Liquid Crystals at Interfaces

DOI: 10.1002/ijch.201200030

Liquid Crystals at InterfacesRaj Kumar Gupta*[a] and V. Manjuladevi[a]

1. Introduction

In this era of modern electronic devices, the liquid crystaldisplay (LCD) occupies the major part of the market,from wristwatches to very large television screens. In anLCD, the molecules collectively reorient from the onstate to the off state upon application of an electric field.Liquid crystal (LC) phases are exhibited by shape aniso-tropic organic molecules.[1] In an LC phase, the mesogensflow like a liquid and exhibit anisotropic properties likea crystal. The LC molecules have to be aligned on a sur-face for device fabrication.[2] The surface induces orderingto the bulk material which is grown over it.[3] Thus, the as-sembly of molecules on the surface is interesting and hasbeen given great importance by the research community.The phase behavior of liquid crystals can be influencedby the presence of interfaces. Even in the isotropic phaseof a nematogen, the nematic ordering can be observed atan interface. There are reports that indicate that smecticordering can be induced at the surface of the nematicphase.[4] There are also numerous reports on aggregationand pattern formation by LC molecules on surfaces.[5–7]

Studies into the factors governing the patterns on the sur-face are important. The aggregation of molecules on sur-faces depends on the nature of the substrate, molecularinteraction, and the experimental conditions for its depo-sition. A Langmuir monolayer (LM), a single layer ofmolecules adsorbed at an air-water (AW) interface toyield a stable two-dimensional film, provides greater flex-ibility for tuning these parameters and hence controllingthe aggregation over a surface or an interface. Langmuirmonolayers of fatty acids exhibit a variety of surfacephases. These phases are classified depending on molecu-

lar density and the positional and orientational orders ofthe molecules.[8] The various phases of Langmuir mono-layer can be transferred to a solid substrate througha layer-by-layer deposition mechanism known as theLangmuir-Blodgett (LB) technique. In the LB technique,the substrate is dipped vertically in and out of the watersubphase while holding the monolayer at a target surfacepressure. The LB deposition technique is advantageous asit provides the flexibility to tune the number of layersand surface phases that can be deposited onto a varietyof solid substrates. The substrates can be treated chemi-cally to yield functionalized surfaces. Hence, the interac-tion parameters between the solid substrate and the ad-sorbing molecules can be controlled during the LB depo-sition, and therefore the ordering of molecules in the LBfilms can be maneuvered. A Langmuir monolayer at anAW interface can be studied by numerous techniques.Generally, the thermodynamical aspect of the LM can beprobed by employing a surface balance.[9] This is com-posed of a Teflon trough, barriers and a surface pressuresensor, and allows the surface pressure-area per moleculeisotherm to be recorded. The other techniques for charac-terizing Langmuir monolayers are listed in Table 1.

[a] R. K. Gupta, V. ManjuladeviDepartment of PhysicsBirla Institute of Technology and Science – PilaniFD-3, Pilani – 333031, Rajasthan (India)phone: +919828 041535fax: +91 1596244183e-mail: [email protected]

Abstract : The ordering in liquid crystals (LCs) can be influ-enced by an interface. Some of these molecules adsorbed atthe air-water interface yield a stable Langmuir monolayer.They exhibit numerous surface phases, which are classifiedon the basis of intermolecular separation and ordering.These surface phases are governed by the molecular interac-tions and the ambient experimental conditions such as tem-perature, humidity, pH, and ion content of the subphase. Inthis article, the role of molecular interactions on the surfacebehavior of several rodlike LCs are discussed. The Langmuir

monolayer of a cholesteric LC exhibits an interesting lowdensity liquid (L1’) phase with tilted molecules. Brewsterangle microscopy reveals stripe patterns, which arise due tothe precession of the tilted molecules. It is demonstratedthat this molecular precession can be controlled by the rela-tive humidity, presence of cations in the subphase, and in-corporation of appropriate molecular species. The Langmuirmonolayer and Langmuir-Blodgett films of a novel rod-dischybrid LC are discussed.

Keywords: Brewster angle microscopy · cholesteric acid · Langmuir-Blodgett films · Langmuir monolayers · liquid crystals

Isr. J. Chem. 2012, 52, 1 – 11 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1&

These are not the final page numbers! ��

Review

2. Calamitic Liquid Crystals at Air-Water Interface

There are several reports indicating that thermotropicliquid crystals form a stable Langmuir monolayer at theAW interface.[50–53] Monolayers of such molecules exhibita variety of surface phases. These phases essentiallydepend on the molecular interactions, which are governedby the functional groups present and the shape of themolecules. The rodlike thermotropic LCs form insolublemonolayers at the AW interface.[54–57] A report of LMstudies on 4-cyano-4’-octylbiphenyl (8CB) indicated the

