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

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1 Indian Journal of Chemistry Vol. 33A, April 1994, pp Ionic association and ion-solvent interactions: Conductance of N-ethyl-4-cyanopyridinium iodide in ethanol-water mixtures Ajoy Mukhopadhyay, Ashis Nandi & Mohan Pal* Chemistry Department, Syamsundar College, Syamsundar, Burdwan and Sanjib Bagchi Department of Chemistry, Burdwan University, Burdwan 71314, India Received 1 June 1993; revised and accepted 4 October 1993 Conductance of N-ethyl-4-cyanopyridinium iodide in ethanol-water mixture has been measured as a function of the concentration of solute. The data have been analysed taking the existence of solvent separated ion pairs into consideration. Temperature variation of the association constant has been studied to get the thermodynamic parameter tlho as a function of the solvent composition. N-Alkylpyridinium iodides (Py+, 1-) provide a suitable system for studying ion-association phenomena. Interionic attractions and thermal motion lead to the existence of contact ion pair (c.i.p.), solvent separated ion pair (s.s.i.p.) and free ions in equilibrium in a solution'. The distribution among the different species in solution is determined by constants characterising the different equilibria. Although much work has been done on the thermodynamics of ion association very little has been done on determining the equilibrium constant for the interconversion of ion pair subspecies/". In a solution of low dielectric constant these compounds show a characteristic absorption band in the UV-visible region, originating from a charge transfer (c.t.) process within the contact ion pair species?". Ion association in solutions may be studied by monitoring the band as a function of solute concentrations". Bagchi and coworkers! studied the extent of equilibrium by spectrophotometric procedure but the method is less precise, less informative and wave length dependent. A major tool for the investigation of ion pair association is conductance measurement which yields information regarding the distribution between ionic and nonconducting species and limiting molar conductance (An). Studies in mixed solvents pose another problem. Besides providing with a system of varying dielectric constant, a binary mixture of two polar solvents also provides a medium where solvent separated species involving the two solvents may be present. Moreover there is a scope for the variations of solvent-solvent interaction as composition of the solvents in a binary mixture is varied. Thus studies in mixed solvents may give information regarding both the nonspecific and specific solvation effects on ion association phenomena. The present work addresses the problem of ionic association in a mixed binary (polar + polar) solvent mixture. Conductance studies have been carried out using solutions of N-ethyl-4-cyanopyridinium iodide at various temperatures. Materials and Methods N-ethyl-4-cyanopyridinium iodide was prepared by quaternising 4-cyanopyridine (Koch light) with ethyl iodide in the dark 1. Ethanol was treated by the standard procedure!'. Doubly distilled and degassed' water was used. All solvents were distilled immediately before use: The specific conductances of solvents were < 3 x 1 - I> s.cm - '. All solutions were prepared by dissolving weighed samples of N-ethyl-4-cyanopyridinium iodide in solvent mixtures prepared as v/v solutions. All the viscosities and dielectric constants were interpolated from literatures values 12.L~. The properties of solvent mixtures at different temperatures have been listed in Table 1. Precision conducitivity bridge (Systronics-34) capable of measuring conductance with an accuracy of ±.5% in the range of 2 j.j.s to 2 s, and a dip-type immersion conductivity cell with smooth electrodes were used for the measurement of electrolytic conductivities. The measurements were made over the temperature range 2-35 C (± O.2 C). The instrument was standardised with KCl solution of known concentration equilibrated at the

$2~ INDIAN J CHEM. SEe. A, APRIL 1994 Table1- Propertiesofethanol-watermixtureat 2-35 C Mole fraction 2 25 3 35 of ethanol X, I](C.P.) I](C.P.) I](C.P.) I](C.P.) O~OO 1.5 8.36.8937 78.~4.87 76.65.$

