Columnar to Nematic Mesophase Transition: Binary Mixtures of Copper Soaps with Hydrocarbons

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Mixtures of Copper Soaps with Hydrocarbons

R. Seghrouchni, A. Skoulios

To cite this version:

R. Seghrouchni, A. Skoulios. Columnar to Nematic Mesophase Transition: Binary Mixtures of Cop- per Soaps with Hydrocarbons. Journal de Physique II, EDP Sciences, 1995, 5 (9), pp.1385-1405.

�10.1051/jp2:1995189�. �jpa-00248240�

Classification

Physics

Abstracts

61.30-V 64.70Md

Columnar to Nematic Mesophase Transition: Binary Mixtures of

Copper Soaps with Hydrocarbons

R.

Seghrouchni

and A. Skoulios

Groupe

des Mat6riaux

Organiques,

Institut de

Physique

et Chinde des Matdriaux de

Strasbourg(*),

23 _rue du

Loess,

67037

Strasbourg

Cedex, France

(Recejved

5

April

1995,

accepted

17

May 1995)

Abstract.

Copper (II)

_{soaps are} known _to

produce

columnar

mesophases

at

high

temper-

atures The

polar

_groups of the soap molecules

are stacked _over

one another within columns surrounded

by

the

paraffin

chains _{m a} disordered conformation and

laterally arranged according

to _a two-dimensional

hexagonal

lattice.

Upon

addition of _a

hydrocarbon,

the

mesophases

swell

homogeneously

The

hydrocarbon

molecules locate themselves _among the disordered chains of the soap

molecules,

the columnar _{cores remain}

perfectly unchanged, keeping

_a constant intra-

columnar

stacking period,

and the

hexagonal

lattice

expands

_m

proportion

to the _amount of

hydrocarbon

added to the system.

Beyond

_a certain

degree

of

swelling,

the columnar

mesophases suddenly

turn into _a nematic

mesophase through

_a first-order

phase

transition. The structural

elements that

align parallel

to the nematic director _are the very same molecular columns that

are involved in the columnar

mesophases

The columnar to nematic

mesophase

transition _was studied

systematically

_{as a} function of the molecular _size of the soaps and

hydrocarbons

used _as diluents and discussed _on

a molecular level,

emphasizing

such aspects _as the

persistence length

of the

paraffin

chains and the location of the solvent molecules _among the columns

1. Introduction

Copper (III

_{soaps are}

amphiphilic

molecules with _a

polar part

made up of two metal atoms

bridged by

four bidentate

carboxylate

_groups. Their chemical formula is:

[CH3 (CH2 )n-2 C02)4 Cu2 (abbreviated

in the

following

_as

Cncu). They

_are well known to show

thermotropic

colum-

nar

mesophases

above

temperatures

^{of about 100 °C}

Ill.

^The

polar parts

^{of the}

molecules,

held

together through apical ligation

of the metal atoms with the _{oxygen atoms} of the

neigh- boring molecules,

_are

regularly

stacked over one another within columns. Surrounded

by

the

paraffin

chains in _a disordered

conformation,

the columns _are

laterally arranged according

to a two-dimensional

hexagonal

lattice.

In _a recent paper

[2],

^the

liquid crystalline

behavior of mixtures of

C12Cu

with

decahydron- aphthalene

used _{as a} solvent _was studied

by polarizing microscopy

^and

X-ray

diffraction. It

(*)

UMR 0046

(CNRS-ULP-EHICS)

©

Les Editions de

Physique

1995

was found

that,

at

high temperatures,

the

mesophase

turned from columnar to nematic when the

weight

fraction of the solvent _was increased

beyond

_a value of about

50%.

The main fea- ture of the nematic

mesophase

observed _is that the structural elements which

align parallel

to

the nematic director _are not individual

molecules,

but the _very columns which constitute the

columnar

mesophase

itself and which have lost their

long-range

lateral

positional

order. This

phase

transition _was then

briefly

discussed in the

light

of two theoretical models

published recently _{[3, 4].}

While the _occurrence of nematic

ordering

at

high

^dilutions is

simply

due to the

Onsager

interactions of the

rigid columns,

the columnar

ordering

at

high

concentrations is due to the existence of soft

short-ranged repulsions

between the columns. In these theoretical

models,

the

repulsions

_are related either to electrostatic interactions when the columns _are ionic in _nature

[3],

or to an effective _energy

penalty

due to

thermally

activated undulations of the columns when these _are not

totally rigid

_[4]. ^It was

suggested,

however

[2], that,

in the

case of _{copper soaps,} the

repulsive potential might equally

well be attributed _to the excluded volume

entropic penalty

connected with the

interpenetration

of the disordered

paraffin

chains

belonging

to

neighboring

columns.

The present paper is intended to

study

the columnar to nematic

phase

transition in further detail

by varying

the

length

of the

paraffin

chains of the _soap molecules and the molecular

weight

of the

hydrocarbon

solvents. It is also intended _to discuss the transition on a molecular

level, emphasizing

such aspects as the

persistence length

_or the location of the solvent molecules

among the columns

2.