formation of gas and liquid expanded phases prior to col-lapse.[50] In the liquid expanded phase, the molecules tiltwith respect to the surface normal. This tilting is due tothe strong dipolar repulsion between the -CN groups ofthe 8CB molecules.[58] In an interesting work by Zhanget al. , the role of molecular interactions on the phase be-havior of LMs of rod-shaped thermotropic liquid crystalswas investigated.[58] The LMs of 4-pentyl-4’-cyanoterphen-yl (5CT), 8CB, and 4-cyano-4’-pentylbiphenyl (5CB) havebeen formed at the AW interface and characterized bysurface pressure (p)-area per molecule (Am) isothermsand sum frequency generation (SFG) vibrational spec-troscopy.[58] The p-Am isotherm of 5CT is remarkably dif-ferent compared to those of 5CB and 8CB. The isothermsof 5CB and 8CB are much more expanded than that of5CT, which indicates a large intermolecular repulsion be-tween the biphenyl molecules. The stability of the films isachieved by a delicate balance between the dipolar repul-sion of the �CN groups and the attractive interaction be-tween the phenyl rings of the LC molecules. The attrac-tive interactions of the 5CB and 8CB molecules are lessthan those of 5CT. This will lead to instability in the LMsof 5CB and 8CB. However, experimental evidence re-veals a stable LM for 5CB and 8CB molecules. SFG stud-ies on the LMs of 5CB, 8CB and 5CT molecules showstrong evidence of penetration of water molecules only inthe monolayers of 5CB and 8CB. Due to the expansion ofthe biphenyl monolayer, water molecules penetrate andoccupy the space created due to dipolar repulsion of theCB molecules. These water molecules provide screeningof the electrostatic repulsion between the dipoles of the-CN groups and therefore provide a stable balance be-tween the repulsive and attractive forces between the bi-phenyl molecules. The LM of 8CB shows interestingproperties in the collapsed region. After the collapse, theisotherm shows a large plateau followed by a small rise in

Raj Kumar Gupta obtained his PhDdegree in physical sciences from theRaman Research Institute, Bangalore,in the field of thin films and surfacescience. He joined BITS Pilani as a fac-ulty member in the Department ofPhysics and continues as an assistantprofessor. His research interests in-clude thin films of liquid crystals andnanomaterials, surface plasmon reso-nance, and image processing.

V. Manjuladevi acquired her PhD fromthe Raman Research Institute in thefield of physical properties of thermo-tropic liquid crystals. From 2005–2006,she carried out research in the field offerroelectric liquid crystals as a postdoc-toral fellow at Trinity College, Dublin,Ireland. She is currently an assistantprofessor in the Department of Phys-ics, BITS Pilani. Her research interestsare the electro-optic and dielectricproperties of liquid crystals, thin filmsof liquid crystals, and nanomaterials.

Table 1. Techniques for characterizing Langmuir monolayers.

Technique Information obtained References

Surface pressure-area isotherm Information on monolayer formation, molecular area, monolayer phases, compressibility, mon-olayer stability, interaction of dopant in subphase with monolayer, level of mixing in mixedmonolayers.

[9–15]

Surface potential-area isotherm Orientation of molecular dipoles, molecular aggregation, course of chemical/photochemicalreaction, interaction of species in the subphase with the monolayer.

[16–19]

Conductance measurement H-bonding, proton hopping, interaction between head groups. [20,21]Brewster angle microscopy Monolayer phases, phase co-existence, domain formation, aggregation, morphological fea-

tures.[22–26]

Fluorescence microscopy Monolayer phases, phase co-existence, domain formation. [27–30]UV-visible spectroscopy Intermolecular interaction, aggregation, presence of chromophores. [31–33]Infrared spectroscopy H-bonding, orientation of head/tail groups, presence of functional groups. [34–37]X-ray diffraction Monolayer thickness, nanostructure of the surface and interface, monolayer and counterion in-

teractions, positional order, in-plane lattice structure, monolayer phases.[38–42]

Second harmonic generation Orientation of molecules within the monolayer, surface density of the monolayer. [43–45]Sum-frequency generation Orientational and conformational changes, surface density of the monolayer, structure of the

interface, phase transitions.[46–48]

Rheological measurement Viscoelastic behaviour of the monolayer, shear modulus, microstructural phases. [49]

&2& www.ijc.wiley-vch.de � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Isr. J. Chem. 2012, 52, 1 – 11

�� These are not the final page numbers!

Review R. K. Gupta and V. Manjuladevi

http://www.ijc.wiley-vch.de

surface pressure.[57] The natural tendency of the LM is totransform into 3D structures in its collapsed state. Duringthe collapse, the 8CB monolayer at the AW interface un-dergoes first order transition to a stable phase which iscomposed of a monolayer and an interdigitated bilayerover the monolayer.[50] This phase also collapses due tocompression, leading to multilayer domains with smecticordering.[59] The transition temperatures for bulk 8CB are20.5 8C (crystal-smectic A), 33.3 8C (smectic A-nematic),and 41.5 8C (nematic-isotropic). It is interesting to ob-serve that the smectic phase will be induced from Lang-muir monolayers even at the AW interface at tempera-tures corresponding to crystal or nematic phases of bulk8CB.[59,60]

