Ionic liquids Walden rule

The conductivity and viscosity of an ionic liquid is often combined into what is termed Walden s rule [Equation (3.6-4)] [54],... 

Ionic liquids with discrete anions have a fixed anion structure but in the eutectic-based liquids at some composition point the Lewis or Bronsted acid will be in considerable excess and the system becomes a solution of salt in the acid. A similar scenario also exists with the incorporation of diluents or impurities and hence we need to define at what composition an ionic liquid is formed. Many ionic liquids with discrete anions are hydrophilic and the absorption of water is found sometimes to have a significant effect upon the viscosity and conductivity of the liquid [20-22], Two recent approaches to overcome this difficulty have been to classify ionic liquids in terms of their charge mobility characteristics [23] and the correlation between the molar conductivity and fluidity of the liquids [24], This latter approach is thought by some to be due to the validity of the Walden rule... 

The Walden rule is interpreted in the same manner as the Stokes-Einstein relation. In each case it is supposed that the force impeding the motion of ions in the liquid is a viscous force due to the solvent through which the ions move. It is most appropriate for the case of large ions moving in a solvent of small molecules. However, we will see here that just as the Stokes-Einstein equation applies rather well to most pure nonviscous liquids [30], so does the Walden rule apply, rather well, to pure ionic liquids [15]. When the units for fluidity are chosen to be reciprocal poise and those for equivalent conductivity are Smol cm, this plot has the particularly simple form shown in Figure 2.6. 

Schreiner C, Zugmann S, Hartl R, Gores HJ (2010) Fractional Walden rule for ionic liquids examples from recent measurements and a critique of the so-called Ideal KQ line for the Walden plot. J Chem Eng Data 55 1784-1788... 

The molar conductivity Am of the liquid salt is obtained from the specific conductivity k and the molar volume Vm (Eq. Id) of the salt that is available from density measurements. The modified Walden rule is applied for comparing ILs. The deviation from the Walden rule represented by a is interpreted in terms of ionic-ity. A small ionicity is caused by strong ion-ion interaction. 

Schreiner et al. recendy published a critique of the use of the Walden rule with ionic liquids [17]. They suggested that use of the enpirical approach from Stokes, Equation 1.7. explains the observed deviations from the so-called ideal Walden plot, since each salt will have a different value of C and a ... 

For liquid electrolytes, ionic conductivity, self-diffusivity, and viscosity are three key properties. Though originally based on dilute aqueous electrolyte solutions, the Walden rule [52] has been proposed as a tool to provide insight to the proton transfer and ion association. The rule suggests that the molar cmiductivity of an electrolyte, A, is proportional to the fluidity, which can be expressed as the inverse of the shear viscosity i/. In other words, the product of the molar conductivity and viscosity of an electrolyte is a constant, as shown in (3.10). 

Once again, the first studies of the viscosity where related to the development of ionic liquids as nonaqueous battery electrolytes as this property is often related to the ico-nicity of die solution. Classical Walden rule diagrams are used to determine the ionicity of ionic liquids and electrolytes. From that the ionic mobilities were represented in a log scale through the equivalent conductivity, A (S cm mol" ) as function of fluidity (Poise" ),... 

Fig. 7.11 The Walden rule for the ionicity of Walden plot log(A) versus og(rf ) of the Li[N(Tf)2]/ MAc DES at A)LiTFsi =1/4 filled circle and Pyrrolidinium nitrate protic ionic liquid [HPyr][NO ] and its mixture with propylene carbonate ([HPyr][N03] + PC) hinaiy mixtures as a function of the temperature at Xpu.=0.25 filled triangle, 0.10 open square, 0.22 filled square, 030 Reverse filled triangle, 0.50 open diamond, 0.80 open circle, 1. Reproduced from [40] with permission from Elsevier...

Room temperature ionic liquids (for simplicity, ILs) are becoming increasingly popular as solvents for many chemical applications, such as catalysis, fuel cells, spectroscopy, etc. Their lattice energies are low and even at room temperature thermal energy is sufficient to overcome them. As early as in 1914, the Latvian chemist Paul Walden, famous for his discoveries of the Walden Turnover and Walden s Rule, synthesized ethylammonium nitrate having a melting point of 285 K. But there was no strong interest in such compounds, except from an electrochemical point of view. 

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