A C T A U . H V E R S I T A 1 I S L O D Z I E N S I S ________________ FOLIA CHIMICA 3 , 1984__________________
S t e f a n i a T an iew sk a-0 % in sk a, M arian Woldan THEflMOCHEMICAL INVESTIGATION OF Nal SOLUTIONS
IN WATER-ACETAMIDE MIXTURES*
The e n t h a l p i e s o f s o l u t i o n o f Nal i n w a te r-a c e ta m id e m ix tu re s have been d e te rm in e d c o l o r i m e t r i c a l l y o v e r th e te m p e ra tu re ra n g e 25-85 C. The therm odynam ic r e s u l t s o b ta in e d a r e c o r r e l a t e d w ith r e s u l t s from e a r l i e r mea su re m e n ts on w a te r-a m id e m ix tu re s .
Amides a t t r a c t c o n s id e r a b le i n t e r e s t i n view o f t h e i r a b i l i t y t o d i s s o l v e v a r io u s s u b s ta n c e s ; f o r t h i 3 re a s o n th e y a r e employed i n a v a r i e t y o f te c h n o l o g ic a l p r o c e s s e s . Owing t o t h e i r h ig h d i e l e c t r i c c o n s ta n t s th e y a r e a ls o good s o lv e n t s q f e l e c t r o l y t e s , t h e p r e s e n c e o f H-bond3 in u n s u b s t i t u t e d and monosub s t i t u t e d am ides may s u g g e s t a c e r t a i n s i m i l a r i t y t o w a te r . P a r t i c u l a r l y good e l e c t r o l y t e - s o l v i n g a b i l i t y has been o b s e rv e d in t h e c a s e o f w a te r- a c e ta m id e m ix tu re s [1 , 2 ] .
The r e s u l t s o b ta in e d in [ 1 ] and [ 2 ] s u g g e s t t h a t th e th erm o d y n an ic s i m i l a r i t i e s betw een w a te r and a c eta m id e s o lu t i o n s o f e l e c t r o l y t e s may be r e l a t e d to th e r e m a in d e r s , i n th e l i q u i d s t a t e , o f th e th r e e - d im e n s io n a l H-bond l a t t i c e w hich i s p r e s e n t b o th i n s o l i d a c e ta m id e [3 -5 ] and in i c e [ 5 ] .
I n v iew o f t h i s we found i t i n t e r e s t i n g to measure th e h e a t o f e l e c t r o l y t e d i s s o l u t i o n i n a mixed «solvent made up o f w a te r an d a c e ta m id e . The e l e c t r o l y t e c h o sen was Nal which i s th e m ost th o ro u g h ly i n v e s t i g a t e d s a l t i n o r g a n ic and mixed s o l v e n t s . The r e s u l t s o b ta in e d f o r w a te r^ a e e ta m iiie s o l u t i o n s can th u s be com pared w ith th o s e f o r o t h e r mixed s o l u t i o n s .
* P ra c a z o s t a ł a n a d e s ła n a n a I I I p o l s k o - r a d z i e c k i e sympo2jum "Termodynamika i e le k tro c h e m ia roztw orów e l e k t r o l i t ó w " .
E x p e rim e n ta l
The e n t h a l p i e s o f s o l u t i o n o f Mai were m easured c a lo r im e t- r i c a l l y f o r a s e r i e s o f w a te r-a c e ta m id e m ix tu re s o f d i f f e r e n t acetam 'ide c o n te n t in the, te m p e ra tu re ra n g e 2 5 -8 5 °3 . The mea su rem en t a w ere c o n d u c te d i n a n o n is o th e rm a l n o n - a d i a b a t i c c a l o r i m e t e r . The c a lo r im e te r p ro p e r was a 220 cm^ Dewar v e s s e l s u p p lie d w ith a s tix v r e r , c a l i b r a t i o n h e a te r and t h e r m i s t o r . Tem perature changes d u rin g th e m easurem ents were d e te rm in e d by means o f a 0-2 mV MKT r e c o r d e r (ODit). The c a lo r im e te r wa3 s e n s i t i v e t o 5 • 10” ^°C. The m easurem ent e r r o r was e s tim a te d t o be ab o u t 1
The w a te r-a c e ta m id e s o lv e n t s employed i n th e i n v e s t i g a t i o n were p r e p a r e d by w e ig h t u s in g t w i c e - d i s t i l l e d w a te r and d r ie d and p u r i f i e d a c eta m id e p . a . p ro d u ced by X enon-W di [ 7 1• Thef m e ltin g p o i n t o f a c e ta m id e was 81-82°C [ 8] , Nal was p u r i f i e d by a p ro c e d u re d e s c r ib e d i n [ 9] .
