Relationship beetwen the enthalpy of transfer of a solute and the thermodynamic mixing functions of mixed soluent components

A C T A U N I V E R S I T A T I S L 0 O Z I E N S I S FOLIA CHIMICA 9, 1991

Jo la n ta Barczyriska*

RELATIONSHIP BEETWEN THE ENTHALPY OF TRANSFER OF A SOLUTE ANO THE THERHOOYNAHIC MIXING FUNCTIONS

OF MIXED SOLVENT COMPONENTS

o On the b a sis of date on the enthalpy of s o lu tio n s, AHS , of CsCl2 and Nal in aqueous m ixtures of methanol, ethanol

and propan-l-ol and of NaCl and KC1 in water * tetrahydro- furan m ixtures - the enthalpy of tr a n s fe r , AHt , of the so­ lu te s from water to m ixtures has been c a lc u la te d . The de­ pendence of AH° on the so lv en t com position has been com­ pared w ith the analoqous r e la tio n s h ip for the enthalpy of mixing, AHE , of the two so lven t components. Ihe c o r r e la ­ tio n suggested by Feakins between: AHt / * 2 * F(AHE/x2) has

not been found lin e a r in the whole r ic h regio n.

DISCUSSION

The appearence of extremal values of some thermodynamic fun­ c tio n s (and also of s e v e ra l physicochem ical p ro p e r tie s ) is ch ara­ c t e r i s t i c fo r s o lu tio n s of s a lt s in w ater-alcohol so lv e n ts as w ell as fo r so lve n ts themuelves.

For example: in v e s tig a tio n s performed e a r l i e r [ l , 2] proved th at the en th a lp ie s of t r a n s fe r , AH®, of a lc a li- m e ta l h a lid e s from water to water-methanol m ixtures pass trough pronauced maxima in the water r ic h region (0.1 < x2 < 0 .2 ). The enthalp y of mixing, AH^, of the two so lv e n t components [3, 4] e x h ib its minimum c o rre s ­ ponding to x2 ca. 0.3 of methanol ( F ig . 1).

F ig . 1. a) P lo ts of AH°/xMe0K. ag ain st AHE/xMe«H for A L1C1 and O NaCl in methanol-water so lv en t systems from r e f . [5] and b) e n th a l­ p ie s of mixing AHE ploted ag ain st the mol fr a c t io n of alc o h o ls

in m ethanol-water; ethano l-w ater; propan-l-ol ♦ water and tetrahy- drofuran-water systems [12]

Feakins and co-workers [5] made an e f f o r t to c o r r e la te both thermodynamic fu n ctio ns fo r the m ethano l-w ater-alcali metal h a lid e s systems. By means of th e o r e tic a l co n sid e ra tio n s the autors p re d i­ cted the lin e a r c o r r e la tio n of the standard tr a n s fe r enthalpy of e le c t r o ly t e s from water to the mixed s o lv e n t, AH?, and the

ent-r x

halpy of mixing o f water w ith a lc o h o l, AH , according to equa­ tio n :

AH° ■ -aAH^ + Xgb □r AH^ _ AH 4 . -a— — ♦ b B

(

)

I t turned out th at in water-methanol s o lu tio n s of a l k a l i metal c h lo rid e s expression (1 ) is f u l f i l l e d over the whole range of the b in ary so lve n t com positions only fo r LiCl-water-m ethanol system ( F ig . 1). In the case of other c h lo rid e s good l i n e a r i t y is shown

a h; XM»OH [kJ ird'l

fo r 0 < x2 < 0.7 in NaCl so lu tio n s anti fo r 0 < x2 < 0.45 in KC1 s o lu tio n s [5 ]. According to the authors [5] the maxima observed in A H ° curves seem to be only a consequence of the appearance of minima of the enthalpy of mixing vs. alcoh ol content corresponding to the s t a b iliz a t io n of water s tru c tu re . Therefore, maxima of AH° do not prove the change of s tru c tu re r e s u ltin g from the e f f e c t of e le c t r o ly t e s o lv a tio n on the so lven t s tru c tu re .

F ig . 3. P lo ts of AH|/xcf0H again st AH /xET0H fo r o Nal and □ CaClo in ethanol-water solven t systems

In th is work we t r ie d to apply the proposed c o r r e la tio n in w ater-alcohol m ixtures fo r e le c t r o ly t e s 1 : 1 type i . e Nal [6] and 2 : 1 type i . e . CaC^ [7] and in water*THF m ixtures fo r NaCl and KC1 [8, 9 ]. The re s u lts of using eqn. 1 fo r water-methanol s o lu ­ tio n s of Nal and CaCl2 are presented in F ig 2. According to the suggestion of the autors [5] F ig . 2 shows the p lo ts of AH°/xCH QH ag ain st AH^/x^^ AH^ from r e f . [10 ].

As I t Is seen to r Nal and C aClj s o lu tio n s the lin e a r c o r r e la ­ tio n between two thermodynamic fu n ctio n s is observed only w ith in the range 0 < x2 < 0 .2 methanol i . e . to such co n cen tratio n of methanol in water which corresponds to the maxima of the AH°

[6. 7 ].

