Indian J01ll'llAl of ChemistryVol. 16A, April 1978, pp. ~91-~93

Interactions of Tetraalkylammonium Ions in Dipolar Aprotic SolventsUTPAL SEN

Central Electrochemical Research Institute, Karaikudi 623006

Received 29 July 1977; accepted 1 November 1977

The literature partial molal volume data of tetraalkylammonium salts in four dipolaraprotic solvents, vlz. dimethylformamide dimethylsulphoxide, ethylene carbonate, and propy-lene carbonate, have been analysed and a comparative study of the specific interactions caused bythe tetraalkylammonlum ions in these solvents has been attempted.

SOLUTIONS of symmetrical tetraalkylammo-nium salts in aqueous and other solvents showa variety of interesting properties which

warrant investigations. In that respect, partialmolal volume study of tetraalkylammonium ions(R4N+) may be useful for understanding the differenttypes of ion-solvent interactions in various solventsystems. Wen! proposed that partial molal volumeof R4N+ ions in water at infinite dilution may besplit into the four following components [Eq. 1).VYon= V?nt + V.3ect+ VfI,o+ V~aging ... (1)where V1nt is the intrir.sic volume of the ion, V~lcct isthe volume change due to electrostriction, VfI.o isthe volume change of water due to hydrophobicstructure formation, and V~agingis the volume changedue to "caging effect" or packing effect, i.e. thevolume loss due to partial hiding of the hydrocarbontails of the R4N+ ions into the cC'ge formation causedby the hydrophobic interaction. Later it hrs beenargued by the au thor- that in Eq. (1) V~lect iscomparatively .smcll for R4N+ ioi.s because of theirsmall surface charge densities, and hence can beneglected. It woosalso assumed that Eq. (1) shouldhold good for other hydrogen-bonded solvent systemsas well, and the author proposed that VfI.o (orV~ for all solvents in general) and V~aging can beequated to Ane z.nd Bln; respectively, where A r.ndB are empirical constants (different for differentsolvents), and ne is the number of cc.rbon atoms inthe R4N+ ion concerned. Accordingly, Eq. (1)becomesV?on = V~rYSd-Ant+Blnc ... (2)where V~ryst' the crystal volume of the ion, h: s beentaken 1 s the V?nt.

From Eq. (2) the values of A and B were deter-mined ar.d it WC'S shown that relative to otherhydrogen-bonded solvent systems, the hydrophobicinteraction in water is the strongest. In the samecontext, though it may be interesting to examinethe usefulness of Eq. (2) for studying ion-solventinteractions of R4N+ ions in dipolar aprotic solvents,it has to be kept in mind that the hydrophobic effectof R4N+ ions may not be present 2S such in thesesolvents where hydrogen-bonding is ruled out.However, other kind of associarion among thesolvent molecules may be possible in dipolar aprotic~olve~ts like dimethyl sulphoxide (DMSO) whichIS believed to form sulphur oxygen chain structures

of varied dimensions", Moreover, the partial molalionic volume in these solvents may be described assum of the effects of (i) 'hole' formation due torearrangements among the solvent molecules, and(ii) the actual accommodation of the solute R4N+ions into these' holes', thus formed. Though, itmay be difficult to draw a sharp dividing line betweenthese two volume effects in solution, similar analysisha s been proved useful earlierl,4.

In the present paper the partial molal volume ofR4N+ ions in four dipolar aprotic solvents, viz.dimethylformamide (DMF)6, dimethylsulphoxide(DMSO)6, ethylene carbonate (EC)6, and propylenecarbonate (PCJ7, obtained from literature data, havebeen used to determine the values of A and B forthese solvents and the results have been examinedon a comparative basis.

Calculations and Results

Ionic volumes of R4N+ ions in dipolar aproticsolvents - Several methods+ ha ve been used fromtime to time to calculate the partial molal volume ofindividual ions from the corresponding values of thesalts. Except the ultrasonic vibration potential(UVp)8 method all are empirical. Since uvp results(with the exception of DMFI') are not available inthe solvents studied in this paper, extrapolationmethodw has been used to obtain the ionic volumes.Though in some solvents the results of extrapolationmethod agree reasonably well with those obtainedby some other methods such as the T ATB assumptionmethodll'12, the validity of the extrapolation methodin solvents other than water, particularly in dipolaraprotic solvents, is not beyond all doubts=P,Therefore the possibility of having some uncertaintyin the values of the ionic volumes, determined byextrapolation method, may not be completely ruledout. However, in the present work it is believedthat as far as the comparative study is concernedthese uncertainties mav not affect the ultimateconclusion. Due to the same reason, though inliterature partial molal volume data of tetraalkyl-ammoniun S2!tS6,7 are available at different tem-peratures ranging from 250 to 750

, only data at 500,

where partial molal volumes of tetraalkylammoniumsalts are available for all the four solvents, are usedin the present work, and the study has not beenextended at other temperatures.

