Magnetic Resonance in Colloid and Interface Science

6 Chemical Aspects of the Hydrophobic/Hydrophilic Interface by Nuclear Magnetic Resonance Methods

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L. W. REEVES Instituto de Quimica, Universidade de São Paulo, Caixa Postal 20,780, São Paulo, Brazil F. Y. FUJIWARA and M. SUZUKI Chemistry Department, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Introduction Amphiphilic species such as detergent ions, lipids, soaps and certain polar compounds dissolve in water to form simple molecularly dispersed systems only at small concentrations, below what is termed the critical micelle concentration. Micelles are agglomerates of these amphiphilic compounds in modest number on the order 10 or less in which the solution structure seeks to minimize the hydrophobic/hydrophilic interactions by forming a hydrophobic inner core in an aqueous medium with an interface between them. At much more concentrated levels of ampiphilic substances the micellar structure becomes liquid crystalline in nature but the overall division into aqueous, and hydrophobic compartments with a structured interface between them remains (1). The nature of these liquid crystals, which can take several forms, has been attributed to superstructure arrays which have been studied in low angle x-ray scattering experiments (2). The concentration of interface area per unit volume is extremely high and furthermore some mesophases are oriented by magnetic fields (3). The completely oriented phases have advantages in magnetic resonance studies because the anisotropic parts of the parameters available become the dominant features of the spectrum which is generally of high resolution quality. With modern nuclear magnetic resonance (NMR) spectrometers almost all elements in the periodic table become accessible by the same sensitivity enhancement techniques available for more commonly studied low abundance nuclei such as C-13 and variation of chemical elements involved in lyotropic liquid crystals can be very wide, especially in the counter ions which reside in the electrical double half layer. It is evident therefore that the NMR technique can attack the interface problem on a broad front, which includes both averaged and dynamic information. The anisotropic NMR parameters such as chemical shift, pseudo-dipolar coupling, dipole-dipole coupling and nuclear quadrupole interaction energies all depend on the angular factor ½(3Cos θ-l) where θ is the angle between some vector or principal 2

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Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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tensor component and the s t a t i c magnetic f i e l d used i n the NMR spectrometer. T h i s angular f a c t o r becomes p a r t i a l l y averaged by the a n i s o t r o p i c tumbling of the l i q u i d c r y s t a l l i n e component and i t i s always non-zero i n a n i s o t r o p i c f l u i d s (4-6). The v a l u e [1] has been c a l l e d the degree of o r i e n t a t i o n of the a p p r o p r i a t e a x i s w i t h r e s p e c t to the s t a t i c magnetic f i e l d . I t i s an important a c c e s s i b l e q u a n t i t y and g i v e s a good i n d i c a t i o n of how h i g h l y ordered an a x i s i s and sometimes the d i r e c t i o n of that o r d e r , from the s i g n . (S = + 1 and S = -% f o r p e r f e c t a l i g n m e n t s ) . The a n i s o t r o p i c tumbling of a molecule o r i o n i s d e s c r i b e d by a 3 x 3 order m a t r i x of 5 independent elements (4,5)· M o l e c u l a r or i o n i c symmetry reduces the number of elements r e q u i r e d t o d e s c r i b e the motion and w i t h a t h r e e f o l d or more a x i s only one degree of order parameter i s r e q u i r e d . The number of independent S a x i s v a l u e s r e q u i r e d to d e s c r i b e the a n i s o t r o p i c motion of a r i g i d molecule i s thus r e l a t e d to molecular symmetry. The new questions t h a t may be posed concerning the i n t e r f a c e r e l a t e to processes a t the molecular and i o n i c l e v e l . While i t i s evident from p r e v i o u s s t u d i e s that the i n t e r f a c e r e g i o n i s the most h i g h l y s t r u c t u r e d and that water p l a y s an important r o l e , there has been no e x p e r i m e n t a l work on the c o n c e n t r a t i o n d i s t r i b u t i o n of ions i n the e l e c t r i c a l double l a y e r (6). The d i s t r i b u t i o n of d i s s o l v e d substances between hydrophobic and h y d r o p h i l i c compartments i s a l s o i n d i c a t e d by NMR measurements (7). There i s a g r e a t d e a l of h e t e r o g e n e i t y of order between the water, i n t e r f a c e and hydrocarbon c h a i n r e g i o n s and component molecules or ions r e v e a l t h e i r l o c a t i o n from the micro degrees of order of axes i n them. The i o n i c head groups and counter i o n s i n f l u e n c e the hydrocarbon c h a i n motions. The present t a l k w i l l seek to i l l u s t r a t e these and other questions on the g e n e r a l i n t e r f a c e problem. |§

