PRESSURE DEPENDENCE OF THE HYDROPHOBIC EFFECT A NMR STUDY IN THE SYSTEM T-BUTANOL/D2O

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PRESSURE DEPENDENCE OF THE

HYDROPHOBIC EFFECT A NMR STUDY IN THE SYSTEM T-BUTANOL/D2O

M. Woznyj, E. Lang, H.-D. Lüdemann

To cite this version:

M. Woznyj, E. Lang, H.-D. Lüdemann. PRESSURE DEPENDENCE OF THE HYDROPHOBIC EFFECT A NMR STUDY IN THE SYSTEM T-BUTANOL/D2O. Journal de Physique Colloques, 1984, 45 (C7), pp.C7-179-C7-183. �10.1051/jphyscol:1984720�. �jpa-00224285�

JOURNAL DE PHYSIQUE

Colloque C7, supplCment au n09, Tome 45, septembre 1984 page C7-179

PRESSURE DEPENDENCE OF THE HYDROPHOBIC E F F E C T A NMR STUDY I N THE SYSTEM T - B U T A N O L / D ~ O

M. Woznyj, E.W. Lang and H.-D. ~iidemann

I n s t i t u t fur Biophysik und PhysikaZische Biochemie, Universitat Regensburg, Postfach 397, 0-8400 Regensburg, F. R. G.

Resume - La dynamique m o l @ c u l a i r e de l a phase l i q u i d e du systeme b i n a i r e t-bu- ' m l ' e a u l o u r d e a e t 6 6 t u d i e e par RMN. n t e des r e s u l t a t s s u r l e s temps de r e l a x a t i o n spin-rereau T~ pour ql) :;'I! 0 des molecules d'eau, s u r l e s v i t e s s e s de r e l a x a t i o n i n t e r m o l e c u l a i r e s proton-proton e t s u r l e s c o e f f i c i e n t s d ' a u t o - d i f f u s i o n des mol6cules de t - b u t a n o l . Les mesures couvrent l e s games de temperature e n t r e 273 K e t 430 K e t de p r e s s i o n e n t r e 0 , l e t 200 MPa. L ' a s - s o c i a t i o n des mol6cules t - b u t a n o l n ' a l i e u que dans l a gamme de temperature e n t r e 310 K e t 360 K. En dehors de c e t t e gamme on ne peut pas observer d'asso- c i a t i o n . On n ' a pas pu d e t e c t e r d ' e f f e t s i g n i f i c a t i f de l a p r e s s i o n s u r ce comportement .

A b s t r a c t - The molecular dynamics o f the l i q u i d phase o f t h e b i n a r y system techniques. Presented are the s p i n l a t t i c e r e l a x a t i o n 0 o f the water molecules, the

e s e l f d i f f u s i o n c o e f f i c i e n t s of the t-butanol molecules. The measurements cover the temperature range between 273 K and 430 K and the pressure range from 0.1 t o 200 MPa. A s s o c i a t i o n of the t-butanol molecules occurs o n l y i n t h e temperature range between 310 K and 360 K. Outside t h i s r e g i o n no a s s o c i a t i o n can be observed. No s i g n i f i c a n t pressure e f f e c t o f t h i s behaviour c o u l d be detected.

I N T R O D U C T I O N

The i n f l u e n c e o f pressure upon the hydrophobic a s s o c i a t i o n i s s t i l l a m a t t e r of controversy. Due t o the very low s o l u b i l i t i e s o f p u r e l y apolar substances i n water i t i s necessary t o study model systems, i .e. substances which have q u i t e a l a r g e a p o l a r s u r f a c e r e g i o n as we1 1 as p o l a r groups (-OH, -NH2,. . . ^.). From s t u d i e s i n such model systems i t i s claimed t h a t pressure severely d e s t a b i l i z e s hydrophobic i n t e r - a c t i o n s , though the evidence presented i s scanty and i n d i r e c t /1,2/. A w i d e l y used model system f o r hydrophobic e f f e c t s i s t h e system t-butanol/water /3-6/. This system e x h i b i t s f u l l m i s c i b i l i t y over t h e whole composition range and i s therefore w e l l s u i t e d f o r a s s o c i t a t i o n s t u d i e s o f t h e "hydrophobic" component by NMR tech- niques.

