Bound fraction surfaces

Trace elements can be adsorbed on the surface of calcite, influencing their solubility in calcareous soils of arid and semi-arid zones. The carbonate bound fraction is the major solid-phase component for many trace elements (Cd, Pb, Zn, Ni and Cu) in arid and semi-arid soils, especially in newly contaminated soils (Table 5.3). In Israeli arid soils treated with metal nitrates, the carbonate bound fraction is the largest solid-phase component (60-80%, 50-60%, 40-60%, 30-40%, and 25-36% for Cd, Pb, Zn, Ni, and Cu respectively). Divalent metallic cations at low aqueous concentrations first associate with calcite via adsorption reactions. Then they may be incorporated into the calcite lattice as a co-precipitate by recrystallization (Franklin and Morse, 1983 Komicker et al., 1985 Davis et al., 1987 Zachara et al., 1988 Reeder and Prosky, 1986 Pingitore and... 

An illustrative example is the work of Clark et al, on the conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on silica 0). These authors determined bound fractions from magnetic resonance experiments. In one instance they added acetone to an aqueous solution of PVP in order to achieve theta conditions for this polymer. They expected to observe an increase in the bound fraction on the basis of solvency effects as predicted by all modern polymer adsorption theory (2-6), but found exactly the opposite effect. Their explanation was plausible, namely that acetone, with ability to adsorb strongly on silica due to its carbonyl group, would be able to partially displace the polymer by competing for the available surface sites. 

Several experimental parameters have been used to describe the conformation of a polymer adsorbed at the solid-solution interface these include the thickness of the adsorbed layer (photon correlation spectroscopy(J ) (p.c.s.), small angle neutron scattering (2) (s.a.n.s.), ellipsometry (3) and force-distance measurements between adsorbed layers (A), and the surface bound fraction (e.s.r. (5), n.m.r. ( 6), calorimetry (7) and i.r. (8)). However, it is very difficult to describe the adsorbed layer with a single parameter and ideally the segment density profile of the adsorbed chain is required. Recently s.a.n.s. (9) has been used to obtain segment density profiles for polyethylene oxide (PEO) and partially hydrolysed polyvinyl alcohol adsorbed on polystyrene latex. For PEO, two types of system were examined one where the chains were terminally-anchored and the other where the polymer was physically adsorbed from solution. The profiles for these two... 

Morrissey 53) used transmission infrared spectroscopy to study protein adsorption onto silica particles in a heavy water (DzO) buffer. By observing the shift in the amide I absorption band, he could deduce the fraction of protein carbonyl groups involved in bonding to the silica surface. He found that bovine IgG had a bound fraction of 0.20 at low bulk solution concentrations, but only about 0.02 at high solution concentrations. However, neither prothrombin nor bovine serum albumin exhibited a change in bound fraction with concentration. Parallel experiments with flat silica plates using ellipsometry showed that the IgG-adsorbed layers had an optical thickness of 140 A and a surface concentration of 1.7 mg/m2 at low bulk solution concentration — in concentrated solutions the surface amount was 3.4 mg/m2 with a thickness of 320 A (Fig. 17). 

It is not always possible to measure c(z) in full detail so that introduction of some average properties remains useful. One such parameter is the train fraction or bound fraction p. In terms of fig. 5.6 it is the amount of polymer within the thickness f of a segment on the surface, divided by the total adsorbed amount. Spectroscopic and other techniques can often be successfully applied to determine p. The general trends found support the picture sketched above, i.e., p Is close to unity when the adsorbed amount is low, and it decreases as the pseudo-plateau of the adsorption Isotherm (see fig. 5.7) is approached. Reported bound fractions vary widely, depending on the system and method chosen. It should be said that bound fraction measurements leave much room for differences in Interpretation some results are still not well understood. We will briefly discuss the methods in sec. 5.6, and give some exjjerimental data in secs. 5.7 and 5.8. 

For the determination of the fraction of segments in trains or, equivalently, the bound fraction p, one relies on features which allow differentiation between free (non-adsorbed) segments and segments in contact with the surface. In some cases, it is also possible to discriminate between parts of the solid substrate covered with polymer segments, and bare surface parts. One can then also determine the train density. 

We divide this section into three main themes structure, adsorbed amount, and polydispersity. In sec. 5.7a we discuss the structure of the adsorbed layer, paying attention to the volume fraction profile, the composition in terms of loops and tails, the bound fraction, and the layer thickness. Section 5.7b deals with the adsorbed amount as a function of the polymer concentration, the chain length, the solvency and the surface affinity. These two sections are mainly concerned with monodisperse polymer. Polydispersity, where competition between different chain lengths shows up, is treated In sec. 5.7c. 

Distribution between trains and loops in molecules adsorbed at solid/Iiquid interfaces is also possible and has been shown to occur for flexible polymers. There are some indications that protein molecules at solid/liquid interfaces do not always undergo the drastic conformational changes that occur at fluid/fluid interfaces. At a solid/liquid interface, an adsorbing molecule cannot penetrate the solid phase. Furthermore, adsorption may be confined to sites and thus be localized. Using infrared difference spectroscopy, Morrissey and Stromberg (1974) found a bound fraction (number of carbonyl surface... 

A novel solid phase used by three commercial immunoassay kit manufacturers employs antibody covalently bound to cellulose particles that contain an iron(III) oxide core.9 These small, paramagnetic particles do not spontaneously sediment, and have a large surface area. The particles are retained magnetically in a tube during decantation and rinsing steps, allowing easy separation of the bound fraction after equilibration with Ag and Ag. ... 

Easily exchangeable, surface-bound, amorphous hydrous oxide—bound, and crystalline hydrous oxide—bound fractions... 

Water soluble, surface adsorbed, associated with Fe- and Al-oxo-hydroxides, and carbonate-bound fractions... 

Metal Surface Composition To determine which metal bound to surfaces of road dust, soil dust was collected that contained larger particles (400 to 3000 nm diameter in air) and that compared closely to amorphous silica dust (6, 7). This dust was thought to be composed of both particles ground from the Earth s emst and particles generated by transportation vehicles that drive on the roads from which road dust was collected. Dust was collected near a road in four fractions. <56 nm, <100nm, both ultrafine (nano) particle aerosols, a fraction <2500 nm (a fine aerosol), and one < 10,000 nm (a coarse aerosol). Different elements were enriched on the surface of different-sized particles (6, 7). 

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