Indian Journal of ChemistryVol. 33A, April 1994, pp. 297-302

Ionic association and ion-solvent interactions: Conductance ofN-ethyl-4-cyanopyridinium iodide in ethanol-water mixtures

Ajoy Mukhopadhyay, Ashis Nandi & Mohan Pal*

Chemistry Department, Syamsundar College, Syamsundar, Burdwan 713 424and

Sanjib BagchiDepartment of Chemistry, Burdwan University, Burdwan 713104, India

Received 1 June 1993; revised and accepted 4 October 1993

Conductance of N-ethyl-4-cyanopyridinium iodide in ethanol-water mixture has been measuredas a function of the concentration of solute. The data have been analysed taking the existence of sol-vent separated ion pairs into consideration. Temperature variation of the association constant hasbeen studied to get the thermodynamic parameter tlHo as a function of the solvent composition.

N-Alkylpyridinium iodides (Py+, 1-) provide asuitable system for studying ion-association phen-omena. Interionic attractions and thermal motionlead to the existence of contact ion pair (c.i.p.),solvent separated ion pair (s.s.i.p.) and free ions inequilibrium in a solution'. The distribution amongthe different species in solution is determined byconstants characterising the different equilibria.Although much work has been done on the ther-modynamics of ion association very little has beendone on determining the equilibrium constant forthe interconversion of ion pair subspecies/".

In a solution of low dielectric constant thesecompounds show a characteristic absorption bandin the UV-visible region, originating from a chargetransfer (c.t.) process within the contact ion pairspecies?". Ion association in solutions may bestudied by monitoring the band as a function ofsolute concentrations". Bagchi and coworkers!studied the extent of equilibrium by spectropho-tometric procedure but the method is less precise,less informative and wave length dependent. Amajor tool for the investigation of ion pair asso-ciation is conductance measurement which yieldsinformation regarding the distribution betweenionic and nonconducting species and limiting mo-lar conductance (An).

Studies in mixed solvents pose another prob-lem. Besides providing with a system of varyingdielectric constant, a binary mixture of two polarsolvents also provides a medium where solventseparated species involving the two solvents maybe present. Moreover there is a scope for the var-iations of solvent-solvent interaction as composi-

tion of the solvents in a binary mixture is varied.Thus studies in mixed solvents may give informa-tion regarding both the nonspecific and specificsolvation effects on ion association phenomena.

The present work addresses the problem ofionic association in a mixed binary (polar + polar)solvent mixture. Conductance studies have beencarried out using solutions of N-ethyl-4-cyanopyr-idinium iodide at various temperatures.

Materials and MethodsN-ethyl-4-cyanopyridinium iodide was pre-

pared by quaternising 4-cyanopyridine (Kochlight) with ethyl iodide in the dark 10. Ethanol wastreated by the standard procedure!'. Doubly dis-tilled and degassed' water was used. All solventswere distilled immediately before use: The specif-ic conductances of solvents were < 3 x 10 - I>

s.cm - '. All solutions were prepared by dissolvingweighed samples of N-ethyl-4-cyanopyridiniumiodide in solvent mixtures prepared as v/v solu-tions. All the viscosities and dielectric constantswere interpolated from literatures values 12.L~. Theproperties of solvent mixtures at different temper-atures have been listed in Table 1. Precision con-ducitivity bridge (Systronics-304) capable of mea-suring conductance with an accuracy of ± 0.5% inthe range of 2 j.J.S to 2 s, and a dip-type immer-sion conductivity cell with smooth electrodeswere used for the measurement of electrolyticconductivities. The measurements were madeover the temperature range 20-35°C (± O.02°C).The instrument was standardised with KCl solu-tion of known concentration equilibrated at the