E X P E R I M E N T A L

The samples were prepared by weighing t h e c a r e f u l l y p u r i f i e d substances and f r e e d from oxygen by several freeze-pump-thaw cycles.

The s p i n l a t t i c e r e l a x a t i o n times TI were obtained w i t h t h e usual

- T - ^{- 'r} - 1 pulse sequence on a Varian XL-100 FT NMR spectrometer o p e r a t i n g

2 2

a t 100.1 MHz f o r protons. The s e l f d i f f u s i o n c o e f f i c i e n t s D were measured by t h e s p i n echo technique i n a m o d i f i e d V 4415 probe using t h e same instrument. D e t a i l s o f experimental design and procedure can be found elsewhere /7/.

The measurements o f the p r o t o n p r o t o n i n t e r m o l e c u l a r r a t e o f t h e t-butan01 molecules were performed i n t e r n a r y mixtures o f (CH3)3COD, (CD3)3COD and D20 w i t h mole f r a c t i o n s x g - p , x g ( l - p ) and (1-x ) r e s p e c t i v e l y xg denoting the mole f r a c t i o n o f t-butanol and p the f r a c t i o n o f t-Eutanol molecules w i t h protonated methyl groups.

For a1 1 experiments t h e extreme narrowing 1 im i t applied. No observable i s o t o p e e f f e c t was seen when s u b s t i t u t i n g (CH3)$OD by (CD3)3COD. The r e l a x a t i o n r a t e Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984720

C7-180 JOURNAL DE PHYSIQUE

1

R1 ! of t h e methyl protons i n t h e s e t e r n a r y mixtures i s given by / 8 , 9 / :

1 i n t r a i n t e r i n t e r

R1 = R ₁

+ R1, XOD-CH3 ⁺ ( P ' ( ' - P ) . ~ ) R1, C H ~ - C H ~ ( I )

with I ( I +1

K = ) ^{. ) =} 1.189.10- 1

I H ( I H + ~ ) ( 2 )

YD H and I D , H a r e t h e gyromagnetic r a t i o s and s p i n quantum numbers of t h e deuteron and proton, r e s p e c t i v e l y . Varying p one can t h e r e f o r e determine t h e intermolecular proton-proton r e l a x a t i o n r a t e of t h e methyl aroups R1, i n t e r CH3-CH3 d i r e c t l y .

T H E O R E T I C A L

Neglecting intramolecular motions t h e i n t e r m o l e c u l a r p a r t of t h e methyl proton r e - l a x a t i o n r a t e can f a i r l y well be approximated by /ID/:

YH denotes t h e gyromagnetic r a t i o of t h e proton, D t h e d i f f u s i o n c o e f f i c i e n t of t h e t-butanol molecules. p ( r ) i s t h e r a d i a l p a i r d i s t r i b u t i o n f u n c t i o n of t h e i n t e r - a c t i n g methyl protons and a t h e i r d i s t a n c e of c l o s e s t approach.

Noreover one can d e f i n e t h e a s s o c i a t i o n parameter A /11/:

i n t e r . ^- D

A := R1, C H ~ - C H ~ n ( 4 )

with n t h e average d e n s i t y of t h e methyl protons. I n s e r t i n g ( 3 ) one obtains:

The experimentally a c c e s s i b l e q u a n t i t y A i s a measure of t h e molecular a s s o c i a t i o n of t h e t-butanol molecules. With t h e t-butanol molecules s t a t i s t i c a l l y d i s t r i b u t e d i n t h e s o l u t i o n p ( r ) i s proportional t o n and thus ^A i s independent of t h e t-butanol concentration. An i n c r e a s e of A with decreasinn t-butanol concentration i n d i c a t e s a s s o c i a t i o n of t h e t-butanol molecules.

R E S U L T S A N D D I S C U S S I O N

In Fig. 1 t h e deuteron spin l a t t i c e r e l a x a t i o n times of t h e water molecules a r e presented a t 5 MPa and 200 MPa a s function of t h e t-butanol concentration.

