7 Enthalpies of Solution of Some Electrolytes Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 10/26/17. For personal use only.
with a Large Cation in Mixtures of Water with Aprotic Solvents C. DE VISSER, W. J. M. HEUVELSLAND, and G. SOMSEN Department of Chemistry, Free University of Amsterdam, De Boelelaan 1083, Amsterdam, The Netherlands
Measured enthalpies of solution of tetra-n-butylammonium bromide (Bu4NBr)in mixtures of water (W) with acetonitrile (ACN) and with ethylene carbonate (EC) are compared with those in mixtures of water with some other aprotic solvents which were reported earlier. The results can be fairly well described by an equation which can be derived either from a cooperative hydration model or from a chemical, pseudo equilibrium model. This equation is tested by varying sys tematically the nature of the solute and of the cosolvent. In order to describe the experimental results in W-ACN and in W - E C as such, it will be necessary to extend the equation with a term which comprises any specific (nonhydrophobic) interactions of the solute with the (inert) cosolvent.
' " p h e e n t h a l p y o f s o l u t i o n o f a solute i n v a r i o u s solvents is a n i m p o r t a n t Α
d a t u m i n s t u d y i n g the b e h a v i o r of different solutions b e c a u s e i t m i g h t
g i v e d i r e c t i n f o r m a t i o n o n t h e energetic aspects of b o t h s o l u t e - s o l u t e a n d s o l u t e - s o l v e n t interactions. D u r i n g the last d e c a d e i n o u r l a b o r a t o r y
(see
Réf. 1 a n d l i t e r a t u r e c i t e d t h e r e i n ) enthalpies o f s o l u t i o n o f a l k a l i - a n d tetraalkylammonium halides have been measured i n water a n d i n several nonaqueous
solvents
including formamide
(F),
N-methylformamide
( N M F ) , N , 2 V - d i m e t h y l f o r m a m i d e ( D M F ) , IV,2V-dimethylacetamide ( D M A ) , and dimethylsulfoxide ( D M S O ) . (AH °) tr
T h e standard
enthalpies
of
transfer
( t h e difference b e t w e e n the s t a n d a r d enthalpies of s o l u t i o n o f a
c e r t a i n solute i n t w o s o l v e n t s ) for b o t h a l k a l i b r o m i d e s a n d t e t r a a l k y l 0-8412-0428-4/79/33-177-099$05.00/l © 1979 American Chemical Society
100
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
a m m o n i u m b r o m i d e s b e t w e e n t w o n o n a q u e o u s solvents are a l m o s t i n d e p e n d e n t of the size of t h e c a t i o n , e s p e c i a l l y i f o n l y a p r o t i c solvents are considered.
O n t h e o t h e r h a n d , i f w a t e r is one of t h e solvents, àH
0
tr
of
t h e t e t r a a l k y l a m m o n i u m b r o m i d e s d e v i a t e as c o m p a r e d w i t h t h e c o r r e s p o n d i n g a l k a l i b r o m i d e s since i n w a t e r these salts s h o w a c o n s i d e r a b l e e x o t h e r m i c s h i f t of t h e e n t h a l p i e s of s o l u t i o n . B e s i d e s t h a t , e n t h a l p i e s of
Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 10/26/17. For personal use only.
s o l u t i o n ( Δ Η ° ) of b o t h a l k a l i b r o m i d e s a n d t e t r a a l k y l a m m o n i u m b r o mides
are a l m o s t p r o p o r t i o n a l to
m i x t u r e s of t w o n o n a q u e o u s
t h e solvent
composition
solvents, w h i l e i n aqueous
i n binary
m i x t u r e s the
a l k a l i b r o m i d e s s h o w a s i m i l a r b e h a v i o r b u t t h e Δ / ί ° of the t e t r a a l k y l a m m o n i u m bromides reach large endothermic m a x i m a i n the water-rich r e g i o n (see
Ref. 2 a n d literature cited therein).
G e n e r a l l y these
phe
n o m e n a are a t t r i b u t e d to the w e l l - k n o w n h y d r o p h o b i c h y d r a t i o n b e h a v i o r of the t e t r a a l k y l a m m o n i u m salts i n w a t e r (3). is absent i n n o n - h y d r o g e n - b o n d e d
Since this special behavior
solvents, m i x t u r e s of these
solvents
w i t h w a t e r c a n serve as m o d e l systems to s t u d y t h e h y d r o p h o b i c h y d r a t i o n effect m o r e s y s t e m a t i c a l l y .
