Numerous assays were developed to measure the antioxidant activity, but each has limitations and the results obtained by different methods are not always comparable. Popular examples are the DPPH and ...ABTS assay. Our aim was to study similarities and differences of these two assay regarding the measured antioxidant potentials of 24 phenolic compounds using the same measurement and evaluation methods. This should allow conclusions to be drawn as to whether one of the assays is more suitable for measuring specific subgroups like phenolic acids, flavonols, flavanones, dihydrochalcones or flavanols. The assays showed common trends for the mean values of most of the subgroups. Some dihydrochalcones and flavanones did not react with the DPPH radical in contrast to the ABTS radical, leading to significant differences. Therefore, to determine the antioxidant potential of dihydrochalcone or flavanone-rich extracts, the ABTS assay should be preferred. We found that the results of the flavonoids in the DPPH assay were dependent on the Bors criteria, whereas the structure-activity relationship in the ABTS assay was not clear. For the phenolic acids, the results in the ABTS assay were only high for pyrogallol structures, while the DPPH assay was mainly determined by the number of OH groups.
Due to their antioxidant properties, secondary plant metabolites can scavenge free radicals such as reactive oxygen species and protect foods from oxidation processes. Our aim was to study structural ...influences, like basic structure, number of hydroxyl groups and number of Bors criteria on the outcome of the oxygen radical absorbance capacity (ORAC) assay. Furthermore, similarities and differences to other
antioxidant assays were analyzed by principal component analysis. Our studies confirmed that the antioxidant behavior in the ORAC assay is dominated by the number and types of substituents and not by the Bors criteria, as long as no steric hindrance occurs. For example, morin (MOR) with five hydroxyl groups and two Bors criteria reached an area under the curve of (3.64 ± 0.08) × 10
, which was significantly higher than quercetin-7-D-glucoside (QGU7) (
< 0.001), and thus the highest result. Principal component analysis showed different dependencies regarding structural properties of Folin-Ciocalteu (FC)- and 2,2-diphenyl-1-picrylhydrazyl (DPPH)-assays or 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)- and ORAC-assays, respectively. Therefore, we conclude that they are based on different reaction mechanisms. The number of hydroxyl groups showed a stronger influence on the antioxidant activity than the Bors criteria. Due to these differences, the correlation of these rapid tests to specific applications should be validated.
Plants produce a diverse array of secondary metabolites that are generally nonessential but facilitate ecological interactions. Fruits, vegetables, seeds and nuts can accumulate bioactive secondary ...metabolites with health-promoting properties, including the potent antioxidant activities of phenolic compounds. Several in vitro assays have been developed to measure the polyphenol content and antioxidant activity of plant extracts, e.g., the simple and highly popular Folin-Ciocalteu (FC) assay. However, the literature contains a number of different descriptions of the assay and it is unclear whether the assay measures the polyphenol content or reducing capacity of the sample. To determine the influence of phenolic structures on the outcome of the FC assay, we tested phenols representing different subgroups (phenolic acids, flavonols, flavanols, dihydrochalcones and flavanones). We observed different results for each reference substance and subgroup. Accordingly, we concluded that the FC assay does not measure the polyphenol content of a sample but determines its reducing capacity instead. Assigning the substances to five structural classes showed that the FC results depend on the number of fulfilled Bors criteria. If a molecule fulfills none of the Bors criteria, the FC results depend on the number of OH groups. We did not find a correlation with other single electron transfer assays (e.g., ABTS and DPPH assays). Furthermore, the FC assay was compatible with all five subgroups and should be preferred over the DPPH assay, which is specific for extracts rich in dihydrochalcones or flavanones.
