Preferential Solvation of Some Electrolytes by Water and Diethyl Ether

10 Preferential Solvation of Some Electrolytes by Water and Diethyl Ether in Sulfolane Downloaded by UNIV OF NEW SOUTH WALES on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch010

E. MILANOVA, S. Y. LAM, B. DESJARDINS, and R. L . BENOIT Département de Chimie, Université de Montréal, Montréal, Québec, Canada

Comparisons are made of the effects of water and diethyl ether—two oxygen bases (R O)—on the thermodynamic properties of acids in sulfolane. The acids (HA) include HSbCl , HCF SO , HCH SO , HCF CO , and HCH CO . Vapor pressure and conductivity measurements as well as calorimetric determinations were made on 0.1-0.5M acid solutions as a function of R O concentration up to 1M (XR O < 0.1). Equilibrium constants and enthalpy changes are obtained for the formation of both ionic species H (R O) and uncharged species R O · · HA. The proton solvates H (R O) have n = 1, 2, 3, 4 . . . for H O but only n = 1 for Et O. The species H O · · HA are more stable than Et O · · HA. The formation of H O is more exothermic than that of Et OH . These results are discussed in relation to gas phase data. The very large solvation enthalpy of gaseous H O is noteworthy. 2

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Π Ρ h e s t u d y of t h e b e h a v i o r o f electrolytes i n m i x e d solvents is c u r r e n t l y a r o u s i n g c o n s i d e r a b l e interest because of its p r a c t i c a l a n d f u n d a m e n t a l i m p l i c a t i o n s (1 ) . A m o n g t h e s i m p l e r b i n a r y s o l v e n t m i x t u r e s , those w h e r e w a t e r is o n e c o m p o n e n t are o b v i o u s l y o f p r i m a r y i m p o r t a n c e .

W e have

r e c e n t l y c o m p a r e d t h e effects of s m a l l quantities o f w a t e r o n the t h e r m o d y n a m i c properties o f selected 1:1 electrolytes i n sulfolane, a c e t o n i t r i l e , p r o p y l e n e carbonate, a n d d i m e t h y l s u l f o x i d e ( D M S O ) .

These four c o m

p o u n d s b e l o n g to t h e d i p o l a r a p r o t i c ( D P A ) class o f solvents t h a t has r e c e i v e d a great d e a l o f a t t e n t i o n ( 2 ) b e c a u s e o f t h e i r w i d e u s e as m e d i a for p h y s i c a l separations a n d c h e m i c a l a n d e l e c t r o c h e m i c a l reactions. W e i n t e r p r e t e d o u r v a p o r pressure, c a l o r i m e t r y , a n d N M R results i n terms o f p r e f e r e n t i a l s o l v a t i o n o f s m a l l cations a n d anions b y w a t e r a n d o b t a i n e d 0-8412-0428-4/79/33-177-145$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.

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THERMODYNAMIC BEHAVIOR OF ELECTROLYTES

II

free e n e r g y a n d e n t h a l p y v a l u e s for the b i n d i n g of the first w a t e r m o l e cule.

T h e b a s i c i t y of t h e solvent w a s f o u n d to b e t h e m a i n f a c t o r i n

d e t e r m i n i n g the energetics of the b o n d f o r m a t i o n ( 3 ) . T h e w o r k w i t h w h i c h w e are chiefly c o n c e r n e d h e r e is a n extension of these i n v e s t i g a t i o n s of t h e effects of w a t e r o n t h e t h e r m o d y n a m i c p r o p e r t i e s of electrolytes i n D P A solvents.

T h e electrolytes

considered

are acids ( H A ) , w h o s e i m p o r t a n c e as a class of electrolytes derives f r o m t h e i r i n v o l v e m e n t i n m a n y c h e m i c a l reactions, e i t h e r as reactants or as

Downloaded by UNIV OF NEW SOUTH WALES on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch010

catalysts. I n c o n j u n c t i o n w i t h these i n v e s t i g a t i o n s , a p a r a l l e l s t u d y w a s c a r r i e d out; w a t e r w a s r e p l a c e d b y d i e t h y l ether ( E t 0 )

to d e t e r m i n e

2

t h e extent to w h i c h the h y d r o g e n b o n d d o n o r p r o p e r t i e s of t h e w a t e r m o l e c u l e affect the i n t e r a c t i o n s b e t w e e n H A , H 0 , a n d the solvent.

