3390 R. GOPAL AND 11. A. SIDDIQI

A Study of Ion-Solvent Interaction of Some Tetraalkylammonium and

Common Ions in N-Methylacetamide from Apparent Molal Volume Data

by Ram GopaP and Mohammed Adam SiddiqiZb Department of Chemistry, Lucknow University, Lucknow, India (Received November 18, 1068)

The apparent molal volumes, $v - s, of six tetraalkylammonium iodides, namely, Et4NI to HepdhrI, have been determined in N-methylacetamide (NMA), at different temperatures and concentrations from which the limit- ing apparent molal volumes, &' - s, have been obtained for these salts. The results indicate that the +v vs. .\/I. curves are almost straight lines for all the salts in the concentration range studied here. The curves for Et4NI and Pr4NI salts have a positive slope, and those for BudNI, Pen&I, Hex" and Hep4NI have a negative slope. Also qh0 of the RdiS iodides increases with the rise in temperature; however, d4"O/dt decreases with the rise in temperature for the smaller R4N iodides, while it remains almost temperature independent for the larger R4N iodides. Common electrolytes behave more or less like the smaller R4N iodides although d&O/dt even becomes negative in these cases. An attempt has been made to explain these results from the point of view of electrostatic solvation and electrostriction in the case of smaller R4N+ ions and from the penetration and the relative volume effects for the larger RkN+ ions.

Introduction A study of the variation of the apparent molar vol-

umes of some tetraalkylammonium iodides with tem- perature and concentration in aqueous solution has been reported in a previous communication.* The results obtained indicate that the limiting apparent molar volume, &", of ;Ile4NI, Et4NI, Pr4NI, and Bu41\TI salts increases with the rise in temperature; also the larger the R&+ ion, the larger the d$,"/dt and i t ap- pears to increase with the rise in temperature. These observations were explained on the well-known hy- pothesis of Frankj43j according to which the R4Y+ ions are, in general, hydrophobic structure promoters in water. To have a more general idea about the be- havior of the R4N+ ions and to test the applicability of the Frank hypothesis to solvents other than water, i t is desirable to extend the work to other solvents which, like water, have a high dielectric constant so that com- plete dissociation of electrolytes is assured. Water is known to have regular tetrahedral structural features, and in order to assess the structural influence on the behavior of the R4N+ ions, the solvent should be such that no such regular three-dimensional structural fea- ture is involved so that its effect can be eliminated. It may be mentioned that the icelike nature of water has been claimed to be responsible for some of the properties of the aqueous solution^^-'^ of the ions in general and of the R4N+ ions in particular. One of the solvents which fit in these requirements is N-methylacetamide (XhlA) which has a very high dielectric constant (e360 = 171.711) and is a fairly good solvent for elec- trolytes.12 It has no structural features except associa- tion through hydrogen bonding.13 It may also be noted that the RqX+ ions in NJIA behave, in some respects, like common univalent cations (e.g., alkali

The Journal of Physical Chemistry

metal ions) in water,14 quite in contrast to their behavior in aqueous solutions in which the alkyl groups of the R4N+ ions exhibit a "hydrophobic" charactera4v5 It would, therefore, be interesting to investigate the apparent molal volumes of the salts containing R4N+ ions in NMA. The present communication reports a study of the solute-solvent interaction of some R4IL'I salts, as well as of some common electrolytes like KI and LiCl for the sake of comparison, in NMA from the point of view of the variation of apparent molal volume with temperature and concentration. It may be men-

