Indian Journal of Chemistry Vol. 43. March 2004, pp. 505-510

Viscosities and electrical conductances of some tetraalkylammonium and common ions in aqueous binary mixtures of N,N-dimethylacetamide at 298.15K

Debashis Das & Dilip K Hazra*

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

Received 18 Mav 2003: revised 9 January 2004

Precise measure ments on e lectrical conductances in the concentration range 0.022-0.044 mol dm-3are reported for

tetraalkylammonium bromides, R4 NBr (R =ethyl to hexyl). sod ium bromide (NaBr) and sodium tetraphenyl borate (NaB Ph) in binary mixture of N.N-dimethylacetamide (DMA) and water at 0.5 mole fraction of DMA at 298.15K. Viscosities of these salts except tetrahexylammonium bromide (Hex4 N Br) have also been measured in the concentration range 0.008-0. 10 mol dm-

3 in this aqueous binary mixture at the same temperature. The conductance data have been analysed by the 1978

Fuoss conductance- concentration equation in terms of the limiting molar conductance (A..). the association constant (KA)

and the association diameter (R). Viscosity data have been analysed by means of Jones-Dole equation and B-coeiTicients evaluated. The limiting ionic conductances and the ionic contributions to the viscosity B-coe fficients have been estimated from the appropriate divi sion of the limiting molar conductivity and the B-cocfficient values using the ' reference electrolyte' method. The results have been interpreted considering the selective solvation. size and structure forming effect of ions and also the movement of ions in a strongly hydrogen bonded solvent system formed between water and the peptido dipole in DMA.

The use of amide + water mixtures has attracted much attention in recent years, as solvents in the study of various physico-chemical properties of electrolytic solution

1-3

• The present work is to study such a system through conductance and viscosity measurements which are very much useful to provide information regarding ion-ion and ion-solvent interactions in this medium4-6. Recently, we have reported the transport properties of some lithium and tetraalkylammonium salts in pure DMA from the measurements of their viscosities and conductances6

-8

. The study has now been extended to binary mixtures with water as cosolvent, in order to understand the nature of ion­solvent interactions and other structural changes that may occur due to the addition of water to DMA. Moreover, N,N-dimethylacetamide is a good model of a peptide bond. So it is of interest to make a study of different tetraalkylammonium and some common salts in aqueous binary mixtures ofDMA to understand how the transport properties of these salts will be effected by the chemical activities of polypeptides in aqueous medium.

Materials and Methods N,N-dimethylacetamide (G R E Merck , Indi a,

>99.5%) was distilled twice in an all-glass distillation set immediately before use and the middle fraction was collected. The purified solvent had a density of 0.9365 g cm-3, a coefficient of viscosity 0.937 mPa-s and a specific conductance of about l.Olx 10-6 S em·' at 298.15 K. These values are in good agreement with the literature values9

• Triply distilled water with a specific conductance of less than lO-u S em_, was used

for the preparation of the mixtures. Freshly distilled solvents were always used, and the solvent mixture was prepared immediately prior to use . Solvent mixture, prepared by weight had a density of 0.9985 g cm-3

, coefficient of viscosity 3.3 18 mPa-s, specific conductance of about 3xl0-6 S cm-

1 and

dielectric constant of 59.92 at 298.15 K. The tetraalkylammonium bromides were of purum

or puriss grade (Fluka) and were purified as described earlier

4"10

• These salts were purified by recrystallization and the higher homologues were recrystallized twice to ensure maximum purity. The recrystallized salts

506 IND IAN J CHEM, SEC A, MARCH 2004

were dried in vacuo at e levated temperatures for J 2 h.

Sodium tetraphenylborate was recrys talli zed from

acetone and dried in vacuo at 253 K for 72 h. Sodium

bromide was ofFiuka's pururn grade and was dried in vacuo for long time immediate ly prior to use and was

used without further purification.

Conductance measurements were carried out on a

Pye-Un icarn PW9509 conductivity me ter at a

frequency of 2000Hz us ing a dip-type cell of cell

constant 1.14 ern·' and having an accuracy of 0.1 %.

