Conductance of Tetra-alkylammonium Perchlorates inAcetonitrile+Methanol Mixtures at 25°CBY ALESSANDRO D'APRANO, MARIO GOFFREDI* AND ROBERTO TRIOLOInstitute of Physical Chemistry, University of Palermo,Via Archirafi 26, 90123 Palermo, ItalyReceived 14th April, 1975The association and the hydrodynamic behaviour of some quaternary ammonium perchloratesin acetonitrile+ methanol mixtures have been investigated by precise conductance measurements at25°C. The dependencies of association and mobility on solvent composition are primarily due tocation size and specific short-range ion-solvent interactions.It is we1 established that the extent of association of a series of electrolytes indifferent solvea ts is specific for any ion-solvent combination, rather than being di-electric constant dependent.The association behaviour of alkali metal and quater-nary ammonium perchlorates in an aprotic solvent such as acetonitrile and a proticsolvent such as methanol is just one examp1e.l This paper extends the measurementsto some tetra-alkylammonium perchlorates in the approximatively isodielectric solventmixture acetonitrile + methanol, with a view to obtaining information on the effectof short-range interactions between ions and adjacent solvent molecules on the ion-pairing process.EXPERIMENTALTetramethylammonium (Me4NC104), tetraethylammonium (Et4NC104), tetrabutyl-ammonium (Bu4NC104) and tetrahexylammonium perchlorates (Hex4NC104) wereprepared, purified and dried as reported previously. Methanol and acetonitrile were purifiedas already de~cribed.~The Pyrex conductance cells were of the Erlenmeyer type as described by Daggett, Bairand Kraus5 with bright platinum electrodes.The cell constants were determined at25.000+0.002"C by the method of Lind, Zwolenick and FuossP All the conductance runswere carried out by the concentration method and the measured resistances were correctedfor the usual frequency dependence. The bridge has been described elsewhere.'All solutions were prepared by weight, vacuum corrected and volume concentrations inequivalents per litre were calculated using the density of the solvents. Solvent mixtures werealso made up by weight; their properties, interpolated from literature data,8 are given intable 1,* where w is the weight % of methanol, d is the density, q is the viscosity and D thedielectric constant.RESULTS AND DISCUSSIONThe measured equivalent conductances A(ohm-l equiv.-l cm2) at correspondingconcentration c(equiv.dm-3) are summarized in table 2, where each set of data isidentified by reference to the corresponding dielectric constant value of the solventmixture.* Tables are collected in an Appendix.780 CONDUCTANCE OF TETRA-ALKY LAMMONIUM PERCHLORATESThe conductance data were analysed by means of eqn (1)A = A0 - SJCY + ECY log CY + J c ~ + JZcsyt- KAcyf 2A (1)where the symbols have their usual significance. The parameters Ao, KAY a and thestandard deviation percent (a/ho)lOO obtained by a computer analysis are given intable 3 together with the Walden products A0q. In fig.1, log& vs. 1/D plots areshown for all the tetra-alkylammonium salts here analysed. Note that log& decreasesmonotonically from pure methanol to pure acetonitrile when Hex4NC104, Bu4NC104and Et4NC104 are considered. Moreover, a minimum appears in the acetonitrile-rich region for Me4NC104 as already found for KC104 lo in the same mixtures.2.00 2.90 3 . 0 0 3.1ClOO/DFIG. l.