Langmuir monolayers of mesomorphic monomers and polymers

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Langmuir monolayers of mesomorphic monomers and polymers

K.A. Suresh, A. Blumstein, F. Rondelez

To cite this version:

K.A. Suresh, A. Blumstein, F. Rondelez. Langmuir monolayers of mesomorphic monomers and poly-

mers. Journal de Physique, 1985, 46 (3), pp.453-460. �10.1051/jphys:01985004603045300�. �jpa-

00209984�

453

Langmuir monolayers ^of mesomorphic ^{monomers and} polymers

K. A. Suresh (*), A. Blumstein (+) ^{and F. Rondelez}

Laboratoire de Physique de la Matière Condensée, Collège ^de France, 11, Place Marcelin Berthelot, ^{75231 Paris Cedex} 05, ^France

(Reçu le 5 octobre 1984, accepté le 5 novembre 1984)

Résumé.

Nous avons préparé ^des monocouches de Langmuir ^{avec des molécules} organiques qui forment des

phases cristal liquide ^{en volume. Tous les} polymères ^et monomères utilisés présentent ^{le même} type ^{de structure}

avec un c0153ur central aromatique rigide ^contenant plusieurs groupes ^esters hydrophiles ^{et deux ou} plusieurs ^chaînes aliphatiques hydrophobes. ^Un étalement bi-dimensionnel satisfaisant est obtenu dans la plupart ^des cas, ^{avec une}

augmentation ^{monotone de la} pression superficielle ^en fonction de la concentration dans la monocouche. Aux

pressions ^les plus fortes, l’aire ^minimum correspond ^{à une} orientation verticale des molécules avec décollement de certains groupes hydrophiles initialement en contact avec le substrat _aqueux. Dans la région ^{de basses} pressions,

les données expérimentales ^{tendent à prouver} que les molécules ^sont orientées parallèlement ^à l’interface.

Abstract.

Monolayers ^of organic compounds which exhibit liquid crystalline ^{behaviour in their bulk} phases ^have

been formed at an air-water interface. All selected polymers ^{and monomers were of the same basic structure with a} rigid aromatic central core containing ^several hydrophilic ^{ester groups and two or more} aliphatic hydrocarbon

chains. Spreading ^was satisfactory ^{in most cases} and the surface pressure ^was found to increase monotonously ^with

surface concentration. At the highest pressure attainable the minimum surface area per molecule corresponds ^{to a}

vertical orientation, ^{with some of the} anchoring ^ester groups lifted up from the water subphase. ^In the lower pres-

sure region, ^{there is some evidence} that the molecules lie flat on the interface.

J. Physique ⁴⁶ (1985) ^{453-460 MARS} 1985,

Classification

Physics ^Abstracts

82.65

61.30

61.40K - 64.30

1. Introduction.

It has been known for a long ^{time that} amphiphilic

molecules such as soaps and phospholipids sponta-

neously spread ^at the air-water interface to form two- dimensional monolayers [1]. On the other hand, only recently has it been realized that thermotropic liquid crystalline compounds ^{can also be} considered as

amphiphiles. Their molecular structure is rather

complex ^{but the} existence, ^{on the same} molecule, ^of hydrophilic ^and hydrophobic groups is generally recognized ^without great difficulty. ^{As a} consequence of this dual character, ^{it is} expected ^that they ^should spread ^as Langmuir monolayers. ^{Moreover, since}

these molecules are fairly rigid, ^it should suffice in

principle ^{to have two} anchoring points ^{on the water}

to insure planar orientation. Unfortunately, ^{the exist-} ing monolayer ^{data on} such materials are very

scarce. To the best of our knowledge, there exists only

two papers in the literature which describe mono-

layers ^{of calamitic} (rod-like) ^molecules [2, 3] ^{and one}

which deals with discotics [4]. Moreover, ^{all these} studies have been performed ^{on low molecular} weight, monomeric, ^materials exclusively.

In this _paper, we will describe a systematic study ^of

monomers and polymers bearing hydrophilic ^ester

groups and exhibiting liquid crystalline phases ^{in the}

bulk. We will show how the planar configuration (with

the molecules lying ^{flat on the} interface) ^{is in} compe- tition with the vertical orientation when the surface concentration is gradually increased. We will also suggest that it is possible ^to form in this _way sweetie phases composed ^{of a} single ^molecular layer, ^{which is}

even closer to an ideal two-dimensional system ^than the freely-suspended ^two layers smectic films obtained

by Moncton and Pindak [5, 6].

