the membrane, however, for still unclear reasons, the transport was not selective. Despite this failure, the results provide new insights into formation of biomimetic membrane on water permeable polymeric substrates, as a generic approach towards biomimetic water filters.

Stepwise Ordering of Imidazolium-Based Cationic Surfactants during Cooling-Induced Crystallization

Surfactants bearing imidazolium cations represent a new class of building blocks in molecular self-assembly. These imidazolium-based cationic surfactants can exhibit various morphologies during phase transformations. In this work, we studied the self-assembly and phase behavior of 1-hexadecyl-3-methylimidazolium chloride (C(16)mimCl) aqueous dispersions (0.5-10 wt %) by using isothermal titration calorimetry, differential scanning calorimetry, synchrotron small- and wide-angle X-ray scattering, freeze-fracture electron microscopy, optical microscopy, electrical conductance, and Fourier transform infrared spectroscopy. It was found that C(16)mimCl in aqueous solutions can form two different crystalline phases. At higher C(16)mimCl concentrations (>6 wt %), the initial spherical micelles convert directly to the stable crystalline phase upon cooling. At lower concentrations (0.5 or 1 wt %), the micelles first convert to a metastable crystalline phase upon cooling and then transform to the stable crystalline phase upon further incubation at low temperature. The electrical conductance measurement reveals that the two crystalline phases have similar surface charge densities and surface curvatures. Besides, the microscopic and spectroscopic investigations of the two crystalline phases suggest that the metastable crystalline phase has preassembled morphology and a preordered submolecular packing state that contribute to the final stable crystalline structure. The formation of a preordered structure prior to the final crystalline state deepens our understanding of the crystallization mechanisms of common surfactants and amphiphilic ionic liquids and should thus be widely recognized and explored.

Electrophoretic mobility of latex particles: effect of particle size and surface structure

The influence of particle size on the electrophoretic mobility of negatively charged latex particles was examined by a comparison between theory and experiment. Theoretical values for the dependence of the mobility on electrolyte concentration were calculated by a modified White–O’Brian model (Hidalgo-Alvarez et al., Adv. Coll. Interf. Sci. 67 (1996) 1) which enables the consistent calculation of the zeta ( ζ) potential. For three polystyrene latexes of different size but similar surface charge density the measured mobilities increased with increasing radius for the electrolyte range under consideration. The theoretical calcalations resulted in a qualitatively correct prediction of the experimental data. The experimental comparison of the mobilities of hydrophobic and hydrophilic particles of similar size and surface charge density lead to the conclusion that hydrophilic surfaces lower the electrophoretic mobility. The same theoretical model was able to describe correctly this observed behavior by assuming a greater distance of the plane of shear. The effect of a spatial distribution of the charges was examined by characterizing an electrosterically stabilized latex. Contrary to all standard latices with surface charges this latex didn't show any mobility maximum as a function of electrolyte concentration.

Optimization of the heterocoagulation process of polymer colloids with different particle size

In this work, the coagulation rates of aqueous dispersions of uniform spherical particles of surfactant-free cationic and anionic latexes were measured with a spectrometer. Two polymer colloids with similar surface charge densities and different particle sizes were used. Owing to the changes in the surface charge density of the cationic latex with pH, this is a factor which influences the heterocoagulation experiments; especially near the isoelectric point of the cationic latex where homocoagulation can occur. From previous homocoagulation studies the initial particle number and the wavelength were selected. The heterocoagulation process was studied as a function of the ratio of the cationic to anionic particle numbers, the total initial particle number and the wavelength of the spectrometer. To calculate the heterocoagulation rate constant, K D12, experimental conditions were selected to eliminate the homocoagulation of cationic or anionic latexes. The proportion of cationic to anionic particles was varied between 1 and 10 and vice versa. This proportion was shown to influence K D12 when the larger sized particles were in the majority. However, by reducing the total initial particle number of the dispersion, a constant K D12 value can be obtained which is similar to that found in the homocoagulation process.

