Journal of Molecular Liquids 111(2004) 15–18

0167-7322/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.molliq.2003.09.017

Ultrasonic velocities and isentropic compressibilities of some symmetricaltetraalkylammonium salts inN,N-dimethylacetamide at 298.15 K

Debashis Das, Bijan Das*, Dilip K. Hazra

Department of Chemistry, North Bengal University, Darjeeling 734430, India

Received 13 May 2003; accepted 7 September 2003

Abstract

Ultrasonic velocities of tetrapropylammonium bromide(Pr NBr), tetrabutylammonium bromide(Bu NBr), tetrapentylammonium4 4

bromide(Pen NBr), tetrahexylammonium bromide(Hex NBr), tetraheptylammonium bromide(Hep NBr), and tetraoctylammon-4 4 4

ium bromide(Oct NBr) in N,N-dimethylacetamide have been measured at 298.15 K. Apparent molar isentropic compressibilities4

k of these electrolytes were derived from these data supplemented with their densities. The limiting apparent molar isentropicf

compressibilitiesk were obtained by extrapolation from the plot ofk vs. the square root of the molality. Thek values of theo of f f

electrolytes were split into approximate limiting ionic compressibilities on the basis of the assumption thatk (Br )s0. Theo o ykf" f

results have been interpreted in terms of specific constitutional and structural factors of the solvent molecules and of solute ions.� 2003 Elsevier B.V. All rights reserved.

Keywords: Ultrasonic velocity; Apparent molar isentropic compressibility; Tetraalkylammonim salts;N,N-Dimethylacetamide

1. Introduction

Recently, we have initiated a comprehensive programin order to study the solvation and association behaviorof several 1:1 electrolytes in different non-aqueoussolvents from the measurements of various transport,thermodynamic, and spectroscopic propertiesw1–6x. Asa part of this series of investigations, we have recentlyreportedw7–9x the results of conductance and viscositymeasurements on a number of tetraalkylammonium saltse.g. tetrapropylammonium bromide(Pr NBr), tetrabu-4

tylammonium bromide(Bu NBr), tetrapentylammonium4

bromide (Pen NBr), tetrahexylammonium bromide4

(Hex NBr), tetraheptylammonium bromide(Hep NBr)4 4

and tetraoctylammonium bromide(Oct NBr) in N,N-4

dimethylacetamide at 298.15 K. We have now extendedw9x this work to study the compressibility behavior ofthis electrolyte solution to unravel the nature of varioustypes of interactions prevailing in these solutions.

2. Experimental

N,N-Dimethylacetamide(G.R.E. Merck, India,)

*Corresponding author.E-mail address: slg_bijandas@sancharnet.in(B. Das).

99.5%) was distilled two times in an all-glass distillationapparatus immediately before use and the middle frac-tion was collected. The properties of the purified solventagree well with the literature valuesw10x.For the preparation of the solutions of each salt, a

different batch of solvent was used and the propertiesof the pure solvent used in each case are recorded inTable 1.The tetraalkylammonium bromides were of purum or

purissimum grade (Fluka) and were purified asdescribed in the literaturew11x. These salts were furtherpurified by recrystallization and the higher homologueswere recrystallized twicew9x to ensure maximum purity.The recrystallized salts were dried under vacuum at373.15–383.15 K for 24 h. Owing to the hygroscopicnature of the salts, these were stored in a vacuumdesiccator over calcium chloride and were dried for 3–4 h at 373.15 K immediately prior to use.In order to minimize moisture contamination, all

solutions were prepared in a dehumidified room withutmost care. A stock solution for each salt was preparedby mass and the working solutions were obtained bymass dilution. The conversion of molality to molaritywas done using the density values, measured with anOstwald–Sprengel type pycnometer having a bulb vol-

16 D. Das et al. / Journal of Molecular Liquids 111 (2004) 15–18

Table 1Concentration(c), density(r), ultrasonic velocity(u) and apparent molar isentropic compressibility(k ) of the electrolyte solutions inN,N-f

dimethylacetamide at 298.15 K

cymol dmy3 ryg cmy3 uycm sy1 10 k y10f cymol dmy3 ryg cmy3 uycm sy1 10 k y10

f

cm mol bar3 y1 y1 cm mol bar3 y1 y1

Pr NBr4 Bu NBr4

0.00000 0.93670 1460.35 – 0.00000 0.93686 1459.50 –0.02789 0.93876 1465.82 y69.99 0.01322 0.93768 1461.98 y16.580.04946 0.94033 1468.44 y46.74 0.03247 0.93871 1464.42 8.270.07420 0.94211 1470.27 y25.64 0.05727 0.93987 1466.69 30.960.09897 0.94388 1471.07 y7.81 0.07930 0.94076 1468.02 47.100.11155 0.94478 1471.13 0.31 0.10131 0.94155 1468.80 60.87