2. Experimental.

The materials used for this study ^are listed in table I.

They ^{were chosen on the} grounds ^that they ^exhibit thermotropic mesomorphic behaviours in the bulk or

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01985004603045300

Table I.

Characteristics of ^the polymers ^{and monomers used in this} study. ^{See text} for ^the signification of ^the

acronyms used for ^{the various} compounds. ^The thermotropic ^bulk liquid crystalline phase transitions are indicated in column 4. K is for solid, ^{N : nematic, I :} isotropic, SA : ^smectic A, Dc : unspecified ^discotic phase, DB : ^rectan- gular disordered columnar discotic phase (Ref. [7]). ^The transition temperatures ^{are indicated in} °C. Parentheses indicate a monotropic transition. The polymer ^molecular weights ^{have not} been indicated here but viscosity ^mea-

surements in chlorinated hydrocarbon ^solvents (1, 1, 2, ² tetrachloroethane, chloroform, m-cresol) have showed them to be in the _range 5,000 ^to 8,000. ^{The minimum areas} Ao per ^{molecule (or per monomer in the case} of ^a polymer chain) ^{measured in the} monolayer experiments ^are given ^{in column 6.} They ^{were derived} from ^an extrapolation

to zero pressure of ^the highest surface pressure data points.

455

are closely ^{related to} well-known liquid-crystals. ^Their

transition temperatures ^are listed in column 4 of table I. Note that rufigallol ^hexa n-octanoate (labelle-

das RGH-8) ^{has no nematic} phase ^{but two columnar} phases (DB ^and DJ [7] ^while 4-4’ azoxy a-methyl n-nonyl ^cinnamate (AMNC) ^has only ^{a smectic A} phase.

All the listed materials are composed ^{of a} rigid

aromatic central core surrounded by long, flexible, hydrocarbon ^chains. They ^also possess several ester groups which are known to provide good anchoring points ^{to the water surface} [1]. ^{With the} exception ^of rufigallol ^{which is an} anthraquinone derivative, ^the

central part ^is either of the azoxy ^or the azobenzene type, eventually substituted with methoxy groups in the ortho position. ^These structures are mostly hydro- phobic ^and prevent direct solubilization of the spread

molecules into the water subphase. ^{In the case of the} polymers, ^{the same} monomeric unit is repeated n ^times (here, n ^is relatively ^low, of the order of 10).

Separate ^studies [8] have shown that polyester

macromolecular chains behave as flexible coils in dilute solution and also in the melt. Indeed their

persistence length, which describes the local chain

rigidity, ^{is found to be no} greater ^{than the} length ^of

the monomeric unit. In that respect, ^these polymers

thus belong ^{to a} different class than the typical ^semi- rigid polymers ^{such as} the aromatic polyamides, polyisocyanates ^or polybenzyl glutamates.

The polymers ^were synthesized by ^a condensation reaction between diacids or diacid chlorides and diols

or diphenols [9]. Depending ^{on the} synthesis, ^the mesogenic ^{unit was} contained in the di-acid and the flexible _spacer in the diol or vice versa. For instance,

PC azoxy-8 ^{and PC} azoxy-10 ^{refer to} polyesters ^of

p-azoxy benzoic acid with octane diol and decane diol

respectively. Their full names are poly(4,4’-carboxy azoxy benzene-1, 10-dioxyoctane) ^and poly(4,4’-car- boxy azoxy benzene-1, 10-dioxydecane). ^{On the other} hand, ^PO azoxy-12 ^{was obtained} by condensation of 4-4’ azoxy phenol with tetradecane dioic acid. Its full

name is poly(4,4’-oxyazoxybenzene ^dodecane dioyl).

Similarly PC azo-8 and PC azo-10 refer to polyesters

of p-azobenzoic acid with octane diol and decane diol respectively ^{while PO azo-10 and PO azo-11}

refer to polyesters ^of 4,4’ azophenol with dodecane and tridecane dioic acids. Their full names are poly(4-4’- carboxy-azoxybenzene-1, 10-dioxy ^{octane or} decane)

and poly(4-4’-oxyazobenzene ^decane dioyl ^{or unde-}

cane dioyl) respectively. The molecular weights ^of

these various polymers, ^as obtained from intrinsic

viscosity data, ^are fairly low, typically in between 5,000 ^and 8,000.

