Transfer energetics of tetraalkylammonium ions in aquo-organic

Jun 13, 1991 - (BF4)2,5/Au sandwiches were prepared, each containing. = ... The solvent effect on hydrophobic hydration has been studied by measuring ...
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J . Phys. Chem. 1992, 96, 970-975

(BF4)z 5 / A sandwiches ~ were prepared, each containing rT = 2.0 x IO-$ mol/cm2 deposited on 0.002-cm2 Pt wire tip electrodes, with evaporated gold overlayer contacts that covered the film and spread out onto the surrounding glass shroud to different extents (28 and 6 mm2 of Au in area). Each sandwich was immersed in 0.1 M Bu4NBF4/CHZCl2and electrolyzed a t E O ' , so as to produce a mixed-valent film where Cos(nI)= then potentiostatic control was released, and the potential of each film (Pt electrode) vs a SSCE reference electrode was monitored vs time. In both cases, Efilmdecayed with time in a direction indicating loss of Os111states (Figure 9), and the decay was faster for the sandwich electrode having the larger area of Au exposed to the solution. This result shows that either oxidation of Au or of some trace solution component at the Au surface occurs to discharge the Os"' states. The dimensions are such that less than 1 X 1O-Io equiv/cmz of charge (less than one molecular layer) must pass across the Au/solution interface in order to completely discharge the Os"' layer.

Our experience indicates that effects such as Figure 9 are greatly retarded in films dried of solvent, which is the reason that, in preparing mixed-valent films, it is important to retain potential control of the film until it has been raised from the solution, rinsed, and dried.

Acknowledgment. This research was supported in part by grants from the National Science Foundation. We gratefully acknowledge the Pt IDA'S supplied for this research by Masao Morita and Osamu Niwa of the NTT Basic Research Laboratories, Nippon Telegraph and Telephone Company, Tokai, Ibaraki, 319-11, Japan. R&tW NO. Vbpy, 74173-48-1; BFC, 14874-70-5; ([Os(bpy)2(vPY)~] ( B F ~ ) Z - ~138062-06-3; )~, ( [OS(V~PY)J(BF~)~+)~, 138089-55-1; [OS(Vbpy),](PF6)2, 130728-19-7; (NH4)2OSCI,, 12 125-08-5; NHdPF,, 16941-1 1-0; CH2C12, 75-09-2; CH$N, 75-05-8; Bu~NBF~, 429-42-5; ([OS(~PY)Z(VPY)ZI(BFS)~)~, 13806247-4; ([OS(V~PY),I(BF~)Z)~, 138062-08-5; Pt, 7440-06-4; Au, 7440-57-5.

Transfer Energetics of Tetraalkylammonium Ions in Aquo-Organic Systems and the Solvent Effect on Hydrophobic Hydration Himansu Talukdar and Kiron Kumar Kundu* Physical Chemistry Laboratories, Jadavpur University, Calcutta- 700 032, India (Received: June 13, 1991, In Final Form: August 20, 1991)

The solvent effect on hydrophobic hydration has been studied by measuring the solubilities of some tetraalkylammonium tetraphenylborate(R.,N+PbB-) salts with R = methyl (Me), ethyl (Et), n-propyl (Pr),and n-butyl (Bu) spectrophotometrically at 25 OC in some aqueous mixtures of tert-butyl alcohol (TBA) and acetonitrile (ACN). The observed results, which yield standard free energies of transfer (AGlo)of KN+P4B- from reference solvent water to these cosolvents, have been dissected into individualion contributions using the preevaluated values of P4B- which are based on the widely used tetraphenylarsonium tetraphenylborate (TATB) reference electrolyte (RE) assumption. The AGlo(i)values of &N+ cations were then combined with the previously evaluated standard enthalpies of transfer (Ml0(i)) based on the same RE assumption, to give standard entropies of transfer (ASto(i)). The AGto(i) values were analyzed in light of the cavity effect and electrostatic effect and the rest in light of the hydrophobic hydration (HH) effect, which was found to decrease with cosolvent. The corresponding A S W H O values when analyzed in light of Kundu et ala's four-step transfer process and the marked influence on water structure by the three-dimensional (3D) structure promoter TBA and 3D structure breaker ACN molecules reflect the respective solvent effect on hydrophobic hydration as induced by R4N+cations.

