REVISTA MEXICANA DE FislCA 4S SUl'LEl\IENTO 1,42-46

Structure and surface properties of Iyotropic Iiquid crystals

A.M. Figucircdo NC10Instituto de FÚicll, Ullil'ersidac!e de Süo Paulo

Caixa Postal 66318,05315-970, S<ioPOli/O, SP, 1Jrmi/e-mail: aJif?ueireclo@ifllsp.br

Recibido ellO de febrero de 1998; aceptado c125 dc ahril de 1995

JUNIO 1999

Dne of Ihe Illost intcresling cxamples of complex fluids are the ¡¡quid C1y\lal.\'. Thcy wcre ohserved for the first time in 1888 by FriedrichReinitl.cr, ilfld sincc liJen they have been studied and used in differenl deviccs. Liquid crystals are mesomorphic states of the matter, hctweenIhe crystalline solid and Ihe is()(ropic liquid. In these systems. propcrties characteristic of a Huid coexist with propcrties observed in crystallinesolids. In (his papero lhc strllcture amI surfacc propcrties 01"these complcx fluids, in panicular for lyotropic liquid crystals. are discussed.

Kl'YlI'onJs: Liquid crystals; complcx l1uids; slructure

Los cristales líquidos est{lIl entre los ejemplos mús interesantes de los !luidos complejos. Fueron ohservados primeramente en 1888 porrriedrich R('init/eL Desde entonces, los cristales líquidos han sido ampliamcnte estudiados y empleados en diversos aparatos. Los cristaleslíquidos son estados Illesomórflcos entre sólido cristalino y el líquido isotrópico. En di chos sistemas. las propiedades características deun Huido coexisten con las propiedades observadas en los sólidos cristalinos. En el presente trnbajo. han sido discutidas las estructuras ypropied;-¡des L1elas superficies de los nuidos complejos, en particular. de los cristales líquidos liotrópicos.

[)eJcrif'ton'.~:Cristales líquidos; fluidos complejos; estructura

PACS, 61.30Gd, 61.30J, 75.30

I. Intrnduclion

Liquid (rystals represenl a (ertain nllmoer of particular sIatesorthe mallero where lhe orlÍer of their oasic unils is intermedi-ate hclween Ihe long rangc positionnl and orienlational order01'cryslalline solids amI Ihe long range disorder of isotropiclIuids [1\.

The lirst experimenlal onservalion of a Iiquid crystalwas done by rriedrich Reinilzer [2J in 1888. Observing un-dcr aplical microscopy sorne derivatives of lhe cholesterol,Reinilzer foulld remarkable properties : two melting poinlsami a sclc((ive relleclion of the Iight. lncrcasing the Icm-pcraturc or lhe sample. slarting on Ihe cryslalline phase, itmelis al a givcn lemperalurc, bullhe fluid remains lurbid. In-crcasing more Ihe lcmpcrature, the samplc rcaches Ihc clear-ing poi",. where il losses ils lurhidity. In lhe flrsl case. lhesamplc prcscntcd a phase transilion lo lhe Iiquid crystallincslatc, and artcr Ihe Iransition lo lhe isolropic lluid phase. DuoLchmann [~qmade a carefull synlhesis of lhe materials ob~ser ved hy Rcinitzl'~r.lo prevenl any possihle conlaminJlion ofthe samples, ano reproouced the Rcinilzcr cxperimenls find~ing Ihe samc results. As Ihese materials presenled at the sameliml' propcrtil's of cryslallinc solids ano Iiquids, Lehmannproposed tlll' llame Jiqflid crysra/s 10 thelTI.Fricdcl [4] in 1922<lnalyzed Ihe availahle data unlilthis year and concluded thatthese matcrials wcrc examples 01'a l/eH' state o/ the /lw{fcr.He proposed Ihe tL'rl1\IIICSOJ}/IllXCor mesomorphic s{(Ite oftheIII{/t/a lo c!lar¡¡ctcrizc t!lese lluids. Nowaoays, however, lheIl'nn ¡¡quid crystal is cOJllmonly used lO name these complexlluids.

Arter more (hen IDO ycars of the flrst ohservation, theI'hysics, Chemislry alllllhe tcchnological applications of ¡¡q-uid cryslals continue to propuse fundamental questions lo beinvestigaleJ for sciClllislS ano engineers.

