The Experts below are selected from a list of 28458 Experts worldwide ranked by ideXlab platform
Claudine Buess-herman - One of the best experts on this subject based on the ideXlab platform.
-
Adsorption sequence of the alkali cations at the tungsten trioxide–water interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1999Co-Authors: F. Dumont, Paul Verbeiren, Claudine Buess-hermanAbstract:Abstract The stability of pyrogenic tungsten trioxide hydrosols is studied in the presence of various monovalent cations between pH 2 and 12. The hydrosol critical coagulation concentrations (c*) are determined by extrapolation of the experimental log W–log c curves up to log W=0. The c* of K+, Rb+and Cs+ slightly increase with the pH of the solution while the c* of Li+ presents a pronounced maximum at pH 4.5. A less marked maximum is also observed at the same pH in the presence of Na+. The corresponding Ionic Adsorption sequence at the water–WO3 interface deduced from these experimental data is Cs+>Rb+>K+>Na+>Li+ in the pH domain under investigation. This overall behavior is successfully explained by the model proposed by Gierst who has generalized the Gurney concept describing the ion–ion interactions in solution to the ion–surface interactions. The results reported in this work are also compared with those obtained previously for other oxides; they confirm the validity and the generality of the conclusions already drawn concerning the Ionic Adsorption sequences at the oxide–water interface.
-
Adsorption sequence of the alkali cations at the tungsten trioxide–water interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1999Co-Authors: Freddy Dumont, Paul Verbeiren, Claudine Buess-hermanAbstract:The stability of pyrogenic tungsten trioxide hydrosols is studied in the presence of various monovalent cations between pH 2 and 12. The hydrosol critical coagulation concentrations (c*) are determined by extrapolation of the experimental logW-logc curves up to logW=0. The c* of K+, Rb+ and Cs+ slightly increase with the pH of the solution while the c* of Li+ presents a pronounced maximum at pH 4.5. A less marked maximum is also observed at the same pH in the presence of Na+. The corresponding Ionic Adsorption sequence at the water-WO3 interface deduced from these experimental data is Cs+>Rb+>K+>Na+>Li+ in the pH domain under investigation. This overall behavior is successfully explained by the model proposed by Gierst who has generalized the Gurney concept describing the ion-ion interactions in solution to the ion-surface interactions. The results reported in this work are also compared with those obtained previously for other oxides; they confirm the validity and the generality of the conclusions already drawn concerning the Ionic Adsorption sequences at the oxide-water interface. Copyright (C) 1999 Elsevier Science B.V.SCOPUS: cp.jinfo:eu-repo/semantics/publishe
-
Stability of TiO2 hydrosols synthesized by hydrolysis of titanium tetraethoxide
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998Co-Authors: E. Ghenne, F. Dumont, Claudine Buess-hermanAbstract:Abstract The TiO2 hydrosols studied in this work were synthesized by hydrolysis of titanium tetraethoxide according to Zukoski's method. Critical coagulation concentrations (CCC) were measured for pH values ranging from 2 to 12 in the presence of various monovalent ions. The observed Ionic Adsorption sequences are explained in terms of an extension of the Gurney concept of ion-ion interactions. The influence of the volume fraction of the solid is also reported and explained by the coagulation-repeptization theory.
-
Properties of TiO2 hydrosols synthesized by hydrolysis of titanium tetraethoxide
Progress in colloid and polymer science, 1997Co-Authors: E. Ghenne, F. Dumont, Claudine Buess-hermanAbstract:The TiO2 hydrosols studied in this work were synthesized by hydrolysis of titanium tetraethoxide according to Zukoski's method. Critical coagulation concentrations (CCC) were measured for pH values ranging from 2 to 12 in the presence of various monovalent ions. The observed Ionic Adsorption sequences are explained in terms of an extension of Gurney concept of ion-ion interactions. The influence of the volume fraction of the solid is also reported and explained by the coagulation-repeptization theory.
F. Dumont - One of the best experts on this subject based on the ideXlab platform.
-
Adsorption sequence of the alkali cations at the tungsten trioxide–water interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1999Co-Authors: F. Dumont, Paul Verbeiren, Claudine Buess-hermanAbstract:Abstract The stability of pyrogenic tungsten trioxide hydrosols is studied in the presence of various monovalent cations between pH 2 and 12. The hydrosol critical coagulation concentrations (c*) are determined by extrapolation of the experimental log W–log c curves up to log W=0. The c* of K+, Rb+and Cs+ slightly increase with the pH of the solution while the c* of Li+ presents a pronounced maximum at pH 4.5. A less marked maximum is also observed at the same pH in the presence of Na+. The corresponding Ionic Adsorption sequence at the water–WO3 interface deduced from these experimental data is Cs+>Rb+>K+>Na+>Li+ in the pH domain under investigation. This overall behavior is successfully explained by the model proposed by Gierst who has generalized the Gurney concept describing the ion–ion interactions in solution to the ion–surface interactions. The results reported in this work are also compared with those obtained previously for other oxides; they confirm the validity and the generality of the conclusions already drawn concerning the Ionic Adsorption sequences at the oxide–water interface.
