and Protic Solvents - American Chemical Society

6 Conductance and Ionic Association of Several Electrolytes in Binary Mixtures Involving

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Sulfolane (TMS) and Protic Solvents

1

GIUSEPPE PETRELLA , ANTONIO SACCO, and MAURIZIO CASTAGNOLO Institute of Physical Chemistry, University of Bari, Via Amendola, 173-70126 Bari, Italy

Conductometric and spectrophotometric behavior of several electrolytes in binary mixtures of sulfolane with water, methanol, ethanol, and tert-butanol was studied. In water-sulfolane, ionic Walden products are discussed in terms of solvent structural effects and ion-solvent interactions. In these mixtures alkali chlorides and hydrochloric acid show ionic association despite the high value of dielectric constants. Association of LiCl, very high in sulfolane, decreases when methanol is added although the dielectric constant decreases. Picric acid in ethanol-sulfolane and tert-butanol-sulfolane behaves similarly. These findings were interpreted by assuming that ionic association is mainly affected by solute-solvent interactions rather than by electrostatics. Hydrochloric and picric acids in sulfolane form complex species HCl - and Pi(HPi) -. 2

2

T n r e c e n t years, i n o r g a n i c syntheses studies, d i p o l a r a p r o t i c solvents o r m i x t u r e s o f these w i t h p r o t i c solvents h a v e b e e n u s e d m o r e a n d m o r e f r e q u e n t l y as m e d i a i n w h i c h t o c a r r y o u t t h e r e a c t i o n , b e c a u s e t h e rate of r e a c t i o n is m u c h h i g h e r i n these solvent systems t h a n i n p r o t i c ones. T h e s e findings a r o u s e d t h e interest o f r e s e a r c h w o r k e r s , so t h a t electro1

To whom correspondence should be addressed. 0-8412-0428-4/79/33-177-077$05.50/l © 1979 American Chemical Society Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

78

T H E R M O D Y N A M I C BEHAVIOR O F E L E C T R O L Y T E S

II

c h e m i c a l studies m u l t i p l i e d w i t h t h e a i m of o b t a i n i n g u s e f u l i n f o r m a t i o n o n ion-solvent a n d i o n - i o n interactions i n aprotic a n d m i x e d p r o t i c a p r o t i c solvents. I n t h i s r e g a r d a s y s t e m a t i c i n v e s t i g a t i o n has b e e n p l a n n e d i n o u r laboratory to study the behavior

of

s e v e r a l electrolytes

i n sulfolane

( T M S ) a n d i n its m i x t u r e s w i t h different p r o t i c solvents. T M S possesses a h i g h v a l u e of t h e d i p o l a r m o m e n t (μ

— 4.65 D . U . ) (I) a n d a d i e l e c t r i c

30

constant of i n t e r m e d i a t e v a l u e ( c

=

30

43.33) ( 2 ) .

T h e s m a l l changes

of

these t w o q u a n t i t i e s o v e r a w i d e t e m p e r a t u r e r a n g e (1 ) shows t h a t T M S is a s c a r c e l y s t r u c t u r e d solvent, e v e n t h o u g h i t has l o w AH a n d A S f u s i o n Downloaded by GEORGETOWN UNIV on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch006

v a l u e s (2.84 c a l / g ( 3 ) a n d 1.1 e u ( 4 ) , r e s p e c t i v e l y ) , w h i c h are c o n n e c t e d t o t h e f a c t t h a t i t first solidifies i n t o p l a s t i c crystals of a

mesomorphic

p h a s e at 2 8 . 4 5 ° C a n d t h e n at 15.43 ° C i n t o a n o n r o t a t i o n a l c r y s t a l l i n e p h a s e w i t h t r a n s i t i o n AS h i g h e r t h a n t h a t of f u s i o n ( 8 e u (4) ). T M S has a l o w a u t o p r o t o l y s i s constant ( p K = 25.45) ( 5 ) , a n d , i n spite of its h i g h d i p o l e m o m e n t , i t has v e r y w e a k a c i d i c a n d b a s i c p r o p e r t i e s ( p K H B

—12.88 ( 6 )

and p K

a

> 31 ( 7 ) .

+

—

T M S has b e e n c h o s e n for o u r studies

b e c a u s e i t appears a n o m a l o u s a m o n g n o n a q u e o u s

solvents since i n t h i s

m e d i u m s e v e r a l ions h a v e v e r y h i g h W a l d e n p r o d u c t s , e v e n h i g h e r t h a n i n w a t e r (8,9,10).

M o r e o v e r , i o n i c a s s o c i a t i o n constants greater t h a n

e x p e c t e d o n t h e basis of its d i e l e c t r i c constant w e r e r e p o r t e d i n l i t e r a t u r e f o r s o m e salts ( 2 1 ) .

These

findings

l e d us t o b e l i e v e t h a t r e s e a r c h o n

i o n i c m o b i l i t y a n d a s s o c i a t i o n to i o n p a i r s e x t e n d e d t o T M S m i x t u r e s w i t h p r o t i c solvents a n d m i g h t p r o v i d e us w i t h some i n t e r e s t i n g results. Experimental

Data

I o n i c m o b i l i t i e s u n d e r c o n s i d e r a t i o n h e r e are b a s e d o n t h e results of c o n d u c t o m e t r i c m e a s u r e m e n t s c a r r i e d o u t o n d i l u t e d solutions ( 1 0 " ^ c ^ 7 . 1 0 ~ m o l / L ) of B u N C l , B u N B r , B u N I , BU4NCIO4, i A m B u N I ( T A B I ) , N a B P h , a n d N a l i n w a t e r - T M S at 3 0 ° C (2,12). Experimental 3

3

4

4

4

3

4

Table I.

Limiting

A,

0 0.0208 0.0744 0.1586 0.3098 0.6080 1

v(cP)

«

0.8004 0.9304 1.282 1.869 2.860 4.988 10.29

76.77 73.95 68.32 62.51 55.79 48.40 43.33

Bu NBr

BuiNCl

h

107.57 91.277 66.297 47.109 31.298 19.12 11.722

105.29 90.782 66.83 45.59 30.36 17.69 12.27

•For B u N C l , B u N B r , B u N I , B u N C 1 0 , iAmaBuNI, N a B P h , and N a l in water-TMS mixtures at 30°C. 4

4

4

4

4

4

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

6.

PETRELLA

Conductance

ET AL.

and Ionic

79

Association

d a t a w e r e a n a l y z e d a c c o r d i n g to F u o s s - O n s a g e r - S k i n n e r t r e a t m e n t (13) a n d t h e v a l u e s of d e r i v e d l i m i t i n g e q u i v a l e n t c o n d u c t a n c e s Λ are s u m m a r i z e d i n T a b l e I t o g e t h e r w i t h T M S m o l e f r a c t i o n x , v i s c o s i t y η, a n d d i e l e c t r i c constant c of solvent m i x t u r e s . L i m i t i n g i o n i c e q u i v a l e n t c o n d u c t a n c e s h a v e b e e n o b t a i n e d o n t h e basis of the h y p o t h e s i s s u g g e s t e d b y C o p l a n a n d F u o s s (14) V ( i A m B u N ) = V ( B P h " ) = [ A ( i A m B u N B P h ) ] / 2 . M o r e o v e r , since i A m B u N B P h ( T A B B P h ) was not s o l u b l e i n w a t e r - s u l f o l a n e m i x t u r e s , t h e v a l u e of its l i m i t i n g e q u i v a l e n t c o n d u c t a n c e has b e e n c a l c u l a t e d b y t h e e q u a t i o n : 0

2

+

3

4

Λ

0

3

(TABBPh ) — Λ 4

0

(TABI) +

4

0

4

Λ

0

3

4

(NaBPh ) 4

Λ

0

(Nal)

