A C T A U N I V E R S I T A T I S L O D Z I E N S I S FO LIA C H I M I C A 10, 1993 Cez ar y M. K i n a r t * S T U D I E S O N I N T E R M O L E C U L A R I N T E R A C T I O N S IN M O N O E T H A N O L A M I N E - W A T E R L I Q U I D M I X T U R E S BY M E A N S OF * H - N M R S P E C T R A A N D E L E C T R I C P E R M I T T I V I T I E S The H-N MR s pe c t r a of liq ui d b i n ar y m i x t u r e s m ad e of wa ter and m o n o e t h a n o l a m i n e (MEA) at 2 9 8 .1 5 K w i t h i n the n e a r l y w h o l e r an ge of s o l ve nt c o m p o s i t i o n s (i.e. f ro m 3.22 to 72.95 m ol % of MEA), were rec orded. F ro m these e x p e r i m e n ta l data, the c he m i c a l shi ft d i f f e r e n c e s , S (M EA - H 20), b e t w e e n the p r o t o n s i g n a l s of w at er and the m e t h y l e n e g ro ups of m o n o e t h a n o l a m i n e were d e t e r m i n e d and, s u b s e q u e n t l y , the va lues of the s pe c t r a l p a r a m e t e r A S ( M E A - H 20) w er e found g r a p h i c a l l y for the s t u d i e d mi x tu re s . F ro m the m e a s u r e m e n t s of e l e c t r i c p e r m i t t i v i t i e s (£) at 293.15, 298.15 and 3 03 .15 K and over the w h o l e range of s o l v e n t c o m p o s i t i o n s , v a l ue s of the t e m p e r a t u r e c o e f f i c i e n t s of £, viz. a = (l/£)-[dE/d (1/T)] , were det e rm i ne d.
The thus r e s u l t s o b t a i n e d w ou ld in d ic at e at the f o r m a tion of some r e l a t i v e l y most s ta ble " c o m p l e x e s " ( s u b - un i ts ) MEA • 2 H 20 in the liq ui d m i x t u r e s u nd er i n v e s t i g at io n .
This p a p er c o n t i n u e s my p r e v i o u s s t u di es on the i nt erna l s t r u c t u r e s of v a r i o u s liq uid b i n ar y mi x tu re s , with e m p h a s i s put on m u t u al i n t e r m o l e c u l a r i n t e r a c t i o n s t h r ou gh the h y d r o g e n b o n d ing [1-4] . The ^H-NMR spe ct ra p r e s e n t e d h e r e i n were r e c o r d e d for b i n ar y s o l u t i o n s of m o n o e t h a n o l a m i n e (MEA) and w at er ( H 20) w i t h i n the n e a rl y full range of c o m p o s i t i o n s . S u b s e q u e n t l y , ba sed on the se data, the r el a ti v e d i f f e r e n c e s in the ch e mi ca l sh ift val ue s b e t w e e n the c en ter of the p r o t o n sig na l of H 20 and
the c e n t er of the MEA m e t h y l e n e g r o u ps signal, 6(ME A - H 20), were c al c u l a t e d . The g r a p h i c a l m e t h o d was used as p r e v i o u s l y [1-4] , for d e t e r m i n a t i o n of the u s e fu l s pe c t r a l p a r a m e t e r A<5(MEA - 1^0) in the m i x t u r e s studied.
I n d e p e n d e n t l y , the m e a s u r e m e n t s of e l e c t r i c p e r m i t t i v i t i e s (£) were p e r f o r m e d for the a n a l y z e d s y s t e m at three t e m p e r a t u r e s and the v a l ue s of the t e m p e r a t u r e c o e f f i c i e n t s of £ were c a l c u l a t e d at 298 .1 5 K.
The c h o i ce of the said binary m ix ture for the p r e s e n t s t u di es was due to the fact that m o n o e t h a n o l a m i n e , b ei ng the si m pl es t a m i n o a l c o h o l , p o s s e s s e s both the - N H 2 and the -OH g r o u ps w hi ch are apt to s t r o n g i n t e r m o l e c u l a r i n t e r a c t io ns . Thus, it is able to f or m the f ai rly s t a bl e h y d r o g e n b o n d i n g w it h w at er mol e cu l es , wh ere e i t he r the a m i n o or the hy d ro xy l group is i nv o l v e d or the both [5-10]. The a q u e o u s s o l u t i o n s of ME A were also the "model" s ys t em for the s t u d i e s on some b i o l o g i c a l o b j ec ts [ l l ] . They are as well w i d e l y used in c h e m i c a l t e c h n o l o g y as s u r f a c t a n t s [ l l ] .