For

2

c o m p a r i s o n , s o m e a d d i t i o n a l experiments w e r e i n c l u d e d t h a t u s e d as electrolytes a l i t h i u m salt a n d a c h l o r i d e salt a n d H S i n s t e a d of H 0 . 2

2

S u l f o l a n e ( t e t r a m e t h y l e n e s u l f o n e ) w a s selected as the most s u i t a b l e D P A solvent f o r this w o r k b e c a u s e ( 1 ) its v e r y l o w v o l a t i l i t y m a k e s o u r t o t a l v a p o r pressure m e a s u r e m e n t s easier to i n t e r p r e t , a n d ( 2 ) its w e a k b a s i c i t y favors b o t h a h i g h e r a c t i v i t y of w a t e r a n d stronger i n t e r a c t i o n s b e t w e e n w a t e r a n d acids. T h e acids s t u d i e d r a n g e f r o m s t r o n g to w e a k ( i n sulfolane) a n d include hexachloroantimonic a c i d ( H S b C l ) , trifluoroe

methanesulfonic

acid

trifluoroacetic a c i d

(HCF3SO3), methanesulfonic acid ( H C H S 0 ) , 3

(HCF C0 ), 3

s i m p l i c i t y , the f o r m u l a H S b C l

a n d acetic a c i d

2

6

(HCH C0 ). 3

2

3

(For

is u s e d to represent the h y p o t h e t i c a l a c i d

f o r m e d w h e n a n t i m o n y p e n t a c h l o r i d e is a d d e d to H C l solutions.

The

h e x a c h l o r o a n t i m o n a t e a n i o n , S b C l ~ , is w e l l c h a r a c t e r i z e d , a n d salts s u c h 6

as E t O H S b C l " are k n o w n ( 4 ) . ) 2

+

T h e l i t h i u m a n d c h l o r i d e salts w e r e

6

lithium perchlorate ( L i C 1 0 ) and tetraethylammonium chloride ( N E t C l ) . 4

4

T h e experiments i n c l u d e m e a s u r e m e n t s of t o t a l v a p o r pressure a n d elec t r i c a l c o n d u c t i v i t y as w e l l as c a l o r i m e t r i c d e t e r m i n a t i o n s o n 0 . 1 - 0 . 5 M electrolyte solutions as a f u n c t i o n of a d d e d w a t e r a n d ether c o n c e n t r a tions u p to 1 M ( X ~ Experimental

0.1).

Section

M a t e r i a l s . Sulfolane ( 9 9 % p u r i t y ) ( A l d r i c h ) was treated w i t h c a l c i u m h y d r i d e a n d d i s t i l l e d u n d e r r e d u c e d pressure. T h e f r e s h l y p r e p a r e d solvent h a d a specific c o n d u c t i v i t y of 1.0 Χ ΙΟ" Ω" c m " a n d a r e s i d u a l w a t e r content of 8 X 1 0 " M as d e t e r m i n e d b y K a r l F i s h e r t i t r a t i o n . C o n d u c t i v i t y w a t e r a n d reagent g r a d e ether ( B a k e r ) w e r e u s e d . G l a c i a l acetic a c i d ( C I L ) , t r i f l u o r o a c e t i c a c i d ( B a k e r ) , a n d t r i f l u o r o m e t h a n e s u l f o n i c a c i d ( 3 M ) w e r e u s e d as r e c e i v e d . A l l these acids h a d a m i n i m u m p u r i t y of 9 9 . 5 % as d e t e r m i n e d b y t i t r a t i o n w i t h s t a n d a r d sodium hydroxide. Methanesulfonic acid ( E a s t m a n ) , distilled under r e d u c e d pressure, h a d a p u r i t y of 9 9 . 6 % . S u l f o l a n e solutions of these acids w e r e p r e p a r e d b y w e i g h t , a n d the a c i d concentrations w e r e c h e c k e d b y a c i d i m e t r y after the samples w e r e flooded w i t h w a t e r . T h e solutions 7

1

3


1

10.

MiLANOVA E T AL.

