Freeze-fracture electron microscopy study of hexagonal phase defects in a sodium dodecylsulfate-formamide system

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Freeze-fracture electron microscopy study of hexagonal phase defects in a sodium dodecylsulfate-formamide

system

M. Abiyaala, P. Duval

To cite this version:

M. Abiyaala, P. Duval. Freeze-fracture electron microscopy study of hexagonal phase defects in

a sodium dodecylsulfate-formamide system. Journal de Physique II, EDP Sciences, 1994, 4 (10),

pp.1687-1698. �10.1051/jp2:1994225�. �jpa-00248070�

/. Phi'.<. II Fianie 4 (1994) 1687-1698 OCTOBER 1994, PAGE 1687

Classification Fhv.iici Ab,in.ai _Is

61.~0 07.80 61.70

Freeze-fracture electron microscopy study of hexagonal phase

defects in

_a

sodium dodecylsulfate-formamide system

M.

Abiyaala

and P. Duval

Groupe de Mdtallurgie Physique de Rouen (*), Facultd des Sciences de Rouen. 76821 Mont Saint Aignan, France

(Ret en,ed /9 Aptii /994, ieceii.e(I _m final faint 8./u/_1' /994, aiiepted /3./uly /994)

R4sumd. Notre Etude _en microscopie dlectronique par cryofracture du systkme ^SDS-FA a

montrd l'existence de deu~ varidtd~ de pha,es h cylindre~ l'une dans laquelle [es cylindres sont relativement

rigide~

et regroupds en domaines de

grande

dimen~ion, comprenant ^{des ddfaut~}

classiques, l'autre constitude de cylindres trb, flexibles regroupds _en domaines plus petits et

comportant, en plus des ddfauts prdcddents des ddfauts inhabituels que nous avons en partie identifids, et pour le~quel~ nou~ proposons une explication de leur ongine.

Abstract. The lyotropic H,, pha,e formed in _a SDS formamide ~ystem ^{has been studied with}

TEM by _mean~ of

a cryofracture method. Two morphologies _were ob~erved _: at low SDS concentration, long straight ^rod; grouped in large domain~ with cla~~ic liquid

cry~tal

defect~ _{; at} higher concentration, ~maller domains and _sinuou~ con~titutive fibers. Unusual defects _were then

detected and analyzed. An explanation of their origin is proposed.

1. Introduction.

Defects in

lyotropic liquid crystal~

_are

usually

studied

by Optical microscopy,

which allow~

their indirect vizualisation from the

Optical anisotropy they produce.

Transmi~sion electron

microscopy,

which in

principle

should allow _a direct vizualisation of these defects~ is

rarely

used, because of the constraint~ involved in

preparing

observable

samples.

In effect, _one should _use

sophisticated cryogenic

methods, the main stage ^{Of which is} a vitrification obtained

by ultra-quenching

in order _tO pre~erve in _a vitrous state the structure

existing previously

in the

liquid

state. The conditions for the _succe~s of this

operation

_{are very}

dependent

On the system

as a

general

rule, _poor re~ults _are Obtained with water-rich systems ^which

crystallize frequently during

the

quenching.

Nevertheless, _a number Of articles

concerning

^defects in the lamellar

L~

have been

published,

in which

samples

_were

prepared by cryofracture [I ].

This method led to the direct

(±) URA CNRS 808.

1688 JOURNAL DE PHYSIQUE II N° 10

observation Of classic defects, and also _« structure defects _» the existence Of which _was

previously proposed

to account for the anomalies of

X-ray

and NMR spectra.

Using

last

method,

_{some papers} deal with the cubic

phase Q~,

which is the

object

of

important

theoretical and

experimental

studies in order to define and

explain

^its structure

[2, 3].

Studies with TEM _on the bidimensional

phases

such _as

H~

are uncommon, except ^for

studies

dealing

with the

lipid-water

_systems in which _two variants of

H~ phase

exist

[4, 5]

_an

« oil-in-water

»

phase (Hi phase)

and _{a «} water-in-oil

»

phase (H,, phase).