Experimental

2.I. MATERIALS. The copper soaps used in the present

work, except

^for

Cscu

and

C12Cu

obtained from LCC in

Grenoble(~),

_were

synthesized following

a method described in the literature

[5]. Copper

acetate

monohydrate

>

99%, Merck)

_was reacted with the

corresponding fatty

acid

(> 99%,

Merck for _{n =}

18,

^Aldrich for _n

= 20 and

22,

Janssen for

n =

24)

^for two

weeks at _room

temperature,

in

anhydrous

ethanol

solution,

in the presence of _a few

potassium

iodide

crystals;

the crude

precipitate

obtained _was washed with

anhydrous ethanol,

_vacuum- dried at 50 ^°

C,

and

recrystallized

twice from hot

heptane. Satisfactory

elemental

analyses

_were

obtained for

carbon, hydrogen

and _copper.

The molecular characteristics of the _{soaps are} shown in Table I. As the

specific

volumes of the soaps with short

paraffin

chains

(n

_<

18)

could not, for technical _reasons

[6],

^{be measured}

experimentally, by dilatometry,

their values _were calculated from

crystallographic

data

relating

to the columnar

mesophases

at 130 °C

[I b]:

from the intercolumnar distance D and the intracolumnar

stacking period

of the _soap molecules h

(=

4.65

I),

one can

easily

deduce both the molecular volume of each soap:

V(i~)

=

QiD~h/2(= _183.21

₊

lls.7gn)

and its

specific

volume:

=

NV/M (where

M is the molecular

weight

of the _soap and N

Avogadro's number).

Most

hydrocarbons

considered in this work _were linear

paraffins

with _p carbon atoms in their molecules

(designated

in the

following by C~).

Two further

hydrocarbons

_were considered _as well-

decahydronaphthalene (dec),

which is

bicydic

in

architecture,

and

squalane (squ),

which is branched. Their chemical characteristics _are

reported

in Table II. The

specific

volume of the linear

paraffins

at 130 °C _was deduced from densities measured

experimentally

_as a function of molecular

weight

and

temperature

_[7]. That of the other two

hydrocarbons

_was calculated from their

density

at room

temperature [8], using

a thermal

expansion

coefficient of

lo~~ K~~.

Binary

blends of copper soap and

hydrocarbon

_were

prepared by

mechanical

mixing

of the constituents at _room temperature. ^As

good homogeneity

is hard to obtain in

samples

with low

(~) Laboratoire de Chimie de

Coordrnation,

Centre d'Etudes Nucldaires _de Grenoble.

Table I. Characteristics

of

the _{copper soaps}

(Cncu)

used. M: molecular

weight; V,

_@:

molecular volume and

specific

volume at 130 ° C.

n M V

(A3j (cm3jg)

8 699.9 109 O 954

12 924.4 1573 .024

18 1261.O 2267 1.082

20 1373.2 2499 1.096

22 1485.4 2730 1-lo?

24 1597.6 2962 1.l16

Table II. Characteristics

of

the

hydrocarbons

used.

C~

^stand

for

linear

hydrocarbons

with p carbon atoms _m the

molecule,

dec

for decahydronaphthalene,

and _squ

for squalane.

M:

molecular

weight;

_mp,

bp:

are the

corresponding melting

and

boiling temperatures;

_@h~

specific

volume _at 130 ^° C.

(AL: Aldrich;

FL:

Fluka;

EK:

Eastman-Kodak;

W: Dr. J C.

Wittmann;

LA:

Lancaster).

purity M mp bp ~

(%l 1°Cj (°Cj (cm3/gj

C~2 AL 99 170.3 .12 216 .SOD

C~~ FL 99 198.4 7 250 1.465

C~e EK 99 254.5 28 Al 8

C~z EK 99 450.9 69 .346

C~ AL 98 507.0 76 265/~ l .336

C~ _{AL 99 619} 2 88 1.321

C~ w 843.8 .303

dec LA 98 138.3 .125 192 1.264

squ AL 99 422.8 .38 176%os 1.345

hydrocarbon

_contents, attention _was focused

only

_on concentrations

higher

than about

15%.

In

addition,

to reduce shifts of

composition

to _a

minimum,

_{care was} taken to _use

hydrocarbons

of low

volatility,

to hold

samples

in

tight cells,

and to

keep

the

temperature during

^the

experiments

below 160 °C.

2.2.

TECHNIQUES.

Transmission

optical

observations of thin films (+~ 20

pm)

_{were per-}

formed with _a Leitz

polarizing microscope

^{fitted with} a Mettler FP82 hot

stage. Owing

to the thermal

degradation

of the _{copper soaps} at

high temperatures,

^{the usual method of}

getting

Fig.

1

Optical

texture of C24Cu _m the columnar state, observed _at 130 °C in the _presence of 30 wt% of C14

(x300)

and

showing "developable

domains".

well-developed

characteristic _textures of the

mesophases (slow cooling

from the

isotropic melt)

could not be

applied.

Clear textures with

developable

units for the columnar and schlieren features for the nematic

mesophases

could nevertheless be obtained.

X-ray

diffraction

patterns

of

powder samples (in

Lindemann

capillaries

_or in

tight

metallic

cells)

_were recorded either

photographically

_or with _a curved

position-

sensitive detector

(INEL- CPS120) using

Guinier

focusing

_cameras

equipped

with _a bent quartz monochromator

(Cu- K~i).

Patterns of

magnetically

oriented

samples (samples

submitted to a

magnetic

induction of lA T

during

the

X-ray experiment

at

high temperatures)

_were recorded

photographically

with _a custom- made

pinhole

_camera

using

Ni-filtered _copper radiation.

