A nematic gap in mixtures of smectics A1 and Ad

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A nematic gap in mixtures of smectics A1 and Ad

K. Czupryński, R. Dabrowski, J. Baran, A. Żywociński, J. Przedmojski

To cite this version:

K. Czupryński, R. Dabrowski, J. Baran, A. Żywociński, J. Przedmojski. A nematic gap in mixtures of smectics A1 and Ad. Journal de Physique, 1986, 47 (9), pp.1577-1585.

�10.1051/jphys:019860047090157700�. �jpa-00210357�

A nematic gap in mixtures of smectics A1 ^and Ad (*)

K. Czupry0144ski, ^R. Dabrowski, ^J. Baran, ^A. 017Bywoci0144ski (+) ^{and J.} Przedmojski (+ +)

Military ^Technical Academy, ^00-908 Warsaw, ^Poland

(+) Institute of Physical Chemistry of the Polish Academy ^{of Sciences, 01-224 Warsaw, Poland} (++) Institute of Physics, ^{Warsaw Technical} University, ^00-662 Warsaw, ^Poland

(Reçu ^le 7 janvier 1986, révisé le 30 avril 1986, accepte ^{le 5 mai} 1986)

Résumé.

Des études de l’influence du rapport

^des épaisseurs des couches smectiques ^des composants ^d’un mélange ^binaire

^{l’allure du} diagramme ^de phase ^{ont été} effectuées. Les mélanges ^{de 80CB} (smectique Ad) ^et

d’un des composés de la série homologue ^des 5-n-alkyl-2-(4-isothiocyanatophenylo) dioxane-1,3 (DBT-smectiques A1) ^{ont été} pris

exemples. Quand

augmente ^{la stabilité de la} phase smectique

mélange ^{diminue et}

pour r ~ 1,4

^observe

intervalle nématique séparant ^les régions ^des smectiques A1 ^et Ad. ^{Pour le} système

binaire particulier ^{80CB-4DBT on}

^{examiné les} densités, les viscosités et les données de la diffusion des rayons X

fonction de la composition ^et température. ^{Enfin les} enthalpies ^des transitions de phase ^{ont été} mesurées. Le nema-

tique séparant ^les smectiques présente ^{une viscosité et structure interne} caractéristiques ^des nématiques typiques.

Le volume molaire du système ^binaire dépasse ^{ceux des} composants; l’excès ^est maximal pour des concentrations

en

80CB inférieures à 0,2.

Abstract 2014 The effect is tested of the smectic layer spacing ratio, r, ^on the phase diagram ^{for the} binary systems consisting ^{of 80CB} (smectic Ad) ^and

of the twelve compounds ^{of the} 5-n-alkyl-2-(4’-isothiocyanatophenyl)

dioxane-1,3 homologous ^series (DBT compounds-smectics A1) has been studied The stability of the smectic

phase ^{in the mixture} decreases with increasing ^r, and for r ~ 1.4

nematic gap separating the smectics A1 ^and Ad ^is

observed The density, viscosity ^and scattering ^of X-rays

^{function of} temperature ^{are measured and the} enthalpy ^{of the} phase transitions is determined for the binary system 80CB-4DBT. The nematic phase ^{reveals in}

the nematic gap

viscosity ^{and structure} characteristic for typical nematics. The binary system increases its molar volume

result of mixing ^{and assumes}

maximum in the concentration range x8OCB 0.2.

Classification Physics ^Abstracts

61.30E - 74.70M

1. Introduction.

The induction of a smectic A phase in mixtures of

suitably selected nematic compounds ^{is a well}

known phenomenon, ^see e.g. [1-7]. ^{If one or two com-}

ponents of the mixture have a smectic A phase ^{in the}

pure state, the smectic phase may be enhanced in the mixture. However, ^an opposite behaviour is also

possible. ^{In a mixture} consisting ^{of two smectic A} compounds, ^{a nematic} phase is induced [8], ^{or the}

nematic phase is enhanced if the mixture is composed

of compounds showing the nematic and smectic

phases. ^{Next we can} observe that the smectic phases

of both compounds ^are separated by ^{a nematic} phase.