The 4-4’-nonanoyloxy-(2,2’ methoxy) ^azobenzene (NOMAB) ^and 4,4’-hydroxy(-2,2’ methoxy) ^azoben-

zene (HMAB) monomers were prepared by ^standard

chemical procedures ^and purified by recrystallization.

Their purity ^{was checked} by ^thin layer ^chromato- graphy. ^The rufigallol hexa-n-octanoate (RGH-8)

and the 4,4’ azoxy a-metyl n-nonyl ^cinnamate (AMNC)

had been synthesized previously by others and were

used as received

Spreading ^{solutions were} prepared in the usual _way

by dissolving 1 mg of powder ^{in 25 ml} of chloroform.

Clear solutions were generally obtained after stirring

for a few days ^and occasionally heating ^{to 40 OC.}

Only ^{in the case of the PC azo-10 and PO azo-10} poly-

mers were we not able to achieve the desired dissolu- tion and these materials were not studied further. It is

possible that the molecular weight ^{of these two} poly-

mers was too large ^{to allow} good solubilization into chloroform.

Drops (4-10 yl) of these solutions were deposited

onto the free surface of tri-distilled, surfactant-free,

water contained in a cylindrical quartz ^container.

The monolayer ^forms spontaneously, following ^the evaporation ^{of the} spreading ^{solvent. Its surface} density ^{can be} gradually ^increased by ^successive deposition up ^{to a state} of saturation is finally ^reached

At this point, ^{the surface} density is maximum and additional drops ^will stay ^{as lenses} floating ^{on the}

interface. These lenses will slowly disappear by ^sol-

vent evaporation, leaving ^on the surface a small flake of solid material which is detectable with the naked _eye. On the contrary, when the molecules do not have the correct hydrophilic-hydrophobic ^balance

for spreading, ^the monolayer ^{does not form and satu-}

ration can never be observed even after deposition

of large ^amounts of solution. PO azoxy-12 ^{is a} good example ^{of such a behaviour.}

The surface _pressure exerted by ^the monolayers ^was

measured by ^the Wilhelmy hanging plate technique

with a platinum ^foil (2 ^{x 0.9 x 0.01} cm’) ^{attached to}

a force transducer (Model FTAI-1, Sanbord, Waltham, MA) ^{fed into a} phase-sensitive amplifier (Hewlett

Packard HP 8805 B). ^The sensitivity ^was of the order of 10 gN ^m-1. The accuracy of the measurements were further limited by ^a noticeable drift of the order of 0.2 gN m - ’ min - ’. ^{This drift, which was more} pronounced ^at large ^surface concentrations, ^was due,

in our opinion, ^{to material} adsorption ^{onto the blade.}

The temperature ^{of the} trough ^was controlled to

± 0.1 OC by ^a circulating ^{water bath} (Haake, ^model F-3). ^{In addition to the} cylindrical trough (6.24 ^cm diameter, ^{0.5 cm} height), ^a rectangular trough (25 ^{x 9.5 x I} cm’) equipped ^{with a} movable barrier

was also used to perform compression-expansion cycles within the monolayer. ^The compression ^rate

was adjusted ^{with a} stepping ^{motor to an} average value of 0.05 nm2 molecule-1 min-’. The _pressure

was never allowed to exceed the equilibrium spreading

pressure (i.e. ^the pressure reached at saturation in the

drop deposition method) ^{in order to} avoid the forma- tion of metastable states in the monolayer.

3. Results.

The surface _pressure isotherms obtained by ^the

drop deposition ^{method at or near room} temperature

456

Fig. ^1.

^Surface pressure isotherm of the polymer poly(4,4’- carboxyazoxybenzene-1, 10-dioxydecane), (PC azoxy-10),

at 20 °C. Substrate is pure ^water. The solid line extrapolates

to a minimum monomer area A0 ~ ^{0.25 nM2 at zero} pres-

sure.

Fig. ^2.