The cyanobiphenyls are rod-shaped molecules and pos-sess a large intermolecular separation and tilt whenspread onto a water surface. The monolayer stabilizesdue to intermolecular interactions and the water mole-cules trapped between neighboring molecules.[58] There-fore, if the molecular interaction is parameterized, newphases can be introduced or some of them may be re-moved. The molecular interaction can be parameterizedin a large number of ways, e.g., by doping the CB mono-layer with some appropriate molecule, or changing thetemperature, pH, or salt content of the subphase. Astable condensed phase is achieved in the mixed mono-layer of the 8CB and p-(ethoxy)-p-phenylazo phenyl hex-anoate (EPPH) binary system. The EPPH molecule ex-hibits isomeric transitions of either cis-trans or trans-cisupon irradiation at 440 nm and 360 nm, respectively. Ifthe mixed monolayer is irradiated at 360 nm, the EPPHmolecules undergo isomeric transition from trans to cisand, due to shape incompatibility (rod- and bow-shapedmolecules), they expel out of the mixed monolayer. Onthe other hand, upon irradiation at 440 nm, the cis iso-mers transform to trans, leading to complete miscibility ofthe 8CB and EPPH molecules.[61]

2.1. Cholesteric Liquid Crystals at Air-Water Interface

In an attempt to study the interfacial behavior of choles-teric LCs,[62] 6-(cholest-5-ene-3-lyoxy)-6-oxohexanoic acid(cholesteric acid, ChA) has been synthesized in the labo-ratory.[63] This molecule exhibits the cholesteric phaseupon heating in the temperature range of 146.7–148.5 8Cand shows a right-handed specific optical rotation of+2328. The chemical structure is shown in Figure 1. Thehydrophilic -COOH group has more affinity towardscomplexation with metal cations. Therefore the molecularinteraction can be controlled by the ion content of thesubphase. An LM of ChA has been formed on an ultra-pure ion-free water subphase and the p-Am isotherm hasbeen recorded. The isotherm indicates changes in theslope at various points (Figure 1, marked a, b, and c, asindicated by the arrows). Such changes in slope can be

treated as phase transitions.[9] The monolayer of ChA atthe AW interface is found to be very stable.

The limiting areas per molecule for the regions of theisotherm between a–b, b–c, and c–collapse were found tobe 56.5, 51.0 and 40.0 �2, respectively. The molecularcross-section for the normal orientation of the ChA mole-cule at the AW interface is estimated to be around 40 �2.Therefore, the region of the isotherm between c–collapsecan be considered as the untilted condensed (L2) phase.The larger values of limiting area per molecule are ac-counted for by the tilting of the molecules. Thus, the re-gions a–b and b–c are considered as low- and high-densityliquid phases with tilted molecules, respectively. Thephases corresponding to these regions are named L1’ andL2’ phases, respectively. The Brewster angle microscope(BAM) images of the different phases of the ChA mole-cules show interesting results. The dark region in theimage represents the gas phase (Figure 2a). The brightgray region appears liquid-like (Figure 2a, 2b). Interest-ingly, the bright gray region shows stripe-like patterns(Figure 2b), consisting of linear stripes, circular stripes,and spirals. These patterns vanish upon compression,leading to a very uniform gray texture (Figure 2c). Uponfurther compression the monolayer collapses and verybright 3D domains appear in the BAM images (Fig-ure 2d).

The stripe pattern may arise due to variation in eitherdensity or tilt azimuth of the molecules in the monolayer.In order to verify this, epifluorescence microscope (EM)images were obtained and compared to the BAM images.

Figure 1. Surface pressure-area per molecule (p-Am) isotherm ofa cholesteric acid monolayer at the air-water interface. Kinks in theisotherm are denoted by a (Am = 57 �2), b (Am = 52 �2), and c (Am =35 �2). The chemical structure of the ChA molecule is shown.

Isr. J. Chem. 2012, 52, 1 – 11 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.ijc.wiley-vch.de &3&


Liquid Crystals at Interfaces


Any variation in molecular density would have led thevariation in density of the dye in the monolayer due towhich the stripe pattern might have appeared. The EMimage shows uniform texture in the L1’ phase (Figure 3).Therefore, the stripe pattern is due to variation in the azi-muthal angle of the tilted molecules in the L1’ phase. TheBAM images in the L2’ phase show uniform texture. Thisphase is characterized as a high density liquid phase withuniformly tilted molecules.

The patterns observed in the L1’ phase vary with time.Immediately after compression to the L1’ phase, a fewstripes and small spirals form. The number of stripes andthe size of the spirals grow over time. Figure 4 shows thewell-developed spirals and stripes observed after 24 h.

The stripe width varies from 20 to 100 mm. Further-more, the spirals rotate with time. Figure 5 shows BAMimages of ChA in the L1’ phase at two different time-points, showing right-handed clockwise rotation of the

spirals. This rotation can be related to the molecular chir-ality. It is reminiscent of the effect of molecular chiralityon the shape of liquid-condensed domains in Langmuirmonolayers of dipalmitoyl phosphatidylcholine (DPPC),as studied by Kr�ger and Lçsche.[64] They observed right-and left-handed spiral structures for l-DPPC and d-DPPC, respectively.

