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

    Interactions of Tetraalkylammonium Ions in Dipolar Aprotic Solvents UTPAL 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 dipolar aprotic solvents, vlz. dimethylformamide dimethylsulphoxide, ethylene carbonate, and propy- lene carbonate, have been analysed and a comparative study of the specific interactions caused by the 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, partial molal volume study of tetraalkylammonium ions (R4N+) may be useful for understanding the different types of ion-solvent interactions in various solvent systems. Wen! proposed that partial molal volume of R4N+ ions in water at infinite dilution may be split 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 is the volume change due to electrostriction, VfI.o is the volume change of water due to hydrophobic structure formation, and V~agingis the volume change due to "caging effect" or packing effect, i.e. the volume loss due to partial hiding of the hydrocarbon tails of the R4N+ ions into the cC'ge formation caused by the hydrophobic interaction. Later it hrs been argued by the au thor- that in Eq. (1) V~lect is comparatively .smcll for R4N+ ioi.s because of their small surface charge densities, and hence can be neglected. It woosalso assumed that Eq. (1) should hold good for other hydrogen-bonded solvent systems as well, and the author proposed that VfI.o (or V~ for all solvents in general) and V~aging can be equated to Ane z.nd Bln; respectively, where A r.nd B are empirical constants (different for different solvents), and ne is the number of cc.rbon atoms in the R4N+ ion concerned. Accordingly, Eq. (1) becomes V?on = V~rYSd-Ant+Blnc ... (2) where V~ryst' the crystal volume of the ion, h: s been taken 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 other hydrogen-bonded solvent systems, the hydrophobic interaction in water is the strongest. In the same context, though it may be interesting to examine the usefulness of Eq. (2) for studying ion-solvent interactions of R4N+ ions in dipolar aprotic solvents, it has to be kept in mind that the hydrophobic effect of R4N+ ions may not be present 2S such in these solvents where hydrogen-bonding is ruled out. However, other kind of associarion among the solvent molecules may be possible in dipolar aprotic ~olve~ts like dimethyl sulphoxide (DMSO) which IS believed to form sulphur oxygen chain structures

    of varied dimensions", Moreover, the partial molal ionic volume in these solvents may be described as sum of the effects of (i) 'hole' formation due to rearrangements among the solvent molecules, and (ii) the actual accommodation of the solute R4N+ ions into these' holes', thus formed. Though, it may be difficult to draw a sharp dividing line between these two volume effects in solution, similar analysis ha s been proved useful earlierl,4.

    In the present paper the partial molal volume of R4N+ ions in four dipolar aprotic solvents, viz. dimethylformamide (DMF)6, dimethylsulphoxide (DMSO)6, ethylene carbonate (EC)6, and propylene carbonate (PCJ7, obtained from literature data, have been used to determine the values of A and B for these solvents and the results have been examined on a comparative basis.

    Calculations and Results

    Ionic volumes of R4N+ ions in dipolar aprotic solvents - Several methods+ ha ve been used from time to time to calculate the partial molal volume of individual ions from the corresponding values of the salts. Except the ultrasonic vibration potential (UVp)8 method all are empirical. Since uvp results (with the exception of DMFI') are not available in the solvents studied in this paper, extrapolation methodw has been used to obtain the ionic volumes. Though in some solvents the results of extrapolation method agree reasonably well with those obtained by some other methods such as the T ATB assumption methodll'12, the validity of the extrapolation method in solvents other than water, particularly in dipolar aprotic solvents, is not beyond all doubts=P, Therefore the possibility of having some uncertainty in the values of the ionic volumes, determined by extrapolation method, may not be completely ruled out. However, in the present work it is believed that as far as the comparative study is concerned these uncertainties mav not affect the ultimate conclusion. Due to the same reason, though in literature 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 tetraalkylammonium salts are available for all the four solvents, are used in the present work, and the study has not been extended at other temperatures.


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


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


    Ion-solvent interaction - It can be seen from Table 2 ~hat the v



    -1000"'O----;;2±OO::--------:4...L O


    n3 C

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


    OEe OPC

    oo:.:r •• ~'e2S0



    (-L )2 nc

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


    hydrophobic interaction in water. Data on trans- port properties of R4N+ ions in various dipolar aprotic solvents obtained by different workers16-23 also point out that the ionic mobilities of larger R,N; ions in aqueous solution are significo.ntly lower than the corresponding values for dipolar aprotic

    solvents, siggesting compare tively WN ker specific interaction in these solvents. The values of ionic mobilities of R4N+ ions in various solvents (obtained from experimented conductivity dr.ta through some empirical assumptior.s like TATB12.21), however, do not indicate structure formation in dipolar r.protic solvents by the incorporated R4N+ ior s. The present analysis, nevertheless, shows the t Eq. (2) m"y be useful for studying ion-solvent interr ctior.s in dipolar aprotic solvents on a compr ra.tive b. sis, and the specific inter" ction ma y be put in the order: EC>DMF ~ Me2SO> Pc. The differer.ces < mongst the extent of the specific ion-solvent interrction in these dipolr r c.protic solvents are, however, rot ; s pronour.ced r s those rmor-g protic solvents". Ex- perimentrl UVp8drta regardir g partial molal volume for R4N+ iOTS in vrrious dipolar r protic solvents seem to be necessr ry for providing a more cor-elusive picture 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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