Defects and curvatures of the interfaces in the « birefringent microemulsions »

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Defects and curvatures of the interfaces in the “

birefringent microemulsions ”

J.M. Di Meglio, M. Dvolaitzky, R. Ober, C. Taupin

To cite this version:

J.M. Di Meglio, M. Dvolaitzky, R. Ober, C. Taupin. Defects and curvatures of the interfaces in the

“ birefringent microemulsions ”. Journal de Physique Lettres, Edp sciences, 1983, 44 (6), pp.229-234.

�10.1051/jphyslet:01983004406022900�. �jpa-00232185�

Collège France, 11,

place

Marcelin-Berthelot, 05, (Re~u le 20 decembre 1982, revise le _{19 janvier} 1983, accepte le _{27 janvier 1983) .}

Résumé. 2014 L’état local dans le film interfacial de nouvelles

_{phases qui}

ressemblent aux

microémul-sions, a été étudié _{par marquage de}

spin.

Ces

systèmes

sont des

_phases

lamellaires _ayant des

_propriétés

particulières

associées au mécanisme de formation des microémulsions : ₁₎ la

_présence

de défauts

très courbés

_pourrait

être une

_étape

de la rupture du film et ₂₎ les ondulations à

_grande

échelle des

lamelles, dont

_{l’amplitude}

_augmente avec le taux de

_gonflement,

_prouvent la flexibilité du film,

théoriquement prévue

par ailleurs. Abstract 2014

The local state in the interfacial film of new

_{phases resembling}

microemulsions was

studied

_{by spin-labelling}

_techniques. These _systems are lamellar

_phases

with unusual characteristics

which

_might

be correlated to the mechanisms of microemulsion formation. These are : ₁₎ the presence

of

_highly

curved local defects which may be a _{necessary step} for the _rupture of the interfacial film, and 2)

large

scale undulations of the lamellae, the

_amplitudes

of which increase with the

_swelling

of the

phase,

which indicate the interfacial

_{flexibility theoretically predicted.}

In the

_past

few years much interest has been shown in

lyotropic

systems

containing

the

immiscible

_liquids

oil and water

_{[1]. Usually}

one of two

_{things happens :}

either the oil and water

regions organize

themselves into

_periodic

_arrays

_(based

on lamellae or rods or more

_complex

objects)

or

_they

form

fluid,

isotropic dispersions,

called microemulsions

[2].

In a

_previous

_paper

[3],

we

presented

a

_preliminary

_study

of new

_phases

which appear at cosur-factant concentrations of 50 to 80

_%

of that needed to form a

_{microemulsion,}

and which exhibit features of both extremes :

_they

resemble microemulsions in their

_transparency

and chemical

composition,

yet

their

_{optical birefringence}

indicates an

_organized

structure.

In the

_preliminary

_experiments,

characteristic

_optical

textures and

_X-ray

diffraction

diagrams

were observed

only

for mixtures of _very low oil content. The

_present

_experiments

which were

performed

on

_systems

with minimum amounts of cosurfactant

_{(corresponding}

to the bottom of the

_{phase domain) give}

rise to classical lamellar

_optical

textures when the

_samples

are

_planar

(Fig.

1 A).

X-ray

diffraction

_{peaks, corresponding}

to

_spacings

_{of up} to 150

_angstrom

units are observed over a wide _range of oil-water

compositions.

We

_present

here new results

_concerning

the local state in the interfacial film as reflected in the

(*) La version _fran~aise de cet article a ete _proposee aux

_Comptes

Rendus de l’ Académie des Sciences. (**) Associe au C.N.R.S.

L-230 JOURNAL DE _{PHYSIQUE -} LETTRES

Fig. 1. _{- (a)}

_Optical

_aspect of a

_{sample (0.4}

g SDS, 1 ml

_{H20, 4}

ml

_{C~H~, 0.43}

ml

_{P1 OH)}

observed between crossed _polarisers in a flat

_glass

cell _(0.1 mm _thick). The

_drawing

_{(b) represents} the distribution of the

consti-tuents in such a

phase.