E X P E R I M E N T A L
For the p r e s e n t *H-NMR s pe c tr a l s t u di es and the m e a s u r e m e n t s of e l e c t r i c p e r m i t t i v i t i e s , m o n o e t h a n o l a m i n e (p.a., Flu ka) was used. After its p r e l i m i n a r y d r y i n g over m o l e c u l a r sie ve s 4 A, MEA was p u r i f i e d and d ri ed by a f ra c t i o n a l d i s t i l l a t i o n under r e d u c e d p re ssur e. For all the e x p e r i m e n t s , r e d i s t i l l e d water from a q ua rtz g la ss d i s t i l l e r was used.
All the b i n a ry s o l u t i o n s s t u di ed were p r e p a r e d by weight. The ^H-NMR s p e ct ra (at 298 .1 5 + 1 K) were r e c o r d e d on a Tesla s p e c t r o m e t e r of the type BS 487C (80 MHz). The c h e m i c a l shi ft va lues for the p r o t o n s i g n a l s of MEA and w at er in ea ch bin ary m i x t u r e we re m e a s u r e d with an a cc u r a c y of +0.2 Hz in r e l a t i o n to an e x t e r n a l s ta n dar d, HMDS ( h e x a m e t h y l d i s i l o x a n e ) .
The e l e c t r i c p e r m i t t i v i t y m e a s u r e m e n t s were p e r f o r m e d with an a c c u r a c y of +0.2%, u si ng a b r i d ge of the type 0H- 30 1 (made in H un g ary ). All the m e a s u r e m e n t s of e l e c t r i c p e r m i t t i v i t i e s were p e r f o r m e d at 293.15, 2 98 .15 and 3 03 .15 K. Each t e m p e r a t u r e was m a i n t a i n e d with an a c c u r a c y of +0.01 K, u si ng a c a s c a d e t h e r m o s t a t ic system.
R E S U L T S AND D I S C U S S I O N
M o n o e t h a n o l a m i n e (MEA) b e l o n g s to a group of o r g a n i c c o m p o u n d s n am ed a m i n o a l c o h o l s . In liv ing o r g a n i s m s it is for med from s e r in e by its d e c a r b o x y l a t i o n [12-13]. The IR s tu d i e s [14] have r e v e a l e d that in d i l ut e t e t r a c h l o r o e t h y l e n e s o l u t io n s, three d i f f e r e n t forms of ME A are s i m u l t a n e o u s l y p r e s e n t Fig. 1.
H ,
---g' Gg’ gGt
F i g . 1
The m os t s t a b le is the form ( G ,GG ,), in w h i c h the int erna l h y d r o g e n bond, 0 -H ...N , is formed b et w e e n the h y d r o x y l group (pr ot on d on or) and the a mi no group (pr ot on a cc e pto r). Two other fo rms are: the form gGy with no i nt erna l h y d r o g e n bond (less s t a bl e than the for me r by ca. 2.8 k J/ mol) , and the form jG-p w h e re - N H 2 is a p r o to n d on or and -OH is a p r o t o n a cc e p t o r (it is less s t a bl e than the form g > ^ G , by 5.6 kJ/ mol) . The p re s e n c e and the r e l a t i v e s t a b i l i t y of the c o n f o r m e r g > G G , was c o n f i r m e d bo th by ab ini tio c a l c u l a t i o n s [5] and by m i c r o w a v e m e a s u r e m e n t s
in the gas phase [6-9] and in the s ol id p ha se [10]. The s t r u c t u r e of liq ui d water was b r i ef ly d i s c u s s e d in the p r e v i o u s p ap er [3]. There are s ev eral r e p or ts in the l i t e r a t u r e d i s c u s s i n g mut ua l i n t e r a c t i o n s b e t w e e n MEA and h^O t h r ou gh i n t e r m o l e c u l a r hydrogen bon ds [5-10]. R o d n i k o v a [15, 16] has s t u d i e d dil ute a q u e o u s s o l u t i o n s of MEA, u si ng the d i e l e c t r o m e t r i c method. She has c o n c l u d e d that up to 8 mol% of MEA, c o n s i d e r i n g the s t r u c t u r e and the d i r e c t i o n s of b on ds in the m o l e c ul e s, MEA does not a p p r e c i a b l y i n f l u en c e the i nt e rn a l s t r u c t u r e of l iq uid water. C a- b a n i [17] and B r u s a l e v a [18] have m e a s u r e d the heat c a p a c t i e s and the e n t h a l p i e s of m i x i n g in the s t u di ed mixtures, d e t e r m i n i n g the d on or n u m be r of MEA to be equal to 41.
