Lyotropic Liquid Crystals

2 Lyotropic Mesomorphism

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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.


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.


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


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 .



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


2.

DANEELSSON

Lyotropic

Mesomorphism

17

d(Â)

80


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


18

L Y O T R O P I C LIQUID C R Y S T A L S

40" VISCOUS ISOTROPIC

NEAT


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


2.

Lyotropic

DANIELSSON

19

Mesomorphism

0.08 o Έ

Ζ ο < rr

i-


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



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 .



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.


22

LYOTROPIC

LIQUID

CRYSTALS


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


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


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


24

LYOTROPIC

LIQUID

CRYSTALS

0.9 0.8

a

0.7 ! pentanol 0.6 0.5


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:


2.

DANEELSSON

Lyotropic

25

Mesomorphism


( 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.



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


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.


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.


Lyotropic Liquid Crystals

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