Lyotropic phase behaviour of n-octyl-1-O-β-D-glucopyranoside and its thio derivative n-octyl-1-S-β-D-glucopyranoside

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Lyotropic phase behaviour of

n-octyl-1-O-β-D-glucopyranoside and its thio derivative n-octyl-1-S-β-D-glucopyranoside

P. Sakya, J. Seddon, R. Templer

To cite this version:

P. Sakya, J. Seddon, R. Templer. Lyotropic phase behaviour of n-octyl-1-O-β-D-glucopyranoside and

its thio derivative n-octyl-1-S-β-D-glucopyranoside. Journal de Physique II, EDP Sciences, 1994, 4

(8), pp.1311-1331. �10.1051/jp2:1994202�. �jpa-00248045�

Classification Physic-s Ahsfi.act.I

61.30 64.70 61.30E

Lyotropic phase ^{behaviour of} n.octyl.I.O.fl.D.glucopyranoside

and its thio derivative n.octyl-I-S-fl-D-glucopyranoside

P.

Sakya,

J. M. Seddon

(*)

and R. H.

Templer

Department of Chemistry, Imperial

College,

Exhibition Road, London SW? 2AY, U-K-

(Receii>ed 9 Maiih 1994, ieceii>ed _m final f(Jim 29 April 1994, accepted 4 Ma~ 1994j

Rdsumd. On ddterrnine les

diagrammes

de phase ^{biiiaires de}

n-octyl-I-O-p-D-glucopyranoside

et de n-octyl-I-S-p-D-glucopyranoside dans l'eau, _en utili~ant la

microscopie

polarisante et la diffraction des _rayons X. Une comparaison des deux composds _{nous permet} de pdn6trer les interactions complexes qui ddterrninent le comportement ^de phase. Les deux compo8ds forment des phases

lyotropes

de typel (normale), _avec l'adoption d'une pha~e lamellaire fluide h des

hydratations basses _et d'une phase cubique h des hydratations plus ^hautes.

Cependant,

la phase hexagonale observde pour le

systdme n-octyl-I-O-p-D-glucopyranosideleau adjacente

h la solution

micellaire est

supprimde

dans le systkme

n-octyl-I-S-p-glucopyranosideleau.

En

plus,

on trouve que dans les deux systbmes la phase

cubique

croit spontandment en

grands

monodomaines, _comme

r6vkle le

facettage

de bulles d'air coincdes darts la phase. L'indexation des diagrammes de diffraction monodomaine montre que, dans les deux _cas, le groupe

d'espace

de la phase

cubique

est Ia3d (no. 230).

Abstract. We have determined the phase diagrams of n-octyl-I -O-p-D-glucopyranoside and its

thio derivative

n-octyl-I -S-p-D-glucopyranoside

in water,

using polarizing microscopy

and

X-ray

diffraction. Comparison of the two compounds gives _{us an} insight into the complex interactions which determine

phase

behaviour. Both compounds ^form type ^I (normal) lyotropic phases, with the adoption of _a fluid lamellar phase at low hydrations and _a cubic phase at higher

hydrations.

However, the

hexagonal

phase observed in the n-octyl- I-O-p-D-glucopyranoside/water system adjacent _to the micellar wlution is suppressed in the

n-octyl-I-S-p-D-glucopyranoside/water

system. ^In addition, _we have found that in both systems ^the cubic phase spontaneously _grows into large monodomains, _as revealed by

faceting

of entrapped air bubbles. Indexing of the monodomain

diffraction patterns ^{shows that in both} cases the space group of the cubic phase is Ia3d (no. 230).

1. Introduction.

It has

only fairly recently

^been

appreciated

that

glycolipids

constitute _a

large

class of

theimon.opic liquid crystals.

The

growth

of research in this field _{over recent} years has been

(*1 Author to whom correspondence should be addressed.

extensively

reviewed

[1, 2].

A number of

cyclic

and

acyclic glycolipids

with

single n-alkyl

_or

acyl

chains of _more than six carbon _{atoms are} known _to form

smectic, probably A~ (partial bilayer), phases.

Prominent among these is

n-octyl-I-O-p-D-glucopyranoside (p-OG) (Fig. la),

_a non-ionic surfactant which has been used _to solubilize membrane

proteins [3, 4].

