2 Lyotropic Mesomorphism
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
Phase Equilibria and Relation to Micellar Systems INGVAR DANIELSSON Department of Physical Chemistry,ÅboAkademi, Porthansgatan 3-5, SF-20500Åbo(Turku) 50, Finland A survey is given of lyotropic mesophase structures characterized by the special effect of the solvent in aqueous lipid systems. They are compared with micelle formation. The equilibria between homogeneous micellar solutions and mesophases follow Gibb's phase law. The solutions have no long range order while the structural parameters of the mesophases are determined by the compositions. The short range order, which reflects the internal structure, is the same for different aggregates. The mesophases have fixed transition temperatures. Polar groups are completely hydrated, but the mesophases also bind "intercalated liquid." Association in an aqueous environment is caused by hydrophobic interactions. Possibilities of analyzing molecular structure by NMR and Raman spectrometry are discussed. Attention is also drawn to the thermodynamic criteria influencing the formation of mesophases: of these only the activity of water is known so far.
* " p h e t e r m l y o t r o p i c m e s o m o r p h i s m is u s e d t o d e s c r i b e t h e f o r m a t i o n o f t h e r m o d y n a m i c a l l y stable l i q u i d c r y s t a l l i n e systems t h r o u g h t h e p e n e t r a t i o n of a solvent b e t w e e n t h e m o l e c u l e s o f a c r y s t a l lattice. I n contrast to t h e t h e r m o t r o p i c m e s o m o r p h i s m s h o w n b y m a n y p u r e
substances,
l y o t r o p i c m e s o m o r p h i s m a l w a y s r e q u i r e s t h e p a r t i c i p a t i o n o f a solvent. L y o t r o p i c a l l y m e s o m o r p h o u s systems, h o w e v e r , a r e u s u a l l y as sensitive t o changes
i n t e m p e r a t u r e as t h e r m o t r o p i c systems.
So f a r , l y o t r o p i c
m e s o m o r p h i s m has b e e n o b s e r v e d almost e x c l u s i v e l y i n l i p i d containing water.
systems
L i p i d s that s h o w l y o t r o p i c m e s o m o r p h i s m f r e q u e n t l y 13
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
14
LYOTROPIC
LIQUID
CRYSTALS
c. Figure
1.
Structures of aggregates in solutions of association colloids (a) Pre-micellar aggregates in water solution (b) Ordinary micelles (Hartley micelles) in water solution (c) "Inverted? miceUes in oil solution
f o r m t h e r m o t r o p i c mesophases tures.
w i t h o u t a n y a d d i t i o n s at h i g h t e m p e r a -
R e c e n t research c o n c e r n i n g the l y o t r o p i c m e s o m o r p h i s m a n d its
r e l a t i o n to m i c e l l a r system covers a w i d e field, a n d o n l y essential features c a n b e g i v e n here. T h e s t r u c t u r e of the l y o t r o p i c a l l y m e s o m o r p h o u s lattice is m a d e u p of m u l t i m o l e c u l a r u n i t s c a l l e d mesoaggregates. an
intervening liquid.
T h e s e are s u r r o u n d e d b y
L y o t r o p i c m e s o m o r p h i s m is therefore
r e l a t e d to the t e n d e n c y of l i p i d s to a c c u m u l a t e at interfaces.
closely
T h e surface
a c t i v i t y is a consequence of t h e same d u a l i s t i c p o l a r / n o n - p o l a r m o l e c u l a r structure that causes the f o r m a t i o n of m i c e l l e s i n solutions of association c o l l o i d s (1, 2,
3,4,5,6).
M i c e l l e s are large p o l y m o l e c u l a r aggregates i n solutions. T h e y are t h e r m o d y n a m i c a l l y stable b e c a u s e of i n t e r m o l e c u l a r interactions.
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Some
2.
Lyotropic
DANiELSSON
Mesomorphism
15
types of m i c e l l a r aggregates are i l l u s t r a t e d i n F i g u r e 1. S m a l l aggregates w i t h f e w anions h a v e b e e n f o u n d i n solutions of s h o r t - c h a i n salts ( F i g u r e la).
N o r m a l H a r t l e y micelles
(Figure l b )
p r e d o m i n a t e i n surfactant
solutions a b o v e the c r i t i c a l m i c e l l e c o n c e n t r a t i o n ( C M C ). T h e l i p o p h i l i c g r o u p s a c c u m u l a t e i n t h e l i q u i d - l i k e i n n e r p a r t of the aggregates. hydrophilic (Figure
groups
lc)
are
directed towards
water.
"Inverted"
The
micelles
i n a hydrocarbon environment have their polar
groups
p i l e d u p i n t h e i n n e r p a r t of the m i c e l l e s . T h e s e i n v e r t e d m i c e l l e s m a y
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
also b i n d w a t e r .