2.1.1. Effect of Relative Humidity on Monolayers of CholestericAcid

The pattern observed in the L1’ phase is dependent on therelative humidity (RH), and the rate of rotation of thespirals has been measured as a function of RH. It was ob-served that with increasing RH, the rate of rotation of

Figure 2. Brewster angle microscope (BAM) images of a ChA mon-olayer at the air-water interface. The images were captured at theAm shown below the respective image. Scale bar = 500 mm.[62]

Figure 3. Epifluorescence image of a ChA monolayer on ion-freewater, captured in the L1’ phase (Am = 54.0 �2). The gray uniformtexture represents the uniform L1’ phase. Scale bar = 312 mm.[62]

Figure 4. BAM image of the ChA monolayer on ion-free water,captured at an Am of 54 �2 after 24 hours of holding the barriers.Scale bar = 500 mm.

Figure 5. BAM images of the ChA monolayer on the ion-freewater, captured at an Am of 54 �2 after four minutes of holding thebarriers. The time interval between images A and B is two seconds.The arrow drawn on one arm of the spiral shows its sense of rota-tion. Scale bar = 335 mm.[62]





the spirals decreases (Figure 6). At higher RH, the rate ofevaporation of water from the subphase is lower andtherefore so is the driving force responsible for the rota-tion. As argued by Tabe and Yokoyama,[65] the spatiotem-poral patterns arise due to collective precession of chiralmolecules at the air-water interface, driven by transfer ofwater across the monolayer during evaporation througha process related to Lehmann rotation. The optimal fre-quency of molecular precession linearly depends on theconcentration gradient of water across the interface, andon the molecular chirality. If the monolayer were uni-form, with no boundaries, domain walls, or topologicaldefects, then all molecules would precess with the samefrequency and no patterns would form. However, if thereare nonuniformities in the monolayer, then there are non-trivial boundary conditions on the director, which canlead to complex patterns. A theoretical model has beenproposed by considering the dynamical equation for theazimuthal rotation, and the patterns observed in theBAM images have been simulated (Figure 7).

2.1.2. Effect of Subphase Cations on Monolayers of CholestericAcid

The spiral pattern arises due to the collective precessionof molecules. Such phenomena can be employed for thefabrication of artificial molecular motors. The rate of pre-cession of the molecules can be controlled by controllingthe RH of the medium. In an attempt to control the mo-lecular precession, the aqueous subphase has also beendoped with divalent (Cd2+) and trivalent (Al3+) cations.These cations are known to form complexes with the car-boxylic acid at the AW interface.[12] The effect of the pres-ence of such subphase cations on the p-Am isotherm isshown in Figure 8.

The isotherm shifts towards lower Am due to the pres-ence of very minute quantities of the cations. This mightbe due to condensation of the monolayer during the com-plexation of -COOH with the metal cations. The BAM

images for such complexed monolayers do not revealstripe patterns, even with a very small concentration ofcations in the subphase. This is due to the fact that the-COOH/metal cation complexed units become rigidlyanchored to the water surface and do not allow the mole-cules to precess. Due to this lack of molecular precession,the BAM images do not reveal the stripe patterns.

2.1.3. Effect of Cholesterol Doping on Monolayers of CholestericAcid

The presence of cations completely removed the L1’

phase and therefore no stripe patterns were seen in theBAM images. The ChA monolayer has also been dopedwith cholesterol and the effect on the phases has beenstudied by surface manometry and Brewster angle mi-croscopy. Cholesterol (Ch) is structurally similar to ChA,and there are numerous studies on Ch monolayers at theAW interface.[66,67] It exhibits gas and untilted condensed(L2) phases. The p-Am isotherms of mixed ChA-Ch mon-olayer systems on ion-free water for various mole frac-tions of Ch (XCh) are shown in Figure 9.

With increasing XCh the lift-off Am shifts towards lowervalues, indicating the condensation of the monolayer. Thetrends of the isotherms for XCh values of 0.0, 0.1, 0.3 and0.5 remain similar. The isotherms reveal three kinks rep-

Figure 6. Rate of rotation of the spiral as a function of relative hu-midity.[62]

Figure 7. Spiral patterns for a single vortex surrounded bya domain wall. (a) Experimentally observed BAM image (size of fullimage = 1100 mm). (b) Theoretical prediction for the preliminarysingle Frank constant approximation, with rigid anchoring on theinner and outer boundaries. (c) Prediction in the case of unequalFrank constants. Here, the anisotropy of Frank constants gives aneffective pinning of the director field at the vortex core, even with-out any imposed anchoring at the inner boundary.[62]





resenting three phases. The isotherms of the monolayersfor higher mole fractions (XCh>0.5) indicate the suppres-sion of one of the phases, and reveal two kinks indicatingtwo phases. The extent of the steep region of the iso-therms corresponding to the L2 phase increases with in-creasing XCh.