I t is p o s s i b l e t o d e s c r i b e t h e results
f o r b o t h the e n t h a l p i e s of d i l u t i o n (4) (2)
a n d f o r t h e e n t h a l p i e s of s o l u t i o n
i n terms of a c o o p e r a t i v e h y d r a t i o n m o d e l (2,5)
although a chemical
m o d e l ( 6 ) leads to exactly t h e same final e q u a t i o n . B o t h m o d e l s i n v o l v e (n)
surrounding
a h y d r o p h o b i c p a r t i c l e a n d the e n t h a l p i c c o n t r i b u t i o n to
o n l y t w o p a r a m e t e r s : the n u m b e r of w a t e r m o l e c u l e s
hydrophobic
hydration i n pure water, H b W . I n a n e a r l i e r p u b l i c a t i o n (6) of the m o d e l s .
w e c o n s i d e r e d t h e b a s i c assumptions
T h i s c h a p t e r b r i e f l y r e v i e w s this subject a n d extends t h e
tests b y a d d i n g n e w results of the ΔΗ°
of t e t r a b u t y l a m m o n i u m b r o m i d e
( B u N B r ) i n m i x t u r e s of w a t e r w i t h a c e t o n i t r i l e ( A C N ) a n d w i t h e t h y l 4
ene c a r b o n a t e
(EC)
a n d p r e l i m i n a r y results of s o m e s u b s t i t u t e d a n d
u n s y m m e t r i c a l t e t r a a l k y l a m m o n i u m salts i n m i x t u r e s of w a t e r w i t h D M F . Experimental
and Results
T h e e n t h a l p i e s of s o l u t i o n w e r e m e a s u r e d w i t h a L K B 8700-1 p r e c i s i o n c a l o r i m e t r y system. T h e e x p e r i m e n t a l p r o c e d u r e a n d test of t h e i n s t r u m e n t h a v e b e e n g i v e n b e f o r e (6,7). E C ( F l u k a , purissimum) was d i s t i l l e d u n d e r r e d u c e d pressure a n d the m i d d l e f r a c t i o n w a s s t o r e d o v e r m o l e c u l a r sieves ( 4 Â ) f o r at least 48 h r . A C N ( M e r c k , p r o a n a l y s i s ) w a s d r i e d over m o l e c u l a r sieves a n d u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . T h e p u r i t y of b o t h solvents ( d e t e r m i n e d s h o r t l y b e f o r e u s e ) , as d e d u c e d f r o m G L C , w a s a l w a y s b e t t e r t h a n 9 9 . 8 % . T h e v o l u m e f r a c t i o n of w a t e r , d e t e r m i n e d b y K . F i s c h e r t i t r a t i o n (8) w a s a l w a y s less t h a n 3.10" . T h e m i x e d solvents w e r e p r e p a r e d b y w e i g h t as s h o r t l y as p o s s i b l e b e f o r e the m e a s u r e m e n t s . Δ Η ° of B u N B r i n W - A C N m i x t u r e s h a v e b e e n m e a s u r e d at 2 5 ° C w h i l e those i n W - E C are at 4 5 ° C , w h i c h is a b o v e the m e l t i n g p o i n t of p u r e E C . 4
4
7.
D E VISSER E T A L .
T*ble I.
Enthalpies
of
Standard Δ Η of B u N B r in Mixtures of Water and A C N at 2 5 ° C and in Mixtures of Water and E C at 4 5 ° C 0
4
InW-ACN
In kj
Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 10/26/17. For personal use only.