Introduction Hydrophobic solutes or ions with apolar residues induce water molecules around them to organize in a way similar to clathrate hydrates,' causing hydrophobic hydration (HH)'-' and resulting in a significant increase of free energy and decrease of entropy of the system.' The phenomena of hydrophobic hydration and the related hydrophobic interactions (HI)693camong apolar sites are of great significance in the realms of micelles, mixed micelles, microemulsions, bilayer membranes, and particularly biopolymers including protein^.^ Studies on thermodynamic and transport b e h a v i ~ r ~ . ~of, ~aqueous . ~ - ' ~ solutions of tetraalkylammonium salts (1) Wen, W. Y. Water and Aqueous Solutions; Horne, R . A,, Ed.; Wiley-Interscience: New York, 1972; p 613. (2) Conway, B. E. Ionic Hydration in Chemistry and Biophysics; Elsevier: Amsterdam, 1981; Chapters 20 and 24. (3) (a) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley-Interscience: New York, 1980. (b) Faraday Symp. Chem. SOC.1982,17. (c) Kauzmann, W. Adu. Protein Chem. 1959, 14, 1. (4) Abraham, M. I+.; Matteoli, E. J. J. Chem. SOC.,Faraday Trans. 1 1988, 84, 1985 and relevant references therein. ( 5 ) Privalov, P. L.;Gill, S.J. Pure Appl. Chem. 1989, 61, 1097. (6) Ben-Naim, A. J. Chem. Phys. 1971, 54, 1387, 3696. (7) Engberts, J. B. F. N.; Nusselder, J. J. H. Pure Appl. Chem. 1990.62, 47. (8) Cox, B. G.; Waghorne, W. E. Chem. SOC.Rev. 1980, 9, 381.

with and without ionic and nonionic cosolutes/cosolvents and salient features of hydrophobic hydration have been reported. We have shown recentlyI5 that tetraalkylammonium picrates and especially the corresponding hydrophobic cations induce more hydrophobic hydration in aqueous sodium nitrate solutions than in pure water. Analysis of the observed standard free energies (Acto) and entropies (Uto) of transfer data specifically indicated the alteration of water structure around the incoming ions and thus provided measures of the salt effect on hydrophobic hydration. It should be equally important and interesting to understand the systematic behavior of these tetraalkylammonium salts in various aquc-organic solvent systems. Various thermodynamic studies2~8-'2J4 on these salts have been made particularly from calorimetric measurements, which helped evaluate standard enthalpies (9) (a) Heuvelsland, W. J. M.; de Visser, C.; Somsen, G. J. Phys. Chem. 1978, 82, 29. (b) Heuvelsland, W. J. M.; Bloemendal, M.; de Visser, C.; Somsen, G. J. Phys. Chem. 1980,84, 2391. (IO) Juillard, J. J. Chem. SOC.,Faraday Trans. 1 1982, 78, 37, 43. (11) Miyaji, K.; Morinaga, K. Bull. Chem. SOC.Jpn. 1983, 56, 1861. (12) Carthy, G.; Feakins, D.; Waghorne, W. E. J. Chem. SOC.,Faraday Trans. 1 1987.83.2585. (13) Nakayama, H.; Kuwata, H.; Yamamato, N.; Akagi, Y.; Matsui, H. Bull. Chem. Soc. Jpn. 1989,62, 985. (14) Feakins, D.; Mullally, J.; Waghorne, W. E. J . Chem. Soc., Faraday T r a m 1 1991, 87, 87. (15) Talukdar, H.; Kundu, K. K. J. Phys. Chem. 1991, 95, 3796.

0022-365419212096-970$03.00/00 1992 American Chemical Society

Transfer Energetics of R4N+

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 971

TABLE I: Solubilities (S, mol d d ) and Solubility Products (pK, M) of Tetraalkylammonium Tetraphenylborate Salts in Water and Aqueous Mixtures of TBA and ACN at 298 K"