Since Ch and ChA are structurally similar, it was ex-pected to have a high degree of miscibility of these mole-cules in the monolayer. For the ideal case of completemiscibility or immiscibility of a two-component monolay-er system, the area of the mixed monolayer is given bythe rule of additivity:

Aid ¼ A1X1 þA2X2 ð1Þ

where X1 and X2 are the mole fractions of components1 and 2, respectively, and A1 and A2 are the Am values ofthe individual pure component monolayers. However, fora mixed system, the monolayer area can deviate from theideal case. Such deviation in the monolayer area dependson the nature of the interaction between the componentmolecules and is known as the excess area per molecule,Aex. The Aex is defined as:

Aex ¼ A12�Aid ð2Þ

where A12 is the experimentally determined value of theAm of the mixed monolayer, and Aid is the ideal Am valuecalculated from Equation 1. Positive or negative values ofAex indicate repulsive or attractive interactions betweenthe component molecules in the mixed monolayer, re-spectively.

The stability and the degree of miscibility of mixedmonolayers have been studied by calculating the excessGibbs free energy (DG).[68,69] The DG is given by:

DG ¼ Na

Zp

0Aexdp ð3Þ

where Na is the Avogadro number.The variation of DG with respect to XCh is shown in

Figure 10. The variation indicates negative values for allof the compositions. Thus, the Ch molecules are readilymiscible in the ChA monolayer and the most stable com-position is XCh =0.7. The miscibility of Ch and ChA couldbe due to attractive interactions between the componentmolecules.

The phases of the two-component monolayer systemwere investigated using BAM. The characteristic stripepattern of the L1’ phase was seen to exist in the BAM

Figure 8. Surface pressure (p)-area per molecule (Am) isotherms ofChA monolayers on aqueous subphases containing different molarconcentrations of (a) CdCl2 and (b) AlCl3. The arrows drawn in oneof the isotherms indicate the kinks.

Figure 9. The surface pressure (p)-area per molecule (Am) iso-therms of ChA-Ch mixed monolayers at different mole fractions ofCh in ChA (XCh).





images of the mixed monolayers with compositions up toXCh =0.5. Figure 11a shows the stripe pattern for an XCh

value of 0.3. The features tend to weaken with increasingXCh, and the BAM image for an XCh value of 0.5 (Fig-ure 11b) shows faint and large stripes. For XCh valuesgreater than 0.5, the BAM images (Figure 11c) of themonolayer do not reveal the stripe pattern, and showonly uniform gray texture. The BAM images of the otherphases of the mixed monolayer show uniform textures.

The spatiotemporal evolution of the stripe patterns inthe L1’ phase of the Ch-ChA mixed monolayer systemhas been observed using BAM, as shown in Figure 12.

These studies clearly indicate that the L1’ phase is sup-pressed slowly due to the incorporation of Ch in the ChAmonolayer. In the L2 phase, the Ch molecules align nor-mally to the water surface, exhibiting a maximum valueof in-plane elastic modulus of around 1000 mN/m.[70] Dueto incorporation of Ch in the ChA monolayer, the in-plane elasticity is expected to increase and the averagetilting of the ChA molecules in the L1’ phase may reduce.This may reduce the rate of precession of the ChA mole-cules in the L1’ phase. Therefore, the rate of precession ofthe molecules is controlled by controlling the XCh in the

ChA monolayer. Such studies give us insights into con-trolling the dynamics of artificial molecular motors.

3. Rod-Disc Hybrid LC Molecules at Air-Waterand Air-Solid Interfaces

Uniaxial calamitic nematic LCs are the active switchingelements in LCD devices, whereas uniaxial discotic nema-togens can be employed as optical compensation films towiden the viewing angle and increase the contrast ratio ofthe LCD. There are numerous advantages of using biaxialnematics in LCD devices, such as faster refresh rate andvery low power consumption. However, it is difficult to

Figure 10. The excess Gibbs free energy (DG) as a function ofmole fraction of Ch in ChA (XCh) at different surface pressures (p).The points denote the calculated values obtained from the experi-mental data. They are joined by smooth solid lines.

Figure 11. BAM images of the Ch-ChA mixed monolayer system. The composition and Am are shown below the respective images. Theimages show part of the concentric ellipses in the background, which are artifacts arising due to the scattering of laser light from very finedust particles on the lens and polarizer of the microscope. Scale bar = 500 mm.

Figure 12. The temporal evolution of the patterns obtainedduring BAM imaging of the Ch-ChA mixed monolayer. The compo-sition of the monolayer is shown below the respective images.Images were captured in the L1’ phase. The time difference be-tween images A and B is 5 minutes. Scale bar = 500 mm. A part ofthe concentric ellipses in the background is due to artifacts.





obtain a biaxial nematic phase. Theoretical studies indi-cate that a biaxial nematic phase could be obtained ina mixed state of discs and rods. However, due to shapecompatibility issues, rods and discs are immiscible. Ina novel attempt where rods and discs were covalentlylinked, a nanophase-segregated layered phase (similar tothe smectic A phase) with alternating rod and disc layerswas obtained.[71] The rods and discs forming the Langmuirmonolayer are immiscible at the AW interface. Therefore,it is interesting to probe the surface behavior of the rod-disc hybrid LC where the discotic core is laterally andsymmetrically substituted with cyanobiphenyl at the pe-riphery. The molecular structure of a rod-disc hybrid mol-ecule is shown in Figure 13.