101
Solution
0 0.025 0.060 0.109 0.175 0.244 0.329 0.400 0.496 0.578 0.601 0.636 0.720 0.750 0.797 0.850 0.901 0.920 0.953 0.976 1.000
W-EC ΔΗ°
ΔΗ° mol'
kj
1
0 0.030 0.052 0.101 0.151 0.250 0.354 0.499 0.625 0.740 0.850 0.900 0.950 0.970 1.000
16.80 11.69 10.83 9.79 9.96 10.79 12.28 13.53 14.87 15.56 15.80 15.95 16.64 17.05 17.82 18.77 18.24 16.58 10.30 2.09 -8.42
moi
1
33.08 27.95 25.71 23.59 22.26 21.65 21.86 22.62 23.05 22.87 22.35 21.80 18.48 14.92 5.89
T h e c o n c e n t r a t i o n of B u N B r i n the m i x t u r e s v a r i e d b e t w e e n 0.003 a n d 0.01 m o l k g " i n w h i c h r a n g e a n y c o n c e n t r a t i o n d e p e n d e n c e a p p e a r e d t o b e m a s k e d b y t h e e x p e r i m e n t a l error. T h e r e f o r e , Δ ί ί ° w a s t a k e n t o b e t h e average of 2 - 4 i n d e p e n d e n t m e a s u r e m e n t s a g r e e i n g w i t h i n 150 J m o l " . F i n a l results i n Δ Η ° as a f u n c t i o n of t h e m o l e f r a c t i o n of w a t e r x are g i v e n i n T a b l e I . 4
1
1
w
Discussion Recently (6)
w e f o u n d t h a t the Δ ί / ° of B u N B r i n W - D M F , 4
W -
D M S O , a n d W - D M A m i x t u r e s c a n b e d e s c r i b e d f a i r l y w e l l i n terms of a c o o p e r a t i v e h y d r a t i o n m o d e l (2,5) AH°(M)
=
(1 - X )
l e a d i n g to the f o l l o w i n g e q u a t i o n :
A f f ° ( S ) +XAH°(W)
+
( X
n
- X ) H b W
(1)
w h e r e Δ / / ° ( Μ ) is t h e s t a n d a r d e n t h a l p y of s o l u t i o n i n a m i x t u r e of w a t e r w i t h a cosolvent S a n d X is t h e m o l e f r a c t i o n of w a t e r .
As mentioned
before, the same e q u a t i o n is o b t a i n e d c o n s i d e r i n g t h e h y d r a t i o n of tetraalkylammonium ion ( R N ) 4
+
a
b y η cooperatively interacting water
m o l e c u l e s as a p s e u d o c h e m i c a l e q u i l i b r i u m , w h i c h c a n b e d e s c r i b e d b y :
102
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
Κ η H 0 *± R N .
R N
+
+
I n this e q u a t i o n R N
+
. . . (H 0)
4
4
2
+
4
2
n
. . (H 0) 2
II
(2)
n
denotes t h e r e s u l t i n g h y d r o p h o b i c
e n t i t y a n d Κ is a n e q u i l i b r i u m constant.
T h e e n t h a l p i c effect of h y d r o
p h o b i c h y d r a t i o n t h e n c a n b e c o n s i d e r e d as t h e r e s u l t of t h e f o r m a t i o n o f this h y d r a t i o n c o m p l e x .
I n b o t h m o d e l s the c h o i c e of the
cosolvent
Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 10/26/17. For personal use only.
o u g h t to b e r a t h e r u n i m p o r t a n t as l o n g as this s o l v e n t does n o t specific i n t e r a c t i o n s l i k e h y d r o g e n b o n d i n g . HbW
show
A s a consequence η a n d
s h o u l d n o t v a r y w i t h t h e different cosolvents.
B e s i d e s t h a t , the
b a s i c a s s u m p t i o n i n the concepts is t h a t i n t h e absence of h y d r o p h o b i c hydration Δ / /
0
w o u l d c h a n g e p r o p o r t i o n a l l y to t h e s o l v e n t c o m p o s i t i o n .
I n t h i s c h a p t e r w e w i l l i n v e s t i g a t e m o r e s y s t e m a t i c a l l y b o t h aspects. V a r i a t i o n of t h e solute gives i n f o r
Influence of Different Solutes. m a t i o n o n the f o l l o w i n g q u e s t i o n s :
(1)
what happens i f hydrophobic
h y d r a t i o n c a n n o t or does not o c c u r ? I n the case of the n o n h y d r o p h o b i c
τ
I 0
• 0.1
I
1
1
I
I
0.3
1
τ
•
0.5 X
Γ
•
•
0.7
•
I
0.9
1 1
DMF
Figure I . Δ Η of various solutes in mixtures of DMF with water and with DMSO as a function of the mote fraction of DMFX at25°C 0
DMF
Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 10/26/17. For personal use only.