(1)

g i v e n t h a t t h e salts T A B I , N a B P h a n d N a l are s o l u b l e i n w a t e r - T M S mixtures. Information on ionic association phenomena have been obtained c o n d u c t o m e t r i c a l l y i n w a t e r - T M S at 3 5 ° C f o r d i l u t e d solutions of L i C l ( 1 5 ) , N a C l ( 1 6 ) , K C 1 ( 1 7 ) , H C 1 ( 1 8 ) , a n d N a C 1 0 ( 1 9 ) . T h e s t u d y of a s s o c i a t i o n to i o n p a i r s has b e e n e x t e n d e d c o n d u c t o m e t r i c a l l y t o d i l u t e d solutions of L i C l i n m e t h a n o l - T M S at 35 ° C ( 2 0 ) , a n d s p e c t r o p h o t o m e t r i c a l l y t o p i c r i c a c i d ( H P i ) i n solutions of e t h a n o l - T M S , a n d tertb u t a n o l - T M S at 3 0 ° C (21). A l s o , i n this case, c o n d u c t i v i t y d a t a w e r e a n a l y z e d b y F u o s s - O n s a g e r - S k i n n e r e q u a t i o n s a n d l i m i t i n g e q u i v a l e n t c o n d u c t a n c e s A a n d a s s o c i a t i o n constants K are c o l l e c t e d i n T a b l e I I t o g e t h e r w i t h p h y s i c a l p r o p e r t i e s of solvent m i x t u r e s . F u r t h e r m o r e , T a b l e I I I shows e t h a n o l and ferf-butanol concentration i n the mixture [ R O H ] , the relevant dielec t r i c constant c, a n d t h e p K of p i c r i c a c i d .


4

4

A

Discussion Ionic Walden Products.

Fundamental work by K a y and Evans

(22)

has s h o w n t h a t a c o r r e c t i n t e r p r e t a t i o n of t h e c o n d u c t o m e t r i c

behavior

of ions i n w a t e r c a n n o t b e m a d e w i t h o u t c o n s i d e r i n g b o t h t h e

complex

t h r e e - d i m e n s i o n a l s t r u c t u r e of w a t e r a n d t h e s t r u c t u r e - b r e a k i n g , s t r u c t u r e - m a k i n g p r o p e r t i e s of ions. O n t h e other h a n d , i f w a t e r is a n a t y p i c a l Equivalent

Conductances

0

A,-0 Bu NI t

106.14 87.46 60.854 43.205 29.744 18.703 9.996

B NClO Ui

95.84 78.76 52.635 37.249 26.34 16.771 9.481

iAm BuNl s

107.43 86.09 59.86 42.29 29.29 18.47 9.80*

NaBPhi 77.63 67.30 49.93 36.46 24.85 14.19

Nal 140.23 118.33 85.67 61.56 41.31 24.25 10.87"

* Value obtained from the data of A . P . Zipp in Ref. 10. •Value obtained from the data of R . Fernandez-Prini and J . E . Prue i n Ref. 11.



Table II.

Limiting Equivalent Conductances and Association Constants 0

η(θΡ)

Χ*

c

Λ

Κ

0

A

Water-TMS


LiCl 0 0.0190 0.0633 0.1522 0.3115 0.4445 0.6259 1

0.7194 0.8225 1.093 1.620 2.574 3.398 4.543 9.033

74.64 72.43 68.05 61.82 54.87 51.08 47.33 42.71

140.22* 122.83 96.19 66.61 42.45 31.51 24.33 15.92

—

— —

2 10 13 36 14595

NaCl 0 0.0206 0.0731 0.1585 0.3119 0.4323 0.6248 0.7443 0.8298

0.7194 0.8304 1.150 1.665 2.602 3.338 4.537 5.466 6.117

74.64 72.27 67.20 61.42 54.69 51.34 47.34 45.60 44.54

153.84" 133.76 101.90 71.85 45.93 35.53 26.10 21.89 19.67

— — — •— 3 7 19 25 40

KCl 0 0.0200 0.1584 0.2735 0.4335 0.6282

0.7194 0.8271 1.665 2.428 3.347 4.555

74.64 72.33 61.43 56.03 51.31 47.30

180.53" 157.93 81.21 55.29 37.58 26.56

3 7

HCl 0 0.0206 0.0731 0.1583 0.3116 0.6248 1

0.7194 0.8304 1.150 1.663 2.602 4.537 0.033

74.64 72.27 67.18 61.42 54.69 47.34 42.71

489.92 426.80 310.33 199.13 99.30 32.63

—


— — —

26 51 76 10*

II

6.

PETRELLA

ET AL.

Conductance

and Ionic

81

Association

Table I I . Continued v(cP)

X*

Λ

€

K

A

0

NaClOj,


0 0.0205 0.0731 0.1573 0.3050 0.6111 1

74.64 72.30 67.24 61.47 55.09 47.57 42.71

0.7194 0.8287 1.145 1.658 2.545 4.441 9.033

142.15 120.94 87.22 61.79 42.99 25.52 11.74

Methanol-TMS LiCl 0 0.0440 0.1566 0.2960 0.4973 0.7927 1

0.4813 0.5178 0.6832 1.010 1.790 4.408 9.033

104.63 99.16 84.19 64.08 42.36 24.30 15.92

30.71 32.37 34.52 36.90 39.27 41.55 42.71

16 25 32 121 14595

For L i C l , N a C l , KC1, HC1, and N a C 1 0 in w a t e r - T M S mixtures and L i C l in methanol-TMS mixtures at 35°C. Ref. 49. Ref. 60. β

4

b

0

s o l v e n t because of its s t r u c t u r a l c h a r a c t e r i s t i c s , w e c a n w h o l l y agree w i t h F . F r a n k s w h e n h e says, " I f t h e r e is one w a y of p r o d u c i n g a s o l v e n t e v e n m o r e e c c e n t r i c a n d a n o m a l o u s t h a n p u r e w a t e r , t h e n this is a c h i e v e d b y t h e a d d i t i o n of s m a l l a m o u n t s of s o m e o r g a n i c c o m p o u n d s " ( 2 3 ) . N u m e r ous c o n d u c t o m e t r i c studies o n electrolytes i n m i x t u r e s of w a t e r

with

o r g a n i c solvents h a v e b e e n r e p o r t e d i n t h e l i t e r a t u r e ; b u t i t is o n l y r e c e n t l y , t h a n k s to K a y a n d B r o a d w a t e r , t h a t i o n - s o l v e n t i n t e r a c t i o n s i n these b i n a r y m i x t u r e s h a v e b e e n s t u d i e d , p a r t i c u l a r l y w i t h respect t o w h a t h a p p e n s i n w a t e r - r i c h m i x t u r e s . T h e y use (24),

teri-butanol

ethanol ( E t O H ) (25), a n d dioxane ( D i o x )

(26)

(terf-BuOH) as co-solvents.

A m o n g these solvents, t h e a l c o h o l s , w h e n a d d e d i n s m a l l a m o u n t s , are a b l e to generate m i x t u r e s w h i c h are m o r e s t r u c t u r e d t h a n p u r e w a t e r . T h i s effect is greater f o r tert-BuOH

than for E t O H .

does n o t e n h a n c e t h e l o n g - r a n g e o r d e r i n w a t e r ( 2 6 ) .

