Lubrication, adsorption, and rheology of aqueous polysaccharide solutions

Aqueous lubrication is currently at the forefront of tribological research due to the desire to learn and potentially mimic how nature lubricates biotribological contacts. We focus here on understanding the lubrication properties of naturally occurring polysaccharides in aqueous solution using a combination of tribology, adsorption, and rheology. The polysaccharides include pectin, xanthan gum, gellan, and locus bean gum that are all widely used in food and nonfood applications. They form rheologically complex fluids in aqueous solution that are both shear thinning and elastic, and their normal stress differences at high shear rates are found to be characteristic of semiflexible/rigid molecules. Lubrication is studied using a ball-on-disk tribometer with hydrophobic elastomer surfaces, mimicking biotribological contacts, and the friction coefficient is measured as a function of speed across the boundary, mixed, and hydrodynamic lubrication regimes. The hydrodynamic regime, where the friction coefficient increases with increasing lubricant entrainment speed, is found to depend on the viscosity of the polysaccharide solutions at shear rates of around 10(4) s(-1). The boundary regime, which occurs at the lowest entrainment speeds, depends on the adsorption of polymer to the substrate. In this regime, the friction coefficient for a rough substrate (400 nm rms roughness) is dependent on the dry mass of polymer adsorbed to the surface (obtained from surface plasmon resonance), while for a smooth substrate (10 nm rms roughness) the friction coefficient is strongly dependent on the hydrated wet mass of adsorbed polymer (obtained from quartz crystal microbalance, QCM-D). The mixed regime is dependent on both the adsorbed film properties and lubricant's viscosity at high shear rates. In addition, the entrainment speed where the friction coefficient is a minimum, which corresponds to the transition between the hydrodynamic and mixed regime, correlates linearly with the ratio of the wet mass and viscosity at ～10(4) s(-1) for the smooth surface. These findings are independent of the different polysaccharides used in the study and their different viscoelastic flow properties.     

Dendritic structures based on bis(hydroxymethyl)propionic acid as platforms for surface reactions

In this paper we present results related to the self-assembly of different generations of disulfide-cored 2,2-bis(hydroxymethyl)propionic acid-based dendritic structures onto gold surfaces. These molecular architectures, ranging from generation 1 to generation 3, contain removable acetonide protecting groups at their periphery that are accessible for hydrolysis with subsequent formation of OH-terminated surface-attached dendrons. The deprotection has been investigated in detail as a versatile approach to accomplish reactive surface platforms. A special focus has been devoted to the comparison of the properties of the layers formed by hydrolysis of the acetonide moieties directly on the surface and in solution, prior to the layer formation.     

Cross-linking of atoms and thermal stability of new MDF compositions

We have summarized Al, Fe, C and P atoms incorporations within cross-linking interaction and location of those after MDF procedure of sulphobelitic clinker — hydroxypropylmethyl cellulose (hpmc) — sodium salt of polyphosphates (poly-P) compositions. Design of Al/Fe-O-C/P cross links is given and discussed. Thermoanalytical patterns (i) give the evidence of the presence of polymers in the reaction products and (ii) confirm the linkages of solidified polymers throughX=C or P of Al/Fe-O-X cross links in interphase regions with decomposition temperature interval higher than this in classical hydraulic materials. Designed local structure of interphase regions gives atomic level explanation of the densification of a bulk, its microstructure and exceptional technological properties. Similar phenomenon has been reported for the high aluminium — polyvinylalcohol/acetate MDF compositions. Moreover, variant clinker as well as polymers employed in syntheses represent one of the proposals aimed to the increase of the moisture resistence; our results are in the power of the above alternative for phases of sulphobelitic clinker.This study was funded under Project 85/92 granted by the Slovak Grant Agency for Science and represents the output of joint research agreed by the Institute of Inorganic Chemistry SAS (Bratislava, Slovak Republic) and the Chemistry Department of University Exeter (Exeter, U.K.). We thank Chemische Fabrik R.A. Oetker (Budenheim, F.R.G.) for provision of sodium polyphosphate glass samples. This study also forms part of EC contracted Copernicus Project (ERB CIPA-CT 94-0105).     

Mucin-chitosan complexes at the solid-liquid interface: multilayer formation and stability in surfactant solutions

The adsorption of a biologically important glycoprotein, mucin, and mucin-chitosan complex layer formation on negatively charged surfaces, silica and mica, have been investigated employing ellipsometry, the interferometric surface apparatus, and atomic force microscopy techniques. Particular attention has been paid to the effect of an anionic surfactant sodium, dodecyl sulfate (SDS), with respect to the stability of the adsorption layers. It has been shown that mucin adsorbs on negatively charged surfaces to form highly hydrated layers. Such mucin layers readily associate with surfactants and are easily removed from the surfaces by rinsing with solutions of SDS at concentrations > or =0.2 cmc (1 cmc SDS in 30 mM NaCl is equal to 3.3 mM). The mucin adsorption layer is negatively charged, and we show how a positively charged polyelectrolyte, chitosan, associates with the preadsorbed mucin to form mucin-chitosan complexes that resist desorption by SDS even at SDS concentrations as high as 1 cmc. Thus, a method of mucin layer protection against removal by surfactants is offered. Further, we show how mucin-chitosan multilayers can be formed.     

Chemical oscillatory reaction in a hydrodynamic oscillator

The periodic mixing of the reactants (bromate-thiocyanate or bromate-hydroxylamine) by the hydrodynamic oscillator gives rise to both periodic and aperiodic chemical oscillations.

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Are uncatalyzed bromate oscillators truly gas-free?

Gas evolution is a common byproduct of chemical oscillations but is seldom treated as anything but a complication. Its precise quantification is usually underestimated, and if not obvious enough, gaseous products are easily neglected completely. Reported herein is how evolution of gas from uncatalyzed bromate oscillators with pyrocatechol, pyrogallol, and 1,4-cyclohexanedione was measured by a high-precision batch-mode gasometric method, and the data obtained with unprecedented time resolution are presented. Even though within this completely closed arrangement of measurement no oscillations in rate of gas evolution were observed, it is verified that neither of the substrates can be considered totally free of gas production. Nevertheless, the amount of gas evolved from 1,4-cyclohexanedione was confirmed to be very small, and near negligible, if only oscillatory phase is of interest. Conversely, under identical conditions, the peak rates of gas evolution from the other substrates were reached much sooner, and even though the results in all three cases suggest some extent of suppression of gas production by oscillations, as much as 1 equiv of gas in total can be produced from the phenols, especially from pyrogallol, where the degradation was measured to be approximately twice as extensive. Greater caution is, therefore, recommended whenever the gas-free nature of these oscillators is considered.