S m a l l " i n v e r t e d " aggregates,
analogous
to the
pre-
m i c e l l e s f o r m e d i n aqueous solutions, are f r e q u e n t l y o b s e r v e d i n o r g a n i c s o l v e n t s — f o r e x a m p l e , i n e x t r a c t i o n systems u s e d i n c h e m i c a l t e c h n o l o g y . W h e n m i c e l l a r aggregates are f o r m e d i n solutions a n d t h e i r aggre g a t i o n n u m b e r s are not v e r y large, t h e y are r a n d o m l y d i s p e r s e d , o w i n g to t h e r m a l m o t i o n . W e a k i n d i c a t i o n s of a n i s o t r o p y are f o u n d at v e r y h i g h concentrations o n l y . Structure
of
the
Mesophases
X - r a y investigations b y L u z z a t i a n d S k o u l i o s ( 5 , 7 ) s h o w e d that t h e aggregates i n l y o t r o p i c mesophases micelles ( F i g u r e 2 ) .
are
structurally very
I n contrast to these, h o w e v e r , the
similar
to
mesoaggregates,
are u s u a l l y of i n f i n i t e d i m e n s i o n s i n one or t w o d i r e c t i o n s .
They form
o r d e r e d lattices, w h i c h cause the c h a r a c t e r i s t i c a n i s o t r o p y . T h e m a c r o s c o p i c flow p r o p e r t i e s of the mesophases d e p e n d o n the f a i r l y free transl a t i o n a l m o b i l i t y of the aggregates i n one or t w o d i r e c t i o n s ( F i g u r e 2 ) . T h e a m p h i p h i l i c m o l e c u l e s are a n c h o r e d t h r o u g h t h e i r p o l a r g r o u p s o n the b o r d e r s b e t w e e n the mesoaggregates regions i n the mesoaggregates
( t y p e 2 Ε or 2 D )
or i n the
that are r i c h i n w a t e r ( t y p e 2 F ) .
The
t y p e 2 D l a m e l l a r phase consists of d o u b l e layers of a m p h i p h i l i c m o l e cules—i.e., t h e i r structure membranes.
Between
is analogous
these layers
there
to that
of b i m o l e c u l a r
is w a t e r ;
the
lipid
hydrocarbon
moieties of the m o l e c u l e s f o r m l i q u i d - l i k e l i p o p h i l i c i n n e r layers
that
may solubilize oil. T h e r o d - s h a p e d aggregates,
2 E , also h a v e i n n e r , l i p o p h i l i c a n d
outer, p o l a r regions w i t h the p o l a r g r o u p s d i r e c t e d o u t w a r d s . U s u a l l y t h e lattices are h e x a g o n a l l y a r r a n g e d , b u t m o r e c o m p l i c a t e d configurations have been encountered. T h e aqueous r e g i o n b e t w e e n the aggregates 2 Ε a n d 2 D is s i m i l a r t o a n electrolyte s o l u t i o n . T h e e l e c t r i c a l c o n d u c t i v i t y is g o o d b e c a u s e of the f a i r l y h i g h m o b i l i t y of t h e c o u n t e r i o n s , a n d the v a p o r pressure of t h e w a t e r is h i g h since almost a l l l o n g c h a i n ions are b o u n d
(4).
T h e r e are n o c o n t i n u o u s w a t e r regions i n mesophases of t y p e 2 F . T h e i r structure resembles that of i n v e r t e d m i c e l l e s .
Their conductivity
is l o w , a n d t h e i r p r o p e r t i e s are l i p o p h i l i c .
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
16
LYOTROPIC LIQUID
CRYSTALS
Figure 2.
Some structures in liquid crystalline systems of association colloids (E) Hexagonally packed hydrophilic rods with water-rich regions between the mesoaggregates (neat soap) (D) Lamellar structure (middle soap) (F) Hexagonally packed lipophilic rods with hydrophilic inner regions and oil-Hch regions between the mesoaggregates. T h e aggregates d i s c u s s e d a b o v e are a l l a n i s o d i m e n s i o n a l , w h i c h is t h e reason f o r the a n i s o t r o p i c character
of the mesophases.
In
some
systems i t has b e e n p o s s i b l e to p r o v e the existence of i s o t r o p i c h i g h l y viscous phases of s i m i l a r structure b u t w h i c h c l e a r l y consist of almost i s o d i m e n s i o n a l aggregates.
T h e exact structure of these phases is s t i l l
the subject of d i s c u s s i o n , as is also the case w i t h the c o m p l e x phases.
meso-
T h e r e l a t i o n b e t w e e n the i s o t r o p i c phases a n d g l o b u l a r p r o t e i n s
a n d p l a s t i c crystals of n o n - a m p h i p h i l i c substances has b e e n d i s c u s s e d b y Gray and Winsor
(5).
T h e nomenclature
f o r the different mesophases v a r i e s ; t r a n s l a t i o n
lists f o r the terms u s e d b y different authors h a v e b e e n p u b l i s h e d b y
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
2.