H o w e v e r , dioxane K a y and Broad

w a t e r observe t h a t i n t h e case of a l c o h o l i c m i x t u r e s , i n w a t e r - r i c h r e g i o n s , a l k a l i a n d h a l i d e ions s h o w m a x i m a i n W a l d e n p r o d u c t s . T h i s c o u l d b e e x p l a i n e d b y t h e f a c t t h a t , i n these regions of t h e m i x t u r e s , solvent s t r u c t u r e is h i g h e r t h a n t h a t of p u r e w a t e r , so t h a t t h e m o b i l i t y of s t r u c t u r e -


82

T H E R M O D Y N A M I C BEHAVIOR OF E L E C T R O L Y T E S

II

Table III. T h e p K Values of Picric A c i d in E t h a n o l - T M S and i e r f - B u t a n o l - T M S Mixtures at 3 0 ° C [ROH](mol/L)

€

K

v

HPi

Ethanol-TMS


16.95 15.36 12.07 8.10 5.31 1.93 0

23.8 25.2 28.2 31.8 36.0 40.5 43.3

3.66 3.62 3.72 3.72 4.40 5.54 7.6

£er£-Butanol-TMS 10.46 9.44 7.39 5.33 3.26 1.12

11.5 13.3 18.3 24.6 31.6 39.3

4.60 4.82 4.56 4.22 4.82 5.90

b r e a k i n g ions is e n h a n c e d . H o w e v e r , t h e m o b i l i t y increase is greater f o r s m a l l e r ions, a n d this does n o t agree w i t h w h a t w o u l d b e e x p e c t e d i f t h e m a x i m a w e r e c a u s e d s o l e l y b y s t r u c t u r e - b r e a k i n g effects, g i v e n t h a t these effects are m o r e m a r k e d f o r l a r g e r ions. T h u s K a y a n d B r o a d w a t e r s u g gest t h a t m a x i m a i n W a l d e n p r o d u c t s c a n b e e x p l a i n e d b y

supposing

t h a t the i n t e r a c t i o n s b e t w e e n t h e c o m p o n e n t s of t h e solvent m i x t u r e a n d t h e ions are m a i n l y of t h e a c i d - b a s e t y p e . T h e r e f o r e , as a r e s u l t of i o n i c c h a r g e d e n s i t y a n d the great differences i n a c i d - b a s e p r o p e r t i e s b e t w e e n w a t e r a n d t h e o r g a n i c co-solvent, the i o n c o s p h e r e is e n r i c h e d w i t h w a t e r t o a greater d e g r e e t h a n t h e b u l k m i x t u r e . C o n s e q u e n t l y , t h e ions h a v e a h i g h e r m o b i l i t y since t h e v i s c o s i t y a r o u n d t h e m is less t h a n t h e b u l k v i s c o s i t y , as c a n b e seen f r o m t h e t r e n d o f t h e v i s c o s i t y of t h e t w o s o l v e n t m i x t u r e s (24,25).

S o r t i n g effect is o b v i o u s l y greater f o r s m a l l e r ions a n d

this agrees w i t h e x p e r i m e n t a l results. R e g a r d i n g w a t e r - d i o x a n e mixtures, the m a x i m a observed i n W a l d e n p r o d u c t s are s m a l l e r t h a n i n w a t e r - a l c o h o l m i x t u r e s a n d i n these m i x tures too t h e ions h a v i n g a l o w e r s t r u c t u r e - b r e a k i n g c a p a c i t y s h o w

a

h i g h e r increase i n W a l d e n p r o d u c t s t h a n i n w a t e r . I n t h e case of w a t e r - T M S m i x t u r e s t h e r e is e v i d e n c e i n l i t e r a t u r e t h a t T M S b r e a k s d o w n w a t e r s t r u c t u r e i n w a t e r - r i c h m i x t u r e s . T h i s has b e e n p r o v e d b y studies c o n c e r n i n g t h e influence of s m a l l a d d i t i o n s of T M S o n t h e t e m p e r a t u r e of t h e m a x i m u m d e n s i t y of w a t e r (27),

the heat

of m i x i n g , a n d t h e v a p o r p r e s s u r e of w a t e r - T M S m i x t u r e s ( 2 8 ) .

Figure 1


6.

PETRELLA

ET AL.

Conductance

and Ionic

83

Association

reports i o n i c W a l d e n p r o d u c t s n o r m a l i z e d t o t h e i r v a l u e s i n w a t e r R = [K>^)s]/[\ ^)w] 0

as a f u n c t i o n o f m o l e p e r c e n t T M S f o r N a , C l " , B r " , T , +

CICV, B u N \ T A B \ and BPh ". 4

4

L e t us n o w c o n s i d e r i n o r g a n i c i o n i c b e h a v i o r .

Na

+

shows R v a l u e s

greater t h a n u n i t y t h r o u g h o u t a l m o s t the e n t i r e r a n g e o f t h e solvent c o m position w i t h a m a x i m u m at about 30 m o l % T M S . C I " and B r " , u p to 6 0 m o l % i n T M S , possess n e a r l y constant v a l u e s , a n d are r o u g h l y e q u a l t o those i n w a t e r , w h i l e I " a n d C 1 0 " , w h i c h are t h e b e s t s t r u c t u r e 4

b r e a k i n g ions i n w a t e r , s h o w a m i n i m u m i n W a l d e n p r o d u c t s a t a b o u t 10 m o l % T M S . T h e r e f o r e N a , c o n t r a r y t o a n i o n s , b e h a v e s i n w a t e r Downloaded by GEORGETOWN UNIV on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch006

+

T M S as i t does i n t h e m i x t u r e s s t u d i e d b y K a y a n d B r o a d w a t e r .

MOLE X S U L F O L A N E Journal of Solution Chemistry

Figure 1. Ionic Walden products normalized to their values in water as a function of mole percent TMS at 30°C: (O), TAB* = BPhf; (β), Bufl ; (Μ), Na ; (A), Cl; (Π), Br'; (Α), Γ; (φ), ClOf (12). +

+


84


A m o b i l i t y increase s h o w n b y N a

+

II

w i t h respect t o t h a t i n w a t e r

c a n n o t b e i n t e r p r e t e d b y s u p p o s i n g t h a t its s t r u c t u r e - b r e a k i n g a b i l i t y is e n h a n c e d i n w a t e r - T M S m i x t u r e s , because t h e y are less s t r u c t u r e d t h a n p u r e w a t e r . T h e r e f o r e , t h e b e h a v i o r of N a i o n m a y b e a t t r i b u t e d t o t h e +

s o r t i n g of w a t e r i n its cosphere.

I n fact, T M S possesses v e r y l o w a c i d -

base p r o p e r t i e s , a n d viscosities i n the m i x t u r e s s t e a d i l y increase i n p a s s i n g f r o m w a t e r to T M S , as s h o w n i n T a b l e I . O n the c o n t r a r y the b e h a v i o r of C I " , B r " , Γ , a n d C 1 0 ~ c e r t a i n l y c a n 4

n o t b e e x p l a i n e d b y the s o r t i n g effect since i n t h a t case one w o u l d expect


a m o b i l i t y increase a n d t h u s a n R m a x i m a . O n t h e other h a n d , t h e o b s e r v e d o r d e r i n R values at a b o u t 10 m o l % T M S ( C I " > C10 ")

agrees

4

Br" >

Γ

w i t h t h e i n c r e a s i n g s t r u c t u r e - b r e a k i n g a b i l i t y of

>

ions.

B e c a u s e the a d d i t i o n of T M S b r e a k s d o w n w a t e r s t r u c t u r e , t h e b e h a v i o r of these anions m a y b e e x p l a i n e d b y s u p p o s i n g t h a t t h e i r m o b i l i t y is affected m a i n l y b y s t r u c t u r a l effects. W i t h r e g a r d to anions, i t is i n t e r e s t i n g to c o m p a r e o u r d a t a w i t h those r e p o r t e d b y K a y a n d B r o a d w a t e r . I n F i g u r e 2, R v a l u e s i n H terf-BuOH,

2

0 -

H 0 - E t O H , H 0 - D i o x , a n d H 0 - T M S are p l o t t e d a g a i n s t 2

2

2

m o l e p e r c e n t of n o n a q u e o u s

solvent i n w a t e r - r i c h regions.