6

~CH20H

Hf~

_~ 2

~

i

O,

7

~@(

₉

~@(

ii

~@(13 ~CH3

^~~

~ C ₈ C

io C12 C _j4

Q~ H~ _H~ H~ H~

~

6

~

CH~OH

~~~~'~~~~~~~~~~~~~~~~

^~~

OH

H2

H2 _H~ H~

H

Fig.

I. Schematic

diagrams

of (a)

n-octyl-I-O-p-D-glucopyranoside

(p-OG) (b)

n-octyl-I-S-p-D- glucopyranoside

(p-thio-OG).

The

thermotropic properties

of this

compound

have been studied

by

several authors.

Goodby [5]

has studied

aligned samples

of the

mesophase

^and concludes that it is

likely

to be of the

smectic

Ad

_type. The

thermotropic phase

^{behaviour of}

binary

mixtures of the _ar and p-

glucopyranosides

has also been

studied,

and the two _anomers

compared [6].

Many

of the

glycolipids

that exhibit

thermotropic

behaviour _may also possess

lyotropic properties [2, 7].

A number of

compounds

that form smectic A

mesophases

^when

dry

have been found _to form fluid lamellar

L~ phases

_on the addition of _water. In _{a recent} article _on this

subject, Chung

and

Jeffrey

have

analysed

the

lyotropic properties

of _a number of

n-alkyl pyranosides,

and

they

have found that

p-OG,

in

particular,

^exhibits three

lyotropic liquid crystalline phases

at room temperature

[8]. They

state that, _on addition of _water, this

compound

forms _a fluid lamellar

L~ phase

at low

hydrations,

and type ^{I cubic and}

hexagonal phases

at

higher hydrations.

On further addition of _{water a} micellar solution is formed.

However,

phase diagrams

_were not

presented. They tentatively

claimed that the space group of the cubic

phase

observed is Fm3m.

In

addition,

Loewenstein _et al, have used the

technique

of deuterium NMR to

study

the

phase

behaviour of the

n-alkyl-ar-

and

n-alkyl-p-D-glucopyranosides, including p-OG,

both with water

(D~O)

^{and also with several}

organic

solvents

[9-1Ii.

This method enabled them _to

obtain information about the orientational order of the

hydrocarbon

chain in the

L~ phase,

and to follow the

dependence

of the order parameter ^with

position along

the

chain,

water content

and temperature.

By monitoring

the

changes

in

quadrupolar splitting

between different

phases they

have drawn up a

phase diagram

for the

p-OG/water

system. ^Unlike

X-ray diffraction,

NMR does not

provide

_{a proper} structural characterization of the

liquid crystalline phases,

and

as

only

^{the lamellar and}

hexagonal phases produce quadrupolar splittings

there is _no way of

distinguishing

the cubic and micellar

phases.

It

does, nonetheless, give

much valuable information about the

degree

^of orientational

ordering

in

lyotropic phases

and

is,

in

general,

complementary

to diffraction studies such _as

reported

here.

We have extended the

study

of the

lyotropic properties

of

p-OG

in _water to

produce

_a full

binary phase diagram.

All three

lyotropic phases

_were characterized

using

_a combination of

polarizing microscopy

and

X-ray

diffraction. From

analysis

of these

X-ray

diffraction pattems

we have found that the space group of the cubic

phase

is Ia3d, which is the _{most common} of the type ^I

(oil-in-water)

cubic

phases

known to exist, rather than Fm3m, _as

previously reported [8].

This conclusion is

supported by

the remarkable similarities which _we have found between the

p-OG/water

and

hexa-ethyleneglycol

_mono

n-dodecyl

ether

('j (Cj~EO~)/water

systems.

Like

p-OG,

the

polyoxyethylene

molecule

Cj~EO~

is _a non-ionic surfactant. The

lyotropic phase

behaviour of

Cj~EO~ [12]

is rather similar _to that of

p-OG

_: both systems ^{form fluid} lamellar and type ^{I cubic and}

hexagonal Hi phases,

and the space group of the cubic

phase

of

Cj~EO~

is also

thought

to be Ia3d

[13]. However,

there is _{a more}

intriguing similarity

between these

compounds.

Sotta

[14]

has studied the _occurrence of _a

highly

unusual

phenomenon

within monodomains of the cubic

phase

of

Cj~EO~.

He has found that _an air bubble

trapped

within this

phase

^will

develop faceting

and tend towards an

equilibrium shape

which reflects the structure of the transparent ^cubic

phase surrounding

it. In this article _we show that the cubic

phase

of

p-OG

also

develops

faceted air bubbles and that the structure of these air bubbles

closely

matches those found

by

Sotta in

Cj~EO~.

In

addition,

_we have studied the

lyotropic phase

behaviour of the thio derivative of

p-OG,

_n-

octyl-

I

-S-p-D-glucopyranoside (p-thio-OG) (Fig, lb).

In this molecule the oxygen which links the

glucopyranoside ring

to the

alkyl

chain has been

replaced by

a

sulphur

_atom. This is the

only

^{difference between the} two

compounds.

This modification is of

particular

interest because the

linkage

atom affected sits very close to the

polar/non-polar

interface. Previous work _on the

related ar-anomeric

compound n-heptyl-I -S-a-D-glucopyranoside [15]

found metastable cubic and

Hi phases

to form _on addition of _water to _a

supercooled

SA

Phase

of the

dry compound.

The effect _on the

lyotropic phase diagram

of this

apparently

^minor

change

^{of chemical}

structure is dramatic. In

p-thio-OG

the

hexagonal phase

which is _seen with

p-OG

is

suppressed

(at least for temperatures ^{above 0}

°C),

but the cubic

phase

extends _{over a} far wider range of

compositions,

and has

sloping phase

boundaries. As with

p-OG,

the cubic

phase

of

p-thio-OG

is of space group Ia3d and tends _to form faceted monodomain air bubbles.