(1) Work supported by the Council of Scientific and Industrial Re- search (CSIR), India. (2)(a) To whom requests for reprints should be addressed at the De- partment of Chemistry, University of North Carolina, Chapel Hili, N. C. (b) Junior Research Fellow, CSIR, India. (3) R. Gopal and M. A. Siddiqui, J. Phys. Chem., 72, 1814 (1968). The authors regret the use of "partial" molar volume (ij) for apparent molar volume (9") in the communication. (4) H. S. Frank and M. W. Evans, J . Chem. Phys., 13, 507 (1945). (5) H. S. Frank and W. Y . Wen, Discussions Faraday SOC., 24, 133 (1957) I (6) Th. Aokerman and F. Schreiner, 2. Elektrochem., 62, 1143 (1958). (7) W. Y. Wen, Ph.D. Thesis, University of Pittsburgh, 1957. (8) J. D. Bernal and R. H. Fowler, J . Chem. Phys., 1, 515 (1933). (9) K. Fajans and 0. Johnson, J. Amer. Chem. SOC., 64, 668 (1942). (10) B. E. Conway and R. E. Verrall, J. Phys. Chem., 70, 3952 (1966). (11) 6. J. Bass, W. I. Nathan, R. M. Meighan, and R. H. Cole, ibid., 68, 509 (1964). (12) R. Gopal and D. Chandra, J . Indian Chem. SOC., 45, 351 (1968). (13) R. Lin and W. Dannhauser, J . Phys. Chem., 67, 1805 (1963). (14) R. Gopal and 0. N. Bhatnagar, J . Indian Chem. Soc., 44, 1082 (1967). (15) A. J. Ellis, J . Chem. Soc., A , , 1579 (1966). (16) F. J. Millero and W. D. Hansen, J . Phys. Chem., 72, 1758 (1968).

IONSOLVENT INTERACTION OF SOME TETRAALKYLAMMONIUM AND COMMON IONS 3391

4&\ 33 z

Figure 1. Bu4NI in NMA at various temperatures.

Apparent molal volumes of Et4NI, Pr4NI, and

tioned that this property has proved to be a simple and convenient tool for studying solute-solvent interac- tionSr15r16 in solutions.

Experimental Section The R4NI salts, obtained from D.P.I., were puri-

fied in the manner described in the literature." N-methylacetamide (NMA), also from D.P.I., was kept overnight on freshly ignited quicklime and re- crystallized twice. It was then vacuum distilled. The process of purification was repeated until the electrical conductance of the sample was reduced to mho or less. Solutions were prepared on a molal basis. The density of solutions was determined with a dilatom- eter of about 30-ml capacity, the stem of which was graduated in 0.01 ml (accuracy "A" from VEB Glas- werke, Stutzerbach, Thiiringen, Germany). The vol- ume changes on the stem could be read to 0.002 ml within an error not exceeding kO.001 ml with the help of a magnifying glass. The temperature of the thermo- stat was regulated to within f0.02" in the lower tem- perature range and T'0.05" in the higher range. The dilatometer was calibrated with conductivity water,

' " I I

539.4 __._ , i

39 I 3s" 341.1 - - - -

3 8 9 3 4 'L

''J;;J '5 Figure 2. and Hep4NI in NMA.

Apparent molal volumes of Pen4NI, HexrNI,

and the accuracy of measurements was checked against some test solutions of known accurate density. The results agree within *0.002 to =t0.004% in the 25-40" temperature range. The apparent molal volume data agreed within f 0.1 to =t 0.4%. From the density data thus obtained, the apparent molal volume, c $ ~ , was ob- tained from the relation

(1) 1000(do - d ) M

mddo +d 9 Y =

The terms used in (1) have their usual significance. From the Cby data, rby vs, 4; curves have been obtained for different salts a t different temperatures and are given in Figures 1-3. It may be noted that these plots are almost straight lines and so the Masson equation, namely

should be applicable within the temperature and con- centration ranges studied here. The slope S, appears to be positive for Et4N1, Pr,NI, KI, and LiCI, but i t is

(17) B. E. Conway, R. E. Verrall, and J. E. Desnoyers, Trans. Para- dag ~ o c . , 62,2738 (1966).

Volume 78, Number 10 October 1909

3392 R. GOPAL AND M. A. SIDDIQI

Table I : Limiting Apparent Molal Volumes of Some Salts a t Different Temperatures

I----- Limiting apparent molal volume, +vD, ml/mol at------- 7

Salt0 36' 40° 4 6 O 60° 60° 70' 800

EtrNI PrdNI

Pen4NI Hex4NI Hep4NI KI LiCl

B u ~ N I

175.4 247.5 322.9 391.1 461.2 527.3 45 .9 20 .5

177.1 249.2 324.6 393.4 463.1 530.0 46 .2 20.2

178.2 251.0 325.8 395.6 464.7 532.4 46 .5 20 .0

Me4NI salt could not be studied owing to its very low solubility in NMA.