DANEELSSON
Lyotropic
Mesomorphism
17
d(Â)
80
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
40
0
be c
1
2 Acta Crystal I ogra phi θ
Figure 3 . X-ray long spacings (A) of systems of cetyltrimethylammonium bromide and water as a function of the ratio gram water/gram association colloid ( 8 ) O O O O lameUar phase XX XX hexagonal phase · · · ·
crystals
E k w a l l a n d c o - w o r k e r s ( I , 2, 3 ) , as w e l l as b y others. T h e m a c r o s c o p i c structure of l y o t r o p i c mesophases is r e m i n i s c e n t of e m u l s i o n s that are s t a b i l i z e d b y surfactants.
M o s t o f t e n t h e y are opales
cent a n d g e l - l i k e . It is sometimes difficult to ascertain the
difference
b e t w e e n m i c e l l a r a n d l i q u i d c r y s t a l l i n e systems o n the one h a n d , a n d heterogeneous
dispersions o n the other.
Heterogeneous, non-crystalline
systems d o n o t s h o w x-ray d i f f r a c t i o n as d o h o m o g e n e o u s m i c e l l a r a n d m e s o m o r p h o u s phases.
T h e i n t e r p l a n e a r spacings c o r r e s p o n d i n g to t h e
aqueous a n d h y d r o c a r b o n regions i n the mesophases v a r y w i t h the v o l u m e f r a c t i o n of w a t e r , as p r e d i c t e d b y t h e s t r u c t u r a l m o d e l s ( F i g u r e 3 ) ( 8 ) . T h e mesophases h a v e w e l l - d e f i n e d t r a n s i t i o n temperatures a n d o c c u r i n w e l l - d e f i n e d c o m p o s i t i o n regions ( F i g u r e s 3 a n d 4 ) ( 8 , 9 ) . T h e state of t h e h y d r o c a r b o n chains i n mesophases a n d m i c e l l e s is r e f l e c t e d i n the K r a f f t p h e n o m e n a .
I n aqueous solutions of surfactants
t h e K r a f f t p o i n t is d e f i n e d as t h e t e m p e r a t u r e at w h i c h t h e s o l u b i l i t y reaches the
critical micelle concentration;
w h e n the
temperature
is
i n c r e a s e d f u r t h e r , the s o l u b i l i t y rises r a p i d l y s i n c e t h e m o n o m e r s f o r m m i c e l l e s ( F i g u r e 5)
(JO).
L i p i d s that d o n o t f o r m m i c e l l e s f r e q u e n t l y
start to s w e l l b y the u p t a k e of w a t e r at a w e l l - d e f i n e d t e m p e r a t u r e ; t h e y are t r a n s f o r m e d i n t o a m e s o m o r p h o u s state ( F i g u r e 6) ( 11 ). T h e r e l a t i o n b e t w e e n these t w o K r a f f t p h e n o m e n a is e x p l a i n e d to some extent b y t h e
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
18
L Y O T R O P I C LIQUID C R Y S T A L S
40" VISCOUS ISOTROPIC
NEAT
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
3W ω ο ζ υ
g (Λ Ο
ο 3OJ
< oc
25 10
20
30
40
50
60
70
T E M P PC) Journal of Physical Chemistry
Figure 4. X-ray long spacings of a 69.6% dimethyldodecylamine oxide in deuterium oxide solution as a function of temperature (9) f a c t that t h e aqueous l a y e r a r o u n d t h e p o l a r g r o u p s o f t h e surfactant facilitates t h e t r a n s i t i o n o f t h e l i p o p h i l i c p a l i s a d e l a y e r t o t h e l i q u i d - l i k e state t h a t is c h a r a c t e r i s t i c
o f t h e mesophases
(1, 2, 3).
A weakened
hydrocarbon-hydrocarbon
interaction
b y bulky
hydrocarbon
caused
g r o u p s o r d o u b l e b o n d s has a s i m i l a r effect Phase
(11).
Diagrams
T h e phase
e q u i l i b r i a a n d d i a g r a m m a t i c structures
p o n e n t system a r e i l l u s t r a t e d i n F i g u r e 7 (12).
i n a two-com
T h e phase
diagrams
have often been determined b y inspection w i t h t h e n a k e d eye. M o r e exact m e t h o d s i n c l u d e x - r a y a n d N M R analysis, d e n s i t y
measurements,
s e p a r a t i o n b y h i g h - s p e e d c e n t r i f u g a t i o n o r d i f f e r e n t i a l t h e r m a l analysis. T e r n a r y systems o f surfactant, s l i g h t l y p o l a r a d d i t i v e s a n d w a t e r a r e particularly interesting.
A t y p i c a l e x a m p l e i s t h e c l a s s i c a l m o d e l system
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
2.
Lyotropic
DANIELSSON
19
Mesomorphism
0.08 o Έ
Ζ ο < rr
i-
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
s2 ο ο
006h
ο.ο4μ
CMC
SOLUBILITY CURVE KRAFFT
20°
CURVE
POINT
30°
TEMPERATURE
40°
"Colloidal Surfactants"
Figure
5.