Studies of these

phases

_concern

especially phenomena accompanying

the

Hjj

-

Qa

_or the

H,,

_-

L~

transitions

[6, 7].

The

possible

defects in these

phases

^have been the

subject

of theoretical

investigation,

supported by experimental

results,

by Bouligand [8, 9]

and Kleman

[10].

These authors

reviewed the different linear and surface defects in 2D

phases. Recently~

a lamellar

gel-H~

transition has been observed

by

means of

optical microscopy by

Mac-Grath _et al.

].

In this paper, we present a TEM

study

of the

morphology

and defects _we have observed in

an

H~ phase

of the

sodium-dodecylsulfate-formamide

system

(SDS-FA).

This system

belongs

to a

recently

discovered class of

lyotropic

systems [12, 13,

25]~

which,

unusually,

includes _a non-aqueous solvent. Given _an

amphiphilic

molecule, these systems reveal

simpler diagram phases

than those obtained with water as a solvent. This is the case with the SDS-FA system,

for which

Auvray

et al.

[14, 15]

have determined the

following

_sequence of

phases, by

increasing

the SDS concentration

(at

temperatures ^above

solid-lyotropic phase equilibrium)

_:

Micelles

- H

~ -

Qa

_-

L~

This sequence which is

simpler

than in the SDS-water system ^{is in} agreement ^{with the}

theoretical sequence derived from steric considerations _on the

polar

heads and

paraffinic

chains of SDS molecules

[16].

The present work deals with the

H~ phase

of this system, the concentration range of which extends from 42 fb (T

> 57 °C _to 75 fb

(T

> 79.8 °C ).

2.

Experimental techniques

and

methodology.

Our solvent-surfactant solutions _were

prepared by weighing

what, ^and put ^{in closed tubes} to

prevent _any variation of

composition during

the

homogeneization

_process. In order to obtain

homogeneous mixtures,

the tubes of solution _were sonicated

and,

_at the _same

time,

heated for half _an hour at 80 °C.

For the observations under the electron

micro;cope~ wmples

_were

prepared by cryofracture

films of the solution~ less than ten micrometer~ thick~ were enclosed between _two thin copper

plates

j3 _mm diameterl. The

« sandwiche~ _» thus formed _were

kept

in _a furnace at the

assigned

temperature ^for a ~ufficient time _to form the

H,, phase.

and

directly quenched

from the furnace in _a

refrigerating

bath. In our case,

liquid nitrogen

which is _a bad

refrigerant

for

lyotropic phases

with aqueous solvent _was found ~ufficient (at least for the H~,

phase)

to inhibit any

crystallization

and any molecular

displacement larger

^than about ? nm. Then the vitrified

sandwiches _were fractured under

high

_vacuum la few

10~~

_{torr) at 150 °C in}

a

Cryofract

apparatus. ² nm Pt reinforced

by

lo _nm C

replicas

were

evaporated

_on the fractures. After

extracting

and

washing~

the

replicas

_were

ready

for the observations under _a 2000 FX JEOL electron

microscope.

Two methods _were used to _measure the

periodicities

_on the fractures

optical

diffraction _on the

negatives

with _a He-Ne laser beam this clas~ical method presents ^the

advantage

of

giving

a

diffractogram

with

adaptable

size. but the process ^is very

sensitive to the

particles

of dust in the

beam,

which

produce

_{a strong}

background

noise _on the

diffractograms

N° 10 TEM STUDY OF H,~ PHASE IN SDS-FORMAMIDE SYSTEM 1689

small

angle

electron diffraction _{: we} believe this is the first time that this method has been used in this type ^of

problem. Although

the coherence is lower in the electron beam than in the laser beam, the first method _can be used

during

the observations under the

microscope~

and

produces

much cleaner

diagram~.