The kinemat ic v iscos iti es we re meas ured us ing a

suspended-level Ubbe lohde type viscometer with a

preci s ion of 0.05 % and results were then converted

into abso lute viscosities by mul tiply ing the former with

the dens ity va lues. Measureme nts were made in a

thermostatic wate r bath maintained within spec ified

te mperature as desc ribed earl ier10

• Soluti ons were

prepared by mass for viscosity and conductance run s,

the molali ties being converted to molarities using of

densities measured with an Ostwald-Sprengel type

Pycnometer of about 25 em' capacity. The precision

of density measure me nts was greater than ±5x 10·5g crn-3

. Several independent solu tions were prepared and

the runs were performed to e nsure the reproducibility

of the results. A ll data were corrected wi th the spec ific

conductance of the solvent and the corrected data were

analysed.

Results and Discussion

T he mea sured molar conduc tanc e ( i\) of

elec t ro lyte solutions as a function of mo lar

concentration ( c) at 298. 15 K are g iven in Table I. The conductance data have been analysed by the

1978 Fuoss conductance - concentration eq uation 11

•

For a given set of conductivity values (ci, Ai' j = 1,

. .... , n), three adjustable parameters - the limiting

molar conductivity ( i\ 0), association constant (KA) and

the cosphere d ia mete r (R), a re derived from the

following set of eq uations :

A = p [ i\ o (1 + R.) + E,J ... (l)

p = 1 - a(1 - y) ... (2) 2 2

y=l-KAcyf

-lnf = ~K I 2 (1 + KR)

13 = e 2 I D K[lT

KA = K/(l- a)= KR ( 1 + K s)

... (3)

... (4)

... (5)

... (6)

Table 1- Molar conductances and corresponding molarities of e lectro lytes in aqueous binary mi xture of N,N-dimethyl acetamide at 298. 15K

IO'c i\ IO' c /\. (mol dm.') ( S cm2mol" 1

) (mol dm.') (S cm~mo l"' )

Et,NBr Pr,NBr

442.48 24.25 435.5 1 21 .57 411 .96 24.60 405.47 2 1. 83 38 1.44 24.98 375.44 22.09 350.93 25.39 345.40 22.33 320.41 25 .78 3 15.37 22.62 290.00 26.2 1 285.34 22.92 259.40 26.67 255.30 23 .23 228.87 27. 14 225.26 23.54

Bu) 1Br Pc n, NBr

435.90 2 1.1 4 434.23 20.47 405.84 2 1.33 404.29 20.64 375 .78 2 1.5.'i 374.34 20.85 345.72 2 1.77 344.39 2 1.05 3 15 .65 21.98 3 14.44 2 1.28 285.59 22. 13 284.50 2 1.50 255.53 22.42 254.55 2 1.77 225.47 22.68 224.60 22.01

Hex,NBr NaBr

436.55 19.57 436.72 3 1.41 406.44 19.72 406.60 3 1.59 376.33 19.89 376.48 3 1.78 346.22 20.08 346.36 3 1.95 31 6.12 20.24 316.24 32. 14 286.0 1 20.43 286.12 32.33 255.90 20.62 256.01 32.55 225.80 20.72 225.89 32.78

Na[B(Ph)4 ]

436.50 20.00 406.40 20.10 376.29 20.20 346. 19 20.29 3 16.09 20.40 285.98 20.50 255.88 20.62 225 .78 20.75

where the terms have the ir usual sign ificance. For

computation, the initial J\0

values were obta ined from

Shed lovsky extrapolation 12

of the data. In practice,

ca lculations are made by finding the va lues of /\.0

and

cr which minimize the standard deviation, cr,

cr2 = L: [ i\i (calc.)- J\0i (obs.)]

2 I (n-2) .. . (7)

for a sequence of R values and then plotting cr against

R, the best-fit R corresponds to the minimum in cr vs

R curve. However, s ince a roug h scan us ing unit

DAS el a /.: VISCOS ITI ES AND ELECTR ICAL CONDUCTANCES OF TETRAALKYLA MMON IUM IONS 507

Table 2- Measured conductivity parameters of electrol ytes in aqueous binary mixture of N,N-dimethylacetamide at 298.15K

Salt A .. K" R (J

(S cm2rnor') (dm3rnor' ) ("A) (%)

Et4 NB r 34.74± 0.15 18.12± 0.41 5.80 0.09

Pr4 NBr 27.34±0.03 9.3 1±0.08 18.40 0.04

Bu4 NBr 25.54±0.08 6.86±0.20 16.00 0. 10

Pen,NBr 25.43±0.03 8.50±0.10 8.00 0.04

Hcx4 NBr 23.40±0. 11 6. 19±0.30 7.40 0.16

NaBr 35.76±0.02 3.88±0.04 8.00 0.02

Na[B(Phl.J 25.52±0.0 1 3. 12±0.03 6.00 0.02

increment ofR values from 4 to 20 gave no significant

minima in the cr(%) vs R curves, the R va lue was

assumed to be R = a + d, where a is the sum of the

crystallographic radii and d is given by 11

:

... (8)

where M is the molecular weight of the sol vent and Po its density.