It is _{a source} of frustration to _us that the

crystal

structures of

p-OG

and

p-thio-OG

remain

unknown.

Jeffrey

has obtained _a

crystal

structure of die

relatively

insoluble _ar-anomer

n-octyl- l-O-ar-D-glucopyranoside [16].

However~ because the p-anomer is

highly

soluble it has not, as

yet, ^been

possible

_to obtain

single crystals

of sufficient size and

quality

to enable its

X-ray

structure to be solved.

Jeffrey

has,

instead,

succeeded in

obtaining

structures for both a-OG and

p-OG

from I _: I

binary

mixtures of the _{two anomers}

[17].

He has found that the molecules

pack

^{in the}

crystalline phase

to form head-to-head

bilayer

structures with

interdigitated

chains.

2.

Experimental.

The

p-OG

and

p-thio-OG

were obtained from Fluka Chemicals Ltd.

(~

99 %

pure)

and used without further

purification.

The

optical

observations of the

lyotropic phases

_were made

using

a Nikon

Labophot polarizing microscope equipped

with _a Linkam

heating

stage. The

penetration technique

_was used to form concentration

gradients

_a few

milligrams

of

sample

was

placed

_{on a}

glass

slide beneath _a

coverslip

^{and heated} and cooled _to form _a

glassy

solid.

Water _was then added around the

edges

of the

coverslip

and allowed to penetrate the solid for

(')

The chemical _structure of

Cj~EO~

_is

CH~-(CH~)j,-(O-CH~-CH~)~-OH.

several minutes. The _rate of

penetration

_was increased

by heating

the solid into its

mesophase

for several minutes before

cooling

it back down to room temperature.

To construct detailed

phase diagrams

for both

p-OG

and

p-thio-OG,

a number of

samples

of

precise composition

were

prepared.

The

proportion

of _water in each

sample ranged

from 4 _to 35 %

(w/w),

and

samples

_were

prepared

at

roughly

2 % _water

intervals, though

_{more were}

prepared

at

compositions

close _to the

phase

boundaries.

The

required

amounts of

compound

and _{water were}

weighed

in Lindemann

capillary

tubes

(diameter

1.5 mm). ^{About 10-15} mg of the

compound,

in the form of a fine

polycrystalline powder,

_was added and

centrifuged

to the base of the tube. The

required

_mass of _{water was} added

using

a very fine

glass pipette

and

centrifuged down,

and the tube _was then sealed. The

uncertainty

in the surfactant concentration Ac/c is estimated to be 3-4 %. It _was found that the

water

penetrated

fine,

powdery samples

_more

easily

than

samples

which had been melted and

cooled _to form _a

glassy

solid, and thus the former

produced homogeneous

mixtures much _more

rapidly

than the latter.

To

homogenize

the

sample,

it _was

repeatedly

heated into the micellar

phase using

a

heating

stage, ^{and then cooled} to room temperature. ^{After each}

heating,

water which had condensed at the top ^{of the tube} was

centrifuged

back down _to the base. To _test the

homogeneity

of the

sample

it _was examined

by polarizing microscope

and heated until _a

phase

transition took

place.

If the

sample

_were

truly homogeneous~

the transition would _{occur at} the _same temperature

throughout

its

length.

The three

lyotropic phases

could be

distinguished

from each other

using polarizing

microscopy

(see

Fig. 2).

The cubic

phase

is

optically isotropic

and

black,

whereas both the lamellar and

hexagonal phases

are

birefringent.

However, the

hexagonal phase

tends _to form

Fig. 2.-Polarizing microscopy penetration scan with crossed polarizers of n-octyl-I-O-p-D-

glucopyranoside

in water.

much

larger

domains than the lamellar

phase,

and exhibits _a characteristic mosaic texture. To

distinguish

the cubic and micellar

phases,

which _are both

isotropic

under the

polarizing microscope,

the

shape

^of the air bubbles

trapped

within the

phases

_was examined. Bubbles present ^{in the} cubic

phase

_{were «} frozen _» into fixed

positions

and tended to

develop faceting (see

Sect.

6),

whereas bubbles in the micellar

phase

were

spherical

and free _{to move} about.

The

identity

of the

phase

could, of _course,

subsequently

be verified

by X-ray

diffraction.

Points _on the

phase

boundaries above _room temperature were found

by heating

the

capillary

tubes

(hydrated samples)

or

glass

slides

(dry samples) using

the

heating

stage ^{of the}

polarizing microscope

and

observing

the

phase

transitions

(accuracy

± 2 °C for

hydrated samples

and

±I °C for

dry samples).

Phase transitions below _room temperature were obtained

by circulating nitrogen

_gas cooled

by

solid carbon dioxide

through

the

sample

_stage

(accuracy

± 2

°C).

To find the

enthalpies

of the

phase transitions,

differential

scanning calorimetry (DSC)

_was used

(Perkin-Elmer DSC-2C). Approximately

lo-15 mg of surfactant _was

weighed (accuracy

± 3-4

%)

into aluminium pans. The

uncertainty

^{in the values of the transition} temperatures is estimated _to be _± I °C. Evidence for the formation of metastable

phases

on

cooling

was

observed with both

p-OG

and

p-thio-OG,

and _so

only

the first

heating

_scan for each

sample

was

analysed.

Two

X-ray

diffraction

techniques

were used _to characterize the various

lyotropic phases

_:

line diffraction and

point

diffraction. The Huber _camera

(Robert

Huber, 821

Rimsting,

Germany)

is _a Guinier type camera which

produces

a focused line beam. It is fitted with _a bent

quartz

