Ion swarm

Viehland L A and Robson R E 1989 Mean energies of ion swarms drifting and diffusing through neutral gases Int. J. Mass Spectrom. Ion Processes 90 167-86... 

Fig. b shows a schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ("s"), inner-sphere complexes ("a"), outer-sphere complexes ("P") and the diffuse ion swarm ("d"). (Modified from Sposito, 1984)... 

The anions Cl, NO3, CIO, for some oxides also SO " and SeO are considered to adsorb mainly in outer-sphere complexes and as diffuse ion swarm. 

In a more restrictive sense, the term "ion exchange" is used to characterize the replacement of one adsorbed, readily exchangeable ion by another. This circumscription, used in soil science (Sposito, 1989), implies a surface phenomenon involving charged species in outer-sphere complexes or in the diffuse ion swarm. It is not possible to adhere rigorously to this conceptualization because the distinction between inner-sphere and outer-sphere complexation is characterized by a continuous transition, (e.g., H+ binding to humus). 

There are a number of more loosely defined terms for different types of adsorption that are related to the form of surface complexation. Specifically adsorbed ions are held in inner-sphere complexes whereas non-specifically adsorbed ions are in outer-sphere complexes or the diffuse-ion swarm. Readily exchangeable... 

Another point in favor of the simpler, but less accurate, fluid approach is that discharge diagnostics are still quite primitive. In studies of electron and ion swarms, experimentalists routinely measure mobilities and diffusiv-ities with a precision in the order of a few percent. A sophisticated model must be used to properly interpret such experiments. However, for discharges, even relative concentration profiles for a few of the dozens of important neutral and charged species are difficult to attain. Thus, an overly complex and expensive model is probably inappropriate, and the fluid model is a good compromise at present. 

Modified Gouy-Chapman theory has been applied to soil particles for many years (Sposito, 1984, Chapter 5). It postulates only one adsorption mechanism -the diffuse-ion swarm - and effectively prescribes surface species activity coefficients through the surface charge-inner potential relationship contained implicitly in the Poisson-Boltzmann equation (Carnie and Torrie, 1984). Closed-form... 

The constant-capacitance model (Goldberg, 1992) assigns all adsorbed ions to inner-sphere surface complexes. Since this model also employs the constant ionic medium reference state for activity coefficients, the background electrolyte is not considered and, therefore, no diffuse-ion swarm appears in the model structure. Activity coefficients of surface species are assumed to sub-divide, as in the triplelayer model, but the charge-dependent part is a function of the overall valence of the surface complex (Zk in Table 9.8) and an inner potential at the colloid surface exp(Z F l,s// 7). Physical closure in the model is achieved with the surface charge-potential relation ... 

Fig. 2. Schematic representation of a mobility spectrometer. Ions created in the ion source are separated in the drift region based on their mobility. The ion swarms reach the detector where their drift times are recorded and plotted in the form of a mobility spectrum (Originally published in the article of Buryakov et al. [9]).

When an ion swarm is injected into the drift region of the drift tube, spatial resolution of ions of differing mobility can be separated as differences in drift velocity as the ions move toward the detector, here at virtual ground. Separate packets or swarms of ions develop with the separation as shown in Fig. 2, where three ion swarms have been resolved in time and space. As ions collide with the detector, commonly a simple metal disc or Faraday plate, neutralization of ions is accompanied by electron flow in the detector plate this is amplified and shown in the inset of Fig. 2. Thisplot of detector response(current or voltage) versus time (in ms) is called a mobility spectrum and is the... 

Fig. 3. The mobility coefficient K describing the movement of an ion swarm in an electric field. The mobility coefficient depends on the cross-section (collision area) ft, the reduced mass /i, and the effective temperature re f of the ion. Collision area ftD will depend on moisture, temperature, drift gas, and molecule.

The mechanism by which increasing electrolyte concentration decreases Wexp is essentially a screening of particle surface charge by an ion swarm.39 This mechanism is represented in the factor Z dm in Eq. 6.75. Another mechanism for decreasing Wexp is represented in the factor a in Eq. 6.75, which is a function of the electric potential near the particle surface a tanh(ZF0 /4RT) in the model equation for < (d), where 0, is a ncar-surlace electric potential). 1... 

II. ION SPECIATION ON CLAY MINERAL SURFACES A. Diffuse Ion Swarm... 

The simplest, self-consistent model of the diffuse-ion swarm near a planar, charged surface like that of a smectite is modified Gouy-Chapman (MGQ theory [23,24]. The basic tenets of this and other electrical double layer models have been reviewed exhaustively by Carnie and Torrie [25] and Attard [26], who also have made detailed comparisons of model results with those of direct Monte Carlo simulations based in statistical mechanics. The postulates of MGC theory will only be summarized in the present chapter [23] ... 

FIG. 4 Comparison between MGC theory (solid curves) and Monte Carlo simulation (circles and triangles) of the diffuse-ion swarm on a planar charged surface. Distributions of cations (c+) and anions (c ) are shown for a 1 1 electrolyte solution and two surface charge densities (oq). 

If the sole mechanism of ion adsorption is via the diffuse-ion swarm, the anions in an electrolyte solution in which clay mineral particles are suspended will, in general, be excluded from a portion of the suspension volume near the particle surface [23,27]. If q- is the specific adsorbed charge of anions resulting from this exclusion and c is their bulk concentration in a 1 1 electrolyte... 

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