H20 TBA

ACN

wt Iof cosolvent 0 10 20 30 40 20 40b 60b

s x 104 Me4NX 0.20 0.30 0.93 2.63 3.70 2.50 39.90 99.50

E4NX 0.19 0.20 0.56 1.52 2.31 1.60 19.51 73.23

PK,,

Pr4NX

Bu4NX

Mo4NX

Et4NX

Pr4NX

0.18 0.23 0.66 1.79 3.59 1.71 46.50 127.00

0.15 0.14 0.25 0.63 0.83 0.65 7.00 50.21

9.40 9.05 8.06 7.16 6.86 7.205 4.89 4.18

9.44 9.40 8.50 7.64 7.27 7.59 5.48 4.42

9.49 9.28 8.36 7.49 6.89 7.525 4.76 3.99

Bu4NX 9.65 9.73 9.20 8.40 8.16 8.375 6.35 4.73

'X = (C6H5),B-. bpK,,values have been obtained after considering the molar activity coefficient. In other cases, this is likely to be unity (see text). TABLE Ik Transfer Free Energies of Tetraakylammonium Tetraphenylborate (AGlo(R,NBPh,,)) Salts and Transfer Energetics ( A c t o , AHlo, TASto) of Tetraakylammonium Cations (I(,N+) from Water to Aaueous TBA and ACN at 298 K (Units:kJ mol-' and Mole Fraction &le) AHt0(R4N+)" TMt0(R4N+) AGo(R" Acto( &N+BPhc) wt Iof Et Pr Bu cosolvent Me,NX EtrNX Pr,NX BuPNX Me Me Et Pr Bu Me Et Pr Bu 1.7 6.6 13.3 TBA 10 -2.5 -0.7 -1.7 0.0 1.1 2.9 1.9 3.5 -0.4 -1.5 -1.2 4.6 9.8 20 -8.6 -6.3 -7.4 -3.5 1.5 3.8 2.7 6.6 15.9 20.3 32.1 50.6 14.4 16.5 29.4 44.0 30 -14.3 -11.8 -13.0 -8.7 0.1 2.6 1.5 5.7 5.5 11.0 24.9 44.0 5.4 8.4 23.5 38.3 40 -16.7 -14.6 -17.0 -10.7 -0.3 1.9 -0.6 5.7 0.9 7.3 22.6 40.4 1.2 5.5 23.2 34.7 ACN 20 -13.2 -11.3 -12.0 -8.0 -1.7 0.2 -0.5 3.5 -9.0 1.0 13.0 25.0 -7.3 0.8 13.5 21.5 40 -27.4 -24.3 -28.7 -20.5 -4.2 -1.1 -5.5 7.0 16.0 -8.8 -4.9 12.5 13.3 2.7 -13.0 -6.0 60 -32.5 -31.4 -34.1 -30.8 -4.1 -3.0 -5.7 -2.4 -16.0 -6.0 7.0 13.0 -11.9 -3.0 12.7 15.4 "See refs 10 and 1 1 .

of solution (AHs")and the related standard enthalpies of transfer (AHto).8-12But, lack of knowledge of transfer entropies left no clear understanding of the structural changes induced by the tetraalkylammonium ions. We are, therefore, reporting AG," of some tetraalkylammonium tetraphenylborate (R.,N+BPh4-)salts with R = methyl (Me), ethyl (Et), n-propyl (Pr),n-butyl (Bu), and phenyl (Ph) by measuring the solubilities at 25 "C in aqueous tert-butyl alcohol (TBA), a threediiensianal(3D) water structure maker,I0J6and acetonitrile (ACN), a 3D water structure breaker."*'' Our present data in combination with reported AH,"values will yield AS," values which are likely to serve as relative measures of the solvent effect on hydrophobic hydration. Experimental Section

Purification of the solvents, TBA and ACN (both A.R; S.D.), has been described.18 The tetraakylammonium tetraphenylborate salts have been prepared as before.Ig The required Me4NI (Fluka AG) has been used after recrystallization from water. The other salts, Le., Et4N+Br-(Sigma), Pr4N+Br-(Aldrich), and Bu4N+Br(Sigma) have been used as received. Salts were used after drying for 2-3 days in a vacuum desiccator. The purity of the salts was excellent when the melting points of the dried samples were compared with the literature va1~es.I~ Water employed was triply distilled and C 0 2 free. The rest of the experimental procedure was essentially similar to that described in our previous paper.I5 Nitrogen gas was passed through the saturated solutions to maintain an inert atmosphere. The required A, and molar extinction coefficient values were obtained from our previous studies.20

Results Observed solubilities (S, mol dm-7 of the salts in water, TBA-water, and ACN-water are given in Table I. Assuming (16) Koga, Y.; Sin, W. W. Y.; Wong, T. Y. H. J. Phys. Chem. 1990,94,

7700.

(17) Robertson, R. E.; Sugamari, S . E. Can. J . Chem. 1972, 50, 1353. (18) (a) Base,K.; Das, A. K.; Kundu, K. K. J. Chem. Soc., Faraday Trans. 1 1976,72, 1620. (b) Das, K.; Das, A. K.; Kundu, K. K. Electrochim. Acta 1981, 26, 471. (19) (a) Accascina, F.;Petrucei, S.;Fuoss, R. M. J. Am. Chem. Soc. 1959, 81, 1301. (b) Berns, D. S.;Fuoss,R. M. J . Am. Chem. SOC.1960,82,5585. (20) Talukdar, H.;Rudra, S. P.; Kundu, K. K. Can. J. Chem. 1989, 67, 321.

that the salts are completely ionized, the thermodynamic solubility product Kspof each salt was computed on the molar scale by the relation PKsp = -2 1% (SYd (1) where ya is the mean molar activity coefficient of the salt. Since the solubilities are very low particularly in water, TBA-water, and 10 wt % ACN-water, the mean molar activity coefficients were calculated only for the cases of 20 and 40 wt % ACN-water using a modified form of the DebyeHiickel equation2Idescribed in our previous paper.15 The required dielectric constants are taken from literature?% pKspvalues thus obtained at 25 "C on a molar scale are given in Table I. The AG," values of the salts on a mole fraction scale were then calculated for each composition by the relation AG," = 2.303RT[,pKSp- ,pKsp] 2RT In (Mwds/Msdw) (2)

