摘要 :
Les effets du vieillissement hygrothermique sur la tenue en fatigue de composites sont analyses dans le contexte specifique de la flexion des unidirectionnels a matrices organiques renforcees par des fibres de verre. Dans une prem...
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Les effets du vieillissement hygrothermique sur la tenue en fatigue de composites sont analyses dans le contexte specifique de la flexion des unidirectionnels a matrices organiques renforcees par des fibres de verre. Dans une premiere partie, les modes d'interactions entre la physico-chimie du vieillissement humide et les mecanismes de corrosion sous contrainte du renfort son detailles ainsi que leurs implications sur le choix de descripteurs appropries de l'etat de degradation physico-chimique du composite. Dans une seconde partie, une approche predictive du couplage entre vieillissement et tenue en fatigue est introduite dans le cadre d'un modele de corrosion sous contrainte prenant en compte de fa?on explicite les effets de l'humidite, de la temperature, du temps et de la contrainte appliquee.
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The polydisperse nature of asphaltenes is not usually considered in studies of asphaltenes adsorption effects at interfaces, e.g., water–oil interfaces. We recently proposed a methodology that takes into account the mixture natur...
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The polydisperse nature of asphaltenes is not usually considered in studies of asphaltenes adsorption effects at interfaces, e.g., water–oil interfaces. We recently proposed a methodology that takes into account the mixture nature of asphaltenes and showed that a binary mixture model for diffusion-limited adsorption at water–oil interfaces could describe qualitatively all of the features of asphaltenes’ interfacial dilatational rheology [ Liu, F. ; <i>Langmuir 2017, 33 , 1927−1042, DOI: 10.1021/acs.langmuir.6b03958 ]. On the quantitative side, however, use of only two pseudocomponents did not adequately predict some other aspects of their behavior, such as dynamic interfacial tension over the full range of time scales. To address these limitations, a methodology for calculating interfacial rheological properties for an <i>n -component mixture was first developed [ Liu, F. ; <i>Colloids Surf. A 2017, 532 , 140−143, DOI: 10.1016/j.colsurfa.2017.05.080 ]. To capture, first, the interfacial tension behavior and then the rheological properties within the same methodological structure, we discuss here an approach using a multicomponent model that inversely solves the Ward–Tordai equations and extracts the properties of individual pseudocomponents (concentration and adsorption coefficient) from dynamic interfacial tension measurements. Using ternary mixture models proves sufficient to capture the data obtained for asphaltenes over large adsorption time scales (up to 24 h) and large frequency range. Quaternary mixture models do not significantly improve the predictions. Another feature revealed by this methodology is the aggregation behavior of the different pseudocomponents. For dilute solutions, the calculated sum of the pseudocomponents’ concentrations falls in the range of the actual asphaltenes concentration. As the actual asphaltenes concentration is increased, the calculated concentration of the most surface-active pseudocomponents levels offs, indicating that the most surface-active asphaltenes are also the most prone to aggregate due perhaps to π–π interactions. This result would be expected as asphaltenes adsorption at the water–oil interface appear to be driven by the interactions of the π electrons of their aromatic cores as previously demonstrated [ Rane, J. P. ; <i>Energy Fuels 2015, 29 , 3584−3590, DOI: 10.1021/acs.energyfuels.5b00179 ]. Finally, the result obtained by this model indicates that the presence of a very small fraction of extremely surface-active asphaltenes components could explain both the “everlasting” interfacial tension decay observed and the apparent irreversibility of adsorption during washout experiments.
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The flexural fatigue behaviour of GFRP beams has been analysed in the light of a stress corrosion cracking model. This approach was developed in the context of unidirectional glass/epoxy composites, where the initial fatigue damag...
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The flexural fatigue behaviour of GFRP beams has been analysed in the light of a stress corrosion cracking model. This approach was developed in the context of unidirectional glass/epoxy composites, where the initial fatigue damage is known to be mostly associated to the delayed failure of the fibres. Theoretical expressions for the stiffness loss have been derived, which require the determination of the statistical distribution of fibres strength and the assessment of the sub-critical-crack growth rates within the glass filaments. From stiffness loss measurements under monotonic and static fatigue conditions, consistent values of the Weibull modulus, m, and the stress-corrosion parameter, n, have been obtained. These values were successfully used in the model to predict the experimental stiffness loss of the composite beams under cyclic fatigue conditions.
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Le comportement en fatigue par flexion de poutres composites renforcees par des fibres de verre a ete analyse par un modele de corrosion sous contrainte. L'approche a ete developpee dans le contexte d'un renforcement unidirectionn...
