A spin labelling study of swollen lyotropic lamellar phases

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A spin labelling study of swollen lyotropic lamellar phases

Jean-Marc Di Meglio, Patricia Bassereau

To cite this version:

Jean-Marc Di Meglio, Patricia Bassereau. A spin labelling study of swollen lyotropic lamellar phases.

Journal de Physique II, EDP Sciences, 1991, 1 (2), pp.247-255. �10.1051/jp2:1991158�. �jpa-00247510�

Classification

Physics

Abstracts

68 IO 61 16N 61 30E

A spin labeJJing study of swollen lyotropic lamellar phases

Jean-Marc di

Megl~o (')

and Patricia Bassereau (~)

(~) Laboratoire de

Physique

de la Matidre Condens6e

(*), Colldge

de France, 75231 Paris Cedex 05, France

(~)

Groupe

de

DynanJique

des Phases Condenskes

(**),

Umversitk des Sciences et

Techniques

du

Languedoc,

34095

Montpellier

Cedex 05, France

(Received June 29, 1989, revned October 12, 1990,

accepted

October 22,

1990)

Rksumk.-Nous

prdsentons

_une Etude par marquage de spin de

phases

lanJellaires

lyotropes

dont la distance rdticulaire peut attemdre

8000A

_{Nous estimons}

amsi la

ngiditd

du film interfacial

(de

l'ordre de kJJ

n

et nous montrons que les lamelles des phases les

plus gonfldes

I l'huile _sont certamement

parsemdes

de trous

Abstract.-We report a

study by

Electron Spin Resonance

(ESR)

of _spin labelldd surfactant molecules

incorporated

_m the mterfacial film of

lyotropic

lamellar

phases

with reticular distances

as

large

_as

8000A

_{We find that the}

rigJdity

constant K of such systems compares to kJJ T and _we find evidence that the lamellae of

highly

oil swollen

phases

include curved defects

1. Introduction.

Lyotropic

lamellar

phases ill

_are made of water

(resp. oil)

lamellae stabilized

by

_a surfactant

bilayer

and embedded _m oil

(resp water)

Their

stnking

and

challenging

_{property is} that the

repeat

^distance can reach 10 000

h _[2]

_{and thus these}

phases

^exhibit

Bragg light scattenng.

They

also show _an

iinportant birefnngence indicating

_a

long

_range orientational

order]

_These

systems appeal

to _a lot of interest and

activity

because

they

offer _{a unique}

opportunity

to

study

the interactions between lamellae and _may also constitute _a very first model

system

for

an infinite _»

fluctuating

membrane [3]~ The interactions

usually

invoked in colloidal science

(dispersion,

electrostatic or

hydration interactions)

are not sufficient to

-explain

the ex~stence of these

systems

^{A clue} to the

comprehension

of them _was

proposed by

Helfnch

[4] initially

for swollen lecithin systems

(bilayers

of double-tailed surfactant

separated by water).

^Helfnch

pioneered

_a

long

_range

repulsive

interaction due to the low

ng~dity

and the self

avoiding

character of the membranes This interaction _was

experimentally undericored

_using different

techniques qu~te recently [5-10].

Most of the

properties

^of these

phases

_are related to the elastic constants

aid particularly

to

(*)

URA 792 du CNRS ^'

(**)

URA 233 du CNRS

248 JOURNAL DE

PHYSIQUE

II M 2

the

bending rigidity

K of the fluid membranes

[11-16].

Different

experiments

have been

performed

_{on some} lamellar

phases

and have shown that this

ng~dity

constant _is

small,

of order

kB

T

In this paper, we report ^results on the local _state of the interfacial film obtained

by

electronic spin resonance

(ESR)

of labelled surfactants Two different systems ^{have been} studied.

CPCI

(cetylpyndmium chloride)/bnne/hexanol (bilayer

of surfactant and cosurfactant molecules

separated by bnne)

Salt _is added to _screen Coulombian

repulsions

OBS

(octylbenzene sulfonate)/water/pentanol/decane (water

swollen lamellae'embed- ded _m

oil).