+

where M stands for molar mass, d is the density (kg d m 3 , and subscripts s and w refer to solvent and water, respectively. The required densities are obtained from the literature.18aJ2bThe AG,O values are shown in Table 11. Since the AGt0(Ph4B-)values, which are based on the tetraphenylarsonium tetraphenylborate (TATB) a ~ u m p t i o n ?are ~ known for the two solvent ~ y s t e m s , ~ ~ 4 ~ ~ we have computed the AG,"(R4N+) values by the additivity principle. The AGt0(Ph4B-)values in 40 wt % TBA have been obtained by interpolation. The reported values of AH,"(R.,N+) (also based on the TATB assumption) as obtained by Juillard and co-worker~'~J~ in TBAwater and by Miyaji and Morinaga" in ACN-water when combined with the present AG,"(R4N+) values yield ASt"(R4N+) values. It has been found that AHt"(Bu4NBr)evaluated by Cox et aLZ6is in close agreement with that of Miyaji and Morinaga. The values of AG,O and AS,"as well as AH," so obtained are listed in Table 11. The uncertainties of the values lie within hO.1 kJ. (21) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions; Butterworths: London, 1968. (22) Moreau, C.; Douheret, G. (a) J. Chem. Thermodyn. 1976,8,403;(b) Thermochim. Acta 1975, 8, 385. (23) Cos, B. G.; Parker, A. J. J . Am. Chem. SOC.1973, 95,402. (24) Basu-Mullick, I. M.; Kundu, K. K. Ind. J . Chem. 1984, 23A, 812. (25) Pointud, Y.; Juillard, J.; Avedikian, L.;Morel, J. P.; Ducros, M. Thermochim. Acta 1974, 8, 423. (26) Cox, B. G.; Natarajan, R.; Waghorne, W. E. J. Chem. Soc.,Faraday Trans. I 1979, 75, 86.

Talukdar and Kundu

972 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992

- -.

p--

5

/'

-".&I

/'

t

-330

.a c*

- - _ _ _ e -

c

k < -I

%d

9

I

I

I

I

4

8

12

16

mol% of TBA-

t

L

f

mol '/,

c

0

4

8 mol '1. of TBA

-

of A C N

-

Figure 3. Variation of AGLmo(i)of tetraalkylammoniumions with (mol %) TBA (A) and ACN (B) at 298 K. 12

Figure 1. Variation of AG,(R4NBPh4)(-) and AG,(R4N+) (- - -) with (mol %) TBA at 298 K.

In fact, both Et4N+and Pr4N+,unlike other tetraalkylammonium ions, are stably accommodated within a pentagonal dodecahedron, giving rise to unusual elevation of solubility of their salts in water.I3 Our recent solubility data2*in water and sodium nitrate solutions showed that both Et4NI and Pr4NI are highly soluble compared to Me4NI and Bu4NI, indicating the formation of more stable "cagelike" structure around Et4N+and Pr4N+. Siar apparently inconsistent results have also been reported by Johnson27and Wen.' In order to understand the relative behavior, AGto(R4N+)composition profiles in both aqueous TBA and ACN are illustrated in Figures 1 and 2 (dotted lines), respectively. While the ions are destabilized from the initial composition in both cosolvents with the relative order Bu4N+> Et4N+> Me4N+(Pr4N+being the odd man out), the same become less destabilized at higher compositions particularly in ACN-water. Also, while hydrophobic Ph4B- is increasingly stabilized in both mi~tures'8&~~ due to possible dispersion interaction, the same is not true in the case of hydrophobic tetraalkylammonium ions. Previous analy~is'~3 of AG,O-commition profiles of single ions shows that speculation about the hydrophobic hydration effect of these ions can be made if one considers interactions arising due to cavity and electrostatic effects. Thus, the observed free energy of transfer AGto(i) may be ascribed as = AG~,cavo(i) + AGt,elo(i) + AGt,HHo(i)

mol '/.

of ACN -C

Figure 2. Variation of AG,0(&NBPh4)(-) and AG,"(&N+) (---) with (mol %) ACN at 298 K.

Discussion Free Energies of Transfer. AGto-composition profiles for the salts (&N+BPhJ are illustrated in Figures 1 and 2 by solid lines in aqueous TBA and ACN, respectively. In both the cases, the salts are increasingly stabilized in the relative order Bu4N+BPh4< Et4N+BPh4-< Me4N+BPh,. The behavior of Pr4N+BPh, is seemingly the odd man out, which is possibly due to the unusual variation of solubility of the salts in these aquc-organic mixtures (Table I). It was not previously e s t a b l i ~ h e d ' ~ that ~ * l ~although ,~~ the anionic volume has a marked influence on the solubility behavior of tetraalkylammoniumsalts, the disparity in the same and also in the Actovalues for a series of these salts depends on the structure enforcing capabilities of the tetraalkylammonium ions. (27) Johnson, D. A.; Martin, J. F. J. Chem. SOC.,Dalron Trans. 1973, 1585.