Note that the interfacial film contains both SDS and

1-pentanol.

electron resonance

_spectra

of

_spin

labelled

_{amphiphiles [4].}

It was

_already

observed in refe-rence

_[3]

that these

_phases

exhibit

_larger

order

_parameters

_(S

=

0.46) [5]

than the

corresponding

microemulsions

_(S

=

0.32)

[6, 7].

In the

present

study,

these

_phases

are

investigated

in more

detail,

with the

_object

of

_following

the events which occur in the interfacial film before the micro-emulsion appears.

1.

_ExperimentaL

- These

_phases

are

_{typically prepared}

from 400 _mg of sodium

dodecyl-sulfate

_(SDS-1.39

_mmole),

1 ml of water, 0 to 8 ml of

_C6H12

and the minimum of

_1-pentanol

(P 1 OH)

needed to obtain a clear

birefringent

_system

(volumic

fraction

_{C6H 12}

+

_{P 1 OH/water :}

from 0.37 to

_8.70).

The

_spin

labelled

_amphiphile

₁

which resides

_exclusively

in the interfacial film

_[8]

is used at a molar concentration of 0.1

_%

of

that of the SDS.

_Samples

were observed in

glass

containers of different

_geometries

as shown in

figure

2 :

_sample

A is a thin film

_{(0.1 mm)}

Fig.

2. -

Various types of

_sample

holder. The orientation of the smectic

_layers

is

_{represented schematically}

in each case.

problem

analysis

spectra

sample

type

samples

were

_perfectly

oriented

_{by glass}

surfaces,

the molecules

being

perpendicular

to the smectic

_layers,

the maximum and minimum

_experimental

_splittings

of the lines measured as a function of the static

_magnetic

field orientation would

_give

the

_Tíl

and

_7~

_parameters

of the

_averaged

_hyperfine

tensor. These

_parameters

and the

_{corresponding}

linewidths would be identical to the

_parameters

needed to

_synthesize

the B «

_powder

_spectrum

». This

_identity

was not found here.

These two

_points

are

discussed,

in turn, below.

Fig.

3. -

Comparison

of the _spectra of

samples

B and C. L refers to the lamellar

_powder

spectrum; the

M1, M2

and

M 3

arrows indicate the three _components ofM.

3. Discussion. - 3. 1 THE M SPECTRUM IS DUE TO HIGHLY CURVED LOCAL DEFECTS IN THE

LAMELLAE. - The M

spectrum

in

figure

3

_(reflecting

fast label

_{reorientation) might}

be attributed

to the coexistence of lamellar and micellar

_phases

in the

_sample.

This

_possibility

is excluded

by

the fact that

_samples

of identical

composition (e.g.

B and

_C)

exhibit different

relative

inten-sities of the M

_spectrum.

Moreover M is not observed for the thin

_planar

A

_sample.

M is thus

clearly

a manifestation of the core volume of the C

_type

_sample,

which is not oriented

_by

the

_glass

surface and contains

_regions

where the molecules

_undergo

a fast reorientation.

_Highly

curved

L-232 JOURNAL DE _PHYSIQUE- LETTRES

Fig.

4. - Influence of the

curvature radius on the

_spin

labelled molecule disorientation

_by

the lateral diffusion _process over a time re.

has been invoked

_previously

in studies on the curved film in

_hexagonal

_phases

_[10,

_11].

We pro-pose that

highly

curved

_regions

exist also in the _present lamellar

_phases.

If one infers the _pure M

spectrum

by subtracting

the lamellar L

_spectrum,

the correlation time is found to be several nanoseconds. A reasonable value for the lateral diffusion coefficient is several millionths of a

cm2/s

[10,

12].

These considerations lead to a calculated radius of curvature of a few tens of

angstrom

units

_(below

the

_repeat

distance of the

_lamellae).

Such a low radius of curvature must

correspond

to _very local defects. In order to _{propose models for these}

_defects,

several facts must

be taken into account. Let us recall

_[3]

that the internal

_{phase organization}

is

_{easily disrupted :}

stirring

decreases the

_{birefringence}

and transforms the two line

_D20

NMR

_powder

_spectrum

into a narrow

_averaged

_spectrum.