On the other hand~ the values of the

periodicities

ⁱⁿ the

H,, phase

lead to diffraction spots very close to the central spot, ^which can

partially

mask

them~ even when the

largest

diffraction distance of the

microscope j250

cm) is used.

3. Results.

According

to the

X-ray investigations by Auvray

et al. [14~ 15

],

the domain of existence of the

H,, phase

extends from 42 §l _to 75 §l _wt SDS.

Actually~

we observed that the

cryofracture

process gave rise to _a lo-15 ill shift towards the

high

concentration. The _cause of this shift i~

probably

related to a loss of formamid

during

the treatment~

preceding

the

quench (elaboration

of the sandwiches, isothermal treatment... ). In

principle.

_a few

precaution~

^{could often} prevent this

phenomenon~

but could

give

^rise to other

problems [17~ 18].

^The

object

of this work

being

only

to observe the

H,, phase,

we did not attempt to solve this

problem.

Under these conditions _two very different forms of the

H~ phase

have been detected _:

I) at low SDS

concentration, longer straight (or slightly curved)

rods

iii

_at

high concentration,

sinuous and shorter fibers.

In both _cases, the _mean fracture

plane

was

approximately parallel

to the faces of the

sandwiches. This shows that the orientation of the

H~ phase

_on the copper walls of the

sandwiches is

parallel

to the _axes of the fibers. It is for such fractures that the

quality

of the

replicas

_was the best, the size of the Pt

grain being

thus smaller than 2 _nm.

Oblique

sections of the fibers _are sometimes observed but the resolution of the

replica

is not so

good.

3.I MORPHOLOGY _{oF THE STRAIGHT RoDs.} The most

frequent

^{fractures exhibit}

equally-

spaced long straight

fiber domains. with _a

length

of the fibers sometimes _over I micrometer.

The space between the

fibers,

either measured

directly

on the

negative

_or deduced from the

optical

or electronic

diffractions,

is 4 _± 0.2 _nm, in agreement ^{with the}

parameter

a~ determined

by X-ray

observations. These fracture

planes

_can ^{thus be} identified _as

lo) planes

and the fibers

correspond

to the

cylinders

of the

H~ phase (Fig. I).

50

_nm

Fig. 1. (10) section of H~ phase~ and corresponding electron diffraction pattem.

1690 JOURNAL DE

PHYSIQUE

II N° 10

These domains contain _some

defects, mainly

_screw dislocations revealed

by

a step, one side

of which ends inside _a domain.

Edge

dislocations _are sometimes observed

(Fig.

2). The

existence of both defects is

probably

related _to the

neces~ity

to balance the _curvatures

imposed by

_any relief of the sandwich walls. Some fractures

parallel

to the

cylinders

and

cutting

two

mutually slightly

disoriented domains _were observed,

revealing

interface dislocations. So.

figure

3 shows fracture

planes

which _{are not}

lo)

but

« vicinal

»

planes.

The fractures then form

regular

terraces

separated by oblique (60°)

_steps, In

figure

3, the film of

platin-carbon

is too thick _to reveal the

cylinders

but allows the steps to be shown. It is easy to determine with _a

good precision

(either

by

calculation _or

graphically)

the number M of

cylinders

in _{a terrace} and N in _a step~ as

reported

in the

caption

^of

figure

3.

00 _nm

Fig. 2. Straight fiber morphology. The _arrow show~

an edge dislocation circles ~how _screw dislocations.

j '/

"'~.

q

-

750fim

Fig. ^3. -Fractureplanes

«

vicinal »to(10) ~ection,

showingtwomutually iightlydisorienteddomain~.

_a) 6a~ width terraces, 2ah high steps. B)7 _uh width terraces, 2 _u~

high

_step~.

