Viscosities and electrical conductances of some tetraalkylammonium and common ions in aqueous binary mixtures of N,N-dimethylacetamide at 298.

Transcription

1 Indian Journal of Chemistry Vol. 43. March 2004, pp Viscosities and electrical conductances of some tetraalkylammonium and common ions in aqueous binary mixtures of N,N-dimethylacetamide at K Debashis Das & Dilip K Hazra* Department of Chemistry. North Bengal University. Darjeeling India Received 18 Mav 2003: revised 9 January 2004 Precise measure ments on electrical conductances in the concentration range mol dm- 3 are reported for tetraalkylammonium bromides, R 4 NBr (R =ethyl to hexyl). sodium 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 K. Viscosities of these salts except tetrahexylammonium bromide (Hex 4 N Br) have also been measured in the concentration range 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-coefficients have been estimated from the appropriate division 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 conductances 6-8. 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, India, >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 g cm- 3, a coefficient of viscosity mpa-s and a specific conductance of about l.olx 10-6 S em ' at K. These values are in good agreement with the literature values 9 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 g cm- 3, coefficient of viscosity mpa-s, specific conductance of about 3xl0-6 S cm- 1 and dielectric constant of at 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

2 506 IND IAN J CHEM, SEC A, MARCH 2004 were dried in vacuo at elevated temperatures for J 2 h. Sodium tetraphenylborate was recrystallized 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 immediately 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 using a dip-type cell of cell constant 1.14 ern ' and having an accuracy of 0.1 %. The kinematic viscositi es we re measured using a sus pended-level Ubbelohde type viscometer with a preci sion of 0.05% and results were then converted into absolute viscosities by mul tipl ying the former with the density values. Measureme nts were made in a thermostatic wate r bath maintained within specified te mperature as described earl ier 10 Solutions were prepared by mass for viscosity and conductance run s, the molalities 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 ti ons were prepared and the runs were performed to e nsure the reproducibility of the results. A ll data were corrected with the specific conductance of the solvent and the corrected data were analysed. Results and Discussion T he measured molar conductance ( i\) of electrolyte solutions as a function of mo lar concentration ( c) at K are given in Table I. The conductance data have been analysed by the 1978 Fuoss conductance - concentration equation 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 meter (R), a re derived from the following set of equations : 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 lectrolytes in aqueous binary mi xture of N,N-dimethyl acetamide at K IO'c i\ IO' c /\. (mol dm.') ( S cm 2 mol" 1 ) (mol dm.') (S c m ~m o l"' ) Et, NBr Pr, NBr Bu) 1 Br Pc n, NBr 'i Hex,NBr NaBr Na[B(Ph) 4 ] where the terms have the ir usual significance. For computation, the initial J\ 0 values were obtained from Shedlovsky extrapolation 12 of the data. In practice, calculations are made by finding the values of /\. 0 and cr which minimize the standard deviation, cr, cr 2 = L: [ i\i (calc.)- J\ 0 i (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, since a roug h scan using unit

3 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 K Salt A.. K" R (J (S cm 2 rnor') (dm 3 rnor') ("A) (%) Et 4 NB r 34.74± ± NBr 27.34± ± Bu 4 NBr 25.54± ± Pen,NBr 25.43± ± Hcx 4 NBr 23.40± ± NaBr 35.76± ± Na[B(Phl.J 25.52± ± increment ofr values from 4 to 20 gave no significant minima in the cr(%) vs R curves, the R value 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 A 0, KA and R obtained by thi s procedure have been recorded in Table 2. In the absence of accurate transference data in the mixed solvent, we have to use the 'refere nce electrolyte' method for the division of A o into singleion val ues. The Bu 4 NBPh 4 has been used as the 'reference e lectrolyte', and the A 0 - value has been deri ved from th~ relation : (9) A Bu 4 NB Ph 4 = A Bu 4 NBr +A NaBPiq - A NaBr 4 To obtain si ngle ion va lues we have div ided A o val ues of Bu 4 NBPh 4 using the method similar to that proposed by Krumgalz 13 : A 0 Bu 4 N+ frb 4 a _ 5.35 A 0 Ph 4 B fsu 4 N = ( 10) 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 electrolytes in the molarity range mol dm 3 at K are given in Table 4. The relative viscosity (Y],) data of the electrol ytes in solution have been analysed with the Jones-Dole equatton. " : Table 3- Lim iting ion ic conductances (Ao± ), Wa lden products (/.."± 11.,) and Ionic Stokes' Radii (r,) in aq ueous binary mixtu re of N,N-dimethylacetamide at K Ion /..." ± /..."± 1l, (S cm 2 rnor') (S cm'mor'p., ) 5 c i Et 4 W N' Bu.N' N Hex 4 N Na' Br" l B(Ph).r Y],= 1 + Ae "' - + Be... (11) where A and 8 are the characteristic parameters fo r salt and solvent, depend on ion-ion and ion-solvent interactions respectively. Plots of (11,- 1) l Ye against.yc for the electrolytes have been found to be linear and the experimental A values have been calculated usi ng the least squares method. The A coefficients have also been calcul ated at K from the physical properties of the solvent using Falkenhagen-Vernon equation 16 : A ~ [ ;... o +_;...-] 2 o ] A,hco= (12) 11' llo( ET) -!l.o!l.o Ao The conductance data req ui red 111 these calculations have been taken from the present work. A coefficients (A,!,eo) thus calcul ated fro m Eq. ( 12) are recorded in Table 5. These A, 11 co va lues have been used for the a nalysis of viscosity data. Viscosity 8- coefficients calculated by using the least squares method are also presented in Table 5. Viscosity 8- coefficient 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 relationships are given in Table 6. Ionic B-values have also been analysed 4 on the basis of Einstein's equation and the corresponding ionic radii evaluated : R 3 ± N B± = 2.5 (4/3) (13) 1000 The number ns of solvent molecules bound to the