The structural

properties

of these systems ^have

already

received _a lot of attention

[2, 8, 9, 17]

^It has been shown

by

scattenng

techniques

that the first system is stabilized

by

Helfrich's

steric interaction and that the membranes _were defect-free But the second

system

^retains

some of its mystery

the smectic order becomes _« harder _»

(2

or even 3

Bragg peaks

_m

light scattenng)

for the extreme-swollen

phases

while there _{is no}

peak

_m the intermediate range

[2],

the lamellae composition as well _as the apparent

thickiiess change

when going from the dilute to

extremely

dilute reg~me

[2]

2. Materials and metbod.

CPCI _is obtained from Fluka

~purum grade)

and further

purified by

two

recrystalhsations

in water and _{one m} wet acetone

(2

_g of water m 100 _cc

acetone).

Hexanol _is also obtained from Fluka

~puriss grade

_~ 99

fb,

controlled

by

_gas

chromatography)

and used w~th _no further

purification

Nacl _is obtained from Merck

~pro analysis grade

_~ 99 5

fb)

and water is

doubly

distilled OBS _is.

synthesized

and

punfied

_as, described elsewhere

[18] 1=pentanol (Merck PA)

and decane

(Fluka purum)

_are used _as received The

samples

_are

prepared by weigh~ng

^and

the compositions are g~ven m table1.

Table I.

Composition of

the

samples fin

volume

fraction)

Lamella Diluent

System Surfactant Cosurfactant Core Solvent Cosurfactant

(A) CPCI (46 2§6) hcxanol (538§6) bnnc 0 2 M Nacl (99 6 §6) hcxanol (0 4§6)

OBS (B) OBS (32 §6) pcntanol (23fb) water (45 §6) dccanc (905§6) pcntanol (95

The labelled surfactant

N-((2-dodecyl)-N-oxy-2-tetrahydrooxazolyl) propyl N-methyl

_mor-

phohmum

methanesulfonate of formula _:

I

_ffi

CH~- (CH~)jj

C

-(CH~)~

-M

O, CH~SOi

/ '

O N" O

~

has been

synthesized by Dvolhitzky [19].

Th~s quatemary ammonium salt _is

highly

amphiphilic

and thus is

strongly

anchored with~n the mterfacial film

[19]

The concentration of

labelled surfactant molecules with

respect

to unlabelled surfactant molecules _was between

10~~mol/mol

_{to 6}

x10~~mol/mol

for the

hyper-swollen phases (this

concentration was

h~gher

_m the _more swollen

phases

to ensure a measurable

signal)

We have

carefully

checked that within th~s range

(and

_even for _a test

sample

between 5 _x 10~ ^~ and 8 _x 10~ ^~

mol/mol)

the

incorporation of labels inside the film Ad _not induce any modification of the

spectra,

th~s

rejects

any

possibility

of label

partitioning

^between the lamellae and the bulk

(this

will be

addressed furthermore _m the

following

of the

paper)

All ESR experiments ^{have been carried} out _on a E-9 Vanan

spectrometer

at _room

temperature (m20°C)

Two kinds of cells have been used

spherical

cells

(Spin

et

Techniques, Paris)

with _a 4-mm diameter _m order to obtain _an isotropic onentation and _a

powder spectrum (the

lamellae coat the inside of the

cell)

and

parallel-walls rectangular

cells

(200

_~m

path length) ~vitro Dynamics, Rockaway,

New

Jersey)

The orientation of the

samples

in

rectangular

cells _is achieved

by

several heat treatments until

getting

_a

perfect homeotropic

orientation as checked _{using a m~croscope} with crossed

polarizers

3. Results and discussion.

3,I ORDER _PARAMETER

[20]

The very first information that _{we can} obtain from the

spectra

^{is the order} parameter ^{S of the labelled}

alkyl

chain

(S

₌

(3cos~

a

I)

with

2

a the

angle

between the normal to the labelled

cycle

and the normal to the

lamellae)

All the recorded

spectra

were characteristic of lamellar

phases

the order parameter is about 0 4 _{as m} prev~ous studies

[21].