Maxima of the tra n s fe r enthalpy s h i f t towards the sm aller alcoh ol content together w ith the in crease of the s iz e of alco h o l molecule and are observed at x2 ca. 0.10 ethanol and x2 ca. 0.B propan-l-ol [6, 7 ]. Minima of the mixing enthalpy show an analoqous tendency ( F ig . 1) and pass trough pronounced minima in the region 0.10 < x2 < 0.15 [11 ].

The la rg e r the alco h o l molecule is the more narrow the s tr a ig h t lin e a r region is . The p lo ts fo r Nal and CaCl2 are lin e a r to x2 ca. 0.10 ethanol (F ig . 3) and to x2 ca. 0.8 propan-l-ol (F ig . 4 ).

I t can be concluded th at the re la tio n s h ip between the AH* of

C t

the so lu te and AH in a b in ary mixed so lv e n t concerns only the large amount of water i . e . from pure water up to the composition

corresponding to maximum of AH®. I t seem th a t the c o r r e la tio n proposed by F e a-k i n s e t a l . depends d i s t i n c t l y on extrema of curves AH° ■ f ( x 2) .

This conclusion is supported by in v e s tig a tio n s of NaCl and KC1 s o lu tio n s in te trahydrofuran-w ater m ixtures. The lin e a r p lo t does not exceed the composition range 0 < x2 < 0.25 of THF ( F ig . 5) i . e . from water to maxima of AH° [12 ].

The a n a ly s is of experim ental data fo r the systems mentioned above does not g e n e ra lly confirm the F e a k i n s ' e t a l . [5] suggestion. I t appears, th e re fo re , th at fo r the water - alco h o l systems, the tra n s fe r enthalpy of the e le c t r o ly t e shows some more complex fe a tu re s . For example, i t e x h ib its minima in the range of high alcoh ol content [7 ]. However, Feakins and co-workers make no assumption about the nature of the s o lu te .

REFERENCES

[1] M. S 1 a n S k y, 3. Am. Chem. S o c ., 6£, 2430 (1940).

[ 2 ] E . de V a l e r a , D. F e a k i n s , W . E . W a g h o r - n e, 3. Chem. Soc. Faraday Trans. I , 76, 560 (1980).

[3] L. B e n j a m i n , G. C. B e n s o n , 3. Phys. Chem., £7, 858 (1963). [4] B. M a r o n g i u, 3. F e r i n o, R. M o n a c i , U. S o l i n a s, S. T o r r a z z a , Jo u rn a l of M olecular L iq u id s , 28, 229 (1984). [5] E. de V a l e r a , D. F e a k i n s , W. E. W a g h o r- n e , 3. Chem. Soc. Faraday Trans. I . 22» 1061 (1983).

[6] H. P i e k a r s k i , Can. 3. Chem,. 61, 2203 (1983).

[7] S. T a n i e w s k a-0 s i r i s k a , 3. B a r c z y r t s k a , 3. Chem. Soc. Faraday Trans. I , 80* 1409 (1984).

[8] S . T a n i e w s k a - O s i ń s k a , B. P i e s t r z y ń ­ s k a , R. L o g w i n i e n k o , Can. J . Chem., 5£J, 1584 (1980).

[9] S. T a n i e w s k a-0 s i r i s k a , B. N o w i c k a , Thermo­ c h i« . A cta, 115. 129 (1987).

[10] R. F . L a m a , B. C-Y. L u, 3. Chem. Eng. Data, H), 216 (1965).

[11] A. C. B r o w n , 0. J . G. I v e s , J . Chem. Soc. A, 1608 (1962).

[12] 0. K 1 y o h a r a, G. C. B e n s o n , Can. J . Chem., 55, 1354 (1977).

[13] W. P. 8 e l o u s o v , A. G. M o r a c h e v s k i J , Tep- lo ty Smieshenla Z h y tk o s tij, Iz d . Khim ja, Leningrad 1970.

Jo la n ta Barczyrtska

ZALEŻNOŚĆ MIĘDZY ENTALPIĄ PRZENIESIENIA SUBSTANCJI ROZPUSZCZONEJ A TERMODYNAMICZNYMI FUNKCJAMI MIESZANIA ROZPUSZCZALNIKÓW

Na podstawie danych e n t a lp ii rozpuszczania, AH°, e l e k t r o l i ­ tów: CaCl? oraz Nal w mieszaninach wody z metanolem, etanolem i n-propanolem (p ro p a n - l- o l), a także NaCl i KC1 w mieszaninach wody z tetrahydrofuranem obliczono e n ta lp ie p rz e n ie s ie n ia , AHf, wymie­ nionych e le k tr o litó w z wody do mieszanych rozpuszczalników. Porów­ nano ‘przebieg zależności AHt i AHfc w fu n k c ji składu badanych m ieszanin. Sprawdzając koncepcję Feakinsa podjęto próbę k o r e la c ji* AH?/ x_ - f ( AHe/ *j ) we wszystkich analizowanych układach i stwierdzono, że lin io w a zależność między funkcjami termodynamiczny­ mi n ie obejmuje całego zakresu składów badanych mieszanin.