A s can be

seen, f o r e a c h solvent m i x t u r e , m o b i l i t y increases w i t h a decrease i n i o n size, i n a g r e e m e n t w i t h the s o r t i n g effect.

A t t h e same t i m e , h o w e v e r , R

v a l u e s f o r e a c h i o n decrease i n p a s s i n g f r o m one m i x t u r e t o a n o t h e r w i t h a decrease i n t h e o r g a n i c solvent's a b i l i t y t o e n h a n c e l o n g - r a n g e o r d e r i n w a t e r - r i c h m i x t u r e s , as e x p e c t e d o n t h e basis of s t r u c t u r a l effects.

We

b e l i e v e t h a t these findings suggest t h a t s o r t i n g a n d s t r u c t u r a l effects exist at t h e same t i m e i n s o l u t i o n a n d t h a t b o t h affect t h e b e h a v i o r of ions i n solution. I t is difficult to e x p l a i n t h e different b e h a v i o r s h o w n i n w a t e r - T M S b y N a a n d h a l i d e s a n d C 1 0 ~ ions; m o r e o v e r , i t w a s n o t f o u n d i n t h e +

4

m i x t u r e s s t u d i e d b y K a y a n d B r o a d w a t e r . T h i s difference p r o b a b l y arose e i t h e r f r o m t h e differences b e t w e e n c a t i o n - T M S a n d a n i o n - T M S i n t e r actions or f r o m t h e fact t h a t t h e s o r t i n g effect p r e v a i l s over s t r u c t u r a l effects for N a , c o n t r a r y to w h a t h a p p e n s t o anions. +

L e t us n o w c o n s i d e r t h e o r g a n i c ions. A s f a r as t h e B u N 4

+

i o n is

c o n c e r n e d , F i g u r e 1 shows t h a t R v a l u e s , after some d e v i a t i o n s f r o m t h e u n i t y w h i c h is p r o b a b l y c a u s e d b y e x p e r i m e n t a l errors, increase w i t h i n c r e a s i n g o r g a n i c solvent p e r c e n t a g e , as K a y a n d B r o a d w a t e r f o u n d for t h i s i o n i n the m i x t u r e s t h e y s t u d i e d . T h i s t r e n d is c a u s e d b y a r e d u c t i o n of h y d r o p h o b i c effects c h a r a c t e r i s t i c of B u N 4

v e r y difficult to e x p l a i n t h e b e h a v i o r of T A B a m i n i m u m at a b o u t 30 m o l %

+

+

i n pure water. It appears

a n d B P h " ions w h i c h s h o w 4

T M S . O n the other h a n d , t h e l a c k of

c o n d u c t o m e t r i c d a t a for these ions i n o t h e r aqueous m i x t u r e s p r e c l u d e s any useful comparison.



6.

PETRELLA E T AL.

Conductance

and Ionic

MOLE % ORGANIC

85

Association

SOLVENT

Figure 2. Ionic Walden products normalized to their values in water as a function of mole percent organic solvents: (- · · · -), tert-BuOH-H 0; ( λ EtOH-H 0; ( λ Diox-H 0; (~ · ~), TMS-H 0; (O), CI'; (Ώ), Br; (A) Γ. 2

2

2

2

f

Association

Phenomena

According

to

the

theoretical model

spheres i n a d i e l e c t r i c c o n t i n u u m t h e ions are r e p r e s e n t e d as

of

rigid,

c h a r g e d spheres t h a t d o n o t i n t e r a c t w i t h solvent, w h i c h is c o n s i d e r e d to b e a m e d i u m w i t h o u t a n y k i n d of s t r u c t u r e . T h e o n l y i n t e r a c t i o n is t h a t w h i c h occurs b e t w e e n t h e ions, a n d t h e f o r m a t i o n of i o n p a i r s is c o n t r o l l e d o n l y b y electrostatic forces.

O n these bases, t h e a s s o c i a t i o n

constant m a y b e expressed b y t h e F u o s s e q u a t i o n (29) :


86


K

=

A

4πΝα

II

3

3000

exp

(2)

\aekT)

T h u s , a c c o r d i n g to this e q u a t i o n , the a s s o c i a t i o n constant increases w i t h a decrease of d i s t a n c e of closest a p p r o a c h of ions a a n d t h e d i e l e c t r i c constant c. A g r a p h of l o g K

A

vs 1/c s h o u l d b e l i n e a r . N e v e r t h e l e s s , sev

e r a l d a t a i n t h e l i t e r a t u r e s h o w t h a t a s s o c i a t i o n to t h e i o n p a i r is n o t expressed a d e q u a t e l y b y E q u a t i o n 2, a n d t h a t s o l u t e - s o l v e n t i n t e r a c t i o n s are f u n d a m e n t a l l y i m p o r t a n t i n d e t e r m i n i n g t h e existence a n d m a g n i t u d e of a s s o c i a t i o n p h e n o m e n a .

T h i s w a s o b s e r v e d also b y us i n t h e m i x t u r e s


of T M S a n d s o m e p r o t i c solvents. A s s o c i a t i o n b e h a v i o r of electrolytes i n these m i x t u r e s c a n b e u n d e r s t o o d b e t t e r b y first e x a m i n i n g t h i s p h e nomena i n pure T M S . Pure T M S .

G i v e n t h e i n t e r m e d i a t e v a l u e of t h e d i e l e c t r i c c o n

stant of T M S , n o n o t i c e a b l e i o n i c a s s o c i a t i o n o n t h e basis of t h e F u o s s equation should be expected.

Moreover, m u c h higher association con

stants t h a n e x p e c t e d b y E q u a t i o n 2 h a v e b e e n o b s e r v e d f o r L i C l ( K =

14595 L / m o l )

(15)

A

3 5

w h i l e the t h e o r e t i c a l v a l u e , c a l c u l a t e d f o r a

°

C

=

2.413, the s u m of c r y s t a l l o g r a p h i c r a d i i of L i a n d C I ' ions, is 7 L / m o l ) . +

T h i s h i g h a s s o c i a t i o n also w a s o b s e r v e d at 30 ° C b y P r i n i a n d P r u e ( K =

13860 L / m o l )

(9)

3 0

A

w h o also r e m a r k e d o n t h e i n a d e q u a c y of

°

C

the

t h e o r y a n d e x p l a i n e d i t i n terms of d i e l e c t r i c s a t u r a t i o n . F u r t h e r m o r e , Garnsey a n d P r u e gave apparent p o l y m e r i z a t i o n numbers for L i C l

(3).