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Le comportement en fatigue par flexion de poutres composites renforcees par des fibres de verre a ete analyse par un modele de corrosion sous contrainte. L'approche a ete developpee dans le contexte d'un renforcement unidirectionnel pour lequel l'endommagement initial est domine par la rupture differee des fibres de verre sous l'action combinee de la contrainte et de l'humidite. Des expressions theoriques de la perte de raideur ont ete etablies qui font appel a la distribution statistique des caracteristiques a rupture des fibres et a la vitesse de croissance sous critique des defauts de surface du verre. A partir de mesures de raideur sous chargement monotone et de relaxation, des valeurs realistes du module de Weibull et du parametre de corrosion sous contrainte ont pu etre identifiees. L'application du modele a ete validee sous des cas de chargements periodiques a differents rapports de sollicitation, deformations appliquees et frequences.
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The emulsion stabilizing properties of a low-total-acid-number (TAN) crude oil, which had initially been attributed to asphaltenes and calcite precipitation, were re-analyzed with regard to the role of organic acids. Despite high ...
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The emulsion stabilizing properties of a low-total-acid-number (TAN) crude oil, which had initially been attributed to asphaltenes and calcite precipitation, were re-analyzed with regard to the role of organic acids. Despite high asphaltenes content, this crude oil exhibits features classically observed with acidic oils, such as the increase in emulsion stability upon pressure decrease/pH increase or the poor efficiency of demulsifiers. The potential for a significant role of organic acids was confirmed by the high interfacial activity of indigenous acids, as extracted from the crude oil by means of an ion-exchange resin. This was further addressed analyzing the molecular chemistry of the interfacial layer and its rheology. The interfacial material was found to be composed of a mixture of asphaltenes and organic acids. These acids exhibit a wide range of structures (mono-versus dicarboxylic, fatty versus naphthenic and benzoic) and molecular weights (from 200 to 700 g/mol), contrary to the medium molecular weight fatty monocarboxylic acids that are generally believed to cause "soap emulsions". The interfacial rheology is indicative of a 2D gel, with an assumed glass transition temperature of approximately 40℃. In conclusion, this study shows that a co-precipitation of asphaltenes and organic acids can promote the build up of a very cohesive interface. The disruption of this interface not only requires the drainage of individual molecules but also a collective yield of the gel. This paper is part one of two: it confronts physical and chemical data, the latter being further detailed in an associated paper.
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Previous studies indicated that asphaltenes adsorbed as monomers on oil- water interfaces and the early stage kinetics of the process was controlled by diffusion and hence dependent on oil viscosity. By measuring interfacial tensi...
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Previous studies indicated that asphaltenes adsorbed as monomers on oil- water interfaces and the early stage kinetics of the process was controlled by diffusion and hence dependent on oil viscosity. By measuring interfacial tension (IFT) as a function of surface coverage during droplet expansions in pendant drop experiments, it was also concluded that the IFT data could be interpreted with a Langmuir equation of state (EoS), which was independent of oil viscosity, time of adsorption, and bulk asphaltenes concentration. The surface excess coverage was calculated to be ~0.3 nm~2/molecule, which suggested adsorption in face-on configuration of asphaltenes monomers at the interface and average PAH core per molecule of about 6 for the asphaltenes investigated, consistent with the Yen-Mullins model. The current study focuses on the kinetics of asphaltenes adsorption at longer times and higher interfacial coverage. Long-term IFT data have been measured by the pendant drop method for different asphaltenes concentrations and for different bulk viscosities of the oil phase (0.5-28 cP). The data indicate that when coverage reaches 35-40%, the adsorption rates slow down considerably compared to the diffusion-controlled rates at the very early stages. The surface pressure increase rate (or IFT decrease rate) at these higher coverages is now independent of oil viscosity but dependent upon both surface pressure itself and asphaltene monomer concentration. The long-term asymptotic behavior of surface coverage is found to be consistent with the predictions from surface diffusion-mediated random sequential adsorption (RSA) theory which indicates a linear dependency of surface coverage on 1/√t and an asymptotic limit very close to 2D random close packing of polydispersed disks (85%). From these observations RSA theory parameters were extracted that enabled description of adsorption kinetics for the range of conditions above surface coverage of 35%.
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Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble
in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery an...
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Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble
in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop−
drop coalescence. As a result, asphaltenes unfavorably impact water−oil separation processes and consequently oil production
rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water−oil interfaces. This
study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop
experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The
pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the
interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as
monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than
about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer
times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling
mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with
interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed
to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barriercontrolled
regime rather than to a conformational relaxation regime.
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In previous studies, the adsorption kinetics of asphaltenes at the water?oil interface were interpreted utilizing a Langmuir equation of state (EOS) based on droplet expansion experiments.~(1?3) Long-term adsorption kinetics follo...