Oh [9] and Holden et ale [10] ^{as well as} Engelen

(*) ^{This work}

presented ^{at the 6th} Liquid Crystal Conference of Socialist Countries, ^26-30 August 1985, Halle,

GDR.

et al. [11] have observed a nematic _gap between the induced smectic A phase and the smectic A phase ^{of the}

pure component. ^{In this case} the concentration _range in which the nematic phase ^{occurs is narrow. For the}

first time we observed the nematic _gap separating ^two

smectic A regions without enhancement of the smec-

tic A phase ^{in another} concentration _range in mixtures

consisting ^of alkylcyclohexylbenzoic ^{acid esters} [12].

In this case the observed nematic _gap ^occurs in a wide range of concentrations. The first component - ^an

ester with an isothiocyanate group ^or iodine atom in the terminal position of the molecule

is a mono-

layered ^{smectic A} (A, type), and the second _compo- nent

an ester with _cyano or nitro _group in the ter- minal position

^is partially ^a double-layered ^smec-

tic A (Ad type).

Next, ^we observed the nematic _gap in binary ^mix-

tures made _up of smectic esters whose molecules have the same polar group (cyano group), ^{in the terminal}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019860047090157700

1578

positions, ^{but one ester is a smectic} Al and the other

one a smectic Ad [13]. Recently ^{we have} also observed such a behaviour in mixtures composed ^{of esters and} pyrimidines having ^{a cyano group in} the terminal

position [14].

The pairs ^of compounds in which the nematic _gap

separating the smectic regions ^was observed hitherto reveal high melting points. Therefore these pairs ^are

not a convenient choice for ^more exhaustive investi-

gations ^{of their} physical properties. ^{We have found} compounds ^{which are more} convenient for this _pur- pose. One of the components of all the mixtures tested in the present ^{work is a} compound ^{selected from the} 2-[4’-isothiocyanatophenyl]-5-alkyldioxane-1,3 ^homo- logous ^series (referred ^{to as} compounds DBT). ^The compounds ^{from this series have a smectic} Al phase

and their spacing ^{is almost} equal ^{to the} length ^{of the}

molecule. They ^{have an} advantageous property ^con- sisting in that the members with 4 to 12 carbon atoms in the alkyl ^{tail have} only ^{one smectic} phase ^{which is} enantiotropic. ^The clearing points ^{of these com-} pounds ^{lie in the} range of 70-80 OC and their melting points between 30 and 60 OC [15]. 4-octyloxy-4-cyano- biphenyl ^was always ^{used as} the second component of the tested mixtures. This compound ^{is a} partially bilayer ^smectic Ad ^{with a} spacing ^{of 3.2 nm and a ratio}

of the smectic layer spacing ^{to the} length ^{of the mole-}

cule equal ^{to 1.37.}

In the present ^work it is shown how the character of the phase diagram changes with the smectic layer

spacing ratio of the components making up the mix- ture. Next, ^the variations of density, viscosity ^and enthalpy ^{of the N - I and} SA --+ ^N phase transitions with mixture composition ^are described for the binary

4DBT-80CB system by way of example. ^To explain

the changes taking place in the internal structure of the

mesophase, X-ray studies of this binary system ^have also been carried out.

2. Experimental.

All the compounds used in the present ^{work have been} prepared ^{in our} laboratory. ^{The DBT} compounds

have been synthesized ^and purified ^{as described in} [15] ; ^{the 80CB} compounds ^were prepared according

to the procedure ^described by Gray [16]. ^The phase

transition temperatures characteristic of the com-

pounds used in the present ^{tests are} summarized in table I.

In the table the lengths (I) of the molecules and the smectic layer spacings (d) ^{are also} given.