W e b e l i e v e t h a t this h i g h a s s o c i a t i o n constant v a l u e also m a y b e

ex

p l a i n e d b y k e e p i n g i n m i n d t h a t T M S , l i k e other d i p o l a r a p r o t i c solvents, s c a r c e l y solvates a n i o n s , p a r t i c u l a r l y the smallest ones s u c h as C I ' ( 3 0 ) . T h u s c h l o r i d e i o n is p a r t i c u l a r l y r e a c t i v e i n T M S a n d this increases t h e i n t e n s i t y of i n t e r a c t i o n w i t h L i . +

T h e f a i l u r e of the F u o s s e q u a t i o n to r e p r o d u c e e x p e r i m e n t a l d a t a a p p e a r s p a r t i c u l a r l y e v i d e n t b y c o n d u c t o m e t r i c m e a s u r e m e n t s of h y d r o c h l o r i c a c i d (18)

a n d b y s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s of p i c r i c a c i d

(21 ) w h i c h w e h a v e c a r r i e d out i n T M S at 3 5 ° a n d 3 0 ° C , r e s p e c t i v e l y . R e g a r d i n g h y d r o c h l o r i c a c i d , i n a c o n c e n t r a t i o n r a n g e of 30.10" 300.10'

4

4

to

m o l / L , e q u i v a l e n t c o n d u c t a n c e assumes a n e x t r e m e l y l o w a n d

constant v a l u e of 0.03 S c m / m o l , as seen i n F i g u r e 3. 2

This behavior

c e r t a i n l y c a n n o t b e e x p l a i n e d o n t h e basis of s i m p l e d i s s o c i a t i o n p h e n o m e n a . T h u s w e h a v e i n t e r p r e t e d these results o n t h e basis of t h e o r e t i cal work b y Caruso a n d co-workers

(31)

w h o consider the

conducto

m e t r i c , p o t e n t i o m e t r i c , a n d s p e c t r o p h o t o m e t r i c b e h a v i o r of w e a k a c i d s and

bases i n n o n a q u e o u s solvents. I n these solvents a w e a k a c i d , H A ,

besides u n d e r g o i n g s i m p l e i o n i c d i s s o c i a t i o n , also m a y u n d e r g o c o n j u g a tion phenomena b y the H ionic complex

species

+

a n d A " ions w h i c h l e a d to t h e f o r m a t i o n of

A(HA)/"

or H ( H A ) / .

C a r u s o shows t h a t t h e



6.

Conductance

PETRELLA E T AL.

and Ionic

87

Association

Electrochimica Acta

Figure 3.

Conductance

of HCl in pure TMS at 35°C (18)

existence o f these species i n s o l u t i o n reflects o n t h e s h a p e o f — l o g Λ vs. — l o g c, Ε vs. — l o g c, a n d — l o g [ A " ] v s . — l o g c p l o t s i n c o n d u c t a n c e , potentiometry, a n d spectrophotometry, respectively. Particularly regard i n g c o n d u c t o m e t r i c m e a s u r e m e n t s , i f t h e r e is u n i l a t e r a l c o n j u g a t i o n b y the H

+

o r A " ions w i t h a n e u t r a l m o l e c u l e o f t h e a c i d H A , t h e c u r v e

— l o g Λ vs. — l o g c assumes a slope e q u a l t o z e r o , w h i c h a p p r o a c h e s

—0.5

o n l y f o r v e r y l o w v a l u e s o f a c i d c o n c e n t r a t i o n . T h i s b e h a v i o r is o b s e r v e d f o r H C l i n T M S a n d w e suppose t h a t i t is c a u s e d b y t h e H C 1 ~ c o m p l e x 2

species, so t h a t the e q u i l i b r i a e s t a b l i s h e d i n s o l u t i o n a r e : HC1^±H

+

+ C1-

K ^ ^ o ç i -

(

3

)

GHCI

CI" + H C l ? ± H C 1 " 2

K

2

=

a H C l 2

"

&C1 - &HC1

(4)

T h e h y p o t h e s i s o f t h e f o r m a t i o n o f t h e H C 1 " i o n agrees w i t h t h e 2

p r o p e r t i e s w h i c h T M S shows i n f a v o r i n g t h e f o r m a t i o n o f l a r g e h y d r o g e n d i h a l i d e ions r a t h e r t h a n s m a l l h a l i d e i o n s , as f o u n d b y B e n o i t a n d c o w o r k e r s (32,33).

M o r e o v e r , a c i d - a n i o n c o m p l e x e s also w e r e


observed

88


f o r 2 . 6 - d i h y d r o x y l b e n z o i c a c i d i n T M S (34,35)

Π

and for phenol and

p h e n o l a t e i o n i n 3 - m e t h y l - s u l f o l a n e (36), w h i c h h a s a c i d - b a s e

proper

ties s i m i l a r t o those o f T M S . T h e theoretical equation found b y Caruso for the intercept of the l i n e a r p o r t i o n o f t h e p l o t — l o g Λ v s . — l o g c enables us t o e v a l u a t e t h e c o n s t a n t Κχ: intercept = - ^ log 4

Λ ( H HC1 ") +

0

(5)

2


I n E q u a t i o n 5, i n s e r t i n g t h e v a l u e s o f 1.52 f o r t h e i n t e r c e p t , 3 5 0 f o r K (32,33),

a n d 13 f o r A ( H

+

2

H C 1 ) , calculated approximately, o n the 2

_

d a t a f r o m t h e l i t e r a t u r e ( 8 , 3 5 ) , afirst a p p r o x i m a t i o n v a l u e o f 1 0 ' m o l / L 9

w a s o b t a i n e d f o r Κχ. M o r e c o m p l e x e q u i l i b r i a t h a n those c o n c e r n e d w i t h s i m p l e a s s o c i a tion to i o n pair have been observed spectrophotometrically i n T M S i n t h e case o f p i c r i c a c i d . F i g u r e 4 shows t h a t Σ%ο [ ^ ( H P i ) / ] v s . — l o g c

Canadian Journal of Chemistry

Figure 4. Spectrophotometric data of picric acid in TMS. The line is the theoretical curve for equilibria 6-8 (K = 10' - , K = 10 - , and K = 10*) (21). HPi

7

6

(HIH)t

1

Pi(HP4)2


9

6.

PETRELLA E T AL.

(^J

= 0

[Pi(HPi(/]

Conductance

and Ionic

89

Association

is t h e s u m of a n i o n i c p i c r a t e species, a n d c is t h e

a n a l y t i c a l c o n c e n t r a t i o n of H P i ) . A s m a y b e seen, t h e r e are t w o l i n e a r p o r t i o n s w i t h slopes of 0.46 f o r c < 0 . 0 8 M a n d 1.56 f o r c > 0 . 0 8 M . v e r y s i m i l a r g r a p h has b e e n o b s e r v e d b y K o l t h o f f a n d c o - w o r k e r s H P i i n a c e t o n i t r i l e (37)

A for

w i t h slopes of 0.48 a n d 1.45 f o r c o n c e n t r a t i o n s

b e l o w a n d a b o v e 0.1 A i , r e s p e c t i v e l y . T h e a n a l y s i s c a r r i e d o u t b y C a r u s o a n d c o - w o r k e r s (31)

o n these d a t a confirms t h e h y p o t h e s i s m a d e b y K o l t

hoff a n d c o - w o r k e r s t h a t the p r e d o m i n a t i n g species i n a c e t o n i t r i l e at h i g h e r c o n c e n t r a t i o n s is the i o n P i ( H P i ) " .

Moreover,

2

conductometric

measurements on H P i i n T M S have been interpreted b y E l l e r a n d C a r u s o Downloaded by GEORGETOWN UNIV on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch006

p r e s u m i n g t h a t H P i p o l y m e r s are f o r m e d i n solutions ( 3 8 ) .

