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Langmuir monolayers of mesomorphic monomers andpolymers

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. 2014 Nous avons préparé des monocouches de Langmuir avec des molécules organiques qui forment desphases cristal liquide en volume. Tous les polymères et monomères utilisés présentent le même type de structureavec un c0153ur central aromatique rigide contenant plusieurs groupes esters hydrophiles et deux ou plusieurs chaînesaliphatiques hydrophobes. Un étalement bi-dimensionnel satisfaisant est obtenu dans la plupart des cas, avec uneaugmentation monotone de la pression superficielle en fonction de la concentration dans la monocouche. Auxpressions les plus fortes, l’aire minimum correspond à une orientation verticale des molécules avec décollement decertains 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. 2014 Monolayers of organic compounds which exhibit liquid crystalline behaviour in their bulk phases havebeen formed at an air-water interface. All selected polymers and monomers were of the same basic structure with arigid aromatic central core containing several hydrophilic ester groups and two or more aliphatic hydrocarbonchains. Spreading was satisfactory in most cases and the surface pressure was found to increase monotonously withsurface concentration. At the highest pressure attainable the minimum surface area per molecule corresponds to avertical 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 Abstracts82.65 - 61.30 - 61.40K - 64.30

1. Introduction.

It has been known for a long time that amphiphilicmolecules such as soaps and phospholipids sponta-neously spread at the air-water interface to form two-dimensional monolayers [1]. On the other hand, onlyrecently has it been realized that thermotropic liquidcrystalline compounds can also be considered as

amphiphiles. Their molecular structure is rather

complex but the existence, on the same molecule, ofhydrophilic and hydrophobic groups is generallyrecognized without great difficulty. As a consequenceof this dual character, it is expected that they shouldspread as Langmuir monolayers. Moreover, sincethese molecules are fairly rigid, it should suffice in

principle to have two anchoring points on the waterto insure planar orientation. Unfortunately, the exist-ing monolayer data on such materials are very

scarce. To the best of our knowledge, there exists onlytwo papers in the literature which describe mono-

layers of calamitic (rod-like) molecules [2, 3] and onewhich deals with discotics [4]. Moreover, all thesestudies have been performed on low molecular weight,monomeric, materials exclusively.

In this paper, we will describe a systematic study ofmonomers and polymers bearing hydrophilic estergroups and exhibiting liquid crystalline phases in thebulk. We will show how the planar configuration (withthe molecules lying flat on the interface) is in compe-tition with the vertical orientation when the surfaceconcentration is gradually increased. We will alsosuggest that it is possible to form in this way sweetiephases composed of a single molecular layer, which iseven closer to an ideal two-dimensional system thanthe freely-suspended two layers smectic films obtainedby 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 exhibitthermotropic 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 theacronyms used for the various compounds. The thermotropic bulk liquid crystalline phase transitions are indicatedin 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. Parenthesesindicate 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 showedthem to be in the range 5,000 to 8,000. The minimum areas Ao per molecule (or per monomer in the case of a polymerchain) measured in the monolayer experiments are given in column 6. They were derived from an extrapolationto zero pressure of the highest surface pressure data points.

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are closely related to well-known liquid-crystals. Theirtransition temperatures are listed in column 4 oftable I. Note that rufigallol hexa n-octanoate (labelle-das RGH-8) has no nematic phase but two columnarphases (DB and DJ [7] while 4-4’ azoxy a-methyln-nonyl cinnamate (AMNC) has only a smectic Aphase.

All the listed materials are composed of a rigidaromatic central core surrounded by long, flexible,hydrocarbon chains. They also possess several estergroups which are known to provide good anchoringpoints to the water surface [1]. With the exception ofrufigallol which is an anthraquinone derivative, thecentral part is either of the azoxy or the azobenzenetype, eventually substituted with methoxy groups inthe ortho position. These structures are mostly hydro-phobic and prevent direct solubilization of the spreadmolecules into the water subphase. In the case of thepolymers, the same monomeric unit is repeated n times(here, n is relatively low, of the order of 10).