(3)

where AGt,ULVo, AGt,elo,and AGt,HHo stand for the free energy contribution of the cavity effect, the electrostaticeffect comprising Born, ion-dipole, ion-induced dipole, and ionquadrupole interactions, and the hydrophobic hydration effect, respectively. In order to have a semiquantitative picture, we have tentatively calculated the cavity effect AG,,c,,o(R4N+) in light of scaled particle theory Born electrostatic AGt,bmo(R4N+)r and i ~ n - d i p o l eAG, ~ ~ido(R4N+),using the appropriate formulations described earlier.i0 Because the ion-induced dipole, AGwo(&N+) was negligibly mall,^^*^ these were not included in AG,,Io(%N+). Similarly, the extent of ion-quadrupole interaction could not be estimated and accounted for, as the data of quadrupole moment for both TBA and ACN are not Of course, this interaction term, king usually less than 10% of the total electrostatic (28) Talukdar, H.; Kundu, K. K. Unpublished data; Third Symposium on Solubility Phenomena, Surrey, U.K., 1989. (29) Pierotti, R. A. (a) J. Phys. Chem. 1965, 69, 281; (b) Chem. Reo. 1976, 76, 717. (30) (a) Kim, J. I. J. Phys. Chem. 1978,82, 191. (b) Kim, J. I.; Cocal, A.; Born, J. J.; Comaa, E. A. 2.Phys. Chem. Neue Folge 1978, 110, 209.

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 913

Transfer Energetics of R4N+

TABLE III: Valws of Free Energies and Eath.lpies of cavity Formation (c, R,) a d Standard Free Ewrgia of Transfer (AG$=,O(i), TaSwo(i)) of TetnrrlLylammoniPm Cations from ACLsono(i), AGwo(i), AG,mo(i)) and Entropies of Transfer (TA&=.O(i), TAS-O(i), Water to Aawous TBA a d ACN at 298 K (Units:W mot' and Mole Fraction serle) ion

water

solvent

water

solvent

AG,,,O(i)

AGhMo(i)

AGtj+O(i)

AG,,HHo(i) TUh,,O(i) V. = 19.86 dm-) .3.i 6.0 6.5 8.5 10.5 6.7 9.7 13.1

T U h m o ( i ) TU,HHo(i)

10 wt % TBA = 2.63 mol % TBA: a = 351 X 10" K-I:

Me4N+ Et4N+ Pr4N+ Bu~N'

50.6 72.9 89.1 110.5

48.1 69.1 84.2 104.4

6.1 8.6 10.3 12.5

9.4 13.1 15.7 19.2

-2.7 -4.0 -5.1 -6.5

0.4 0.3 0.3 0.3

0.1 0.1 0.0 0.0

-0.8 -0.6 -0.6 -0.5

-6.7 -9.2 -5.3 -2.8

Me4N+ Et4N' Pr4N+ Bu~N"

45.6 65.4 78.0 98.4

20 wt % TBA 15.6 21.9 26.2 31.8

= 5.73 mol % TBA; a = 60 4 X 10" K-'; V, = 21.97 dm-' -5.4 0.9 0.2 5.8 14.9 -8.0 0.7 0.1 11.0 21.3 -11.6 0.7 0.1 13.5 27.5 0.1 18.6 32.0 -12.7 0.6

-2.4 -2.0 -1.8 -1.6

1.9 -2.8 3.7 13.6

Me4N+ Et4N' Pr4N+ Bu~N'

42.9 60.8 74.4 91.9

30 wt % TBA = 9.44mol 4% TBA; a = 759 X IO" K-I; V , = 24.58dm-' 18.8 -8.5 1.5 0.3 6.8 21.1 1.3 0.2 14.0 30.6 26.3 -12.9 31.5 -15.4 1.2 0.2 15.5 36.6 38.3 -19.4 1.O 0.1 24.1 45.2

-4.2 -3.5 -3.2 -2.9

-11.5 -18.7 -9.9 -4.0

Me4N+ Et4N' Pr4N+ Bu~N'

39.8 56.6 68.7 84.6

40 wt % TBA 16.8 23.6 28.2 34.3

= 13.97 mol % TBA; a = 725 X 10" K-I; V, = 27.91 dm-3 -9.4 2.5 0.4 8.8 22.6 -14.1 2.1 0.3 16.8 32.4 -21.5 1.9 0.3 18.7 39.4 1.7 0.2 30.9 48.8 -27.0

-6.3 -5.3 -4.8 -4.3

-15.2 -21.6 -1 1.4 -9.8

Me4N+ Et4N' P4N+ Bu~N"

47.7 68.3 83.5 103.3

20 wt % ACN 16.6 23.2 28.0 33.9

= 9.89mol % ACN; a = 610 X 10-6 K-I; V , = 21.16 dm-' -3.3 0.4 -1.4 2.6 13.8 -4.9 0.3 -1.2 6.0 19.6 -5.9 0.3 -1.1 6.2 23.6 -1 .o 11.9 29.0 -7.6 0.2