Phase diagram
close to the Krafft point (10)
Concentration, c "Form and Function of Phospholipids"
Figure 6. Phase diagram of the 1,2-dipalmitoyl-glycero3-sn phosphorylchotin-water system ( 1 1 ) of s o d i u m o c t a n o a t e - d e c a n o l - w a t e r at 2 0 ° C w h i c h has b e e n s t u d i e d b y E k w a l l a n d c o - w o r k e r s ( F i g u r e 8 ) (13). S o m e d i a g r a m m a t i c structures are s h o w n ; a l l m u l t i p h a s e regions h a v e b e e n l e f t o u t . I n t h e system, o n e finds
normal H a r t l e y micelles
( L i ) , inverted micelles
( L ) , lamellar 2
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
20
L Y O T R O P I C LIQUID
CRYSTALS
cM Molecular Crystals and Liquid Crystals
Figure 7. Schematic of a soap phase diagram and structure of some phases. The boundanes for the main phases of the system are 30-60% for the hexagonal phase and 71-82% for the lamellar phase at 20°C (12). structures
( D ) , mesophases
c o n s i s t i n g o f p o l a r r o d s i n a n aqueous
c o n t i n u u m ( E ) , a n d n o n - p o l a r rods i n a h y d r o c a r b o n c o n t i n u u m ( F ) . T h e structures o f phases Β a n d C are n o t d e f i n i t e l y settled. T h e same phase e q u i l i b r i a are s h o w n i n F i g u r e 9 , b u t h e r e t h e t w o p h a s e a n d three-phase regions h a v e b e e n i n t r o d u c e d . T h e r e a r e eight three-phase Li-C-D,
triangles
Li-E-D,
representing
L -D-F, D-E-G, 2
the equilibria L i - L - D , 2
a n d L2-F-G.
Li-B-D,
I n the region of the
t r i a n g u l a r d i a g r a m close t o t h e c r y s t a l l i n e s o d i u m octanoate
the very
s m a l l a m o u n t s o f s o l u t i o n a n d mesophases m a k e i t d i f f i c u l t t o separate t h e e q u i l i b r i u m phases. I n r e g i o n G t h e e x t e r n a l a p p e a r a n c e o f t h e soap is c r y s t a l l i n e o r c u r d - l i k e ; t h e soap i n c l u d e s c o n s i d e r a b l e a m o u n t s o f decanol a n d water.
T h e heterogeneous
systems b e t w e e n L i , B , a n d C
d o n o t separate w e l l o n c e n t r i f u g a t i o n . T h e c o m p o s i t i o n s v a r y w i t h t h e strength a n d duration of the centrifugal forces b e t w e e n t h e mesoaggregates
field.
I n regions Β a n d C , t h e
are possibly weak enough to b e
i n f l u e n c e d b y c e n t r i f u g a t i o n . T h e s e p h a s e e q u i l i b r i a a r e also e x t r e m e l y sensitive t o t e m p e r a t u r e .
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
2.
DANEELSSON
Lyotropic
21
Mesomorphism
Ε
D "MTP International Review of Science"
Figure 8. Schematic of mesomorphous structures in phase Ε, D , and F in the three-component system: sodium octanoate-decanol-water (1) T h e exact extension of t h e different phase regions i n t h e d i a g r a m is less i n t e r e s t i n g t h a n t h e fact that t h e l y o t r o p i c l i p i d systems a l w a y s f o l l o w t h e phase r u l e f r o m a m a c r o s c o p i c p o i n t o f v i e w . I n t h e t e r n a r y d i a g r a m ( F i g u r e 9 ) there are n e v e r m o r e t h a n three phases present a t any one time.
E a c h three-phase t r i a n g l e is s u r r o u n d e d b y t w o - p h a s e
regions, a n d t h e corners o f t h e three-phase regions e n d i n one-phase regions. T h i s shows that t h e mesophases are r e a l l y h o m o g e n e o u s e q u i l i b r i u m systems; t h e y are n o t , f o r e x a m p l e , h i g h l y d i s p e r s e d e m u l s i o n s . S i n c e t h e mesophases m a y c o n t a i n m o r e t h a n 9 0 % w a t e r , t h e pres ence o f a f o u r t h c o m p o n e n t , e v e n as a s m a l l i m p u r i t y , m a y distort t h e w h o l e phase d i a g r a m . T h e r e q u i r e m e n t s o f t h e p h a s e r u l e w e r e o f t e n n e g l e c t e d i n earlier studies, b u t e v e n i n r e c e n t p a p e r s c o n t r a d i c t o r y diagrams can b e found. Because of t h e microscopic a n d macroscopic validity of t h e phase r u l e t h e m i c e l l e s i n t h e i s o t r o p i c solutions L
x
and L
2
must b e regarded
as p o l y m o l e c u l a r complexes i n s o l u t i o n a n d n o t as a separate m i c e l l a r phase.
