Chemical Models of Electrolytes

TVn efficient description of ion pairing is based on the chemical model of electrolytes. Chemical models of electrolytes take into account local structures of the solution due to the interactions of the ions and solvent molecules. The underlying information stems from spectroscopic, kinetic, and electrochemical experiments, as well as from dielectric relaxation spectroscopy. The postulated structures include ion pairs, higher ion aggregates, and solvated and selectively solvated ions [183]. 

The formation of these structures is represented with the help of chemical equilibria. The equilibrium constants can consistently be determined with the help of experimental methods. The association of solvated ions can be described by the overall equilibrium reaction 

According to Eigen and Tamm, ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C SA ] and finally a contact ion pair [C A ]° [184, 185]. AU these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

In the last two decades experimental evidence has been gathered showing that the intrinsic properties of the electrolytes determine both bulk properties of the solution and the reactivity of the solutes at the electrodes. Examples covering various aspects of this field are given in Ref. [16]. Intrinsic properties may be described with the help of local structures caused by ion-ion, ion-solvent, and solvent-solvent interactions. An efficient description of the properties of electrolyte solutions up to salt concentrations significantly larger than 1 mol kg is based on the chemical model of electrolytes. 

The association of solvated ions can be described by the overall equilibrium reaction 

---

Recent developments of the chemical model of electrolyte solutions permit the extension of the validity range of transport equations up to high concentrations (c 1 mol L"1) and permit the representation of the conductivity maximum Knm in the framework of the mean spherical approximation (MSA) theory with the help of association constant KA and ionic distance parameter a, see Ref. [87] and the literature quoted there in. 

Fig. 2. The chemical model of electrolyte solutions. O observer, i ion X, j ion Xj in an arbitrary position, fjj, with regard to the ion X, special positions (contact, separation by one or two orientated solvent molecules) are sketched with broken lines, r, a, R distance parameters W mean-force potentials and relative velocities of ions X, and Xj...

The chemical model of electrolyte solutions introduces short-range interactions by means of potentials of mean force W , which can be considered as contributions to ion-pair formation. 

P. Fletcher and G. Sposito, The chemical modelling of clay-electrolyte interactions for montmorillonite, Clay Minerals 24 375 (1989). See also R. S. Mansell, S. A. Bloom, and W. J. Bond, A tool for evaluating a need for variable selectivilies in... 

Fletcher, P. and Sposito, G.S., The chemical modeling of clay/electrolyte interactions for montmorillonite, Clay Miner., 24, 375, 1989. 

Fletcher, P., and Sposito, G. (1989) The Chemical Modeling of Clay-Electrolyte Interactions for Montmorillonite, Clay Miner. 24, 375-391. 

Pabalan, R. T., and Pitzer, K. S. (1990) Models for Aqueous Electrolyte Mixtures for Systems Extending from Dilute Solutions to Fused Salts. In Chemical Modeling of Aqueous Systems, Vol. II, R. L. Basset and D. C. Melchior, Eds., American Chemical Society, Washington, DC. 

Kalyuzhnyi, Yu.V., and Stell, G. Solution of the polymer msa for the polymerizing primitive model of electrolytes. Chemical Physics Letters, 1995, 240, p. 157-164. 

In order to describe the photoelectrochemistry of d-band materials it will be necessary to introduce to a higher degree formalisms and energy tenti considerations from coordination chemistry into physical chemical models of the semiconductor/electrolyte interface. 

In connexion with dielectric and other spectroscopic relaxation methods, e.g. NMR, the group of ultrasonic relaxation, temperature- and pressure-jump methods 340 - 342) jjg mentioned. These yield information on the processes in electrolyte solution and confirm the basic chemical model of free ions and ion pairs in the solution... 

The coefficients J R) and J2 R) depend on the cutoff distance R and thus include the influence of the short-range forces on the transport phenomenon for the activity coefficient of the chemical model, see Electrolyte Solutions,... 