crystal

monochromator which is

adjusted

to isolate the

CuKarj

radiation with _a

wavelength

of 1.5405

h.

_{The radiation}

was

produced by

a

Philips

PW

2213/20 Cu-target

X- ray generator

operating

at 40kV and 30 mA. This _{camera was} used for the lamellar and

smectic

phases,

and enabled the _accurate determination of

layer spacings

and lattice

parameters. For the

hexagonal

and

particularly

the cubic

phases,

where

large

monodomains _are present~ ^it was not

possible

to obtain

satisfactory

diffraction patterns ^{with this line focus}

camera, due to «

spotting

_» of the pattems.

For these

phases point

diffraction,

employing

toroidal

optics,

was used. The

CUK~ X-rays (1.542 h)

_were

produced by

_an Elliott GX20

rotating

anode

X-ray

generator

(Enraf-Nonius,

Netherlands) operating

at 30 kV and 25 mA, with _a 0, I _mm focus cup. ^A nickel filter _was used

to remove the

Kp

^{line. A stack of film} was

placed

in the

holder,

_so that the first film

produced

the most intense

image, allowing

the faintest

Bragg

_spots to be

indexed,

whereas the

subsequent

films

produced

less intense

images

which

permitted

the

indexing

of the strongest spots.

3.

Thermotropic properties.

Although

the bulk of this report ^{is concerned with}

lyotropic phase

behaviour, we also studied the

thermotropic

behaviour of

p-OG

and

p-thio-OG.

As has been

mentioned,

the

single crystal

structures of

p-OG

and

p-thio-OG

have not, as

yet, ^been determined. From

powder

diffraction patterns we have found the

bilayer spacing

of the

crystalline

Iamellar L~

phase

of

p-OG

to be

29.31.

_{This compares}

_favourably

_{with the}

values of 29.4

I

_and

29.01 _{given by Jeffrey}

_and

Bhattacharjee [18]

and Dorset and

Rosenbusch

[19] respectively.

For

p-thio-OG,

the

bilayer spacing

^is

substantially _greater,

at 32.3

1, _suggesting

_{that the}

crystalline packing

^of

p-thio-OG

differs

significantly

from that of

p-OG.

On

heating, p-OG undergoes

transitions from the

crystalline phase

to a

smectic, probably

S~~, phase

at 68-69 °C and then to the

isotropic liquid

at 106.5-107.5 °C. This agrees well with the literature values. We have found that

p-thio-OG

forms _a smectic

phase

^{which, from}

the similar

X-ray

diffraction data, _we also expect to be

S~~.

It exists _{over a} temperature range

more than twice _as great as that for

p-OG,

with _a lower

melting point

of 41-42 °C, and _a

higher clearing point

of 126-127 °C. This agrees well with the values of 41.9-43.8 °C and 125- l25.7 °C

previously reported [20].

Thus the thio

linkage

destabilizes the

crystal packing,

but stabilizes the smectic

phase.

The average

d-spacing, (d),

of the smectic SAd

Phase

of

p-OG

was 25.5

h, _ranging

_{from 25.6}

h

at 70 °C _to 25.3

h

_{at 105 °C. These values}

are in line with

Dorset's

figure

of 25.4

A _{[2 Ii,}

_but

are rather smaller than

Jeffrey

and

Bhattacharjee's

value of 26,

I1 _[18].

_The

layer spacing

for the smectic

S~~ phase

of

p-thio-OG

was 26.2

A.

What _are the factors which affect these transition temperatures ^{? The increase in chain}

length

of

p-thio-OG

_over

p-OG

is too small _to affect the transition temperatures

significantly.

Other factors must be at work. At the

crystal melting point

the

hydrocarbon chains,

which _are

relatively loosely

bound

together by

van der Waals interactions, _« melt _»,

disengaging

from the

crystal

lattice and

losing

their conformational

rigidity,

but the sugar moieties remain

partially hydrogen

^bonded

together

in clusters. It is

only

at the

clearing point

that the

hydrogen

bonds _are

(largely)

broken, the

long-range ordering

is lost and _an

isotropic liquid

forms.

The lower temperature for the

crystalline

to smectic transition of

p-thio-OG points

to weaker

van der Waals interactions between the

hydrocarbon chains, allowing

these chains to _« melt

» more

easily.

This is

likely

_to be due _to the

larger

size of the

sulphur

atom

(visible

in

Fig. 3),

which

prevents

^{the chains from}

getting

_as close

together

_as

they

do in

p-OG, reducing

their mutual interactions.

H

H H

~ H ~

a)

~ H

~ H

H H

H

H H

H b)

~ H

~ _H

H H

H H

Fig. 3. Three-dimensional space filling molecular models of la) n-octyl-I-O-p-D-glucopyranoside 16)

n-octyl-I-S-p-D-glucopyranoside.

The

higher

temperature ^{for the smectic} to

isotropic

transition of

p-thio-OG

is harder _to

explain,

but it suggests ^{that the}

bonding

between

headgroups

is stronger ^{than for}

p-OG.

The

explanation

for this is

unclear, although

the thio

linkage

_may alter the orientation of the

headgroup

relative to the

chaingroup,

thus

allowing

a greater

degree

of

hydrogen bonding

between the

headgroups. Alternatively,

there could be _a lesser

degree

of intramolecular

hydrogen bonding

in

p-thio-OG, freeing hydroxyl

_groups to interact with

neighbouring

molecules.

The

enthalpies

of the

p-thio-OG

transitions _are

given

in table I. The

enthalpy

of

melting

is

larger

than

Goodby's

value of

13.31cat/gm

for the

corresponding p-OG

transition

[5].

However,

the

enthalpy

of

clearing

matches

closely

the value of1.37

cal/gm

for

p-OG given by Goodby.

Table I. Transition teniperatures

(T~)

^and

enthalpies for n-octyl-I-S-p-D-glucopyranoside,

determined

by differential scanning calorimetry.

~

AS/R

Transition

~