2~ INDIAN J CHEM. SEe. A, APRIL 1994

Table1- Propertiesofethanol-watermixtureat 20-35·CMole fraction 20· 25° 30· 35°

of ethanolX, I](C.P.) 0 I](C.P.) 0 I](C.P.) 0 I](C.P.) 0

O~OO 1.005 80.36 0.8937 78.~4 0.8007 76.65 0.7225 75.00

0.089 2.183 68.66 1.8150 67.09 1.5530 611.68 1.3320 63.66

0.281 2.870 50.39 2.400 49.11 2.020 47.84 1.7200 46.57

0.4772 2.370 39.14 2.037 38.07 1.767 37.01 1.5290 35.94

0.7788 1.610 29.14 1.424 28.20 1.279 27.35 1.147 26.49

1.OG 1.200 25.07 1.096 24.28 1.003 23.50 0.914 22.70

same temperature. The constancy of measurementwas checked at intervals and the final equilibriumvalue was noted. The reproducibility of the resultswas ensured by performing several independentexperiments. Molar conductivities corrected forthe conductivity of the solvent of about eight so-lutions in the concentration range 1.3 x 10-4-

2.5 X 10-1 mol dm >' of Nsethyl-q-cyanopyridmi-um iodide were examined at 20°, 25°, 30° and35°C.

Data treatmentAssociation constants were calculated from the

conductance data by both Fuoss and Kraus!'method and Shedlovsky' ~ method. Equation (1) isused in Fuoss and Kraus method

... {l)

A is the molar conductance at a concentration Cin mol dm -'. A 0 is the limiting conductance. K isthe ohserved association constant.

4 , [1 - I ,1i" )]F=3Cos- 3CoS (-3 Z/2

Z = [S/(Ao)3!2](CA )I/\S = aAo + f3;f3= 82.501l11(DT)II2, D is the dielectric constant;T is the temperature in K; 11is the viscosity (CP);a = 0.8204 x lO"/(DT V/2. The degree of dissocia-tion (y) is related to F by the equation:

y= A/(AoF)f + is the mean activity coefficient of the free ionsand is calculated using the equation:

A[I- log f ± = 1+ BR fl

1.8247 x lOt.A= (DT)3/2

0.5029 X 1010

B= (DT)112where

I is the ionic strength of the solution and R is themaximum centre to centre distance between theions in the ion pair. There exists at present nomethod of determining the value of R uniquely 10.

According to Justice 'I'!" R should be numericallyidentified with Bjerrum radius which for the uni-valent electrolyte is given by the expression,q = e2/2DkT; where e is the electronic charge andk is Botzmann's constant!", In the present workthe distance parameter has been set equal to theBjerrum radius as well as a fixed distance of 5 A;approximately, the sum of ionic radii in a contaction pair. The variation ofthe distance parameterdoes not significantly alter the A 0 values, as it hasbeen reported previouslyi-". This means that Aois independent of the assumed model. The asso-ciation constant K, however, depends on R in twoways: R affects strongly the degree of associationand influences also the activity coefficient of freeions. It has been found that for solutions withweak ionic association, (K < 100) the differencebetween the two models became negligible". Forother solutions the K values at 5 A are higher byabout 3% than those at Bjerrum radius. In suchsolutions the distance parameter is numericallyidentified with Bjerrum radius.

In Shedlovsky method Eq. (2) is used

1 1 2 2---'<-=-+ K Cf + S(Z)/Ao ... (2)AS(Z) Ao -

whereS(Z)=(Z!2+jI+(Z!2)2/and Z is thesame as defined in Fuoss-Kraus method. The de-gree of dissociation (y) in this case is given by,

r= AS(Z)/ Ao

Both the methods are iterative. A preliminaryvalue of An was obtained by extrapolating the ex-perimental plot of 11A versus A· C as a first ape.proximation. Calculations were carried out usingabove equations iteratively by the method of least

MUKHOPADHYAY et al: CONDUCfANCE OF N-ETIIYI.r4-CYANOPYRIDINIUM IODIDE 299

Table 2- Values of An, K and An 'I in ethanol-water mixtures at 20-35°C

Mole fraction 20° 25° 30° 35°'of ethanol(XI) K(dm-' All A,,'1 K(dm-' A" A,,'1 K(dm-' A" A" 'I K(dmJ A" All 'I

mol-I) (s.cm? mol-I) (s.cm? mol-I) (s. ern? mol : ") (s.cm?

mol') mol"") mol-I) mol-I)

