Diffuse ion swarm

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... 

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 ... 

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... 

Figure 9.10. (a) Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ( chemical bond ), an outer-sphere complex (ion pair), or be in the diffuse swarm of the electric double layer. (The inner-sphere complex may still retain some aquo groups toward the solution side.) (From Sposito, 1989.) (b) A schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ( s ), inner-sphere complexes ( a ), outer-sphere complexes ( /3 ), and the diffuse ion swarm ( d ). (Adapted from Sposito, 1984.)... 

Na MAS NMR has provided useful information about the interaction between NaCl and the calcium silicate hydrate phases typically occurring in hydrated cement pastes (Viallis et al. 1999). The results suggest that the Na is absorbed together with its hydration sphere on the surface of dry calcium silicate hydrate, whereas in the hydrated material, the cations are located in a diffuse ion swarm on the calcium silicate surface. 

A detailed model of the interfacial region requires the specification of the position of the plane where the diffuse ion swarm begins, A popular choice in the literature of soil chemistry has been jc = 0, which means that outer-sphere surface complexes are neglected entirely and inner-sphere surface complexes are ignored if they would protrude beyond the plane to which (Tin, the intrinsic surface charge density, refers. (See Secs. 1.5 and 3.1 for a discussion of trjn ) That this choice is not reasonable physically, however, can be seen from a simple calculation involving Eq. 5.16. Consider a 1 1 electrolyte at the concentration Cq == 100 mol m" and suppose that /r(0) = SRT/F, a value that is not unrealistic for a smectite siloxane surface. Then k = = 1.04 x 10 m" at 298 K, a =... 

Surface complexation involves all four of the planes depicted in Fig. 5.4. Complexed protons and hydroxide ions reside in the s plane, inner-sphere complexes containing trace metal cations or oxyanions are assigned to the a plane, outer-sphere complexes with the ions of a background electrolyte are assigned to the j3 plane, and the d plane marks the beginning of the aqueous solution phase, where the diffuse ion swarm is found. 

Equations 5.58c, 5.76, and 5.77 define the charge-potential relations in the objective model for any set of adsorbed species in any plane or in the diffuse ion swarm. 

---