~~~/~m

kJ/mot

Melting

41.2 19.01 24.54 9.55

Clearing

126.5 1.38 1.78 0.54

4.

Lyotropic phase

behaviour.

The

phase

behaviour of

p-OG

and p-thio-OG _was

initially analysed by observing

how _water

penetrated

a

sample

of the

dry glassy

solid _{on a}

glass slide,

_as described

by Chung

and

Jeffrey [8].

After several minutes _a concentration

gradient developed,

and all the

lyotropic phases

which _are stable _{at room}

temperature

^{could be observed. For}

p-OG

all three

lyotropic liquid crystalline phases

formed, as is shown in the

photograph

in

figure

2. Two distinctive _areas of

birefringence

_are visible. The smooth mosaic _texture of the

hexagonal phase

can be _{seen at} the top ^{of the}

photograph.

This contrasts with the

mottled,

_«

oily

streak _{» texture} of the lamellar

phase, consisting

of _a

large

number of

tiny

_« Maltese _{crosses »,} which is visible in the lower half of the

picture.

Between these lies _{a narrow} band which is

isotropic

and black _: this is the cubic

phase.

This

phase

_can be

distinguished

from the micellar solution

by

its

high viscosity.

When pressure is

applied

to the

coverslip

the cubic

phase

moves

only slightly

before

returning

to its

original shape,

whereas the other

phases

flow far _more

easily.

A similar

penetration

_scan for

p-thio-OG produced

_a lamellar

phase

and _a cubic

phase,

but

no

hexagonal phase

_was observed.

The

phase diagrams

for the

p-OG/water

and

fl-thio-OG/water

_{systems are} shown in

figures

4 and 5

respectively.

We observe that the

L~ (fluid lamellar), L~ (crystalline lamellar), Qi (normal cubic), Hi (normal hexagonal)

and

Lj

(normal

micellar) phases

are present ⁱⁿ the p-

OG/water

phase diagram,

and all but the

Hi phase

are present ^{in the}

p-thio-OG/water phase diagram.

The borders between _two

phases

_are

actually

_narrow

regions

where

phase

coexistence _occurs. These _are shown _on the

diagram by

solid and dotted lines. The dotted lines indicate that the thickness of these

biphasic regions

is

only

^estimated

approximately.

The measured

layer spacings

for the

p-OG L~ phase ranged

from 26.6

I ₍₅

_{% water)}

to

29.7

1 ₍₁₉

_%

_water),

_{while the lattice}

_parameters

_{for the}

Qi

and H~

phases

of

p-OG

_are 73.

01

(21% water)

and 38.6

I ₍₃₃

_{% water)}

respectively.

The

layer spacing

for the

p-thio-OG

40

zo

)loo

8° L

L

fi

^a

g

⁶⁰

(

W 40

Q

20 l~~~~~ H

L~ ^'

O

lo 20 30 40

H~O(%w/w)

Fig. 4. -Binary phase diagram of n-octyl-I-O-p-D-glucopyranoside in _water. The _narrow regions between the wlid and dotted lines indicate _areas of two phase coexistence.

40

zo

~il00

i~ 80

~a ~l

3

f

60

3 40

Q~

(la3d)

o

O lo 20 30 40

H~O (%w/w)

Fig. ^5. -Binary phase diagram of

n-octyl-I-S-p-D-glucopyranoside

in _water. The _narrow regions between the solid and dotted lines indicate _areas of two pha~e coexistence.

L~ phase

at a

hydration

^{of 6} % water was 27.8

i~,

while the lattice parameter ^{for the}

Q, phase

of the thio

compound ranged

from 74,

1 ₍₁₃

_%

_water)

to

80.61 ₍₂₅

_%

_water).

The

d-spacing

increases in _a

fairly smooth,

linear fashion _across all three

phases,

_as

figure

6 shows. This

implies epitaxial

relations between the

phases,

which _are discussed in section 5.

Loewenstein _et al.

[9-1Ii

have studied the

lyotropic phase

behaviour of

p-OG using

deuterium NMR. Our results for the lamellar _to micellar

(isotropic)

transition

generally

^fit the

biphasic

to

isotropic

data

points given by

Loewenstein. There

is, however,

_a

discrepancy

at room temperature. Loewenstein's results suggest ^{that the lamellar}

phase

is stable up ^to a

composition

of 26 % deuterated _water

(at

_room

temperature),

whereas _our results indicate _a

34

A

32

Gi ~

~~ Q ~

j

30

~ ~

~~ ~

~

Qj Hi

26

0 10 20 30

H~O

(% w/w)

Fig. 6. Plot of

d-spacing

_i'eisus water content at room temperature, ^for p-OG (hollow symbols) and p- thio-OG (filled symbols). The L,,(dt»ii I,

Qi(d~j

and Hi (djt>) Phases are represented

by

circles, triangles and squares respectively. Dotted lines indicate phase boundaries _at

room temperature.

phase

^transition to the cubic

phase

at 19-20 fb water. As the data at most other

compositions

^fit

our

results,

_{we presume} that their data at 26 %

composition

_are incorrect.

Loewenstein and his coworkers have also examined the

phase

behaviour of

p-OG

_at

higher hydrations (cf. Fig.

2 from Ref.

[10]). They

have found that

phase

transitions between the

hexagonal

and micellar and the cubic and micellar

phases

occur at 21°C and 56°C

respectively,

I.e, around K below the

temperatures

we measured. This may be because the

phase

transitions _we observed _were recorded _on

heating,

whereas Loewenstein's results _were recorded _on

cooling. Consequently

a

degree

of

supercooling

_may have occurred.

Both

p-OG

and

p-thio-OG display

_type I

lyotropic

behaviour, _as the

headgroup

_cross-

sectional _area is

large

^relative to that of the

chaingroup

(as shown in

Fig. 3),

and _so the balance

between

headgroup

and chain

packing

tends to induce

positive

interfacial _curvature. The

sequence of

phases

observed with

increasing hydration

is in the

expected

order,

progressing

from the lamellar

phase

to the curved cubic

phase,

the _more

highly

curved

hexagonal phase (for p-OG only),

and

eventually

to the micellar solution.

For both

p-OG

and

p-thio-OG,

the lamellar

phase

is most

thermally

stable at

hydrations

of around lo

%,

rather than when

dry.