0.12773 0.94240 1469.03 75.68

Pen NBr4 Hex NBr4

0.00000 0.93692 1461.09 – 0.00000 0.93654 1459.50 –0.01887 0.93813 1464.26 22.55 0.01184 0.93712 1461.62 57.650.05031 0.94003 1468.05 43.73 0.03154 0.93802 1464.32 78.280.08180 0.94184 1470.82 58.60 0.05126 0.93888 1466.43 92.900.11010 0.94340 1472.45 70.63 0.07096 0.93971 1467.86 104.920.15100 0.94556 1474.09 84.00 0.09066 0.94050 1469.02 115.330.22015 0.94902 1474.43 103.57 0.11431 0.94143 1469.79 126.43

Hep NBr4 Oct NBr4

0.00000 0.93610 1456.48 – 0.00000 0.93617 1458.54 –0.01930 0.93715 1459.63 93.26 0.01142 0.93677 1460.87 98.930.03858 0.93814 1462.01 109.04 0.03045 0.93770 1463.98 117.860.06003 0.93918 1464.05 122.06 0.04951 0.93858 1466.58 131.300.07930 0.94010 1465.40 133.06 0.06854 0.93943 1468.68 142.300.09860 0.94098 1466.40 142.32 0.08952 0.94032 1470.58 152.780.12001 0.94193 1466.99 152.27 0.11042 0.94118 1472.08 162.04

Table 2Limiting apparent molar isentropic compressibilities(k ), the exper-o

f

imental slopes(S ) and the standard deviations for the electrolytek

solutions inN,N-Dimethylacetamide at 298.15 K

Electrolyte 10 k y10 of 10 S y10

k s

cm mol bar3 y1 y1 cm mol kg bar3 y3y2 1y2 y1

Pr NBr4 y140.37"0.09 421.23 0.29Bu NBr4 y60.33"0.18 380.93 0.34Pen NBr4 y11.07"0.39 244.65 0.29Hex NBr4 25.00"0.12 299.90 0.25Hep NBr4 53.37"0.89 283.66 0.22Oct NBr4 69.00"0.01 280.00 0.23

ume of 25 cm and an internal diameter of the capillary3

of approximately 0.1 cm.The velocities of sound were measured with an

accuracy of 0.3%, using a single-crystal variable pathultrasonic interferometer(Mittal Enterprises, New Delhi,India) operating at 4 MHz. The interferometer wascalibrated with water, methanol and benzene. The tem-perature stability was maintained within"0.01 K bycirculating thermostated water around the measuringcell.

3. Results and discussion

Adiabatic compressibility coefficients were derivedfrom the following equation

2k s1yu r (1)s

wherer is the density andu is the velocity of sound inthe solution. The apparent molar isentropic compressi-bility k of the electrolyte solutions was calculated fromf

o ok s1000(k r yk r)ymrr qM k yr (2)f s o s o s o

wherem is the molal concentration of the solution andthe other symbols have their usual significance.The molalitym, the densityr, the sound velocityu

and the apparent molar isentropic compressibilityk off

the electrolyte solutions at 298.15 K are given in Table1.The limiting apparent molar isentropic compressibili-

ties k were obtainedw12x by extrapolating the linearof

plots ofk vs. the square root of the molal concentrationf

of the solutes to zero concentration by the method ofleast-squares

o yk sk qS m (3)f f k

where S is the experimental slope. Thek and Sok f k

values are listed in Table 2.The limiting apparent molar isentropic compressibili-

17D. Das et al. / Journal of Molecular Liquids 111 (2004) 15–18

Table 3Ionic standard apparent molar isentropic compressibilities inN,N-Dimethylacetamide at 298.15 K

Ion 10 o10 kf" Ion 10 o10 kf"

cm mol bar3 y1 y1 cm mol bar3 y1 y1

Pr Nq4 y140.37 Hep Nq4 53.37

Bu Nq4 y60.33 Oct Nq4 69.00

Pen Nq4 y11.07 Bry 0.00Hex Nq

4 25.00

ties (k ) of the electrolytes investigated here are foundof

to increase in the order:

Pr NBr-Bu NBr-Pen NBr-Hex NBr-Hep NBr4 4 4 4 4

-Oct NBr.4

It is also interesting to note that thek values ofof

Pr NBr, Bu NBr and Pen NBr are negative, while those4 4 4

of the next three higher homologues are found to bepositive.The negativek values of the electrolytes can beo

f

interpreted in terms of loss of compressibility of thesolventN,N-dimethylacetamide due to the presence ofthese solutes in solution. The positivek values, how-o

f

ever, indicates an increase in the compressibility of thesolution compared to the pure solvent.Several factors may contribute to the compressibility

of the solution and we will now examine the importantfactors in order to elucidate the effects of the solutes onthe solvent structure and their role as compressibility.The bromide ion has been shown to remain unsolvated

in N,N-dimethylacetamide from our earlier conductivityand viscosity studyw8,9x. Moreover, it is not a molecularion, thus ruling out the possibility to have intrinsiccompressibility. This ion, therefore, does not influencethe compressibility of the medium.Most of the interpretations ofk in terms of solvationo

f

effects w13–18x assume thatk of unsolvated ions isof

negligible, although Conway and Verrallw18x recognizedthat large organic ions could have some intrinsic com-pressibility due to the intermolecular free space whichmakes the solution more compressible. This could beexpected for the unsolvated tetraalkylammonium ions inour study also.Another effect that is possible for large organic ions

is the penetration of the solvent molecules into theintraionic free space. This is the result of the interactionof the positively charged nitrogen central atom of thetetraalkylammonium ions with the neighbouring solventmolecules. This is essentially an electrostriction effectand causes constriction in the solution volumes, resultingin a more compact and, hence, a less compressiblemedium.In view of the above factors, it can be concluded that

since the bromide ions have no effect on the compress-ibility of the solution, the interplay of the other twofactors actually controls the compressibility of thesolution.The negativek values for Pr NBr, Bu NBr, ando

f 4 4

Pen NBr solutions can be attributed to the predominance4

of the penetration effect over the effect of intrinsiccompressibility of the tetraalkylammonium ions.For Hex NBr, Hep NBr and Oct NBr, however, the4 4 4

effect of intrinsic compressibility of these ions overridesthe penetration effect.

In order to investigate the behavior of the individualions comprising these electrolytes, it is necessary to splitthe limiting apparent molar isentropic compressibilitiesdown into their ionic components. However, there areno reliable methods for the division ofk values intoo

f

ionic contributions. Millerw19x has suggested dividingthe limiting partial molar volumes of Ph As Ph Bq y

4 4

into ionic components, but such a method cannot beused here since we do not know the ratio of thecompressibilities of the pure(solid or gaseous) ions.Also the extrapolation method as suggested by Conwayet al. w20x cannot be used for the division ofk valueso

f

since the variation of the limiting apparent molar isen-tropic compressibilities of these salts with the formulaweight of the tetraalkylammonium ions is not linear.The methodw21x used for acetonitrile, which assumesk (Ph B )s0 is also not appropriate as the Ph B iono y y

f 4 4

is large (rs0.535 nm), and, therefore, its intrinsiccompressibility contribution cannot be taken as zero.Under these circumstances, and also in view of the

compressibility behavior of bromide ion inN,N-dime-thylacetamide as discussed above, the choice ofk (Br )s0 seems to be the best at the moment. Ionico y

f

k values based on this assumption are given in Tableof

3, which seem to fit quite well with the discussion madeabove on the basis of thek values of the electrolyteso

f

as a whole thus substantiating the present protocol forthe division of limiting apparent molar isentropic com-pressibilities of these electrolytes inN,N-dimethyl-acetamide.

4. Conclusion

Apparent molar isentropic compressibilities of sixsymmetrical tetraalkylammonium bromides, namelyPr NBr, Bu NBr, Pen NBr, Hex NBr, Hep NBr and4 4 4 4 4

Oct NBr, inN,N-dimethylacetamide have been obtained4

from ultrasonic velocity measurements at 298.15 K. Thelimiting apparent molar isentropic compressibility valueswere obtained by the extrapolation method and thesewere separated into ionic contributions. The effects ofpenetration of the solvent molecules into the intra-ionicfree space of the tetraalkylammonium ions have beenfound to predominate over the effect of their intrinsiccompressibility for Pr NBr, Bu NBr, and Pen NBr solu-4 4 4

18 D. Das et al. / Journal of Molecular Liquids 111 (2004) 15–18

tions. For Hex NBr, Hep NBr and Oct NBr, however,4 4 4

the effect of intrinsic compressibility of these ionsoverrides the penetration effect. The bromide ion, how-ever, has been found not to influence the compressibilityof the medium.

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