O n these

bases w e h a v e h y p o t h e s i z e d t h a t p i c r i c a c i d i n T M S too forms the c o m p l e x species P i ( H P i ) " a c c o r d i n g to the f o l l o w i n g r e a c t i o n s : 2

HPi^±H* +

Pi-

2HPi-(HPi)

Κ

K

2

P i " + (HPi) ^±Pi(MPi) 2

Kp

2

(

1 ( H

" "

Η

P

H

P

i ) 2

i

)

[

2

- -

Η

[ Η Κ ] "

1

Ρ

(

= ^Hpffi

6

)

( 7 )

^ - ^ H P i ) ! ]

( 8 )

S p e c t r o p h o t o m e t r i c d a t a h a v e b e e n a n a l y z e d m a t h e m a t i c a l l y as s u g gested b y C a r u s o a n d the t h e o r e t i c a l c u r v e w h i c h fits the e x p e r i m e n t a l data very w e l l was Kp HPi) - = i(

4

2

and H S 0 2

2

obtained

for

10 . C o m p a r i s o n of p K 4

Κ ρι=10 · , Η

A

7

6

K pi) = ( H

2

10" , 1 9

and

values i n T M S for H C 1 0 , H S 0 F , 4

(2.7, 3.3, a n d 5, r e s p e c t i v e l y ( 3 9 ) )

3

w i t h t h e ones o b t a i n e d

f o r H C l ( 9 ) a n d H P i (7.6) shows t h a t the w e a k e s t a c i d i n this solvent is h y d r o c h l o r i c a c i d . T h i s is f u r t h e r p r o o f t h a t i n T M S l a r g e anions are s t a b i l i z e d b y s o l v a t i o n m o r e t h a n l i t t l e ones. T h e results o b t a i n e d for L i C l , H C l , a n d H P i i n T M S s h o w the f u n d a m e n t a l i m p o r t a n c e of the effect of i o n - s o l v e n t i n t e r a c t i o n s o n a s s o c i a t i o n phenomena.

These

effects also h a v e b e e n

evidenced

when studying

T M S - p r o t i c solvent m i x t u r e s w h e r e i o n a s s o c i a t i o n is c o n d i t i o n e d m a i n l y b y t h e presence of T M S . Water-TMS

Conductometric

Mixtures.

studies o n

L i , Na, and

Κ chlorides a n d hydrochloric a c i d i n w a t e r - T M S have s h o w n association constants h i g h e r t h a n t h e F u o s s e q u a t i o n p r e d i c t s i n these m i x t u r e s too. I n t h e case of H C l , K

A

v a l u e s e q u a l to 26 ± 5, 51 ± 9, a n d 76 ± 4 c o r r e

s p o n d e d to d i e l e c t r i c constant v a l u e s of 61.42, 54.69, a n d 47.34, respec tively.

O n the c o n t r a r y , K

A

v a l u e s for t h e same systems c a l c u l a t e d o n

the basis of the F u o s s e q u a t i o n u s i n g the r e a s o n a b l e v a l u e of 4 A f o r t h e


90

T H E R M O D Y N A M I C BEHAVIOR OF

ELECTROLYTES—Π

d i s t a n c e of closest a p p r o a c h of ions a are 1.5, 1.9, a n d 2.8, r e s p e c t i v e l y . T h e difference b e t w e e n e x p e r i m e n t a l a n d t h e o r e t i c a l t r e n d s of l o g K

A

vs.

1 0 0 / c is s h o w n i n F i g u r e 5. I n the case of L i C l , N a C l , a n d K C 1 , F i g u r e 6 enables e x p e r i m e n t a l v a l u e s t o b e c o m p a r e d w i t h t h e o r e t i c a l ones c a l c u l a t e d b y t h e F u o s s equation, substituting for the a parameter the lowest value w i t h a p h y s i c a l m e a n i n g (a = 2.413, t h e s u m of L i a n d C I " c r y s t a l l o g r a p h i c r a d i i ) , +

i n o r d e r to get l a r g e r t h e o r e t i c a l v a l u e s f o r K . A s seen f r o m F i g u r e s 5 A

a n d 6, the o b s e r v e d trends i n a l m o s t t h e w h o l e r a n g e of solvent c o m p o s i


t i o n are l i n e a r b u t w i t h different slopes f r o m the t h e o r e t i c a l one i n the

100/f Figure 5. Dependence of association constants on the dielectric constant for HCl in water-TMS at 35°C: ( ), association constants calculated from the Fuoss equation (a = 4 A).



Figure 6. Dependence of association constants on the dielectric constant for LiCl, NaCl, and KCl in water-TMS mixtures at 35°C: (---), association constants calculated from the Fuoss equation (a = 2.413 A); (O), LiCl; (Π), NaCl; (Δ), KCl.

case of a l k a l i c h l o r i d e s , a m o n g w h i c h those of N a a n d K +

+

are s c a r c e l y

s o l u b l e i n T M S . I n t h e case of L i C l a n d H C l , as t h e s o l v e n t b e c o m e s richer i n T M S , log K

A

a b r u p t l y increases.

H i g h e r a s s o c i a t i o n constants t h a n those e x p e c t e d

o n t h e basis of

E q u a t i o n 2 h a v e b e e n o b s e r v e d also i n s e v e r a l p r o t i c solvents b y E v a n s a n d co-workers

(40),

between cation M

+

who

explained it b y assuming that association

a n d a n i o n A " i n a p r o t i c solvent S H is a t w o - s t e p


92


II

process w h e r e t h e r e is, first, f o r m a t i o n of a s o l v e n t - s e p a r a t e d i o n p a i r a n d t h e n , f o l l o w i n g the loss of a solvent m o l e c u l e b y t h e a n i o n , f o r m a t i o n of a c o n t a c t i o n p a i r . T h i s h a p p e n s a c c o r d i n g to the e q u a t i o n s :

(SH)

m

M + A(SH) " 5 M ( S H ) A ( S H ) +

M(SH) A ( S H ) Downloaded by GEORGETOWN UNIV on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch006

.!

(9)

. i + SH

(10)

n

m + n

_i 5 MA(SH)

m + n

Ki m a y be calculated b y the Fuoss equation a n d K

2

m + n

b e c o m e s l a r g e r as

t h e b o n d b e t w e e n the a n i o n a n d t h e solvent m o l e c u l e w e a k e n s .

The

a s s o c i a t i o n constant o b s e r v e d is t h e r e f o r e :

^^•(' + ϊΕτ)

u

T h i s e q u a t i o n accounts f o r e x p e r i m e n t a l a s s o c i a t i o n constant v a l u e s h i g h e r t h a n t h e o r e t i c a l ones b e c a u s e K t h a t w h e n cations are e q u a l , K

A

> Ki. Moreover, w e m a y deduce

A

is s m a l l e r f o r s m a l l e r a n i o n s , w h i c h m e a n s

t h a t t h e y h a v e a greater c h a r g e d e n s i t y a n d thus are m o r e b a s i c , since K

2

is l o w e r f o r t h e m . O n t h e c o n t r a r y , K

values d e c r e a s i n g w i t h a n

A

increase i n t h e size of t h e a n i o n h a v e b e e n o b s e r v e d i n a p r o t i c solvents l i k e T M S (II), a c e t o n i t r i l e (44),

acetone (41),

nitrobenzene

(42),

a n d 1,1,3,3-tetramethylurea (45).

nitromethane

(43),

T h i s order can be

u n d e r s t o o d i f one considers t h a t i n t h i s class of solvents the anions are s c a r c e l y s o l v a t e d so t h a t a s s o c i a t i o n is affected m a i n l y b y t h e s t r e n g t h of c a t i o n - a n i o n i n t e r a c t i o n , w h i c h , w i t h t h e cations b e i n g e q u a l , increases w i t h a n increase i n the a n i o n c h a r g e d e n s i t y . I n o r d e r to see w h e t h e r t h e h i g h K

A

values w e f o u n d i n w a t e r -

T M S mixtures can be explained b y the multiple-step mechanism sug gested b y E v a n s a n d c o - w o r k e r s w e h a v e s t u d i e d c o n d u c t o m e t r i c b e h a v i o r of N a C 1 0 i n these m i x t u r e s (19).

T h e results of these m e a s u r e m e n t s

4

s h o w t h a t N a C 1 0 is n o t associated, w h e r e a s f o r N a C l a s s o c i a t i o n a p 4

pears to start f r o m w

2

~ 70 w t %

K

A

T M S (c~55).