This

suggests that,

at low

hydrations,

water molecules bind _to the

polar headgroups

and form _an intermolecular

hydrogen-bonded

network which

promotes

binding

between the

headgroups

and thus increases the

rigidity

of the lamellar

phase.

But, as further _water is added, the rise in the effective volume of the

headgroup

increases the desire for curvature, destabilizes the lamellar

phase

^and

eventually

_{causes a} transition _to the cubic

phase.

The two most

prominent

features of the

p-thio-OG/water phase diagram

are the

complete

absence of the

hexagonal phase

(above

0°C)

and the increased

stability

with respect to

composition

of the cubic

phase.

^{The thio}

linkage

appears ^to stabilize the cubic

phase

and destabilize the

hexagonal phase.

Also, the

phase

boundaries between both the lamellar and cubic and the cubic and micellar

phases

have _a

significant gradient, allowing

_{us to} observe

phase

transitions

by changing

the temperature ^alone.

We _are

unable,

_{as yet,} to

explain

the differences in

phase

behaviour between

p-OG

and

p-

thio-OG in _terms of their molecular _structures alone. However, _we suggest ^{that the thio}

linkage

does differ from the oxygen

linkage

in three

important

_ways. We

estimate,

from molecular

modelling

and literature values for similar

compounds [22, 23],

that there is a decrease from 113° for the

C(I )-O-C(7)

bond

angle

^of

p-OG

to 96° for the

corresponding

bond

angle

of the

linking sulphur

in

p-thio-OG.

There is _a resultant

change

in the

angle

of the sugar

ring

to the

hydrocarbon

chain, which

figure

3 illustrates.

Secondly, sulphur

has _a greater ^steric

bulk,

with

a van der Waals radius of 1.85

I, _compared

_to

a van der Waals radius of 1.40

I

_{for oxygen.}

Sulphur

also has

considerably

less ionic character than oxygen, and _so forms

hydrogen

bonds

only

_very

weakly

if _at all. This lack of

hydrogen bonding capability

_may affect the

degree

of intra- and intermolecular

hydrogen bonding

which takes

place.

The fact that the thio

linkage

stabilizes the cubic

phase

but destabilizes the

hexagonal phase implies

that

p-thio-OG

has _a

positive

desire for

negative

Gaussian interfacial curvature, and finds it

energetically

unfavorable to form _a

phase

with

purely

_mean curvature. Thus _{we can} infer that the

apparently

minor substitution of

sulphur

^for _oxygen has _a

large,

^and as yet

unexplained,

effect _on the relative

magnitudes (and, possibly, signs)

of the _mean and Gaussian

curvature elastic moduli.

It therefore appears that the structure of the interfacial

region plays

the crucial role in

controlling

the types ^{of curved}

phase

which will be

adopted.

In

particular,

_one cannot

explain

the differences of these

phase diagrams using simple

models of

headgroup hydration

and chain

packing.

It _seems that the subtle

interplay

between molecular conformation and intra- and

intermolecular

hydrogen bonding (headgroup-headgroup

and

headgroup-water),

and their

hydration

and temperature

dependences,

_are the

principal

factors behind the observed

phase

behaviour.

5.

Indexing

of the cubic

phase.

It

proved impossible

_to obtain reasonable

quality X-ray

diffraction patterns

using

Guinier

cameras due to the formation of

large crystallites (monodomains).

To circumvent these

problems,

the Toroid

point

diffraction _{camera was} used. This

produced

a

single

intense

point

beam which enabled _us to

analyse

individual

single crystals

of the cubic

phase.

Indexing

of the monodomain patterns demonstrated

unambiguously

that the spacegroup of both the

p-OG

and

p-thio-OG

cubic

phases

is Ia3d

(no. 230).

This is contrary to the

previous study

^of

p-OG,

which

tentatively

indexed the cubic

phase

as Fm3m

[8]. However,

in the latter

study

too few reflections _were

observed,

and

only

the ratios of the

reciprocal spacings

_were

employed

in

assigning

the spacegroup. In the present

study

_we have obtained _more

hkf

reflections, and,

_more

importantly,

_we have also

analysed

the orientation of the various

hkf

reflections in sections

through reciprocal

_space.

Example

diffraction patterns ^for both

p-OG

and

p-thio-OG

_are shown below,

along

with the

indexing

of the

Bragg

spots onto an Ia3d cubic lattice. Each

image

_can be considered _{as a} slice which the Ewald

sphere

makes

through reciprocal

_space.

We have found that many of the diffraction patterns obtained gave a

hexagonal

arrangement of spots

corresponding

to a section

through

^the Ewald

sphere

normal, or

nearly

normal, to a

[I

I I axis. The

indexing

of this central

hexagon

of spots can be _{seen as} the innermost

hexagon

in

figure

7. In this _{case we are}

looking along

the

[lit]

axis. This

hexagon

consists of

(211)

reflections at the

midpoints

of the sides and

(220)

reflections _at the vertices.

Figure

7 shows the

complete

theoretical pattern one would expect ^{if the}

X-ray

beam

passed along

the

[lit]

_axis of the

phase.

There is _a

hexagonal

arrangement ^of

spots

^{with certain} gaps

corresponding

to

systematic

absences caused

by

the symmetry ^{elements of the Ia3d lattice. We}

can see that the central

hexagon

^of

Bragg

_spots is

just

the first of several concentric

hexagons

of

spots occurring

at

regular

intervals from the centre of the

pattern.

How does this relate _to the _structure of the real cubic lattice ? The structure of the Ia3d cubic

phase

is described

by

Luzzati

[24].