L

Micro-Degrees of Order of Species i n the Hydrophobic/Hydrophilic I n t e r f a c e Systems I t i s p o s s i b l e to prepare two types of l y o t r o p i c l i q u i d c r y s t a l s , which o r i e n t i n a magnetic f i e l d . We have designated these phases as Type I and Type I I , the f i r s t a l i g n s w i t h the symmetry or d i r e c t o r a x i s of the phase p a r a l l e l to the magnetic f i e l d and the second i n a p e r p e n d i c u l a r manner (8). The degree of o r i e n t a t i o n of phase components and s m a l l d i s s o l v e d molecules has been i n v e s t i g a t e d i n a wide range of chemical systems and r e p r e s e n t a t i v e r e s u l t s a r e l i s t e d i n Table I (8-12)· The degree of o r i e n t a t i o n or order parameter S, i s always r e f e r r e d t o the magnetic f i e l d d i r e c t i o n i n the ordered sample, but samples of Type I and Type I I d i f f e r i n the d i r e c t i o n of the phase d i r e c t o r , w i t h r e s p e c t t o the a p p l i e d f i e l d , as i n d i c a t e d above. For comparison of the order parameters of i o n s , molecules or hydrocarbon c h a i n segments the longest l o c a l symmetry a x i s i n the


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E T

A L .

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Interface

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species i s u s u a l l y chosen i n each case, and ranges of v a l u e s a r e g i v e n i n Table I w i t h respect t o the magnetic f i e l d d i r e c t i o n . Since a l l S v a l u e s a r e r e f e r r e d t o a long a x i s i n the s p e c i e s w i t h r e s p e c t t o the magnetic f i e l d they may be compared i n magnitude as an i n d i c a t i o n , more o r l e s s , o f p e r f e c t l o c a l o r d e r . The water has c l e a r l y a low average l o c a l o r d e r , the experimental v a l u e s being t h e exchange averaged v a l u e f o r i n t e r f a c e s t r u c t u r e water and the remotely l o c a t e d molecules i n the aqueous compartment . The hydrocarbon segments near the head group and the -ND head group a r e the most h i g h l y ordered r e g i o n s o f these o r i e n t e d l y o t r o p i c phases (13)« The order parameter of long axes i n i o n s v a r i e s a great d e a l from phase t o phase, as a l s o they do i n p o l a r molecules. The more mobile c o n s t i t u e n t s o f t h e e l e c t r i c a l double h a l f l a y e r may o r may not p l a y an i n t e g r a l r o l e i n the h i g h l y s t r u c t u r e d i n t e r f a c e r e g i o n between the hydrophobic and hydrop h i l i c compartments(6). I n the case of the d i m e t h y l t h a l l i u m i o n , which i s p o s i t i v e l y charged, the degree of o r i e n t a t i o n of t h e a x i s i s v e r y low i n the c a t i o n i c mesophase b u t almost two orders of magnitude h i g h e r i n the a n i o n i c phase. I n both i n s t a n c e s t h e i o n was added i n about 2 weight % as the e l e c t r o l y t e d i m e t h y l t h a l l i u m n i t r a t e (9,10). I n these cases i t i s c l e a r t h a t i n the c a t i o n i c phase the complex c a t i o n r e s i d e s i n a r e g i o n r e l a t i v e l y remote from the i n t e r f a c e , but i n the a n i o n i c phase i t i s s u f f i c i e n t l y ordered t o suggest a r o l e i n forming the i n t e r f a c e . Such s t u d i e s on one i o n cannot g i v e us a model of what i s important among the f a c t o r s i n f l u e n c i n g s t r u c t u r e a t t h e i n t e r f a c e , b u t t h e s t u d i e s must be extended as w i d e l y as p o s s i b l e , c o n s i d e r i n g d i f f e r e n t chemical s t r u c t u r e s . Aromatic organic i o n s and r e l a t e d molecules a l s o v a r y a great d e a l i n the degree o f o r i e n t a t i o n of t h e i r long axes, though they appear t o be l e s s s e n s i t i v e t o t h e s i g n of the charge on the i n t e r face (9,10,14). Those aromatic organic i o n s , which are h i g h l y water s o l u b l e seem t o be l e s s ordered along t h e i r para axes, w h i l e those w i t h a para s u b s t i t u e n t , which i s hydrophobic, such as para c h l o r o b e n z o i c a c i d become as h i g h l y ordered i n a d i r e c t i o n p e r p e n d i c u l a r t o t h e i n t e r f a c e as the f i r s t segment o f the hydrophobic hydrocarbon c h a i n . The r e s u l t s i n Table I c o n f i r m the i n f l u e n c e of charge and h y d r o p h o b i c i t y i n the examples i n v e s t i g a t ed. I n s e r t i o n of o r g a n i c i o n s and molecules i n the hydrophobic r e g i o n i n f l u e n c e s t h e i r degree of o r i e n t a t i o n a g r e a t d e a l . I t i s c l e a r t h a t such s t u d i e s , as i n d i c a t e d i n Table I , must be broadened t o i n c l u d e as many s t r u c t u r a l types as p o s s i b l e w i t h i n the regime t h a t the l y o t r o p i c l i q u i d c r y s t a l l i n e r e g i o n s can be o r i e n t e d i n a magnetic f i e l d ; such wide chemical f l e x i b i l i t y i s evidently a v a i l a b l e to us. +