179.9 252.5 327.0 397.8 466.4 535.4 46 .7 19.7

181.9 254.8 329.0 402.1 468.7 539.4 4 6 . 4 19.2

182.8 256.0 330.6 406.2 471.1 543.5 4 5 . 4 18.4

183.7 256.9 331.8 409.9 473.4 547.6 4 4 . 8 17.8

I V / K r

Figure 3. Apparent molal volumes of LiCl and K I in NMA at various temperatures.

negative for Bu4N1, Pen4NI, Hex4NI, and Hep4NI. From the usual extrapolation of the I+v us. 4 curves to infinite dilution, the limiting apparent molal volume,

(or Fo), of various salts a t different temperatures have been obtained and are given in Table I.

Results and Discussion From Table I i t appears that the &" values of the

smaller R4NI salts in YMA are somewhat smaller than the corresponding values in water3 (e.g., C$~"(ECNI) = 187.4 and & O p r 4 ~ ~ ) = 254.0 at 40"). This appears to indicate a stronger R4N+-NMA i n t e r a c t i ~ n ~ s j ~ ~ leading to a larger electrostriction in NMA as compared to that in water; however, due to the possibility of different types of ion-solvent interactions in the two solvents (e.g., hydrophobic in water), the deduction may be questionable. In NMA the positive slope in cpv us. 4; curves for KI, LiC1, EtdNI, etc., is similar to that for common electrolytes and Me4NI in water, while the negative slope for Bu4NI, Pen4N1, Hex4NI,

and Hep4NI salts is similar to that for PrdNI and Bu4NI in aqueous solutions. The concentrations used in the present investigation are comparatively high and so the limiting slope of the curves in Figures 1-3 cannot be reasonably expected to provide a basis for testing the applicability of the Debye-Huckel limiting law. 17,20--2z

Also it is not possible to calculate, theoretically, the limiting slope since the calculationz2 would require the pressure dependence of the dielectric constant and the compressibility of the solvent, both of which are not available. Further, the slope is positive in some and negative in others in the concentration range studied here. It appears, therefore, advisable not to attach too much quantitative significance to the 8, values and make deductions therefrom about the ion-solvent interaction in KMA. Only the dependence of apparent molal volume on temperature and concentration should be used to throw light on the mode of ion-solvent inter- action in such cases.

It has been mentioned earlier that in aqueous solu- tions only Me4N13,23 exhibits a positive slope in the (bv usa 4 curve while the other R4NI salts, containing the larger R4N+ ions, exhibit a negative lope.^^^^ It is also well known that the positive slope is characterk- tic of the common electrolytes in aqueous solutions and that this behavior of common ions has been as- sumed to be caused by the depolymerization (structure breaking) and electrostriction effects of the ions on water medium. Now referring to solutions in NMA, a study of the conductance and viscosity of the solu- tions of these salts in NMA appears' to show that these (specially those containing the smaller R4Xf ions)

(18) R. Gopal, 0. N. Bhatnagar, and M. M. Husain, J . Indian Chem. Sac., 44,1005 (1967). (19) J. M. Notley and M. Spiro, J . Phy8. Chem., 70, 1602 (1966). These workers expect a stronger ion-solvent interaction in formamide as compared to that in water. (20) F. Franks and H. T. Smith, Trans. Faraday Soc., 63, 2686 (1967). (21) L. A. Dunn, ibid., 62, 2348 (1966). (22) 0. Redlich and D. M. Meyer, Chem. Rev., 64, 221 (1964). (23) This salt could not be studied in the present investigation owing to its low solubility in NMA.