The smectic layer spacings ^{were measured} by ^a

standard method consisting ⁱⁿ recording ^{on a film the} small-angle diffraction of X-ray ^beams by ^the liquid

thin glass capillary, ^{heated to the} isotropisation ^tem- perature ^and subsequently ^{cooled to the} measuring temperature ^{in a 0.85 T} magnetic ^held.

The length ^of the DBT molecules was calculated

making ^use of the results of Hartung et ^al. [18] ^obtained

from X-ray investigations ^{of the} 5-alkyl-2-(4’-bromo-

Table I.

The structural formulae, phase transition temperatures, lengths of ^molecules (1) ^{and smectic} layer

spacing (d) for compounds used for preparing ^the binary ^{mixtures :}

phenyl)dioxane-1,3 molecule. The length ^{of the} alkyldioxylphenyl ^{radical was taken} according ^{to the}

measurements of the authors quoted, while the NCS group ^was assumed to be linear and 0.434 nm in

length. ^{In the} calculations, ^{account was taken of the}

Van der Waals radii of the terminal atoms.

The phase transition temperatures ^{were measured} both in the heating ^and cooling cycles. ^{For this} pur- pose ^a heated stage ^VEB Analytic ^Dresden polariza-

tion microscope ^{was used The} points ^{of the} phase diagram ^were determined by preparing separately weighed ^out portions ^for every mixture composition

to be tested In order to homogenize ^{the mixture} sample, ^the portions ^{of the} components weighed ^on

an analytical ^{balance were heated} together ^{to the} isotropization temperature ^{and mixed} accurately.

The measurements of density ^versus temperature

were carried out according ^to procedures ^described previously [19, 20]. ^{The other} type of dilatometer used in this work consists of a 7 cm 3 volume connected at the bottom with ^a capillary ^{0.5 mm in diameter, 35 cm} long, ^{and at the} top ^{with a} teflon needle valve. At the bottom of the vessel and of the capillary, ^about

2 + 4 cm 3 of degassed mercury is placed ^{The dilato-}

meter was then filled with degassed liquid crystal

under vacuum. The capillary ^{was connected to an}

external manometer allowing ^{a constant} pressure ^to be

kept during ^the measurements. The dilatometer ^was immersed in a circulating bath thermostat containing

701 of water and stable to ± ^{0.2 mK. The} temperature

was determined with a Tinsley ^Pt resistance thermo- meter and the heights ^{of a} mercury meniscus were

measured with the aid of ^a Wild cathetometer. The absolute volume of the dilatometer could differ in individual runs by less than 0.001 cm’ owing ^{to the use}

of a Teflon valve, thus the accuracy of the density

measurements is about 0.03 %. ^The determined _spe- cific density of the pure compounds (4DBT ^and 80CB)

and of their mixtures allowed us to calculate the molar

excess volumes of mixing ^{defined as :}

where : M80CB ^and V8ocB, ^and M4DHT ^and y4DBT ^are

the molar weights and volumes of _pure 80CB and 4DBT. The viscosities of the _pure compounds ^{and their}

mixtures were determined by ^{means of a viscosime-}

ter with a capillary diameter of 1 mm. The values of

viscosity ^{were found} by comparing with those of the standard oil (in ^{our case} p2ooc

0.8943; f/2ooC = 19.00 mPa. s).

The phase transition enthalpy ^{was measured} by

means of a Unipan ⁶⁰⁰ microcalorimeter with a DSC

adapter. The values of enthalpy ^{were read} directly

off the calorimeter integration ^{curves after} prelimi-

nary calibration by using ^standards (In ^{of 99.999} % purity, ^{Sn of 99.999} % purity).