N° lo TEM STUDY OF H,~ PHASE IN SDS-FORMAMIDE SYSTEM 1691

More

rarely

_some fractures _are observed in which the _mean

plane

is tilted with respect to the

cylinder

_axes. In these _cases, the

replicas

show that the fractures _{are not}

planes

but formed

by

very ~mall and

irregular

steps as a result, the

«

readability

_» of such fractures is poor and identification of

possible

defects is

quite impossible. Nevertheless,

it is

possible

to check from the electron _or

optical

diffractions (which show a 2D

lattice)

whether such fractures

really

are

H~ phase

^{fractures. Indeed} measurement of the side A and B of the centered cells of the

diffractograms

allows us to go back to the parameter _a~ ^and to calculate the

angular

parameters and _~

characterizing

the orientation of the fracture

plane

normal in _a reference

given

in

figure

4. This is

given by

the

following

relations _:

,

(A~ + 3 B~ ) ±

,/(A~

₃

_B~)~

₊ 12

A~

B~

cos~

a

al = ~

~°~ ~ ~~ ^~

~ _{~°~ ~}

~

"3 B

~

~~

B~

Then _we could confirm~ for

example~

that the fracture

planes

lI and lI' shown in

figure

5

correspond

to an

oblique

section of _H~~

phase

with the

appropriate

_{parameter u~.} This check

was made _on every

oblique plane

observed.

,/

_/

j

⁶ /

1'

_,/

1' ,/ ,/

_/

q~

~. ~/

/

l'

loll ,1' ~,/

/

f~,I'

,/ _,/

/

_

,/ ,I'

TC

~ ^,/

~

Ii C

,

~~

/

,/ ~, j

~jj' ^~'Q

^/

~' ~,

^/

,l' _I' l~°]/j

/

jjjj ^~ii~ _,/

/ p _q~

~j~j'

I

Fig. 4. A Drawing of

a

plane

~ection of the H,~ pha~e.

3.2 MORPHOLOGY OF THE siNuous FIBERS. For nominal concentrations

beyond

40 §b~ the

fractures still ~how _a fibrous _structure, but the fibers _are now very sinuous. The sections

parallel

to the fiber _axes and the

jrare) oblique

^{sections show that the fractures} can still be identified _as

H~ phase

but with _an apparent parameter a~ which is less stable than in the

previous

case

j4

to 5 nm).

1692 JOURNAL DE PHYSIQUE II N° lo

50

_nm

Fig. ^5. A) fracture

parallel

to the _axes of the fibers. B)

oblique

section. 0

= 45°~ _~ 21°.

It is still

possible

to define domains _as

regions

in which the fibers _are

parallel~

but the

curvature of the fibers varies inside _a domain. We _can define boundaries of _a domain either as

a

discontinuity

of the fibers _or, at least, _{as a}

discontinuity

of their _curvature. If such _a definition of domain boundaries is

adopted~

one observes that the domains _{now are} much smaller than in the

previous

case

(120

to 300 _nm instead of _a few micrometers _see

Fig. 6).

Due _to the

difficulty

of

interpreting

the

oblique

fractures, tran~versal dislocations _or disinclinations _are the

only

linear defects _{one can} expect to observe [8~

9].

^{Such defects} are in fact observed

(edge

dislocations, _r

disinclinations),

where the

Burgers

vectors are

multiples

of the

spacing

^between

neighbouring

^fibers

(Figs.

7a and

7b).

Moreover, the observed

edge

dislocations _are most often associated with _a

global

rotation of the fibers

(Figs.

7a and 7cl.

The structures of interfacial defects _are varied and their

analysis

is difficult. The most

frequent

walls result from _a

divergence

of the direction of two

neighbouring

fibers. Such _a wall

N° lo TEM STUDY OF H,~ ^{PHASE IN} SDS-FORMAMIDE SYSTEM 1693

50

_nni

Fig.

6. A general view of the morphology of sinuous fibers.

thus ends inside _a domain. Some other walls _are similar to flexion walls in solids, that is the rotation _vector needed _to pass from _one side of the wall _to the other is located in the wall, which is itself

generally

curved. A

frequent

case is observed where the orientation of the wall passes from _a

position tangential

to the fibers of _one of the

adjacent

domains to a

position tangential

to the fibers of the other domain

(Fig.