0.00 70 106.6~ 107.17 ~~ 11~.~9 103.30 ~ 120.8~ 96.7~ 23 133.02 96.10

0.089 83 ~4.7~ 119. ~9 ~~ 63.~~ 11~. 3~ 36 70.2~ 109.08 2~ 80.16 106.77

0.281 127 40.74 116.92 91 ~~.37 108.88 69 ~7 .14 9~.20 ~6 ~~.96 96.2~

0.4772 172 ~0.07 94.96 118 42.98 87.~~ l~ 48.0~ 84.90 88 ~8.17 88.94

0.7788 24~ 40.91 6~.86 149 ~6.~0 66.21 117 ~.~6· 64.66 100 ~~.77 63.96

1.00 306 41.29 49.!l~ 311 48.10 ~2. 71 323 ~.99 ~1.1~ 388 ~~. 40 ~O.63

3.0240 400

2.5300

~O200

••0:.~ 10~

U)

0 0.25 os

~5 X,

o 0.25 \Co0·5 0.75

X,

Fig. I=-The Waldon product (A ()'I) and viscosity ('I) as afunction of XI' the mole fraction of ethanol in ethanol-water

mixtures at: e, 20°, 0, 25°, .to, 30° and A, 35°C.

squares till the error of estimation was 0.01 %. Allthe calculations were carried out using Fortranprogramme on mM PC-AT /386.

The A 0 values obtained using Eqs (1) and (2)are found to be almost identical; but the associa-tion constants (K) are some times different. As Kvalues are in the approximate range 1 < K < 1000,extrapolation by means of Eq. (2) according tothe recommendation of Shedlovsky" is consid-

0.75 I 0

Fig. 2-K as a function of XI'

ered. At this point it may be mentioned that themeasured A 0 and K values are of the same orderas determined by Hemrnes et al.22 for pyridiniumiodide in aqueous ethanol mixtures. The results ofA 0, K and A 0 'YJ at different temperatures are col-lected in Table 2.

Results and DiscussionThe. variations in A ()'YJ with mole fraction of

ethanol are shown in Fig. 1. The viscosity of etha-nol-water mixture increases up to a mole fractionof 0.3 in ethanol and then decreases. It is interest-ing to note that the A 0 values of the solute de-crease upto this mole fraction and then increasein ethanol region indicating maximum ethanol-wa-ter interactions in this composition. The Walden

300 INDIAN J CHEM. SEe. A. APRIL 1994

~01o

2·5

2·3

Fig. 3-Plot of log Kyersus 100/0.

product (Ao 1]) increases upto a mole fraction of0.125 in ethanol and then decreases. If variationsin Walden product reflect the change in solvationof an electrolyte", it indicates a significant de-crease in the total solvation size of the solute. Inwater rich mixtures a noticeable reduction of thesolvation size of other solutes has been foundpreviously by others?' .

The negative temperature coefficients of theWalden products may be due to the thermal ex-pansion of solvated ions-". The experimentally de-termined association constants (K) are found toincrease with an increase in the mole fraction ofethanol {Xl) showing an increased association asethanol is added to water. An approximately line-ar relation is shown from the plot of K versus Xl(Fig. 2) upto Xl < 0.8. The significantly large va-lues of K and exothermic ion-pair formation inthe solvents indicate the presence of specific shortrange iriteraction within the ion pair. Thus forpure water the value of K is equal to 45 dm 'mol- I at 298 K and decreases with increasingtemperature while electrostatic theories for ion-pair formation predict usually K < - 2 drn ' mol- Iand endothermic one for 1: 1 electrolyte in purewater. The phenomenon of ion-pair formationdue to the interplay of electrostatic and shortrange interaction may be represented by thescheme

F· 4 PI 1[4:nNaJ

, ]ig, - at of K --exp(e-.DakT) versusX3000 I'

KAS «;A+(S)+B-(S)~ (A+IIB-)~ (A+,B-) ... (3)Here (A+liB-) and (A+, B -) represent solvent~eparated ion pair and contact ion pair respect-ively. !<AS may be thought as consisting of twoparts, one due to electrostatic contribution andthe other part due to specific short range interac-tion. The conductometrically determined associa-tion constant, K, may be shown to be given byEq. (4). .

K= KAs[l + Ks1 = KAS' Ks[l + II Ks1 = KAc[l + K~l

... (4)KAC represents the formation of contact ion pairspecies from the solvated ions.