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
22
LYOTROPIC
LIQUID
CRYSTALS
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
Decanol
"MTP International Review of Science"
Figure 9. Phase diagram for the three-component system: sodium octanoate-n-decanol-water at 20° C ( 1 ) . Concentra tions expressed as weight percent. Li'. Region with homogeneous, isotropic aqueous solu tion. L : Region with homogeneous, isotropic decanolic solu tion. B, C, D, E, F: Regions with homogeneous mesomor phous phases. E: Region with homogeneous mesophase with two-di mensional, hexagonal structure; amphiphilic rods; "normcX structure. F: Region with homogeneous mesophase with two-di mensional hexagonal structure; water rods; "reversed" struc ture. D: Region with homogeneous mesophase with lamellar structure; one-dimensional swelling. 2
9
Thermodynamics T o c h a r a c t e r i z e t h e mesophases
thermodynamically, i t is desirable
t o h a v e d e t a i l e d k n o w l e d g e o f t h e regions i n w h i c h phases exist i n t h e m o d e l system s o d i u m octanoate—decanol-water.
Β and C T h e heats
at w h i c h t h e mesophases a r e f o r m e d are s o m e w h a t easier t o o b t a i n t h a n t h e c h e m i c a l potentials. T a b l e I gives some examples o f t h e f e w c a l o r i m e t r i c values a v a i l a b l e (14, 15). T h e h e a t o f s o l u b i l i z a t i o n , heat o f m i c e l l e f o r m a t i o n , a n d heats o f f o r m a t i o n o f phases D a n d Ε a r e s m a l l i n c o m p a r i s o n w i t h o r d i n a r y
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
2.
DANDELSSON
Lyotropic
Mesomorphism
23
Table I. Apparent Heats of Aggregation in Micellar Solutions and Mesomorphic Phases in the System: Sodium Octanoate—w-Pentanol-Water (14,15) Aqueous
Solution
30% N a O O C C H
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
7
1 5
AH
Aggregation +
Micelle formation T r a n s f e r of n - p e n t a n o l f r o m w a t e r to micelles F o r m a t i o n of phase Ε from saturated solution
Δΐ Εθ Ν
Al
H 2
o
8
7.6 k J / m o l 4.5 k J / m o l
= =
23 J / m o l 300 J / m o l
c h e m i c a l reactions. T h e e n t r o p y of the c h a n g e i n G i b b s e n e r g y o n aggre g a t i o n is t h e r e f o r e of the same o r d e r as the e n t h a l p y . T h e f o r m a t i o n of m i c e l l e s a n d mesoaggregates f r o m t h e i r c o m p o n e n t s i n s t a n d a r d states is therefore g o v e r n e d b y s i m i l a r factors. ( a ) T h e h y d r o c a r b o n chains h a v e m o r e t o r s i o n a l f r e e d o m i n t h e aggregates t h a n i n the r i g i d m o l e c u l a r c o n f i g u r a t i o n s t a b i l i z e d b y t h e surrounding water (16). ( b ) T h e e n t r o p y increases as the s t r u c t u r e d w a t e r a r o u n d t h e h y d r o c a r b o n chains returns to " n o r m a l " b e h a v i o r . T h i s is sometimes expressed, r a t h e r i n a d e q u a t e l y , as a n " e x p u l s i o n of h y d r o c a r b o n chains f r o m w a t e r " (17). T h e b i n d i n g of counterions a n d v a n d e r W a a l s ' a t t r a c t i o n f u r t h e r c o n t r i b u t i o n s to the association. m i n e d so far, h o w e v e r , are insufficient.
make
T h e heats of f o r m a t i o n deter M o r e definite k n o w l e d g e a b o u t
t h e n a t u r e of the b i n d i n g has b e e n o b t a i n e d b y s p e c t r o s c o p i c i n v e s t i g a t i o n . So f a r i t has n o t b e e n p o s s i b l e to m e a s u r e the c h e m i c a l potentials of t h e c o m p o n e n t s i n the mesophases.
T h i s m e a s u r e m e n t is p o s s i b l e , h o w
ever, i n solutions w h i c h are i n e q u i l i b r i u m w i t h t h e mesophases.
If p u r e
w a t e r is t a k e n as the s t a n d a r d state, t h e a c t i v i t y of w a t e r i n e q u i l i b r i u m w i t h the D a n d Ε phases i n the system N a C - d e c a n o l - w a t e r is m o r e 8
t h a n 0.8 (4).
F r o m these activities i n m i c e l l a r solutions, the a c t i v i t y of
the f a t t y a c i d salt has sometimes b e e n c a l c u l a t e d . T h e salt is i n c o r r e c t l y t r e a t e d as a c o m p l e t e l y d i s s o c i a t e d electrolyte.
T h e a c t i v i t y of t h e f a t t y
a c i d i n solutions of short c h a i n carboxylates has also b e e n d e t e r m i n e d b y gas c h r o m a t o g r a p h y ; f r o m these d e t e r m i n a t i o n s the c a r b o x y l a t e a n i o n a c t i v i t y c a n be d e t e r m i n e d (18). are o b t a i n e d (15).