3.

Swelling

of the Columnar

Mesophase

In _a first

stage,

we

systematically

studied the

swelling

behavior of the columnar

mesophase

of _{copper soaps} in the _presence of _a wide

variety

of

hydrocarbons

at 130 °C.

Up

to a certain concentration of

hydrocarbon,

the swollen

samples

^looked

homogeneous,

viscous and

sponta- neously birefringent.

Their

optical

textures, as observed with _a

polarizing microscope

_upon

heating

from _room

temperature,

were

generally complex

and difficult _to

interpret safely.

But after _a few hour

stay

at

high

temperature,

especially

with dilute

blends,

the textures became

clearer in the

end, acquiring

the

specific

features of _a columnar

mesophase (Fig. I).

Their

powder X-ray patterns

^showed a series of at least three

sharp Bragg

reflections _in the small-

angle region,

with

reciprocal spacings

in the ratio I:

Qi: vi: Vi,

_{indicative of the}

_hexagonal

packing

of the columns.

They

also showed _a diffuse halo at 4.5

I

_{in the}

wide-angle region,

related _to the disordered conformation of the

paraffin

chains

(Fig. 2) and, eventually,

_an ad- ditional weak band at 4.65

I,

_{connected with the}

_{stacking period}

_{of the}

soap molecules inside the columns.

In the presence of _a

hydrocarbon,

the

small-angle Bragg

reflections remain

extremely sharp

and the D

parameter,

that

is,

the distance between the _axes of two

adjacent columns,

increases both with the molecular

weight

of the _soap and the concentration of

hydrocarbon

m the

l~?'

'~'

~T~'

~-

"~[i@~

t'

~ 'iii IS I,I ~/~~

~ C'

ifh '~ii i'~

,J.~i~'

"' '~'L~~4', (~'

.~<~

'[j _1:.~,'

_>[..

'[~;j~ ^.~'

mfl?.~

/3j'

_{Qli. "( ~}

f-'

~ _~20

~/~

Fig.

2 Powdei

X-ray

pattern of C20Cu in the columnar state, observed with _a Guinier

focusing

camera at 130 ^°C _m the _presence of 25 wt% of C14

fi

i ^{' Cm}

. C18

6

32~

o C36

o C44

~

~

~

<

l 25

000 0 25 0 50 0 75 00

a

Fig

3.

Swelling

of C24Cu in the columnar state at 130 °C _m the _presence of _{a series} of

Cp hydrocarbons. Straight

line represents

theory:

A

=

Ao(1

+ 4l)

mixture. The _area of the

hexagonal cell,

A

=

QiD~/2,

_grows

_linearly

_{with 16}

=

C@h/(I _C)@,

the volume of

hydrocarbon

_per unit volume of _soap in the mixture

(C

is the

weight

fraction of the

solvent,

and _@h and the

specific

volumes of the solvent and _soap,

respectively,

_see Tables I and

III.

Such _a behavior shows that the

stacking period

of the molecules inside the columns is

not affected

by

the

swelling

and that the solvent molecules _are distributed

uniformly

_among the

hexagonal

cells of the

mesophase:

the mixture _is

crystallographically homogeneous

in the _sense of Bravais

[9]. Indeed, simple geometrical considerations,

based _on the

assumption

^of

additivity

of the molar volumes of the

species present

in the

mixture,

show that the volume of _one swollen soap

molecule,

that

is,

the volume of _{one soap} molecule increased

by

the

corresponding

volume of added

hydrocarbon,

is Ah

= V ₊ Vlb

=

Aoh

+

Aohlb, leading

to:

A=Ao(I+16),

where

Ao represents

^the area of the

hexagonal

cell of _{pure soap} in the columnar state.

Figure

3 shows the

swelling

^behavior of

C24

Cu in the _presence of _a series of

hydrocarbons

and

Figure

⁴

that of _a series of _soaps in the _presence of

C14.

The

question

now arises of how the

hydrocarbon

molecules distribute themselves _{on a} molec- ular scale inside the

hexagonal

cells. Before

discussing

this

problem,

it is

helpful

to take the

following

three

points

into account. Let _us first consider the behavior of disordered

paraffin

chains immobilized at _one of their extremities For stereochemical _reasons, the freedom of the successive

methylene

_groups in _a

single chain,

and

consequently

their

disorder,

_grows

rapidly

JOmNAL DEPHYBIQIJBD T5~N°_9~ WnEMBER1995 ~

200

1

. Cui8

a Cu20

a Cu22

a Cu24

°~

~

i

0 00 0.25 0 50 0 75 00

a

Fig.

4.

Swelling

of _{a series} of Cncu _soaps jn the columnar state at 130 °C in the presence of C14

Straight

^line represents theory A

=

Ao(1+

_4l)

with their distance from the

anchoring point.

This _was confirmed

by

nuclear

magnetic

reso-

nance studies in the _case of

liquid crystals _[10].

In _a dense

assembly

of such

paraffin chains,

excluded volume interactions

play

_an

important part,

^{and the freedom of the}

methylene

_groups

is

globally

reduced. This effect is _more

pronounced

when the available _space around the

methy-

lene _groups, that is their molecular _area in _a

plane

normal to the chain

axis, gets

smaller

Thus,

when the chains _are attached onto a thin

rod,

the freedom decreases

dramatically

_near the rod.