(C1-) > K

A

So, t h e o r d e r

(CKV)

ob-

(12)

s e r v e d i n w a t e r - T M S is a n a l o g o u s to t h a t f o u n d i n a p r o t i c solvents. I t therefore excludes the f a c t t h a t anions f o r m i n g i o n p a i r s i n w a t e r - T M S m i x t u r e s are p r e f e r e n t i a l l y s o l v a t e d b y w a t e r , since i n t h a t case s o d i u m p e r c h l o r a t e s h o u l d b e m o r e associated t h a n c h l o r i d e , as suggested b y the E v a n s m e c h a n i s m . I n s t e a d i t suggests t h a t i o n i c a s s o c i a t i o n p h e n o m e n o n


6.

PETRELLA

Conductance

ET AL.

and Ionic

93

Association

i n w a t e r - T M S m i x t u r e s are m a i n l y affected b y T M S . I n fact w e

have

a l r e a d y seen t h a t t h e C I " i o n is s c a r c e l y s t a b i l i z e d i n T M S a n d t h u s tends t o f o r m v e r y stable i o n p a i r s . I o n i c a s s o c i a t i o n i n the case of a l k a l i chlorides a n d hydrochloric a c i d i n w a t e r - T M S mixtures m a y

therefore

b e e x p l a i n e d b y a s s u m i n g t h a t the s m a l l f r a c t i o n of c h l o r i d e i o n w h i c h interacts w i t h T M S forms v e r y stable i o n p a i r s , d e s p i t e t h e h i g h v a l u e of t h e d i e l e c t r i c constant. Alcohol-TMS

Furthermore, convincing

Mixtures.

c e r n i n g the i n a d e q u a c y

of t h e t h e o r e t i c a l m o d e l

evidence

con

i n describing

ionic


a s s o c i a t i o n m a y b e o b t a i n e d o n t h e basis of results of studies o n

con-

d u c t o m e t r i c a l b e h a v i o r of L i C l i n M e O H - T M S a n d s p e c t r o p h o t o m e t r i c a l b e h a v i o r of H P i i n E t O H - T M S a n d

terf-BuOH-TMS

mixtures.

R e g a r d i n g L i C l , F i g u r e 7 shows t h a t e v e n i n a l c o h o l i c m i x t u r e s experimental K

A

is h i g h e r t h a n t h e t h e o r e t i c a l one i n n e a r l y the w h o l e

r a n g e of the solvent c o m p o s i t i o n . is t h e n o n c o u l o m b i c

F u r t h e r m o r e , the m o s t i n t e r e s t i n g f a c t

t r e n d of t h e a s s o c i a t i o n constant w h i c h decreases

e v e n t h o u g h the d i e l e c t r i c constant decreases o n p a s s i n g f r o m T M S to m e t h a n o l (c ~ 31).

43)

(t^

G i v e n t h a t L i C l is not associated i n M e O H ,

i o n i c association is a s c r i b a b l e to T M S , w h i c h also occurs i n its m i x t u r e s w i t h water.

T h e t r e n d of l o g K

A

vs. 100/e

also c a n b e e x p l a i n e d b y

a s s u m i n g t h a t i o n - s o l v e n t i n t e r a c t i o n s s t r o n g l y i n f l u e n c e i o n i c associa t i o n , m u c h m o r e t h a n t h e d i e l e c t r i c constant does. T h e K

A

decrease t h a t

o c c u r s w h e n m e t h a n o l is a d d e d to T M S is c a u s e d m a i n l y b y the greater c a p a c i t y of m e t h a n o l as c o m p a r e d w i t h T M S to solvate the ions.

This

is p r o v e d b y t h e s t r o n g l y n e g a t i v e v a l u e s of transfer e n t h a l p i e s

from

TMS

to m e t h a n o l of b o t h L i (-9.1 +

K c a l / m o l ) a n d C l " (-4.8

K cal/mol)

(46). N o n c o u l o m b i c v a r i a t i o n of i o n i c a s s o c i a t i o n w a s o b s e r v e d f o r H P i too, as s h o w n i n F i g u r e 8 b o t h for the E t O H - T M S a n d the TMS

mixtures. L o g K

A

vs. 100/c

is not l i n e a r a n d K

A

tert-BuOH-

decreases i n these

m i x t u r e s w i t h a decrease i n t h e d i e l e c t r i c constant, i n spite of the t h e o r y b a s e d o n electrostatic c o n t r o l of i o n a s s o c i a t i o n (c toH = 23.8,

etert-Buon =

E

11.5). L i k e L i C l i n M e O H - T M S , the greatest decrease of K

A

is o b s e r v e d

w i t h s m a l l a d d i t i o n s of alcohols t h a t are better a b l e to solvate p i c r i c a c i d than T M S w i t h w h i c h they establish hydrogen bonds. is stronger i n a l c o h o l s ( p K ( p K p i = 7.6). H

E T

I n fact p i c r i c a c i d

0 H = 3.7 a n d p K i - B u O H = 4.6) ier

T h e trends of K

A

than i n T M S

values m a y b e e x p l a i n e d b y p r e f e r e n t i a l

s o l v a t i o n of H P i b y the a l c o h o l s . O n this basis a n a t t e m p t w a s m a d e t o e x p l a i n m o r e t h o r o u g h l y t h e k i n d of s o l u t e - s o l v e n t i n t e r a c t i o n s i n t h e case of p i c r i c a c i d . G i v e n t h e n o t i c e a b l e difference b e t w e e n p K v a l u e s i n T M S a n d those i n E t O H a n d

terf-BuOH,

we hypothesized that T M S

b e h a v e s l i k e a n i n e r t solvent a n d p i c r i c a c i d m a i n l y reacts w i t h a l c o h o l s a c c o r d i n g to t h e reactions

(47):



II

H P i + n-FvOH ± ? H P i · n R O H K

HPi · nROH^±ROH

2

+

[HPi · nROH] 1

=

(13)

[HPi][ROH]»

· ROH(„.i)Pi" 2

=

[ROH * · ROH 2

( ( t

.i)Pi-]

[HPi · nROH]


K

Zeitschrift fur Naturforschung

Figure 7. Dependence of association constants on the dielectric constant for LiCl in methanol-TMS mixtures at 35°C: ( ), association constants calculated from the Fuoss equation (a = 2.413 A) (20).


(14)


6.

PETRELLA E T AL.

Conductance

and Ionic

95

Association


Figure 8. Association constants of picric acid in ethanol-TMS and tertbutyl alcohol-TMS mixtures: (O), EtOH-TMS; (9), tert-BuOH-TMS (21).

ROH

2

+

·ROH^.DPi-^RO^

+ ROHin.DPi [ROH^tROH^pPi-] · R O H ^ . D P I - ] -

[ROW

U

O

J

T h u s p i c r i c a c i d reacts w i t h η a l c o h o l m o l e c u l e s f o r m i n g a c o m p l e x ( E q u a t i o n 13) w h i c h rearranges t o f o r m a n i o n p a i r ( E q u a t i o n 14) w h i c h t h e n dissociates ( E q u a t i o n 15 ). T h e processes i n v o l v e d i n E q u a t i o n s 13 a n d 14 are c o n d i t i o n e d b y specific s o l u t e - s o l v e n t i n t e r a c t i o n s , w h i l e t h e process d e p i c t e d b y E q u a t i o n 15 is c o n t r o l l e d b y electrostatics. equal to 1 / K , where K A

A

K

3

is

is t h e t h e o r e t i c a l association constant g i v e n b y

E q u a t i o n 2. I t c a n b e easily s h o w n t h a t t h e e x p e r i m e n t a l a s s o c i a t i o n constant is g i v e n b y t h e e q u a t i o n :

Atexp) — [

R

0

H

] »

K

l

K

2

-


(16)