3. Results.

3.1 EFFECT OF THE SMECTIC LAYER SPACING RATIO ON THE CHARACTER OF THE PHASE DIAGRAMS OF

80CB-nDBT ^BINARY MIXTURES. - In figures ^la-

1 d, 2a-2d and 3a-3b phase diagrams of 80CB-nDBT

binary ^{mixtures are} presented, ^{where n} varies successi-

vely ^{from 12 to 2} in the order of the increasing ^smectic layer spacing ^{ratio, r}

dSOCB/ dnDBT. The latter assu- mes the lowest value of 1.08 for the pair ^80CB-12DBT

and the highest ^one of 2.09 for the pair ^80CB-2DBT.

The first pair characterized by the smectic layer spac-

ing ratio close to unity ^{has N - S and I}

^S phase

transition temperatures lying ^{close to the} straight ^line connecting ^the phase transition points ^{of the pure} components. ^{In the case of the next} pair, ^80CB- 10DBT, ^{with a} larger ^r

1.22, ^{we observe a minimum} of the thermal stability of the smectic phase (Fig. 1 b).

This minimum deepens for the successive pairs ^{and it is}

most pronounced for the 80CB-8DBT pair (Fig. 1 d)

whose ^r is 1.36. The latter is the last pair in this series for which the smectic regions A, ^and Ad ^{are conti-}

nuous. For the next pair, ^80CB-7DBT (r

1.46) ^the continuity ^{of the smectic} regions At ^and Ad ^is interrupt-

ed (Fig. 2a). ^{The smectic} phase decays in the middle of the concentration range and is replaced by ^the

Fig. ^1.

The influence of the smectic layer spacing ^ratio

on the phase diagram. Bicomponent ^{systems with} lowering stability of smectic A phase. ^The phase transitions are

denoted by : 0 - melting point, 0 - smectic-isotropic transition, ^{0 -} nematic-isotropic transition, ^{+ - smectic-}

nematic transition.

1580

Fig. ^2.

The influence of the smectic layer spacing ^ratio

on the phase diagram. Bicomponent systems ^with

^nematic gap.

Fig. ^3.

^{The influence} of the smectic layer spacing ^ratio (r)

on the phase diagram. Bicomponent systems with nematic gap and high

nematic phase. ^{The observed} nematic gap is immedia-

tely wide, it appears in the concentration interval of 0.4 to 0.75 molar fraction of 80CB. For the successive

pairs ^of compounds ^with increasing ^{r, no further} great changes ^{are observed in the} diagrams, ^{however, the}

nematic _gap becomes wider, especially ^on the side of 80CB excess. For the 4DBT-80CB mixture the nematic _gap is observed in the concentration interval of 0.3 to 0.9 molar fraction of 80CB. Besides. the smectic Ad region ^{has a} parabolic shape ^{which makes}

it possible ^{to observe the reentrant nematic} phase, ^{in a}

limited _range of concentrations. Such a shape ^{of the}

SAd ^phase border is characteristic of 80CB [21 ] and

also of other smectics Ad [13,14]. ^On the side of 4DBT

excess we observe (Fig. 2d) ^an interesting ^extension

of the temperature range in which the smectic and nematic phases coexist in equilibrium. ^The tempera-

ture difference between the point ^{at which the smectic} phase appears and that at which the nematic phase disappears amounts to 5 deg ^{in the case of this} pair.

This phenomenon is observed on the right-hand ^{side of}

the triple point ^{at the} concentration _range of 0.1- 0.25 80CB molar fraction. Among ^other interesting changes observed in the phase diagrams is the shift of the triple point I, SA, ^{N towards} greater ^{nDBT con-} centrations in the mixture accompanied by ^{an increase}

of ^r. As soon as the smectic phase stability ^minimum

appears, the triple point ^shifts fairly rapidly ^{from the}

molar fraction value of 0.32 for 80CB-12DBT pair ^to

0.2 for the 80CB-IODBT _one; further more it has almost constant values. The next greater shift of the

triple point ^to the 80CB molar fraction value of 0.12 is observed for the pair 80CB-7DBT, which is the first to reveal a nematic gap. The solidus ^curves (solubility

curves of the solid phase ^{in the} mesophase) ^{of all the}

tested 80CB-nDBT pairs agrees with the theoretical CSL equation, fairly ^{well and this} equation ^{is in} parti- cularly good agreement with the section of the ^curve from the eutectic point ^to pure 80CB. This _proves that nDBT in the solid phase ^{has no} solubility ^{in 80CB.}