This _was confirmed

by X-ray

diffraction studies of

poly(di-n-alkylsilanes)

in _a columnar

state, showing

that the

methylene

_{groups are}

packed

much _more

densely

_near the columnar axis than far away from it

ill].

This _was also confirmed

by

incoherent

quasi-elastic

neutron scat-

tering

^{studies of} columnar copper soaps,

showing

that the

mobility

of the successive

methylene

groups, almost

negligible

next to the columnar

axis,

increases

rapidly

with the distance from the columnar axis _to level off

beyond

the fourth carbon atom

[12].

Let _us then examine the

hexagonal packing

mode of the molecular columns in the columnar

liquid crystals

of _{copper soaps.} From _a

crystallographic standpoint,

this

packing

is described

completely by

_a two-dimensional

hexagonal

Bravais lattice

specifying

the

periodic

_array in which the

repeated

columns _are

arranged.

From _a

geometrical standpoint, however,

it is _more

convenient to _use the

corresponding Wigner-Seitz

lattice

defining

the cells in which the columns

are located

individually (Fig. 5).

Attached _onto the columnar _cores at the center of the

cells,

the

paraffin

chains of the _soap molecules

spread

outwards in _a disordered

conformation, filling

the

remaining

_space

completely.

Because of the steric hindrance effect mentioned

above,

the

polar

_cores of the columns _are surrounded

by

_a thin shell of immobile

methylene

_groups, the radius of which _may be estimated to be of the order of 7

I(~)

_This

_high

value _is

probably responsible,

at least to _some

extent,

for the

rigidity

of the columns and their

high persistence length (of

about 200 250

I

as measured

by low-angle X-ray scattering

of dilute copper soap

(~) ^{As the}

increasing mobility

of the

methylene

_groups reaches its

asymptotic

value

beyond

the fourth carbon atom from the

carboxyllc

_{group [12], we may}

place

the

rnob1llty

threshold

halfway

between the fourth and fifth methylene _groups. We may then

easily

evaluate the radius of the frozen

region.

r =

11(Gt

₇

I),

using the value Ao

("

lsl.5

i~)

of the

hexagonal

cell _area of the fictitious C4 5Cu soap.

Fig

5.

Arrangement

of the columns of _copper soap m the cells of _a

hexagonal Wigner-Seitz

lattice Solid circles stand for the

polar

_cores of the

columns,

and open circles for the regions of low

mob1llty

of the

methylene

_groups.

solutions

[13]).

^In this frozen

shell,

the molecular _area of the

paraffin

chains

hardly

exceeds 50

i~ (~),

and the

interdigitation

of the disordered chains is

hardly possible. Interdigitation

and conformational

deployment

of the

paraffin

chains

actually

make their _appearance

only

in the _space between the frozen

regions.

As _a

result,

the

Wigner-Seitz

walls cannot be viewed _as

impassable

barriers for the

intermingled paraffin chains,

but

just

_as fictitious surfaces

marking

the boundaries of the _average volume of each individual

column,

and _as the

places

where the

methylene

_{groups are} most disordered. It is useful to notice in this connection that the thickness of the matrix between the frozen

regions (D 2r)

is rather

small,

of about 6

I

_for

C12Cu

and

131

_for

_C24Cu.

Let _us

finally

recall the

swelling

behavior of sodium _soaps in _a columnar

liquid crystalline

state

[14].

^{As shown}

long

_ago

by X-ray diffraction,

the

polar endgroups

of these

systems

are

close-packed

in ribbon-like columns surrounded

by

the

paraffin

chains in _a disordered confor- mation and

laterally arranged according

to a two-dimensional

rectangular-centered-lattice.

^In

the presence of _a

hydrocarbon,

the

rectangular symmetry

remains intact. The

length

of the

rectangular

cell

expands proportionally

to the amount of

hydrocarbon

added while its width is

kept

constant. This

highly anisotropic swelling suggests

that the

hydrocarbon

locates

itself,

as illustrated in

Figure 6, along

the

glide planes

normal to the ribbon flat

sides,

that _is where the

paraffin

chains _are most disordered and the cohesion of the

liquid crystal

is weakest.

With these

points

in

mind,

the discussion of the

swelling

of the columnar

mesophases

of copper soaps is

straightforward.

Excluded both from the

strongly incompatible polar

heads of the _soap molecules and from the frozen shell of immobile

methylene

_groups next to the columnar

axis,

the

hydrocarbon

solvent cannot but be located within the continuous

paraffin

(~) ^Each soap molecule contains four

paraffin

chains and

occupies

_a

length

of h

= 4.65

I along

the columnar _axis. Calculated _on the surface of _a

cylinder

of radius _r, the molecular _area of _one

paraffin

chain _is then-

2~rh/4

= 7.3r. At _a radial distance of 7

I,

where

mobility

_appears, the molecular _area is of about 51 i~.