The values of A0 , KA and R obtai ned by thi s

procedure have been recorded in Table 2.

In the absence of accurate transference data in the

mixed so lve nt , we have to use the 'refe re nc e

elec tro lyte' method for the division of A o into single­

ion val ues. The Bu4NBP h4 ha s been used as the ' reference e lectrolyte', and the A 0- va lue has been

de ri ved from th~ relation :

0 0 0 0 (9) A Bu4NBPh4 = A Bu4NBr +A NaBPiq - A NaBr4 · • ·

To obtain single ion va lues we have div ided A o

val ues of Bu4NBPh4 using the method similar to that

proposed by Krumgalz13

:

A0Bu

4N+ frb

4a· _ 5.35 = 1.07 .. . ( 10)

A0 Ph

4B· fsu

4N+ 5.00

The r-values have been taken from the works of Gill and Sekhri 14• The limiting ionic conductances based

on the above relationship are presented in Table 3.

The density (p ) and relative viscosity (ll J data for the e lectrolytes in the molarity range 0.008- 0.10 mol dm·3 at 298.15 K are given in Table 4.

The relative viscosity (Y] ,) data of the e lec trol ytes

in solution have been analysed with the Jones-Dole . " equatton · :

Table 3- Limiting ion ic conductances (Ao± ), Wa lden products (/.."± 11.,) and Ionic Stokes ' Radii (r,) in aq ueous binary mixtu re of N,N-dimethylacetamide at 298.15K

Ion /..." ± /..."± 1l, (S cm2rnor' ) (S cm'mor'P., 5) c i •

Et4 W 17.11 0.056 1.44

Pr4N' 9.71 0.032 2.55

Bu.N' 7.91 0.026 3.1 3

Pen4N• 7.80 0.025 3.17

Hex4N• 5.76 0.019 4.30

Na' 18.13 0.060 1.36

Br" 17.63 0.058 1.40

l B(Ph).r 7.39 0.024 3.35

"' Y],= 1 + Ae - + Be ... (11)

where A and 8 are the characteristic parameters fo r

salt and solvent, depend on ion-ion and ion-sol vent

inte ractions respective ly.

Plots of (11,- 1) l Ye against .Yc for the e lectrolytes

have been found to be linear and the experimental A

va lues have been calculated usi ng the least squares

method . The A coefficients have also been calculated

at 298 .1 5 K from the physical propert ies of the solvent using Falkenhagen-Vernon equation

16:

A o o 0.2577 ° ~ [ ;... +_;... -]2]

A,hco=----- 1-0.6863 . .. (12) 11'1 + 1 -

llo( ET) -!l.o !l.o Ao

The condu c tanc e data req ui red 111 the se

calcu lations have been taken fro m the present work . A coefficients (A,!,eo) thus calculated fro m Eq. ( 12) are

recorded in Table 5. These A,11co va lues have been used for the a nal ys is of v iscos ity data. Viscosity 8-

coefficients calcu lated by using the leas t squares

method are also presented in Table 5. Viscosity 8-

coeffic ient values of the electrolytes have been di vided

into ionic components using similar method described above in Eqs (9) and (10) for di vision of the limiting

molar conductances. The ionic B-coefficients obtained

from these re lationships are given in Table 6. Ionic B-values have also been ana lysed

4 on the

basis of Ei nste in 's eq uation and the corresponding ionic radii evaluated :

R3 N ±

B± = 2.5 (4/3 ) 11 ... (13) 1000

The number ns of solvent molecules bound to the

508 INDIAN J C HEM, SEC A, MARCH 2004

Table 4- Concentration (c), density (p) and re lative viscosity (ll,) of e lectrolytes in aqueous binary mixture at 298.15K

c p T],

(mol dm.3) (g cnf-')