-0.4 -0.3 -0.3 -0.2

-20.6 -18.5 -9.8 -7.3

Me4N+ Et4N+ Pr4N+ Bu~N'

43.8 62.4 71.8 88.7

40 wt % ACN 19.4 27.2 29.3 35.5

= 22.64 mol % ACN; a = 750 X 10" K-I; V, = 25.43 dm-' -7.6 1.O -3.3 5.7 20.9 0.8 -2.8 12.2 26.9 -11.3 0.7 -2.5 14.4 37.1 -18.1 0.6 -2.3 27.2 45.7 -22.8

-1.3 -1.1 -1.0 -0.9

-28.4 -30.7 -23.6 -31.5

Me4N+ Et4N+ Pr4N+ Bu~N'

39.6 56.1 68.3 84.0

60 wt % ACN 21.1 29.6 35.5 43.2

= 39.70 mol % ACN; a = 880 X -12.4 1.7 -5.8 1.4 -4.9 -18.1 -22.2 1.3 -4.4 1.2 -4.0 -28.0

-3.0 -2.5 -2.2 -2.0

-36.3 -39.6 -32.4 -41.3

lod K-I; V, = 31.37 dm-' 12.4 18.6 19.6 28.4

27.4 39.1 47.3 58.7

V, = molar volume and a = -d In d,/dT, d, being the densities (kg dm-') obtained from refs 18a and 22b. CY^^^ has been taken as 223 X 10" K-I.'* The required diameters of H20(0.276 mm), TBA (0.497 nm), and ACN (0.412 nm) and also the dipole moments of the solvents have been taken from: Rudra, S. P.; Chakraborty, B. P.; Kundu, K. K.; Basu-Mallick, I. N . Z.Phys. Chem. Neue Folge 1986, 150, 211.

contributions, can be tentatively taken to be hardly e f f e c t i ~ e . ~ ~ The required parameters for calculating the AGt,cavo(i), AGt,Bomo(i), and ACLid0(i)quantities are given in Table 111. In the computations, the hard-sphere radii of the cations are taken from Marcus.3' The same for Bu4N+agrees fairly well with the data obtained recently.32 These values (see Table 111) when subtracted from the AG,0(R4N+)values yield the hydrophobic hydration effect, Le., AGLHHo(R4N+), and Figure 3 illustrates the AG,HHo(&N')-CXXllpC6itiOn profdm in aqueous mixtures Of TBA (Figure 3A) and ACN (Figure 3B). In both solvent systems, the ions become more desolvated compared to an earlier observation and follow the order Bu4N+> Pr4N+> Et4N+ > Me4N+, indicating more hydrophobic hydration for the ions in water as compared to the case in aquo-organic mixtures. Significantly enough, for hydrophobic ions that are likely to induce more hydrophobic or dispersion interaction with the organic cosolvents, the reverse effects have been observed. This is indicative of the effect of decreased hydrophobic hydration caused by the organic cosolvents. In fact the water-like skin-phase environment that is likely to form around the apolar sites or hydrophobic groups due to hydrophobic hydration in pure water gets partly nullified by the solvophilicity of R4N+ions in these aqueous organic cosolvents. Besides, TBA (31) Marcus, Y . (a) Pure Appl. Chem. 198658, 1721; (b) IonSolvation; Wilcy-Interscience: London, 1985; Chapter 3. (32) Kontturi, A.; Kontturi, K.; Murtomaki, L.; Schiffrin, D. J. J . Chem. Soc., Faraday Trans. 1990,86, 931.

being a 3D water structure maker,16 which shifts the equilibrium (H20)b (H20)d(b = bulky, d = dense) toward more bulky water, decreases the number of monomeric water molecules in the mixtures, thereby inducing less hydrophobic hydration in TBA-water as compared to that in water. Also, as the size of the &N+ cations increases,more monomeric water molecules will be required for hydrophobic hydration. As a result, the AGmo-c"position profiles in TBA-water should be in the order observed in Figure 3A. On the other hand, ACN, being a 3D water structure breaker," increases the number of monomeric water molecules. Some of these water molecules and ACN molecules however may in situ lead to the formation of intercomponent hydrogen-bonded complexes A and B as indicated previously.18b Thus, the observed +6