Since the labelled surfactant molecule _is

notably

different from the unlabelled surfactant molecules

(CPCI

_or

OBS)

that surround it

(the

side

cycle

constitutes a non

neghg~ble bulge),

_we cannot claim that the order

parameter

^that we

actually

_{measure is}

the order parameter ^{of the molecules of the} interfacial film nevertheless it does reflect the molecular

packing

of the mterfacial film

(the

denser the

film,

the

higher

the

parameter).

The order

parameter

measurements of the water-swollen

(referred

_as A _m the

following)

and oil- swollen

(B) phases

_are

reported

_m

figure

I

The order parameter is

higher

_m the _water swollen

sample

A than _m the oil-swollen B Th~s

s

i~

i j iijj ^I

j _i ii _~i i~ ^{i i}

3 5 7 9

Log d

Fig

I Order parameter versus the

loganthm

of the reticular distance d

(.) sample

A

(CPCI),

(o) sample

B

(OBS)

250 JOURNAL DE

PHYSIQUE

II M 2

indicates _a

h~gher

_compacity of the A film. The

principal

_{reason we can imag~ne} for this _is that

an A lamella

(of

thickness

do =2651)

is a true

bilayer

while B _is water swollen

(do

=

351)

The

polanty

of the

samples

_can be deduced from the trace of the

hyperfine

tensor of the interaction of the electronic _spin and the

nitrogen

^nuclear spin ^it remains constant

throughout

the whole dilution _range for both systems ^this proves that the

anchoring quality

of the labelled surfactant does not

change

_upon

>welling (and

this _is another

proof

of the _non-

occurrence of

partiomng

of the labelled

molecules)

3 2 LAMELLA FLUCTUATIONS AND RIGIDITY CONSTANT MEASUREMENTS

3 2 Method -<A method for

determining

the

ngidity

constant of lamellar

phases

has been

developed

and _is

fully

described m a recent conference

proc6eding

book

[22].

We w~ll nevertheless expose ^it here _once agam~ The basic

principle

_is to compare the ESR spectra ^of

samples

contained in

spherical

cells

inducing

an isotropic orientation _(«

powder spectra)

w~th the ESR

spectra

^obtained m

rectangular

cells which induce _an

homeotropic anchoring

of the lamellae _(« oriented spectra, _see materials and

method)

The

spectroscopic

^{data of} the label embedded _m the mterfacial film

(i,e, hyperfine splittings

and line

widths)

have been obtained for each

sample by

simulation of the _«

powder

_»

_spectra

_using Lorentzlan

hneshapes

and

weighing

each orientation of _resonance

by

_{a sin} o factor

(isotropic).

We then determ~ne the

angular spread

of disorientation of the lamellae _m the

rectangular

cells due _to fluctuations We

arbitranly

_assume that

locally

the interfacial film is _a

spherical

cap

(which

_{is a} deformation _at minimal

surface)

and determine

oo

^which is the half

angle

of the _cone defined

by

the

spherical

_cap

(by integrating

_over o until _{Ho w~th a sin} o

ponderation)

,

we compare these simulated spectra ^{with the} onented spectra, ^{obtained either} m a

parallel geometjy (the

normal of the cell walls

parallel

to the

magnetic field) ^or"m

_a

perpendicular

geometry

(this

_maximum disorientation

oo

is of

ciurse equal

to

~r/2

for the

isotropic case).