^{Same as} figure ^{1 but for the} polymer poly(4,4’-car- boxyazobenzene-1, 10-dioxyoctane), (PC azo-8) ^{at 20°C.}

Ao N ^{0.16 nm2.}

Fig. ^3.

^{Same as} figure ¹ but for the polymer poly(4,4’- oxyazobenzene ^undecane dioyl), (PO azo-11), ^{at 20 °C.}

A0 ~ ^{0.15 nm2.}

are shown in figures ^{1 to 5.} The first three curves are

for polymeric ^materials (PC azoxy-10, PC azo-8 and PO azo-11) ^{while the next two are} for the RGH-8 and AMNC monomers. All display ^similar behaviours.

At _very large ^areas per molecule A > 2.5 nm2 (in ^the

Fig. 4. - Surface _pressure isotherm of rufigallol ^hexa

n-octanoate (RGH-8) ^at 40 °C. Substrate is _pure water. The solid line extrapolate ^{to a minimum area} per molecule

Ao N ^{0.33 nm2 at zero pressure.}

Fig. ^5.

Surface pressure isotherm of 4-4’ azoxy a-methyl n-nonyl ^cinnamate (AMNC) ^at 20°C. Substrate is acidified water (0.01 ^N HCI, pH

2.0). The solid line extrapolates

to a minimum area per molecule Ao N ^{0.22 nm’ at zero}

pressure.

case of polymers, the abscissa scale is calculated _per

monomer), ^the pressure H is low and typically ^less

than 10- 5 N. m - 1. As the area is further reduced,

the pressure increases gradually and reaches values of about 10-4-10-3 N . m-’ at 0.5 nm’. In this

region ^the monolayer compressibility, which is the

reciprocal ^{of the} slope, ^{is still} fairly high. The pressure

variation, however, becomes much more rapid ^for

even lower areas per molecule. Typically ^Il changes by ^{several 10- 3 N. m-1} between 0.3 and 0.2 nm2. The

compressibility ^{is then so} low that the curve is _prac-

tically ^vertical. Extrapolation ^{of the} slope ^{at zero}

pressure yields limiting ^{minimum areas} per monomer,

Ao, between 0.15 and 0.33 nm’ for all compounds

studied (see ^{column 6 of Table} I).

We have also measured the surface _pressure iso-

therms for AMNC at several temperatures. ^The

results are shown in figures ^{6 and 7. The most}

457

Fig. ^6.

^{Same as} figure ^{5 but at} three different tempera-

tures (20, 30, ³⁵ °C). The solid lines are just ^a guide ^{to the}

eye.

Fig. ^7.

^{Same as} figure 6 but the surface pressure isotherms have now been plotted ^{as a} function of the surface concen-

tration. Note the _very sharp pressure increase around C ~ 0.6 ^x 10- 3 g. m - 2.

interesting feature is the sharp ^surface pressure increase observed around 1.4 nm’ for the two higher tempe-

ratures. The change ^{is about 1 x 10- 3 N . m -1 at}

30 °C and 2 ^x 10-3 N. m-1 at 35 °C (see Figs. ^{6 or} 7).

From there down to molecular areas of about 0.5 nm2,

the _pressure variations are more gradual, ^{and the} slope ^is fairly similar whatever the temperature ^{is. In} this region ^the compressibility ^is large again. ^The

surface _pressures measured at even lower areas and for the two temperatures ^{of 30 and 35} OC, ^{have not} been plotted ⁱⁿ figures ^{6 and 7} which focus more on

the low _pressure regimes. ^{It suffices to} say that they

are qualitatively ^{similar to the one} plotted ⁱⁿ figure ⁵

at room temperature. Extrapolation ^{of the} slopes ^at

zero pressure yields Ao

^{0.22 nm2 at 20 °C, 0.3 nm2}

at 30 °C and 0.5 nm2 at 35 °C.

A word of further comment should be made concern-

ing ^the spreading conditions for the AMNC mono- mers and also for the azo materials, although ^for

different reasons. In the case of AMNC, ^and contrary

to all other experiments, ^the aqueous subphase ^has

been acidified, by addition of dilute HCI, ^{down to a} pH ^{value of 2.0. This is a routine} procedure ^{which is}

known to favour spreading ^{in the case} of surface- active molecules bearing carboxylic ^{or amine} groups.