96


Π


Figure 9. Test of Equation 17 ( n = 4) for ionization of picric acid in ethanol-TMS and text-butyl alchol-TMS mixtures: (O), EtOH-TMS; tert-BuOH-TMS (21). E q u a t i o n 16 m a y b e r e w r i t t e n as f o l l o w s : logK

A ( e x p )

l

0

g

+nlog[ROH] =

-3ÔÔÔ- "

l

0

g

2

(

+ ïôô^kT ( τ " )

1

7

)

I t m a y b e a s s u m e d t h a t p i c r i c a c i d reacts w i t h f o u r a l c o h o l m o l e cules. O n e o f t h e m n e u t r a l i z e s t h e a c i d f o r m i n g t h e i o n R O H

+ 2

a n d the

r e m a i n i n g t h r e e m o l e c u l e s solvate t h e p i c r a t e a n i o n b y h y d r o g e n b o n d s w i t h the three nitro groups. D ' A p r a n o a n d Fuoss have observed a similar m e c h a n i s m i n m i x t u r e s o f a c e t o n i t r i l e a n d p r o t i c solvents (48). I f this h y p o t h e s i s is correct, w h e n s u b s t i t u t i n g t h e v a l u e o f f o u r f o r η i n E q u a t i o n 17, a g r a p h o f l o g K

MeTp)

straight line.

+ 4 l o g [ R O H ] vs. 100/c should b e a

T h i s , i n fact, c a n b e seen f o r b o t h s o l v e n t m i x t u r e s , as

s h o w n i n F i g u r e 9. Literature Cited 1. Lamanna, U., Sciacovelli, O., Jannelli, L., Gazz. Chim. Ital. (1966) 96, 114. 2. Petrella, G., Castagnolo, M., Sacco, Α., DeGiglio, Α., J. Solution Chem. (1976) 5, 621.



6.

PETRELLA ET AL.

Conductance and Ionic Association

97

3. Garnsey, R., Prue, J. E., Trans. Faraday Soc. (1968) 64, 1206. 4. Della Monica, M., Jannelli, L., Lamanna, U., J. Phys. Chem. (1968) 72, 1068. 5. Kreshkov, A. P., Aldarova, N. S., Tanganov, Β. B., Zh. Fiz. Khim. (1970) 44, 2089; Chem. Abstr. (1970) 73, 124160. 6. Hall, S. K., Robinson, Ε. Α., Can. J. Chem. (1964) 42, 1113. 7. Bordwell, F. G., Imes, R. H., Steiner, E . C.,J.Am. Chem. Soc. (1967) 89, 3905. 8. Della Monica, M., Lamanna, U., Senatore, L., J. Phys. Chem. (1968) 72, 2124. 9. Della Monica, M., Lamanna, U.,J.Phys. Chem. (1968) 72, 4329. 10. Zipp, A. P.,J.Phys. Chem. (1973) 5, 718. 11. Fernández-Prini, R., Prue, J. E., Trans. Faraday Soc. (1966) 62, 1257. 12. Petrella, G., Sacco, Α., Castagnolo, M., Della Monica, M., De Giglio, Α., J. Solution Chem. (1977) 6, 13. 13. Fuoss, R. M., Onsager, L., Skinner, J. T.,J.Phys. Chem. (1965) 69, 2581. 14. Coplan, Μ. Α., Fuoss, R. M., J. Phys. Chem. (1964) 68, 1177. 15. Petrella, G., Castagnolo, M., Sacco, Α., Lasalandra, L., Z. Naturforsch., Teil A (1972) 27, 1345. 16. Castagnolo, M., Jannelli, L., Petrella, G., Sacco, Α., Ζ. Naturforsch., Teil A (1971) 26, 755. 17. Sacco, Α., Petrella, G., Castagnolo, M., Z. Naturforsch., Teil A (1971) 26, 1306. 18. Castagnolo, M., Petrella, G., Electrochim. Acta (1974) 19, 855. 19. Petrella, G., Castagnolo, M., Sacco, Α., Ζ. Naturforsch., Teil A (1975) 30, 533. 20. Petrella, G., Castagnolo, M., Z. Naturforsch., Teil A (1973) 28, 1149. 21. Castagnolo, M., De Giglio, Α., Dell'Atti, Α., Petrella, G., Can.J.Chem. (1975) 53, 1651. 22. Kay, R. L., Evans, D. F.,J.Phys. Chem. (1966) 70, 2325. 23. Franks, F., "Physico-Chemical Processes in Mixed Aqueous Solvents," F. Franks, Ed., p. 50, Heineman, London, 1967. 24. Broadwater, T. L., Kay, R. L., J. Phys. Chem. (1970) 74, 3802. 25. Kay, R. L., Broadwater, T. L., J. Solution Chem. (1976) 5, 57. 26. Kay, R. L., Broadwater, T. L., Electrochim. Acta (1971) 16, 667. 27. Macdonald, D. D., Smith, M. D., Hyne, J. B., Can. J. Chem. (1971) 49, 2818. 28. Benoit, R. L., Choux, G., Can. J. Chem. (1968) 46, 3215. 29. Fuoss, R. M., J. Am. Chem. Soc. (1958) 80, 5059. 30. Parker, A. J., Chem. Rev. (1969) 69, 1. 31. Sellers, N. G., Eller, P. M. P., Caruso, J. Α., J. Phys. Chem. (1972) 76, 3618. 32. Benoit, R. L., Beauchamp, A. L., Domain, R., Inorg. Nucl. Chem. Lett. (1971) 7, 557. 33. Benoit, R. L., Rinfret, M., Domain, R., Inorg. Chem. (1972) 11, 2603. 34. Coetzee, J. F., Bertozzi, R. J., Anal. Chem. (1973) 45, 1604. 35. Ibid. (1971) 43, 961. 36. Morman, D. H., Harlow, G. Α., Anal. Chem. (1967) 39, 1869. 37. Kolthoff, I. M., Bruckenstein, S., Chantooni, M., J. Am. Chem. Soc. (1961) 83, 3927 38. Eller, P. M. P., Caruso, J. Α., Can. J. Chem. (1973) 51, 448. 39. Benoit, R. L., Buisson, C., Choux, G., Can. J. Chem. (1970) 48, 2353. 40. Evans, D. F., Matesich, S. Μ. Α., "The Physical Chemistry of Aqueous Systems," R. L. Kay, Ed., p. 95, Plenum, New York and London, 1973. 41. Evans, D. F., Thomas, J., Nadas, J. Α., Matesich, S. Μ. Α.,J.Phys. Chem. (1971) 75, 1714. 42. Witschonke, C. R., Kraus, C. Α.,J.Am. Chem. Soc. (1947) 69, 2472.


98

THERMODYNAMIC BEHAVIOR OF ELECTROLYTES II

43. 44. 45. 46. 47.

Wright, C. P., Murray-Rust, D., Hartley, H.,J.Chem. Soc. (1931) 199. Evans, D. F., Zavoyski, C., Kay, R. L.,J.Phys. Chem. (1965) 69, 3878. Barker, B. J., Caruso, J. Α.,J.Am. Chem. Soc. (1971) 93, 1341. Choux, G., Benoit, R. L.,J.Am. Chem. Soc. (1969) 91, 6221. Shedlovsky, T., "Electrolytes," B. Pesce, Ed., p. 146, Pergamon, New York, 1962. 48. D'Aprano, Α., Fuoss, R. M.,J.Phys. Chem. (1969) 73, 400. 49. Kay, R. L.,J.Am. Chem. Soc. (1960) 82, 2099. 50. Goffredi, M., Shedlovsky, T.,J.Phys. Chem. (1967) 71, 2176.


RECEIVED February 21, 1978.


and Protic Solvents - American Chemical Society

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