Enthalpies of Solution of Several Solutes in Aqueous-Organic Mixed

17 Enthalpies of Solution of Several Solutes in Aqueous-Organic Mixed Solvents C. DE VISSER and G. SOMSEN

Downloaded by UNIV OF ARIZONA on March 11, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch017

Department of Chemistry, Free University of Amsterdam, de Lairessestraat 174, Amsterdam, The Netherlands

Measured enthalpies of solution of n-Bu NBr at 5°, 25°, and 55°C in mixtures of DMF and water are compared with those of other tetraalkylammonium bromides, rubidium chloride, and urea, reported earlier. For the latter salts, the enthalpies of solution in the mixtures are almost proportional to the mole fraction of water, but for n-Bu NBr a large endothermic maxi mum occurs. From this profile it appears to be possible, using a simple hydration model, to calculate both the number of water molecules which surround a hydrophobic alkyl group and the enthalpic effect of the hydrophobic hydration. The latter effect appears to diminish strongly with increasing tem perature, whereas the number of water molecules involved to the hydrophobic hydration changes very little with tempera ture. 4

4

O

ne difference i n behavior between the h y d r o p h i l i c a l k a l i halides a n d h y d r o p h o b i c solutes l i k e the larger t e t r a a l k y l a m m o n i u m halides i n water is expressed b y the enthalpy. T h e enthalpies of solution of the larger tetr a a l k y l a m m o n i u m halides i n water are more exothermic than those of the corresponding alkali halides but i n other solvents, e.g., several amides, propylene carbonate (PC), and dimethylsulfoxide ( D M S O ) , the reverse is true. Generally, this phenomenon is attributed to a n enhanced h y d r o g e n b o n d i n g i n the h i g h l y structured solvent water i n the v i c i n i t y of the t e t r a a l k y l a m m o n i u m ions (hydrophobic hydration) (J). This idea is substantiated b y the absence of the effect i n solvents like N,N-dimethylformamide ( D M F ) , P C , a n d D M S O (2), where specific structural effects are not present i n the p u r e solvents. In order to see how the hydrophobic hydration breaks d o w n w h e n an aprotic cosolvent is a d d e d to water a n d h o w it starts to contribute w h e n water is a d d e d to the aprotic solvent, w e measured the enthalpies of solution of h y d r o p h o b i c and, for comparison, h y d r o p h i l i c solutes i n m i x e d solvent systems of water a n d 289

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

290

THERMODYNAMIC BEHAVIOR O F ELECTROLYTES

T a b l e I.

Standard Enthalpies o f Solution i n k j m o l

-

1

of

Tetra-n-butylammonium Bromide in Mixtures of A ^ N - d i m e t h y l f o r m a m i d e a n d W a t e r a t 5 ° , 2 5 ° , a n d 5 5 ° C as a F u n c t i o n o f the M o l e Fraction o f Water, 5°C AH° (sol.)

XH 0


2

0 0.224 0.421 0.601 0.676 0.806 0.890 0.949 1

Xp H

55°C

25°C

+9.32 +16.31 +22.10 +25.86 +25.69 +18.45 +5.26 -8.19 -23.76

+ + + ± ± ± ± ±

0.01 0.06 0.03 0.02 0.01 0.02 0.02 0.03

AH° (sol.)

^H 0 2

0 0.192 0.311 0.488 0.623 0.695 0.760 0.869 0.945 1

+12.5 +18.48 +22.07 +27.12 +29.63 +29.77 +27.89 +18.21 +5.26 -8.58

+ + + + ± + + ± +

0.05 0.05 0.04 0.05 0.04 0.03 0.06 0.02 0.06

^:i o 0 0.124 0.321 0.519 0.681 0.778 0.874 0.930 1 2

AH° (sol.) +15.90 +19.88 ± +25.68 ± +31.78 ± +35.02 ± +34.31 ± +29.38 ± +23.92 ± +13.08 ±

0.04 0.04 0.05 0.05 0.05 0.02 0.03 0.04

an aprotic solvent like D M F . I n the present paper, the enthalpies of solution of t e t r a - n - b u t y l a m m o n i u m b r o m i d e ( n - B u N B r ) i n D M F - w a t e r i n particular are 4

considered a n d the results w i l l be c o m p a r e d w i t h those of other t e t r a a l k y l a m m o n i u m bromides, R b C l , a n d urea published before (3, 4, 5). Since it might be expected that the hydrophobic hydration effect is very sensitive to temperature changes, the enthalpies of solution of n - B u 4 N B r i n D M F - w a t e r have been measured at 5 ° , 2 5 ° , a n d 5 5 ° C .

Table II.