It consists of _two interwoven networks of

amphiphilic

rods

6% W 6© 6@ 615 62T 6@ _{660 671 682 693}

5% 5h ₅₁₄

521 532 54i

~ 4W 4@ 40T 411 422 _{43i 440}

3© W

312 32i 352 374

2© W 2© 202 21i 220

231

_{253 275}

I@ 1% _{132 154}

[ill]

X

% W 123 145

W W W fit

213 235

W W _{Ml fi2}

W W dl _©2

W

W

1322 JOURNAL DE PHYSIQUE II N° 8

' ~$i~i~

~"-

~ ~

(ooi)

--

)

L~ Q~ H~

Fig. 8.

Representation

of the epitaxial relationship between the lamellar phase (left), the Ia3d cubic

phase

(middle), and the hexagonal phase (right),

adapted

from reference [25].

815 631 65i

4di

f4 lo

jiTi]

«

iii

»

W

W

a)

Fig.

the

pattern.

a)

62i 63i

31i 32i 332 34i ₃₆₁

2g

lfl

143 154

044

fly

___ _

242 .

iii

fl5 I[6

[13 [14

Table II.

X-ray diffraction

data

for p-thio-OG

at room temperature. ^The

reflections correspond

to the

diffraction

pattern ^{shown in}

figure

9. s~~~ is the obser»ed

reciprocal spacing

and _s~~j is the

reciprocal spacing

calculated

for

_an Ia3d cubic- lattice w>ith _a

=

76.0

A.

I~~~ are the observed intensities, which _were

i>isually

estimated and, in the tables which

follow,

range

from

_i>vs

(extremely strong), through

_m

(medium),

to _vvw

(extremely weak).

hkl'

h~ ₊ k~ ₊

f~

Sob,

Job,

Smi

x io- ³

A-

'

) (x

lo- ³

A-

'

)

21 6 32.92 _s 32.23

220 8 37.82 _vs 37.22

422 24 66. 62 _s 64.46

431 26 69.05 _s 67.09

541 42 87. 91 _m 85. 27

633 54 93.68 _m 96. 69

642 58 99.37 _w 100.21

651 62 100.39 _m 103.61

Table III.

X-ray diffraction

data

for p-thio-OG

_{at room} temper"ature. _s~~~ is the obser>'ed

reciprocal spacing

and _s~~j is the

reciprocal spacing

calculated

for

_an Ia3d cubic lattice w>ith

a =

76.0

I.

_The

reflections correspond

to the

diffraction

_pattern shown in

figure

lo.

hkf

h~ ₊ k~ ₊

f~

_Sob,

lob,

_Sol

(x

lo- ³

A-

'

(x

lo- ³

A-

'

211 6 33.12 _vvs 32.23

220 8 37.24 _vvs 37.22

420 20 57.15 _s 58.84

332 22 59.89 _m 61.72

422 24 64.67 _s 64.46

431 26 66.03 _w 67.09

521 38 71.49 _vw 72.07

541 42 86.77 _w 85.27

631 46 88.12 _vvw 89.14

543 50 92.52 _m 93.04

640 52 95.88 _w 94.88

552 54 97.91 _w 96.69

633 54 95.89 _vvw 96.69

642 56 97.23 _vw 98.46

in the first of the _two

images,

is _so intense _{as to} obscure the spots themselves. Dotted lines _are used to indicate this _scatter in

figure

lob _{we can see} that it tends _{to occur} in lines between the

spots.

^{It has been}

suggested

that this diffuse _scatter is due _to

dynamic

fluctuations

along

the

II

I I directions

[13, 27].

Figure

I I shows _a similar diffraction pattern ^{for the cubic}

phase

of

p-OG. Again

_{we are}

looking along

a

[I

I I axis 38

Bragg

_{spots are} indexed. The second diffraction

image clearly

shows all the 211 and 220 reflections. In the first diffraction

image

_{we can} also _see

larger

al

5fl 53i

~62

3fl

3$~

lg '3f3

. » ,,' ~

234~ 211 2fli 21i _22fl

'~

~

' O .

;

. .

ill ₁₂₁

* O

"

oil ^Jill]

'122

« .

ill iii

~ii il)~f23

₁₃₄

; . * ~ . .

fl

^{i~fl Ill I}

_g'

^§24

O--«- .

Ml hi

_{ill lf2}

i~4 i~5

~ * . .

'fli

ill i12 i13 its

~ . . . .

b)

Fig.

II. (a)

X-ray

diffraction pattern ^for a p-OG/23 wtfb water cubic

phase sample.

^The first

(upper)

and second (lower) films from the film stack _are shown. (b)

Indexing

of the pattern.

concentric

hexagons

with _a considerable _amount of diffuse

scattering

between the spots. ^The presence of _a small number of

spurious

_spots in this pattern ^indicates that the beam

actually passed through

two or more

single crystals

orientated _at

slightly

different

angles

to each other.

Table IV shows that there is _a close match between the theoretical and observed

reciprocal spacings

for

p-OG.

Table IV.

X-ray dijfiaction

data

for p-OG

at loom tempeiatui"e. _s~~~ is the observed

reciprocal spacing

and s~~j is the

reciprocal spacing

calculated

for

_an Ia3d cubic lattice with

a =

73.01.