_{This proves that the disorientation}

_{of D20}

occurs in less than

10 - 3

s. If the

_D20

diffusion coefficient

_parallel

to the lamellae is taken to be

_equal

to

10-5

cm2/s,

then the

_organization

has been

_disrupted

at a scale smaller that one micron. Moreover

bire-fringence

and NMR

_powder

_spectrum

are

_restored,

whilst the M

_spectrum

_disappears

with time. These results indicate that all these defects cost _{little energy} to _{form. We propose} that

they

could be the ends of

_layers

or lamellae defects such as _{pores. Helfrich has}

_recently

discussed the existence of defects in

_{amphiphilic bilayers [13]}

and has

_suggested

that the B and C

_phases

described

_by

Eckwall

_[I],

where the interfacial film is also a mixture of two

_amphiphiles,

are

lamellar

_phases

with a

_significant

concentration of pores in the lamellae. Whatever the

_precise

model may

be,

the

_experimental

facts demonstrate how

_{exceptionally}

easy the

rupture

and recons-titution of the interfacial film is in these _systems.

Another

_interesting

_aspect

of these defects is that their number increases with the amount of

cosurfactant,

i.e. their number increases as the microemulsion

_region

is

approached.

3.2 THE INTERFACIAL FILM IS FLEXIBLE.

_{- Figure}

5

(a)

shows the

_experimental

_{spectrum of}

sample

A when the

_magnetic

field is

_{applied perpendicularly}

to the

_{glass plates,}

which

corres-ponds

to the maximum

_splitting

of the lines. This is to be

_compared

to the

_spectrum

_(b)

which is

expected

on theoretical

_grounds

from the

_parameters

of the

_{corresponding powder}

_spectrum.

There is

_disagreement

in two

_{respects :}

in the

_experimental

_spectrum

_(a),

the

_splitting

_T~

is smaller and the linewidths are

_larger

than

_{predicted. Conversely, T~}

is

larger

than

predicted.

This effect increases with the oil concentration in the

_phase,

i.e. as the

_separation

of the water

layers

increases

(Fig. 1&).

Such a

_discrepancy

has also been observed in

_{thermotropic [14]}

and

lyotropic [15]

_systems.

It indicates that a

significant

fraction of the molecules are not

_{aligned parallel}

to the

_magnetic

Fig.

5. -

Comparison

of the

_sample

A _{spectrum (a)} with the

_magnetic

field

_{corresponding}

to the maximum

lines

_splitting

₍₂

_Tíí)

with the

_{corresponding}

theoretical _spectrum (b) which _gives 2

_T~.

The arrows indicate

a small contribution of the molecules oriented

by

the small sides of the cell. Their

splitting gives 27~.

field. Two different

_geometrical

models werp used to

_explain

these differences. The « tilt »

_model,

where the molecules form a well-defined

angle

with the smectic

_layers

normal,

leads to a decreased order

_parameter

but unmodified

linewidths.

This model has been verified for

_thermotropic

smectics B and C. The second model which allows a continuous

_{angular spread}

of molecules

around a mean

_orientation,

modifies both

_splittings

and linewidths. It

_corresponds

to smooth

undulations of the lamellae in the oriented

_sample

_(Fig.

4a).

The table shows that it is

_possible

to

_predict

the

_{experimental splittings}

and linewidths

_{by introducing}

a static

_{angular spread}

_{80 ;}

0o

increases with the

_swelling

of the oil

_{layer. By taking}

the same value as above for the lateral

diffusion coefficient and

_noting

that these curvatures _appear as « static effects »

_(the

_relaxation time is

_greater

than

10-’

_s),

the

_{corresponding}

radius is calculated to be of the order of several thousands of

_angstrom

units.

Table. - Variation

_{of the spread}

_angle

₀₀

with the oil content

_expressed

as the volumic

fraction

ø =

C6H12

₊

PiOH/~~.