8). Twist walls _were also observed

(rotation

vector

perpendicular

to the

wall),

_as shown in

figure

9 where the wall is

parallel

to the _mean

fracture

plane.

The fibers _can also

wind,

thus

forming

focal domains. We therefore

interpret

the _texture of

figure

lo _as

resulting

^from two focal domains

Dj

and

D~

the _core of which is formed

by

two

nearby

b _axes

(see Bouligand [9]) perpendicular

to the fracture. A

breaking

of the fibers

appears ^at the wall between

Dj

and

Dj.

A domain of

straight

fibers

D~

is linked with _no

discontinuity

to

D~.

However, a wall exists between

D~

^and

D~.

JOURNAL DE PH~SiQUE ii T 4 N' Jo n(TOBER it>t>4 M

1694 JOURNAL DE

PHYSIQUE

II N° lo

,r~

~ 'i'

Jill

j+,

iii

cl

j.(["

~l.J

-

i'

~

~°'l"? ~(i~~_

i, ^~

a)

b> c)

Fig. 7. Edge dislocations a~~ociated with _{a curvature} of the fibers. One notes that the Burgers vector _is a

multiple

_{of a~.} When the fibers

diverge,

_a wall is

generated.

b) Sketch of _a transver~al

edge

diiocation.

c) A transversal edge dislocation

accompanied

by _a curvature of the fibers.

4. Discussion.

Concerning

the parameter a~ of the

H~ phase,

_our results _are in agreement ^with

X-ray

data

[15]

(taking

into _account the above-mentioned

composition

shift _to

explain

the apparent

displace-

ment of the

composition

domain of

H~).

However, the

morphology

^of the

H~ phase

^is _very

different _at low and at

high

SDS concentrations.

At low concentrations, the fibers _are

long

and

straight.

This fact

might

_appear to conflict with the observations

by optical microscopy

which show the existence of focal domains and

therefore the

possibility

for fibers to be curved

[15].

However, the size of such domains is very

large (several

hundreds of

micrometers). Consequently,

the

density

of the associated 3

singularity

is very low and the

probability

of

observing

such _a

region by

electron

microscopy

is also very small. At the very most, the

possibility

of a

large

curvature radius has been observed. Classical defects, _e.g. screw and

edge

dislocations, _were observed in this domain of

composition.

N° lo TEM STUDY OF H~, PHASE IN SDS-FORMAMIDE SYSTEM 1695

50

_niii

,

Fig. 8. The wall between the I and II domains i~ tangent to domain I (top left), and tangent to domain II (bottom).

50

_am

Fig. 9. A twi~t wall. The wall is parallel to the picture plane.

At

high

concentrations, the fibers _are shorter and sinuous.

Consequently,

a new class of defects appears, the energy of which is

normally high.

The existence of such defects, the

pos~ibility

of _a strong

bending

of the fibers, and the fluctuations of the a~ parameter ^could be, in _our

opinion,

the consequence ^of a

collapse

of the elastic constant~ of the fibers, that reduces the energy of formation of these defects. A

possible

cause of this

phenomenon

^{could be the}

proximity

of the cubic Ia3d domain.

Compared

to the

H,, phase,

this last

phase corresponds

to

1696 JOURNAL DE PHYSIQUE II N° lo

50nm

_~..,

.,- .~

~

Fig. lo- Two focal domains Dj ^and D~, with parallel _axes~ and perpendicular to the photo plane (see text).

a drastic modification of the SDS formamide interface

configuration

which passes from _a finite

curvature to a zero mean curvature

[19].

A

supporting

argument to this

hypothesis

is the

impossibility

of

preserving

the Ia3d symmetry ^{of the}

Q« by using

our

quenching

method

[20].

