| 1. |
- Feiler, A., et al.
(författare)
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Atomic force microscopy measurements of adsorbed polyelectrolyte layers. 1. Dynamics of forces and friction
- 2003
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 19:10, s. 4173-4179
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Tidskriftsartikel (refereegranskat)abstract
- The measurement of surface forces and friction between silica substrates bearing adsorbed layers acrylamide-1% [3-(2-methylpropionamide)propyl]trimethyl ammonium chloride (AM-MAPTAC) was examined using atomic force microscopy. The cationic polymer had a large molecular weight (900 000) and a very low charge density (1% MAPTAC units). The force curves in the presence of adsorbed polyelectrolyte showed a very long range repulsive interaction and large deformation on compression typical of electrosteric interactions and consistent with the expected adsorption profile. There was also a strong dependence on the scan rate with increased repulsion during approach and increased attraction during separation as the scan rate increased. The hysteresis was attributed entirely to a hydrodynamic interaction induced by the polyelectrolyte. At slow enough scan rates, the hysteresis between approach and retract curves was absent. The friction force measurements were sensitive to both applied load and scan rate. With increasing applied load, a critical load was apparent, above which the friction force increased. Upon decrease of the load, the friction force remained higher than that before the yield point, suggesting a metastable deformation of the polymer layer. In addition, the friction coefficient increased with increasing scan rate. At low scan rates the friction coefficient was lower than the bare surfaces, whereas at high scan rates the friction coefficient was significantly larger that obtained than for the bare surfaces. Subsequent force curves taken after friction measurements confirm that a permanent change in the polyelectrolyte layer had occurred. The magnitude of the repulsive interaction was reduced to about half of its original force.
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| 2. |
- Plunkett, M. A., et al.
(författare)
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Atomic force microscopy measurements of adsorbed polyelectrolyte layers. 2. Effect of composition and substrate on structure, forces, and friction
- 2003
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 19:10, s. 4180-4187
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Tidskriftsartikel (refereegranskat)abstract
- An investigation on the effect of the conformation of preadsorbed polyelectrolyte layers of acrylamide-1% [3-(2-methylpropionamide)propyl]trimethylammonium chloride on the normal and lateral interactions between surfaces has been conducted. It was shown that when bridging mechanisms increased the adhesion, huge increases in the friction were also seen. When the polymer adsorbed in an extended layer, it resulted in a steric repulsion in the direction normal to the interface. However, the resulting friction measurements were shown to be far more complicated. For example, in cases where the integrity of the polymer layer was maintained under compression, the layer was able to act as a lubricant; however when the layer integrity was affected by the load and shear rate, then friction increased due to energy losses resulting, from disrupting the polymer conformation. The conformation was controlled by varying both the polymer charge density and the nature of the surface. The interaction between the polyelectrolyte and gold proved to be significantly stronger than that with silica, rendering the polymer layer more resistant to damage under shear and, consequently, a significantly different friction-load relationship. The dynamics of the interactions were also investigated and were highly dependent on the polyelectrolyte-surface interaction. As an aside, a novel observation of interference effects between cantilever and substrate is also made.
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| 3. |
- Heinonen, M., et al.
(författare)
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Investigation of the Equivalence of National Dew-Point Temperature Realizations in the -50 C to + 20 C Range
- 2012
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Ingår i: International journal of thermophysics. - : Springer Science and Business Media LLC. - 0195-928X .- 1572-9567. ; 33:8-9, s. 1422-1437
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Tidskriftsartikel (refereegranskat)abstract
- In the field of humidity quantities, the first CIPM key comparison, CCT-K6 is at its end. The corresponding European regional key comparison, EUROMET.T-K6, was completed in early 2008, about 4 years after the starting initial measurements in the project. In total, 24 NMIs from different countries took part in the comparison. This number includes 22 EURAMET countries, and Russia and South Africa. The comparison covered the dew-point temperature range from -50 C to +20 C. It was carried out in three parallel loops, each with two chilled mirror hygrometers as transfer standards in each loop. The comparison scheme was designed to ensure high quality results with evenly spread workload for the participants. It is shown that the standard uncertainty due to the long-term instability was smaller than 0.008 C in all loops. The standard uncertainties due to links between the loops were found to be smaller than 0.025 C at -50 C and 0.010 C elsewhere. Conclusions on the equivalence of the dew-point temperature standards are drawn on the basis of calculated bilateral degrees of equivalence and deviations from the EURAMET comparison reference values (ERV). Taking into account 16 different primary dew-point realizations and 8 secondary realizations, the results demonstrate the equivalence of a large number of laboratories at an uncertainty level that is better than achieved in other multilateral comparisons so far in the humidity field.