-
Stability of TiO2 hydrosols synthesized by hydrolysis of titanium tetraethoxide
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998Co-Authors: E. Ghenne, F. Dumont, Claudine Buess-hermanAbstract:Abstract The TiO2 hydrosols studied in this work were synthesized by hydrolysis of titanium tetraethoxide according to Zukoski's method. Critical coagulation concentrations (CCC) were measured for pH values ranging from 2 to 12 in the presence of various monovalent ions. The observed Ionic Adsorption sequences are explained in terms of an extension of the Gurney concept of ion-ion interactions. The influence of the volume fraction of the solid is also reported and explained by the coagulation-repeptization theory.
-
Properties of TiO2 hydrosols synthesized by hydrolysis of titanium tetraethoxide
Progress in colloid and polymer science, 1997Co-Authors: E. Ghenne, F. Dumont, Claudine Buess-hermanAbstract:The TiO2 hydrosols studied in this work were synthesized by hydrolysis of titanium tetraethoxide according to Zukoski's method. Critical coagulation concentrations (CCC) were measured for pH values ranging from 2 to 12 in the presence of various monovalent ions. The observed Ionic Adsorption sequences are explained in terms of an extension of Gurney concept of ion-ion interactions. The influence of the volume fraction of the solid is also reported and explained by the coagulation-repeptization theory.
-
Influence of the point of zero charge of titanium dioxide hydrosols on the Ionic Adsorption sequences
Journal of Colloid and Interface Science, 1990Co-Authors: F. Dumont, Jean Warlus, André WatillonAbstract:Abstract Several TiO2 hydrosols with point of zero charge (pzc) ranging from 2.8 to 6 were prepared depending on the method of synthesis. The Ionic Adsorption sequences at the different interfaces were deduced from the critical coagulation concentrations of the corresponding hydrosols. The general behavior can be summarized as follows: on positively charged surfaces, the structure-maker anions are always more adsorbed than the structure-breakers; on negatively charged surfaces, for “high pzc” hydrosols, the catIonic sequence is the same as that observed for the anions, whereas, for “low pzc” hydrosols, the reverse sequence is found. The model based on the generalization to the ion-surface interactions of the Gurney concept of the ion-ion interactions, as proposed by Gierst, is used to explain the observed properties of the different hydrosols.
Jose M. Guisan - One of the best experts on this subject based on the ideXlab platform.
-
Reversible Immobilization of Glucoamylase by Ionic Adsorption on Sepabeads Coated with Polyethyleneimine
Biotechnology Progress, 2008Co-Authors: Rodrigo Torres, Benevides C. C. Pessela, Cesar Mateo, Claudia Ortiz, Manuel Fuentes, Jose M. Guisan, Roberto Fernandez-lafuenteAbstract:Glucoamylase (GA) from Aspergillus niger was immobilized via Ionic Adsorption onto DEAE-agarose, Q1A-Sepabeads, and Sepabeads EC-EP3 supports coated with polyethyleneimine (PEI). After optimization of the immobilization conditions (pH, polymer size), it was observed that the Adsorption strength was much higher in PEI-Sepabeads than in Q1A-Sepabeads or DEAE-supports, requiring very high Ionic strength to remove glucoamylase from the PEI-supports (e.g., 1 M NaCl at pH 5.5). Thermal stability and optimal temperature was marginally improved by this immobilization. Recovered activity depended on the substrate used, maltose or starch, except when very low loading was used. The optimization of the loading allowed the preparation of derivatives with 750 IU/g in the hydrolysis of starch, preserving a high percentage of immobilized activity (around 50%).