We should stress at this

point

that _:

i)

this method _is

sensitive

_to

very small undulations of the

lamellae, ii)

it is not sensitive _to the sign of

the'curvature

_and

_iii)

_it

is

_not

very ^accurate for

large

disorientations

The charactenstic _time of the spin

labelhng technique

_is

equal

to

10~~

s this _means

thit

molecular motions with charactenstic _times smaller than

10~~

_s will not be observed

(the

average position is then

recorded). Concerning lamellae,

_{we can} estimate from the

dispersion

of the undulation mode

[6]

^that undulations with

wavelengths

smaller than

1001

are not

observed

(the

lamellae _{are seen as} flat _at this

scale)

The recorded spectrum _is then _a

superposition

of

spectra corresponding

to all the orientations of the lamellae

(of wavelengths larger

than

1001)

_the

maximum

disonintation oo represents

an average of the _maximum

disonentations _over time and space. It _is then difficult to

dptermine exactly (o~)

=

(o(0) o(r))

from

oo

If _we omit the time fluctuations of the

lamellae,

_we found from _a

numerical integration ^that

(o~) m0.90(,,includmg

these

fluctuations,

we will write

(o

_~)

= a

o(

with

a a numencal factor of order unity

~loote

that _our previous studies

[5, 21, 22]

would

correspond

to a ₌ 2 _m this

frame)

3 2 2 Fluctuations

theory

The curvature energy

~per

unit

area)

of the interfacial film reads

ill, 23]

~ l

~~

^{l I}

^~

~2

_R

Jlo

wherq

^K is the

ngidity

constant

(with

_energy

dimension),

R _is the total radius of curvature and

Jlo

is the spontaneous radius of curvature which _is the radius of _curvature that the interface

would

adopt

_in absence of any interaction

Dealing

w~th

lamellae,

it _is

leg~timous

to _assume

(kB T)~

that

Rp

= 0. To this energy, we add the stenc

repulsive

term of Helfnch U

= ao

Kd ~

where _ao is _{a constant.}

Using

these two

ingredients

and

making

_a classical mode

analysis,

it _can be

eisily

shown

[11]

that the variation of the

quadratic

_mean of the disonentation o

obeys [24]

_:

(o~)

m

~~(

ln

~~ (l.)

" a

fu

_{is a} correlation

length governed by

the

competition

between the

flexibility

of _one lamella and the interactions with its

neighbours

J~ I/4

fu

= ~ w~th U" the second derivative of the interaction

potential

U with respect ^to U

d,

the repeat ^distance

,

a is the _m~mmum radius of curvature

possible (we

will take it

equal

to

the half thickness of _a

lamella)

Notice that

fu

~

f~

when

(o

_~)

~

l,

I _e that the correlation

length

becomes

eqial

_{to the} persistence

length

introduced

by

de Gennes and

Taupm [11]

(f~

= aexp

~£~

when the lamella has lost the memory of its orientation. The

T

computation

^of U"