Separate studies [8] have shown that polyestermacromolecular chains behave as flexible coils indilute solution and also in the melt. Indeed their

persistence length, which describes the local chainrigidity, is found to be no greater than the length ofthe monomeric unit. In that respect, these polymersthus 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 diolsor diphenols [9]. Depending on the synthesis, the

mesogenic unit was contained in the di-acid and theflexible spacer in the diol or vice versa. For instance,PC azoxy-8 and PC azoxy-10 refer to polyesters ofp-azoxy benzoic acid with octane diol and decane diol

respectively. Their full names are poly(4,4’-carboxyazoxy benzene-1, 10-dioxyoctane) and poly(4,4’-car-boxy azoxy benzene-1, 10-dioxydecane). On the otherhand, PO azoxy-12 was obtained by condensation of4-4’ azoxy phenol with tetradecane dioic acid. Its fullname is poly(4,4’-oxyazoxybenzene dodecane dioyl).Similarly PC azo-8 and PC azo-10 refer to polyestersof p-azobenzoic acid with octane diol and decanediol respectively while PO azo-10 and PO azo-11refer to polyesters of 4,4’ azophenol with dodecane andtridecane 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 ofthese 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 standardchemical 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 wereused as received

Spreading solutions were prepared in the usual wayby dissolving 1 mg of powder in 25 ml of chloroform.Clear solutions were generally obtained after stirringfor 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 solubilizationinto chloroform.

Drops (4-10 yl) of these solutions were depositedonto the free surface of tri-distilled, surfactant-free,water contained in a cylindrical quartz container.The monolayer forms spontaneously, following theevaporation of the spreading solvent. Its surface

density can be gradually increased by successive

deposition up to a state of saturation is finally reachedAt this point, the surface density is maximum andadditional drops will stay as lenses floating on theinterface. These lenses will slowly disappear by sol-vent evaporation, leaving on the surface a smallflake of solid material which is detectable with thenaked eye. On the contrary, when the molecules donot have the correct hydrophilic-hydrophobic balancefor spreading, the monolayer does not form and satu-ration can never be observed even after depositionof large amounts of solution. PO azoxy-12 is a goodexample of such a behaviour.The surface pressure exerted by the monolayers was

measured by the Wilhelmy hanging plate techniquewith a platinum foil (2 x 0.9 x 0.01 cm’) attached toa force transducer (Model FTAI-1, Sanbord, Waltham,MA) fed into a phase-sensitive amplifier (HewlettPackard HP 8805 B). The sensitivity was of the orderof 10 gN m-1. The accuracy of the measurementswere further limited by a noticeable drift of the orderof 0.2 gN m - ’ min - ’. This drift, which was morepronounced 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, modelF-3). In addition to the cylindrical trough (6.24 cmdiameter, 0.5 cm height), a rectangular trough(25 x 9.5 x I cm’) equipped with a movable barrierwas also used to perform compression-expansioncycles within the monolayer. The compression ratewas adjusted with a stepping motor to an averagevalue of 0.05 nm2 molecule-1 min-’. The pressurewas never allowed to exceed the equilibrium spreadingpressure (i.e. the pressure reached at saturation in thedrop 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

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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 extrapolatesto 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 arefor polymeric materials (PC azoxy-10, PC azo-8 andPO azo-11) while the next two are for the RGH-8 andAMNC monomers. All display similar behaviours.At very large areas per molecule A > 2.5 nm2 (in the

Fig. 4. - Surface pressure isotherm of rufigallol hexan-octanoate (RGH-8) at 40 °C. Substrate is pure water. Thesolid 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-methyln-nonyl cinnamate (AMNC) at 20°C. Substrate is acidifiedwater (0.01 N HCI, pH = 2.0). The solid line extrapolatesto a minimum area per molecule Ao N 0.22 nm’ at zeropressure.