Increasingly restricted availability and environmental impact of mineral oils have boosted the interest in sustainable lubrication. In this study, the thickening properties of sodium carboxymethyl ...celluloses (CMCs) were investigated in order to assess their potential as viscosity modifiers in aqueous gear and bearing fluids. The pressure, temperature and shear dependence of viscosity was studied at different concentrations and molecular weights MW. The tribological properties were investigated at different viscosity grades in both sliding and rolling contact, and compared to rapeseed oil and polyethylene glycol 400. The viscosity of the CMC solutions was adjustable to all application-relevant viscosity grades. Viscosity indices were similar or higher compared to the reference fluids and mineral oil. Temporary and permanent viscosity losses increased with MW. Permanent viscosity loss was highest for high MW derivatives, up to 70%. The pressure-viscosity coefficients α were low and showed a high dependency on shear and concentration. In rolling contact, low MW CMC showed up to 35% lower friction values compared to high MW, whereas no improvement of lubricating properties was observed in sliding contact. The results suggest that low MW CMC has great potential as bio-based thickener in aqueous lubrication.
Plant extracts from natural and renewable resources have the potential to replace conventional antioxidants in many applications and to increase the stability of lubricants. By specific selection of ...raw materials and adaptation of extraction parameters, the properties of the additives can be specifically optimized for the respective application. Easy-to-perform photometric assays enable targeted and rapid process adjustment and pre-selection of extracts. Later, they can be evaluated in application-oriented analytical methods, such as the rancimat test, with regard to their effect in the oil. In our analyses, secondary plant compounds increased the oxidation stability by 300 %, whereas the conventional antioxidants only achieved an increase of 200 %.
Pflanzenextrakte aus natürlichen und nachwachsenden Rohstoffen haben das Potential in vielen Anwendungsbereichen herkömmliche Antioxidantien zu ersetzen und die Oxidationsstabilität von Schmierölen ...zu steigern. Durch spezifische Auswahl der Rohstoffe und Variation der Extraktionsparameter lassen sich die Eigenschaften der Additive spezifisch für den jeweiligen Einsatzzweck anpassen. Einfach durchzuführende photometrische Assays ermöglichen dabei eine gezielte und schnelle Prozessanpassung und Vorauswahl der Extrakte. Diese können später in anwendungsnahen Analysenmethoden, wie dem Rancimat-Test, hinsichtlich ihrer Wirkung im Öl untersucht werden. In unseren Analysen konnten sekundäre Pflanzenstoffe die Oxidationsstabilität um bis zu 300 % erhöhen, wohingegen die konventionellen Vergleichsantioxidantien lediglich eine Erhöhung um 200 % erreichten.
•Temperature dependent quasi-stationary approximations were derived and validated.•Theoretical approximation reveals effect of temperature and pressure on permeation.•Temperature dependency of ...steady-state permeation is determined by silicon oxide.•Silicon oxide doubles the activation energy for permeation of the substrate film.•Lag time is mainly influenced by heat of sorption of polymeric intermediate layer.
Barrier layers are often used to protect sensitive organic devices from the detrimental effects of oxygen and water vapor. We investigated the effect of temperature on the time-dependent behavior of water vapor permeation in multilayer barrier films, focusing on the water vapor solubility, diffusion and permeability coefficients of individual layers and layer sequences. Activation energy measurements helped to explain the permeation mechanisms and showed the possible interactions between water and silicon oxide (SiOx). The activation energy for permeation (EP) through a two-layer film of polyethylene terephthalate (PET) coated with SiOx was 5.7 kJmol−1, about twice the EP value of the uncoated PET substrate. An intermediate ORMOCER® layer, providing a smooth surface for a second SiOx layer in an alternating structure, had only a negligible effect on the EP. Temperature dependent quasi-stationary approximations were derived for permeation through films with at least one inorganic/polymer/inorganic triplet, and were validated according to experimentally determined values. These equations showed the temperature and pressure dependencies of steady-state permeation and lag time in multilayered structures. Accordingly, the temperature dependency of steady-state permeation is determined by the EP of SiOx, whereas the lag time was mainly influenced by the heat of sorption of the intermediate polymeric layer. Furthermore, the increase in water vapor transmission rate was dominated by the partial pressure change with increasing temperature. Using these equations and experimentally determined parameters, we can predict the influence of temperature and humidity on the barrier performance of multilayered barrier structures.