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In previous studies, the adsorption kinetics of asphaltenes at the water?oil interface were interpreted utilizing a Langmuir equation of state (EOS) based on droplet expansion experiments.~(1?3) Long-term adsorption kinetics followed random sequential adsorption (RSA) theory predictions, asymptotically reaching ～85% limiting surface coverage, which is similar to limiting random 2D close packing of disks. To extend this work beyond this slow adsorption process, we performed rapid contractions and contraction?expansions of asphaltene-laden interfaces using the pendant drop experiment to emulate a Langmuir trough. This simulates the rapid increase in interfacial asphaltene concentration that occurs during coalescence events. For the contraction of droplets aged in asphaltene solutions, deviation from the EOS consistently occurs at a surface pressure value ～21 mN/m corresponding to a surface coverage ～80%. At this point droplets lose the shape required for validity of the Laplace?Young equation, indicating solidlike surface behavior. On further contraction wrinkles appear, which disappear when the droplet is held at constant volume. Surface pressure also decreases down to an equilibrium value near that measured for slow adsorption experiments. This behavior appears to be due to a transition to a glassy interface on contraction past the packing limit, followed by relaxation toward equilibrium by desorption at constant volume. This hypothesis is supported by cycling experiments around the close-packed limit where the transition to and from a solidlike state appears to be both fast and reversible, with little hysteresis. Also, the soft glass rheology model of Sollich is shown to capture previously reported shear behavior during adsorption. The results suggest that the mechanism by which asphaltenes stabilize water-in-oil emulsions is by blocking coalescence due to rapid formation of a glassy interface, in turn caused by interfacial asphaltenes rapidly increasing in concentration beyond the glass transition point.
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Stirred tanks experiments were conducted to investigate water droplets size in asphaltenes solutions. After emulsification, droplets were kept suspended by a gentle agitation. Pictures were periodically captured by means of an in ...
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Stirred tanks experiments were conducted to investigate water droplets size in asphaltenes solutions. After emulsification, droplets were kept suspended by a gentle agitation. Pictures were periodically captured by means of an in situ microscope. For low asphaltenes content, droplets were found not be stable upon completion of emulsification but to grow in size upon the decrease in agitation. This coalescence process was fast yet limited: droplets quickly reached an equilibrium size. Equilibrium size was found to be independent from emulsification conditions. On the other hand, varying emulsion composition yielded to a much larger variation of equilibrium size than expected from the impact of interfacial tension on break-up. Equilibrium droplet size proved proportional to the ratio of the mass of emulsified water to the mass of molecular asphaltenes present in the organic phase. Such a dependency is reminiscent of limited coalescence within Pickering emulsions. Upon coalescence, the total interfacial area of an emulsion decreases. For irreversibly adsorbed particles, surface coverage increases up to a critical value (close to 2D maximum packing) blocking further coalescence. In the present case, irreversible adsorption of asphaltenes was favored by the large aliphatic content of the organic phase. The estimated critical mass coverage for asphaltenes (3.2–3.5 mg/m~2) is close to results of direct titration found in the literature. On the other hand for high asphaltenes content (i.e. high enough to crowd water surface during emulsification), droplets do not coalesce after reduction of agitation confirming again the limited coalescence principle. Finally, the critical mass coverage was converted into a critical molecular coverage from the estimate of the molar mass of asphaltenes. This critical molecular coverage corresponds to 80-85% of a recent estimate of the surface excess coverage for asphaltenes of the same origin. Those observations tend to disqualify interfacial cross-linking as the cause for emulsion stabilization by asphaltenes. This stabilization could be due to steric jamming in dense monolayers, like for nanoparticles or proteins. Such a scenario is compatible with most of the phenomenological observations previously ascribed to slow cross-linking.
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An alternative approach for deriving the equation of state for a two-dimensional lattice gas is proposed, based on arguments similar to those used in the derivation of the Langmuir-Szyszkowski equation of state for localized adsor...
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An alternative approach for deriving the equation of state for a two-dimensional lattice gas is proposed, based on arguments similar to those used in the derivation of the Langmuir-Szyszkowski equation of state for localized adsorption. The relationship between surface coverage and excluded area is first extracted from random sequential adsorption simulations incorporating surface diffusion (RSAD). The adsorption isotherm is then obtained using kinetic arguments, and the Gibbs equation gives the relation between surface pressure and coverage. Provided surface diffusion is fast enough to ensure internal equilibrium within the monolayer during the RSAD simulations, the resulting equations of state are very close to the most accurate equivalents obtained by cumbersome thermodynamic methods. An internal test of the accuracy of the method is obtained by noting that adsorption RSAD simulations starting from an empty lattice and desorption simulations starting from a full lattice provide convergent upper and lower bounds on the surface pressure.
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