3

Changes i n the Phase D i r e c t o r w i t h Respect t o the Magnetic F i e l d The chemical nature of l y o t r o p i c l i q u i d c r y s t a l l i n e systems, which can be o r i e n t e d i n magnetic f i e l d s , has been v a r i e d i n our


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previous studies i n several respects (7-14). The phases can be binary mixtures of detergents and water, but often require the addition of electrolyte i n order to lower the viscosity and f a c i l i t a t e the act of orientation (15). Mesophases of ternary composition can include cationic, anionic and neutral amphiphiles or mixtures of them with and without added electrolyte. The experimenter has some control over the chemical nature of the head group, which can be any of -OH, -NH , -NH , -N(CH ) , -SOi*~, - C O 2 " -COOH, etc. The charge density of the head groups per unit area of interface i s a useful experimental variable, but the chemical nature of the head group and of counter ions i s also evidently extremely important, though often ignored i n model theories. The hydrocarbon chain length i s usually Ce, Ci 0 or C i 2 in phases which can be oriented near room temperature, though some phases can be oriented at higher temperatures and cooled slowly to remain oriented at room temperature with other chain lengths. The counter ion can be any soluble simple or complex ion stable i n water between pH~0.5 and ~12. While some counter ions are d i f f i c u l t to orient i f they constitute 100% of the total counter ions, they can often be introduced by substitution of less than 5% of the total ionic detergent content, or as part of the added electrolyte. One important physical result of these chemical variations i s the fact that two types of mesophase emerge, a Type I with the phase director p a r a l l e l to the applied magnetic f i e l d and Type II with the director perpendicular to the magnetic f i e l d (8). The direction of this director with respect to the magnetic f i e l d can be determined i n the following manner. A typical NMR sample tube i s shaken well to randomise the mesophase before entering the magnetic f i e l d and a spectrum recorded immediately of the D 0, deuterium doublet, preferably, i n the interests of speed of execution, by giving a single pulse and transforming the free induction decay to the frequency mode spectrum. A powder diagram spectrum results. The sample i s allowed to remain for a s u f f i c i ent time i n the magnetic f i e l d to become homogeneously oriented. At this point i n time another deuterium NMR spectrum i s recorded and the sharp doublet observed may have twice the powder diagram doublet splitting (Type I phase) or the same s p l i t t i n g (Type II phase) (16). An example of the general behaviour of mesophases is given i n figure 1, for the binary system decylammonium chloride/water and the ternary system with added electrolyte. +

2

3

+

3

3

2

In binary mixtures between 35 and -49 weight per cent water there i s a mesophase which gives a powder doublet, the s p l i t t i n g decreasing with added water. This phase i s viscous, not oriented i n detectable times at 28°C by the magnet and i s probably a lamellar type. Between ^ 49 and 52 weight per cent water two phases separate, the upper one i s clearly the lamellar type phase, the powder spectrum having constant s p l i t t i n g of the D 0 doublet. A second phase separates and orients 9


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E T AL.