A

(i)

a

,/

(

~ ^l jj~~

(11)

< >

b

Fig

6. Schematic representation of the columnar mesophases of _pure

(i)

and

paraffin

swollen

(it)

sodium _soaps at

high

temperature _a and b _are the cell parameters of the

rectangular-centered-lattice

matrix,

_among the mobile

parts

of the disordered

chains,

that

is,

ⁱⁿ the

vicinity

of the

Wigner-

Seitz walls

4. Occurrence of _a Nematic

Mesophase

Beyond

_a certain concentration of

hydrocarbon,

_we noticed that the

binary

mixtures became

suddenly

^{fluid while}

remaining birefringent. Although

deformed due _to the _easy flow of the material between the

glass slides,

their "schlieren" textures

(Fig. 7) displayed

the

specific

features of _a nematic

mesophase.

Their

powder X-ray _patterns (Figs.

8 and

9) fully

confirmed this

interpretation. They

showed that the

low-angle sharp Bragg

reflections of the columnar

mesophase

_were

replaced by

a diffuse

band,

located in the

equatorial plane

when the

sample

_was submitted _{to an} external

magnetic field, indicating

that the

long-range positional

order of the columnar

phase

_was lost in favor of

short-ranged

columnar

cybotactic

_groups.

Overexposed X-ray patterns

of

magnetically-oriented samples

showed _a weak meridian band at 4.65

I,

connected with the

stacking period

of the soap molecules inside the structural elements of the nematic

phase. Quite clearly,

these elements _are identical to those

(columns)

of the columnar

phase.

Fig.

7. "Schlieren" texture of C12Cu _m the nematic state, ^observed at 130 °C in the presence of 50 wt% of C14

(x300)

~~ ~~

(200)

0 4 8 12

26

(deg)

Fig.

8 X-ray

powder

pattern of _pure C12Cu in the columnar state

(solid circles)

and of swollen

(40

^{wt% of}

C14)

C12Cu in the nematic state

(open

circles

),

observed at 130 ^°C with _a Guimer

focusing

camera

equipped

with _a curved

position-sensitive

detector

To

analyze

the

swelling

of the nematic

phase,

_we considered the

angular position

of the low-

angle

^{diffuse band} as a measure of the average distance D between the _axes of _two

adjacent

columns in the

cybotactic

_groups. We thus could

get

_an estimate of the _{average area} of the unit cell of the

corresponding

^local

hexagonal

lattice.

Figure

lo shows the

swelling

behavior of _a series of copper soaps in the nematic

phase

added with

C14i

this is

obviously

identical to

that observed in the columnar

phase.

The local

hexagonal

lattice does

expand proportionally

Fig.

9. X-ray diffraction pattern ^of a

magnetically

oriented nematic sample of C20Cu added with 50 wt% of

Cm,

recorded

photographically

at 130 °C _with

a

pinhole

_camera.

3.0

1

~'~ ~~~

w Cu22

o Cu24

O

o 2.0

<

~ o

~

<

0.0 DA 0.8 1.2 1.6 2.0

w

Fig.

10.

Swelling

of _{a series} of Cncu _{soaps m} the nematic state at 130 °C in the _presence of Cm-

Straight

line represents

theory:

A

=

Ao(1

+ 4l)

to the amount of

hydrocarbon

in the

mixture, suggesting

that the solvent is shared out

equally

among the

columns,

and that the columns themselves remain

unchanged. Figure 11, dealing

with the

swelling

^{behavior of}

binary

mixtures of

C12Cu

with _a wide _range of

hydrocarbons, brings

out _a further

point

of

interest,

the stabilization of the unit cell _area above _a certain

degree

of

dilution, indicating

the

phase separation

of _a dilute solution of _soap from the nematic

phase.

While the

limiting

dilution of the nematic

phase

is easy ^to measure

accurately

from the break in the evolution of

A/Ao

_vs. lb (16* = 0.54 for

C36

and 16* ₌ 0.73 for

C32),

^the upper limit of the

biphasic region

is hard _even to

detect,

either

by polarizing microscopy owing

to the

extremely

weak

birefnngence

of the

mixture,

_or

by X-ray

diffraction

owing

to the

likely

presence of

cybotactic

_groups also in the

isotropic

solution.

30

. C14

2.5 ° C18

+ C32

. c36

* D6c

o 6 SqU

<

~

< , ^A

~

i.5 "

0.0 0.4 0.8 1.2 1.6 2.0

w

Fig.

II.

Swelling

of C12Cu

m the nematic state at 130 °C in the _presence of _a series of

Cp hydrocarbons. Oblique straight

fine represents

theory:

A

=

Ao.(I +16).

A final comment may be added

concerning

the

spatial

extension of the columnar order in the nematic

phase.

To

get

an estimate of this

extension,

we

analyzed

the

profile

of the X- ray

cybotactic

band

semi-quantitatively (without proceeding

to the usual

intensity

corrections for

polarization,

Lorentz

factor,

etc.

), fitting

the

experimental

data to _a Lorentzian function

(Fig. 12).

As

expected,

the range of columnar correlations extends _over

only

_a few

hexagonal

cells and declines with

increasing temperature

and

particularly

with

increasing

dilution

(Fig.

13).

5. Columnar to Nematic Phase Transition

After

having

shown

that,

at _a

given

solvent concentration 16*, the columnar

phase

of _copper soaps

suddenly

transforms into a nematic

phase,

_we studied this transformation _more exten-

sively

_{as a} function of molecular size and

temperature.

As observed with

polarizing microscopy,

the

change

from columnar to nematic

proceeds discontinuously,

within _{a narrow} range of _con-

centration.