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| 4. |
- Macakova, Lubica, et al.
(författare)
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Influence of ionic strength changes on the structure of pre-adsorbed salivary films. A response of a natural multi-component layer
- 2010
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Ingår i: Colloids and Surfaces B. - : Elsevier BV. - 0927-7765 .- 1873-4367. ; 77:1, s. 31-39
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Tidskriftsartikel (refereegranskat)abstract
- Salivary films coating oral surfaces are critically important for oral health. This study focuses on determining the underlying nature of this adsorbed film and how it responds to departures from physiological conditions due to changes in ionic strength. Under physiological conditions, it is found that pre-adsorbed in vitro salivary film on hydrophobic surfaces is present as a highly hydrated viscoelastic layer. We follow the evolution of this film in terms of its effective thickness, hydration and viscoelastic properties, as well as adsorbed mass of proteins, using complementary surface characterisation methods: a Surface Plasmon Resonance (SPR) and a Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). Our results support a heterogeneous model for the structure of the salivary film with an inner dense anchoring layer and an outer highly extended hydrated layer. Further swelling of the film was observed upon decreasing the salt concentration down to 1 mM NaCl. However, upon exposure to deionised water, a collapse of the film occurs that was associated with the loss of water contained within the adsorbed layer. We suggest that the collapse in deionised water is driven by an onset of electrostatic attraction between different parts of the multi-component salivary film. It is anticipated that such changes could also occur when the oral cavity is exposed to food, beverage, oral care and pharmaceutical formulations where drastic changes to the structural integrity of the film is likely to have implications on oral health, sensory perception and product performance.
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| 5. |
- Macakova, Lubica, et al.
(författare)
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Influence of ionic strength on the tribological properties of pre-adsorbed salivary films
- 2011
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Ingår i: Tribology International. - : Elsevier BV. - 0301-679X .- 1879-2464. ; 44:9, s. 956-962
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Tidskriftsartikel (refereegranskat)abstract
- We have studied tribological properties of pre-adsorbed salivary films formed in vitro on compliant hydrophobic surfaces. The adsorbed salivary film significantly decreases boundary friction under physiological ionic strength, which is related to a hydrophilic character of the adsorbed film and its structure. Decrease in the ionic strength below physiological conditions affects film's structure, but it does not significantly affect boundary lubrication at low loads. Applications of high loads led to a gradual loss of lubrication due to shear-induced wear of the films. The wear became more extensive as the ionic strength of the solvent was lowered below physiological conditions.
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| 6. |
- Plunkett, M. A., et al.
(författare)
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Adsorption of a cationic polyelectrolyte followed by surfactant-induced swelling, studied with a quartz crystal microbalance
- 2002
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 18:4, s. 1274-1280
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption and subsequent surfactant-induced swelling of a 10% charged cationic polyelectrolyte (AM-MAPTAC-10), on a gold surface, was monitored by means of a quartz crystal microbalance with dissipation (QCM-D). This instrument gives information on the total adsorbed amount including any adsorbed solvent and on the manner of adsorption. In this case the total adsorbed amount from a 20 ppm AM-MAPTAC-10 solution registered by the QCM-D device was approximately 0.6 mug cm(-2). X-ray photoelectron spectroscopy results showed that the polyelectrolyte adsorbed mass was 0.16 mug cm(-2); thus the water trapped within the polyelectrolyte layer constitutes about 70% of the mass measured by the quartz crystal microbalance. The adsorption process was found to be rather complex, though the time evolution of the adsorbed mass indicated that the majority of the process was diffusion controlled, Toward the end of the adsorption process, the rate of adsorption drops off and the dissipation rate increases, indicating that as the surface becomes crowded the layer extends further in the direction normal to the surface. The effect of addition of sodium dodecyl sulfate (SIDS) to a preadsorped AM-MAPTAC-10 layer was also investigated. It was found that some swelling of the preadsorbed layer occurred once the bulk surfactant concentration reached 20% of the critical micelle concentration (cmc). Between 60% of the cmc and twice the cmc, the adsorbed layer swelled significantly and desorption started to occur. Rinsing the surface with the surfactant-free electrolyte solution results in a rapid decrease in dissipation and adsorbed mass indicating the removal of the surfactant but not the polyelectrolyte.