Journal of Physical Chemistry
Figure 2. Δ Η ° of tetraalkylammonium bromides in mixtures of water with DMF as a function of the mole fraction of water at 25°C(6) u r e a w e h a v e f o u n d t h a t Δ / / ° i n W - D M F i n d e e d are a l m o s t p r o p o r t i o n a l to t h e solvent c o m p o s i t i o n ( 9 ) l i k e f o r s o m e a l k a l i h a l i d e s i n W - D M S O g i v e n i n the l i t e r a t u r e ( 1 0 ) . P r e l i m i n a r y results i n o u r l a b o r a t o r y o f C s B r i n W - D M F d i s p l a y t h e same b e h a v i o r ( F i g u r e 1 ). I n this context i t is also i m p o r t a n t to m e a s u r e Δ Η ° of a h y d r o p h o b i c particle i n mixtures of two
non-hydrogen-bonded
solvents. P r e l i m i n a r y results f o r B u N B r i n m i x t u r e s o f D M S O a n d D M F 4
y i e l d e d a s t r a i g h t l i n e . A l l o f these results a r e s u m m a r i z e d i n F i g u r e 2 w h i c h c l e a r l y demonstrates that o n l y i n the case o f h y d r o p h o b i c h y d r a t i o n is a l a r g e e n d o t h e r m i c m a x i m u m i n the ΔΗ° present.
( 2 ) W h a t is the
i n f l u e n c e o f the size o f t h e h y d r o p h o b i c solute? I n F i g u r e 2 Δ Η DMF
0
inW -
are given f o r M e N B r , E t ^ N B r , P r N B r , B ^ N B r , a n d P e n N B r . 4
4
4
D e t a i l s o f t h e measurements h a v e b e e n r e p o r t e d e a r l i e r ( 6 ) . A s t h i s figure s h o w s , a l l five t e t r a a l k y l a m m o n i u m salts d i s p l a y s i m i l a r profiles a l t h o u g h i n the case o f M e N B r t h e h y d r o p h o b i c c h a r a c t e r a l m o s t 4
has d i s a p p e a r e d . A c a r e f u l analysis o f these results i n terms o f E q u a t i o n 1 a n d after c o r r e c t i o n o f t h e i n f l u e n c e o f t h e B r " i o n y i e l d e d a v a l u e o f t h e p a r a m e t e r η o f a b o u t 10, r a t h e r i n d e p e n d e n t o f t h e n u m b e r o f C atoms
104
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
Π
a n d a v a l u e of H b W o f a b o u t — 4 k j m o l " p e r C H g r o u p . S i m i l a r results 1
2
h a v e b e e n f o u n d b y L i n d e n b a u m et a l . ( 5 )
i n the case of t r i a l k y l p h o s -
phates i n W - D M F . T h e r e f o r e , i n these cases, E q u a t i o n 1 p r o v i d e s a g o o d p o s s i b i l i t y to d e s c r i b e the e x p e r i m e n t a l results. ( 3 ) W h a t is t h e i n f l u e n c e of different s u b s t i t u t i o n g r o u p s o n the h y d r o p h o b i c c h a r a c t e r of a t e t r a a l k y l a m m o n i u m salt? I n F i g u r e 3 p r e l i m i n a r y results of t h r e e different solutes are c o m p a r e d w i t h those of P r N B r i n W - D M F .
When in Pr NBr
Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Downloaded from pubs.acs.org by UNIV OF MISSOURI COLUMBIA on 10/26/17. For personal use only.
4
the
four terminal C H
((EtOH) NBr) 4
composition
groups
3
the Δ Η °
4
OH
groups
a g a i n is a l m o s t p r o p o r t i o n a l to t h e
solvent
showing that
are s u b s t i t u t e d b y
(EtOH) NBr 4
is n o
four
longer
a
hydrophobic
solute i n w a t e r n o r i n m i x t u r e s of w a t e r w i t h D M F . T h i s c o n c l u s i o n is i n a g r e e m e n t w i t h results f r o m o t h e r m e a s u r e m e n t s f o r this salt i n w a t e r (11).
O b v i o u s l y , i n t r o d u c t i o n of t h e h y d r o p h i l i c O H g r o u p s converts this
salt to a r a t h e r n o r m a l n o n h y d r o p h o b i c one. W h e n i n P r N B r t h e a l k y l 4
τ
1— ( Et OH) NBr 4
30 h [6.6] Β Γ
ο Ε