Physical Characterization of Heparin by Light Scattering

Dynamic and electrophoretic light scattering were used to measure the size and charge heterogeneity of a commercial preparation of heparin. For this preparation of porcine mucosal heparin (Mr = 10–20 kDa), the diffusion coefficient was 1.2 ± 0.5×10−8cm2/s and the mobility was 4.4 ± 0.9×10−4cm2/V s for an unfiltered solution at 22°C in distilled water. This diffusion constant is 2 orders of magnitude smaller than expected for a molecule the size of heparin. A fast diffusion component of 5.8 ± 1.0×10−7cm2/s, corresponding to the individual molecule, was observed in the presence of 2M NaCl, where single molecule motion is better observed. This indicates that a portion of the heparin population is in an aggregated state, which produces a higher scattering intensity than individual heparin molecules. The weight percentage of the aggregates was 5–10 as measured by high performance exclusion chromatography. These aggregates were stable up to a temperature of 75°C as measured by light scattering. This suggests that the aggregates are made up of tightly bound heparin molecules. From the diffusion coefficients we estimate the average aggregate to be made up of about 50 heparin monomers, and from the mobility, we estimate the electrophoretic charge on the aggregate to be about 1000 or about 20 electrophoretic charges for each heparin monomer. The electrophoretic light‐scattering measurements also indicate that the aggregate scattering species have very similar surface charge densities resulting from the aggregates being formed from heparin molecules. These results can provide a framework to interpret future light‐scattering data for various preparations of heparin.

Effects of surface roughness (hairiness) of latex particles on their electrokinetic potentials

In a previous paper, we demonstrated that the electric properties of the bitumen/water and crude oil/water interfaces could be explained by the dissociation of carboxyl groups belonging to natural surfactants present in bitumen. Electrophoresis of bitumen-in-water emulsions indicated very high zeta potentials (−100 mV in 10 −2 M NaCl) especially when compared to a carboxylated latex (−50 mV in 10 −2 M NaCl) having similar surface charge density. This difference was attributed to the surface roughness of the latex particle, which results in a larger value for the location of the shear plane resulting in a smaller value for the zeta potential. To test this hypothesis we modified the surface roughness of the latex by heating it above its glass transition temperature. The heat treatment resulted in a significant increase of the zeta potentials calculated from the measured electrophoretic mobilities. This observation suggests that surface roughness is one of the most important factors in determining zeta potentials from electrophoresis. Results of electrophoretic mobilities of the original and heat-treated latexes will be shown as a function of pH in a wide range of NaCl concentrations, and will be compared with those predicted by the Ionizable Surface-Group model. In addition, results of measured mobilities of the heat-treated latex in NaCl, CaCl 2, and La(NO 3) 3 below 10 −4 M suggest that the computer solution of O'Brien and White overestimates the electrokinetic relaxation effect of itκa < 100, where κ is the reciprocal Debye-Huckel length and a is the particle radius.

Involvement of surface potential in regulation of polar membrane lipids in Acholeplasma laidlawii.

Upon induced variation of membrane lipid acyl chain unsaturation in Acholeplasma laidlawii, the cells strongly change in a characteristic manner the proportions of individual (charged and noncharged) polar lipids synthesized. Monolayer analysis of polar lipid extracts revealed different mean lateral molecular areas but similar surface charge densities. Microelectrophoresis of these lipids indicated an almost constant lipid membrane zeta-potential of about -35 mV. Simulation by the Gouy-Chapman-Stern (GCS) relations verified that the zeta (surface)-potentials remain constant. Exposing cells to increasing concentration of Na+ yielded a substantial increase in amounts of charged lipids synthesized. In model systems consisting of mixtures of A. laidlawii phosphatidylglycerol (anionic) and glucolipid (diglucosyldiglyceride, noncharged) microelectrophoresis showed; (i) increasing PG amounts resulted in an increased-, and (ii) increasing Na+ concentration resulted in a decreased zeta-potential, respectively, (iii) at physiological ionic strength and lipid surface charge densities the zeta-potential was approximately -35 mV, and (iv) simulation according to the GCS theory yielded an acceptable fit with experimental data. This behavior of the phosphatidylglycerol/diglucosyldiglyceride mixtures is very similar to that of phosphatidylserine/phosphatidylcholine mixtures. It is concluded that the changes in lipid surface charge densities (and surface potential) in A. laidlawii membranes brought by variation in lateral areas of lipid acyl chains and the concentration-dependent quenching of lipid charge by Na+, is compensated for by the cellular regulation of charged lipid amounts thereby maintaining a constant lipid surface potential.