Adequately

selected

samples displayed simultaneously optical

textures of both the columnar and nematic

phases (Fig. 14), separated by fairly sharp boundaries, suggesting

a

first-order transition

going through phase separation. Although

_{very narrow}

(<

I

wt%),

the concentration range of

phase separation is, however,

^wider than the concentration deviations due to an eventual

inhomogeneity

of the

samples prepared. Upon heating,

the

phase

boundaries shift indeed

progressively,

the nematic texture

invading

the columnar _one within _a

temperature

interval of _more than 20 ° C. The first-order _nature of the

phase

transition _was

fully

confirmed

by X-rays (Fig. Is),

the diffraction

patterns registered showing clearly

the coexistence of _a

columnar and _a nematic

phase.

The

hydrocarbon

concentrations measured

experimentally

at the

phase

transition for _{a num-}

0 4 8 26

(deg)

Fig.

12.

Least-squares

Lorentzian fit of the

low-angle X-ray

diffuse band of C12Cu _m the nematic

state at 130 °C _m the presence of 40 wt% of Cm

~

26%

~.

~.

Cl

~ ~~~

~ 40% .

~ 50%

100 120 140 160

T(°c)

Fig

13 Variation of the columnar correlation

length

_m the nematic

phase

ofC12 Cu _{as a} function of temperature and

weight

fraction of C14 Correlation

length (

_is

compared

to the _average intercolumnar distance D.

Fig.

14.

Optical

texture of C24Cu observed at 130 °C

m the _presence of 36 wtslo of

C14, showing phase

separation between columnar and nematic

phases.

Columnar

~

#

Nematic

#

~

m

I

_{".._}

(

~fl ~

Fig.

15. X-ray powder _pattern of C12Cu added with 22.I wt% of C14,

registered

at 150 °C with _a Guinier

focusing

_camera

equipped

with _a curved

position-sensitive detector, showing

the coexistence of _a columnar and _a nematic phase

Experimental

data _were fitted with the _sum of two Lorentzian

functions. Dots represent ^{the lorentzian function}

corresponding

to the nematic

phase

ber of

binary

mixtures _are listed in Table III. The

particular

behavior of the

binary

mixtures of

C12Cu

with _a

variety

of linear

hydrocarbons

is summarized in

Figure

16.

Clearly,

the _ex-

tension of the

stability

domain of the columnar

phase

_grows when the

hydrocarbon

molecules

get shorter, especially

when the number of carbon atoms in the latter falls below fifteen. The

hydrocarbon

enters the columnar _structure all the _more

easily

since its molecular

weight

is

Table III. Solvent concentrations

(C*

_m

wt%

and 16* _m

parentheses)

at the columnar

/

nematic transition

for

a

vartety of binary

mut~res

of

_{copper soap} with

hydrocarbons. Symbol

$ designates

direct transitions

from

columnar to

isotropic liquid (concentrations correspond

then to the _upper limit

of

the columnar

phase).

C12Cu ciecu _C~OCU C~CU C~4Cu

C12 25.5 36.O 38.O

(O.501) (O.779) (O.824)

Cm 22.1 33.O 35.O 36.O 36.O

(O406) (O.666) (O.720) (O.744) (O.738)

C~e 18.6 28.4 31.O

(0.316) (0.520) (0 571)

Cz~ 15.1 23 2 27.O

(O.234) (0.376) (O.446)

Cm 14.O 22.6 26.2

(O.212) (O.360) (O.425)

C« ₊ 21.5 25.5

(O.172) (O.334) (O.405)

C~ _{+ 21.O} 25.O

(O.Og7) (0.320) (O.389)

Squ 16.O 24 25.2 26.4 27.4

(O.250) (O.395) (0.413) (0.436) (O.455)

Dec 40 O 51.6 52.4 53.4 54.O

(O.823) (1 246) (1.269) (1.308) (1.330)

lower. The _same holds for the nematic

phase.

For

high dilutions,

the nematic

phase gives

way ^to an

isotropic liquid through

_a wide concentration gap of

phase separation.

The

capacity

of the nematic

phase

to

incorporate hydrocarbons collapses suddenly

when the

hydrocarbon

molecules

get longer

than about

fourty

carbon atoms. This is

why

the nematic

phase

does not appear with

long hydrocarbons,

the columnar

phase entering

_upon ^dilution

directly

in coexistence with the

isotropic

solution. The enhanced

stability

of the columnar with

regard

to the nematic

phase

_may be understood _as

being

^related to

stronger

van der Waals attractions of the columns due _to shorter

spacings.

The

abrupt shrinking

of the nematic domain for

sufficiently long hydrocarbon

chains is

strikingly

similar to what

happens

to solutions of flexible

polymers

in _a calamitic nematic

,

,, fi

~,

~12~~

l Col+lso

I

,

45 ,

j

Nem+lso

---~_

~--~~

~~~-~~

Cu '-o~,

~~~~

,~ ~~.~_

~~ Cal

Nem

15

0.0 0.2 0A 0.6 0.8

W*

Fig.

16. Phase

diagram

of C12Cu

/ Cp binary

mixtures at 130 °C

as a function of the

swelling

4l*

and

length

_p of the

hydrocarbon

molecules Full line stands for the first-order columnar to nematic

phase

transition

(through

_{a narrow}

biphasic region);

dashed line represents the

limiting swelling

of either the columnar

or the nematic

phase

_m coexistence with the isotropic

liquid.

solvent. For

instance,

the

solubility

of

poly(ethylene oxide)

in

p-azoxyanisole

falls

dramatically

with

growing

molecular

weight

^{of the}

polymer,

to even vanish

completely

for molecular

weights higher

than

40,000.