Standard Enthalpies o f Solution i n k j mol""

1

Tetra-n-propylammonium Bromide, of Rubidium Chloride, and Water MeJSfBra ^H 0 2

0 0.100 0.263 0.364 0.506 0.674 0.759 0.849 0.949 1

4

4

± 0.12 ± 0.03 ± 0.03 ± 0.01 ± 0.03 ± 0.04 ± 0.01 ± 0.06 ± 0.05

^H o

AH°

0 0.147 0.310 0.446 0.563 0.683 0.820 0.928 1

9.4 12.86 15.30 16.39 15.96 13.86 9.91 7.40 6.2

2

o f the

n-Pr NBrb

Et NBra

AH° (sol.) 11.3 16.52 20.32 21.72 23.32 22.76 22.35 21.97 23.33 24.57

as a F u n c t i o n

(sol.) ± 0.06 ± 0.04 ± 0.05 ± 0.01 ± 0.03 ± 0.05 ± 0.04

X

HO 2

0 0.143 0.309 0.494 0.600 0.705 0.865 1

AH° 9.9 13.64 17.20 20.28 20.65 18.92 9.87 -4.25

Reference 3. b Reference 6. Reference 5. Reference 4.

a

c

d


(sol.) ± 0.02 ± 0.05 ± 0.01 ± 0.01 ± 0.02 ± 0.03 ± 0.05

17.

DE VISSER AND SOMSEN

Enthalpies of Solution

291

Experimental and Results A l l enthalpy of solution measurements were carried out w i t h an L K B 8700-1 precision calorimetry system.

T h e experimental procedure a n d tests of the

calorimeter have been reported previously (3, 4, 5).

T h e p u r i f i c a t i o n of the

solvent D M F (Baker Analyzed Reagent) and of all solutes used has been described i n the same papers.

T h e solvent mixtures were p r e p a r e d b y w e i g h i n g a n d the

mole fraction of water i n the D M F - w a t e r mixtures was corrected for the original water content of the a m i d e as measured b y K a r l F i s c h e r titration. T h e enthalpy of solution measurements of n - B u 4 N B r i n D M F - w a t e r were


made i n very dilute solutions (0.02-0.001 mole k g

- 1

) so that, i n v i e w of the ex-

p e r i m e n t a l error, a n y concentration dependence of the enthalpies of solution i n these solutions was neglected.

Consequently, the enthalpy of solution at i n -

finite d i l u t i o n , AH°(sol.), was taken to be the average of three or more independent measurements agreeing w i t h i n 150 J m o l . - 1

F i n a l results of AH°(sol.)

of n - B u N B r w i t h their mean deviations at 5 ° , 2 5 ° , a n d 5 5 ° C are given i n Table 4

I.

T h e results for the other t e t r a a l k y l a m m o n i u m bromides, for R b C l , a n d for

urea i n D M F - w a t e r at 2 5 ° C are s u m m a r i z e d i n T a b l e II. Discussion Previously w e f o u n d that the enthalpies of solution of n - B u N B r i n mixtures 4

of D M F and n - m e t h y l f o r m a m i d e ( N M F ) are almost proportional to the solvent composition a n d m a y be regarded as close to i d e a l (6). I n this context, ideal behavior means that AH

id

= X AH ° A

A

+ X B A H B i n w h i c h AH ° 0

A

and A H

B

0

are

the enthalpies of solution i n two pure solvents A a n d B . As a consequence, the of T e t r a m e t h y l a m m o n i u m - , Tetraethylammonium-, a n d of U r e a at 25°C i n M i x t u r e s o f A V V - D i m e t h y l f o r m a m i d e a n d Mole Fraction o f Water, X

H

RbCl?

uread

H0

AH° (sol.)

^H 0

0 0.513 0.651 0.749 0.828 0.934 1

0.89 13.22 16.10 16.34 16.13 16.04 17.21

0 0.100 0.200 0.300 0.400 0.500 0.601 0.700 0.800 0.928 1

X

2

O

± 0.06 ± 0.05 ± 0.02 ± 0.03 ± 0.03 ± 0.02

2

AH° (sol.) 5.85 ± 5.44 ± 5.48 ± 5.75 ± 6.32 ± 7.31 ± 8.70 ± 10.25 ± 11.89 ± 14.05 ± 15.28 ±

0.03 0.03 0.01 0.03 0.04 0.03 0.03 0.01 0.01 0.03 0.04


292

THERMODYNAMIC BEHAVIOR OF ELECTROLYTES

excess enthalpy of solution A H ( s o l . ) is d e f i n e d as AJF/ (SO1.) = A H E

e

P -

E X

AH

id

= 0 for n - B u N B r i n D M F - N M F . 4

In D M F - w a t e r mixtures, the A H ( s o l . ) of n - B u N B r deviates substantially E

4

f r o m zero, but for M e N B r , urea, a n d R b C l , AH (sol.)

is small, especially w h e n

E

4

c o m p a r e d w i t h the values for n - B u N B r (see F i g u r e 1). 4

In view of the different behavior of n - B u N B r i n mixtures of D M F and N M F 4

a n d of D M F a n d water, w e recently (6) d e r i v e d a n equation for the excess enthalpy of solution i n the D M F - w a t e r m i x t u r e ( A H ( M ) ) b y use of a simple h y E

drophobic hydration model.