case of polymers, the abscissa scale is calculated permonomer), the pressure H is low and typically lessthan 10- 5 N. m - 1. As the area is further reduced,the pressure increases gradually and reaches values ofabout 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 pressurevariation, however, becomes much more rapid foreven lower areas per molecule. Typically Il changesby several 10- 3 N. m-1 between 0.3 and 0.2 nm2. Thecompressibility is then so low that the curve is prac-tically vertical. Extrapolation of the slope at zeropressure yields limiting minimum areas per monomer,Ao, between 0.15 and 0.33 nm’ for all compoundsstudied (see column 6 of Table I).We have also measured the surface pressure iso-

therms for AMNC at several temperatures. Theresults are shown in figures 6 and 7. The most

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Fig. 6. - Same as figure 5 but at three different tempera-tures (20, 30, 35 °C). The solid lines are just a guide to theeye.

Fig. 7. - Same as figure 6 but the surface pressure isothermshave now been plotted as a function of the surface concen-tration. Note the very sharp pressure increase aroundC ~ 0.6 x 10- 3 g. m - 2.

interesting feature is the sharp surface pressure increaseobserved 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 theslope is fairly similar whatever the temperature is. Inthis region the compressibility is large again. Thesurface pressures measured at even lower areas andfor the two temperatures of 30 and 35 OC, have notbeen plotted in figures 6 and 7 which focus more onthe low pressure regimes. It suffices to say that theyare qualitatively similar to the one plotted in figure 5at room temperature. Extrapolation of the slopes atzero pressure yields Ao = 0.22 nm2 at 20 °C, 0.3 nm2at 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 fordifferent reasons. In the case of AMNC, and contraryto all other experiments, the aqueous subphase hasbeen acidified, by addition of dilute HCI, down to apH 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 groupssuch as esters [10]. For a given surface concentration ofAMNC we have repeatedly observed that the surfacepressures are typically 10 % higher on an acidicsubphase, which indeed implies better spreading. Inorder to rule out possible hydrolysis effects by theacid, we have performed a separate experiment inwhich a chloroform solution of AMNC was stirred withan equal volume of acidified water during 24 hours.After decantation, the AMNC solution was spreadonto a pure water subphase. The surface pressure datawere absolutely identical to those of a fresh AMNCsolution, 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 cisisomerization under ultra-violet light excitation in therange 300-380 nm [11]. With PO azo-11, a decreasein the optical absorption spectrum was observed at350 nm, together with the appearance of a newshoulder at 400 nm, when the solution was illuminatedwith alms ultra-violet pulse through a UG-1 Schottfilter. Such a photo-isomerization is not desirablehere since there is a considerable conformation

change between the two isomers. As a consequence,all solutions of azo compounds were stored in brownbottles and the corresponding monolayer experimentswere always conducted under dim yellow light condi-tions.

Last, there are several compounds listed in table Iwhich have only been studied qualitatively and atroom temperature. PC azoxy-8 spreads and shows apressure isotherm comparable to that of PC azoxy-10.Its minimum molecular area is Ao - 0.15 nm2. Onthe contrary, no surface pressure could be detectedwith PO azoxy-12, which therefore should be consi-dered as non-spreading at the air-water interface. Asdiscussed earlier, PC azo-10 and PO azo-10 could notbe dissolved in chloroform, making it impossible tocheck their spreading behaviour. Finally the twomonomers NOMAB and HMAB are observed to

spread satisfactorily and their surface pressure iso-therms yield a minimum molecular area of about0.23 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-definedsurface pressure isotherms observed in our experi-ments. On the whole, the curves are typical of mono-

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layers exhibiting a gaseous phase at large areas and acondensed phase at smaller areas. By analogy withthe known behaviour for the simpler long-chainfatty acids [1], the gaseous phase corresponds toa state of weakly-interacting molecules and whereboth the polar heads and the aliphatic chains arein contact with water. Similarly, the condensed phasecorresponds to a state where the molecules are muchmore densely packed and where some of their chemicalgroupings, generally the methylene units, have beenlifted up from the air-water interface.

