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| 2. |
- Ojeda, M., et al.
(författare)
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Synthesis of Rh nano-particles by the microemulsion technology - Particle size effect on the CO+H-2 reaction
- 2004
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Ingår i: Applied Catalysis A. - : Elsevier BV. - 0926-860X .- 1873-3875. ; 274:1-2, s. 33-41
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Tidskriftsartikel (refereegranskat)abstract
- Several alumina-supported rhodium-based catalysts have been prepared using the microemulsion technology. The change of microemulsion properties led to catalysts with different Rh particle size and narrow size distribution while keeping the same metal loading. The metal particle size was determined by XRD and TEM. These catalysts were tested in the CO hydrogenation reaction in order to investigate the influence of the Rh particle size. The catalytic results indicated that turnover frequency (TOF) increased about four times when the Rh particle size increased approximately from
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| 3. |
- Stubenrauch, C., et al.
(författare)
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Comparison between interaction forces at air/liquid interfaces in the presence of non-ionic surfactants
- 2004
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Ingår i: Tenside Surfactants Detergents. - : Walter de Gruyter GmbH. - 0932-3414 .- 2195-8564. ; 41:4, s. 174-179
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Tidskriftsartikel (refereegranskat)abstract
- Interactions in thin liquid foam films and between solid/liquid surfaces coated with the non-ionic surfactant hexaoxyethylene dodecylether (C12E6) were investigated with TFPB (thin film pressure balance) and the MASIF (Measurement and Analysis of Surface Interaction Forces) technique, respectively. For foam films the formation of common black films (CBF) and Newton black films (NBF) was observed depending on the surfactant concentration and the applied pressure. With increasing surfactant concentration and increasing pressure the CBF is destablized, whereas a stabilization of the NBF is observed. In addition, MASIF measurements with two different hydrophobic surfaces were made. In one case silanated glass surfaces and in the other case thiolated gold surfaces were used. Differences and similarities of the interactions between these two surfaces are shown and discussed. Furthermore, the results of the MASIF measurements are compared with the ones of the thin films pressure balance measurements and the influence of the surfactant concentration on the interactions is discussed.
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| 4. |
- Stubenrauch, C., et al.
(författare)
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Interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl Ether C12E6 and the origin of surface charges at the air/water interface
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:12, s. 4977-4988
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Tidskriftsartikel (refereegranskat)abstract
- The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces were chosen. The interactions between the air/water interfaces were measured with a thin film pressure balance in terms of disjoining pressure as a function of film thickness. The interactions between the solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the nature of the surface on the interaction forces was investigated at surfactant concentrations below and above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged silanated glass and the charged air/water interface results in a lowering of the surface charge density. The reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at the air/water interface as well as its decrease with increasing surfactant concentration is discussed and a new mechanism for generation of OH- ions at this particular interface is proposed.
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| 5. |
- Claesson, Per M., et al.
(författare)
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Polyelectrolytes as adhesion modifiers
- 2003
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Ingår i: Advances in Colloid and Interface Science. - 0001-8686 .- 1873-3727. ; 104, s. 53-74
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Tidskriftsartikel (refereegranskat)abstract
- Adsorbed layers of polyelectrolytes have been studied with atomic force microscopy (AFM) and the interferometric surface force apparatus (SFA). Particular emphasis was put on determining the effect of the polyelectrolyte charge density on surface topography, and the effect of the polyelectrolyte coating on the adhesive properties. The AFM was employed to image individual polymer chains at low adsorption densities and to characterize the layer topography and coverage at higher adsorption densities. The adhesive properties between two polyelectrolyte-coated surfaces in air were determined as a function of the number of contacts made at any given spot. The data provide evidence for formation of electrostatic bridges, particularly when highly charged polyelectrolytes are used. Further, material transport between the surfaces is observed when the polyelectrolyte is either highly charged or have a very low charge density. For intermediate charge densities we could not observe any indication of material transfer. The adhesion between one polyelectrolyte-coated surface and one bare surface was initially higher than that between two polyelectrolyte-coated surfaces. However, due to material transfer between the two surfaces the adhesion decreased significantly with the number of times that the surfaces were driven into contact. For the polyelectrolytes of the lowest charge density the results suggest that entanglement effects contribute to the adhesive interaction. The modification of the adhesion by polyelectrolytes in practical systems such as in the case of dry-strength additives to improve paper resistance is also considered.