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| 7. |
- Plunkett, M. A., et al.
(författare)
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Adsorption of pNIPAM layers on hydrophobic gold surfaces, measured in situ by QCM and SPR
- 2003
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 19:17, s. 6837-6844
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Tidskriftsartikel (refereegranskat)abstract
- In situ surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) measurements have been employed to measure the adsorption kinetics and absolute adsorbed amount of the poly(N-isopropyl acrylamide) (pNIPAM) from bulk aqueous solution onto a hydrophobized gold substrate. The adsorption was carried out at 31 degreesC, which is just below the lower critical solution temperature of pNIPAM in water. We find that the shift in the coupling angle of the surface plasmon (proportional to the optical thickness) and the shift in the resonance frequency of the quartz crystal (proportional to the acoustic thickness) increase in parallel for most of the adsorption. Also, the change of dissipation is proportional to the change in frequency. These observations suggest that the buildup of the polymer layer proceeds via growth in thickness rather than by densification of a layer with constant thickness. We interpret this finding in the sense that the dense high-temperature phase wets the hydrophobic gold surface. The wetting layer has a fixed density and grows in thickness. In addition, the QCM has been used to study the temperature-induced conformational change for pNIPAM around the critical temperature. It was found that the technique was able to monitor additional adsorption that occurs when crossing the critical point, which was due to bulk phase separation. Desorption was also noted when crossing the critical point from the opposite direction, and for the given system the process was entirely reversible.
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| 8. |
- Plunkett, M. A., et al.
(författare)
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Comparison of the adsorption of different charge density polyelectrolytes : A quartz crystal microbalance and X-ray photoelectron spectroscopy study
- 2003
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 19:11, s. 4673-4681
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of a series of six cationic polyelectrolytes onto a gold surface was monitored via a quartz crystal microbalance with dissipation (QCM-D). The series of polyelectrolytes were chemically similar but differing in the ratio of two randomly ordered constituent monomers, one of which was charged, the other neutral. Thus the series of polyelectrolytes differed systematically in their charge densities, ranging from a high charge density (100% of monomers charged) to a low charge density (1% charged). It was determined that high charge density polyelectrolytes adsorbed in a relatively flat and rigid layer, while the low charge density polyelectrolyte, as expected, adsorbed in a much more extended structure that coupled strongly to the bulk solvent. By comparison to X-ray photoelectron spectroscopy results, we have also calculated the relative solvent mass hydrodynamically coupled to the adsorbed polymer, which ranges from almost 80% solvent for the 1% charged case down to close to zero for the 100% charged case. Since the QCM-D results are measured relative to uncoated gold in aqueous solution, the latter results should be interpreted as showing that the amount of water hydrodynamically coupled to gold and that coupled to gold coated with the 100% charged polyelectrolyte is very similar. It is believed that this systematic study on the effect of polyelectrolyte structure on the measured dissipation change in the QCM-D may serve as a first guide when inferring structural and viscoelastic information based solely on the QCM-D technique for other similar systems. In addition, a preliminary study on the ability of one polymer to replace another preadsorbed polymer layer was conducted that showed that a steric layer was able to prevent the adsorption of a thermodynamically more favorable polymer. In the reverse case, greater exchange was possible.
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| 9. |
- Plunkett, M. A., et al.
(författare)
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Dynamic adhesion of grafted polymer surfaces as studied by surface force measurements
- 2002
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Ingår i: Journal of Adhesion Science and Technology. - : Informa UK Limited. - 0169-4243 .- 1568-5616. ; 16:7, s. 983-996
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Tidskriftsartikel (refereegranskat)abstract
- Surface force experiments have been carried out to investigate the dynamics of adhesion for grafted polymer-coated glass spheres in a solvent. The quality of solvent has been varied both by changing the relative concentrations in a mixed solvent system and by variations in temperature in order to investigate the effect on the measured adhesion. Under very poor solvent conditions (far from theta conditions) there is a contact time dependence of the magnitude of the adhesion, that can be explained in terms of polymer interpenetration. This interpenetration is consistent with the diffusion following primitive path fluctuation dynamics. An increase in the maximum applied load leads to a decrease in adhesion which is attributed to a slower diffusion rate due to squeezing out of the solvent from the contact zone. Under slightly poor solvent conditions (near-theta conditions), any polymer interdiffusion is found to be rapid on the experimental time frame. In this case the adhesion is strongly dependent on the separation rate, which is interpreted in terms of viscoclastic losses at the crack tip.
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| 10. |
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