4.1.1 Small molecular areas (less than 0.4 nm2). -For all the compounds investigated here, the minimumarea per molecule Ao (corresponding to maximumcompression of the monolayer) is of the order of0.15-0.35 nm2. For the azo and azoxy derivatives it isincluded in an even narrower range of 0.15-0.25 nm2(see Table I). The value for rufigallol hexa-n-octanoateis higher, of the order of 0.33 nm2, but this is consistentwith its bulkier aromatic core.We can now try and compare these values with esti-

mations from molecular models and crystallographicdata in order to gain information on the molecularorientation at the air-water interface. The azo and

azoxy cores, not counting the aliphatic chains, arecalculated to be 1.36 nm long, 0.65 nm wide and0.36 nm thick [12]. Similarly the anthraquinone coreof rufigallol is 1.17 x 0.86 x 0.36 nm3 [13]. Usingthese sets of values, it is easy to see that in all cases theminimum molecular area Ao observed in our mono-layer experiments is not compatible with a planarorientation. More precisely, if one assumes the ringsystem 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) coresif the short axis is perpendicular to the interface, and0.88 nm2 (1.0 nm2) if the short axis is also parallel tothe interface. The only possibility left, and compatiblewith our experimental Ao values, is that the ring sys-tem is oriented vertically, with its long and short axisrespectively perpendicular and parallel to the watersurface. In that case the cross-sectional areas shouldbe 0.23 and 0.31 nm2 respectively for the two types ofcompounds. This is in excellent agreement with ourexperimental findings. It is also interesting to notethat polyester polymers very similar in structure toPO azoxy-10 have been recently reported to alignhomeotropically on untreated microscope glassslides [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 agoby Adam et ale in their study of molecules with twoethyl ester groups at opposite ends of a long methylenechain [ 15-a] and also with benzene derivatives contain-ing two or more hydroxyl or methoxyl groups in thering [15-b]. When all the molecules are standingvertical, the benzene derivatives in particular can adopt

a close-packed configuration with the faces of theiraromatic rings all in register against the faces of theneighbouring 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 willtry to arrange themselves to fill up the void availablebetween the aromatic cores. At not too large compres-sions, they may form a very thin liquid phase, acting asa diluent and the rigid cores will gather in lamellar,cylindrical and other arrangements similar to the

lyotropic micellar systems in which water is thediluent [16]. At maximum compression, they will beexpelled from the interstitial region and will formtwo thin pure hydrocarbon layers, one below and oneabove the rigid cores. In this configuration, they willno longer contribute to the molecular area occupiedby one monomer in the interfacial plane. This explainswhy the measured Ao values are independent of thealiphatic chain length, at least in the range of seven totwelve carbon atoms investigated here.

4.1.2 Large molecular areas (more than 1.5 nm2). -When the surface occupied per molecule is largerthan the cross-sectional areas in the plane of the ringsystem, it is probable that the hydrophilic ester groupslocated at the farthest ends of the azo or azoxy benzene

ring system tend to maintain the molecule flat on theinterface. The best evidence for this planar orientationis provided by the AMNC data (Figs. 6 and 7). At30 and 35 OC, the surface pressure stays very low downto a molecular area of about 1.4 nm2, and then startsincreasing rapidly over a narrow region. We suspectthat this sudden compressibility change occurs whenthe surface density occupied by the molecules in theplanar configuration becomes of the order of unity.This interpretation is compatible with the cross-

sectional area deduced from molecular models whichis 1.2-1.4 nm2. On further compression, the moleculestilt progressively until they reach a fully verticalorientation. This molecular rearrangement was alsoproposed for the same compound, but at a singletemperature, by Dorfler, Kerscher and Sackmann [2].Here we have the additional information that the

pressure jump occurs at a surface area value which isindependent of temperature. This confirms a purelygeometrical interpretation. Moreover the pressurejump gets larger with increasing temperatures. Thisshows that the new configuration is due to the increasedintermolecular cohesion in the tilted orientation ratherthan to the weakening of the interaction between theester pinning groups and the water subphase. We aretempted to interpret in the same way the observationsof 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 multilayersbelow molecular areas of the orders of 1.0 nm’, wassuggested. We think, however, that the observation of apressure re-increase around 0.2 nm2 is a strong indica-tion of a progressive molecular tilting towards acondensed phase where all molecules are verticallyoriented. In the present experiments, we have carefullychecked, using the trough equipped with the movingbarrier that the pressure isotherms were always fullyreversible on both sides of the transition region. This isclearly not compatible with the formation of multi-layers.

