11 Viscosity of Dilute Solutions of Alkali Halides
Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 14, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch011
in Methanol-Water Mixtures ROBERT A. STAIRS Department of Chemistry, Trent University, Peterborough, Ontario, Canada K9J 7B8
Viscosities of 0-0.1M solutions of lithium, potassium, and cesium chloride and potassium and rubidium iodide in water-methanol mixtures have been measured, and the results were fitted to the Jones-Dole Equation. The Β coeffi cients were interpreted as indicating slight to moderate preferential solvation of the ions by water vs. methanol. Values of the equilibrium constant representing the exchange at a single solvation site: H O (bound) + MeOH (free) H O (free) + MeOH (bound) were estimated as lying be tween 0.3 and 0.7 for all the salts, corresponding to differen tial solvation free energies,ΔG°(H O-MeOH) of 0.8-1.6 kJ/mol of bound solvent. Solvation numbers ranged from 10 to 16. The values of the A coefficients, although scattered, show trends that are in approximate agreement with a pre diction based on Walden's rule, in the absence of the con ductance data required for their calculation. 2
2
2
T n a p r e v i o u s s y m p o s i u m v o l u m e o n this t o p i c ( I ) , w e suggested
that
t h e v i s c o s i t y Β coefficient f o r t h e s o l u t i o n o f a salt i n a m i x e d solvent c o u l d i n d i c a t e p r e f e r e n t i a l s o l v a t i o n o f t h e ions b y one solvent c o m p o n e n t o v e r t h e other.
T h i s suggestion w a s s u p p o r t e d b y some m e a s u r e m e n t s
that w e r e i n t e r p r e t e d q u a l i t a t i v e l y as s h o w i n g preference iodide for water over methanol, for example.
of potassium
A s a start t o w a r d
more
systematic s t u d y o f t h e effect, this c h a p t e r reports t h e m e a s u r e m e n t o f t h e viscosities o f d i l u t e solutions o f s e v e r a l salts i n a s i n g l e solvent water-methanol.
W e also r e p o r t
a s m a l l step t o w a r d a
pair,
quantitative
interpretation. 0-8412-0428-4/79/33-177-167$05.00/l © 1979 American Chemical Society In Thermodynamic Behavior of Electrolytes in Mixed Solvents—II; Furter, William F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
168
THERMODYNAMIC
Let
us define
molecule
BEHAVIOR
OF ELECTROLYTES
Π
a n e q u i l i b r i u m constant f o r t h e s u b s t i t u t i o n of
of solvent species 2 f o r species
a
1 at a s i n g l e s o l v a t i o n site
( a s s u m i n g a l l sites are a l i k e ) :
Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 14, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch011
K =
where / i and f
are the fractions of sites c o n t a i n i n g b o u n d 1 a n d 2,
2
respectively, a n d X i a n d x
2
i n t h e solvent.
(1)
^
are t h e m o l e fractions of t h e free
components
( F o r t h e present, 1 is w a t e r a n d 2 is m e t h a n o l . )
A v o i d i n g the w a t e r - r i c h r e g i o n , w h e r e the s p e c i a l s t r u c t u r e of w a t e r m a y cause p r o b l e m s , c o n s i d e r the Β coefficient at x
=
1 / 2 , 2 / 3 , a n d 1.
VJ )
(2)
2
F r o m previous work w e have B where V , 0
V
lt
= 2.5 ( V + n V i +
E
0
and V
q (V
-
2
are t h e m o l a r v o l u m e s of t h e solute a n d solvent
2
species ( 1 a n d 2 ) , η is t h e n u m b e r of a v a i l a b l e s o l v e n t sites p e r " m o l e c u l e " of solute, a n d q — n / , t h e m e a n n u m b e r of m o l e s of 2 b o u n d p e r 2
m o l e of solute. T h e s u b s c r i p t Ε i n B
E
i n d i c a t e s t h a t this is t h e c o n t r i b u t i o n
to Β c a l c u l a t e d b y E i n s t e i n ( 2 ) , c a u s e d b y t h e s i m p l e b u l k of t h e s o l v a t e d ions. I f w e assume that i n this r e g i o n of s o l v e n t c o m p o s i t i o n t h e n u m b e r of sites, n , r e m a i n s constant a n d t h a t t h e w h o l e c h a n g e i n Β is c a u s e d b y t h e effect o n B
E
o f t h e c h a n g e i n q, w e c a n w r i t e :
β
Bl/2 ~~ B /3
=
2
B /3 — B i 2
=
qi/2 —