In the p a p er s [l9 - 2 l ] , their a u t h o r s have a ss umed , on the b as is of m e a s u r e m e n t s of a n u m be r of v ar ious p h y s i c o c h e m i c a l p r o p e r t i e s of the MEA - H 2 0 mi x tu re s , the p o s s i b i l i t y of f o r m a tion of some r e l a t i v e l y s t a bl e c o m p l e x e s ( s u b -u n it s) b et w e e n MEA and H 20 m o l e c ul e s, wi th the c o m p o s i t i o n s : M E A / h ^ O = 1 : 1 and 1 : 2
D et aile d p h y s i c o c h e m i c a l s t u di es on the MEA - H 20 l iq uid m i x tures were p e r f o r m e d by W e n c e 1 in her PhD t h e s i s [22].
On the basis of the m e a s u r e d v a l u es of d e n i s it i es , v i s c o s i ties, r e f r a c t i v e i n d ic es at sev er al t e m p e r a t ur es , and c a l c u l a t e d from them the d e v i a t i o n s from "i d ea li t y" of the s t u d i e d p r o p e r t i e s as well as the m ol ar p o l a r i z a t i o n s and the t e m p e r a t u r e c o e f f i c i e n t s of v i s c o s i t y and density, the a u t h o r e s s has put f o r w ar d some a s s u m e d i nt erna l str u ct u re s. She has a s s u m e d that the m os t p r o b a b l e fairly sta bl e " c o m p le x es " ( su b - u n i t s ) formed via h y d r o g e n bonds b e t w e e n MEA and H 2 0 m o l e c u l e s are: MEA • 2 H 20 and MEA • 3 H 20.
In o rd er to c o n f i r m or to v er ify the e x i s t i n g l i t e r a t u r e p o i n ts of view c o n c e r n i n g m u t u al i n t e r a c t i o n s b e t w e e n MEA and H 20 m o l e c ul e s, in this work I have n ew ly m e a s u r e d the v a l ue s of c he m i c a l shift d if f e r e n c e s , S ( M E A - H ? 0) (in Hz), b e t w e e n the
i L
c e n t e r s of the H-N MR s i g n a l s of water m o l e c u l e s and the c e n te rs of the *H-NMR s i g na ls of - C H 2 - g ro ups in m o l e c u l e s o ve r a wide range of s o l ve nt c om p o s i t i o n s , 3.22 to 72.95 mo l% of MEA.
Su b s e q u e n t l y , u si ng the same m e t h o d as p r e v i o u s l y [1-4], from these new s pe c t r a l data the val ue s of the s pe c t r a l p a r a m e ter A S ( M E A - H 20) w er e g r a p h i c a l l y d e t e r mi n ed . The S ( M E A - H 20)
and the A S ( M E A - H^O) val ues are shown in Tab. 1, and they are v i s u a l i z e d in Fig. 2 as a f un c ti o n of the m i x t u r e c o m p o s i t io ns . T a b l e 1 R e l a t i v e c he m ic a l shifts, 8 ( H 2 0 - MEA), and 1HN MR sp e ct ra l p a r a m e t e r s ,A 8 ( H 2 0 -- MEA), m e a s u r e d at 298 .15 K m ol % of MEA S ( H 2 0 - MEA) [Hz] A 5 ( H 2 0 - MEA) [Hz] 3.22 96.5 0.0 6.97 95.5 2.0 11. 40 94.0 3.4 16.65 91.5 4.5 23.05 87.5 5.4 31.01 82.0 6.1 41.15 74.0 4.3 54.51 62. 5 2.5 72.95 47.5 0.0
The a n a l y s i s of the data o b t a i n e d i nd i ca t es the p r e s e n c e of a d i s t i n c t m a x i m u m for the p ro p e r t y a 5 ( M E A - H 20) loc at ed at ca 33 ma l^ of MEA. As it was d e s c r i b e d e a r li er [ l - 4 ] , the m a x i m u m for this p a r a m e t e r p o i n ts at the c o m p o s i t i o n w he re the s t r o n g e s t i n t e r a c t i o n s b e t w e e n c o m p o n e n t s with i n v o l v i n g h y d r o g e n bonds, are di s pl ay e d. Thus, from the new s pe c tr a l r e s u l t s thus obtained, the c o n c l u s i o n w ou ld be d ra wn that the most s ta ble "co mple x" ( s u b - un i t) b e t w e e n MEA and water m o l e c u l e s is of the MEA • 2 H 20 - type.