It is also applicable ^{to some} non-ionizable _groups such as esters [10]. ^{For a} given surface concentration of AMNC we have repeatedly observed that the surface pressures ^are typically ¹⁰ % higher ^{on an acidic} subphase, which indeed implies ^better spreading. ^In

order to rule out possible hydrolysis ^effects by ^the acid, ^{we have} performed ^{a separate} experiment ⁱⁿ

which ^a chloroform solution of AMNC was stirred with

an equal ^volume of acidified water during ^{24 hours.}

After decantation, the AMNC solution was spread

onto a pure ^water subphase. ^{The surface} pressure data

were absolutely ^{identical to those of a fresh AMNC} solution, ^never exposed ^{to dilute} hydrochloric ^acid.

This check eliminates the possibility of chemical degra-

dation under the mild conditions of the experiment,

and justifies ^{the data} points ^of figures 5, 6 ^{and 7.}

In the case of the materials containing ^azo linkages,

a different kind of precaution ^{has to be taken.} Indeed,

it is well known that azobenzenes undergo ^{trans to cis}

isomerization under ultra-violet light excitation in the range 300-380 nm [11]. ^{With PO} azo-11, ^{a decrease} in the optical absorption spectrum ^{was observed at} 350 _nm, together ^{with the} appearance of a new

shoulder at 400 nm, when the solution was illuminated with alms ultra-violet pulse through ^a UG-1 Schott filter. Such a photo-isomerization ^{is not desirable}

here since there is a considerable conformation

change between the two isomers. As a consequence, all solutions of azo compounds ^were stored in brown bottles and the corresponding monolayer experiments

were always conducted under dim yellow light ^condi-

tions.

Last, ^{there are several} compounds listed in table I which have only been studied qualitatively ^{and at}

room temperature. ^PC azoxy-8 spreads ^{and shows a}

pressure isotherm comparable ^to that of PC azoxy-10.

Its minimum molecular area is Ao - ^{0.15 nm2. On}

the contrary, ^{no surface} pressure could be detected with PO azoxy-12, which therefore should be consi- dered as non-spreading ^at the air-water interface. As discussed earlier, ^{PC azo-10} and PO azo-10 could not be dissolved in chloroform, making ^it impossible ^to

check their spreading behaviour. Finally ^{the two}

monomers NOMAB and HMAB are observed to

spread satisfactorily and their surface _pressure iso- therms yield ^a minimum molecular area of about 0.23 nm2 in both cases.

4. Discussion.

4.1 MOLECULAR CONFIGURATION IN THE MONOLAYER.

-

That thermotropic ^nematic polymers ^{and mono-}

mers containing ^ester groups ^can be spread ^{as mono-} layers ^{at an} air-water interface and form stable two- dimensional films, is evidenced from the well-defined surface _pressure isotherms observed in our experi-

ments. On the whole, ^the curves are typical ^{of mono-}

458

layers exhibiting ^a gaseous phase ^at large ^{areas and a}

condensed phase ^{at smaller areas.} By analogy ^with

the known behaviour for the simpler long-chain fatty ^acids [1], ^the gaseous phase corresponds ^to

a state of weakly-interacting molecules and where both the polar heads and the aliphatic ^{chains are}

in contact with water. Similarly, the condensed phase corresponds ^{to a state} where the molecules are much

more densely packed ^{and where some of their chemical} groupings, generally ^the methylene ^{units, have been}

lifted _up from the air-water interface.

4.1.1 Small molecular areas (less ^{than 0.4} nm2).

For all the compounds investigated here, ^{the minimum}

area per molecule Ao (corresponding ^{to maximum} compression ^{of the} monolayer) is of the order of 0.15-0.35 nm2. For the azo and _azoxy derivatives it is included in an even narrower range of 0.15-0.25 nm2 (see ^Table I). The value for rufigallol hexa-n-octanoate is higher, of the order of 0.33 nm2, but this is consistent with its bulkier aromatic core.