-8

+6

O-H----N=C--CH,

/ H A

+6 -6 +6 H----NEC-CH3

- / 60 \

H

+s

- - - -N G+C -CH, -8

B

6

914

Talukdar and Kundu

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992

0

8

4

mol '1. of

TBA

-

12

less hydrophobic hydration in ACN-water can be ascribed partly to the solvophilicity of R4N+ions and partly to the fewer monomeric water molecules to undergo hydrophobic hydration. At high ACN composition, possible formation of dimers C, which are shown from IR studies to exist in pure ACN at l e a ~ t , 3 ~may 2~ cause a more hydrophobic hydration effect as compared to that in lower composition ACN-water mixtures. This is perhaps true when we observed that the AGt,~~'(BU4N+)-compositi0n profile at higher mole percent ACN tends to become less desolvated (Figure 3B). Lastly, the solid lines in Figure 6 reflect a comparison of AGt,HHo(i)-composition profiles of R4N+and Bu4N+cations with these observed in our previously studied15NaN0, salt solutions. It may be concluded that the hydrophobic hydration effect in the different solvents as compared to that in pure water decreases in the order aqueous NaNO, < aqueous ACN < aqueous TBA. This of course conforms to what is expected from the above discussion. Apart from solvophilicity of &N+ ions, TBA being a 3D structure maker would contribute less monomeric water molecules and thus impart less hydrophobic hydration as compared to ACN. The latter being a 3D structure breaker will release monomeric water molecules but, being capable of forming hydrogen-bonded (H20),-ACN complexes, will engage free water molecules and hence decrease the hydrophobic hydration as compared to pure water. On the other hand, NaNO, salt being a 3D structure breaker will no doubt induce more monomeric molecules by shifting the equilibrium (H20)b (H20)dto the right. But, some of the monomeric water molecules being again likely to be part and parcel of the ions (particularly at higher compositions) due to ionic force fields round the ions will induce less hydrophobic hydration than in pure water. The slight minimum in AGt,HHo(i)-composition profiles in the salt solution, however, as indicated earlier, is the effect of the formation of solvent-separated ion pairs, Le., Na+. .(H,O),. .NO3-. Entropies of Transfer. The solid lines in Figures 4 and 5 illustrate the TASto(R4N+)-xmpositionprofiles in TBA-water and ACN-water, respectively. The profiles are found to be similar in nature with that of AH,o(R4N+)-compositionprofiles reported (33) Zukhova, E. L. Opt.Spectrosk.1958,4,750. (34) Saum, A. U. J . Polym.Sci.1960, 42,57.

I

10

I

I

I

20

30

40

mol'/. of ACN-

Figure 4. Variation of TASt0(i)(-) and TASIBHo(i) (---) of tetraalkylammonium ions with (mol %) TBA at 298 K.

-

I 0

Figure 5. Variation of TASlo(i)(-) and TASl,HHo(i) (---) of tetraalkylammonium ions with (mol %) ACN at 298 K.

earlier.I0*" The cavity effect ASt,,' and Born-type electrostatic effect have been computed by the SPT and Born formulations, respectively, as described earlier.20 These, on subtraction from the ASl0(R4N+),yield hydrophobic hydration effect (R4N+), as the corresponding ion-dipole, ion-induced dipole, and ion-quadrupole electrostatic effects for Ut0 are likely to be negligibly mall.^^,^' Thus, the dotted lines in Figures 4 and 5 show the TASLHHo(i)-composition profiles of all the cations in both the aqueous mixtures. Evidently, the observed minima around 2-3 mol % TBA and maxima around 6-8 mol % TBA seemingly indicate that two opposing forces are in operation. As per Kundu et a1.k four-step transfer process35the S t , H H ' may be taken to be composed of M,,HHo(i) = ( S I ' + hS2O)

+ (u3'+ As,')

(4)

where MIois the entropy change accompanying the dismantling of the hydration sphere (hydrophobic) around the R4N+cations, azo is that accompanying the formation of 3D water structure released free from step 1, AS30 is that due to disruption of structuredness of the cosolvents, if any, and AS4' is the entropy change associated with the hydrophobicJhydrophilic hydration of the cations in the cosolvent. The composite term AS,' ASz', being related to water molecules, remains constant for an ion and is positive or negative depending upon the relative extents of the primary solvation zone ( p s ~ ) resulting ~~* from hydrophobic hydration and the secondary solvation zone (ssz)',~resulting chiefly from the N+ site of R4N+. The other composite term, AS,' + S 4 0 , related to the solvents, changes with cosolvent composition and is dependent on the relative structuredness and the solvating capacity of the solvents involved in steps 3 and 4, respectively. In the present TBA-water mixtures, in order to solvate the incoming ions, the characteristic 3D structure formed due to addition of TBA in water should be broken. As a result, the entropy change AS30 would be a highly positive quantity. Also, as the R4N+ cations are likely to induce less order due to less hydrophobic hydration compared to pure water, which may be partially opposed by dispersion interaction, As4' would be less negative, resulting in increasing positive magnitudes of AS3' + AS4' from the initial composition. But in view of the fact that

+

(35) (a) Kundu, K. K.; Majumder, K. J. Chem. SOC.,Faraday Trans.1 1973,69,806. (b) Bose, K.; Das, K.; Das, A. K.; Kundu, K. K. J. Chem. Soc., Faraday Trans.I 1978,74,1051.