yields

the final formula _:

~~~

~~iK

~~ ~~

~°~

^~~

li~

^{~~~ ~~~}

This formula will constitute _our

principal

formula to determ~ne the

ng~dity

constant.

3.2 3

Rigidity

_constant estimation. In

figure 2, o(

_is

plotted

w~th respect ^{to In d for the} water-swollen

(A)

and oil-swollen

(B) samples

_up to 800

h

_{For B}

_samples,

_the

« oriented and _«

powder

_{spectra are} identical for dm 800

h

_We

can imag~ne ^two reasons for that

i)

We cannot obtain a

perfect

onentation m the

rectangular

cells. For the most swollen

phases,

the

optical

observation of the usual textural defects

(l~ke oily streaks)

_is

quite

^difficult in 200 _~m

cells,

and this disabled _us to get a

perfect

control of the onentation.

.

5 7

Log d

Fig 2 Square of the disonentation angle _{@o versus} the natural

logarithm

(Log) of the reticular distance d

(.) sample

A (CPCI), (o)

sample

B

(OBS)

252 JOURNAL DE

PHYSIQUE

II M 2

ii)

The lamellae _{are so} much undulated that there _{is no} difference between oriented and

powder

_spectra

From the

slopes

of

figure 2,

we deduce the _same

ng~dity

constant for both systems :

Km

21a kB

^T

The

ngidity

constant can also be obtained from the intercept of the

(o)~

=

f(In d) plot

with the abscissa axis _we obtain Km 0 66 k

B T if _we take _a

= 15

h,

_that

is the half lamella

thickness~

^{as determined}

^{by X-Ray scattering}

^{at small}

^swelling

^ratio

^(we

^{also take}

ao =

~ _"

as m Helfnch~s

original

_paper

_[4]

but note that the value for K _is not very 128

dependent

_on

_ao)

We _can compare this value with the

ng~dities

obtained _on different

systems

^{with the} same

experimental procedure

and equation

(2) (the following

values _{assume a}

=

2)

_:

Km 4

x10~~~

_{erg for}

water_lamellae

embedded _m

cyclohexane

and

protected

w~th sodium

dodecyl

sulfate

(SDS) (a

cosurfactant

(I-pentanol)

is added until the appearance of

birefnngende)

_[5]

Km 4

x10~~~

_{erg for}

bilayers

of

Cj~E~

embedded m water

(repeat

distance

50h,

Cj2E~

is a

nombn~c

_{surfactant where the}

hydrophobic

chain _{is an}

ahphatic

chain of12 carbon atoms and the

hydrophil~c

chain _is made of 5

ethylene oxides) [25].

Km 3 _x 10~ ^'~ erg for

bilayers

of

Cj~E~

embedded m water

(repeat

distance 50

h) _[25]

K

m

4 _x 10~ ^'~ erg for

water/Ukanil 36/heptane

_or

(CH~)~-Si-O-Si- (CH~

_)~

(repeat

distance 65

h,

_surfactant concentration 58 fb _m water before oil

addition,

Ukaml 36 is _an

mdustnal _non~omc surfactant of cloud point 36

°C) [26, 27]

Km15

x10~'~

_erg for

water/Ukaml 36/silanol [26, 27] (of

formula

CH~

HO-

[-Si-O

j~~

-OH) CH~

It _is not surprising ^to find that K _is about kT for all these systems since

they

are in the

vicinity

of

droplet

microemulsions _m

phase diagrams

If K _were much smaller than

kT, only

m~cellar systems ^could be obtained The smaller value for K w~th the last

system

which

incorporates

silanol _may reflect the molecular interaction of the mterfacial film w~th the silanol

(i

_e the OH

end-groups

of the silanol _can interact with the surfactant molecule

[26, 27j).

As

already

mentionned _m

[27],

it has been found for the OBS system ^{that the lamella} thickness

do

as deduced from _structure experiments

(i

e

X-rays

_or

light

_scattenng,

do

_~

d*~L (3)

d is deduced from the

Bragg peak

and

4i~

the lamella concentration from the

sample

composition)

_{is increasing upon} dilution of the membranes This could be

explained

from the

large crumpling

of the

lamellae,

the apparent thickness _is related to the true thickness

d( by

:

do

₌

dl ( ₍₄₎

where S is the true surface of the lamella and S' the

projected

surface

(S

_~

S').

Helfnch

[12]

a)

M

'

b)

Fig ³ Powder spectra ^of a) a d 100

A

_sample

(B), b)

_a d

=

8 300

A sample (B) showing

^the additional spectrum ^M due to defects

has shown that ~

=

(cos @).

This

approach

leads to the renormalization of the

rigidity

constant

[15, 16].

3.3 DEFECTS _{IN THE OIL} HYPERSWOLLEN PHASE As

already

observed _in the first

experiments perforrned

_on these kinds of

systems [21],

the ESR spectra of the most swollen B

phases

_{are composite}

(while

the spectra ^{of A}

phases

and concentrated B

phases

_{are « pure}

»)

_:

they

_are constituted of _a «slow»

spectrum (i e'

_the correlation time stands _m the slow

tumbl~ng

_regime

[20])

characteristic of the lamellae

plus