-LOg& variation as a function of dielectric constant for (0) Me4NC104 ; ( 8 ) Et4NC104 ;(a) Bu4NC104 and (0) Hex4NC104 in acetonitrile + methanol nlixtures at 25°C.The non-linearity of log& plotted as a function of 1/D and the fact that for allthe salts the curves lie above the theoretical straight line of eqn (2) l1e 2bkDTlo@, = logKi + - (2)suggest the existence of strong interactions between the solvent and the electrolyte.As was earlier pointed out,' the different degree of ion-pairing found for thesequaternary ammonium perchlorates in pure methanol and pure acetonitrile can berationalized in terms of a two-step mechanism involving solvent-separated andcontact-solvated ion-pairs,l2 or in other words, the excess of association of a saltin the two solvents is strictly dependent on the contributions of two terms relatedboth to ion-solvent and ion-pair-solvent interactions.As a consequence, by adding acetonitrile to a methanolic solution of a salt inwhich only the anion can be solvated by the protic solvent, we would expect a decreaseof the association extent.If, on the other hand, only cation solvation (by the proticsolvent) occurs we would expect an increase of association. When both cation andanion can be solvated they will exhibit compensating effects, that is to say, a minimumin the log& vs.1 / D plot might appearReturning to the present results, we note that this is the case for Me4NC104A . D’APRANO, M. GOFFREDI AND R. TRIOLO 81while for the other three salts, whose cations are poorly or not solvated at all, theonly effect observed is the one related to anion solvation.Mobility data for these electrolytes are shown in fig. 2 where the Walden productsare plotted against the mole fraction of methanol in the mixtures. The salts showa nearly parallel behaviour in the small variation of Aoq over all the solvent composi-tion range.Furthermore, all sets of data show a systematic deviation from thesmooth curve for 0.70 < XMeOH < 0.90, suggesting a common origin. Such beha-viour seems to be related to the change in the structure of the solvent mixture asshown by the negative deviation of the molar volume excess plotted in the same figure.Present knowledge of the structure of these solvents, however, is not detailed enoughto permit a direct correlation of the hydrodynamic and associative behaviour of thesesolutions.A L - 0 0.25 0 . 5 0 0.75 1.0x2FIG. 2.-Walden product dependence on mole fraction of methanol (X2) for (0) Me4NC104 ;( 0 ) Et4NC104 ; (a) Bu4NC104 and (a) Hex4NC104 (ordinates right). The molar volume excess inper cent (Av/v) x 100 (-.-.-.) against mole fraction of methanol (Xz) (ordinates left).wt.% methanol0.0014.4923.9933.7050.0050.5051.4059.7660.1069.2273.8074.2082.6085.9789.8390.00100.00APPENDIXTABLE PROPERTIES OF SOLVENT MIXTURES AT 25°C3.448 0.776 903.340 0.779 323.350 0.780 663.430 0.782 043.663 0.784 223.672 0.784 283.686 0.784 383.848 0.785 393.854 0.785 434.190 0.786 384.308 0.786 714.316 0.786 924.586 0.787 004.720 0.786 994.880 0.786 964.891 0.786 955.450 0.786 55103 ~ I P d/g ml-1 D36.0235.5235.2034.7634.3234.3134.2834.0033.9833.6733.5233.5133.2233.1032.9832.9732.682 CONDUCTANCE OF TETRA-ALKYLAMMONIUM PERCHLORATESTABLE 2.-EQUIVALENT CONDUCTANCES OF TETRA-ALKYLAMMONIUM PERCHLORATES IN ACETO-NITRILE+ METHANOL MIXTURES AT 25°CD35.5234.2833.6735.5234.7634.2833.6733.22l04c/equiv. dm-3 A/cmz R-1 equiv.-I D3.11405.592 28.524 411.36914.2823.393 68.807 912.54315.76018.6713.907 86.920 613.83217.31721.0934.729 78.229 515.96119.54723.1804.120 87.941 711.53615.1 1618.3924.251 08.586 613.39316.83921.03024.4534.670 38.392 812.42915.58618.87821.9289.650 513.73 118.22422.02525.354Me4NC104199.77 34.76197.08194.55192.52190.65186.57 33.98181.34178.71176.74175.13169.85 33.22166.89161 -92159.98158.04Et4NC104187.86185.02180.47178.84177.35184.28180.99178.64176.66175.08173.64170.12167.23165.48163.58162.24158.34155.34152.87151.23149.68148.41140.36138.01135.74134.15132.9135.2034.3133.9833.5132.98104c/equiv. dm-3 A/cm2 R-1 equiv.