Preferential

147

Solvation

of H S b C l i n s u l f o l a n e w e r e p r e p a r e d b y a d d i n g k n o w n a m o u n t s o f S b C l ( 9 9 . 9 % ) ( B a k e r ) to H C l solutions of k n o w n c o n c e n t r a t i o n , w h i c h , in turn, were obtained b y b u b b l i n g anhydrous H C l ( L i q u i d A i r ) i n the solvent. T e t r a e t h y l a m m o n i u m t r i f l u o r o m e t h a n e s u l f o n a t e w a s p r e p a r e d b y n e u t r a l i z i n g a 2 5 % aqueous s o l u t i o n of N E t O H ( B D H ) w i t h H C F 3 S O 3 . N E t C F S 0 is v e r y s o l u b l e i n w a t e r ( ~ 2 2 m o l / L of w a t e r at 2 5 ° C ) , i n d i c h l o r o m e t h a n e ( ~ 6.7 m o l / L of solvent at 2 5 ° C ) , a n d i n c h l o r o f o r m ( ~ 8.1 m o l / L of solvent at 2 5 ° C ) b u t a l m o s t i n s o l u b l e i n C C 1 . C o n s e q u e n t l y , N E t C F S 0 w a s e x t r a c t e d f r o m its a q u e o u s s o l u t i o n b y t w o p o r t i o n s of d i c h l o r o m e t h a n e . T h e r e s u l t i n g extracts w e r e c o m b i n e d a n d filtered, a n d t h e solvent w a s e v a p o r a t e d . T h e salt w a s r e d i s s o l v e d i n c h l o r o f o r m a n d r e p r e c i p i t a t e d b y a d d i t i o n of C C 1 . T h e d r i e d salt h a d a m e l t i n g p o i n t of 164.5°C. T e t r a e t h y l a m m o n i u m c h l o r i d e ( E a s t m a n ) a n d l i t h i u m p e r c h l o r a t e ( S m i t h ) w e r e d r i e d i n v a c u u m desiccators o v e r phosphorus pentoxide. Calorimetry. H e a t s of s o l u t i o n w e r e m e a s u r e d at 3 0 ° C u s i n g a n L K B m o d e l 8725-2 i s o p e r i b o l c a l o r i m e t e r . D e t a i l s of t h e p r o c e d u r e h a v e b e e n g i v e n ( 5 ) . T h e heats of m i x i n g k n o w n w e i g h t s of w a t e r o r ether w i t h 2 5 m l of 0 . 1 - 0 . 5 M a c i d solutions i n s u l f o l a n e w e r e d e t e r m i n e d . S o m e of t h e a c i d solutions c o n t a i n e d some k n o w n concentrations o f w a t e r as w e l l . I n a d d i t i o n , t h e heats of s o l u t i o n of N E t C F S 0 i n s u l f o l a n e a n d i n a 0 . 2 1 1 M H S b C l s o l u t i o n i n sulfolane w e r e d e t e r m i n e d at t h r e e c o n centrations b e t w e e n 0.02 a n d 0 . 0 4 M . Vapor Pressure Measurements. T o t a l v a p o r pressures w e r e m e a s u r e d at 30 ° C w i t h a Texas I n s t r u m e n t s q u a r t z s p i r a l gauge. T h e p r o c e d u r e u s e d w a s s i m i l a r to t h a t g i v e n p r e v i o u s l y ( 5 ) . T h e c o n c e n t r a t i o n of w a t e r after t h e e x p e r i m e n t w a s c h e c k e d b y K a r l F i s c h e r t i t r a t i o n , w h i l e t h a t o f ether w a s f o u n d b y w e i g h i n g t h e c e l l before a n d after t h e v a p o r pressure d e t e r m i n a t i o n ; t h e loss of w e i g h t w a s that of ether. S i n c e S b ( V ) interferes w i t h the K a r l F i s c h e r t i t r a t i o n of w a t e r , t h e w a t e r c o n c e n t r a t i o n i n t h e H S b C l solutions w a s also o b t a i n e d f r o m t h e loss of w e i g h t of the c e l l . Conductivity. T h e c o n d u c t i v i t y b r i d g e has b e e n d e s c r i b e d ( 6 ) . A 0 . 0 1 0 0 0 M s t a n d a r d K C l s o l u t i o n w a s u s e d to c a l i b r a t e t h e B e c k m a n c o n d u c t i v i t y c e l l . A v a l u e o f 0.488 c m " w a s o b t a i n e d f o r t h e c e l l constant. M e a s u r e m e n t s w e r e m a d e at 3 0 ° C o n 50 m l of 0 . 1 - 0 . 5 M a c i d solutions i n s u l f o l a n e w i t h successive a d d i t i o n s of w a t e r o r ether f r o m a 2 - m l T e f l o n glass s y r i n g e . A l l a c i d solutions w e r e p r o c e s s e d i n a g l o v e b o x . 6

5

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0

1

Results T h e results of s o m e a n c i l l a r y e x p e r i m e n t s to e s t a b l i s h t h e n a t u r e o f the solutions of H C F S 0 3