-
Stabilization of different alcohol oxidases via immobilization and post immobilization techniques
Enzyme and Microbial Technology, 2007Co-Authors: Fernando López-gallego, Cesar Mateo, Roberto Fernandez-lafuente, Lorena Betancor, Aurelio Hidalgo, Gisela Dellamora-ortiz, Jose M. GuisanAbstract:The thermal stability of multimeric alcohol oxidases (AOXs) from three different sources (Candida boidinii, Hansenula sp. and Pichia pastoris) was evaluated. AOX from C. boidinii was markedly more unstable than the other two enzymes, while the enzyme from Hansenula sp. was the most stable of the three. The stability of the enzymes was strongly dependent on the enzyme concentration, suggesting that the first inactivation cause could be subunits dissociation. The two most stable enzymes were immobilized through different immobilization methodologies (covalent and Ionic Adsorption techniques). The best results in terms of stability were obtained when the enzymes were covalently immobilized in glyoxyl agarose. However, with this strategy the quaternary structure of these enzymes could not be fully stabilized. Thus, dextran aldehyde was used to cross-link the enzyme subunits not attached to the support. Although this treatment prevents dissociation of the enzyme subunits and avoids the effect of enzyme concentration on the stability of both enzymes, the final activity recovery in both cases was below 20%. The other very stabilizing immobilization protocol for both enzymes was the Ionic Adsorption on agarose coated with 600 kDa polyethylenimine. The Adsorption of the proteins on the polymeric bed allowed the stabilization of the quaternary structure: the effect of the enzyme concentration on thermal enzyme stability disappeared in these immobilized preparations. Thus, a high stability was obtained without a significant decrease in enzyme activity during immobilization (recovered activity was over 50%).
-
Reversible immobilization of a thermophilic β-galactosidase via Ionic Adsorption on PEI-coated Sepabeads
Enzyme and Microbial Technology, 2003Co-Authors: Benevides C. C. Pessela, Cesar Mateo, Manuel Fuentes, Roberto Fernandez-lafuente, Alejandro Vian, José Luis García, Alfonso V. Carrascosa, Jose M. GuisanAbstract:Abstract The immobilization of the enzyme β-galactosidase from Thermus sp. T2 was performed via Ionic Adsorption onto two different supports: a new anIonic exchanger resin, based on the coating of Sepabeads internal surfaces with polyethylenimine (PEI) polymers ( M w 25,000), and conventional DEAE-agarose. Immobilization proceeded very rapidly in both cases, but the Adsorption strength was much higher in the case of PEI-Sepabeads than in DEAE-supports at both pH 5 and 7 (e.g. at pH 7 and 0.4 M NaCl, less than 5% of enzyme was eluted from PEI-support while more than 70% protein was eluted from DEAE-agarose). Interestingly, the PEI-derivatives remained almost fully active at pH 5 and 7 after several weeks of incubation at 50 °C, conditions that allows the hydrolysis of lactose in milk coupled with the antimicrobial treatment usually performed.
-
Reversible enzyme immobilization via a very strong and nondistorting Ionic Adsorption on support-polyethylenimine composites.
Biotechnology and Bioengineering, 2000Co-Authors: Cesar Mateo, Roberto Fernandez-lafuente, Olga Abian, Jose M. GuisanAbstract:New tailor-made anIonic exchange resins have been prepared, based on films of large polyethylenimine polymers (e.g., MW 25,000) completely coating, via covalent immobilization, the surface of different porous supports (agarose, silica, polymeric resins). Most proteins contained in crude extracts from different sources have been very strongly adsorbed on them. Ionic exchange properties of such composites strongly depend on the size of polyethylenimine polymers as well as on the exact conditions of the covalent coating of the solids with the polymer. On the contrary, similar coating protocols yield similar matrices by using different porous supports as starting material. For example, 77% of all proteins contained in crude extracts from Escherichia coli were adsorbed, at low Ionic strength, on the best matrices, and less than 15% of the adsorbed proteins were eluted from the support in the presence of 0.3 M NaCl. Under these conditions, 100% of the adsorbed proteins were eluted from conventional DEAE supports. Such polyethylenimine–support composites were also very suitable to perform very strong and nondistorting reversible immobilization of industrial enzymes. For example, lipase from Candida rugosa (CRL), β-galactosidase from Aspergillus oryzae and D-amino acid oxidase (DAAO) from Rhodotorula gracilis, were adsorbed on such matrices in a few minutes at pH 7.0 and 4°C. Immobilized enzymes preserved 100% of catalytic activity and remained fully immobilized in 0.2 M NaCl. In addition to that, CRL and DAAO were highly stabilized upon immobilization. Stabilization of DAAO, a dimeric enzyme, seems to be due to the involvement of both enzyme subunits in the Ionic Adsorption. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 98–105, 2000.
L R Evangelista - One of the best experts on this subject based on the ideXlab platform.