Assuming i s o t r o p i c r o t a t i o n a l m 3 t i o n of t h e water molecules i n t h e t-butanol/water mixtures one can c a l c u l a t e t h e H r e l a x a t i o n time of water from t h e 170 d a t a i n the case of those mixtures, t h a t due t o t h e rapid exchange between t h e water deuterons and t h e hydroxylic t-butanol de t e r o n s do n o t allow a d i r e c t experimental B

determination of t h e water m o b i l i t y from lH-T1. E s p e c i a l l y a t lower temperatures a d d i t i o n of t-butanol has a dramatic i n f l u e n c e upon t h e water mobility. This p o i n t s t o a promotion of water s t r u c t u r e through t h e a d d i t i o n of t-butanol molecules.

Fig. 2 shows t h e r e s u l t s of t h e s e l f d i f f u s i o n c o e f f i c i e n t of t h e t-butanol molecules a t two p r e s s u r e s . I t can be seen t h a t f o r low t-butanol concentrations t h e t r a n s l a t i o n a l mobility of t h e t-butanol molecules i s a f f e c t e d by t h e a d d i t i o n of t-butanol s i m i l a r l y t o t h e r o t a t i o n a l rnobi 1 i t y of t h e water molecules.

In Fig. 3 t h e s p i n l a t t i c e r e l a x a t i o n times of t h e t-butanol methyl protons a t 5 MPa i n two t e r n a r y s o l u t i o n s of (CH3)3COD, (CD3) COD and D20 a r e presented.

The mole f r a c t i o n of t-butanol xg i s equal i n both s o 7 u t i o n s . They only d i f f e r i n t h e i r f r a c t i o n o f protonated t-butanol molecules and provide an example f o r t h e determination of t h e methyl-methyl intermolecular r a t e . One can c l e a r l y s e e t h a t the i n t e r m o l e c u l a r r a t e e x h i b i t s a maximum a t about 335 K. (The e r r o r i n t h e i n - dividual TI-measurements being 2 %).

This behaviour has been observed f o r a l l but t h e h i g h e s t t-butanol concen- t r a t i o n s and can be seen i n Fig. 4 . Presented a r e t h e temperature and concen- t r a t i o n dependencies of the product R ~ : ~ ~ ~: ~A - n . - ~One can conclude t h a t ~ ~ - D

the a s s o c i a t i o n o f the t-butanol molecules goes through a maximum a t about 335 K f o r a l l b u t the h i g h e s t c o n c e n t r a t i o n s . This maximum gets q u i t e pronounced i n the lower concentrations. No s i g n i f i c a n t pressure e f f e c t o f t h i s behaviour can be seen.

Fig. 5 presents t h e A parameter (equ. 4) a t t h r e e temperatures and two pressures.

Density data b e i n g o n l y a v a i l a b l e f o r l i m i t e d pressure and temperature ranges we c a l c u l a t e d the p r o t o n d e n s i t i e s by assuming i d e a l s o l u t i o n behaviour and by neg- l e c t i n g pressure e f f e c t s . The e r r o r s i n t r o d u c e d by t h i s procedure should n o t

200 MPo

Tt Is1

t

1 0

01

0 01

-

-

F i g u r e 1 - Concentration and temperature dependence o f the s p i n l a t t i c e r e l a x a t i o n times T1 of the water molecules i n t-butanol/water mixtures a t 5 MPa and 200 MPa.

Closed symbols: d i r e c t l y measured :H-T~ o f D20.

Open symbols: c a l c u l a t e d lH-T1 2 from ' ~ O - T ~ measurements o f ~ ~ under t h e ' ~ 0 assumption o f i s o t r o p i c motion o f t h e water molecules.

F i g u r e 2 - Concentration and temperature dependence o f the s e l f d i f f u s i o n c o e f f i -

c i e n t s D o f the t - b u t a n o l molecules i n (CH3)3COD/D20 mixtures a t 5 MPa and 200 MPa.

C7-182 JOURNAL DE PHYSIQUE

wt-% t-butonol

- - -

- ^{2 3 2 7} - ³¹ 1 0 ~ 1 ~ IK-'I ^{3 5 2 3 2 7}

-

-

Figure 4 - Concentration and temperature dependence o f Ri . D a t 5 MPa and 200 EIPa.

D denotes t h e s e l f d i f f u s i o n c o e f f i c i e n t

1.0: of t h e t-butanol molecules and Ri t h e

- methyl-methyl proton i n t e r m o l e c u l a r r a t e

- _{i n} t h e (CH3)3COD/D20 sol utions.