We can now try ^and compare these values with esti- mations from molecular models and crystallographic

data in order to gain information on the molecular orientation at the air-water interface. The azo and azoxy cores, not counting ^the aliphatic chains, ^are

calculated to be 1.36 ^nm long, ^{0.65 nm wide and}

0.36 nm thick [12]. Similarly ^the anthraquinone ^core

of rufigallol ^{is 1.17 x 0.86 x 0.36 nm3} [13]. Using

these sets of values, ^{it is} easy ^{to see} that in all cases the minimum molecular area Ao observed in our mono-

layer experiments ^{is not} compatible ^{with a} planar

orientation. More precisely, ^if one assumes the ring system ^{to have its} long ^axis parallel ^{to the water sur-} face, Ao should be of the order of 0.49 nm2 (0.42 nm2)

for the azo and _azoxy benzene (anthraquinone) ^cores

if the short axis is perpendicular ^{to the} interface, ^and

0.88 nm2 (1.0 nm2) if the short axis is also parallel ^to

the interface. The only possibility left, ^and compatible

with our experimental Ao ^values, is that the ring sys- tem is oriented vertically, ^{with its} long and short axis

respectively perpendicular ^and parallel ^{to the water}

surface. In that case the cross-sectional areas should be 0.23 and 0.31 nm2 respectively ^{for the two} types ^of compounds. This is in excellent agreement ^{with our}

experimental findings. ^{It is also} interesting ^{to note}

that polyester polymers very similar in structure to PO azoxy-10 ^{have been} recently reported ^to align homeotropically ^{on untreated} microscope glass

slides [14].

Of course this vertical orientation requires ^that

some of the ^ester _groups get ^lifted up from the inter- face. Such a possibility ^{has been} suggested long ago

by Adam et ale in their study of molecules with two

ethyl ^ester groups ^at opposite ^{ends of a} long methylene

chain [ 15-a] and also with benzene derivatives contain-

ing ^{two or more} hydroxyl ^or methoxyl groups in the

ring [15-b]. When all the molecules are standing vertical, the benzene derivatives in particular ^can adopt

a close-packed configuration with the faces of their aromatic rings ^{all in} register against ^the faces of the

neighbouring rings. ^{This is} very favourable energetical- ly ^and may ^more than compensate for the loss of adhe- sional affinity ^{of the} polar ^ester group with the water.

In this vertical configuration, ^the aliphatic chains will try ^to arrange themselves to fill _{up the} void available between the aromatic cores. At not too large compres-

sions, they may form a very thin liquid phase, acting ^as

a diluent and the rigid ^{cores will} gather ⁱⁿ lamellar, cylindrical ^{and other} arrangements similar ^{to the}

lyotropic ^micellar systems ^{in which water is the} diluent [16]. At maximum compression, they ^{will be} expelled from the interstitial region and will form two thin _pure hydrocarbon layers, ^{one below and one}

above the rigid ^{cores. In this} configuration, they ^will

no longer contribute to the molecular ^area occupied by ^{one monomer} in the interfacial plane. ^This explains why the measured Ao ^{values are} independent ^{of the} aliphatic ^chain length, ^{at least in the} range of seven to twelve carbon atoms investigated ^here.

4.1.2 Large ^{molecular areas} (more ^{than 1.5} nm2).

When the surface occupied ^per molecule is larger

than the cross-sectional areas in the plane ^{of the} ring system, ^{it is} probable ^{that the} hydrophilic ^ester groups located at the farthest ends of the azo or azoxy benzene

ring system ^{tend to} maintain the molecule flat on the interface. The best evidence for this planar orientation is provided by the AMNC data (Figs. ^{6 and} 7). ^At

30 and 35 OC, ^{the surface} pressure stays ^{very low down}

to a molecular area of about 1.4 nm2, ^{and then starts} increasing rapidly ^{over a narrow} region. ^We suspect that this sudden compressibility change ^{occurs when}

the surface density occupied by the molecules in the

planar configuration becomes of the order of unity.

This interpretation ^is compatible ^{with the cross-}

sectional area deduced from molecular models which is 1.2-1.4 nm2. On further compression, the molecules tilt progressively ^until they ^{reach a} fully ^vertical

orientation. This molecular rearrangement ^{was also}

proposed ^{for the same} compound, ^{but at a} single temperature, by Dorfler, Kerscher and Sackmann [2].