-
Destabilizing effect of a surface electric field generated by Ionic Adsorption on the molecular orientation of nematic liquid crystals
The European Physical Journal E, 2005Co-Authors: H.a. Pereira, F Batalioto, L R EvangelistaAbstract:The destabilizing effect of a surface electric field, produced by selective Ionic Adsorption, on the molecular orientation of a nematic-liquid-crystal sample is analyzed for a cell in the shape of a slab of thickness d . The electric-field distribution considered in the analysis is the one obtained in the limit in which essentially all the positive ions are adsorbed. Because of the coupling of this surface field with the nematic director, the surface anchoring energy depends on the thickness of the sample as well as on the Adsorption energy characterizing the surfaces. A relation connecting the threshold field for the destabilization of the homeotropic pattern to the Adsorption energy and to the thickness of the sample is established in closed form, after solving a set of two coupled non-linear equations determining the electric-field distribution across the sample. It is shown that the values of surface electric field generated by adsorbed ions that can lead to a destabilization of the homeotropic alignment can be attained by real samples.
-
Contribution of the Ionic Adsorption phenomenon to the effective anchoring energy of a nematic liquid-crystal sample.
Physical review. E Statistical nonlinear and soft matter physics, 2003Co-Authors: H.a. Pereira, F Batalioto, L R EvangelistaAbstract:The effective anchoring energy resulting from the Ionic Adsorption phenomenon in a nematic liquid-crystal sample in the shape of a slab of thickness d is investigated. The electric field distribution is determined in the framework of a general nonlinear Poisson-Boltzmann approach. The analysis is particularized for the case in which d>>lambdaD, where lambdaD is the Debye screening length. In this limit, the spatially dependent electric field distribution across the sample as well as the contribution, of dielectric and flexoelectric origins, to the effective anchoring energy is obtained in an exact manner.
-
Surface electric-field-induced instabilities in a homeotropically oriented nematic liquid crystal sample
The European Physical Journal E, 2000Co-Authors: H.a. Pereira, L R EvangelistaAbstract:The effect of a surface electric field produced by Ionic Adsorption on the molecular orientation of a nematic liquid crystal sample is analyzed. The eigenvalue problem for a semi-infinite medium is analytically solved both for strong and weak anchoring situations. The threshold instabilities are numerically determined and it is shown that the homeotropic pattern can be destabilized also in the situation of strong anchoring. The dependence of the threshold field on the anchoring strength and on the surface polarization is determined by taking into account also the coupling of the quadrupolar component of the flexoelectric coefficient with the field gradient.
-
Asymmetric Ionic Adsorption and cell polarization in liquid crystals
Journal of Applied Physics, 2000Co-Authors: Giovanni Barbero, L R Evangelista, D. OliveroAbstract:A model to determine the steady-state distribution of Ionic charges in a liquid sample, in the shape of a slab, whose limiting surfaces are supposed to adsorb positive ions with different Adsorption energies, is proposed. It is used to explain the asymmetric electro-optical response in a liquid-crystal cell recently observed. The Ionic Adsorption gives rise to a difference of potential between the surfaces which can be responsible for the existence of an internal field. It changes the effective voltage thresholds for the reorientation of the liquid crystal when external fields are applied.
Roberto Fernandez-lafuente - One of the best experts on this subject based on the ideXlab platform.
-
Reversible Immobilization of Glucoamylase by Ionic Adsorption on Sepabeads Coated with Polyethyleneimine
Biotechnology Progress, 2008Co-Authors: Rodrigo Torres, Benevides C. C. Pessela, Cesar Mateo, Claudia Ortiz, Manuel Fuentes, Jose M. Guisan, Roberto Fernandez-lafuenteAbstract:Glucoamylase (GA) from Aspergillus niger was immobilized via Ionic Adsorption onto DEAE-agarose, Q1A-Sepabeads, and Sepabeads EC-EP3 supports coated with polyethyleneimine (PEI). After optimization of the immobilization conditions (pH, polymer size), it was observed that the Adsorption strength was much higher in PEI-Sepabeads than in Q1A-Sepabeads or DEAE-supports, requiring very high Ionic strength to remove glucoamylase from the PEI-supports (e.g., 1 M NaCl at pH 5.5). Thermal stability and optimal temperature was marginally improved by this immobilization. Recovered activity depended on the substrate used, maltose or starch, except when very low loading was used. The optimization of the loading allowed the preparation of derivatives with 750 IU/g in the hydrolysis of starch, preserving a high percentage of immobilized activity (around 50%).