A key ¡lOinl 01' Ihe cumplex and supermolecular lluidsstudy is that it uses cOllcepts ano lechniqucs of Ihe condenscdmaller physics lo investigate collcclivc hehaviors.

2. Classification

Thcrc are [511wo hig families of liquid crystals (LC). thetha11lotropics allll/yot,.opic."i.

2.1. Thl'rlllnlropics

The basic units 01'this lype of Le are anisomclric ITIolecules.!\'lnlecules \\/ilh large shapc anisotropy (elongated or disk-shapcd) can prcsent liquid crystalline phases, The has;e pa-rameler which conlrols the phase transitions is lhe tcmper-:lIurc (l'). rigures I;¡ and Ih show two examplcs of lheseanisolllelric Illole(ules. Ihe MllBA and the hexa-N acilox-itruxelle respcclivL'ly. The liquid crystalline phases arrear bc-tweell the solid amI Ihe isolropic Iiquid stalcs, as a fllnctionorTo This lype 01"Le has Ill<.lnylechnological applicalions inI,el) (1,C d~\'iccs), dilTerent scnsors (thermomctcrs, prcssuresensms, elc.), amI L'k~ctro-optical devices.

STRUCTURE ANO SURFACE PROPERTIES OF LYOTROI'IC L1QUID CRYSTALS 43

CH,

'o--@-CH =N--@-CII,\: FII,\-11, eH,

(a)

(b)

FIGURE l. Anisornetric rnoleculcs which form thcrmotropic liquidcryslals. (a) MRRA; (b) hexa-N aciloxitruxcnc.

2.2. Lyolropics

Lyolropic LC are mixtures 01"amphiphilic molecules (oneor more difTerenl types) and a solvent (usually water). Un-der propcr temperature and concentralion cont!ilions, ahovea critical conccntration of amphiphilic molecules, thesemolecules self-assemhly forming different structures. \Vhenthese molecular aggregates have a small shape anisotropythey are callcd micelles. The liquid cryslalline phases arefound hctwcen the salid and the isotropic liquid stnte, as afunction of the tempcrature and the relative concentralions 01"the mixture compounds.

3. Structures

The hasic units of LC organize in space in different structllrcswith well defined sYllllTIetries. The most common struc-tures are: the Ilematic, the cholesteric and the smectic. LetliS hrielly descrihe thesc structurcs.

In the uniaxial nematic phase structure (Fig. 2) the cen-ters ol"mass of the individual hasic units 01"lhe LC (Iet usimagine rod-like molecules) prescnt a liquid-Iike arder. Onthe other hand, the long axis uf the rods are orienled prcl"er-cntially alung a particular direcLion dehned hy the vector n.This direction deflnes the optical axis 01'the sample, and thevector is named director. This phase presents optical birefrin-gence (.6.n -= un - "1l.l, where 11 and.l refer lo parallel amIperpendicular to n and H is the linear rcfractive index). Fromsymmetry consideraLions [ól three types 0'- nematic phasesare expectcd: two uniaxial and one hiaxial. As we will seclatcr on, only in Iyotropic LC lhese Ihrce phases wcrc clcarlyitlcntilicd [7J.

The choleslcric phase structure (Fig. 3) prcsents a hcli-coidal arrangclllent of the Illolecules. It can he skelched as a

rJGllRE 2. Sketch ofan uniaxinl n~matic pha.s~ .structurc. The rodsrepresenl Ihe <lnisollletric molccules.

FIGURE 3. Sketch oflhe cholesteric phase structure.

superpositiol1 01'nemaLic planes where, from onc plane lo an-other, n twisLsalong thc direction perpendicular to the normalto Lheplanes. Usually lhe ncmatic phase can he considered asa cholcsLeric phase whcre the hclix pitch is infinite.

Thc smcctic phase structure is characterizcd by a onc-dimensional slacking of laycrs. In Lhese layers the cenlers 01'mass 01'the molecules present a Iiquid-like order, hut lhe longaxes 01"the 1ll0lcClllesare oriented preferentially rarallel to agivcn dircclion. In the smectic A phase this dircclion is per-pendicular Lothe layers (Fig. 4a), and in the smectic C phaseLhisdirection is tilted with respect to lhe normal to lhe laycrs(Fig. 4b). In a sense, these ph<tsescan be inlcrprcted as a one-dimensional sol id which coexists with a two-dimensionalliq-uid.