In a binary solvent mixture the solvent separat-ed ion pairs involving either of the solvent mole-cules would be expected in equilibrium and theoverall picture is then given by the followingscheme

[(A+IIB-);][S;l[A+)[B-]

: i= 1,2

(A+IIB-).~(A+ B-)+S' K=[(A+,B-)][S;]1 , "S,1 [(A+IIB-);]

+ _ (+ _ [(A+,B-))A +B :;::! A , B ); KAC [A+)[B-1 ... (5)

The square bracket indicates the activity of a spe-cies. The experimental parameter, K, in this eventwill be given by

K=KAC [1 + [S11+ [S2))Ks,1 KS,2

= KAC[ 1+ KS.I [SI)+ Ks.2[S21) ... (6)

2.7

2.5

2.3

2.1

lie: 1.9CII.2

1.7

1.5

1.332

MUKHOPADHYAY et al: CONDUCTANCE OF N-ETHYL-4-CYANOPYRIDINIUM IODIDE 301

•

33

!IT x 103.

Fig. 5-Plot of log K versus I/Tx 103• Pure alcohol, .;XI =0.7788, -; 0.4772, .; 0.281, o. 0.089, c:Jand 0.00, .1.

Replacing the solvent activities by mole fractionsand noting that for dilute solution of electrolyteX2 may be replaced by (1 - Xl) we have

... (7)

The parameter K AC = K AS: Ks,; denotes the forma-tion of a contact ion-pair species from the solvat-ed ions and can be factored into an electrostaticcontribution (Ksp)' Kel is given by the Fuoss equa-tions " and Eq. (7) may be written as

4.71Na3 2 ,K=-- exp(e IDakT)·Ksp[l + K S'

3000-

... (8)

The exponential part depends on D, the dielectricconstant of the mixed binary solvent and is de-pendent on the mole fraction XI' This may be thereason for the observed non-linearity of K versusXl curve (Fig. 2) in the higher values of XI'

The logarithimic form of Eq. (8) indicates thata straight line should be obtained upon plottinglog K versus liD at constant temperature pro-vided a is independent of the solvent composition.A plot of log K versus liD is shown in Fig. 3. Alinear relation as expected from Eq. (8) is ob-served in the water-rich region. The slope of sucha plot is e~/2.303 akT, from which the value of a,the distance parameter is obtained. It is interest-ing to note that observed a values are in the range

4

0

-46.Ho

-8

0

-12

0

-160·20

oo

0.80.4 0.6X,

1.0

Fig. 6-Plot of flJ.Ho versus X"

8-7 A and decrease slightly with increase in tem-perature. This indicates that extent of solvation isless at higher temperature. Using the value of adetermined by the above procedure electrostaticcontribution (Kel) can be calculated.

Dividing K by the electrostatic contributionone gets

K I[43:~3exp.( D::T) 1= Ksp [1 + K ~.2

+ (K ~.I - K ~,2)XI 1.. , (9)

A plot of the L.H.S. of Eq. (9) versus XI at a par-ticular temperature is given in Fig. 4, It appearsthat for the water-rich region the plot is linearwithin experimental uncertainties. A positive slopeobtained in this region indicates thatK ~.alcohol > K ~."ater as is evident from Eq. (9). Thusin the water-rich region where the three dimen-sional H-bonded net work of water moleculespredominates, water behaves as a poorer donorto the ions with respect to ethanol.

The situation, however, becomes different inthe ethanol-rich region. This is likely in view ofthe difference in water structure in the two Ie-gions. The J\ II values also indicate this in Table 1.The observed association constants except forethanol are found to decrease with an increase inthe temperature. A plot of log K versus 1IT hasbeen shown in Fig. 5.

l'l.Ho for overall association process may he de-termined from the slope of the linear plots. The8Ho values so obtained are found to dependsystematically on the composition of the mixedsolvent (Fig. 6) heing more and more negative(the association process more and more exother-

302 INDIAN J CHEM. SEe. A, APRIL 1994

mic) as the percentage of water is increased. Thusit may be concluded that specific solvation effectis important in the ion association phenomenon inN-alkylpyridinium iodides in mixed aqueous sol-vents.

AcknowledgementMP thanks the UGC, New Delhi, India, for

financial support.

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