-

I I I I I I r

2.3 2.7 3.1 3.5

- - l o 3 / ~ ( K - ~ I

A 5MPa 29LK 200MPa

Figure 3 - Temperature dependence of the ^(,5,,2, proton spin l a t t i c e re1 axation times T1 i n two t e r n a r y mixtures of (CH3)3C0D,

(CD3)3COD and D20 a t 5 MPa. The mole lo-\/p3+-++-+---+

If--*---

f r a c t i o n of t-butanol i s 0.071 i n both s o l u t i o n s .

o: 14 % of the t-butanol molecules a r e protonated

x: 87,7 % of t h e t-butanol molecules

a r e protonated lo-3a 3 3 9 ~

exceed 15 %. Considering t h e e r r o r s i n -

volved i n t h e determination of t h e A

Ifp

parameter, e s p e c i a l l y a t low concen- )*J;+~c +- - -*

.

t r a t i o n s , i t follows from t h e d a t a

presented i n Fig. 5 t h a t a s s o c i a t i o n l o 3 '

of t h e t-butanol molecules i s only observable a t t h e i n t e r m e d i a t e temperature of 339 K. A t 294 K and a t 439 K t h e d a t a a r e b e s t explained by

the a t-butanol s t a t i s t i c a l molecules. d i s t r i b u t i o n No s i g n i - of

::;- ;:,

.:::

f i c a n t pressure dependence could be observed.

0 20 LO 60 80 100 0

-

Figure 5 - Concentration dependence of t h e a s s o c i a t i o n parameter A f o r t-butanol i n (CH3)3COD/D20 mixtures a t various pressures and temperatures.

For a more extensive discussion, t h e various models f o r the i n t e r m o l e c u l a r proton- p r o t o n r e l a x a t i o n r a t e would have t o be a p p l i e d t o t h e data, and i n a d d i t i o n , one would want t o have a v a r i e t y o f thermodynamic and dynamic data i n t h e whole p,t range covered.

Provided, a l l these data were a v a i l a b l e , one would s t i l l have t o answer t h e question, t o what e x t e n t t h e t-butanol/water system can be a u s e f u l model system f o r hydrophobic phenomena, i . e . how the e f f e c t s caused by t h e h y d r o x y l i c group i n t e r f e r e w i t h t h e hydrophobic i n t e r a c t i o n and the s t e r i c a l c o n s t r a i n t s o f t h e t h r e e methyl arouos.

A C K N O W L E D G E M E N T

The work presented here was o n l y f e a s i b l e through t h e e x p e r t t e c h n i c a l assistance o f M r . S. Heyn, R. K n o t t and E. Treml. Generous f i n a n c i a l support was obtained from the DFG and t h e Fonds d e r Chemie.

R E F E R E N C E S

/ 1/ K. SUZUKI, Y. TANIGUCHI, T. WATANABE, J. Phys. Chem., 1973, 77, 1918 / 2/ N.S. ISAACS: L i q u i d Phase High Pressure Chemistry, John Wiley and Sons,

Chichester 1981, p. 354 ff

/ 3/ K. IWASAKI, T. FWIYAMA, J. Phys. Chem., 1977, 81, 1908 / 4/ K. IWASAKI, T. FUJIYAMA, J. Phys. Chem., 1979, 83, 463

/ 5/ K. TAMURA, M. MAEKAWA, T. YASUNAGA, J. Phys. Chem., 1977, 81, 2122

/ 6/ A. HVIDT, R. MOSS, G. NIELSEN, Acta Chem. Scand., 1978, B 32, 274 / 71 F.X. PRIELMEIER, E.W. LANG, H .-D. LDDEMANN, Mol . Phys., i n p r i n t / 8/ H.-D. LUDEMANN, E.W. LANG, This volume

/ 91 M. ZEIDLER i n : F. Franks ed. "Water - A Com~rehensive T r e a t i s e " , Plenum Press, New York 1973, Vol. 2, p. 529 ff

/ l o / H. LEITER, K.J. PATIL, H.G. HERTZ, J . Sol. Chem., 1983, 12, 503 /11/ E.v. GOLDAMMER, H.G. HERTZ, J . Phys. Chem., 1970, 74, 3734