The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 975

Transfer Energetics of R4N+

Evidently, the observed result also shows that ordering negative entropy effect (of ssz water molecules) is more predominant in Me4N+, which sometimes behaves like a simple electrophilic cation,sJOJ and this is gradually nullified by disordering the positive entropy effect as the size and hydrophobicity of the R groups increases. The anomalous behavior of Et4N+is, however, seemingly indicative of the superimposed effect of some specific factors like stable 'cagelike" structure formation around Et4N+, as evident from earlier studies." In the case of ACN-water, since ACN is a 3D structure breaker, AS30 = 0 and the resulting AS30 AS40 is an increasing negative quantity. This is because as ACN might contribute some monomeric water molecules, the cations are not only solvated by dispersion interaction but also more hydrophobically hydrated (compared to TBA-water), thereby accounting for more negative magnitudes of AS40 as compared to TBA-water. As a result, the overall effect is the increasing negative TASt,HHo(i)-xmposition profiles for all the cations from the initial composition (Figure 5 , dotted lines). Thus, the absence of any maximum/minimum is essentially the effect of the well-known structure-breaking propensity of the cosolvent. The relative order as shown in the figure is not of course as expected from the dictates of ionic force field. The observed results are seemingly the superimposed effects of so many involved interactions whose orders are after all hardly predictable. The dotted lines in Figure 6 reflect the comparison of TASt,HHo-composition profiles of Pr4N+ and Bu4N+ in NaN03-water and present solvent systems. It is evident that the characteristic maximum implies the 3D structure making effect of TBA; the absence of the same implies a 3D structure breaking effect of both ACN and NaN03. This has also been shown by some nonelectrolytes in different aquo-organic and aquo-ionic systems.3s

+

mol */. of corolvenl

-

Figure 6. Variation of AGt,HHo(i)(-) and Ths,,HHo(i) (-- -) of tetrapropylammonium cation (A) and tetrabutylammonium cation (B) with (mol 96) cosolvent compositionsof TBA (A), ACN (O), and N a N 0 3 (0) at 298 K.

A S t , H H o values of &N+ ions pass through a minimum (at lower composition), it appears that at least a constant negative quantity results from MIo ASzo for any ion. If we assume that hydrophobic hydration causes the formation of a primary solvation Conclusion sheath (or psi$ around the hydrophobic R groups,j6the contribution of ASl0 AS20 will be a constant positive quantity since It may be concluded that although dispersion interaction bethe system changes from a more ordered (more hydrogen bonding tween the hydrophobic parts (of cosolvent molecules and R groups in the HH region) state (step 1) to a less ordered (normal/less of R4N+ions) are in operation, the overall effect of desolvation hydrogen-bonded water) state (step 2). On the other hand, the of tetraalkylammoniumions is the effect of hydrophobic hydration, water molecules in the ssz past the psz being in the disordered which is suppressed due to depletion of monomeric water molecules state (due to the N+sites of R4N+ions), the combined ASlo + in the mixed aqumrganic systems as compared to that in pure AS2"would result in a constant negative quantity since the system water. This led to the positive magnitudes of real (HH)free changes from a less ordered (less hydrogen-bonded water in ssz) energies of transfer. Besides, the TASto-composition profiles, state (step 1) to more ordered (normal/more hydrogen-bonded unlike AHtO-compositionprofiles, reflect the change in solvent water) one (step 2). Our present results, particularly at lower structure around the &N+ cations in the present systems and thus TBA-water compositions, show that the negative entropy lead to the concept as well as the relative measures of the solvent (structural) effect of the ssz molecules overcomes the positive effect on hydrophobic hydration. entropy effect of hydrophobic hydration for all the R4N+cations, Acknowledgment. Thanks are due to the CSIR, Government contributing an overall constant negative magnitudes of ASl0 of India, New Delhi, for a Research Associateship to H.T. m20.This when combined with the increasing positive magniRegistry No. ACN, 75-05-8; TBA, 75-65-0; Me4NtPh4B-, 15525tudes of h S 3 O AS40 causes the minimum to occur as in Figure 13-0; Et4NtPh4B-, 12099-10-4; Pr4NtPh4B; 15556-39-5; Bu4NtPh4B-, 4. But beyond 6 mol 3'% of TBA, breaking down of the relevant Me4N+, 51-92-3; Et4Nt, 66-40-0; Pr4Nt, 13010-31-6; structure occurs due to the onset of packing i m b a l a n ~ e . ~ ~ , ~15522-59-5; ~ Bu~N', 10549-76-5; H20,7732-18-5.

+

+

+

+

(36) Sarma, T. S.; Ahluwalia, J. C. Chem. Soc. Rev. 1973, 2, 203. (37) Franks, F.; Ives, D. J. G . Q.Rev. 1966, 20, 1 .

(38) Datta, J.; Kundu, K. K. (a) J . Phys. Chem. 1982,86,4055; (b) Can. J . Chem. 1983,61,625.