It should be technically feasible to follow directlythe progressive tilt of the molecular axis upon com-pression. Ellipsometric measurements, which giveaccess to the monolayer thickness, have already beenperformed at an air-liquid interface [17]. Another

possibility would be to build multilayers on a solidreflecting substrate using the horizontal lifting method[18] and then to perform standard interferometricmeasurements. In this last case, however, it is not

absolutely certain that the molecular packing will befully preserved at all surface coverages during thetransfer 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 carefulbalance between the hydrophobic and hydrophilicgroups on the molecule. The comparison between PCazoxy-10 and PO azoxy-12 strikingly illustrates thispoint. Apart from a minor variation by two methyleneunits in the flexible spacer, these two azoxy polyesterchains mainly differ by a mere inversion in the sequenceof their ether and carbonyl linkages. It appears thatthe displacement of the carbonyl group one atom awayfrom the benzene ring is sufficient to jump from aspreading (PC azoxy-10) to a non-spreading (POazoxy-12) behaviour. That the rule is fairly general isevidenced from the comparison between PC azo-8and PO azo-11. Here also there is a reversal betweenthe carbonyl and the ether bonds. Although bothpolymers are observed to spread at the air-water

interface, the spreading was found to be much easierand 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 moleculesflip over to the vertical orientation upon compres-sion. In order to try and keep the molecule flat on theinterface at all surface pressures, it would be temptingeither to increase the number of ester groups permolecule or to exchange the esters with more polargroups (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 moleculesconsist of a single benzene ring to which are connectedsix hydrocarbon chains, each with one ester group.

Surface pressure isotherms have evidenced that thearomatic ring lie flat on the water at all compressionsand that the limiting molecular areas correspond tothe cross section of the molecules parallel to the planeof the ring. Moreover it was possible to show that thealiphatic chains are parallel to the interface in theexpanded liquid monolayer state and perpendicularin the condensed state [19]. By comparison, rufigallolhexa-n-octanoate, which has been studied here, hasalso six ester groups but its aromatic anthraquinonecore is much bulkier. As a consequence the verticalorientation 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 alltake the vertical orientation despite the presence ofhydroxyl groups situated all around the benzene

ring. It was suggested at that time that «the samegroups which must increase the adhesion to the wateralso increase the adhesion between adjacent benzenerings standing upright ». This argument may well bevery general. It will explain why, so far, all experimentson monolayers of liquid crystal-forming compounds(including the present ones) have shown evidence fora vertical reorientation upon compression.

Recently Nakahara and Fukuda [20] have reportedthat molecules of 4,4’-bis-stearyl amino azobenzenealso get gradually lifted up from the interface on

compression despite the presence of two stronglyhydrophilic amide groups. Here also it is possiblethat the strong hydrogen-bonding forces between theamide 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 decreasethis intermolecular interaction and to look if the pla-nar orientation is preserved at larger compressionsthan before.