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| 6. |
- Rojas, Orland J., et al.
(författare)
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Coadsorption and surface forces for selective surfaces in contact with aqueous mixtures of oppositely charged surfactants and low charge density polyelectrolytes
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:8, s. 3221-3230
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Tidskriftsartikel (refereegranskat)abstract
- The coadsorption of a positively charged polyelectrolyte (with 10% of the segments carrying a permanent positive charge, AM-MAPTAC-10) and an anionic surfactant (sodium dodecyl sulfate, SDS) on silica and glass surfaces has been investigated using optical reflectometry and a noninterferometric surface force technique. This is a selective coadsorption system in the sense that the polyelectrolyte does adsorb to the surface in the absence of surfactant, whereas the surfactant does not adsorb in the absence of polyelectrolyte. It is found that the total adsorbed amount goes through a maximum when the SDS concentration is increased. Maximum adsorption is found when the polyelectrolyte/surfactant complexes formed in bulk solution are close to the charge neutralization point. Some adsorption does occur also when SDS is present in significant excess. The force measured between AM-MAPTAC-10-coated surfaces on approach in the absence of SDS is dominated at long range by an electrostatic double-layer force. Yet, layers formed by coadsorption from solutions containing both polyelectrolyte and surfactant generate long-range forces of an electrosteric nature. On separation, adhesive interactions are found only when the adsorbed amount is low, i.e., in the absence of SDS and in a large excess of SDS. The final state of the adsorbed layer is found to be nonhysteretic, i.e., independent of the history of the system. The conditions for formation of long-lived trapped adsorption states from mixed polymer-surfactant solutions are discussed.
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| 7. |
- Rojas, O. J., et al.
(författare)
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Desorption of low-charge-density polyelectrolyte adlayers in aqueous sodium n-dodecyl sulfate solution
- 2001
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 237:1, s. 104-111
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Tidskriftsartikel (refereegranskat)abstract
- The association between low-charge-density polyelectrolytes adsorbed onto negatively charged surfaces (mica and silica) and an anionic surfactant, sodium dodecyl sulfate (SDS), has been investigated using surface force measurements, ellipsometry, and XPS. All three techniques show that the polyelectrolyte desorbs when the SDS concentration is high enough. The XPS study indicates that desorption starts at a SDS concentration of ca. 0.1 unit of cmc (8 x 10(-4) M) and that the desorption proceeds progressively as the SDS concentration is increased. Surface force measurements show that for the polyelectrolyte studied here, having 1% of the segments charged, the desorption proceeds without any swelling of the adsorbed layer. This behavior differs from that observed when polyelectrolytes of greater charge density are used.
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| 8. |
- Rojas, O. J., et al.
(författare)
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Effect of polyelectrolyte charge density on the adsorption and desorption behavior on mica
- 2002
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 18:5, s. 1604-1612
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Tidskriftsartikel (refereegranskat)abstract
- The XPS (ESCA) method was employed to quantitatively determine polyelectrolyte adsorption on the mica basal plane from low ionic strength solutions. Particular emphasis was given to the effect of the polyelectrolyte charge density. By combining the results obtained from XPS and surface force measurements it was possible to analyze the cation exchange at the surface that occurs as a result of polyelectrolyte adsorption. AFM-imaging was used to obtain information on the structure of the adsorbed layer when the polyelectrolyte coverage was low. Further, the desorption of preadsorbed polyelectrolyte layers by addition of inorganic salt and by addition of an anionic surfactant was investigated by XPS and some complementary surface force measurements. The results demonstrate that the lower the polyelectrolyte charge density is, the easier it is to remove the polyelectrolyte from the surface. The surfactant, which by itself does not adsorb to the mica surface, is more efficient in this respect than the inorganic salt. This observation can be rationalized by considering that the surfactant and polyelectrolyte form complexes with each other. Thus, the surfactant brings negative charges into the adsorbed layer that reduces the affinity to the surface. However, high-charge-density polyelectrolytes are removed to a very limited degree even when the surfactant concentration is above the critical micellar concentration, which is explainable by the poor solubility of the polyelectrolyte-surfactant complexes formed.