L o w C M C values f o r the
carboxylate
T h e same m e t h o d has s h o w n that the a c t i v i t y o f
s o l u b i l i z e d p e n t a n o l i n octanoate solutions is s t i l l v e r y l o w w h e n t h e s o l u t i o n is i n e q u i l i b r i u m w i t h p h a s e D ( F i g u r e 10)
(15).
T h e k i n e t i c m i c e l l a r u n i t s i n c l u d e a b o u t h a l f of the c o u n t e r i o n s ; t h e mesoaggregates b i n d counterions e v e n m o r e
firmly.
It c a n b e e s t i m a t e d
f r o m a c t i v i t y measurements, h o w e v e r , that a c o n s i d e r a b l e p r o p o r t i o n of
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
24
LYOTROPIC
LIQUID
CRYSTALS
0.9 0.8
a
0.7 ! pentanol 0.6 0.5
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
0.4 0.3 -
0.01
0.02
0.03 [
NaC
0.06
0.05
0.04
8
15th Scandanavian Chemistry Meeting
Figure 10. The activity of n-pentanol in micellar solutions of sodium octanoate with solubilized pentanol and in equilibrium with the lamellar mesophase D. The abscissa indicates the mole fraction of sodium octanoate in the system (15). the s o d i u m ions r e m a i n free; a c e r t a i n m o b i l i t y a n d c o n d u c t i v i t y are retained.
T h i s d i s s o c i a t i o n gives rise to a D o n n a n p o t e n t i a l , w h i c h ,
a c c o r d i n g t o E k w a l l , explains t h e extreme some mesophases
w a t e r - b i n d i n g c a p a c i t y of
(4).
T h e c h e m i c a l potentials m e a s u r e d so f a r d o n o t a l l o w t h e f o r m u l a t i o n o f t h e r m o d y n a m i c c r i t e r i a f o r the f o r m a t i o n o f l y o t r o p i c mesophases. Some qualitative remarks, however, can b e made.
O f p a r t i c u l a r interest
are E k w a l l ' s studies of t h e relations b e t w e e n t h e w a t e r b i n d i n g of t h e mesophases, t h e i r i o n i z a t i o n , x - r a y p a r a m e t e r s , a n d v a p o r pressures F o r c o m m o n soaps at r o o m t e m p e r a t u r e mesophases o n l y i n t h e presence
(4).
c a n be observed
o f a m o u n t s o f w a t e r that h y d r a t e t h e i o n i c a n d
p o l a r groups. H y d r a t i o n is therefore characteristic o f aqueous l y o t r o p i c mesophases
( I , 2, 3).
T h e b i n d i n g of
counterions to t h e m i c e l l e s a n d t o t h e mesoaggregates
as w e l l as m i c e l l a r systems
seems t o b e of a
s i m i l a r electrostatic
nature.
T h e a d d i t i o n of N a C l g r e a t l y affects t h e
l a m e l l a r phase D a n d , t o a lesser extent, p h a s e E ; i n these phases t h e counterions a r e m o r e s t r o n g l y b o u n d t h a n b y m i c e l l e s i n t h e s o l u t i o n α
2,3). I n t h e t e r n a r y system N a C - d e c a n o l - w a t e r t h e influence of t h e 8
p o l a r / a p o l a r solubilizate o n the formation of micelles a n d
mesoaggre
gates c a n b e seen c l e a r l y ( F i g u r e 9 ) . Q u i t e o f t e n t h e f o l l o w i n g rules o f t h u m b for the influence of the solubilizate can b e used:
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
2.
DANEELSSON
Lyotropic
25
Mesomorphism
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
( a ) N o n - p o l a r s o l u b i l i z a t e s are b u i l t i n t o t h e l i p o p h i l i c m o i e t y of the aggregates w i t h l i t t l e influence o n t h e a g g r e g a t i o n n u m b e r s . T h e h y d r o p h i l i c surfaces of the aggregates a n d t h e aqueous s o l u b i l i t y of the surfactants a r e n o t c h a n g e d ; excess s o l u b i l i z a t e separates as a p u r e substance. ( b ) P o l a r / a p o l a r a d d i t i v e s a r e b u i l t i n t o t h e p a l i s a d e layers a n d s t r o n g l y influence t h e charge d e n s i t y , t h e c o u n t e r i o n b i n d i n g , a n d t h e i n t e r a c t i o n w i t h w a t e r of t h e m i c e l l e s a n d t h e mesoaggregates. F r o m aqueous solutions, excess s o l u b i l i z a t e separates as a mesophase w i t h a h i g h w a t e r content. N o n - e l e c t r o l y t e p o l a r a d d i t i v e s i n p a r t i c u l a r i n d u c e the f o r m a t i o n o f l a m e l l a r mesophases, o w i n g to t h e decrease i n t h e surface c h a r g e d e n s i t y of t h e aggregates. T h e f o r m a t i o n of r o d - l i k e mesoaggre gates o f t y p e Ε is i n d u c e d w h e n t h e charge d e n s i t y is h i g h . ( c ) Surfactants i n w e a k l y p o l a r solvents f o r m i n v e r t e d m i c e l l e s w i t h t h e w a t e r s o l u b i l i z e d i n strongly p o l a r i n n e r e n v i r o n m e n t s . F r o m these solutions t h e r e is also f r e q u e n t l y a separation of mesophases t h a t consists of l a m e l l a r o r n o n - p o l a r r o d l i k e aggregates. I n these aggregates t h e w a t e r is b o u n d as h y d r a t i o n w a t e r a n d cannot b e c o m p a r e d w i t h a n aqueous s o l u t i o n . E x a m p l e s o f these cases a r e g i v e n b y t h e t w o - p h a s e e q u i l i b r i a F - L a n d F - D i n F i g u r e 9. 2
I n t w o - c o m p o n e n t systems o f association o f c o l l o i d a n d w a t e r t h e s e q u e n c e of phases, as the w a t e r content decreases, i s : m i c e l l a r s o l u t i o n -> h e x a g o n a l l y p a c k e d p o l a r rods - » c o m p l e x phases w i t h r o d - s h a p e d aggregates
l a m e l l a r mesophase D - » c r y s t a l l i n e surfactant.