In o rd er to s u p p o r t this c on c l u s i o n , I also p e r f o r m e d the i n d e p e n d e n t m e a s u r e m e n t s of e le c t r i c p e r m i t t i v i t i e s (£) over the w ho le ran ge of s o l v e n t c o m p o s i t i o n s , at the t e m p e r a t u r e s 293.15, 298 .15 and 303 .15 K (the m e a s u r e d val ue s of E are s u m m a r i z e d in Tab. 2). U si ng these data, I have c a l c u l a t e d the val ue s of the t e m p e r a t u r e c o e f f i c i e n t s of £, d en o t e d ot, at 2 98 .15 K. This c o e f f i c i e n t , in a g r e e m e n t with the r e s u l t s of R a e t z s c h
% mol. M E A
Fig. 2. C h a ng es in the f un c tio ns, S ( H 2 0 - MEA) = f (m ol%) and A S ( H 2 0 MEA) = f(mol%), for the liq uid m i x t u r e s w a t e r m o n o e t h a
-n o l ami-n e, at 2 98 .15 K
[23], sho ul d be used as one of the c ri t e r i a for e s t i m a t i n g the m ut ual i n t e r a c t i o n s b e t w e e n p ol ar c o m p o n e n t s of b in ary sol vent s. The c om p o s i t i o n , w h e r e this s pe c t r a l p a r a m e t e r is d i s p l a y i n g a m a x i m u m p o i n t s at the " co m ple x" with a s t r o n g e s t i n t e r m o l e c u l a r h y d r o g e n bond i n t e r a c t i o n b e t w e e n the com p on e nt s. In the p r e v i o u s p a p e rs [1-4] I have s ho wn that there is a fai rly good a g r e e me n t in c o n c l u s i o n s d ra wn from the both maxima: for the s pe c t r a l p a r a m et er A 5 ( M E A - H 20 ) , and for the oi property.
U sing the e x p e r i m e n t a l v al ues of e le c t r i c p e r m i t t i v i t i e s , I have c a l c u l a t e d the val ue s of their d e v i a t i o n s from "i d ea li t y" as a f un c t i o n of the MEA m ol ar f ra c ti o n as follows:
( A E 1 2 ^ i d e a l . = E12 ' ( x l ’ £ 1 + * 2 ' C 2 ) " AC a d d .
T a b l e 2
Dielectric p e r m i t t i v i t i e s for binary liq uid m i x t u re s , H 2 0 - MEA, m e a s u r e d at 293.15, 298 .15 and 303 .15 K m ol % of MEA e 293.15 K 298 .1 5 K 303 .1 5 K 0.00 80.37 78.48 74.95 3.22 78.47 76.51 73.31 6.97 76.18 74.04 70.38 11.40 72.98 70.78 66.79 16.65 69.11 66.87 62.35 23.05 64.28 61.74 55.91 31.01 59.75 56.61 50.36 41.15 55.49 53.43 47.84 54. 51 51.07 49.96 45.47 72.95 45.58 44. 93 41.89 100.00 38.11 37.70 36.88
The val ue s of £ , a and e 12^ idea l as f u n c t i o n s of the s ol v e n t c o m p o s i t i o n are shown in Fig. 3.
A n a l y s i s of the f u n c t i o n a = f ( m ol % MEA) i n d i c a t e s that the in c r e a s i n g a d d i t i o n of MEA to water up to 10 m o l 5-. of MEA n ea rly does not c h a ng e the v a l ue s of a . This s u g g e s t s lack of a p p r e c i a b l e s t r u c t u r a l c h a n g e s in liq ui d w at er upon a d d i t i o n of such small q u a n t i t i e s of MEA, and it also c o n f i r m s the r e s ul ts of R o d n i k o v a [15, 16].