2 2~ INDIAN J CHEM. SEe. A, APRIL 1994 Table1- Propertiesofethanol-watermixtureat 2-35 C Mole fraction of ethanol X, I](C.P.) I](C.P.) I](C.P.) I](C.P.) O~OO ~ OG same temperature. The constancy of measurement was checked at intervals and the final equilibrium value was noted. The reproducibility of the results was ensured by performing several independent experiments. Molar conductivities corrected for the conductivity of the solvent of about eight solutions in the concentration range 1.3 x X 1-1 mol dm >' of Nsethyl-q-cyanopyridmium iodide were examined at 2, 25, 3 and 35 C. Data treatment Association constants were calculated from the conductance data by both Fuoss and Kraus!' method and Shedlovsky' ~ method. Equation (1) is used in Fuoss and Kraus method... {l) A is the molar conductance at a concentration C in mol dm -'. A is the limiting conductance. K is the ohserved association constant. 4, [1 - I,1i" )] F=3Cos- 3CoS (-3 Z/2 Z = [S/(A o )3!2](CA )I/\S = aao + f3; f3= 82.51l11(DT)II2, D is the dielectric constant; T is the temperature in K; 11is the viscosity (CP); a =.824 x lo"/(dt V/2. The degree of dissociation (y) is related to F by the equation: y= A/(AoF) f + is the mean activity coefficient of the free ions and is calculated using the equation: A[I - log f ± = 1+ BR fl where x lot. A= (DT)3/2.529 X 1 1 B= (DT)112 I is the ionic strength of the solution and R is the maximum centre to centre distance between the ions in the ion pair. There exists at present no method of determining the value of R uniquely 1. According to Justice 'I'!" R should be numerically identified with Bjerrum radius which for the univalent electrolyte is given by the expression, q = e 2 /2DkT; where e is the electronic charge and k is Botzmann's constant!", In the present work the distance parameter has been set equal to the Bjerrum radius as well as a fixed distance of 5 A; approximately, the sum of ionic radii in a contact ion pair. The variation ofthe distance parameter does not significantly alter the A values, as it has been reported previouslyi-". This means that Ao is independent of the assumed model. The association constant K, however, depends on R in two ways: R affects strongly the degree of association and influences also the activity coefficient of free ions. It has been found that for solutions with weak ionic association, (K < 1) the difference between the two models became negligible". For other solutions the K values at 5 A are higher by about 3% than those at Bjerrum radius. In such solutions the distance parameter is numerically identified with Bjerrum radius. In Shedlovsky method Eq. (2) is used '<-=-+ K Cf + S(Z)/Ao... (2) AS(Z) Ao - wheres(z)=(z!2+ji+(z!2)2/and Z is the same as defined in Fuoss-Kraus method. The degree of dissociation (y) in this case is given by, r= AS(Z)/ Ao Both the methods are iterative. A preliminary value of An was obtained by extrapolating the experimental plot of 11A versus A C as a first ape. proximation. Calculations were carried out using above equations iteratively by the method of least

3 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 2-35 C Mole fraction ' of ethanol (XI) K(dm-' All A,,'1 K(dm-' A" A,,'1 K(dm-' A" A" 'I K(dm J A" All 'I mol-i) (s.cm? mol-i) (s.cm? mol-i) (s. ern? mol : ") (s.cm? mol') mol"") mol-i) mol-i) ~ ~~ 11~.~ ~ 12.8~ 96.7~ ~4.7~ 119. ~9 ~~ 63.~~ 11~. 3~ ~ ~ ~~ ~7.14 9~.2 ~6 ~~ ~ ~ ~~ l~ 48.~ ~ ~ ~ ~6.~ ~.~ ~~ !l~ ~ ~.99 ~1.1~ 388 ~~. 4 ~O ~O 2 :. 1 ~ U) o \Co X, Fig. I=-The Waldon product (A ()'I) and viscosity ('I) as a function of XI' the mole fraction of ethanol in ethanol-water mixtures at: e, 2,, 25,.to, 3 and A, 35 C. squares till the error of estimation was.1 %. All the calculations were carried out using Fortran programme on mm PC-AT /386. The A values obtained using Eqs (1) and (2) are found to be almost identical; but the association constants (K) are some times different. As K values are in the approximate range 1 < K < 1, extrapolation by means of Eq. (2) according to the recommendation of Shedlovsky" is consid- ~5.25 os X, Fig. 2-K as a function of XI'.75 I ered. At this point it may be mentioned that the measured A and K values are of the same order as determined by Hemrnes et al. 22 for pyridinium iodide in aqueous ethanol mixtures. The results of A, K and A 'YJ at different temperatures are collected in Table 2. Results and Discussion The. variations in A ()'YJ with mole fraction of ethanol are shown in Fig. 1. The viscosity of ethanol-water mixture increases up to a mole fraction of.3 in ethanol and then decreases. It is interesting to note that the A values of the solute decrease upto this mole fraction and then increase in ethanol region indicating maximum ethanol-water interactions in this composition. The Walden