4 508 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 K c p T], (mol dm. 3 ) (g cnf-') EL 4 NBr I Bu.NBr NaBr c p T], (mol dm. 3 ) (g cm. 3 ) NBr Nl3r Na[B(Ph>.J 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 volu 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 K. Table 2 shows that the limiting molar conductivities (A 0 ) 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 Table 5- Theoretical and experimental A-coefficients and viscosity B-coefficients for the electrolytes in aqueous binary mixture of N,N-dimethylacetamide at K Salt (J Pr,NBr Bu.NBr NBr NaBr Na[B(Ph) 4 ]

5 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 K Ions B± Rc R± n, (dm 3 mor') ("A) ("A) Et,N' N' Bu,N' N' Na' Br" Ph,B findings in pure DMA 7 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 A 0 values decrease in the following order Et 4 N+ > Pr4 ~ > Bu N+ > Pen N+ > 4 4 Hex 4 N+ 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) of cations increase in the order Et 4 N+ < N+ < Bu 4 N+ < N+ < Hex 4 N+, so the mobility should be in the reverse order and this explains the order observed for limiting equivalent conductance. In cases of anions the A 0 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 (A 0 ±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 tetraalkyl ammonium 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 4 N+ > ~ > Bu N+ > Pr N+ > Et N+. 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 NBr and some common salts except Hex 4 NBr (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 mixtures 22 In order to make further investigation for the specific behaviour of the individual ions the viscosity B-coefficients values of electrolyte resolved into ionic

6 510 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 viscosity 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, Ph 4 B, however, is fo und to be very efficient 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 ionic interactions. The A -values, calculated theoretically from the physical parameters of the solvent and the limiting ionic equivalent conductances using Falkenhagen and Vernon equation given in Table 5, do not differ very much with the experimental A-value. The values of crystallographic radi us (RJ, ionic radii (R±) and solvation number (n,) shown in Table 6 can provide us important information regarding the solvation of some tetraalkylammonium, Na+, Br- and Ph 4 B- ions in this present aqueous binary mixture. With the exception ofna+, Br- and Ph 4 B- ions, the R± values for the other ions are always found 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. References I. Visser C de & Somsen G,.I Phys Chem, 78 ( 1974) Singh D, Singh N P & Bahadur Lal, Indian.I Chem. 13A ( 1975) McDowall J M, Martinus N & Vincent C A..I Chem Soc, Faraday Trans, I, 72 ( 1976) Nandi D, Das S & Hazra D K..I Chem Soc, Faraday Trans, I. 85 ( 1989) 153 I. 5. Das B & Hazra D K, Bull Chem 3oc.Iapan, 65 ( 1992) Das D, Das B & Hazra D K..I Soln Chem, 3 1 (2002) Das D, Das B & 1-lazra D K,.I Soln Chem, 32 (2003) Das D, Das B & Hazra D K,.I Soln Chem, 32 (2003) Physical Chemistry of Organic Solvent Systems. edited by A K Covington & T Dickinson (Plenum Press. New York) Chap. I. I 0. Dasgupta D, Das S & Hazra D K..I Chem Soc, Faraday Trans, I. 84 ( 1988) I I. Fuoss R M,.I Phys Che ( 1978) Fuoss R M & Shedlovsky T,.I 1\m Chem Soc. 71 ( 1949) Krumgalz B S,.I Ch('lll Soc, Faraday Trans, I. ( 1980) Gill D S & Sekhri 1\1 B,.I Chem Soc, Faraday Tran s. I, ( 1982) Jones G & Dole M,.I 1\m Chem Soc, 51 ( 1929) Falkcnhagcn H & Vernon E L. Phys Z, 33 (1 932) Bahadur L & Ramanamurti M V..I Chem Soc, Faraday Trans I, 76 ( 1980) Evans D F & Kay R L,.I Phys Chem, 70 ( 1966) Aprano A D & Fuoss R M..I Phys Chem, 67 ( 1963) Desnoyers J E & Perron G, J Soln Chem, I ( 1972) Kay R L, Vituccio T, Zawoysky C & Evans D F..I Phys Chem, 70 ( 1966) Patial B S, Chauhan S, C hauhan V S & Syal V K. Indian.I Chem. 41A (2002) 2039.