8 The Apparent Charge of Ionic Micelles in
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Aqueous Binary Solvents CLAUDE TREINER, ANNE L E B E S N E R A I S ,
and CHRISTIAN MICHELETTI Universite Pierre et Marie Curie, Laboratoire d'Electrochimie, 4, Place Jussieu, 75005 Paris, France
The degree of dissociation αof ionic micelles for sodium decylsulfate and trimethyldecylammonium bromide have been determined in water + acetone and water + n-propanol mixtures from conductance measurements. Using these data and assuming a pseudophase model for the equi librium between surfactant ions in the bulk and in the micellar phase the variation of the apparent charge of the micelles Ζ with addition of organic solvents can be calculated. After a small initial decrease, a constant Ζ value is observed in all cases studied; this result is interpreted as the consequence of two concomitant phenomena: an increase of α caused by the preferential solvation of cations by the organic com ponent and a decrease of the aggregation number of the micelles as the water structure is progressively destroyed by that same component.
T o n i c m i c e l l e s are c h a r g e d aggregates f o r m e d essentially i n w a t e r f r o m the c o o p e r a t i v e i o n - i o n i n t e r a c t i o n s o f electrolytes c o n s t i t u t e d b y a linear hydrocarbon chain attached to a n ionic group a n d a counterion. T h e a d d i t i o n o f o r g a n i c substances t o i o n i c m i c e l l e s m a y a l t e r t h e size a n d c h a r g e a n d u l t i m a t e l y d e s t r o y these i o n i c aggregates ( 1 ) ; h o w e v e r , l i t t l e is k n o w n a b o u t t h e b e h a v i o r o f t h e i o n i c constituents o f t h e m i c e l l e s i n t h e m i x e d solvents a n d t h e i r influence o n t h e m i c e l l e itself, a l t h o u g h i n s o m e cases c h a n g e s i n t h e p r o p e r t y o f a m i c e l l a r s o l u t i o n w i t h a d d i t i o n o f o r g a n i c m o l e c u l e s h a v e b e e n a t t r i b u t e d to changes o f t h e i o n - s o l v e n t interactions
(2,3,4,5). 0-8412-0428-4/79/33-177-109$05.00/l © 1979 American Chemical Society
In Thermodynamic Behavior of Electrolytes in Mixed Solvents—II; Furter, W.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
110
T H E R M O D Y N A M I C BEHAVIOR O F E L E C T R O L Y T E S
Π
I n c o r p o r a t i o n o f t h a t effect w a s difficult b e c a u s e r e l i a b l e t h e r m o d y n a m i c data were not available for the standard properties of the ionic components
o f t h e m i c e l l e i n t h e m i x e d solvents.
T h u s w e have deter
m i n e d r e c e n t l y ( 6 , 7 ) t h e s t a n d a r d G i b b s free e n e r g y of transfer, of
two ionic
(nDTMABr)
surfactants,
namely
trimethyldecylammonium
AG °, t
bromide
a n d sodium decylsulfate ( S D S ) from water to water
+
acetone m i x t u r e s at 298.15 K , u s i n g a p r e c i s e v a p o r p r e s s u r e t e c h n i q u e . W e shall show, using the pseudophase
m o d e l i n order to describe the
e q u i l i b r i u m b e t w e e n ions i n t h e b u l k a n d i n t h e m i c e l l a r phase, t h a t t h e Downloaded by OHIO STATE UNIV LIBRARIES on June 11, 2012 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch008
b e h a v i o r of m i c e l l e s at t h e c r i t i c a l m i c e l l e concentrations ( c m c ) c a n b e , i n these m i x e d solvents, essentially d e d u c e d f r o m t h e p r o p e r t i e s of t h e i o n i c constituents at infinite d i l u t i o n . F u r t h e r m o r e , w e h a v e d e t e r m i n e d the d e g r e e o f d i s s o c i a t i o n o f t h e m i c e l l e s u s i n g a c o n d u c t i v i t y m e t h o d , a n d formally treating the electric potential created b y the micelle b y n u m e r i c a l i n t e g r a t i o n of t h e P o i s s o n - B o l t z m a n n E q u a t i o n w e h a v e deter m i n e d t h e v a r i a t i o n of its a p p a r e n t c h a n g e w i t h t h e a d d i t i o n of acetone. T h e case o f w a t e r +
n-propanol mixtures where
m i x e d micelles are
f o r m e d also w i l l b e d i s c u s s e d . Theory and
Experimental
T h e m i c e l l a r s o l u t i o n is d e s c r i b e d u s i n g t h e p s e u d o p h a s e w h i c h i m p l i e s t h a t t h e c m c corresponds
(1,8,9,10),
c o n c e n t r a t i o n of m o n o m e r s
model
to a m a x i m u m
i n the b u l k solution above w h i c h a second
p h a s e ( t h e m i c e l l e p h a s e ) appears.
T h i s is a v e r y c r u d e m o d e l w h i c h
has b e e n d i s c u s s e d e x t e n s i v e l y i n t h e l i t e r a t u r e . W e u s e i t m e r e l y b e cause i t is t h e o n l y o n e t r a c t a b l e i n t h e c o m p l e x m e d i a w e a r e d e a l i n g w i t h (ions + m i c e l l e s + m i x e d s o l v e n t ) . W e m a y w r i t e : i n water, fho — μ*° + 2 R 7 In ( c m c ) , . 7
(f ) ±
w
= /x ° + RT In a ; w
mic
(1)
i n solvent s, μ* = μ ° + 2RT In ( c m c ) , {f ) 8
±
= / w '
8
+ RT In a ,
0
s
w h e r e μ. ' is t h e c h e m i c a l p o t e n t i a l o f t h e m o n o m e r i n t h e b u l k s o l u t i o n Β
a n d i n t h e m i c e l l e , /x ° t h e s t a n d a r d c h e m i c a l p o t e n t i a l o f t h e m o n o m e r mic
i n the micelle, f
±
t h e m e a n a c t i v i t y coefficient of t h e free ions, a n d a t h e
a c t i v i t y of t h e m o n o m e r i n t h e m i c e l l e phase. I f t h e s o l v e n t does n o t p a r t i c i p a t e to t h e m i c e l l e f o r m a t i o n , t h e n t h e m i c e l l e m a y b e c o n s i d e r e d as a p u r e p h a s e , t h e n a
w
p a r t i c u l a r case w e m a y also w r i t e
/A
m i c
°
=
/Ami '°. C
=
a = 8
1. I n t h a t
T h u s w e get:
In Thermodynamic Behavior of Electrolytes in Mixed Solvents—II; Furter, W.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
8.
Ionic
TREINER E T A L .
°
-
μβ
= Δ(?ί° =
μ° κ
111
Micelhs
2RT In . (
c m c )
."
.^j»
(2)
If w e c o n s i d e r that t h e o r g a n i c solvent p a r t i c i p a t e s to t h e m i c e l l e formation, then:
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AG,°
- 2RT In , (
C m c )
" j{ «
+ RT In χ f +
± }
(cmc),
(/ ) ±