30

Hydrophobic/ Hydrophilic

40

Interface

50 WT. % 0^0

59

60

70

Figure I. The mesophase behavior of the binary system decylammonium chloride/D O with and without added Ν H fil electrolyte. The quadrupole splitting in Hertz of the deuterium doublet is used as a monitor of the numbers ana types of mesophases. (a) and (b) refer to two possible Type II mesophases. The large open circles refer to the lamellar and Type I phases. The larger filled circles to the Type II phase which occurs without adding electrolyte. The small fitted points refer to phases to which a few wt % of ammonium chloride has been added. These are Type II. g


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i n the magnet t o g i v e a s i n g l e c r y s t a l sharp doublet. T h i s second phase was shown t o be type I I w i t h the mesophase d i r e c t o r p e r p e n d i c u l a r to the f i e l d . At g r e a t e r than ^ 52 weight per cent water another two phase system o c c u r s . The upper l a y e r i s biréfringent and the lower l a y e r an i s o t r o p i c phase. The upper phase o r i e n t s r a p i d l y i n the magnetic f i e l d . T h i s type I I phase c o - e x i s t s w i t h an i s o t r o p i c phase g i v i n g a s i n g l e deuterium peak a t the center of the type I I doublet. The l a m e l l a r phase which appears i n e q u i l i b r i u m w i t h the type I I phase may be p a r t l y o r i e n t e d by h e a t i n g the sample u n t i l i t becomes i s o t r o p i c then a l l o w i n g i t to c o o l s l o w l y i n the magnet. A w e l l o r i e n t e d type I I and p a r t l y o r i e n t e d l a m e l l a r phase occur a f t e r t h i s treatment. A d d i t i o n of the e l e c t r o l y t e NH^Cl i n a few weight per cent completely changes the phase behavior. Only type I I mesophases are observed as i l l u s t r a t e d i n f i g u r e 1» but there i s a suggestion of two d i f f e r e n t type I I phases (a) and (b) as i n d i c a t e d i n the f i g u r e . I n f i g u r e 2, the behavior of the e q u i l i b r i u m mixture of l a m e l l a r type I I and the second type I I phase of h e a t i n g and c o o l i n g i n the magnet i s i l l u s t r a t e d . There are o b v i o u s l y d i f f e r e n t arrangements of s u p e r - s t r u c t u r e i n type I I mesophases and comparison between phases of micro-degrees of order must be undertaken w i t h some care. Only when d i f f e r e n c e s i n degrees of order w i t h respect to the magnetic f i e l d d i r e c t i o n d i f f e r by f a c t o r s g r e a t e r than 2 or 3 can r e l i a b l e comparative c o n c l u s i o n s be drawn. There i s a l a c k of low angle x-ray d i f f r a c t i o n work on the o r i e n t e d samples r e p o r t e d i n our work.

Changes i n Water Content of a Given Mesophase Some mesophases which can be o r i e n t e d s u s t a i n reasonably l a r g e changes i n water content without undergoing phase t r a n s i t i o n s . The e f f e c t of e n l a r g i n g the volume of the water compartments of phases can be s t u d i e d by always p r e s e r v i n g the mole r a t i o s of a m p h i p h i l i c molecules and merely u s i n g d i f f e r e n t water contents. A good example i s the system 37 p a r t s decylammonium c h l o r i d e , 4 p a r t s ammonium c h l o r i d e and 54 t o 69 p a r t s by weight D 2 O . The deuterium magnetic resonance spectrum of s p e c i f i c a l l y deuterated c h a i n segments was monitored as a f u n c t i o n of D2O content and the doublet s e p a r a t i o n i n o r i e n t e d samples i s shown i n f i g u r e 3A. The composition of the a m p h i p h i l i c compartment was a l s o v a r i ed by s t u d y i n g the corresponding deuterium d o u b l e t s i n s p e c i f i c a l l y deuterated chains f o r mesophases w i t h compositions 0.197 gms c h o l e s t e r o l , 1.11 gms d7-decylammonium c h l o r i d e , 0.12 gms ammonium c h l o r i d e and 2.61 t o 3.30 gms D 0. These deuterium doublet s p l i t t i n g s v e r s u s water weight per cent are p l o t t e d i n f i g u r e 3B. 2