The p-Am isotherm of TCQCB molecules at the AW in-terface is shown in Figure 14. The isotherm indicates pri-marily two liquid phases between 6 and 2.2 nm2 and 2.2and 1.5 nm2. These phases can be labeled as LD1 and LD2.The maximum values of the elastic modulus (E) of theLD1 and LD2 phases are around 46 and 61 mN/m, respec-tively. These values are low compared to that of the con-densed phase of Ch (ca. 1000 mN/m). The average molec-ular areas, as estimated from the isotherms for the LD1

and LD2 phases, were found to be 5.6 and 3.0 nm2, respec-tively. For the face-on conformation, the average molecu-lar area should be 22 nm2. Therefore, the molecular con-formation in these two phases can be predicted to beedge-on, wherein the disc plane lies normal to the inter-face.

Single layers of LB films of the TCQCB molecule havealso been deposited on glass coverslips at different targetsurface pressures (Apex Instruments), and scanned using

an atomic force microscope (AFM; NTMDT, SolverPro).These are shown in Figure 15.

The AFM images of the LB films of TCQCB depositedat different surface pressures show different textures. TheAFM image of the LB films deposited at 10 mN/m (Fig-ure 15a) shows the bright domains grown over a lessbright background. The average height of these bright do-mains was 5.2�0.3 nm, which corresponds to a verticalorientation of the TCQCB molecules. The average heightof the less bright region is around 3 nm. This can be ac-counted for by a tilt of the molecules. The bright domainsdo not show any preferential growth direction, and werefound to have grown randomly. The morphology of thedomains changes upon increasing target surface pressure.The less bright region vanishes, whereas the randomlygrown bright domains assemble to yield interesting pat-terns. The AFM image at 20 mN/m (Figure 15b) showsthe domains to be elongated and oriented in a preferentialdirection. The height variation data also reveal that theaverage height of such elongated domains is around5.5 nm. With a further increase in target surface pressureto 30 mN/m, the elongated domains appeared to vanish(Figure 15c). The AFM image at 35 mN/m (Figure 15d)revealed a grainy texture, where the elongated domainsobserved at 20 and 30 mN/m have completely trans-formed to isotropic small grain-like domains. From thisreport,[72] it can be noted that though the p-Am isothermof TCQCB reveals LD1 and LD2 phases, interesting pat-terns have been observed in the LB films of the mole-cules. The morphological patterns are found to be depen-dent on target surface pressure. Therefore, in addition tothe studies on LMs of LC molecules, the field of LB filmsof such molecules at the air-solid interface has to be ex-plored in detail.

Figure 13. Chemical structure of a rod-disc hybrid LC molecule(TCQCB). It possesses a tricycloquinazoline (TCQ) discotic core withrod-shaped cyanobiphenyl (CB) substituted symmetrically at theperiphery.[72]

Figure 14. Surface pressure (p)-area per molecule (Am) isotherm ofthe monolayer of TCQCB at the AW interface (solid line). The in-plane elastic modulus (E) as a function of Am is also shown (dashedline). The in-plane elastic modulus is calculated using E =�Am(dp/dAm).[72]





4. Summary and Outlook

The rod-shaped calamitic liquid crystals form stable mon-olayers at the AW interface. The surface phases primarilydepend on the intermolecular and molecule-substrate in-teractions. The molecular interactions in monolayers of

LC molecules can easily be tuned by appropriate doping.This provides flexibility to manipulate the surface phase.A new surface phase can either be created or an existingone can be eliminated. The LM of cholesteric LC showsan interesting liquid (L1’) phase wherein the tilted mole-cules precess. The precession of the molecules can be con-

Figure 15. Atomic force microscope images of LB films of the TCQCB molecule. The target surface pressures for LB films (one layer) deposi-tion for images A, B, C and D were 10, 20, 30 and 35 mN/m, respectively. The height profile along the dark line drawn on the images isshown below the respective image. Scale bar = 200 nm.[72]





trolled by various factors such as relative humidity, theion content of the subphase, and doping. The precessionof the molecules is found to be halted due to the presenceof metal cations in the subphase. In general, the rods anddiscs are immiscible and do not yield a stable mixed LM.Therefore, a hybrid rod-disc system has been investigated.The rod-disc hybrid molecules (TCQCB) form a stableLM that exhibits different liquid phases. The molecularconformation in such liquid phases was found to be edge-on. AFM studies on single layers of LB films of TCQCBreveal a morphological transformation from elongateddomains to isotropic small grain-like domains as a func-tion of target surface pressure. In this article we haveshown the capability of some of the thermotropic LCs toform LMs at AW interfaces. These molecules are simpleand hence their aggregation can be suitably controlled byexternal parameters such as temperature, pH and ioncontent of the subphase and through appropriate doping.We have discussed the interfacial behavior of both simplerodlike molecules and a complex rod-disc hybrid mole-cule.

Acknowledgements

The authors are thankful to the University Grants Com-mission, India, for financial support.

References

[1] P. G. de Gennes, J. Prost, The Physics of Liquid Crystals,Clarendon Press, Oxford, 1993.

[2] V. Manjuladevi, Y. P. Pananrin, J. K. Vij, Appl. Phys. Lett.2007, 91, 052911.