_The

reflections anti"espond

to the

dijfiaction

_pattern shown in

figure

11.

hki

_h~

+ k~ ₊

i~

s~b,

I~~,

_.i~~j

(x

10~ ^~

l~ _(x

_10~ ^~

l~ ₎

211 6 33.90 _vvs 33.55

220 8 39.04 _vvs 38.74

321 14 51.51 m 51.26

422 24 67.59 m 67.I1

431 26 69.05 _m 69.85

532 38 85.02 _vw 84.44

541 42 88.63 _w 88.78

633 54 97.28 _vw 100.66

642 58 100.87 _w 104.33

853 98 136.74 _vvw 135.61

It is

striking

that all the monodomain patterns ^extend to

quite high

resolution, with up ^to the

~/98

reflection

(853) being

observed. Reflections at such

high scattering angles

^could not be observed in

powder

_pattems of the cubic

phase (without

much

longer

_exposure times _or the _use of a more intense radiation source) because of the

progressive geometrical spreading

out of the

intensity

towards

higher (hki),

The

indexing

of these diffraction patterns ^{leaves little} doubt about the _correct space group of the cubic

phase

of both

p-OG

and

p-thio-OG.

As the indexed

points

fit the conditions

hki

h ₊ k ₊

I

=

2 _n

0ki

_; k,

I

=

2 _n

hhi

_{2 h +}

I

=

4 _n h00 _; h

=

4 _n, the spacegroup is

confirmed _as Ia3d.

Although

we have determined the space group of the cubic

phase

of these two systems we

have not carried _{out a} full structural

analysis, owing

to _a lack of

knowledge

of the

phases

of the

structure factors. However, it may be

possible

_to deduce these

by

an

«isomorphous

replacement

_»

approach, employing

^mixtures of

p-OG

and

p-thio-OG (sulphur

has _a

larger

atomic

scattering

factor than

oxygen).

The fact that at

compositions

of ?0 _to 25 fl _water both

p-OG

and

p-thio-OG

exist in the cubic

phase

suggests ^that ternary ^{mixtures of}

p-OG

and

p-

thio-OG with _water will also form the cubic

phase

we have found this to be _so,

By mixing

p- OG and

p-thio-OG

in different

proportions

it should be

possible

to vary the

X-ray

contrast at the

polar/non-polar

interface without

altering

the _structure of the cubic

phase significantly,

This may

permit

the

phases

of the reflections _to be deduced.

6.

Faceting

of air bubbles in the cubic

phase.

The presence of air bubbles in the cubic

phase

_can

help

to

highlight

^structural features of the cubic

phase

otherwise invisible _to the eye.

Although

the cubic

phase

^itself _appears

isotropic

and black,

light

is reflected off the facets which form _at the air-cubic

phase

interface,

allowing

the observation of

entrapped

air bubbles.

It _was not found necessary ^to blow bubbles into

samples,

_as the

samples

tended _to form bubbles of their _own accord, This _{was a} consequence of the method of

sample preparation.

Water _was added _on top ^{of the}

solid, trapping

the air present in the pores of the

powdery sample,

On

heating

into the micellar

phase

this air formed

spherical

bubbles

which,

_on

cooling, developed

facets. It took _a

period

of several

days

_or weeks for these bubbles to reach

equilibrium.

Sotta has

already

observed faceted bubbles in the

Ci2EOJwater

system

[14].

He claims that if

given enough

time to

equilibrate,

bubbles present ^within a cubic monodomain will

always

tend towards _a standard

equilibrium shape,

the

faceting

of which

corresponds

to

(21 1) planes,

which _are the

planes

with the

highest density

of _matter (as discussed in the

previous section)

and,

consequently,

the

planes

with the

largest

inter-reticular

spacing.

He has constructed _a

polyhedron

with

(211)

reticular

planes,

which represents ^the

equilibrium shape

which bubbles form in a cubic monodomain of

C12E06.

^These are shown in

figure

12.

Figure

12a, b and _c show the views

along

the

4-fold,

3-fold and 2-fold _axes,

respectively.

jai (hi _ICI Id

Fig. 12. Polyhedron constructed with

(211)

^reticular planes, seen along different _axes taken from reference [14].

We have found similar

faceting

in bubbles present ^{in both the}

p-OG

and

p-thio-OG

cubic

phases. Figure

13 shows bubbles which _were found in the cubic

phase

of

p-OG. Figure

14a exhibits bubbles found in the

p-thio-OG

cubic

phase,

while 14b shows _a

close-up

view of these bubbles. The diameter of the central bubble in

figure14b

is

approximately

0.6 _mm.

The left hand bubble in

figure

14b is

particularly

close _to the standard

equilibrium shape

described

by

Sotta. In this view _{we can see}

along

both the 2-fold and 3-fold _{axes, as} illustrated in

figure

14c. The geometry ^{of this bubble} appears ^to be identical to that of the bubbles found in

CI2EO~ (cf. Fig.

5c from Ref.

[14]).

In _common with the bubble

pictured

in

figure

4 of

Sotta's

article,

this bubble is also

elongated along

its 4-fold axis.

In contrast, the central bubble in

figure

14b is

rounder,

but its

faceting

does not match that of the theoretical

equilibrium shape

as

clearly.

One _can,

however, tentatively

claim to be

looking along

a 4-fold axis.

Many

such bubbles, with

faceting

similar to but not

exactly

the _{same as} that of the standard

equilibrium shape,

were found, These bubbles _are

probably

not

entirely

enclosed in

perfect

monodomains but in _{two or more} different domains. Other distortions may be due _to the fact that _most of these bubbles _were found adhered _to the

capillary

wall,

leading

to

possible anisotropy.

Also, several bubbles contained _more facets than the

equilibrium shape, suggesting

that

they

had not reached full

equilibrium.

A _common defect _was the formation of _{a narrow} facet where

a

sharp edge

should have been present,

causing

these

edges

to appear rounded. Indeed, all these

photographs

were taken at _room temperature, ^{but it} _was

subsequently

found that if the

samples

were cooled _to close to 0

°C,

the

edges

became

sharper

and better defined.