Instead, _a fracture reveals the

possible

section of _a cubic structure, the parameter of which is

only

half the

Q«

Parameter

(4

_nm instead of 8

nm).

This _{seems to us to}

correspond

to the loss of information _on helicoidal symmetry~ ^which

emphasizes

the

necessity

of _an

upheaval

of elastic

properties

to realize the _curvature modification needed for the

phase

transition.

It is

striking

to note the similitude of

morphology

between the _two forms of

H~ phase

in _our systems ^{and the} two variants

Hi

and

H,j

^observed

by cryofracture by

Deamer _et al. _{on some}

lipid-water

systems

[5]. According

to the observations of these authors, the

Hjj phase

resembles _our

rigid

form while the

Hi

looks rather flexible. The

rigid

character of the

Hii Phase

seems to be confirmed

by

other authors

[6, 21].

However, if this shows a certain similitude between the elastic constant behaviour in _our system ^{and that of} some

lipid

systems, the

comparison

cannot be taken any further, _as the

Hi

and the

Hjj phases correspond

to two

different _{modes of}

amphiphile aggregation

while this is

certainly

not the _case for _our system in

which,

for steric _reasons

[22],

the

Hi, phase

cannot exist.

The presence of unusual defects in the sinuous

morphology

is _a consequence of the limited

length

^of the

fibers,

_so that the fibers _can

begin

_or end within _a

domain, frequently

in _a

correlated way. However, it should be noted that the

cryofracture

method

gives only

a section of a defect, which could lead to an

ambiguous interpretation.

As _an

example,

the defect in

figure

7 has been

interpreted

as a transversal

edge

dislocation. This

interpretation implies

a

repetition

of the geometry ^{of the fracture}

plane

above and below this

plane

_up to the walls of

the sandwich where the dislocation would be anchored. However _{one cannot} exclude the

possibility

that the defect does not exist outside the fracture

plane (or

exists above _or below it but in _an uncorrelated

way) (Figs.

I la and

I16).

However, in this _case, the defects would _no

longer

be related to the

boundary

conditions at the external faces of the

sample.

One would

then have _« structure defects _»

[17, 23],

their existence

being possibly

related _to the

proximity

of the

phase

^transition

[23, 24].

N° lo TEM STUDY OF H,, ^PHASE IN SDS-FORMAMIDE SYSTEM 1697

a b

~i~~ ^~~~~

~ _fi

~

~

fi fi

@

p p

fi _fi

Fig. _ll.

L,

nchored the wall of

the sandwich. b) in plane P.

5. Conclusion.

Besides the agreement ^with the

X-ray

results

concerning

the

H~ phase

^{in the} SDS-formamide system, ^the

cryofracture

method

gives

_new information about the

morphology

and _texture of this

phase.

There are

basically

two

morphologies

_one with weak _or zero

longitudinal

curvature of the constitutive fibers and the other in which the fibers _can

easily

bend _or

wind,

producing

unusual defects. An

exploratory analysis

of these defects has been undertaken. The loss of elastic constants, which is a characteristic of the second

morphology, might

exhibit

precursor states of the

Q« Phase.

Acknowledgments.

We would like Pr. C.

Petipas

and Dr. X.

Auvray

who _{were at} the

origin

^{of this} work and have

helped

further

by providing stimulating

discussions.

References

II Kleman M., Williams C. E., Costello M. J. and

Gulik-Krzywicki

T., Phi%.<. Mag. 35 (1977) 33.

[2] ^Delacroix H., Gulik-Krzywicki T.~ Mariani P. and Luzzati V., J Mol. Bio/ 229 (1993) 526-539.

[3] Delacroix H.. Gulik-Krzywicki T., Mariani P. and Risler J. L.~ Liq. Ci=vst. 15 (1993) 605-625.

[4] ^Luzzati V., Biological membranes~ D. Chapman Ed. jAc. Pre~s~ N.Y.~ 1968) _pp. 71-123.

1698 JOURNAL DE PHYSIQUE II N° lo

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