EL4 NBr

0.10186 1.0032 1.040

0.07276 1.0021 1.028

0.05238 1.0012 1.02 I

0.03201 1.0003 1.0 13

0.02037 0.9998 1.008

0.00873 0.9992 1.004

Bu.NBr

0.10470 1.0016 1.074

0.07479 1.0009 1.054

0.05385 1.0003 1.039

0.03290 0.9997 1.024

0.02094 0.9993 1.016

0.00897 0.9989 1.007

NaBr

0.10954 1.0065 1.032

0.07824 1.0044 1.023

0.05633 1.0028 1.017

0.03442 1.0012 1.0 10

0.02191 1.0002 1.007

0.00939 0.9991 1.003

ion in the primary shell of solvation can be calculated by combining the Jones-Dole equation with that of Einstein:

. . . (14)

where vi represents the bare ion molar vo lu me and is related to the crystallographic radius (Rc) of the ion. V, is the solvent molar volume. Table 6 deals with the crystallographic radius (Rc), experimental ionic radii (R±) and solvation number (n,) of ions at 298.15K.

Table 2 shows that the limiting molar conductivities (A0

) of tetraalkylammonium bromides decrease with increase in length of the alkyl chain and the difference becomes very small in cases of higher homologues. This is in agreement with our earlier

c p T],

(mol dm.3) (g cm.3)

Pr4 NBr

0.10233 1.0027 1.061

0.07309 1.0017 1.044

0.05262 1.0010 1.032

0.032 16 1.0002 1.020

0.02046 0.9997 1.013

0.00877 0.9992 1.006

Pen4 Nl3r

0.10235 0.99942 1.083

0.07310 0.99929 1.060

0.05264 0.99914 1.044

0.03217 0.99893 1.027

0.02047 0.99877 1.017

0.00877 0.99858 1.008

Na[B(Ph>.J

0.10386 1.0022 1.105

0.07418 1.0013 1.073

0.05341 1.0006 1.054

0.03264 0.9998 1.034

0.02077 0.9994 1.022

0.00890 0.9989 1.011

Table 5- Theoretical and experimental A-coefficients and vis­

cosity B-coefficients for the electrolytes in aqueous binary

mixture of N,N-dimethylacetamide at 298.15K

Salt (J

0.0068 0.0062 0.366 0.03

Pr,NBr 0.0086 0.0132 0.558 0.04

Bu.NBr 0.0096 0.0088 0.683 0.05

Pen4 NBr 0.0099 0.0082 0.790 0.06

NaBr 0.0064 0.0052 0.279 0.01

Na[B(Ph)4] 0.0110 0.0113 0.964 0.08

DAS et al.: VISCOSITIES AND ELECTRICAL CONDUCTANCES OF TETRAALKYLAMMONIUM IONS 509

Table 6- Ionic B-values. theoretical and experimental ionic radii

(R±) and solvation numbers (n) of ions in aqueous binary mixture

of N,N-dimethylacetamide at 298.15K

Ions B± Rc R± n,

(dm3 mor') ("A) ("A)

Et,N' 0.298 4.00 3.60 -0.358

Pr4 N' 0.490 4.52 4.20 -0.345

Bu,N' 0.615 4.94 4 .60 -0.236

Pen4 N' 0.722 5.29 4.80 -0.303

Na' 0.211 1.17 3.20 0.968

Br" 0.068 1.80 2.20 0.465

Ph,B. 0.753 4.80 4.90 -0.075

findings in pure DMA7• In order to investigate the

specific behaviour of the individual ions comprising these electrolytes, it is necessary to split the limiting molar electrolyte conductances into their ionic components. It is seen from Table 3 that for the tetraalkylammonium ions, the A0

values decrease in the following order Et4N+ > Pr4~ > Bu4N+ > Pen4N+ > Hex4N+ i.e. , the limiting ionic conductivity values

decrease with increase in size of these cations. The trend was similar with the observation made by

Ramanamurti et a/ 11 in this aqueous binary system at

higher temperature and can be attributed to the size and structure forming effect of cations (anion Br· being the same). Size and structure forming effects (hydrophobic solvation) 18

-21 of cations increase in the

order Et4N+ < Pr

4N+ < Bu

4N+ < Pen

4N+ < Hex

4N+, so

the mobility should be in the reverse order and this explains the order observed for limiting equivalent conductance. In cases of anions the A0

values decrease with increase in size of the ions i.e. Br- > [ B(Ph)4 r. Table 2 clearly indicates that in almost all cases except tetraethylammonium bromide, association constants are less than 10, so little emphasis should be given to numerical values of KA. This implies that a preponderant portion of each salt remains dissociated in this solvent medium. However, in solvent mixtures having dielectric constants less than 60, the value of KA seems to decrease with increase in chain length of tetraalkyl-ammonium bromide salts

17• This is expected

because the charge density of tetraalkylammonium ions decrease in the same order.

