Ion depletion

Fig. 3 -13. (a) A ion levels at the surface and in the interior of ionic compound AB, and (b) concentration profile of lattice defects in a surface space charge layer since the energy scales of occupied and vacant ion levels are opposite to each other, ion vacancies accumulate and interstitial ions deplete in the space charge layer giving excess A ions on the surface. 

Setting of the cements occurs by neutralisation, and involves initial formation of calcium or strontium polyacrylate and later formation of aluminium polyacrylate. There is also evidence for a later, slow reaction involving the ion-depleted... 

It is doubtful whether the therapeutic application fo penicillamine in rheumatoid arthritis83 is related to its metal-chelating ability. Metal ion depletion undoubtedly occurs and it has also been observed that patients on protracted penicillamine therapy had decreased taste sensitivity this was restored to normal when oral copper was given84. ... 

Fahey RC, Prise KM, Stratford MRL, Watfa RR, Michael BD (1991) Rates for repair of pBR 322 DNA radicals by thiols as measured by the gas explosion technique evidence that counter-ion condensation and co-ion depletion are significant at physiological ionic strength. Int J Radiat Biol 59 901-917... 

The ED stack is the unit holding together anionic and cationic membranes assembled in parallel as in a filter press between two electrode-end blocks in such a manner that the stream undergoing ion depletion (i.e., the diluate or diluting stream) is kept separated from the other solution (concentrate or concentrating stream) undergoing ion enrichment. Figure 4 shows an exploded view of it. 

An integrated microfabricated cell sorter has been constructed using a control layer and a fluidic layer fabricated on PDMS [890]. The control layer consists of valves that will be pneumatically controlled by pressurized N2. This device is superior to an electrokinetic sorter, which suffers from buffer incompatibilities and frequent voltage adjustments because of ion depletion or pressure imbalance due to evaporation. E. coli cells expressing EGFP were sorted and selected out from a cell mixture which also contained E. coli cells expressing the / -nitrobenzyl (PNB) esterase. About 480,000 cells have been sorted at a rate of 44 cefls/s in 3 h. The recovery yield was 40% and the enrichment ratio was about 83-fold [890]. 

As already noted, the Poisson-Boltzmann equation fails to describe the ion depletion in the vicinity of the surface, which is basically due to the preference of the ions for the bulk liquid, where they can be fully hydrated. 

The Onsager-Samaras theory fails at electrolyte concentrations above approximately 0.2 M. One of the reasons is the assumption that the screening length X does not depend on the distance to the interface. In reality, the screening in the ion-depleted region near the surface is smaller than in the bulk. Only when X is large compared to the thickness of the depleted layer, the above approximation is accurate. Other reasons for the inaccuracy (additional interactions and surface charge) will be examined later in the paper. 

When the interface is charged, the Poisson-Boltzmann equation predicts stronger fields and higher ionic concentrations in the vicinity of the surface, therefore a dielectric constant smaller than in the bulk, and the local Born energy indicates that the ions are repelled by the interface. However, a neutral surface depleted of ions should have a dielectric constant larger than the bulk and the ions should be attracted, not repelled by such an interface. Therefore, other interactions should be included to explain the ion depletion near a neutral surface. 

The differences in the hydration energies of the ions and in their hydrated radii are expected to lead to d1 =d2. As indicated by simulations [28], the negative ions are less repelled, therefore they can approach the interface closer than the positive ions. The asymmetric ion depletions generate a surface potential, even in the absence of an external surface charge. The charge that generates the diffuse double layer is located a few Angstroms from the interface, and is due to the anions. 

When the sample size is small (10-20 pL), it becomes possible to effectively inject most of the ions in the sample in less than 30 s at 5 kV. A small water plug injected hydrodynamically has been shown to further improve sensitivity [51], though this was not the case for the above-mentioned application. Since the sample conductivity severely affects the amount of solute injected, internal standards are important for this mode of stacking. For quantitative analysis, make only a single injection out of each vial, as the sample becomes ion-depleted. 

An injection-related artifact can occur in gel buffers with consecutive electroki-netic injections from the same, low volume (10-200 mL) sample progressively smaller amounts of sample are introduced into the capillary, resulting in peak heights or areas that decrease with each injection (Schwartz et al., 1995). This effect is due to the migration of cations (e.g., Tris) from the gel buffer into the sample, changing its relative ionic strength. One solution to this problem is to perform an electrokinetic injection from a water vial prior to sample injection. This water injection generates a zone of ion depletion (i.e., rela-... 

Figure 2. FTMIR spectra of LDPE treated with 15% fluorine mixture with helium under ion-depleted conditions (A) 2-min, (B) 5-min, and (C) 15-min treatments. Reaction conditions 40 cc/min, 3.0 mm, 50 W spectra at 45° incidence.

Another important feature of the reactions in the ion-depleted zone is that -CF2 type groups are formed predominantly as in the case of reactions in the glow region of the plasma. This is seen by the chemical shift or about 6eV from the original -... 

An apparent upper limit on the depth of fluorination was observed even for reactions under ion-depleted conditions. This was attributed to an insufficient fluorine gradient from the surface of the polymer into the bulk. An increase in this gradient obtained by an increase of fluorine concentration in the plasma resulted in greater depths of fluorination suggesting a mechanism controlled by diffusion of fluorine species into the polymer. 

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