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Hydrophobic/ Hydrophilic Interface

TYPE I PHASE

A

POWDER PATTERN

-400 PARTLY ORIENTED POWDER PATTERN PHASE

Figure 2. (A) The deuterium magnetic resonance spectrum of the equilibrium mixture of lamellar and Type I mesophase from Figure 1 (49-52% D O in binary mixture) is shown. The superimposed powder pattern and single crystal spectrum of the Type I persist after 12 hr in the spectrometer at 28°C. (B) Heating to an isotropic medium and allowing it to cool slowly in the magnet causes both phases to appear more highly ordered at 28°C, but the powder pattern spectrum of the outer doublet partly persists. The phase hading initially to powder pattern spectra appears to be Type II. g


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0

II 55

j

I

I

i

l

l

1

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ι ι

ι

60

ι

ι

ι

RESONANCE

ι

65 Weight

%

ι

1

70

D 0 2

Figure 3. The deuterium quadrupole splittings for deuterium in hydrocarbon chain segments for mesophases (Type II) which differ only in water content. The phases were acidified slightly to prevent rapid proton exchange from the —ND,* group. (A) Without cholesterol content in hydrophobic compartment. (B) With 9.2 mole ratio detergent/cholesterol. Compositions are as in the text. Q indicates ith carbon from the head group.


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The a d d i t i o n of water w i t h i n one phase r e g i o n l e a d s t o a decrease i n the average degree o f o r i e n t a t i o n of the deuterium p r i n c i p a l e l e c t r i c f i e l d g r a d i e n t f o r a l l components and segments [D2O ( f i g u r e 1) c h a i n segments ( f i g u r e s 3A and 3 B ) ] . The temp e r a t u r e of the experiments was 31.3±0.1°C. The phases w i t h add ed c h o l e s t e r o l r e q u i r e l a r g e r water contents i n order t o form the d e s i r e d mesophase. The deuterium e l e c t r i c f i e l d g r a d i e n t tensor has a p r i n c i p a l a x i s approximately a l i g n e d w i t h t h e C-D bond d i r e c t i o n and i t i s g e n e r a l l y considered t o have a s m a l l asymmetry parameter η(0.15 o r l e s s ) which i s u s u a l l y n e g l e c t e d t o the order of t h i s d i s c u s s i o n . The deuterium quadrupole c o u p l i n g s a r e pro p o r t i o n a l t o the order parameter along the C-D bonds. The e f f e c t of the c h o l e s t e r o l i s t o i n c r e a s e the degree of order c o n s i d e r a b l y at a l l p o i n t s i n the hydrocarbon c h a i n s . T h i s has been i n t e r p r e t ed as i n c r e a s i n g the p o p u l a t i o n of a l l t r a n s chains ( 1 3 ) . The s o l i d p o i n t s i n f i g u r e 3 correspond t o a two phase r e g i o n one of which i s i s o t r o p i c (higher d e n s i t y ) . The c h a r a c t e r i s t i c q u a n t i t y , which i s independent of water content, provided counter i o n and hydrophobic components a r e not changed, i s the r a t i o of the quadrupole c o u p l i n g s t o each other i n p a r t i a l l y o r i e n t e d hydrocarbon segments. I n f i g u r e 4 f o r a number of phases, i n which the water content o n l y i s v a r i e d a t constant temperature, t h e quadrupole s p l i t t i n g s of the hydro carbon c h a i n segments a r e p l o t t e d a g a i n s t t h e deuterium doublet s p l i t t i n g s of the -ND + head groups. A s t r i c t l i n e a r i t y i s ob served i n a l l cases w i t h zero i n t e r c e p t . The r a t i o s a r e a f f e c t e d by adding c h o l e s t e r o l as i n f i g u r e 3B. Comparisons of the r a t i o s from r e s u l t s i n 3A and 3B w i t h and without c h o l e s t e r o l a r e ; e.g. 3

(Δνχ/Δν ) - 2.325±0.015 w i t h o u t and 2.42±0.01 w i t h c h o l e s t e r o l , (Δν /Δν^Γ) « 1.215±0.02 w i t h o u t and 1.57±0.02 w i t h c h o l e s t e r o l 9