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| 9. |
- Rojas, Orlando J., et al.
(författare)
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Fluorosurfactant self-assembly at solid/liquid interfaces
- 2002
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 18, s. 8085-8095
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Tidskriftsartikel (refereegranskat)abstract
- Fluorosurfactants have some unique properties that are advantageously used in a range of applications. Their solutions are commonly in contact with solid surfaces onto which the molecules adsorb. Despite this, the adsorption behavior of fluorosurfactants at solid/liquid interfaces is not sufficiently understood, and there is a need for more information. In this study we focus on cationic fluorosurfactant adsorption on negatively charged hydrophilic surfaces, especially with respect to the adsorbed layer structure, long-range interactions and adhesion forces. To this end we combined results obtained from bimorph and interferometric surface force instruments and ellipsometry techniques. The initial adsorption to the oppositely charged surfaces occurs due to the electrostatic attraction between the charged headgroups and the surface. Further adsorption, driven by hydrophobic interactions, occurs readily as the surfactant concentration is increased. Surface force and ellipsometric experiments indicate that the surfactants self-assemble in the form of bilayer aggregates. The thickness of the bilayer aggregates was found to be consistent with the molecular structure. Further, ellipsometric measurements indicate that no complete bilayers were formed but rather that bilayer aggregates were present on the surface even at concentrations well above the cmc. Surface force data for low fluorosurfactant concentrations demonstrate that upon compression the bilayer aggregates assembled on the isolated surfaces are transformed and as a result monolayer structures build up between the surfaces in contact. The force required to attain bilayer-bilayer contact increases with the surfactant bulk concentration due to an increase in the repulsive double-layer force. The force required to drive out surfactant molecules to achieve monolayer-monolayer contact also increases with surfactant concentration. Above the cmc some additional aggregates are present on top of the bilayer aggregates coating the surface. The adhesion found between the monolayer aggregates is an order of magnitude larger than between the bilayer aggregates. However, it is an order of magnitude lower than the corresponding value for Langmuir-Blodgett monolayer films of similar fluorosurfactants.
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| 10. |
- Rojas, O. J., et al.
(författare)
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X-ray photoelectron spectroscopy in the study of polyelectrolyte adsorption on mica and cellulose
- 2000
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:43, s. 10032-10042
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Tidskriftsartikel (refereegranskat)abstract
- X-ray photoelectron spectroscopy was used to estimate the absolute amount of cationic polyelectrolytes that adsorbs on mica and cellulose surfaces in aqueous media. The calculation takes advantage of the knowledge of the mica crystal composition at the basal plane and its ion-exchange properties in aqueous solution. The XPS was operated under monochromatic and unmonochromatic mode and good agreement was observed in the resulting adsorbed amount. The evaluation of the amount of cationic polyelectrolyte adsorbed on cellulose was achieved using calibration curves obtained from adsorption data for the same polyelectrolytes on bare mica surfaces. The adsorption isotherm for polyelectrolytes of low charge density adsorbed on cellulose reveals that their affinity toward cellulose is weaker compared to that observed for highly charged surfaces such as mica. The effect of the polyelectrolyte charge density on the adsorbed amount and the number density of charged segments adsorbed on cellulose were also investigated. From these results it can be concluded that nonelectrostatic interactions are the main contributors to the adsorption of polyelectrolytes on cellulose, but it cannot be ruled out that electrostatic effects also take part in the adsorption mechanism. Finally, it is demonstrated that it is not correct to use the adsorbed amount of polyelectrolytes to determine the surface charge on cellulose surfaces.
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