Tunable rheological-tribological performance of “green” gel-like dispersions based on sepiolite and castor oil for lubricant applications

This work has been focused on the preparation and characterization of gel-like dispersions based on sepiolite and castor oil potentially suitable as eco-friendly lubricating greases. Particularity, the effect of sepiolite content exerted on rheological, chemical, thermal and tribological properties was studied. The system exhibited a gel-like behaviour for all the concentrations considered (20–40 wt%) related to the development of a sepiolite three-dimensional colloidal network due to its characteristic nanoscale structure formed by fibers, laths and bundles. The values of both apparent viscosity and viscoelastic functions in the linear viscoelastic region increased with sepiolite content, as a consequence of a strengthening of the gel network. An empirical correlation between viscoelastic modulus and sepiolite concentration was proposed. From tribological point of view, the friction coefficient values and wear marks obtained after the frictional tests became lower when sepiolite concentrations decrease. Gel-like dispersion formulated with 30 wt% sepiolite showed appropriate rheological and tribological behaviour to be used as biolubricating greases.Departamento de Ingeniería Química, Química Física y Ciencias de los Materiale

Impact of Vegetable Oil Type on the Rheological and Tribological Behavior of Montmorillonite-Based Oleogels

We formulated and characterized oleogels based on montmorillonite clay and vegetable oils that could serve as eco-friendly semi-solid lubricants. In particular, we studied the influence of the physical-chemical properties of olive, castor, soybean, linseed, and sunflower oils on the rheological, chemical, thermal, and tribological properties of the semi-solid lubricants. We prepared the oleogels via the highly intensive mixing of vegetable oils with clay at a concentration of 30 wt.%. The oleogels exhibited shear-thinning, thixotropy, structural recovery, and gel-like behavior commonly related to that of a three-dimensional network. The results were corroborated via XRD measurements showing the presence of intercalated nanoclay structures well-dispersed in the vegetable oil. Empirical correlations between the content of saturated (SFAs), unsaturated (UFAs), mono-unsaturated (MUFAs) and poly-unsaturated (PUFAs) fatty acids and the plateau modulus of the aerogels were found. From these experimental results, we can conclude that the fatty acid profile of the vegetable oils exerts an important influence on the rheological and tribological properties of resulting clay and vegetable oil oleogelsThis research was supported by FEDER European Programme and Junta de Andalucía, grant number PY20_00751. The authors acknowledge the X-ray Diffraction Service (Universidad de Huelva) for providing full access and assistance in X-ray Diffraction measurement

Oleo-Dispersions of Electrospun Cellulose Acetate Butyrate Nanostructures: Toward Renewable Semisolid Lubricants

In this work, the electrospinnability of cellulose acetate butyrate (CAb) solutions, and the ability of the resulting micro- and nano-architectures to structure castor oil are studied aiming to develop eco-friendly lubricating greases. Particles, beaded-fibers, defect-free fibers, and porous nanostructures are successfully prepared by dissolving CAb in N,N-dimethylacetamide/acetone (DMAc:Ac, 1:2 w/w) and methylene chloride/acetone (DM:Ac, 1:1 w/w) solvent mixtures at different concentrations (2.5–15 wt.%). The formation of bead-free nanofibers is favored at concentration above 10 wt.%, when solutions achieve relaxation times of ≈50 ms and shear-thinning in extensional and shear flow tests, respectively. Non-porous and porous CAb nanostructures are successfully used as castor oil thickeners at concentrations of 3–5 wt.%, leading a wide variety of rheological responses which mimic those of traditional semisolid lubricants. The surface properties of the nanofibers have a ignificant impact on the wear and friction performance in metal–metal contact, which has been associated with the oil release ability of the generated 3D network. Oleo-dispersions prepared with smooth fibers show tribological performance comparable to, or even better than, commercial lithium greases. Overall, this study reveals the potential of CAb electrospun nanostructures for the development of next-generation renewable semisolid lubricant formulations.This work is part of different research projects funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe” (grant PID2021-125637OB-I00) and by FEDER/Junta de Andalucía Programmes (grants PY20_00751 and UHU202029). J.F.R.-V. has also received a Ph.D. Research Grant PRE2019-090632 from Ministerio de Ciencia e Innovación (Spain). Funding for open access charge: Universidad de Huelva/CBUA. Authors gratefully acknowledge the financial support.Departamento de Ingeniería Química, Química Física y Ciencias de los Materiale

Assessment of the Tribological Performance of Electrospun Lignin Nanofibrous Web-Thickened Bio-Based Greases in a Nanotribometer

The tribological performance of novel bio-based lubricating greases thickened with electrospun lignin nanostructures was investigated in a nanotribometer using a steel-steel ball-on-disc configuration. The impact of electrospun nanofibrous network morphology on friction and wear is explored in this work. Different lignin nanostructures were obtained with electrospinning using ethylcellulose or PVP as co-spinning polymers and subsequently used as thickeners in castor oil at concentrations of 10-30% wt. Friction and wear generally increased with thickener concentration. However, friction and wear decreased when using homogeneous bead-free nanofiber mats (with higher fiber diameter and lower porosity) rather than nanostructures dominated by the presence of particles or beaded fibers, which was favored by reducing the lignin:co-spinning polymer ratio.This work is part of a research project (PID2021-125637OB-I00) funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe” (EU). The financial support is gratefully acknowledged