The Journal of Physical Chemistry

ION-SOLVENT INTERACTION OF SOME TETRAALKYLAMMONIUM AND COMMON IONS 3393

are net structure b reake r~ .24~~~ The smaller R4N+ ions are also expected to be solvated electrostatically on account of a sufficient electrical charge density on them and also because of the large dipole moment of the NMA molecule ( p = 3.7 D) ; both these factors enhance ion- solvent interaction which would be, more or less, negligible for the larger R4N+ ions. So comparing the results obtained in NMA solutions with those in water, it appears that the positive slope of 4,. us. curves in NMA for the R4NI salts, containing the smaller R4N+ ions, is due to appreciable contraction of the solvent around these ions; the same is true for LiCl and KI. At infinite dilution, the contraction of the system per mole of the solute would be maximum and so (byo would be minimum. As the concentration increases, the con- traction per mole would decrease and 9,. would increase. In aqueous solutions, the smaller dipole moment of the water molecule causes appreciable contraction only in the presence of Me4N+ ion and the common ions, all of which have high surface charge density. In NRIA, electrostriction appears to be appreciable even in the presence of somewhat larger Et4N+ and Pr4N+ ions, apparently because of the larger dipole moment of NMA.

Referring now to the negative slope in the +y us. <c curves for Bu4NI, Pen4NI, HexdNI, and Hep4NI salts in NMA, it may be recalled that the similar behavior of the R4N+ ions in aqueous solution has been ascribed to factors which directly depend on hydrophobic interac- tion4J between water molecules and the organic alkyl groups present in the R4N+ ions.2e Since this dislike for the alkyl groups is not expected to be present in NMA and, indeed, there is independent evidence from the surface tension rneas~rements'~ that the R4N+ ions in NMA behave like common cations in water ( i e . , R4N+ ions are lyophilic in NILLEA), it appears that the cause of the negative slope in NMA must be factors other than lyophobic structure promotion and the hatred for the solvent.

When the salts are dissolved in NMA, some elec- trostatic ion-solvent interaction and electrostriction are expected in presence of the smaller R4N+ and the com- mon ions; this interaction results in a positive slope in the 4,. vs. curves of Et4NI, Pr4NI, KI, and LiC1. The electrostatic ion-solvent interaction and the posi- tive slope will go on diminishing with the increase in the radius of the R4N+ ion. If the ion is sufficiently large, the positive slope may be completely missing or, a t least, may become undetectable experimentally.20 At higher concentrations, when the larger R4N+ ions with weak or negligible electrostatic interaction come nearer to one another, mutual penetration of the ions t h e m s e l ~ e s ~ ~ and their penetration by the iodide ion98 are very likely to occur. The larger the R4N+ ion, the lower the concentration at which this effect would be- come significant. This effect will increase with the rise in concentration, and, as a consequence, 4,. will

decrease. Also in between the voids, formed by the larger R4N+ ions, the solvent molecules and the iodide ions may be isolated and locked in, if the voids are large enough, so that these would not contribute to the volume of the system and hence a reduction in volume would occur. This effect would also be larger, the larger the R4N+ ion. After all the voids have been oc- cupied, any further increase in concentration will cause t& to increaseOz9

Eflect of Temperature on the Limiting Apparent Molal Volume. The limiting apparent (or partial) molal volume, + y o , of the different salts has been plotted against temperature (1')) and the curves, thus obtained, are given in Figures 4 and 5 .

It may be noted that for R4NI salts, containing the smaller R4N+ ions, $,." increases with the rise in tempera- ture although d+,.O/dt decreases so that +,.' us. to curves may pass through a maximum a t the higher tempera- tures. For the common salts like KI, the maximum appears to occur around 50-55", and for LiCl, the maxi- mum has already been passed over a t room temperature and cpyo only decreases with the rise in temperature. It may be remembered that in NRiIA the behavior of R4NI salts (containing the smaller R4N+ ions) and of the common electrolytes like KI and LiCl is exactly similar to that of the common electrolytes in aqueous solutions. On the other hand, $,.' of the R4N iodides, containing the larger R4N+ ions (in NMA), appears to increase proportionally with temperature. It thus appears that the occurrence of maximum in the c&' os. to curves of some electrolytes and its absence in those of others is a general phenomenon and is not confined to aqueous solutions only. This fact indicates that no regular three-dimensional features of the solvent (e.g., icelikeness in water) should be considered to be the cause of this behavior.

