Trace Inorganics In Water

1 The Effect of Water Structure on the Transport Properties of Electrolytes ROBERT L . KAY

Downloaded by 91.204.15.35 on October 9, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0073.ch001

Carnegie-Mellon University, Mellon Institute, Pittsburgh, Pa.

The abnormal transport properties (conductances and viscosities) of electrolytes in aqueous solution are explained in terms of the effect ions can have on water structure. Various theories that treat the solvent as a continuum are shown to be incapable of explaining these abnormalities. The FrankWen model is used to classify ions as electrostrictive struc ture-makers, Li , F , Ca ; hydrophobic structure-makers, Bu N , Ph B ; and structure-breakers, Cs , I , Me N . These effects can be detected by comparing data for aqueous solu tions with nonaqueous solutions, by investigating the tem perature coefficient of ionic mobilities and viscosity Β co efficients, by comparing ionic mobilities in H O and D O, and by comparing the particular hydrophilic properties of the (EtOH) N ion with its alkyl analog, the Pr N ion. +

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In the past few years considerable interest has developed i n the structure of water in electrolytic solutions (25). This renewed interest is the result of a number of extensive experimental investigations and the reali zation that many otherwise unexplainable observations can be accounted for if water is considered as a structured medium rather than as a con tinuum. This paper w i l l consist of a review of some of the more recent advances that have been made i n elucidating the factors determining the properties of electrolytes in aqueous solution. Transport properties w i l l be dealt with almost exclusively since the authors main interests lie i n that direction. Owing to the unique mechanism of proton conduction in aqueous solution, acids and bases w i l l not be considered and the discussion w i l l be limited exclusively to salt solutions. The problem of explaining the magnitude of ionic mobilities i n aque ous solutions has baffled the electrolyte chemist for a considerable time. Two major factors have contributed to the slow progress i n this field. 1

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


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T R A C E INORGANICS I N W A T E R

First, water has been considered for so long as the typical solvent whereas we now know this to be a poor assumption. It is one of the few solvents in which three-dimensional structure can exist. These three dimensional structures are formed by water's ability to form hydrogen bonds i n a tetrahedral configuration. As a result of this high degree of hydrogen bonding, water has a number of unique properties among which are its exceedingly high heat capacity, viscosity, and dielectric constant and its maximum density of 4°C. It is the only known liquid whose viscosity de creases with increasing pressure (22). Possibly better progress would have been made in this field if a non-hydrogen bonded solvent like acetonitrile had first received our undivided attention instead of water. The lack of good data for ionic properties in any solvent other than water has been a big handicap. For instance, for a considerable time we d i d not know the temperature coefficient of ionic conductances in any solvent other than water. This deficiency has been rectified only recently by the measurements of transferance numbers and conductances in methanol a t l O ° C . (36). The second reason for slow progress in the field is the lack of a good theory i n which water is treated as a structured medium rather than as a continuum. Considering the solvent in an electrolytic solution as a uniform medium with the same physical properties as the pure solvent has certain mathematical advantages, and this approach has enjoyed con siderable success. However, this success has been limited generally to the prediction of the concentration dependence of various transport and thermodynamic properties, and then only in extremely dilute solutions. The continuum theory is not as successful in explaining the change in magnitude of these properties as the solvent or temperature are changed, as I shall point out later. The success of the continuum theory in pre dicting the concentration dependence is not difficult to understand. Interionic effects are longer ranged than ion-solvent effects and, therefore, are sizeable effects in very dilute solutions. O n the other hand, one must go well above what is considered the dilute range (ionic strength 0.01) before any one ion has an appreciable effect on the interaction of a second ion with the solvent. One would expect that any effect of water structure on the property of an electrolyte would be fairly concentration independent i n this dilute range and, consequently, would not interfere with a continuum calculation. However, there are exceptions i n the case of very large ions. The continuum theory is not as successful i n explain ing the temperature dependence of various properties of aqueous solu tions. It is interesting to note that the structure existing in water is ex tremely temperature dependent, owing to the fact that thermal energies approach the strength of the hydrogen bonds involved. Thus, an appeal to structural effects to account for the unexplained temperature coeffi cients of transport properties of electrolytes is certainly reasonable.

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

1.

KAY

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Transport Properties of Electrolytes

Experimental and Results In considering the dependence of limiting ionic mobilities on solvent and temperature it is most useful to apply Stokes' law (30). For univalent ions (34) it is given by (1)

λ =15Α/6πτ οηο

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and states that the limiting ionic conductance-solvent viscosity product is inversely proportional to the radius, r , of the moving ion. It should be remembered at this point that the limiting ionic conductance is pro portional to the limiting ionic mobility, the proportionality factor being the Faraday constant. Stokes' law is based on the assumption that an ion moves i n the direction of the field by pushing solvent molecules out of its path. This involves doing work in overcoming the viscous drag of the solvent molecules and this viscous drag is assumed to be identical to that of the bulk solvent. A n application of Equation 1 to experimental results for aqueous and acetonitrile (4, 27) solutions at 25°C. is shown i n Figure 1. Here λο ηο or the Walden product (37, 38) as it is sometimes known, is plotted against the reciprocal crystallographic (33) or estimated (34) radii of the alkali and tetraalkylammonium cations and the halide anions. Equation 1 or Stoke's law is shown as the broken line and at first glance appears to be an exceedingly poor representation of the data. Stake's law is not as poor as it first appears if certain deviations, attributable to deficiencies i n the model on which it is based, are taken into account. Since smaller ions must be solvated to some extent, they must carry a solvent sheath through the solution. Consequently, the ordinate i n Figure 1 should represent the solvated rather than crystallographic radius. This could possibly account for the ions lying below the Stokes' line but it w i l l not account for those lying above the line. If complete "slippage" of the solvent past the ion is assumed to take place, the numerical constant i n Equation 1 is reduced from 6 to 4. This helps but it is not sufficient and a considerable number of ions still lie above the Stokes' line indicating a higher mobility than the theory predicts.


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Another interaction between ions and solvent molecules besides solvation has been identified by Born ( J ) and latter by Fuoss (13). A moving ion can orient the solvent dipoles as they move through the solu tion. O w i n g to the finite time required for the relaxation of such orienta tions, a retarding force is exerted on the moving ion. Zwanzig (42) has quantitatively evaluated this effect and showed that Equation 1 becomes Ao*=15.4/ftr[r,+ A/r »] e

(2)

Here, the constant A depends on various dielectric properties of the solvent. Frank (10) has discussed this result i n considerable detail and Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

T R A C E INORGANICS I N W A T E R

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showed that this theory predicts a maximum λ of about 30 for aqueous solutions at 25 ° C , whereas values as high as 75 have been observed. K a y (27) has made a similar calculation for methanol solutions and obtained a comparable result. There is little doubt that this Born-Fuoss effect exerts some influence on ionic mobilities, but it certainly w i l l not explain the very high mobilities exhibited by some ions i n aqueous solution. 0

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Trace Inorganics In Water

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