Composition operators with linear fractional smbols and their adjoints

We characterize all linear fractional maps of the disk into itself in terms of their coefficients. We also prove the formula for the adjoint of a composition operator with a linear fractional symbol acting on the quotient Dirichlet space due to Gallardo and Montes by a method different from theirs.Ministerio de Ciencia y Tecnologí

Uso de nanoestructuras híbridas de acetato de celulosa/nanoarcilla para el desarrollo de lubricantes ecólogicos

Departamento de Ingeniería Química, Química Física y Ciencias de los Materiale

Ab initio studies of propene epoxidation on oxidized silver surfaces

Producción CientíficaAb initio density functional simulations have been performed to study the adsorption of propene on partially oxidized silver surfaces and its interaction with surface oxygen. Two different adsorption conformations for propene are studied, with the molecule either intact or forming an Ag–C3H6–O oxymetallacycle (OMC) intermediate. Then, pathways for propene oxide, acrolein and propanone formation have been studied in detail, providing insight into the selectivity of the surfaces. We find that formation of acrolein must necessarily take place from OMC intermediates, requiring at least two neighbouring reactive surface oxygen anions. This suggests a strong relationship between the concentration of surface oxygen and the selectivity of these surfaces.Ministerio de Ciencia e Innovación (Proyect MAT2008-06483-C03-01 and MAT 2011-22781)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. A158A11-

About a family of naturally graded no p-filiform lie algebras

Junta de Andalucia, PAICYT y Ministerio de Ciencia y Tecnología BFM 2000-104

Computer simulations of the structure of nanoporous carbons and higher density phases of carbon

The most stable form of solid carbon is graphite, a stacking of graphene 2 layers in which the carbon atoms show sp2 hybridization which leads to strong intra3 layer bonding. Diamond is a denser phase, obtained at high pressure. In diamond the 4 carbon atoms show sp3 hybridization. Metastable solid carbon phases can be pre5 pared also with lower density than graphite (in fact, densities lower than water); for 6 instance the carbide-derived carbons. These are porous materials with a quite disor7 dered structure. Atomistic computer simulations of carbide-derived carbons indicate 8 that the pore walls can be viewed as curved and planar nanographene ribbons with 9 numerous defects and open edges. Consequently, the hybridization of the carbon 10 atoms in the porous carbons is sp2. Because of the high porosity and large specific 11 surface area, nanoporous carbons find applications in gas adsorption, batteries and 12 nanocatalysis, among others. We have performed computer simulations, employing 13 large simulation cells and long simulation times, to reveal the details of the structure 14 of the nanoporous carbons. In the dynamical simulations the interactions between 15 the atoms are represented by empirical many-body potentials. We have also investi16 gated the effect of the density on the structure of the disordered carbons and on the 17 hybridization of the carbon atoms. At low densities, typical of the porous carbide18 derived carbons formed experimentally, the hybridization is sp2. On the other hand, 19 as the density of the disordered material increases, a growing fraction of atoms with 20 sp3 hybridization appear

Influence of surfactants on the electrospinnability of lignin‑PVP solutions and subsequent oil structuring properties of nanofiber mats

This work focuses on the improvement of the electrospinnability of low-sulfonate lignin (LSL)/polyvinylpyrrolidone (PVP) solutions by the addition of surfactants (SDS, CTAB and Tween-20) as well as on the ability of resulting nanofibers to structure castor oil. Solutions with two LSL/PVP weight ratios (70:30 and 90:10) in DMF were prepared by adding variable surfactant concentrations (0–1 wt.%), and physicochemically characterized. Electrical conductivity, surface tension and rheological measurements were performed. Variations of these physicochemical properties were explained on the basis of surfactant-polymer interactions. The addition of surfactants to LSL/PVP solutions improves electrospinnability, producing more compact and uniform fiber mats in 70:30 LSL/PVP systems, generally reducing the average diameter of the nanofibers and the number of beads. In contrast, nanofiber mats were not obtained with 90:10 LSL/PVP solutions, but different nanostructures composed of particle clusters. Dispersions of nanofiber mats obtained by electrospinning from 70:30 LSL/PVP solutions in castor oil were able to generate physically stable strong oleogels. In general, linear viscoelastic functions of oleogels increased with surfactant concentration. In addition, these oleogels exhibited excellent lubrication performance in a tribological contact, with extremely low values of the friction coefficient and wear diameters, which may lead to potential applications as lubricants.Research project (RTI2018-096080-B-C21) funded by MCIN/ AEI/10. 13039/ 501100011033 and by “ERDF A way of making Europe.” Funding for open access charge Universidad de Huelva/CBU