3

and H S b C l

6

i n sulfolane are c o n s i d e r e d

A v a l u e o f t h e i o n i z a t i o n constant K ° of H C F S 0 d

3

the specific c o n d u c t a n c e L of 0 . 1 - 0 . 2 M H C F S 0 3

3

3

first.

was estimated from

solutions, e x t r a p o l a t e d

to z e r o w a t e r c o n c e n t r a t i o n , a n d f r o m values of t h e e q u i v a l e n t c o n d u c t a n c e Λ of H C F S 0 " , o b t a i n e d , i n t u r n , f r o m t h e e q u i v a l e n t c o n d u c t +

3

3

ances of H S b C l ~ , a s t r o n g electrolyte i n sulfolane +

and N E t

4

+

6

CF S0 ". 3

3

(7), NEt SbCl ", 4

+

6

B y e x t r a p o l a t i o n i t is n o t p o s s i b l e to o b t a i n t h e

equivalent conductance c^rj^|(g^p{)^gp||^te dilution because of the

Society Library 1155 16th St. N. W. Furter; Thermodynamic Behavior of in Mixed Solvents—II Washington, D.Electrolytes C. 20036

Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

148

THERMODYNAMIC BEHAVIOR O F ELECTROLYTES

II

s t r o n g i n f l u e n c e of r e s i d u a l w a t e r o n t h e d i s s o c i a t i o n o f t h e a c i d .

Using

t h e r e d u c e d D e b y e - H i i c k e l E q u a t i o n f o r t h e a c t i v i t y coefficients

of H

+

a n d CF3SO3", t h e a c t i v i t y constant K ° f o r HCF3SO3 w a s c a l c u l a t e d t o b e d

2.5 X

1 0 " , so t h a t HCF3SO3 is n e a r l y as s t r o n g a n a c i d as HCIO4 i n 4

s u l f o l a n e (7,8).

T r i f l u o r o m e t h a n e s u l f o n i c a c i d is therefore p r e f e r r e d t o

p e r c h l o r i c a c i d as a reagent f o r a c i d - b a s e

titrations i n sulfolane

(9).

T h e r i s k of explosions w h e n h a n d l i n g s o l u t i o n of HCIO4 i n s u l f o l a n e ( 7 ) must again b e forcefully emphasized. measurements o n H C F S 0


3

3

T h e results o f o u r v a p o r pressure

solutions i n s u l f o l a n e i n d i c a t e t h a t t h e p r e s

sure, Ρ ( m m H g ) , is p r o p o r t i o n a l to t h e a c i d c o n c e n t r a t i o n , C

H

A (M),

up

t o 0 . 2 M . T h e pressures, P , a r e : 0.16 m m ( 0 . 1 0 M ) ; 0.33 m m ( 0 . 2 0 M ) ; 0.57 m m ( 0 . 5 0 M ) ; a n d 0.71 m m ( L O O M ) . T h e v a p o r pressure o f p u r e

HCF3SO3 at

3 0 ° C w a s m e a s u r e d as 3.01 m m , so t h e o b s e r v e d

negative

d e v i a t i o n f r o m H e n r y ' s L a w at i n c r e a s i n g c o n c e n t r a t i o n s of H C F S 0 3

expected.

( T h i s v a l u e is i n g o o d a g r e e m e n t o n a l o g Ρ v s . 1 / Γ p l o t w i t h

v a p o r pressures o b t a i n e d at three h i g h e r t e m p e r a t u r e s (11) i n v a l i d a t e s t h e v a l u e of 1.0 m m H g at 4 2 ° C (11).) (HCF S0 ) 3

3

2

a n d therefore

F o r m a t i o n of a d i m e r

at h i g h c o n c e n t r a t i o n s m i g h t e x p l a i n t h e d e v i a t i o n .