Here we have the additional information that the pressure jump ^{occurs at a surface area} value which is

independent ^of temperature. This confirms a purely geometrical interpretation. Moreover the _pressure

jump gets larger ^with increasing temperatures. ^This shows that the new configuration ^{is due to} the increased intermolecular cohesion in the tilted orientation rather than to the weakening of the interaction between the ester pinning groups and the water subphase. ^{We are} tempted ^to interpret ^{in the same} way the observations of Diep-Quang and Ueberreiter [3] ^on 4-n-heptyl- phenyl (4’-n-hexanoyloxy) ^benzoate,

At that time a « rolling-over » liquid collapse ^mecha-

459

nism, in which the molecules start to form multilayers

below molecular areas of the orders of 1.0 nm’, ^was suggested. ^We think, however, that the observation of a

pressure re-increase around 0.2 nm2 is a strong ^indica- tion of a progressive ^molecular tilting ^{towards a}

condensed phase where all molecules are vertically

oriented. In the present experiments, ^{we have} carefully checked, using ^the trough equipped ^{with the} moving

barrier that the pressure isotherms ^were always fully

reversible on both sides of the transition region. ^{This is} clearly ^not compatible with the formation of multi-

layers.

It should be technically ^{feasible to follow} directly

the progressive ^tilt of the molecular axis _upon com-

pression. Ellipsometric measurements, ^which give

access to the monolayer thickness, ^have already ^been performed ^{at an} air-liquid ^interface [17]. ^Another possibility ^{would be to build} multilayers ^{on a solid} reflecting ^substrate using the horizontal lifting ^method [18] ^{and then to} perform standard interferometric measurements. In this last _case, however, ^{it is not} absolutely certain that the molecular packing ^{will be} fully preserved ^{at all surface} coverages during ^the

transfer onto the solid surface.

4.2 INFLUENCE OF THE HYDROPHILIC GROUPS ON MONOLAYER SPREADING.

We have already ^men-

tioned that the spreading ^is controlled by ^{a careful}

balance between the hydrophobic ^and hydrophilic

groups ^on the molecule. The comparison ^{between PC} azoxy-10 ^{and PO} azoxy-12 strikingly illustrates this

point. Apart ^{from a} minor variation by ^two methylene

units in the flexible _spacer, these two azoxy polyester

chains mainly ^differ by ^{a mere} inversion in the _sequence of their ether and carbonyl linkages. It appears that the displacement ^{of the} carbonyl group ^{one atom} away from the benzene ring ^is sufficient to jump ^{from a} spreading (PC azoxy-10) ^{to a} non-spreading (PO azoxy-12) behaviour. That the rule is fairly general ^is

evidenced from the comparison ^{between PC azo-8}

and PO azo-11. Here also there is a reversal between the carbonyl and the ether bonds. Although ^both polymers ^{are observed to} spread ^at the air-water interface, ^the spreading ^{was found to be much easier}

and reproducible in ^{the case of PC azo-8.}

Another _very legitimate question ^{is related to the}

nature of the anchoring points ^{to the water substrate.}

We have seen that, ^{with ester} groups, all the molecules

flip ^{over to} the vertical orientation upon compres- sion. In order to try ^and keep the molecule flat on the interface at all surface pressures, it would be tempting

either to increase the number of ester groups per molecule or to exchange ^{the esters with more} polar

groups (e.g. hydroxyls).

The former approach has ^{been used} successfully by ^{one of us in an earlier} paper dealing with benzene- hexa-n-alkanoates [4]. ^These disc-like molecules consist of a single ^benzene ring ^{to which are connected}

six hydrocarbon ^{chains, each with one ester} group.

Surface _pressure isotherms have evidenced that the aromatic ring ^{lie flat on the water at all} compressions

and that the limiting ^{molecular areas} correspond ^to

the cross section of the molecules parallel ^{to the} plane

of the ring. ^{Moreover it was} possible ^to show that the

aliphatic ^{chains are} parallel ^{to the} interface in the

expanded liquid monolayer ^{state and} perpendicular

in the condensed state [19]. By comparison, rufigallol hexa-n-octanoate, which has been studied here, ^has

also six ester groups but its aromatic anthraquinone

core is much bulkier. As a consequence the vertical orientation is still favoured over the parallel ^orienta-

tion at maximum compression.

The possibility ^of exchanging ^{esters with more} polar groups has been tried long ago by ^{Adam et} al., however the results have been deceptive [15-b].