4. Phase diagrams of Iyotropic Iiquid crystals

The phase diagrams uf Iyotropic ¡iquid crystals (LLC) arevcry rich [8], presenting many diffcrent phases: nematic,lamellar (Iikc the sllleclic). hexagonal (cylindrical aggregatesof Illolecules arranged in a hexagonal structurc), rectangular,sponge 19, lO], micellar,etc. Let us concentrate our attention011Lhephase diagralll of a tcrnary Iyotropic mixture 01"potas-siul1l laurate/decanol(fixed)/water. Figure 5 shows a sketchof the experimental phase diagram of Lhis mixture [7]: thethrce nemaLic phases were identifleJ, and the transition be-lwccn them are of sccond-order. The critical exponents I"orthe order paramcter and ror Lhesusceptibility [11] agree withthe Iheorctical resulls [121 for the XY modcl, {3= 0.38 ami"(= 1.:l5. Deviations from the mean-field theory cxisL in thetemperatllre range arollnd the transition temperature Te cor-responding lO .6.T ¡Te"""" 10-1. Besidcs Lheisotropic phase at

N('I'. Mex. Fú. ~5 SI (1999) 42--46

44 A.t\.l. FIGUEIREDO NETO

(a)

presses the change in the lllicel1ar fonn and produces a IOPO-logical metamorphosis of lhe standard two-component orderparameler diagram. This lransformalion rcsults in a drasticrestructuring 01' the rhase diagram involving a symmetricfolding 01' the rhases amI singularities. The new thermody-namicai potential \vhich takes inlO accounl this non-criticalorder parameter can he written as

In the case of hillary mixtures (ane amphiphile and a solvent)the micelles are exrectcd to rresent an uniaxial symmetry(a disk, a cylinder) or a spherical shape. In lhese mixtures01111'one ncmatic phase was ohserved. On the olher hand,in ternary mixtures (two amphiphiles or one alllphiphile andall alcohol amI a solvent) it was proposed [14} the rnodel ofintrinsically hiaxial rnicelles. In these mixtures lhe three ne-malic rhases were observed as a function 01' T and the rela-tive concentrations 01' the compounds. In this framework themicel1es have three mutually perpendicular symmetry axes01' arder t\I,'{) (n, ,[J, and ,), as shown in Fig. 6. The differ-ent nematic rhases are ohtaincd hy mean s 01' orientationallluctuations al' lhe intrinsically hiaxial micelles. The shapeanisotropy 01' lhe micelles evalualec! from X-ray diffraetionis ahout 1:2:3, laking into account the micellar dimensions.\\'hen Ihe orienlalional lluctuations degenerate one axis wehave the uniaxial rhase and this axis is lhe optical axis al' lhesample (n). When the orientational Iluctuations are 01' smallamplilude the phase is macroscopically hiaxial. These aspectsare sketched in Figure Ó.

The continuous variation of the micellar shape anisotropyseems lo he one fundamental point lo allow the presence ofthe three nematic phases in the phase diagram. It tunes lhemicellar shape anisotrory necessary to originare the particu-lar orientational lluctuations which gives the nematie phases.This coule! he olle 01' the reasons to cxplain why in ther-rnotropic nematics the topology shown in Fig. 5 was neverobscrved.

5. The symmelry of Ihe micelles in lyolropics

F,(J;. ¡;. ¡;)¡' JI'!. b J' I J'2 J' J" (1)= (/1 1 + (12 1 + 1 '2 + )2 2 + el .'3+ C2 3

\vhereI; = Il:f::T,I~ = h,I~ = T,I1,andharethelwoin-dependenl quadrupolar tensor order parameter invariants, andT = (1113 - 1105)/(1113 + "s) is the non-critical orderparam-eler. /lu and lIS are, per unitary volume, the respective num-her al' predominantly hiaxial and almost spherical micelles. T

is a scalar non-critical order parameler which represents thecontinuous changc in shape of the micellar populalions as thetcmperaturc varies. \Vith minimization of lhe thermodynam-ieal potential 01' Eq. (1) one ohtains [16J the topology 01' theexpcrimental phase diagram of Iyotropic nematic LC. It isimportant to note lhat out of this framework the experimentalphase diagrams could 110t he theorctically completely repro-duecd. The usual approaeh [6] \Vas able to describe qualita-tively ol1ly the rcgion very close lo the Landau point.