[3] H. Knoblach, H. Orendi, M. Bluchel, T. Seki, S. Ito, W.Knoll, J. Appl. Phys. 1995, 77, 481 –487.

[4] B. M. Ocko, A. Braslau, P. S. Pershan, J. Als-Nielsen, M.Deutsch, Phys. Rev. Lett. 1986, 57, 94 –97.

[5] B. Schulz, C. Bahr, Phys. Rev. E 2011, 83, 041710.[6] J. P. Michel, E. Lacaze, M. Alba, M. de Boissieu, M. Gailha-

nou, M. Goldmann, Phys. Rev. E 2004, 70, 011709.[7] R. M. Overney, E. Meyer, J. Frommer, H.-J. G�ntherodt, G.

Decher, J. Reibel, U. Sohling, Langmuir 1993, 9, 341 –346.[8] V. M. Kaganer, H. Mohwald, P. Dutta, Rev. Mod. Phys.

1999, 71,779–819.[9] G. L. Gaines Jr., Insoluble Monolayers at Liquid-Gas Inter-

faces, Interscience, New York, 1966.[10] A. Ulman, A Guide to Ultrathin Organic Films, Academic

Press, Boston, 1991.[11] M. C. Petty, Langmuir-Blodgett Films – An Introduction,

Cambridge University, Cambridge, 1990.[12] G. Roberts, Langmuir-Blodgett Films, Plenum Press, New

York, 1990.[13] R. K. Gupta, V. Manjuladevi, in Molecular Interactions

(Ed.: A. Mughelia), InTech Open, 2012, pp. 81–104.[14] C. M. Knobler, Adv. Chem. Phys. 1990, 77, 397–449.[15] P. Dynarowicz-Latka, A. Dhanabalan, O. N. Oliveira Jr.,

Adv. Coll. Int. Sci. 2001, 91, 221–293.

[16] N. K. Adam, J. F. Danielli, J. B. Harding, Proc. R. Soc.London, Ser. A 1934, 147, 491–499.

[17] H. C. Parreira, J. Colloid Sci. 1965, 20, 742–754.[18] J. Caspers, E. Goormaghtigh, J. Ferreira, R. Brasseur, M.

Vanderbranden, J. Ruysschaert, J. Colloid Interface Sci.1983, 91, 546 –551.

[19] A. Dhanabalan, L. Gaffo, A. M. Barros, W. C. Moreira,O. N. Oliveira Jr., Langmuir 1999, 15, 3944 –3949.

[20] M. Prats, J. Teissie, J. F. Tocanne, Nature 1986, 322, 756 –758.

[21] H. Morgan, D. M. Taylor, O. N. Oliviera, Chem. Phys. Lett.1988, 150, 311–314.

[22] J. M. R. Patino, C. C. Sanchez, M. R. R. Nino, Langmuir1999, 15, 2484 –2492.

[23] E. Teer, C. M. Knobler, J. Chem. Phys. 1997, 106, 1913 –1920.

[24] F. Wu, A. Gericke, C. R. Flach, T. R. Mealy, B. A. Seaton,R. Mendelson, Biophys. J. 1998, 74, 3273 –3281.

[25] D. Honig, D. Mobius, J. Phys. Chem. 1991, 95, 4590.[26] S. Henon, J. Meunier, Rev. Sci. Instrum. 1991, 62, 936.[27] B. Moore, C. M. Knobler, D. Broseta, F. Rondelz, J. Chem.

Soc. Faraday Trans. 1986, 282, 1753 –1761.[28] J. Teissie, Chem. Phys. Lipids 1979, 25, 357–368.[29] M. Losche, H. Mohwald, Colloids Surf. 1984, 10, 217 –224.[30] P. Kruger, M. Schalke, Z. Wang, R. H. Notter, R. A. Dluhy,

M. Losche, Biophys. J. 1999, 77, 903–914.[31] H. Gruniger, D. Mobius, H. Meyer, J. Chem. Phys. 1983, 79,

3701–3710.[32] L. J. Kloeppner, R. S. Duran, Langmuir 1988, 14, 6734 –

6742.[33] T. Ubukata, T. Seki, K. Ichimura, Macromol. Symp. 1999,

137, 25–31.[34] R. A. Dluhy, S. M. Stephens, S. Widayati, A. D. Williams,

Spectrochim. Acta 1995, 51A, 1413 –1447.[35] H. Sakai, J. Umemura, Langmuir 1998, 14, 6249 –6255.[36] R. Mendenlson, J. W. Brauner, A. Gericke, Annu. Rev.

Phys. Chem. 1995, 46, 305–334.[37] T. R. Baekmark, T. Wiesenthal, P. Kuhn, T. M. Bayerl, O.

Nuyken, R. Merkel, Langmuir 1997, 13, 5521 –5523.[38] V. M. Kaganer, G. Brezesinski, M. Mohwald, P. B. Howes,

K. Kjaer, Phys. Rev. E 1999, 59, 2141–2152.[39] I. Watanabe, H. Tanida, S. Kawauchi, J. Am. Chem. Soc.

1997, 119, 12018–12019.[40] Z. X. Li, C. D. Bain, R. K. Thomas, D. C. Duffy, J. Penfold,

J. Phys. Chem. B 1998, 102, 9473–9480.[41] J. R. Lu, T. J. Su, Z. X. Li, R. K. Thomas, E. J. Staples, I.

Tucker, J. Penfold, J. Phys. Chem. B 1997, 101, 10332 –10339.