Multilayer films that comprise alternating inorganic barrier and polymer layers deposited on a flexible substrate are often used to protect organic electronic devices from degradation caused by ...oxygen and water vapor. We tested films consisting of up to 11 inorganic silicon oxide barrier and polymer layers to characterize the water vapor permeability, solubility and diffusivity of each layer based on measurements of water sorption, transient water vapor permeation and lag times. The time-dependency of transient water vapor transmission rates (WVTR) of the structures containing a barrier-polymer-barrier sequence was also estimated using a so-called quasi-steady-state (QSS) approximation. The permeability values and lag times in multilayer structures predicted by QSS approximation agreed with the values determined empirically based on time-dependent WVTR measurements. The steady-state WVTR of the multilayer barrier films was dominated by the permeability of the inorganic barrier layers, whereas the solubility coefficient and thickness of the interleaved polymer layers determined the lag times. The barrier performance and lag time of multilayer structures can be predicted using the parameters obtained from sorption measurements, thus significantly reducing the time required for experiments. The results of this study will allow us to design multilayer barrier films according to the lifetime requirements of flexible organic electronic devices.
•WVTR of silicon oxide is up to five orders of magnitude lower compared to polymers.•Longer lag times for multilayers incorporating barrier-polymer-barrier triplets•Solubility and thickness of the polymer layers in BPB structures increase the lag time.•Permeability of inorganic layer dominates steady-state WVTR.•Prediction of WVTR by QSS approximation reduces experimental timeframes.
The relative humidity level in the immediate vicinity of objects such as foods and technical items has an influence on transpiration and undesired relative humidity dependent reactions on their ...surfaces such as water vapour condensation, chemical reactions and microbial growth. Desirable are therefore materials that regulate the relative humidity to predefined levels in closed spaces such as in packagings and also in housing spaces. Such materials should be able to adjust the surface humidity of objects via water vapour diffusion in the gas phase without being in direct contact. A strategy to create such humidity regulating materials is the use of substances that absorb and desorb high amounts of water vapour, such as sodium chloride. Sodium chloride (NaCl) particles absorb water vapour at a relative humidity above 75% at 23 °C and then they form NaCl solution. NaCl solution desorbs water vapour when the relative humidity in the immediate vicinity decreases below the equilibrium relative humidity of that salt solution. Therefore, this system is able to regulate the relative humidity in its immediate vicinity. A strategy to provide free space in polymeric structures for the NaCl solution is the creation of cavities by foaming and stretching. However, only little knowledge exists about the influence of the combination of both processes on the functional properties of humidity regulating films with salt as active substance. Hence, the aim of this study was to investigate how and to which extend foaming and/or stretching affects the functional properties, i.e. water vapour sorption and mechanical properties, of polypropylene (PP) films containing NaCl particles. For this study, foamed and non-foamed PP with 3 and 6 weight-% NaCl were extruded into films. In the next step, some of these samples were biaxially stretched and their structures, water vapour absorption, porosities and mechanical strengths were analysed. The only-foamed films had a porosity of 0.3, the porosity of only-biaxially stretched films was between 0.1 and 0.2. The porosity increased to 0.7 when the films were first foamed and stretched afterward. Foamed and then stretched films with 6 wt.-% NaCl absorbed a high amount of water vapour from air with a value of 0.8 g H2O/g film at 97% relative humidity. Stretching of filled and non-filled foamed films also resulted in higher mechanical strength of the pure matrix polymer in comparison to the pure matrix polymer of non-stretched films. By the results of this study is shown that humidity regulating films with high water vapour sorption capacity can be produced via extrusion, foaming and stretching processes which are established processes in the polymer industry.
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