S u m m a r i z i n g this derivation, w e conceived the

enthalpies of solution i n the D M F - H 0 system (AH°(M)) 2


two effects:

as b e i n g the result of

(a) W h e n the hydrophobic hydration of t e t r a a l k y l a m m o n i u m ions

is absent, the corresponding enthalpy of solution i n pure water A H ^ H f e O ) a n d i n the m i x t u r e AH (M)

should be correlated by:

l

A H K M ) = X H a o A f f ^ H a O ) + (1 - X

H 2

o ) AH°(DMF)

(1)

(b) H o w e v e r , i n the real case, w e must account for h y d r o p h o b i c h y d r a t i o n w i t h an enthalpic contribution both i n pure water, H b ( H 0 ) a n d i n the aqueous 2

mixture, H b ( M ) .

Consequently, A H ° ( M ) = AH (M)

+ Hb(M).

l

Since also

A H ( H O ) = A H ( H 0 ) + H b ( H 0 ) , the excess e n t h a l p y of solution i n the 0

1

2

2

2

m i x t u r e is given b y : A H ( M ) = Hb(M) - X H b ( H 0 ) E

(2)

2

Values of H b ( M ) w i t h increasing mole fraction X of water can be calculated w i t h the a i d of the m o d e l of M a s t r o i a n n i , P i k a l , a n d L i n d e n b a u m (7), using the f o l l o w i n g assumptions: (a) In aqueous solution a n - B u N 4

i o n is surrounded b y a cage of N water

+

molecules a n d each b u t y l g r o u p is surrounded b y a subcage of N / 4 water molecules.

T h e presence of one or more D M F molecules w i l l prevent the formation

of a subcage but w i l l not affect the structures of the subcages a r o u n d the other b u t y l groups. (b) In a m i x t u r e of D M F a n d water, the distribution of solvent molecules surrounding the ion is random so that the probability of a given solvation site being occupied b y a water molecule is X H O 2

Hence, this probability is X

N

/

4

for N / 4

sites. Table III.

Values o f the Enthalpic Effect of H y d r o p h o b i c H y d r a t i o n

H b ( H 0 ) , t h e N u m b e r o f W a t e r M o l e c u l e s p e r A l k y l G r o u p N/4 2

C o r r e l a t e d w i t h It, a n d the M e a n Deviations o f the C a l c u l a t e d f r o m t h e E x p e r i m e n t a l V a l u e s o f AH

E

M i x t u r e s at D i f f e r e n t T(°C) 5 25 55

Hb(H 0)(kJ 2

-65.6 -52.8 -34.4

mol- ) 1

of n-Bu NBr in DMF-Water 4

Temperatures N/4 6.0 6.4 6.7

Mean Deviation(kJ 0.38 0.23 0.15


mol' ) 1

17.

D E VISSER A N D SOMSEN

Enthalpies

293

of Solution

(c) T h e enthalpic effect of h y d r o p h o b i c h y d r a t i o n is merely the result of subcage formation and each butyl group contributes \ H b ( H 0 ) .

Consequently,

2

H b ( M ) = 4 . X / . ( y 4 ) H b ( H 0 ) = X / H b ( H 0 ) a n d E q u a t i o n 2 becomes: N

4

N

2

Atf

E

4

2

= (X"/ -X)Hb(H 0) 4

(3)

2

E q u a t i o n 3 can be used i n a c u r v e - f i t t i n g p r o g r a m (6) to search for the best f i t of the experimental data b y o p t i m i z i n g both parameters H b ( H 0 ) a n d N / 4 . 2

These calculations were p e r f o r m e d o n a n I B M - 1 1 3 0 c o m p u t i n g system.

The

curve f i t t i n g has been carried out f o r A H values of n - B u N B r i n mixtures of E

4

D M F a n d water at 5 ° , 2 5 ° , a n d 5 5 ° C as seen i n F i g u r e 2. Values of H b ( H 0 ) 2

and N / 4 corresponding to the best f i t together w i t h the m e a n deviation of the calculated A H values f r o m the experimental ones, are g i v e n i n Table III. It


E

should be noted that i n the derivation of Equations 2 a n d 3 clearly it is assumed that any significant deviation of A H ( M ) f r o m zero is caused b y a change of the E

hydrophobic hydration of n - B u N B r i n the m i x e d solvents.

I n v i e w of the very

4

small values of AH (M)

for non-hydrophobic solutes (Figure 1), this idea has been

E

justified as a first approximation. T

I

i

i

i

i

i

i

r

j

i 0.4

i

I 0.6

i

i 0.8

L

30

25

i |

20

— > ^

15

I

Enthalpies of Solution of Several Solutes in Aqueous-Organic Mixed

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