The Walden products (A0±ll

0) of the ions are also

included in Table 3. Walden products are usually

employed to discuss the interactions of the ions with the solvent medium. From Table 3, we see that for the tetraalkyl-ammonium ions, the value of the Walden

product decreases from tetraethylammonium ion to tetrahexylammonium ion. This points out that the

electrostatic ion-solvent interaction is very weak in these cases. Such type of behaviour is also found in pure DMA as reported in our previous paper

7. In

comparison to the Walden product values of these electrolytes in pure DMA it is observed that except tetraethylammonium ion the other tetraalk y l­ammonium ion maintains nearly same value in this present aqueous binary mixture. The phenomenon can

be explained by the hydrophobic dehydration of cations by the cosolvent (DMA). Further, the hydrophobic

dehydration effect will be more in the case of an ion which is more hydrophobic in nature. This hydrophobic character of cations varies in the order Hex

4N+ > Pen4~ > Bu

4N+ > Pr4N+ > Et4N+. Due to the

greater hydrophobic dehydration effect of higher homologues of tetraalkylammonium cation the ability to promote the structure of water becomes reduced and as a result of it the Walden product remains the same

value. The Stokes radii (Table 3) of the tetraalkyl­ammonium cations are, however, found to be either

close to or smaller than their corresponding crystallographic radii . This observation indicates that these ions are scarcely solvated in this medium.

In addition to conductance measurements the result of the viscosity measurements of different tetraalkylammonium salts from Etl•mr to Pen4NBr and some common salts except Hex4NBr (solubility too low to perform viscosity experiments) are also

reported. The viscosity B-coefficients which represents

solute - solvent interactions shown in Table 5 are positive for all the electrolytes studied. The B-values for the tetraalkyl- ammonium bromides increase regularly as we go from tetraethylammonium bromide to tetrapentylammonium bromide. The same kind of behaviour has also been found for these electrolytes in ethyl methyl ketone and ethyl methyl ketone + dimethyl formamide mixtures22

•

In order to make further investigation for the specific behaviour of the individual ions the viscosity B-coefficients values of electrolyte resolved into ionic

510 INDIAN J CHEM, SEC A, MARCH 2004

B± coefficients by using "reference electrolyte" method

and are given in Table 6. This table shows that the

viscosity B-coefficients for all cations and anions are

positive and are also large with the exception of

bromide ion . The observed order may be attributed to

greater e lectrostatic ion-solvent interaction a nd

resistance to the movement of ion due to large size of

cation. In case of Br- ion the very low viscosity B­

coefficient value indicates that the viscos ity of the

solvent mixture is very little modified by its presence

due to the poor solvation of the ion and this is al so

supported by the conductance data . The other anion,

Ph4B·, however, is fo und to be very e fficient in

modifyi ng the solvent viscosity as manifested by its

large and positive viscosity B val ue. The A-coefficients

reported in Table 5 are small and positive for all the

electrolytes indicating the presence of weak ioni c

interactions. The A -values, calc ulated theoretically

from the physical parameters of the solvent and the

limiting ionic equiva lent conductances usin g

Falkenhagen and Vernon eq uat ion given in Table 5,

do not differ very much with the experimental A-va lue.

The values of crystallographic radi us (RJ, ionic radii

(R±) and solvation number (n,) shown in Table 6 can

provide us important in fo rmation regarding the

solvation of some tetraalkylammonium, Na+, Br- and

Ph4B- ions in this present aqueous binary mixture. With

the except ion ofNa+, Br- and Ph4B- ions, the R± values

for the other ions are always fo und to be less than Rc values of the ions indicrting th at they are not so

solvated in thi s mixture. Although the negative values

of solvation number have no physical significance, it

seems to an indication of the poor solvation of these ions. For Na+ and Br- ions, the greater value of ionic

radii (R±) than crystallographic rad ii (RJ and positive

solvation number clearly demonstrate that these ions

are significant!'; solvated in this solvent medium. The

results obtained from viscosity measureme nts are

found to be in good agreement with the conductometric

investigation.

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