R e s u lts and d i s c u s s io n «r^sni-.ng-ii'n ■ w ....
The o b ta in e d h e a t o f s o l u t i o n v a lu e s Af^ f o r Nal d i s s o l v e d in w a te r-a c e ta m id e m ix tu re s a t 2 5 , 4 0 , 6 0 , 75 and 85°C a r e p r e s e n te d i n F ig s . 1-5 a s f u n c t i o n s AH^ ■ f ( m ) . I n th e c a s e o f w a te r and m ixed s o l u t i o n s c o n ta i n in g l e s s th a n 20 w t % o f a c eta m id e ( 1 1 .5 mole %) th e is o te r m s AH » » f(ia) - c o rre s p o n d in g to a te m p e ra tu re 25 C ( f o r w a te r a l s o 4 0 °C) may be c l a s s i f i e d a s ty p e I I Is o te rra s i n t h e M i s h - c h e n k o and P r o n i n a ' s c l a s s i f i c a t i o n [ 1 0 ] ,
The o t h e r is o th e rm s a r e o f ty p e I , i . e . th e e x o th e jrra ic ity o f th e h e a t o f s o l u t i o n d e c re a s e s a s th e e l e s t r o l y t e concen t r a t i o n r i s e 3 . As can be se e n i n F ig s . 1 -5 , chan ges in t h e sh ap e s o f is o th e rm s AH^ => f(m ) f o r th e i n v e s t i g a t e d sy ste m s depend on a c e ta m id e c o n c e n t r a t io n i n th e m ixed s o lv e n t and on t h e te m p e ra tu re o f th e s o l u t i o n .
In th e c a s e o f w a te r s o l u t i o n s o f N a l, th e t r a n s i t i o n from ty p e I I to ty p e I iso th e rm s ta k e s p la c e a t a b o u t 45°C, T h is
F ig . 1 . Dependence o f th e h e a t o f s o l u t i o n <AHm) o f Nal on con c e n t r a t i o n in w a te r-a c e ta m id e m ix tu re s a t 25°C . 1 - w a te r; 2 - 5 wt
% ( 1 , 5 8 mole %) AcNHj; j - 15 w t % ( 5 .1 mole %) AcNHa : 4 - 30 wt #
(1 1 ,5 5 mole %) AcNilj; 5 - 50 wt Si (2 3 ,3 6 mole '/») AcNH2
F ig . 2 . Dependence o f th e h e a t o f s o l u t i o n (AHm ) o f . i cn ¡~ c e n t r a t i o n In w a te r-a c e ta m id e m ix tu re s a t 40 C. 1 - w.r. *-r; - v : Jfc ( 1 ,5 8 m ole M) AC.VH2* 3 - 15 wt % ( 5 ,1 mole D Ac<»Hc ;
.-.a*«.-r f i n s l t i o n p o i n t lo w e .-.a*«.-rs a s th e a c e ta m id e c o n te n t i n th e mixed s o lv e n t i n c r e a s e s .
F ig . 3 . Dependence o f th e h e a t o f s o l u t i o n ( AHm) o f N al oncon- c e n t r a t i o n i n w a te r-a c e ta m id e m ix tu re s a t 60°C. 1 w a te r ; 2 -- 5 w t % ( 1 ,5 8 mole %) AcNH j 3 1 5 w t j 6 ( 5 . 1 mole Jt) AcNH»: 4 -- 70 w t % (1 1 ,5 5 mole Si) AcNH,; 5 - 50 w t % (2 3 ,3 6 mole %) AcNH,;
6 - 70 w t % ( 4 1 ,5 5 mole Ji) AcNH2 e
When th e m ixed s o lv e n t c o n ta in e d 50 wt # o f a c e ta m id e , th e shape o f th e is o th e rm s - f(m ) ch ang es a lr e a d y a t a b o u t 25°C. As i s w e ll known, ty p e I is o th e rm s a r e c h a r a c t e r i s t i c o f s t r o n g l y s o lv a t e d e l e c t r o l y t e s , f o r w hich th e e n e r g e ti c e f f e c t r e l a t e d t o io n d e s o lv a tio n grows w ith i n c r e a s i n g c o n c e n t r a t io n and p re d o m in a te s o v e r th e e x o th e rm ic p r o c e s s o f io n ap p ro a c h in g . On t h e o t h e r h an d, i n ty p e I I is o th e rm s AHffl • f(m ) de c i s i v e r o l e i s p la y e d by p r o c e s s e s o f io n a p p ro a c h in g and de s t r u c t i o n o f th e s o l v e n t s t r u c t u r e by th o s e io n s w hich le a d s t o i n c r e a s e d e x o th e r iu ic ity o f th e d i s s o l u t i o n p r o c e s s a s th e c o n c e n t r a t io n o f th e s o l u t i o n i n c r e a s e s . A d d itio n o f a c eta m id e to w a te r evokes a s i m i l a r e f f e c t a s th e grow th o f te m p e r a tu r e . Thus, p o s s i b l y , a c e ta m id e m o le c u le s p a r t i a l y d i s r u p t t h e o r i
-g i n a l s t r u c t u r e o f w a te r b u t , a a can be se e n i n F ig s . 1-5 th e e f f e c t i s n o t v e ry s i g n i f i c a n t .