-I8.577 86.890 09.934 614.04018.0372.884 75.536 18.842 214.97018.8293.331 86.328 99.234 912.39316.25319.1734.560 69.509 213.00417.11220.2505.889 79.645 713.90217.6482 1 A5227.9095.878 211.51116.59621.12525.18629.8596.859 810.70215.18619.36823.5976.691 010.43714.41518.10126.250196.82193.46191.03188.35186.12180.36177.38174.44170.301 68.08155.91152.90150.51148.36146.06144.54187.51183.54181.41179.35177.90173.21170.45167.96166.12164.47162.03165.27161.26158.61156.50154.91153.31151.43148.79146.32144.37142.63134.68132.08129.79127.96124.5A .D’APRANO, M. GOFFREDI AND R . TRIOLO 83D35.5234.2833.6733.2235.2033.5233.10104c/equiv. dm-32.541 65.233 37.992 010.76713.19315.8372.661 75.256 37.766 610.66913.49416.6664.593 98.439 111.88715.26618.00221.1743.330 27.766 710.71013.98216.64919.4642.644 66.268 810.13214.11819.13622.2632.624 46.100 59.792 713.98722.1002.947 06.296 510.55015.36420.32826.207TABLE 2.-continuedBu4NC104165.35 35.20162.79160.80159.16157.94156.75150.27 34.00147.88146.10144.40143.01141.63134.18 33.51131.54129.69128.14127.01125.85122.46 32.98119.12117.46115.88114.62113.61Hex4NC104153.44 34.32150.32147.91146.02144.04142.96122.82 33.22120.00117.88115.93112.95111.43 32.97108.82106.41104.24102.36100.45104c/quiv. dm-33.259 95.901 27.763 110.52113.48616.0054.261 27.325 910.62713.86016.91520.6605.210 113.30516.37220.64424.6822.592 35.976 58.912 612.12115.85219.1 142.599 66.380 710.44814.95019.99925.3622.563 65.719 09.437 113.23622.8763.163 96.937 811.84416.62922.25328.826163.49161.15159.90158.20156.70155.55142.05139.76137.84136.26134.96133.52129.28124.58123.24121.59120.21115.42112.52110.63108.89107.17105.85141.27138.08135.73133.73131.81130.081 15.02112.43110.21108.38104.78107.30104.48101.7999.6997.6495.584 CONDUCTANCE OF TETRA-ALKYLAMMONIUM PERCHLORATESTABLE 3 .-DERIVED CONSTANTS FOR TETRA-ALKYLAMMOWM PERCHLORATES IN ACETOMTRILE+METHANOL MIXTURES AT 25°CD35.5234.7634.2833.9833.6733.2235.5235.2034.7634.3 134.2833.9833.6733.5133.2232.9835.5235.2034.2834.0033.6733.5133,2232,9835.2034.3233.5233.2233.1032.97207.42+0.010205.0Sk 0.004194.44& 0.005187.52+0.0101 77.89 & 0.01 0163.OOf0.010196.77_+0.010196.34$-0.0201 92.59 0 .O 1 0182.88k 0.004181.82+0.010174.76& 0.030166.46+0.010161.30f0.010151.93f0.020144.15+ 0.002171.36& 0.040170.32+ 0.007156.13k 0.003149.30+ 0.003141.44f0.002136.94_+0.001128.37+0.010120.4640.003159.37f0.010146.85+_ 0.010127.95rt:0.003119.95f0.010116.69f0.003112,73 & 0.002&/dm3 mol-1Me4NC10418.61 f 0.0519.96f 0.0524.84+ 0.0328.24f 0.0630.60f 0.0536.92f 0.04E t4NC10414.16+ 0.0415.02+ 0.0815.63k0.051 8.12f 0.0218.45 + 0.0521.45 + 0.1024.20f 0.0626.62+ 0.0430.52+ 0.0936.95 5 0.01Bu4NClOd11.66f 0.0312.11_+0.0515.8950.0317.83+ 0.0221.46k0.0223.94+ 0.0529.22$- 0.1035.6850.03Hex,NClQ411.20+ 0.0614.17f 0.0722.33f 0.0327.92+ 0.0130.92+ 0.0335.56k0.024.94.94.44.14.03.76.06.06.05.65.65.14.84.54.23.76.96.96.25.95.34.94.33.97.26.75.24.54.23.90.0100.0050.0070.0100.0100.0100.0100.0200.0100.0040.01 00.0300.0100.0100.0300.0050.0080.0100.0080.0060.0050.0020.0300.0100.0200.0200.0100.0030.0100.007AoVlcrn2 9-requiv.-1 P0.6930.7030.7170.7220.7450.7480.6570.6580.6610.6710.6710.6750.6970.6970.6970.7030.5720.57 10.5760.5750.5930.5920.5890.5880.5340.5380.5510.5500.5510.551A. 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