4 3 INDIAN J CHEM. SEe. A. APRIL ~ 1 o Fig. 3-Plot of log Kyersus 1/. product (Ao 1]) increases upto a mole fraction of.125 in ethanol and then decreases. If variations in Walden product reflect the change in solvation of an electrolyte", it indicates a significant decrease in the total solvation size of the solute. In water rich mixtures a noticeable reduction of the solvation size of other solutes has been found previously by others?'. The negative temperature coefficients of the Walden products may be due to the thermal expansion of solvated ions-". The experimentally determined association constants (K) are found to increase with an increase in the mole fraction of ethanol {Xl) showing an increased association as ethanol is added to water. An approximately linear relation is shown from the plot of K versus Xl (Fig. 2) upto Xl <.8. The significantly large values of K and exothermic ion-pair formation in the solvents indicate the presence of specific short range iriteraction within the ion pair. Thus for pure water the value of K is equal to 45 dm ' mol- I at 298 K and decreases with increasing temperature while electrostatic theories for ionpair formation predict usually K < - 2 drn ' mol- I and endothermic one for 1: 1 electrolyte in pure water. The phenomenon of ion-pair formation due to the interplay of electrostatic and short range interaction may be represented by the scheme J F 4 PI 1[4:nNa ig, - at of K --exp(e-.dakt), ] versusx 3 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 respectively.!<as may be thought as consisting of two parts, one due to electrostatic contribution and the other part due to specific short range interaction. The conductometrically determined association constant, K, may be shown to be given by Eq. (4).. K= KAs[l + Ks1 = KAS' Ks[l + II Ks1 = KAc[l + K~l... (4) KAC represents the formation of contact ion pair species from the solvated ions. In a binary solvent mixture the solvent separated ion pairs involving either of the solvent molecules would be expected in equilibrium and the overall picture is then given by the following scheme [(A+IIB-);][S;l ( A+IIB-).~(A+ B-)+S' K=[(A+,B-)][S;] 1, "S,1 [(A+IIB-);] [A+)[B-] : i= 1,2 + _ (+ _ [(A+,B-)) A +B :;::! A, B ); KAC [A+)[B-1... (5) The square bracket indicates the activity of a species. The experimental parameter, K, in this event will be given by K=KAC [1 + [S11+ [S2)) Ks,1 KS,2 = KAC[ 1+ KS.I [SI)+ Ks.2[S21)... (6)