concentratían

NdT(°C)

FIGURE 5. Sketch orthe phase diagrarn orlhe ternary Iyotropic nc-rnatic mixture 01' potassium laurutc/dccanol(l1xcd)/watcr. Ne. Nd,NIl. and ISO statc for lhe uniaxial calamilic, uniaxial disentic. hi-axial and isotropic rhases respectively. Thc tcmpcraturc incrcasesrrom Ihe holtom lo lhe lar nnd lhe water concentration ¡nercasesfrom lhe r¡gh! 10 lhe left.

(h)

FIGURE 4. Sketch 01"lhe Slllcctic rhase struclurc. (a) Slllcctic A.(h) Slllcctic C.

high lcmpcralurc, anotiler one is ohscrvcd al low Icmpcra-Imes, This rcsult is duc lo {he J11odiflcation 01' the miccl-lar shapc anisotropy with lhe tcmpcrature [13J. T\'•...o Lant.laupoinls are prescnt on this rilase diagram. whcrc lwo sccond-ordcr transition hnes converge in a first-order transition line.

A key point in the Physics of Iyotropic nematic LC isthat the micelles change their shape anisotropy with temper-ature. This charactcristic is nol ohscrvcd in thcnnolropic ne-matic Le, wherc the hasic units are Illolccules. On the olhcrhand, there are experimental evidences [14,15] that supporll!lis idea in Iyotropics. The change in lhe Illicellar shapc wasshown [1G}lo strongly influence the specific features reportedrOl" the phase Lliagrallls 01' Iyotropic nClllalic Le. The usualisotroric-hiaxial nematic theorelicalmodel, which considerslhe quadrupolar lensorial order rammeter as a traceless ten-sar [1], ncedcd to he compleled by considering an addilionalnon-critical urder para meter. This ncw order paralllclcr ex-

Rel'. Me.\'. Fú. 45 SJ (1999) 42--46

STRUCTURE ANO SURFACE PROPERTIES OF LYOTROI'IC L1QUID CRYSTALS 45

Id

FIGURE 6. Sketch 01' (he intrinsically biaxial miccllc in a temaryIyotropic mixture. Thc oriclltational flUclUations originmc macro-scopically Ihe thrcc diffcrcnl nemalic phases.

6. Surface properties of miccllar I)'otropics

There are experimental cvidcnccs [1i, 181 that the miccllarIyotropic Le stabilizcs onlo the glass surface a ¡amcHar laycr01"amphiphilic molcculcs (Fig. 7). Thc average surface oricn-wlion 01' thcfmotropic ¡¡quid crystal moleculcs slrongly dc-pcnds 011 Ihe chcmical composition of Ihe surfacc with whichthey are in contact. This tcchniquc is largcly used to oriCOIthcnnotropics in many lcchnological applicatiolls 01' ¡¡quidcrystals. On lhe olher hand, (hefe are experimental evidencesIhat i1 nat glass surface, independcntly 01' lhe trcalmen! orthc ahscnec 01' any trcatment, stahilizcs onlo itsclf a lamel-lar layer (m hilayer) formcd hy the i1mphiphilic molecules 01'Ihe liquid crySlal. Different Langl11l1ir-Blodgett (LB) film de-pOSilS were done onto glass suhslrales of sample holders tostlldy (he elTecl 01' lhis particular surface lreatmen( on the ori-cnlalion of the micellcs near lhe surfacc and in lhe bulk. Theexperimental resuils indicate that independently 01' lhe Sllf-

faee trealment the orienlalion of the nematic director near lhesurface and in lhe hul"- is independenl of the particular surfacetreatment. The possihle existence 01' lhis surface layer com-posetl hy lhe liquid crystal amphirhilic molecules acts likc acarpel that screens lhc elTecl of the LB tIlms on the alignmentof the Iyotropic nematic pllase. This effect was called [18]self-scree"¡,,g effecl of Iyotropic ncmalics.