Palmityl resorcinol and stearyl phloroglucinol ^all

take the vertical orientation despite the presence of

hydroxyl groups situated all around the benzene

ring. ^{It was} suggested ^{at that} time that «the same

groups which must increase the adhesion to the water also increase the adhesion between adjacent ^benzene rings standing upright ^{». This} argument may well be very general. ^{It will} explain why, ^so far, ^all experiments

on monolayers ^of liquid crystal-forming compounds (including ^the present ones) ^{have shown} evidence for

a vertical reorientation _upon compression.

Recently Nakahara and Fukuda [20] ^have reported

that molecules of 4,4’-bis-stearyl amino azobenzene also get gradually ^lifted up from the interface on

compression despite ^the presence of two strongly hydrophilic ^amide groups. Here also it is possible

that the strong hydrogen-bonding forces between the amide _groups belonging ^{to two} neighbouring ^mole-

cules ultimately favours the vertical orientation. It would now be interesting ^to introduce ortho substi- tuents in the ring system ^{in order to} try and decrease this intermolecular interaction and to look if the pla-

nar orientation is preserved ^at larger compressions

than before.

5. Conclusion.

To conclude, ^{we have} prepared Langmuir monolayers

of several polymers ^{and monomers} bearing ^ester

groups and forming liquid crystalline phases ^{in the}

bulk. Surface _pressure data show that _{upon compres-} sion the molecules tilt from a planar ^{to a vertical}

orientation relative to the air-water interface. In the

high pressure region, the molecules have probably

their aromatic ring systems ⁱⁿ register. ^{On the other} hand, ^the aliphatic chains do not seem to contri- bute noticeably ^{to the minimum} molecular area. ^This

is suggestive ^{of a} smectic-type conformation, involving

a single ^molecular layer with the aromatic cores on

the inside and the hydrocarbon ^{chains on the outside.}

A direct experimental check of this hypothesis ^{is not}

easy. However, it may show _up in macroscopic pro-

perties ^{such as surface} elasticity, ^shear viscosity, ^etc.

460

In the lower _pressure regions, the stratified organiza-

tion is probably ^{lost. It is} possible that the aromatic

cores gather ^{in a} variety of two-dimensional lamellar, cylindrical, ^etc... aggregates, ^as in the bulk lyotropic

micellar systems. ^The hydrocarbon chains will then act as a diluent. Modem _x-ray synchrotron ^tech- niques should allow to test this point ^{in the near}

future [21]. Eventually, for the lowest pressures, the

monolayer ^{becomes so} dilute that all organization ^is

lost and the molecules lie flat on the interface with all their hydrophilic groups in direct contact with the water.

Acknowledgments.

We thank P. G. de Gennes for his interest in this study.

RGH-8, ^{and AMNC were} gifts of Drs. J. C. Dubois and J. Billard respectively. HMAB and NOMAB were

synthetized specially ^{for us} by ^V. Surendranath. This work was also made possible by ^{a travel} grant ^under the CNRS-CSIR exchange program between the

College ^{de France and the Raman} Research Institute

and by ^a scholarship from the Joliot Foundation for K. A. Suresh.

Note added in proof.

^{After this work was} comple- ted, ^{we became aware of} experiments by ^{D. Cadenhead}

and M. Philips (J. ^Colloid Interface ^{Sci. 24} (1967) 291)

on fl-estradiol diacetate, ^a sterol molecule bearing

two polar ^heads (acetate) ^{at its} extremities. The

reported ^surface pressure isotherms are strikingly

similar to those of AMNC, ^{with a} break in the curves

for an area of 0.96 nM2 and a limiting value of 0.38 nm2.

As in the present analysis, ^{this was taken as evidence}

of molecular rearrangement ^{from a flat to a} vertical molecular orientation, ^{relative to} the air-water inter- face. The lifting-up ^takes place ^{when the} close-packed

state corresponding ^{to the flat} orientation is reached.

The value of the corresponding ^area per molecule is also independent ^of temperature, which confirms the

geometrical interpretation. ^{On the other} hand, ^the

surface _pressure at the transition is a much larger

function (increasing) ^of temperature ^{in the case of} AMNC than for p-estradiol di-acetate. This reflects differences in the intermolecular interaction energies

for the two types ^of compounds.

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