The v al ues of oi r e a c h a m a x i m u m at the c o m p o s i t i o n h a v in g ca 33 m o l 5. of MEA. Thus, for this c o m p o s i t i o n one w o u l d e x p ec t the s t r o n g e s t i n t e r a c t i o n s b e t we en MEA and w a t er m ol e c u l e s , and the f o r m a t i o n of some fai rl y s t a bl e " co mple x" (s u b- un i t) of the MEA • H 20 - type.
% mol. M E A % mol. M E A
% mol. M E A
Fig. 3. C h a ng es in t e m p e r a t u r e c o e f f i c i e n t s of d i e l e c t r i c p e r m i t t i v i t y d ra wn as a f un c ti o n of c o m p o s i t i o n for the liq ui d m i x
This c o n f i r m s the for mer c o n c l u s i o n s drawn abo ve and b a s e d on the a n a l y s i s of the 1H-NMR s pe c tr a l p ar a m e t e r , A S f M E A - h^O). In the same area of the c o n c e n t r a t i o n s the re is also a m a x i m u m d e v i a t i o n from "i d ea li t y" for e l e c t r i c p e r m i t t i v i t i e s as a f u n c tion of the MEA m o l a r f ra c t i o n (see Fig. 3).
On the basis of the p r e s e n t results, and those found in the literature, it may be c o n c l u d e d that the mo st s t a bl e " co m p l e x e s " ( su b -u n it s) e x i s t i n g in the MEA - H 20 l iq uid b i n a ry m i x t u r e s are those of the MEA • 2 H 2 0 - type. Other " c o m p l e x e s " ( su b - u n i t s ) tho ugh they are q ui te p os sibl e, are a p p a r e n t l y less stable.
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Cez ar y M a c i e j Kin ar t B A D A N I A O D D Z I A Ł Y W A Ń M I Ę D Z Y M O L E K U L A R N Y C H W M I E S Z A N I N A C H M 0 N 0 E T A N 0 L 0 A M I N Y Z WODĄ M E T O D Ą P OM I A R U ICH W ID M XH - N M R I P R Z E N I K A L N O Ś C I E L E K T R Y C Z N E J Z m i e r z o n o w id ma ^H- NMR c i e k ł y c h m i e s z a n i n wody i m o n o e t a n o - l o a mi ny (MEA) w t e m p e r a t u r z e 2 98 ,15 K (w z a k r e s i e od 3,22 do 7 2,95 mo l% MEA) oraz w ar t o ś c i p r z e n i k a l n o ś c i e l e k t r y c z n e j dla tego u k ł ad u w t e m p e r a t u r a c h 293,15, 2 98 ,15 i 303 ,1 5 K, w p e ł ny m z a k r e s i e s k ł a du m i e s z a n y c h r o z p u s z c z a l n i k ó w . Z d a n yc h d o ś w i a d c z a l n y c h w y z n a c z o n o p r z e b i e g i w z g l ę d n y c h p r z e s u n i ę ć c he m ic z n y c h , S (M EA - H 2O), p o m i ę d z y ś ro d k a m i s y g n a ł ó w p o c h o d z ą c y c h od wody i od grup m e t y l e n o w y c h m o n o e t a n o l o a m i n y , a n a s t ę p n i e w ar t o ś c i p a r a m e t r u s p e k t r a l n e g o , A 5 ( M E A - H 2O), w b a d a n y c h m ie s z a n i n a c h . Z t e m p e r a t u r o w y c h z a l e ż n o ś c i p r z e n i k a l n o ś c i e l e k t r y c z n e j w y z n a c z o no dla b a d a n y c h u k ł a d ó w w a r t o ś c i t e m p e r a t u r o w e g o w s p ó ł c z y n n i ka p r z e n i k a l n o ś c i e l e k t r y c z n e j a. O t r z y m a n e w y n ik i w s k a z u j ą na t w o r z e n i e się w z g l ę d n i e t r w a łych " k o m p l e k s ó w " o s k ł a d z i e MEA • 2H„0 w c i e k ł y c h m i e s z a n i n a c h wody i m o n o e t a n o l o a m i n y .