5 MUKHOPADHYAY et al: CONDUCTANCE OF N-ETHYL-4-CYANOPYRIDINIUM IODIDE Ho -8 o o lie: 1.9 CII X,.8 1. Fig. 6-Plot of flj.ho versus X" !IT x 1 3. Fig. 5-Plot of log K versus I/Tx 1 3 Pure alcohol,.; XI =.7788, -;.4772,.;.281, o..89, c:jand.,.1. Replacing the solvent activities by mole fractions and noting that for dilute solution of electrolyte X 2 may be replaced by (1 - Xl) we have... (7) The parameter K AC = K AS: Ks,; denotes the formation of a contact ion-pair species from the solvated ions and can be factored into an electrostatic contribution (Ksp)' Kel is given by the Fuoss equations " and Eq. (7) may be written as 4.71Na 3 2, K=-- exp(e IDakT) Ksp[l + K S' (8) The exponential part depends on D, the dielectric constant of the mixed binary solvent and is dependent on the mole fraction XI' This may be the reason for the observed non-linearity of K versus Xl curve (Fig. 2) in the higher values of XI' The logarithimic form of Eq. (8) indicates that a straight line should be obtained upon plotting log K versus lid at constant temperature provided a is independent of the solvent composition. A plot of log K versus lid is shown in Fig. 3. A linear relation as expected from Eq. (8) is observed in the water-rich region. The slope of such a plot is e~/2.33 akt, from which the value of a, the distance parameter is obtained. It is interesting to note that observed a values are in the range 8-7 A and decrease slightly with increase in temperature. This indicates that extent of solvation is less at higher temperature. Using the value of a determined by the above procedure electrostatic contribution (Kel) can be calculated. Dividing K by the electrostatic contribution one 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 particular temperature is given in Fig. 4, It appears that for the water-rich region the plot is linear within experimental uncertainties. A positive slope obtained in this region indicates that K ~.alcohol > K ~."ater as is evident from Eq. (9). Thus in the water-rich region where the three dimensional H-bonded net work of water molecules predominates, water behaves as a poorer donor to the ions with respect to ethanol. The situation, however, becomes different in the ethanol-rich region. This is likely in view of the difference in water structure in the two Iegions. The J\ II values also indicate this in Table 1. The observed association constants except for ethanol are found to decrease with an increase in the temperature. A plot of log K versus 1IT has been shown in Fig. 5. l'l.ho for overall association process may he determined from the slope of the linear plots. The 8Ho values so obtained are found to depend systematically on the composition of the mixed solvent (Fig. 6) heing more and more negative (the association process more and more exother-

6 32 INDIAN J CHEM. SEe. A, APRIL 1994 mic) as the percentage of water is increased. Thus it may be concluded that specific solvation effect is important in the ion association phenomenon in N-alkylpyridinium iodides in mixed aqueous solvents. Acknowledgement MP thanks the UGC, New Delhi, India, for financial support. References I Pal M & Bagchi S, J chem SOl' Faraday Trans I, 81 (1985) Strauss I M & Symons M C R, J chem Soc Faraday Trans 1,74 (IY78) 2146; 73 (1977) Gupta A & Rao C N R, J phys Chern, 77 (1973) Fuoss R M,J phys Chern, 82 (1978) Hogen-Esch T F & Smid J, J Am chem Soc, 88 (1966) Mulliken R S & Person W B, Molecular complexes (Wiley Interscience, New York) Kosower E M, An Introduction to physical organic chemistry(wiley, New York), B Griffiths T R &. Pugh D C, J sol qern, B (1979) Bagchi S & Chowdhury M, J phys Chern, 8 (1976) Mackey R A & Pozomek E J, JAm chem Soc, 92 (197) Wiesberger A, Techniques in organic chemistry, Vol 7, (Interscience, New York), Handbook of chemistry and physics, edited by C D Hodgman, R C Weast & S M Selby, Vol 38 (Chemical Rubber Publishing Co. Cleaveland, Ohio), Akhadov Y Y, Dielectric properties of binary solutions- A data handbook (Pergamon Press, Oxrford), Fuoss R M & Krauss C A, J Am chem Soc, 55 (1933) Fuoss R M & Shedhovsky T, JAm chem Soc, 71 (1949) Beronius Per, Acta Chern Scand A, 31 (1977) Justice J C, Electrochim Acta, 16 (1971) Renard E & JusticeJ CJJ soln Chern, 3 (1974) Bjerrum N K, Dan vidensk; mat selsk, Fys medd, 9 (1926) 7. 2 Haw1icka E, Naturforscb Z, 42a (1987) Hawlicka E & Grabowski R, Naturforsch Z, 46a (1991) Hemmes P, Costanzo J N & Jordan F, J phys Chern, 82 (1978) Raju U G K, Sethuram B & Rao TN, Bull chem Soc Japan, 55 (1982) Doe H, Kitagawa T & Sasabe K, J phys Chen" 88 (1984) Fuoss R M, JAm chem Soc, 8 (1958) 558.

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