i
F ig . 4 . Dependence o f th e h e a t o f s o l u t i o n (AHm) o f Nal on con c e n t r a t i o n i n v a te r - a c e ta m id e m ix tu re s a t 75°C . 1 - w a te r ; 2 - - 5 vrt % ( 1 ,5 8 mole %) AcNH*; 3 - 15 wt % ( 5 .1 mole %) AcNH2; 4 - 30 wt % (1 1 ,5 5 mole %) AcNII2 : 5 - 50 w t ( 2 3 ,3 6 mole %)
AcNH2 ; 6 - 70 wt 54 ( 4 1 ,5 5 mole %) AcNH*
T herm ochem ical i n v e s t i g a t i o n s o f Nal s o l u t i o n s i n m ixed wa t e r - form am ide s o lv e n t s [11] have shown t h a t t h e iso th e rrtis AH^ * » f(m ) have a v e r y s m a ll s lo p e A(AHm)/Am w hich may be r e l a t e d t o th e h ig h d i e l e c t r i c c o n s t a n t . I n t h e c a s e o f water-Nal-DM F sy ste m [11] t h e sh ap e o f th e is o te rm chan g es much f a s t e r ; a l r e a d y i n th e s o l u t i o n s c o n ta in in g 20 mole 56 o f EE-'.F t h e e x o th e r - m ic it y o f t h e h e a t o f s o l u t i o n d e c re a s e s d i s t i n c t l y depending on th e c o n c e n t r a t io n ( ty p e I i s o th e r m ) . T h is may be r e l a t e d to t h e p re s e n c e o f two m ethy l g ro u p s i n a DMF m o le c u le (lo w e r d j (>~ l e c t r i c c o n s t a n t , l a r g e r m o le c u le , a b se n c e o f th r e e - d im e n s io n a l H-bond l a t t i c e ) .
A com p arison o f th e th erm o chem ical p r o p e r t i e s o f w a te r mix t u r e s w ith form am ido, a c eta m ld e and DMF shows t h a t th e a c e t a - ■nide i n v e s t i g a t e d in t h i s s tu d y has I n te r m e d ia te p r o p e r t i e s .
F ig . 5 . Dependence o f h e a t o f s o l u t i o n (AH,»,) o f Nal on con c e n t r a t i o n i n w a re r-a c e ta m id e m ix tu re s a t .85° C.- 1 - w a te r ; 2 - - 5 w t % ( 1 ,5 8 mole A) AcNHz ; 3 - 15 wt A ( 5 ,1 mole %) AcNH,; 4 - 30 w t % (1 1 ,5 5 mole A) AcNH,; 5 - 50 wt % ( 2 3 ,3 6 mole %) AcHH2 ; 6 - 70 wt A ( 4 1 ,5 5 mole %) AcHH2 ; 7 - 85 wt * ( 6 3 ,3 mo
F ig u re 6 shows th e dependence AH^a I ** f ( x ) f o r w a te r - a c e - tam id e s o l u t i o n s . The .»eat o f s o l u t i o n o f Nal in an i n f i n i t e l y d i l u t e s o l u t i o n w ere o b ta in e d by l i n e a r e x t r a p o l a t i o n o f t h e de pendence AHjjj m tilftn) b ec au se we d id n o t have th e d a ta n e c e s s a r y t o e x t r a p o l a t e t h e f u n c tio n <>m - f{m) by th e m ethod o f C r i s s and C o b b l e [1 2 , 1 3 ],
As can be s e e n i n F ig . 6 th e dependence in w a te r-- a c e ta ra id e s o l u t i o n s on t h e mole f r a c t i o n o f a c e ta m id e i s l i n e a r s u g g e s tin g t h a t i n t e r a c t i o n s betw een w a te r and a c e ta m id e mole c u le s a re v e r y weak and t h a t th e h e a t o f s o l u t i o n o f Nal i n an i n f i n i t e l y d i l u t e s o l u t i o n i s a d d i t i v e .