[42] J. Majewski, T. L. Kuhl, K. Kjaer, M. C. Gerstenberg, J. Als-Nielsen, J. N. Israelachvili, G. S. Smith, J. Am. Chem. Soc.1998, 120, 1469–1473.

[43] T. F. Heinz, H. W. K. Tom, Y. R. Shen, Phys. Rev. A 1983,28, 1883–1885.

[44] Th. Rasing, Y. R. Shen, M. W. Kim, S. Grubb, Phys. Rev.Lett. 1985, 55, 2903 –2906.

[45] K. B. Eisenthal, Chem. Rev. 1996, 96, 1343–1360.[46] P. Guyot-Sionnest, J. H. Hunt, Y. R. Shen, Phys. Rev. Lett.

1987, 59, 1597 –1600.[47] G. R. Bell, Z. X. Li, C. D. Bain, P. Fischer, D. C. Duffy, J.

Phys. Chem. B 1998, 102, 9461–9472.[48] S. R. Goates, D. A. Schofield, C. D. Bain, Langmuir 1999,

15, 1400–1409.[49] C. F. Brooks, G. G. Fuller, C. W. Frank, C. R. Robertson,

Langmuir 1999, 15, 2450–2459.





[50] J. Xue, C. S. Jung, M. W. Kim, Phys. Rev. Lett. 1992, 69,474–477.

[51] S. Reuter, E. Amado, K. Busse, M. Kraska, B. Stuhn, C.Tschierske, J. Kressler, J. Coll. Inter. Sci. 2012, 372, 192 –201.

[52] N. C. Maliszewskyj, O. Y. Mindyuk, P. A. Heiney, J. Y. Jose-fowicz, P. Schuhmacher, H. Ringsdorf, Liq. Cryst. 1999, 26,31–36.

[53] J. Wang, L. Zou, A. Jakli, W. Weissflog, E. K. Mann, Lang-muir 2006, 22, 3198 –3206.

[54] M. C. Friedenberg, G. G. Fuller, C. W. Frank, C. R. Robert-son, Langmuir 1994, 10,1251–1256.

[55] M. N. G. de Mul, J. A. Mann, Langmuir 1994, 10, 2311 –2316.

[56] R. Hertmanowski, T. Martynski, D. Bauman, J. Mol. Struct.2005, 741, 201–211.

[57] K. Inglot, T. Martynski, D. Bauman, Liq. Cryst. 2006, 33,855–864.

[58] Z. Zhang, D. Zheng, Y. Guo, H. Wang, Phys. Chem. Chem.Phys. 2009, 11, 991–1002.

[59] K. A. Suresh, A. Bhattacharyya, Langmuir 1997, 13, 1377 –1380.

[60] B. Rapp, H. Gruler, Phys. Rev. A 1990, 42, 2215 –2218.[61] P. Viswanath, K. A. Suresh, Langmuir 2004, 20, 8149 –8154.

[62] R. K. Gupta, K. A. Suresh, S. Kumar, L. M. Lopatina,R. L. B. Selinger, J. V. Selinger, Phys. Rev. E 2008, 78,041703.

[63] J.-W. Lee, J.-I. Jin, M. F. Achard, F. Hardouin, Liq. Cryst.2001, 28, 663 –671.

[64] P. Kr�ger, M. Lçsche, Phys. Rev. E 2000, 62, 7031 –7043.[65] Y. Tabe, H. Yokoyama, Nature Mater. 2003, 2, 806 –809.[66] S. Lafont, H. Rapaport, G. J. Somjen, A. Renault, P. B.

Howes, K. Kjaer, J. Als-Nielsen, L. Leiserowitz, M. Lahav,J. Phys. Chem. B 1998, 102, 761–765.

[67] R. K. Gupta, K. A. Suresh, Eur. Phys. J. E 2004, 14, 35–42.[68] F. C. Goodrich, in Proceedings of the 2nd International Con-

gress on Surface Activity, Vol. 1 (Ed.: J. H. Schulman), But-terworths, London, 1957, p. 85.

[69] R. Seoane, P. Dynarowicz-tstka, J. Minones Jr., I. Rey-Gomez-Serranillos, Coll. Polym. Sci. 2001, 279, 562–570.

[70] R. K. Gupta, K. A. Suresh, Colloids Surf., A 2008, 320,233–239.

[71] H. K. Bisoyi, V. A. Raghunathan, S. Kumar, Chem.Commun. 2009, 7003–7005.

[72] R. K. Gupta, V. Manjuladevi, C. Karthik, S. Kumar, J. Phys-ics: Conference Series, in press.

Received: May 21, 2012Accepted: July 30, 2012

Published online: && &&, 0000





Liquid Crystals at Interfaces

Documents

Transcript of Liquid Crystals at Interfaces