S o m e of
these steps m a y b e absent, d e p e n d i n g , f o r e x a m p l e , o n t h e t e m p e r a t u r e . Reactions between Various Association
Structures
W e n o w t u r n to t h e q u e s t i o n of w h e t h e r there is a n y successive r e l a t i o n s h i p i n v o l v e d i n l o w e r complexes, m i c e l l e s , a n d t h e different meso aggregates. S u c h a succession c a n f o r m a l l y b e a t t r i b u t e d t o t h e t e n d e n c y of t h e aggregate surfaces to b e convex w h e n t h e y a r e i n c o n t a c t w i t h w a t e r or a h y d r o c a r b o n i n a c c o r d a n c e w i t h t h e R t h e o r y d e v e l o p e d b y W i n s o r ( 5 , 6 ). T h e association processes as s u c h a r e , h o w e v e r , v e r y fast. Present e x p e r i m e n t a l t e c h n i q u e s s h o w that association c a n b e c o n s i d e r e d as a series of s e c o n d - o r d e r reactions, t h e surfactant ions b e i n g successively c a u g h t at a b o u t s i m i l a r rates u n t i l t h e o p t i m a l size o f t h e aggregates h a s b e e n r e a c h e d ( 6 ) . T h e d i s s o c i a t i o n of the aggregates is also v e r y fast a n d is
a
first-order
reaction.
W e have
only investigated
the
stationary
e q u i l i b r i a a n d , so far, n o o n e has b e e n a b l e t o d e c i d e w h e t h e r t h e meso aggregates a r e f o r m e d b y t h e r e s t r u c t u r i n g o f s m a l l e r aggregates o r m i c e l l e s . W e c a n o n l y c o n c l u d e that l y o t r o p i c m e s o m o r p h i s m is almost a l w a y s s h o w n b y m i c e l l e - f o r m i n g surfactants; h o w e v e r , l y o t r o p i c meso phases c a n f r e q u e n t l y b e f o r m e d w i t h o u t a n y association i n t h e aqueous solution.
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
26
LYOTROPIC LIQUID CRYSTALS
1/C.
1/mol "Liquid Crystals and Ordered Fluids"
Figure 11. Na chemical shifts (in ppm) for isotropic solutions of sodium octanoate ( X ) and sodium octylsulfate ( O ) as a function of the inverse soap concentration. A positive δ denotes a shift to lower field ( 2 3 ) . 23
T h e m o s t i m p o r t a n t n e w results c o n c e r n i n g t h e structure o f m i c e l l e s a n d mesophases i n recent years h a v e b e e n a c h i e v e d u s i n g N M R m e t h o d s . These have
been
p a r t i c u l a r l y successful
w h e n s t u d y i n g t h e changes
u n d e r g o n e b y groups of h y d r o c a r b o n s w h e n t r a n s f e r r e d f r o m a n aqueous e n v i r o n m e n t to m i c e l l e s a n d mesoaggregates a n d i n t h e b i n d i n g o f w a t e r a n d counterions to t h e aggregates (19-25).
A s examples of studies o f
this k i n d F i g u r e 11 describes c h e m i c a l changes of N a i n i s o t r o p i c s o l u 2 3
tions o f s o d i u m octanoate a n d s o d i u m o c t y l s u l f a t e b o t h above a n d b e l o w the c r i t i c a l m i c e l l e f o r m a t i o n c o n c e n t r a t i o n (21, 22, 23). Laser-Raman
spectroscopy
is a n e w m e t h o d
with
considerable
potential for providing a n explanation of h o w the surroundings inside the aggregates influence t h e c r y s t a l l i n e state o f t h e h y d r o c a r b o n chains a n d other g r o u p s (25,26).
I t seems p r o b a b l e , h o w e v e r , that a n i m p o r t a n t
a r e a o f research o n t h e phase e q u i l i b r i a p r o p e r w o u l d concentrate o n attempts
to t h r o w l i g h t o n t h e exact t h r e m o d y n a m i c c r i t e r i a f o r t h e
association processes.