^On the other

hand,

the

biphasic region

of the nematic with the

isotropic liquid

is very extended

[15].

^Such a

similarity reflects,

of _course, the conflict that opposes the nematic

molecules, tending

to orient themselves

parallel

to _one

another,

_over

against

the linear

polymer

_or

hydrocarbon molecules, tending

instead to

adopt

_a disordered conformation In the

polymer

_case, the short rod-like nematic molecules tend to

locally

orient themselves

parallel

to the

polymer chains,

and their nematic order is

locally

disturbed. At low

concentration,

each

polymer

random coil behaves like _an

"isotropic liquid droplet"

of

polymer

swollen with the nematic solvent. At

high concentration,

the

polymer

coils _{come near}

enough

to _one another

as to

destroy

the nematic

ordering completely [16].

^{This view is all the} more

satisfactory

since the

length

of the calamitic nematic molecules

(close

to 15

I)

is

comparable

to the

persistence length

of the

polymer

chains. In the _case of _{copper soaps, on} the other

hand,

the

point

is

quite

different. When introduced inside the nematic

system,

^the

hydrocarbon

chains _are confined

to the _{narrow spaces} in between the

long

and

rigid

columnar _cores, and their

solubility

is

dominated

by

the

thermodynamic penalty

of confinement

[17].

This latter view is

supported by

_our

experimental

observations shown in

Figure

17.

Quite evidently,

_as the

hydrocarbon

molecules

get

shorter and the intercolumnar

spacings larger,

the

swelling

_range of the columnar

phase

increases

appreciably.

This

meaningful

result cannot be taken into account

by

the

existing

theoretical models

quoted

in the introduction of the

present

paper.

Indeed, discarding

all

microscopic aspects,

^{these models deal with the}

macroscopic properties

of idealized

systems

^{made of}

aligned

columns

(either rigid

but

varying

in size

[3],

or

infinitely long

but semiflexible

[4])

^{immersed in} a continuum of

liquid solvent; and,

_as

6°

C~~CU C~~CU

40 C~~CU ^Nem

cu

20 Col

O-O 0.3 0.6 0.9

W*

Fig.

17 Phase

diagram

of Cncu

/ _Cp

binary mixtures at 130 °C

as a function of

~*,

_p, and _n

(see

Table

III).

such, they

_are therefore not well

adapted

for

studying

the

liquid crystalline

behavior of _copper soaps in which the dense

layer

of disordered

paraffin

chains round the columns

plays

_a

major part.

^As

suggested

in _a

previous

_paper

[2],

^{the steric effects related to the}

interpenetration

^of the dense

paraffin layers surrounding

the columnar _cores contribute

significantly

to the short-

ranged repulsion

of the

columns,

and to the columnar

stability altogether. Upon swelling,

the columns _move apart and their

paraffin

chains

disentangle progressively;

the transition to the nematic

phase

_occurs when the

short-ranged repulsions

have

ultimately

vanished. It is clear that the _range of the

repulsions

_grows with the chemical

length

of the soap

molecules;

_m

fact,

it

depends specially

_on the conformational extension of the

paraffin chains,

and

therefore,

_on their

mixing

^mode with the

hydrocarbon

solvents _{on a} molecular scale.

To discuss this

point,

_we

analyzed

the columnar to nematic

phase

transition _{as a} function of two

separate

molecular

parameters:

the size of the

hydrocarbon

and the size of the _soap

molecules. The role of the first parameter is illustrated in

Figure

18. For _a

given

_soap, the

swelling

at the transition reduces

rapidly

when the molecular size of the

hydrocarbon

_grows, re-

vealing

the

increasing

difficulties

experienced by

the

hydrocarbon

to enter the columnar

phase,

because of its confinement _to the limited _spaces available between the columns: the

longer

the disordered

hydrocarbon molecules,

the _more

they

must

elastically change

their conformational

shape

in order to enter the

structure, orienting

themselves

parallel

to the columnar _axes. The onentational trend of the deformed coils shows _up

explicitly

in the

levelling

off of the

swelling

observed _at chain

lengths

_greater than about

thirty

carbon atoms.

Beyond

this

value,

the molecular

length

of the

hydrocarbon

_{seems no}

longer

to

play

_a ^decisive

role,

_as the molecular coils _are

probably

all

pointing parallel

to the columnar _axes, in _a direction where the _con- finement has _no effect. The _case of hexacontane

C60

is

exemplary

in this

respect.

^In

spite

of its

important

molecular

length

_(m~ 80

I),

_{which is well}

_beyond

_{the size of the}

_hexagonal

0.9

~~

0.6

$

_C~~CU

°.3 C~~CU

C~~CU

~'~0

_{12 24 36 48 60}

P

Fig

18. Variation of the

degree

of

swelling

at the columnar to nematic

phase

transition of three soaps ^at 130

°C,

as a function of the number of carbon atoms of the linear

hydrocarbons

used _as

a

solvent Solld _{curves are}

least~squares exponential

fits of

experimental

data.

cells (rw 25

I),

it is nevertheless

perfectly

able to enter the columnar structure. To do _so, it

quite clearly

cannot but

adopt

_a

highly elongated

disordered conformation _to reduce its lateral

spatial expansion and, orienting

itself

parallel

to the

columns,

to fit into the

elongated

available

intercolumnar _spaces.