5. Conclusion.

To conclude, we have prepared Langmuir monolayersof several polymers and monomers bearing ester

groups and forming liquid crystalline phases in thebulk. Surface pressure data show that upon compres-sion the molecules tilt from a planar to a verticalorientation relative to the air-water interface. In the

high pressure region, the molecules have probablytheir aromatic ring systems in register. On the otherhand, the aliphatic chains do not seem to contri-bute noticeably to the minimum molecular area. Thisis suggestive of a smectic-type conformation, involvinga single molecular layer with the aromatic cores onthe inside and the hydrocarbon chains on the outside.A direct experimental check of this hypothesis is noteasy. 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 aromaticcores gather in a variety of two-dimensional lamellar,cylindrical, etc... aggregates, as in the bulk lyotropicmicellar systems. The hydrocarbon chains will thenact as a diluent. Modem x-ray synchrotron tech-

niques should allow to test this point in the nearfuture [21]. Eventually, for the lowest pressures, themonolayer becomes so dilute that all organization islost and the molecules lie flat on the interface with alltheir hydrophilic groups in direct contact with thewater.

Acknowledgments.

We thank P. G. de Gennes for his interest in this study.RGH-8, and AMNC were gifts of Drs. J. C. Dubois andJ. Billard respectively. HMAB and NOMAB weresynthetized specially for us by V. Surendranath. Thiswork was also made possible by a travel grant underthe CNRS-CSIR exchange program between the

College de France and the Raman Research Institute

and by a scholarship from the Joliot Foundation forK. A. Suresh.

Note added in proof. - After this work was comple-ted, we became aware of experiments by D. Cadenheadand M. Philips (J. Colloid Interface Sci. 24 (1967) 291)on fl-estradiol diacetate, a sterol molecule bearingtwo polar heads (acetate) at its extremities. Thereported surface pressure isotherms are strikinglysimilar to those of AMNC, with a break in the curvesfor an area of 0.96 nM2 and a limiting value of 0.38 nm2.As in the present analysis, this was taken as evidenceof molecular rearrangement from a flat to a verticalmolecular orientation, relative to the air-water inter-face. The lifting-up takes place when the close-packedstate corresponding to the flat orientation is reached.The value of the corresponding area per molecule isalso independent of temperature, which confirms thegeometrical interpretation. On the other hand, thesurface pressure at the transition is a much largerfunction (increasing) of temperature in the case ofAMNC than for p-estradiol di-acetate. This reflectsdifferences in the intermolecular interaction energiesfor the two types of compounds.

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[9] BLUMSTEIN, A., VILASAGAR, S., PONRATHNAM, S.,CLOUGH, S. B. and BLUMSTEIN, R. B., J. Polym.Sci. Polym. Phys. Ed. 20 (1982) 877.

[10] See Ref. [1] p. 243.

[11] Ross, D. L. and BLANC, J., in Techniques of Chemistry,Photochromism, Vol. 3, ed. by G. H. Brown

(Wiley, New York) 1971.

[12] BROWN, C. J., Acta Crystallogr. 21 (1966) 146.

[13] SEN, S. N., Indian J. Phys. 22 (1948) 347.[14] BLUMSTEIN, A., SCHMIDT, H. W., THOMAS, O., KHA-

RAS, G. B., BLUMSTEIN, R. B. and RINGSDORF, H.,Mol. Cryst. Liq. Cryst. 92 (1984) 271.

[15] a) ADAM, N. K., DANIELLI, J. F. and HARDING, J. B.,Proc. R. Soc. 147A (1934) 493.

b) ADAM, N. K., Proc. R. Soc. 119A (1928) 628.[16] We are indebted to P. G. de Gennes for this suggestion.

For a review on lyotropic systems, see for example :PERSHAN, P. S., J. Physique Colloq. 40 (1979)C3-423.

[17] TENEBRE, L., J. Physique Colloq. 38 (1977) C5-123 ;SMITH, T., J. Opt. Soc. Am. 58 (1968) 1069.

[18] FUKUDA, K., NAKAHARA, H. and KATO, T., J. ColloidInterface Sci. 54 (1976) 430.

[19] RONDELEZ, F., BARET, J. F. and BOIS, A., to be pu-blished.

[20] NAKAHARA, H. and FUKUDA, K., J. Colloid InterfaceSci. 93 (1983) 530.

[21] WINNIK, H. and DONIACH, S., Synchrotron RadiationResearch (Plenum Press, New York) 1980.