(9.2 mole % of hydrophobic components). These r a t i o s a r e o n l y v e r y s l i g h t l y a f f e c t e d by temperature changes (19-50°C) a t const ant composition. An example of t h i s behaviour a r e the changes [Δν /Δν | ] • 1.26±0.01 a t 21.7°C which l i n e a r l y decreases t o 1.10±0.01 a t 45.4°C and [àvÏQ/àvm] - 0.29±0.001 decreasing over 9

Ν

)

the same temperature range t o 0.255±0.003. Other r a t i o s a r e i n variant. Dramatic changes i n the r a t i o s of quadrupole c o u p l i n g s do occur i f there i s s u b s t i t u t i o n of the counter i o n . Phases prepared w i t h decylammonium t e t r a f l u o r o b o r a t e have r a t i o s , [àv /àv ] * 0.994±0.008; • 1.97+0.05 and [Δν Δνιοϊ » 4.63±0.05. I n decylammonium f l u o r i d e phases [Δν Δνιοϊ = 4.46±0.01. The systematic dependence of these r a t i o s on the nature o f the counter i o n has a l r e a d y been p u b l i s h e d ( 1 7 ) . The f a c t t h a t the r a t i o s of the deuterium quadrupole c o u p l ings i n the c h a i n a r e independent of the water content i s i n d i c a t i v e that t h e degree of order p r o f i l e of the c h a i n does not change i n a r e l a t i v e sense. I f the f i r s t segment C-D f a l l s i n Saxis v a l u e the l a s t segment C-D has a corresponding change i n Saxis- T h i s i m p l i e s t h a t s i n c e the degree of order p r o f i l e s

m

9

9


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RESONANCE

Figure 4. The quadrupole coupling constants in —CD —chain segments plotted against those of the —ND * head group for mesophases of deculammonium chloride in which only the water content is varied. The indices Q indicate the carbon number i in the chain numbered from the — N D * head group. t

3

S

DICY l S t f l f A f l

1 4 Sé POSITION

Figure 5. The ratios of deuterium quadrupole coupling constants in specifically deuterated decanol and decylsulfate ions which form part of the same lyomesophase in the oriented state. [AVX/AVJO] represents the ratio of the coupling in the xth carbon to the 10th carbon. The carbon position is indicated on the abcissae.


REEVES

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E T A L .


Interface

65

r a t i o i s a f f e c t e d by changes i n the i n t e r f a c e , such as a change i n counter i o n , then the a l t e r a t i o n of water c o n c e n t r a t i o n does not a l t e r the b a s i c s t r u c t u r e of the i n t e r f a c e i n the systems s t u d i e d , but merely changes the amount of l o c a l motion allowed i n the structure. Degree of Order P r o f i l e as a F u n c t i o n of Head Group Many l y o t r o p i c mesophases which are o r i e n t e d by magnetic f i e l d s , have two or more components i n the hydrophobic compartment. A t y p i c a l example i s t h e sodium d e c y l s u l p h a t e / d e c y l a l c o h o l / H 0 or D 0/sodium sulphate system o r i g i n a l l y d e s c r i b e d by F l a u t t and Lawson (3). I n these phases both decanol w i t h an -OH p o l a r head group and d e c y l s u l p h a t e w i t h -SOi*~ head groups are coa l i g n e d i n the hydrophobic compartment. By s u i t a b l e s p e c i f i c d e u t e r a t i o n of both the decanol and d e c y l s u l p h a t e i o n s i t i s p o s s i b l e t o a s s i g n the deuterium doublets from the Type I I mesophases and o b t a i n the r a t i o s of the deuterium quadrupole c o u p l i n g s i n phases where o n l y water content i s v a r i e d . Although complete assignment of t h e 10 carbon c h a i n i n each case has not been achieved because of the lengthy s y n t h e t i c work needed, enough of both chains were deuterated t o get r a t i o s [Δνχ/Δνιo] f o r each c h a i n , where χ = 1,2,3,4,8 and 9 f o r the d e c y l sulphate i o n and χ = 1,2 and 9 f o r the decanol c h a i n . I n f i g u r e 5 these r a t i o s a r e p l o t t e d a g a i n s t the numerator carbon number i n the c h a i n . I t i s q u i t e c l e a r from t h i s r e s u l t that the degree of order p r o f i l e , and thus d e t a i l e d c h a i n segment motions are de pendent on the head group. I n the d e c y l s u l p h a t e c h a i n there i s evidence of a l t e r n a t i o n of order, w h i l e i n decanol chains they appear t o have a more r e s t r i c t e d motion near the -OH head group r e l a t i v e t o the d e c y l s u l p h a t e c h a i n . T h i s could n o t be accounted f o r by assuming t h a t the -S0i*~ group i t s e l f c o n s t i t u t e d an a d d i t i o n a l segment. That d e t a i l e d c h a i n motions of the hydro carbon r e g i o n should be dependent on the chemical nature of the head group, which i n t u r n must i n f l u e n c e l o c a l i n t e r f a c e hydra t i o n and counter i o n b i n d i n g i s n o t a t a l l s u r p r i s i n g when demonstrated e x p e r i m e n t a l l y , but both b u l k t h e o r i e s of the i n t e r f a c e and work i n molecular b i o l o g y e x c l u s i v e l y ignores t h i s concept. The degree of order p r o f i l e of the chains i s l i k e l y t o become a means of accomplishing i n t e r f a c e chemistry a t the mole c u l a r l e v e l u s i n g an NMR spectrometer and homogeneously o r i e n t e d lyomesophases. 2