HCF3SO3

z a t i o n of Kolthoff

is

3

i n a c e t o n i t r i l e solutions

has b e e n

Dimeri-

postulated

( J O ) o n t h e basis o f c o n d u c t i v i t y m e a s u r e m e n t s

by

although the

i n f l u e n c e of r e s i d u a l w a t e r w a s a p p a r e n t l y n o t t a k e n i n t o a c c o u n t i n t h e interpretation. kcal mol"

1

T h e heats of s o l u t i o n o f s o l i d N E t C F S 0 4

3

3

were +2.17

i n p u r e sulfolane a n d + 0 . 3 6 k c a l m o l " i n a 0 . 2 1 2 M 1

s o l u t i o n i n s u l f o l a n e ; t h u s , t h e heat of i o n i z a t i o n AH °

of

d

HSbCl

HCF3SO3

6

is

+1.81 kcal mol" . 1

C o n c e r n i n g t h e n a t u r e of H S b C l reported ( 7 ) that a d d i n g S b C l

5

solutions i n s u l f o l a n e , w e h a v e

6

(1) to w e a k l y c o n d u c t i n g solutions of

H C l leads to a n i n i t i a l l i n e a r increase o f L w i t h C * b c i a n d a n e x t r a p o l a t e d S

v a l u e of Λ ° n e a r 11.5 Ω" c m e q u i v ' . 1

2

5

W e t o o k this to i n d i c a t e t h a t t h e

1

e q u i l i b r i u m constant K f o r R e a c t i o n 1 is l a r g e t

H C l + SbCl a n d that H S b C l

5

— HSbCl

is a s t r o n g e l e c t r o l y t e .

6

(1)

6

Additional conductivity

deter

minations, i n w h i c h w e varied the initial H C l concentration a n d a d d e d h i g h e r c o n c e n t r a t i o n s of S b C l , l e d us to estimate f o r K

t

5

a v a l u e of t h e

o r d e r o f 1 0 . F u r t h e r v a p o r pressure m e a s u r e m e n t s o f solutions c o n t a i n 25

i n g H C l a n d S b C l , w i t h m o l a r ratios C b c i / C c i s l i g h t l y a b o v e 1, s h o w e d 5

S

5

H

a r e s i d u a l H C l v a p o r pressure that w a s u s e d to estimate t h e H C l c o n c e n t r a t i o n f r o m H e n r y ' s L a w constant f o r H C l ( 5 ) . T h e v a l u e of K

t

calculated, 10 · 2

7±0

thus

· , was retained. 2

W e n o w e x a m i n e t h e v a p o r pressure, c o n d u c t i v i t y , a n d c a l o r i m e t r y results o b t a i n e d w h e n w a t e r a n d ether w e r e a d d e d t o t h e a c i d s o l u t i o n s . T h e v a p o r pressure d a t a i n F i g u r e 1 g i v e t h e m e a s u r e d t o t a l pressure,



10.

MiLANOVA E T A L .

Preferential

149

Solvation

Figure 1. Vapor pressure of sulfolane solutions of electrolytes as a function of water concentration at 30°C: (O), sulfolane; (A), 0.50M HCH C0 ; (-Ο-), 0 . 5 0 M HCF C0 ; (Δ), 0.53M HCH S0 ; (®), 0.Ι0Μ HCF SO ; (Β), 0.20M HCF S0 ; (Π), 0.50M HCF S0 ; (A), 0.50M NEtfil; (O), 0.51M LiClO 3

3

2

S

3

3

3

2

s

3

3

3

k

Ρ ( m m H g ) , as a f u n c t i o n of t h e m o l a r c o n c e n t r a t i o n of a d d e d w a t e r , C

H 2

o , for 0 . 1 - 0 . 5 M a c i d solutions i n s u l f o l a n e .

The corresponding data

o b t a i n e d f o r a d d e d e t h e r a r e p l o t t e d i n F i g u r e 2. T h e v a p o r p r e s s u r e of p u r e s u l f o l a n e at 30 ° C is 0.02 m m H g , so t h e t o t a l pressure, P , c a n b e t a k e n as t h e s u m of t h e v a p o r pressures of u n b o u n d w a t e r or

(PH O or p E t o ) a n d a c i d 2

2

ether

A l t h o u g h t h e c o n t r i b u t i o n of p A to Ρ

(PHA).

H

is s m a l l b e c a u s e t h e acids H A s t u d i e d h a v e a l o w v o l a t i l i t y , i t is t h e decrease of p A t h a t e x p l a i n s the i n i t i a l s m a l l v a r i a t i o n s of Ρ w i t h w a t e r H

concentration w h e n 0
Κ ^ ; , sulfolane; (+), 0.50M HCF S0 ; (O), 0.48MHCH SO ; (Q) 0.51M HCF S0 ; (A), 0.44M HCF,SoJ(n) 0.25M HCF SO ; (·), 0.20U HsUi^C^C^ = 1.00)? s

f v

S

S

9

s

s

s

c o n c e n t r a t i o n the v a l u e s of p HCF C0 3

< HCH S0

2

2

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Preferential Solvation of Some Electrolytes by Water and Diethyl Ether

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