-
Stabilization of different alcohol oxidases via immobilization and post immobilization techniques
Enzyme and Microbial Technology, 2007Co-Authors: Fernando López-gallego, Cesar Mateo, Roberto Fernandez-lafuente, Lorena Betancor, Aurelio Hidalgo, Gisela Dellamora-ortiz, Jose M. GuisanAbstract:The thermal stability of multimeric alcohol oxidases (AOXs) from three different sources (Candida boidinii, Hansenula sp. and Pichia pastoris) was evaluated. AOX from C. boidinii was markedly more unstable than the other two enzymes, while the enzyme from Hansenula sp. was the most stable of the three. The stability of the enzymes was strongly dependent on the enzyme concentration, suggesting that the first inactivation cause could be subunits dissociation. The two most stable enzymes were immobilized through different immobilization methodologies (covalent and Ionic Adsorption techniques). The best results in terms of stability were obtained when the enzymes were covalently immobilized in glyoxyl agarose. However, with this strategy the quaternary structure of these enzymes could not be fully stabilized. Thus, dextran aldehyde was used to cross-link the enzyme subunits not attached to the support. Although this treatment prevents dissociation of the enzyme subunits and avoids the effect of enzyme concentration on the stability of both enzymes, the final activity recovery in both cases was below 20%. The other very stabilizing immobilization protocol for both enzymes was the Ionic Adsorption on agarose coated with 600 kDa polyethylenimine. The Adsorption of the proteins on the polymeric bed allowed the stabilization of the quaternary structure: the effect of the enzyme concentration on thermal enzyme stability disappeared in these immobilized preparations. Thus, a high stability was obtained without a significant decrease in enzyme activity during immobilization (recovered activity was over 50%).
-
Reversible immobilization of a thermophilic β-galactosidase via Ionic Adsorption on PEI-coated Sepabeads
Enzyme and Microbial Technology, 2003Co-Authors: Benevides C. C. Pessela, Cesar Mateo, Manuel Fuentes, Roberto Fernandez-lafuente, Alejandro Vian, José Luis García, Alfonso V. Carrascosa, Jose M. GuisanAbstract:Abstract The immobilization of the enzyme β-galactosidase from Thermus sp. T2 was performed via Ionic Adsorption onto two different supports: a new anIonic exchanger resin, based on the coating of Sepabeads internal surfaces with polyethylenimine (PEI) polymers ( M w 25,000), and conventional DEAE-agarose. Immobilization proceeded very rapidly in both cases, but the Adsorption strength was much higher in the case of PEI-Sepabeads than in DEAE-supports at both pH 5 and 7 (e.g. at pH 7 and 0.4 M NaCl, less than 5% of enzyme was eluted from PEI-support while more than 70% protein was eluted from DEAE-agarose). Interestingly, the PEI-derivatives remained almost fully active at pH 5 and 7 after several weeks of incubation at 50 °C, conditions that allows the hydrolysis of lactose in milk coupled with the antimicrobial treatment usually performed.
-
Reversible enzyme immobilization via a very strong and nondistorting Ionic Adsorption on support-polyethylenimine composites.
Biotechnology and Bioengineering, 2000Co-Authors: Cesar Mateo, Roberto Fernandez-lafuente, Olga Abian, Jose M. GuisanAbstract:New tailor-made anIonic exchange resins have been prepared, based on films of large polyethylenimine polymers (e.g., MW 25,000) completely coating, via covalent immobilization, the surface of different porous supports (agarose, silica, polymeric resins). Most proteins contained in crude extracts from different sources have been very strongly adsorbed on them. Ionic exchange properties of such composites strongly depend on the size of polyethylenimine polymers as well as on the exact conditions of the covalent coating of the solids with the polymer. On the contrary, similar coating protocols yield similar matrices by using different porous supports as starting material. For example, 77% of all proteins contained in crude extracts from Escherichia coli were adsorbed, at low Ionic strength, on the best matrices, and less than 15% of the adsorbed proteins were eluted from the support in the presence of 0.3 M NaCl. Under these conditions, 100% of the adsorbed proteins were eluted from conventional DEAE supports. Such polyethylenimine–support composites were also very suitable to perform very strong and nondistorting reversible immobilization of industrial enzymes. For example, lipase from Candida rugosa (CRL), β-galactosidase from Aspergillus oryzae and D-amino acid oxidase (DAAO) from Rhodotorula gracilis, were adsorbed on such matrices in a few minutes at pH 7.0 and 4°C. Immobilized enzymes preserved 100% of catalytic activity and remained fully immobilized in 0.2 M NaCl. In addition to that, CRL and DAAO were highly stabilized upon immobilization. Stabilization of DAAO, a dimeric enzyme, seems to be due to the involvement of both enzyme subunits in the Ionic Adsorption. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 98–105, 2000.