Oriented spectra

Experimental

Calculated

ø

₀₀

split-

order width without

00 : fitting

ting

para- of 3rd parameters of the with

₀₀

(Gauss)

meter line _{powder spectrum}

T ~~ G

S

_{13 G}

_{T’ 11 G}

S

_{13 G}

_{Tíl G}

_{13 G}

_(degree)

0.37 22.84 0.44 2.5 22.75 0.42 2 22.75 2 3 1.35 22.15 0.41 3.125 22.75 0.43 2.8 22.25 3 19 2.31 22.06 0.41 2.95 22.63 0.42 2.9 22 3.1 20 4.43 22 0.40 3.375 23 0.43 3 21.88 3.7 27 8.70 21.73 0.38 3.5 22.81 0.43 3 21.75 3.8 27

4. Conclusions. As noted in a

_{previous preliminary}

communication

[3],

the

_phases

which

precede

microemulsions are lamellar

_phases

with unusual characteristics.

L-234 JOURNAL DE _{PHYSIQUE -} LETTRES

the interfacial film exhibits

_large

scale curvature. Such undulations in lamellae have been

investi-gated theoretically [16]

and could inhibit the formation of

_{well-organized}

lamellae. In

_addition,

these lamellar

_phases

exhibit

_highly

curved local defects.

The various

_types

of

_experiments

prove that these two features are characteristic of

_being

in

the

_vicinity

of the microemulsion

_phase.

The

_importance

of the interface

_flexibility

in the

micro-emulsion formation has been

_emphasized

_{recently [17].}

The _presence of defects associated with

a lack of

continuity

in the

lamellae,

which was

_{predicted by}

_Helfrich,

_may be a _necessary

_step

for the

_rupture

of the interfacial film before the formation of microemulsions.

References

[1]

EKWALL, P., Advances in

_{Liquid Crystals}

1

₍₁₉₇₅₎

1.

[2]

ROBB, I. D., Microemulsions (Plenum Press, New

York)

1982.

[3]

DVOLAITZKY, M., OBER, R., BILLARD, J., TAUPIN, C., CHARVOLIN, J., HENDRICKS, Y., C.R. Hebd. Séan. Acad. Sci. B 295 (1981) 45.

[4]

BERLINER, L. J., Spin labeling

(Academic

Press, New

_York)

1976.

[5] GAFFNEY, B. J., MCNAMEE, C. M., Biomembranes, Methods

_Enzymol.

32

(1974)

161.

[6]

DVOLAITZKY, M., SAUTEREY, C., TAUPIN, C., C.R. Hebd. Séan. Acad. Sci. B 287 ₍₁₉₇₈₎ 77.

[7]

DVOLAITZKY, M., OBER, R., TAUPIN, C., C.R. Hebd. Séan, Acad. Sci. B 293

₍₁₉₈₁₎

27.

[8]

DVOLAITZKY, M., TAUPIN, C., Nouv. J. Chim, 1 (1977) 355.

[9]

BERLINER, L. _J.,

_Spin

_labeling

_(Academic

Press, New York) 1976, p. 373.

[10]

SEELIG, J., LIMACHER, H., Mol.

_Cryst.

Liq.

Cryst.

25 ₍₁₉₇₄₎ 105.

[11] SCHARA, M., PUSNIK, F., SENTJURC, M., Croat. Chem. Acta 48

₍₁₉₇₆₎

147.

[12] ROBERTS, R. _T., Nature 242 (1973) 348.

[13] HELFRICH, W.,

Physics of Defects,

R. Balian ed. _{(North Holland-Amsterdam) 1981,} p. 713.

[14]

POLDY, F., DVOLAITZKY, M., TAUPIN, C., J.

_Physique

_Colloq

36 ₍₁₉₇₅₎ Cl-27.

[15]

MCCONNEL, H. M., MCFARLAND, B., Ann. N.Y. Acad. Sci. 195

(1972)

207.

[16]

HELFRICH, W., Z.

_Naturforsch.

33a (1978) 305.