18 Deuteron N M R Studies on Soap-Water Mesophases HÅKAN WENNERSTRÖM, NILS-OLA PERSSON and BJÖRN LINDMAN
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Division of Physical Chemistry 2, The Lund Institute of Technology, Chemical Center, P.O.B. 740, S-220 07 L U N D 7, Sweden
Introduction Soap-water systems form in a d d i t i o n to i s o t r o p i c s o l u t i o n s and c r y s t a l l i n e phases s e v e r a l d i f f e r e n t types of mesophases which have p r o p e r t i e s intermediate between l i q u i d s and s o l i d s . The mesophases are often very viscous but the motion o f the i n d i v i d u a l molecules i s normally almost as r a p i d as i n an o r d i nary liquid. The viscous appearence o f the phases i s caused by the formation of l a r g e aggregates and by the presence o f long -range o r d e r . The v a r i o u s mesophase s t r u c t u r e s differ in the types of aggregates and i n t h e i r packing c h a r a c t e r i s t i c s . In a l a m e l l a r mesophase the soap molecules u s u a l l y form double-layered lamellae which are separated by lamellae o f v a r i a b l e thickness c o n t a i n i n g the water molecules and the c o u n t e r i o n s . In a normal hexagonal phase, on the other hand, the soap molecules form long rodshaped aggregates which are arranged i n a hexagonal a r r a y . In a d d i t i o n to these two phases, which are best understood, a hexagonal phase of the reversed type and d i f f e r e n t types of cubic i s o t r o p i c meso phases may form i n a b i n a r y soap-water system. I f a t h i r d compo nent i s added to a soap-water system the phase equilibria may become quite complex and i n a d d i t i o n to those phases mentioned a l s o other types o f s t r u c t u r e s may o c c u r . A thorough account o f the phase equilibria i n amphiphilic systems as w e l l as o f the present s t a t e o f knowledge o f the phase s t r u c t u r e s has recently been given by Ekwall ( 1 ) . In mesophases the molecular arrangement is more ordered than in a s o l u t i o n but the o r d e r i n g is l e s s than in a crystalline phase. In order to understand the s t r u c t u r e and the p r o p e r t i e s of the soap-water mesophases i n more detail it is necessary to have information on the o r i e n t a t i o n e f f e c t s on a molecular level. During recent years it has been demonstrated that some s t a t i c magnetic resonance parameters may provide p e r t i n e n t information i n t h i s r e s p e c t . Previous s t u d i e s have mostly been concerned with o r i e n t a t i o n e f f e c t s i n the soap lamellae whereas the aqueous p a r t has a t t r a c t e d l e s s i n t e r e s t , p a r t l y because o r i e n t a t i o n
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In Colloidal Dispersions and Micellar Behavior; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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C O L L O I D A L DISPERSIONS A N D M I C E L L A R
BEHAVIOR
e f f e c t s here are s m a l l and consequently d i f f i c u l t to study. Recent s t u d i e s have, however, demonstrated t h a t s t a t i c quadrupole e f f e c t s i n NMR s p e c t r a can give r a t h e r d e t a i l e d information on o r i e n t a t i o n e f f e c t s i n the aqueous l a y e r s . The present study was aimed at p r o v i d i n g an i n s i g h t i n t o water o r i e n t a t i o n e f f e c t s , using deuteron NMR, f o r c h i e f l y some l a m e l l a r l i q u i d c r y s t a l l i n e phases o c c u r r i n g i n three-component systems o f soap, a l c o h o l and heavy water. Thereby we have studied how water o r i e n t a t i o n depends on sample composition, on temperature and on the counteri o n . The study a l s o provides some information on the o r i e n t a t i o n e f f e c t s i n the amphophilic lamellae by i n v e s t i g a t i n g the deuteron NMR s i g n a l o f the -0D group o f the a l c o h o l . Deuteron NMR s t u d i e s on soap-water mesophases have been reported p r e v i o u s l y ( f o r a review see Ref. 2) but since some misunderstanding o f the t h e o r e t i c a l b a s i s o f i n t e r p r e t a t i o n seems to e x i s t i n the l i t e r a t u r e we w i l l s t a r t by b r i e f l y g i v i n g the t h e o r e t i c a l background o f deuteron NMR s t u d i e s o f a n i s o t r o p i c systems. E s p e c i a l l y we want to p o i n t out some e f f e c t s which may complicate the a n a l y s i s o f the NMR s p e c t r a . Theory The deuteron has the s p i n quantum number 1 = 1 and thus possesses an e l e c t r i c a l quadrupole moment. Through the quadrupole moment the nucleus i n t e r a c t s with e l e c t r i c a l f i e l d gradients i n the surroundings. The main terms i n the deuteron s p i n hamiltonian are the Zeeman term, H , and the quadrupole c o u p l i n g term, H ^ , z
Η = H
Δ
7
+ Η = - ν I- + 3 Z ( - l ) q OÙ Q^ π
n
q
V A -q q
(1)
Here the V ^ s are the i r r e d u c i b l e components o f the e l e c t r i c f i e l d gradient tensor ( o f second rank) and the A^'s the standard components o f a second rank s p i n tensor operator (_3). $Q i s given by e Q / 2 I ( 2 I - l ) h . The hamiltonian i s expressed i n frequen cy u n i t s . In an i s o t r o p i c l i q u i d the mean value o f HQ i s zero and the quadrupole i n t e r a c t i o n c o n t r i b u t e s only to r e l a x a t i o n . In an a n i s o t r o p i c medium a s , f o r example, a l a m e l l a r o r hexagonal mesophase the mean value o f HQ i s no longer zero and a quadrupole s p l i t t i n g appears i n the NMR spectrum. To e l u c i d a t e the e f f e c t s of the a n i s o t r o p i c medium more c l e a r l y i t i s convenient t o r e w r i t e HQ as (3) H
= β
1
M
L
(
}
(
Σ (-1)* V A „ D ? (Ω ) D ^