F ig . 6. S ta n d a r d s o l u t i o n e n th a lp y o f Nal (AH0 ) i n w a t e r - a c e t a - mide m ix tu re s a s a f u n c t i o n o f s o l v e n t c o m p o sitio n
The dependence on th e te m p e ra tu re i n w a te r-a c e ta s : id e m ix tu re s i s p r e s e n t e d on F i g . , 7 . As we can s e e from F I " , 7 th e c o u rs e o f t h e f u n c t i o n A H ^ j « i ( T ) f o r in v e n t ;"o
i-v e n ts e lra o s t does n o t depend on th e c o n te n ts o f acQ tam ide in th e m ix ;d s o l v e n t .
F ig . 7 . S ta n d a rd s o l u t i o n e n th a lp y o f Nal (AH0 ) i n w a te r - a c e - ' tara id e m ix tu re s a s a f u n c t i o n o f te m p e ra tu re . 1 - w a te r ; 2 - i> w t * AcNH2; 3 - 15 w t % AcNH,; 4 - 30 wt % AcNH2; 5 - 5 0 wt 54
AcNH2; 6 - 70 wt % AcNHj
I n o r d e r t o compare w a te r-a c e ta m id e m ix tu re s w ith o t h e r m ix-0 1
ed s o l v e n t s th e dependence o f f u n c tio n AHm* - f ( x ) o f Nal i n some m ixed s o lv e n t s i s p r e s e n t e d on F ig . 8 ( X - t h e m o lar f r a c t i o n o f th e o r g a n ic component i n th e m ix t u r e ) . I t can be se e n t h a t t h e f u n c tio n f o r a lc o h o l- w a te r m ix tu re s i s d i s t i n c t l y d i f f e r e n t from th o s e f o r o t h e r m ix tu re s i n t h a t t h e r e a p p e a rs a minimum o f h e a t o f s o l u t i o n e x o t h e r m i c i t y . a t x » 0 . 2 f o r me th a n o l an d a t x «* 0 .0 9 f o r is o p r o p a n o l. A c c o rd in g t o S a m o i l o v [ 2 1 , 22] th e a p p e a ra n c e o f a o in ic u a i i s c o n n e c te d w ith f i l l i n g th e g ap s i n w a te r s t r u c t u r e by a lc o h o l m o le c u le s . T h is i n t e r p r e t a t i o n , p r o b a b le a s i t i s , i s n o t nrsAversal and c a n n o t be a p p li e d t o o t h e r n o n - e l e c t r o l y t e s even th o u g h t h e s i z e o f t n e i r m o le c u le s i s su c h t h a t t h e y c o u ld
F ig . 8. Dependence AHft1 o f Nal in some m ixed s o lv e n t s a s a f u n c t i o n o f m ix tu re s c o m p o s itio n [mNoi « 0 ,1 m ole/ k g J a t 25 0 . 1 - water-EM F [1 4 ]; 1 - w a te r - a c e to n e [15]5 3 - w a te r - a c e t o n i t - r i l e [16] ; 4 - w ate r-fo rm am id e [14] ; 5 - w a te r-a c e ta in id e [8;rC j ; 6 - w a t e r - u r e a [17] ; 7 - w a te r- a c e ta m id e ; 8 - w a te r-p ro p io n a -v i de [1 8 ]; 9 - w a te r-m e th a n o l [ 1 9 ]j 10 - w a te r - is o p r o p a n o l [2 0]
f i l l th e gaps in w a te r s t r u c t u r e . I n th e c a s e o f w a te r- a c e to n e 0 1
and w a t e r - a c e t o n i t r i l e sy stem s th e f u n c t i o n s AIIm* • F(x) ex h i b i t a d i s t i n c t maximum in th e ra n g e o f h ig h c o n c e n t r a t io n o f t h e o r g a n ic component w hich e x p la in s th e fo rm a tio n o f com plexes [ 2 3 ] . Such a maximum does n o t a p p e a r i n w a te r-fo rm a m id e , w a te r- -acetam i.de (Q5°C) and water-DMF s y s te m s . I n th e s e sy stem s th e e x o th e r m ic ity o f th e h e a t o f s o l u t i o n o f Nal c o n s t a n t l y r i s e s w ith i n c r e a s i n g n o n - e l e c t r o l y t e c o n te n t i n th e mixed s o l v e n t .
