| 1. |
- Attard, P, et al.
(author)
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Dynamic surface force measurement. I. van der Waals collisions
- 1999
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In: Review of Scientific Instruments. - 0034-6748 .- 1089-7623. ; 69, s. 3852-3866
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Journal article (peer-reviewed)abstract
- Theoretical calculations and experimental measurements are used to show that hitherto neglected inertial effects can be significant in computer- controlled surface force measurement devices such as the atomic force microscope. The problem is analyzed in detail for the case of the van der Waals attraction in air. It is demonstrated that equating the cantilever deflection to the surface force systematically underestimates the magnitude of the surface force, increasingly so as the speed of approach is increased. It is also shown that the surface separation becomes lost at high accelerations due to a dynamic uncoupling of the cantilever deflection and angle. The effects of elastic deformation of the bodies are taken into account, including the collision-induced elastic vibrations in the solids. Experimental data are obtained for the van der Waals attraction and collision of glass surfaces in air using the measurement and analysis of surface forces device. All of the effects found in the theoretical calculations are identified in the experimental data.
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| 2. |
- Poptoshev, E., et al.
(author)
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Surface forces in aqueous polyvinylamine solutions. I. Glass surfaces
- 1999
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 15:22, s. 7789-7794
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Journal article (peer-reviewed)abstract
- A noninterferometric surface force apparatus has been used to measure interactions between glass spheres in dilute aqueous polyvinylamine solutions at two different salt concentrations. Close to the substrate charge neutralization point, an attractive interaction is present mainly because of bridging of the extending polymer tails. Additional adsorption leads to an overcompensation of the glass negative surface charge, and the interaction at this point is dominated by a long-range double-layer repulsion. The results from fitting the Derjaguin-Landau-Verwey-Overbeek theory to the measured force curves demonstrate that the degree of overcompensation increases with polyelectrolyte concentration and increasing ionic strength of the solution (addition of indifferent electrolyte). An increase in ionic strength results in the screening of the electrostatic forces which leads to: (i) a reduced free energy cost of creating a charged interface, (ii) a decreased repulsion between protonated amine groups along the polymer backbone and a corresponding increased chain flexibility, (iii) a reduced electrostatic attraction between the polyelectrolyte and the surface. The first effect is apparently is the most important in the present case.
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| 3. |
- Rutland, M, et al.
(author)
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Calcium soaps in flotation deinking; Fundamental studies using surface force and coagulation techniques
- 1997
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In: Colloids and Surfaces A. - 0927-7757 .- 1873-4359. ; 125, s. 33-46
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Journal article (peer-reviewed)abstract
- The surface force technique was shown to give useful information concerning the interaction of fatty acid flotation collectors and Ca2+ activator with a hydroxylated mica substrate at high pH. Since the surface of ink particles are enriched mainly by hydroxyl groups from natural resins, these experiments enabled some details of the fundsmental mechanisms involved in deinking flotation to be illucinated. The preliminary results (carried out at relatively low Ca2+ and fatty acid concentrations) may be summarised as follows; (a) At pH >10, the hydroxylated surface generated a long range DLVO double_x001E_layer repulsion and the electrical potential at the mica/electrolyte interface could be estimated. However, at short distances (15 Å from mica contact) a repulsive non-DLVO hydration barrier were detected due to adsorbed Na2+. (b) On addition of CaCl2, the Na2+ was exchanged by less strongly hydrated Ca2+ resulting in the elimination of the hydration forces and mica-mica contact. On addition of fatty acid, no change in the force profile was detected suggesting no Ca2+ was removal from the surface and there was no evidence of calcium soap formation in the surface region. This result implies that the Ca2+ does not induce a bridging mechanism in the presence of fatty acid (below the calcium soap precipitation level). In fact, the Ca2+ ions could only operate as bridging agents if they can specific bind to the surface as well as the carboxylated fatty acid. The "Ca2+ dehydration destabilization mechanism" was verified by coagulation studies with quartz suspensions. At higher fatty acid and Ca2+ concentrations, calcium soap was precipitated in bulk solution. It was suggested that microencapsulation of the ink particles with hydrophobic species can only occur through heterocoagulation with the bulk precipitated calcium soap particles.
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