2

Degree of Order P r o f i l e as a F u n c t i o n of Phase Type A Type I I phase d e s c r i b e d e a r l i e r of decylammonium c h l o r i d e / water/ammonium c h l o r i d e s u s t a i n s a d d i t i o n s of c h o l e s t e r o l t o form o r i e n t i n g phases a t higher water content ( f i g u r e 3). A t a mole r a t i o per cent of c h o l e s t e r o l t o detergent between 9 and 12, the Type I I phase passes over i n t o one i n which a powder p a t t e r n deuterium doublet p e r s i s t s i n d e f i n i t e l y i n the spectrometer. By p l o t t i n g the r a t i o s of deuterium quadrupole c o u p l i n g s versus mole


66

MAGNETIC

[Cholesterol] [DACI]

χ

RESONANCE

joo%

Figure 6. Ratios of deuterium quadrupole coupling constants / AVCDX/AVND 7> where χ denotes carbon number for decylammonium chloride mesophases with added cholesterol. The open circles correspond to Type Π (probably hexagonal) meso phases and filled circles to a mesophase which renders a powder pattern (probably lamellar). Only in the case of the water doublet is no direct extrapolation possi ble, and in this case the geometry of the compartments changes drastically. The hydrocarbon chain motions are not perturbed by the phase change. 3

+


6.

REEVES

E T

A L .


Interface

67

ratio per cent cholesterol i t i s possible to compare the degree of order profile i n Type II and the lamellar phases, which do not orient. It i s clear from figure 6 that the hydrocarbon chain motions do not appear to depend on the phase type. Abstract Lyotropic mesophases oriented by a magnetic f i e l d can be used as tools to study interface science by NMR methods. Anisotropic parameters allow the study of internally divided aqueous, interface and hydrophobic regions of these phases. The proton dipole-dipole and deuterium quadrupole couplings measured i n various oriented phases are useful i n probing local degrees of order with respect to the magnetic f i e l d direction. The heterogeneity of local micro order of long axes i n ions, molecules, head groups and hydrocarbon chain segments i s determined and discussed. The order of ions i n the e l e c t r i c a l double layer i s influenced by counter head group charges as well as the hydrophobicity of the ion and complex interface structure. The chemica l f l e x i b i l i t y of the mesophases i s outlined. Water content changes affect the amount of motion i n the interface but not the type of interface motions and structure. The surface motion i s affected by counter ions and amphiphilic components. The degree of order profile of chains with different head groups i n the same phase i s quite distinct, but phase changes do not s i g n i f i c antly affect this p r o f i l e . Acknowledgement Work supported by the National Research Council of Canada, Conselho Nacional de Fesquisas do Brasil and Fundacao de Ampara à Pesquisa do Estado de Sao Paulo, Brazil.