In _an

attempt

to

analyze

the

experimental

data

quantitatively,

_we

ajusted

them

by

_a least- squares fit

method, using

^for convenience _an

exponential

function

(Fig. 18). lb*(p)

₌

16[

+

Kexp (-p/(p). Quite remarkably,

the characteristic

lengths

found _are the _same for all three soaps studied

((p

₌ 7 +1 carbon

atoms),

and their value

corresponds

to the

persistence length

(rw 12

I)

of linear

hydrocarbon

chains

[18] Obviously,

the

penetration

of

hydrocarbons

in the columnar

phase

of _{copper soaps} is controlled

by

their "conformational

rigidity",

that is

by

their

persistence length,

which is of the _saIne order of

magnitude

_as the

hexagonal

cell

parameter

itself.

Another _{way to}

analyze

the

experimental

results _{is to} discuss the _{excess area} of the swollen

hexagonal

cell at the transition _{as a} function of the size of the soap molecules. This _area is

simply equal

to

Ao.4l*(p),

where

Ao

is the cell _area of _{pure soap.} Its

asymptotic value,

calculated for

long hydrocarbon

molecules

using

the value

of16[,

varies

linearly

_upon the size of the soap molecules and _seems to vanish at about _n

= 7

(Fig. 19),

that is

slightly

above the value of _n

= 4.5

corresponding

to the frozen

part

^{of the}

paraffin

chains next to the columnar _core. This

suggests

^{that the}

penetration

of

long hydrocarbons

in the columnar

structure

requires

_soaps with _more than about _seven carbon _atoms in their

fatty

acid

moiety.

Let _{us now} consider the role of the second

parameter controlling

the columnar to nematic

phase transition, namely

the _size of the soap molecules. This is shown in

Figure

20. For _a

given

linear

hydrocarbon,

the

swelling

at the transition reduces

rapidly

with the molecular size of the

280

210

~

~140

<

~i

70

0

4 8 12 16 20 24

Fig.

19. Variation of the asymptotic

[4l*(p).Ao

for _p » 30] excess area of the swollen

hexagonal

cell at the columnar to nematic

phase

transition _{as a} function of the size of the _soap molecule.

C12

0.6 C32

0.3

C36

0.0

8

Fig.

20 Variation of the

degree

of

swelling

at the columnar to nematic

phase

transition _{as a} function of the _size of the soap molecule Cncu for _{a series} of linear

hydrocarbons Cp

used _{as a} solvent.

Solid _{curves are}

least-squares exponential

fits of experimental data.

~

i ~ Dec

~

C14

0.4

~~~

0.0

8 12 16 20 24

Fig.

21.

Comparison

of the

degree

of

swelling

at the columnar to nematic

phase

transition _{as a} function of the size of the soap molecule Cncu for decalin

(dec),

tetradecane

(C14 ),

and

squalane (squ)

used _{as a} solvent Solid _{curves are}

least-squares exponential

fits of experimental data

soap

molecules, showing

the

increasing

confinement constraints discussed above. In _an

attempt

to

analyze

this behavior

quantitatively,

_we

ajusted

the

experimental

data

by

_a

least-squares

fit

method, using

_an

exponential

function _as

previously: 16*(n)

₌

16[

+

K.exp (-n/(n).

This

was easy to carry ^out for the

binary

mixtures with

C14, investigated

_more

extensively.

It is of interest to notice that the

corresponding

^fit

extrapolates

to _zero at n m

7.5,

which is the _same

as

quoted

above for the solubilization of _very

long hydrocarbons (see Fig. 18).

This

suggests

that the _same minimum _size of soap molecules _is

required

to dissolve

hydrocarbons

_no matter how short

they

_are.

Incidentally,

_we could

verify

that the soap with _n

= 8 does indeed dissolve small amounts of

hydrocarbon prior

to

turning

into _a nematic

phase.

For the

binary

mixtures with other linear

hydrocarbons,

for which _our

experimental

observations _were not rich

enough,

we

proceeded

to the

least-squares

^fit

by

trial and _error, with the

arbitrary

constraint of

using

the _same

extrapolation

_{at n 1} 7.5

(see Fig. 20). Quite remarkably,

the characteristic

lengths

found _are the _same for all the

hydrocarbons

studied

((n

= 6.5 +1 carbon

atoms),

and

equal

to the

persistence length (p

r- 7 carbon _atoms.

To

complete

this

work,

_we studied the columnar to nematic

phase

transition of _{copper soaps} in the presence oftwo further non-linear

hydrocarbons

used _{as a}

solvent, decahydronaphthalene

which is _a saturated

bicydic hydrocarbon,

and

squalane,

which is _a branched

hydrocarbon (see

Table

II).

On the

whole,

the transition

proceeds exactly

in the _same way as with linear

hydrocarbons (Fig. 21).

With

decahydronaphthalene,

whose molecules _are rather

small,

the

swelling

at the transition is rather

important and, conversely,

with

squalane,

whose molecules contain 30 carbon

atoms,

the

swelling

is

moderate, comparable

to that obtained with

C32.