291

INDIAN J. CHEM., VOL. 16A, APRIL 1978

Results

The values of r~m 's in various solvents obtainedby the extrapola tion-? method are listed in Table 1.Ta~le 2 contains the difference. Vi'on- V~ryst. forvanous solvents. Eq. 2 has been used to calculatethe values of A and B by graphical method" and thevalues given in Table 3 contains the values ofA and B for these solvents.

Discussion

Ion-solvent interaction - It can be seen from Table 2~hat the v<l:luesof Vfon- vgryst for R4N+ ions in

dIpola~ aprotic solvents are negative and the negativevalne. In gen~ral shows a tendency to increase inmagnitude with the size of the ion. The value ofV~r~st (2'52r3) de~ends upon !he choice of the crystalradius (r) of the IOn while Vion has been determinedfrom experimental data through empirical method.In the present work r has been estimated accordingto the method recommended by Robinson andStokes-e. This method has been used earlier byseveral workers2,4.15.16 without difficulties. It maytherefore be assumed that the uncertainty (if any)In the value of r [i.e, in the value of V~ryst) would

TABLE 1 - PARTIAL MOLALVOLUMESOF TETRAALKYL-AMMONIUMIONS AT INFINITE DILUTlON* IN VARIOUS

DIPOLAR APROTICSOLVENTSAT 50°

Ion -0V ion (ems/mol) of R,N+ in

DMF Me.SO EC PC H.OtEt.N+ 140·6 137·7 138-4 139·8 145·7Pr.N+ 213·6 209·1 209·3 211·7 213·0Bu,N+ 283·0 277-1 278·7 283·0 279-1Pen.N+ 353·6 350·6 348·8 352·4 346·6Hex.N+ 425·0 416·5Hep.N+ 492·1 488·8 486·3 490·2

·Calculated using extrapolation methodw,t Reference 2.

-0 -0TABLE 2- Vion-Vcryst VALUES FOR TETRAALKYL-

AMMONIUMIONS IN VARIOUSDIPOLAR APROTIC-50LVENTS AT 50°

Ions V~ryst --0 -0(ems/mol) inV ion-V cryst

(ref. 2)DMF MesSO EC PC H.O

(ref. 2)

Et.N+ 161·3 -20·7 -23-6 -22-9 -21·5 -15·6Pr.N+ 232·7 -19·1 -23-6 -23-4 -21·0 -19·7Bu.N+ 303-8 -20·8 -26·7 -25·1 -20·8 -24·7Pen.N+ 375·3 -21·7 -24·7 -26·5 -22·9 -28·7Hex.N+ 454·5 -29·5 -38·0Hep.N+ 522-9 -30·8 -34-1 -36·6 -32·7