T h is may be due to d e c re a s in g en d o th erm ic component o f th e h e a t o f s o l u t i o n w hich i s c o n n e c te d w ith t h e d i s t r u c t i o n o f wa t e r s t r u c t u r e . Q u ite l i k e l y th e n o n - e l e c t r o l y t e m o le c u le s i n tro d u c e d t o w a te r g r a d u a l ly d e s tr o y t h e th r e e - d im e n s io n a l H-bond l a t t i c e in w a te r fo rm in g mixed a s s o c i a t e s .
In th e w ater-fo rm am id e system th e s t r u c t u r e o f t h e m ixed s o lv e n t changes g r a d u a l ly and i s l a b i l e [ 2 3 - 2 5 ] , w h ile i n c a se o f water-DMF m ix tu re s a s s o c i a t e s o f d e f i n i t e c o m p o sitio n can b e ' d i s c e r n e d [ 2 6 -2 3 ] . P o s s ib ly i n th e w a te r-a c e ta m id e sy stem mi xed a s s o c i a t e s w ith H-bonds a r e form ed . S i m i la r phenomena may
tak e p la c e i n w a te r-p ro p io n a m id e and W a te r-u re a s y s te m s .
Some in fo r m a tio n c o n c e rn in g th e i n t e r a c t i o n betw een w a te r and n o n - e l e c t r o l y t e can a ls o be d e r iv e d from an a n a l y s i s o f th e te m p e ra tu re c o e f f i c i e n t A(AH°)/AT. As can be se e n i n F ig . 9 t h i s c o e f f i c i e n t r i s e s m ost q u ic k ly in th e c a s e o f w a t o r - f o r - raamide m ix tu re ' a t t a i n i n g th e z e ro l e v e l a t 14 mole % o f f o r - raamide and changes i t s s ig n t o p o s i t i v e . I n water-DMF m ix tu re s , i t a t t a i n s th e z e ro l e v e l and ch an g es i t s s ig n t o p o s i t i v e a t 30 mole ¥> o f DMF, w h ile i n t h e c a s e o f w a te r-a c e ta m id e m ix tu re s i t has n e g a tiv e v a lu e s o v e r th e e n t i r e ra n g e o f c o m p o s itio n s i n v e s t i g a t e d and changes o n ly l i t t l e .
As i s w e ll known, th e te m p e ra tu re c o e f f i c i e n t has n e g a tiv e v a l u e s i n t h e cape o f w a te r s o l u t i o n and p o s i t i v e v a lu e s . f o r most o r g a n i c s o l u t i o n s [ 1 0 ].
Assuming t h a t n e g a tiv e v a lu e s o f t h e c o e f f i c i e n t a r e ch a r a c t e r i s t i c o f w a t e r - l i k e s o l u t i o n s , th e n th e z e ro v a lu e would c o r r e s p o n d t o s o l u t i o n s i n w hich t h e o r i g i n a l s t r u c t u r e o f wa t e r was d e s t r o y e d .
v *
9 snows t h a t form amide h as a s t r o n g e r d e s t r u c t i v e e f f -, on tv:- o r i g i n a l s t r u c t u r e o f w a te r th an DMF, w h ile i n th e
c a s e o f o cetam id e t h i s e f f e c t i s v e ry s m a l l. T his may be r e l a t ed t o th e p o s s i b i l i t y o f fo rm a tio n o f a m ixed th re e -d im e n s io n a l l a t t i c e o f H-bonda w ith b o th w a te r and a c eta m id e m o le c u le s p a r t i c i p a t i n g .
r
F ig . 9 . The te m p e ra tu re c o e f f i c i e n t o f th e N al d i s s o l u t i o n h e a t i n w a te r-a m id e m ix tu re s a t 2‘5°C
The above c o n c lu s io n i s in ag reem en t w ith o p in io n a r i s i n g from d e n s im e tr ic [ 2 9 ] , v i s c o s i m e t r i c [3 0 ] and d i e l e c t r i c [23] s t u d i e s o f w a te r-a c e ta m id e m ix tu re s . R e fe re n c e s [ 1 ] S . T a n i e w s k a - 0 s i n s k a , M. W o l d a n , tiocz. Chem., 4 8 , 1753 (1 9 7 4 ). [ 2 ] S . T a n i e w s k a - 0 s i n s k a , H. W o l d a n , .tocz. Chem., 5 1 , 1933 (1 9 7 7 ).
[3] F. S e n t i , D. H a r k e r , J . Am. Chem. S o c ., o2, 2008 (1 9 4 0 ).
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