It may be noticed from Figures 4 and 5 that, in general, appreciable curvature in the &" os. t o curves toward the temperature axis is present only in cases for which 4,. os. <c curves have a positive slope. This behavior is characteristic of solvation and electrostric- tion, i e . , the contraction of the solvent around the ion. On heating, some solvent may be released from the loose solvation layers; this would increase the volume a little more rapidly than that of the pure solvent and so d&"/dt would be positive in the beginning. However,

(24) R. D. Singh, P. P, Rastogi, and R. Gopal, Can. J . Chem., 46, 3525 (1968). (25) C. M. French and K. H. Glover, Trans. Faraday SOC., 91, 1427 (1965). (26) These factors have been summarised by R. A. Horne and R. P. Young, 1.7. Phys. Chem., 72, 1763 (1968) I and also by F. J. Millero and W. D. Hansen (see ref 16). (27) One might call i t "cation-cation" interaction [cf. W. Y . Wen and K. Nara, ibid., 71, 3907 (1967); R. H. Wood, H. L. Anderson, J. D. Beck, J. R. France, W. E. de Vry, and L. J. Soltzberg, ibid., 71, 2149 (1967); R. A. Horne and R. P. Young, ibid., 72,1763 (1968)l. (28) S. R. C. Hughes and D. H. Price, J . Chem. SOC., A , 1093 (1967). (29) W. Y. Wen and 8. Saito, J . Phys. Chem., 68,2639 (1964).

Volume 73, Number 10 October 1969

3394

,r 481 I 47

R. GOPAL AND M. A. SIDDIQI

KI

476 1 HexrNI

464

460

rqol ' I f B u,. NI

I i 310'

175! <, , , 30 35 40 45 50 6b 7; i 0

TEM~~RATURE -5

Figure 4. apparent molal volume of some RlNI salts in NMA.

The temperature dependence of the limiting

d$,'/dt would decrease with the rise in temperature because the loosely attached solvent molecules around the ions would slowly pass into solution. So a t the higher temperatures, pure liquid solvent would expand more rapidly than the solution containing ions with the firmly attached solvent molecules forming the primary solvation layer; hence, d+,' /dt would decrease and may finally become negative for the ions (at higher temperatures) which are small enough to have a strong influence on the surrounding solvent medium so that some of the solvent in solution would not contribute to

I I

l7 30 is i o 4i' i o do ib do TEMPERATURE -+

Figure 5. apparent molal volume of LiCl and KI in NMA.

The temperature dependence of the limiting

expansion with the rise in temperature to the same extent as the pure solvent itself.

The case of the larger R4N+ ions appears to be differ- ent. On account of the low surface charge density on them, electrostatic solvation and structure breaking of NMA would be negligible in their presence. However, there appears to be a high probability of some penetra- tion, on the average, of the space in between the flicker- ing or even partly coiled-up four alkyl chains, attached to the nitrogen atom by the solvent molecules, specially under the directing influence of the positive charge on the nitrogen atom itself. At the higher temperatures, the penetration would be less due to higher thermal energy and so 4,' would increase. Besides, the coiled-up chains are likely to open out a t the higher temperatures, and this would cause the volume of the system to in- crease even in the absence of the penetration effect. Since the factors which are responsible for d&'/dt decreasing, Le. , the primary solvation and the structure breaking of the solvent, would be missing in the larger R4N+ ions, 4,' would go on increasing with the rise in temperature. It is difficult to decide a t present which (or both) of the two factors is responsible for the in- crease in &' with the rise in temperature. It appears advisable not to dilate these concepts any further until more is known about the behavior of the chains of the RIN + ions when present in solution.

The authors wish to express their gratitude to the Council of Scientific and Industrial Research, India, and to the Society of Sigma Xi and RESA for financial assistance.

Acknowledgment.

(30) NOTE ADDED IN PROOF. Negative Sv values are also found for the common electrolytes like NaN03, NaBr, etc., in N-methylpro- pionamide [A. J. Millero, J . P h y s . Chem., 7 2 , 3209 (1968) ] and NaCl, NaBr, KBr, and KNOa in N-methylacetamdde (R. Gopal and K. Singh, unpublished data). It appears that the relative volumes of the ions and the solvent molecules determine the sign of Sv.

The Journal of Physical Chemistry