TABLE 3 - VALUES OF CONSTANTSA AND B OF Eq. (2)FOR VARIOUS DIPOLAR APROTIC SOLVENTSAT 50°

Solvents A Bems/mol ems/mol

DMF -1·0 -100MesSO -1·0 -130EC -1·2 -110PC -0·9 -120H.O (ref. 2) -1·3 -40

292

not be sufficient to change the sign of Vion- Vgryst.The magnitude of incre, se in Vion- V~ryst valuewith increase in the size of the ion suggests thatele~trostric~ion (i.e. electrostatic ion-dipole inter-action] WhICh should become more prominent whenthe size of the ion decreases causing increase in thesurface charge density. is of little importance in thecase of R4N+ ions in dipolar aprotic solvents. Simi-lar observation has been reported in the case ofinteractions of R4N+ ions with pro tic solvents''.Negative value of Vi'on- Vgryst means. in general.structure formation in the system'. For pro ticsolvents this observation of negative value has beenexplained by considering hydrophobic interaction ofR,N+ ions with the solvent moleculess+, Thoughthere is LO hydrogen-bonding in dipolar aproticsolvents, which is believed to be necessary for hydro-phobic interaction, the negative Vi'on- Vgryst valuesof R4N+ ior.s in dipolar aprotic solvents suggestthat some kind of packing, arising from some specificinteractior.s, is introduced by the incorporated R4N+ions in these solvents. Dack et al.12 earlier triedto explain the cause of different Vion values indifferent solvents from the respective solvent com-pressibility cor.sideration, It has been suggestedthat solute-solvent interaction for uncharged mole-cules or large ions (i.e. ion with very low surfacecharge density) should depend mainly upon theability of the large solute to push back the solventmolecules to create tholes' for themselves. Thussolvent compressibility is of paramount importanceto Vi'on with solvent of high compressibility beingmore able to absorb the intrinsic volume of theneutral or feebly charged solutes. It has also beenobserved that Vi:,n of Ph,As+ ion. a large cation.is smaller in dipolar aprotic solvents than in waterwhich has a comparatively lower compressibility-P.Bhatr.agar and Crissl? from their calorimetricmeasurements of enthalpy of transfer (6.Htr) ofdifferent ions for various sovent pairs. have alsoenvisaged the possible existence of some It specialinteractior.s II of R4N+ ions with various solventsincluding dipolar aprotic solvents. Similar type ofspecific interaction may also be responsible for makingVi'on- Vgryst value for R,N+ ions in dipolar aproticsolvents negative. All these consideration mightallow one to apply Eq. (2). originally proposed todeal wi th the specific ion -solven t in teractions ofR4N+ ions in protic solvents. for dipolar aproticsolvents < s well. The excellent linear plots (Figs. 1and 2) provide additional support for this view.It may be mentioned here that the uncertainty inthe values of VR,N+ in dipolar aprotic solvents,obtained through the extrapolation method-", maynot contradict the above finding. because a similaranalysis of experimental uvp data" in DMF solutionalso gives negative value of Vion- Vgryst leading tothe same conclusion [i.e, the incorporated R4N+ ionsinduce some kind of packing among solvent (DMF)molecules causing volume loss to the system]. Thevalues of the constants A and B of Eq. (2) obtainedfrom uvp datat for DMF at 25°C are -1·5 cms/moleand -190 cms/mol, respectively.

The data in Tables 2 and 3 show that the specificinteraction of R4N+ ions in dipolar aprotic solventsare comparatively weaker than the corresponding

or----------------------------------.SEN: INTERACTIONS OF TETRAALKYLAMMONIUM IONS IN APROTIC SOLVENTS

-800

-1000"'O----;;2±OO::--------:4...LO

-:'"O----'6::'O""O,------nO\,;OO,--'

n3C

1 - Plots of (V~on- V~ryst)nc vs Iln~ for R,N> ions inDMF and DMSO solutions

Fig.

OEeOPC

oo:.:r•• ~'e2S0

-).

';;O----~OO~O~5-----~O,~OI~O----:-O·~O-:-I~~~--~~~OO~O

(-L )2nc

2 - Plots of Wron - V~ryst)/nc vs I/n~ for R,N+ ions inDMF. DMSO. EC. and PC solutions

Fig.

hydrophobic interaction in water. Data on trans-port properties of R4N+ ions in various dipolaraprotic solvents obtained by different workers16-23

also point out that the ionic mobilities of largerR,N; ions in aqueous solution are significo.ntly lowerthan the corresponding values for dipolar aprotic

solvents, siggesting compare tively WN ker specificinteraction in these solvents. The values of ionicmobilities of R4N+ ions in various solvents (obtainedfrom experimented conductivity dr.ta through someempirical assumptior.s like TATB12.21), however, donot indicate structure formation in dipolar r.proticsolvents by the incorporated R4N+ ior s. The presentanalysis, nevertheless, shows the t Eq. (2) m"y beuseful for studying ion-solvent interr ctior.s in dipolaraprotic solvents on a compr ra.tive b. sis, and thespecific inter" ction ma y be put in the order:EC>DMF ~ Me2SO> Pc. The differer.ces < mongstthe extent of the specific ion-solvent interrction inthese dipolr r c.protic solvents are, however, rot ; spronour.ced r s those rmor-g protic solvents". Ex-perimentrl UVp8drta regardir g partial molal volumefor R4N+ iOTS in vrrious dipolar r protic solventsseem to be necessr ry for providing a more cor-elusivepicture of ion-solvent inter, ctions in these solvents.

Acknow ledgement

The e.uthor thanks Dr H. V. K. Udupa, Director,CECRI, for his permission to publish this paper.

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