_a fast _» spectrum

(the hyperfine splittings

_are

totally averaged) (Fig. 3). Very

curved domains have been shown to

be

responsible

for this _extra spectrum

[21]

,

they

should be due _{to intrinsic} defects such _as pores for the system ^under

study (the

radius of curvature of the defects _is half the lamella thickness

= 18

h)

_{We have added to I}

cc of _a d

=

6 000

h _sample

₁₀

_~l

_of

_I-pentanol

m order that the system enters the

isotropic phase (which

should be

L~

^swollen

micelles)

^and we have

actually

checked that the extra

spectrum

^has the _same features _as this isotropic

spectrum

We _were thus able to simulate the composite

spectra by adding

_a defect component ^w~th

proportIon

p

according

to

y(H)

=

(i -p) ~°j~~

^{+ p}

~9

(5)

y(H)

_is the total _microwave

absorption, yo(H)

the lamella component,

yj(H)

the defect

component, p, = y,

(H)

dH where H _is the scanning magnetic ^{field. The} results _are

plotted

m

figure

4 The percentage ^of

highly

curved surface

drastically

_increases for

samples

where the oriented and

powder spectra

are no more discemable. Let _us notice that the

Bragg peak

reappears for the _same dilutions. We do think that these defects _are intnnsic to the

phase

because

they

do not anneal _even after the

repeated

and _severe heat treatments

performed

to orient the

samples.

We have also checked that the presence of defects is not due to the added

254 JO,URNAL QE

PHYSIQUE

II N 2

is

i

0 2 4 6 8

d(xio-3A)

Fig

4 Fract~on of curved surface for

sample (B)

_versus reticular distance

(=

fraction of labelled surfactant molecules _m curved

regions)

]

labelled surfactant, if _we increase the number of labelled molecules inside the

film,

the proportion ^{of curved} area does not

change

We'should _note that the presence ^of defects

changes

the thickness of the lamellae the actual thickness of the lamellae should be

larger

than the thickness _g~ven

by equation (3)

_or

(4)

,

a

rough

estimation would _{g~ve an} increase of the thickness

by

_a factor _as

large

as 4 in the

case of pores of radius

100h

_{for the most swollen}

_samples~

_{We have}

no

satisfactory explanation

for the presence of such _a

large _proportion

of curved _area We have

nivertheless already

noted

[28] that

_spin

labelhng

_may overestimate the number of defects _or could be

sensitive

of very labile defects _as

dynamic

_{pores »} which _{are not seen}

by

other methods. It

wiuld

_be

interesting

_{to understand if these defects}

are necessary ^to the

stability

of these

hyperswollen phases'(which

_{is not}

_{clearly explained}

_{from the}

_entropic repulsive ~interaction)

4. Conclusion.

We have

reported

_{a spin}

labelhng study

of swollen lamellar

phases

Our

ong~nal

_{aim was} to

investigate

the

amplitude

of onentational fluctuations of the lamellae when

they

_are

separated by

thousands of

Angstroms.

For these

hyperswollen phases,

_we cannot

distinguish

between

the oriented _» and

powder spectra

and _{we are}

unfortunately

not able to attnbute this

experimental

behaviour to _a poor orientation of the

samples

_or to

giant

onentational

fluctuations We have thus failed with respect to our

original goal

Nevertheless _we have estimated the

ng~dity

constants of _our systems and found _a value close to the thermal _energy

kB T[

Other estimations of the

ng~dity

constant K on the _same systems _using

light

_scattenng

have been

performed [10]

and _a

comparison

between -the obtained values should be interesting since the

probed

scales _are different But

unfortunately,

these

techniques

cannot

give

_a direct and model-free measurement of the

rigidity (they depend

_on the

hydrodynam~cal mo(el) Moreover,

_we -confirm the absence of intrinsic defects _m the CPCI system

(A).

On

the

qther hand,

we have shown that the very swollen

oily phases (B)

have _a lot of defects _m their lamellae. We have not~ found any pertinent answer to the stronger ^{smectic order} for

extremely high

dilutions of this latter system as observed

by

scattenng

techniques

Acknowledgments.

We would like to thank Drs. J.

Appell,

D

Chatenay,

J.

Mangnan

and G. Porte for

helpful

discussions and encouragement. ^This

study

has received

partial

financial support ^from PIRSEM

(CNRS,

_{grant no} AIP

2004)

and _{is a} collaborat~on of the GRECO Microemulsions.

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