To moJify the surfacc layer deposited hy lhe IyotropicLC it is neccssary to inlroduce magnelic grains on it. Thedoping 01' LC wilh magnelic grains is usually done in Iy-utropics [UJ, 20). This doping malcrial is known asJerroflllid.Ferrolluids are colloidal suspcnsions of ferromagnetic grains,coaled with surfactanl agcnts or eleclrically charged, dis-persed in a ¡iquid carrier 1211. The typical sizc oflhe grains is

FIGURE 7. Sketch of (he lameBar layer stabilized anto the g:lasssurface in contact with (he Iyotropic I¡quid crystal.

ahollt IDO Á. Dne 01' the 1110s1speclaclIlar effeet of this dop-ing is to reduce in lhree onlers 01' magnitude the magncticlield necessary lo mient LC [22]. The sl11all magnetic fieldorients Ilw magnclÍí.: grains ano thelll, hy means 01' a rnechani-cal coupling, the Le is oriented. \Vhen a nematic LC is doped\vith ferrol1uids ami a magnetic tleld gradient is applied on lhesample in a way Ihal lhe grains are forced to move to the in-terface glass-LC [231, it is possible to modify the state of theinterface. The magnelic grains penetrate the lamellar layerand the lilt angle of Ihe director al Ihe interface and in lhehulk are funetions of the density of magnetie grains. It seemsthal (he presence of lhe grains al the interface modifies themolecular organizalion. Even the splay-bcnd elastic conslant1\13 [2.1) is inllllenced 1231 hy the slate 01' the interface. 1\"decreases wilh increasing amounts of ferrofluid on the bound-ing surfaces and the oilference between the huI k angle andthe easy angle al the surface increases for increasing amounts01' fcrrolluid in lhe sample. The inlrinsic contribution [25] tothe elTective !\- t:l is cunnectcd lo the intermolecular interac-tions t1epcnding un Ihe rclalivc molecular orientation and 011

the molecular oricnlalion \vilh respect to lhe relalive molec-ular positioll The intrinsic contrihution 01' /(13 is expected tohe indcpcndent 01' lhe ferrolluid concentration. Its extrinsicpart dcpends on the amplituoc and on Ihe decay length 01' Ihesurfacc tield. which are expccled to he increasing funclionsof the ferrollllid concentration. Sinee 1\-13 decreases with in-creasing concentrations of ferrolluid one can conclude thalthe intrinsic and extrinsic parts DI' lhe splay-bend elaSlic con-slant have opposite signs.

7. Conciuding rcmarks

In sumll1ary, ¡¡quid cryslals provide an inleresting field ofba-sic research and technological applications. Among the basicresearch prohlelTls \Ve can point out Ihe study of phase lransi-lions, investigalion 01' classes of universality, analogies withdi!'ferent physical systel11s (Iike He superlluid), and the study01' the welting 01' substrates by Le. Among the lechnologi-cal applicalions the masl important nowadays are the IiquiJcrystal displays (LCD), but there are 'lIso differelll sensorsfor lempcralure, prcssure, etc. The important characteristic al'Iyotropics wherc lhe micclles changc their shape anisotropywith tempcralure opens a new f1eld 01' rescarch nol only from¡he hasic point uf view hut alsu for possihJe lechnological ap.plicatiol1s.

ReI'. Mer. Fú. 45 SI (t999) 42-46

-------- - --

A.M. FIGUEIREDO NETO

In particular. Iyotropics prcsents a largc interface wilh hi-ology sincc Ihe dOllhle laycr existent in lllal1Y Iyotropic Lehas Ihe samc structurc 01' Ihe ceH mcmbrane. The cosmcticinduslry also has an incrcasing ¡nlcreS! in Iyotropics tille loIhe amphiphilic charaClcr 01' lhe molcculcs prcscnt in lhe 11'-(lImpie mixtures.

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Acknowleligments

The author lhanks to Ihe Fund~H;aoVital', FAPESP (Funtla,;lodc Alllpan) ;ll\:squisa uo Estado dc Sao Pauto) PRONI:X amiCNPq fmm Bra/il rOl" linancial supporL.

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Re\'. ¡\1l'x. FiJ. 45 SI (1999) 42-46