Subsequently, the model has been extended [37, 38] to the case of associated electrolytes by using a recent model for associating electrolytes[39]. Unlike the classic chemical model of the ion pair the effect of the pairing association is included in the computation of the MSA screening parameter F. Simple formulas for the thermodynamic excess properties have been obtained in terms of this parameter when a new EXP approximation is used. The new formalism based on closures of the Wertheim-Ornstein-Zernike equation (WOZ)[40, 41 does accommodate all association mechanisms (coulombic, covalent and solvation) in one single association parameter, the association constant. The treatment now includes the fraction of particles that are bonded, which is obtained by imposing the chemical equilibrium mass action law. This formalism was shown to be very successful for ionic systems, both in the HNC approximation and MSA [42, 43, 44, 45, 46, 47]. 

Fletcher, R, and G. Sposito. 1989. The chemical modelling of clay/electrolyte interactions for mont-morillonite. Clay Mineral. 24 375-391. 

Alkali-metal halides are textbook examples of strong electrolytes. However, conductance and potentiometric measurements reveal that there are some salts which behave differently and do form ion pairs by the strong attraction of the unlike ions. For these systems, a chemical model of ion pairing as proposed in Refs. 17 to 20 can be applied to consider the equilibrium between the completely dissociated electrolyte and the ion pair... 

Table 3 gives HLB values of some of the important emulsifiers. The HLB optimum for a given emulsifier varies with the components of the food system. A coconut oil—water emulsion that shows optimum stabiUty with an HLB of 7—9 shows a shift ia requirements for stabiUty upon addition of caseia and electrolytes to an optimum stabiUty usiag an emulsifier having an HLB of 3—5. In addition, the stabiUty of an emulsion can be affected by the chemical nature of the emulsifier. The optimum HLB for an emulsifier ia a given system is iafluenced by the other iagredients as is illustrated for a model synthetic milk system ia Figures 1 and 2. 

Scale- Up of Electrochemical Reactors. The intermediate scale of the pilot plant is frequendy used in the scale-up of an electrochemical reactor or process to full scale. Dimensional analysis (qv) has been used in chemical engineering scale-up to simplify and generalize a multivariant system, and may be appHed to electrochemical systems, but has shown limitations. It is best used in conjunction with mathematical models. Scale-up often involves seeking a few critical parameters. Eor electrochemical cells, these parameters are generally current distribution and cell resistance. The characteristics of electrolytic process scale-up have been described (63—65). 

The commonly used method for the determination of association constants is by conductivity measurements on symmetrical electrolytes at low salt concentrations. The evaluation may advantageously be based on the low-concentration chemical model (lcCM), which is a Hamiltonian model at the McMillan-Mayer level including short-range nonelectrostatic interactions of cations and anions [89]. It is a feature of the lcCM that the association constants do not depend on the physical... 

According to Vitanov et a/.,61,151 C,- varies in the order Ag(100) < Ag(lll), i.e., in the reverse order with respect to that of Valette and Hamelin.24 63 67 150 383-390 The order of electrolytically grown planes clashes with the results of quantum-chemical calculations,436 439 as well as with the results of the jellium/hard sphere model for the metal/electro-lyte interface.428 429 435 A comparison of C, values for quasi-perfect Ag planes with the data of real Ag planes shows that for quasi-perfect Ag planes, the values of Cf 0 are remarkably higher than those for real Ag planes. A definite difference between real and quasi-perfect Ag electrodes may be the higher number of defects expected for a real Ag crystal. 15 32 i25 401407 10-416-422 since the defects seem to be the sites of stronger adsorption, one would expect that quasi-perfect surfaces would have a smaller surface activity toward H20 molecules and so lower Cf"0 values. The influence of the surface defects on H20 adsorption at Ag from a gas phase has been demonstrated by Klaua and Madey.445... 

Van Luik, A.E. and Jurinak, J.J., Equilibrium chemistry of heavy metals in concentrated electrolyte solution, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symp. Series 93, American Chemical Society, Washington, 1979, pp. 683-710. 

---