E k w a l l s studies o f t h e w a t e r activities o f meso
phases i n t h e system w a t e r - d e c a n o l - s o d i u m c a p r y l a t e a r e a n e x a m p l e o f s u c h research (4).
H o w e v e r , t h e r m o d y n a m i c treatment o f t h e associa
t i o n processes presupposes
measurements
o f t h e activities o f s e v e r a l
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
2.
DANiELSSON
Lyotropic
27
Mesomorphism
d i f f e r e n t c o m p o n e n t s i n t h e systems.
W h e n supplemented b y measure
ments o f the heats o f f o r m a t i o n , s u c h studies s h o u l d p r o v i d e i n f o r m a t i o n a b o u t the e n t r o p y effects i n the systems a n d c o n s e q u e n t l y h e l p t o e x p l a i n t h e f u n d a m e n t a l causes o f t h e association p h e n o m e n o n .
Such
studies
w o u l d also b e v e r y i m p o r t a n t i n h e l p i n g t o i n t e r p r e t s p e c t r o s c o p i c results.
Downloaded by UNIV OF MISSOURI COLUMBIA on June 13, 2013 | http://pubs.acs.org Publication Date: September 1, 1976 | doi: 10.1021/ba-1976-0152.ch002
Literature Cited 1. Ekwall, P., Danielsson, I., Stenius, P., "MTP International Review of Science," Physical Chemistry Series I, Vol. 7, p. 97, A. D. Buckingham and M. Kerker, Eds., Butterworths, London, 1972. 2. Ekwall, P., Stenius, P., "MTP International Review of Science," Physical Chemistry Series 2, Vol. 7, in press. 3. Ekwall, P., Adv. Liq. Cryst. (1975) 1, 1. 4. Ekwall, P., "Liquid Crystals and Ordered Fluids," J. F. Johnson and R. S. Porter, Eds., Vol. 2, p. 177, Plenum, New York, 1974. 5. Gray, G. W., Winsor, P. Α., Mol. Cryst. Liq. Cryst. (1974) 26, 305. 6. Winsor, P.A.,Mol. Cryst. Liq. Cryst. (1971) 12, 141. 7. Luzzati, V., in "Biological Membranes," D. Chapman, Ed., p. 71, Academic, London, 1968. 8. Vincent, J. M., Skoulios, Α., Acta Cryst. (1966) 20, 444. 9. Lawson, K. D., Mabis, A. J., Flautt, T. J., J. Phys. Chem. (1968) 72, 2058. 10. Shinoda, Kozo, Nakagawa, Toshio, Tamamuchi, B.-I., Isemura, Toshizo, "Colloidal Surfactants," p. 7, Academic, London, 1963. 11. Chapman, D., Williams, R. M., "Form and Function of Phospholipids," B.B.A. Library 3, G. B. Ansell, R. M. C. Dawson, and J. N. Hawthorne, Eds., p. 126, Elsevier, New York, 1973. 12. Eins, S., Mol. Cryst. Liq. Cryst. (1970) 11, 119. 13. Ekwall, P., Mandell, L., Fontell, K., Mol. Cryst. Liq. Cryst. (1969) 8, 157. 14. Danielsson, I., Proc. Scandinavian Symp. Surface Chem., 5th, Abo, 1973. 15. Rosenholm, B., Proc. Scandinavian Chem. Meetg., 15th, 1974, p. 64. 16. Aranow, R. H., Whitten, L., J. Phys. Chem. (1960) 64, 1643. 17. Poland, D. C., Sheraga, Η. Α., J. Phys. Chem. (1965) 69, 2431. 18. Backlund, S., Danielsson, I., Proc. Intern. Congr. Surface Active Substances, 6th, Zürich, 1972, p. 1013. 19. Rassing, J., Wyn-Jones, E., Chem. Phys. Lett. (1973) 21, 93. 20. Johansson, Α., Lindman, B., in "Liquid Crystals and Plastic Crystals," G. W. Gray and P. A. Winsor, Eds., Ellis Horwood, Chichester, 1974. 21. Tiddy, G. J. T., J. Chem. Soc. F1 (1972) 68, 369. 22. Ibid., p. 653. 23. Gustafsson, H., Lindblom, G., Lindman, B., Persson, N.-O., Wennerström, H., "Liquid Crystals and Ordered Fluids," J. F. Johnson and R. S. Porter, Eds., Vol. 2, p. 161, Plenum, New York, 1974. 24. Wennerström, H., Lindblom, G., Lindman, B., Arvidsson, G., Chem. Phys. Letters (1974) 12, 4, 261. 25. Larsson, K., Rand, R. P., Biochem. Biophys. Acta (1973) 326, 245. 26. Parsegian, V. Α., Trans. Faraday Soc. (1966) 62, 848. 27. Larsson, K., Chem. Phys. Lipids (1972) 9, 181. RECEIVED November 19,
1974.
In Lyotropic Liquid Crystals; Friberg, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.