68

M A G N E T I C

RESONANCE

Table I . Degrees of O r i e n t a t i o n of Components of L y o t r o p i c Liquid Crystals Phase

Species

Axis

Saxis

DA

CH 0H

C-0

±8x10"

3

3

DS

CH3OH

C-0

±6x10"

3

DA

CH3CO2*

C-C

-6.5x10*

DS

CH3NH3 "

C-N

-1.6xl0"

2

DS

CH SnCH

C-Sn-C

-1.4xl0~

2

DS

D 0

0-D

±1x10-

3

2

DS

D 0

0-D

*2xl0~

3 +

DA

CH T1CH

C-Tl-C

±8x10*

k

DS

CH3TICH3 *

C-Tl-C

±7xl0-

2

DS

0 N^^~C0 -

para

-2xl0"

DA

0 N-^^-C00H

para

-8x10*

2

DS

° ^O^

para

-4x10"

2

DA

H- NI

N-Ï

±4xl0"

3

DA

C lH - ^ 0)>C00H h

para

±0.2

DA

-CD -

first φ segment a x i s

±0.15

DA

-ND

first

±0.17

4

++

3

3

2

3

+ 3

4

2

2

2

2N

cooH

2

+ 3

COOH

3

2

segment a x i s ±0.14 first segment a x i s A l l t h e phases a r e c l a s s i f i e d as type I I e x c e p t , which i s a Type I . Degree of o r i e n t a t i o n f o r the f i r s t segment a x i s of t h e c h a i n r e f e r s t o t h e d i r e c t i o n of a h y p o t h e t i c a l a l l trans c h a i n , which a l i g n s perpendicular t o the i n t e r f a c e . T h i s s e g m e n t ( )

DS

-CDo-

c o n t e


6.

REEVES

E T AL.

Hydrophobic/ Hydrophilic Interface

69

a x i s i s approximately represented by the l i n e j o i n i n g the b i s e c t o r s of the Ci-N and C 1 - C 2 bonds or the C 1 - C 2 and C 2 - C 3 bonds. The S v a l u e of the -ND3"*" head group i s c o r r e c t e d f o r r o t a t i o n of the moeity about t h e -C-N bond. DA and DS represent phases based on t h e detergent c a t i o n decylammonium or the detergent anion d e c y l s u l p h a t e r e s p e c t i v e l y . A l l phases a r e about 90 mole % water and the balance detergent, a l c o h o l and added e l e c t r o l y t e . I n some cases as a consequence of the a n a l y s i s t h e s i g n of S i s known. The temperature of the measurements was 31±1°C and the S v a l u e s correspond t o mid-range compositions w i t h a ±30% change w i t h composition.

Literature Cited 1. Brown, G.H., Doane, J.W., and Neff, V.D., "A Review of the Structure and Physical Properties of Liquid Crystals," Pub. CRC. Cleveland (1971). 2. Luzzati, V., Mustacchi, Η., Skoulios, Α., and Husson, F., Acta Cryst., (1960), 13, 660, 668. 3. Lawson, K.D., and Flaut, T.J., J. Am. Chem. Soc., (1967), 89, 5489. 4. Saupe, Α., Ζ. Naturforsch, (1965), 20a, 572. 5. Saupe, Α., Angew. Chem. Int., Ed. (English), (1968), 7, 107. 6. Winsor, P.Α., Chem. Rev., (1968), 68, 1. 7. Chen, D.M., "Ph.D. Dissertation", University of Waterloo, Waterloo, Ontario (1975). 8. Radley, Κ., Reeves, L.W., and Tracey, A.S., J. Phys. Chem., (1976), 80, 174. 9. Lee, Y., and Reeves, L.W., Can. J. Chem., (1975), 53, 161. 10. Lee, Y., and Reeves, L.W., Can. J. Chem., (1976), 54, 500. 11. Reeves, L.W., and Tracey, A.S., J. Am. Chem. Soc., (1974), 96, 1198. 12. Reeves, L.W., Suzuki, Μ., Tracey, A.S., and Vanin, J.A., Inorganic Chemistry, (1974), 13, 999. 13. Fujiwara, F.Y., and Reeves, L.W., J. Am. Chem. Soc., (In press). 14. Reeves, L.W., Suzuki, Μ., and Vanin, J.A., (unpublished work). 15. Fujiwara, F.Y., and Reeves, L.W., (unpublished work). 16. Cohen, M.H., and Reif, F., Solid State Phys., (1957), 5, 321. 17. Reeves, L.W., and Tracey, A.S., J. Am. Chem. Soc., (1975), 97, 5729.


Magnetic Resonance in Colloid and Interface Science

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