The clearing points show minima in all diagrams : ^the

smallest for the 80CB-12DBT pair ^{and the} deepest

for the 80CB-4DBT one. It is difficult to discuss at

present ^{the nature of this} minimum, ^{since the} position

of the N - I virtual phase transition temperature ^is

not known for the DBT series.

It is interesting ^{to note} that the nematic _gap does not decrease when ^r approaches ^to the value of 2.

Hence the DBT molecules cannot be arranged ⁱⁿ pairs ^of parallel ^or antiparallel orientation between the pairs ^{of 80CB} molecules, ^even if from the _geome- trical point of view it would seem that such an arran-

gement ^is privileged, ^{as is the case with 3DBT or 2DBT.}

The 80CB-4DBT system is characterized by ^{a wide}

nematic _gap and also shows the greatest ^{number of} various specific properties; ^{for these reasons it was}

chosen by ^{us for more} exhaustive testing. ^{For this} system ^{we measured the} density, viscosity, phase ^tran-

sition enthalpies ^{and studied the} scattering of X-rays ^as

a function of composition ^{so as to obtain as much}

information as possible ^{about the} properties ^and

internal structure of the nematic phase occurring

between the smectic regions A, ^and Ad.

3.2 VARIATION OF THE DENSITY OF THE 80CB-3DBT

MIXTURES WITH TEMPERATURE AND COMPOSITION.

In figure ⁴ the results are presented ^as plots P(t)x=const

of 80CB and 4DBT, and of their binary ^mixtures

carried out in the temperature range 50-80 OC. ^In table II the molar volumes of the pure compounds

and their binary ^{mixtures at selected} temperatures

are summarized

Table II.

Interpolated molar volumes (V) of ^4DBT (Mrool

^277.388 g mol-I) ^{and 80CB} _(Mmol

^257.440

g mot’*)./br ^chosen temperatures.

°

Fig. ^4.

^{Densities of the} system ^4DBT-80CB

^function of temperature.

At most temperatures the mixture is in a phase

different from those of its components. Only ^{at 82 OC}

and above, where the pure compounds ^{and their}

mixtures are in the isotropic phase, one can assume

that :

where VE is an excess quantity representing directly

the deviation of the tested solution from the ideal behaviour. The densities of the _pure 80CB and 3DBT

compounds vary with temperature ^{in a somewhat} different _way from those of their binary ^mixtures.

Apart ^{from the} neighbourhood ^{of the} phase ^tran-

sition point ^the density varies with temperature according ^{to the} general linear relation :

However, the thermal expansion coefficients _a, sum-

marized in table III, ^are different at different concen-

trations. In the _pure isotropic phases of 80CB and 4DBT, ^values higher than in the smectic phase ^are

observed This is normal, since the smectic phase

resists compression. ^For comparison, ⁱⁿ mixtures with 0.425 and 0.75 molar fractions of 80CB the thermal

expansion coefficient a is smaller in the isotropic phase than in the nematic phase (absolute value).

This might indicate that the lowering ^of temperature produces greater changes of the internal structure of the liquid in the nematic phase than in the isotropic phase.

In pure 4DBT the transition from the isotropic

to the smectic phase ^is manifested by ^a significant change ^of density (Fig. 4), the observed change ^{of the}

molar volume dY for the phase transition SAt -+ ^I

in this compound being ^2.8 cm3 . mole-1. This volume

change ^{and the} enthalpic effect discussed further in the text indicate that the transition is strongly ^{of the}

first order. An even greater change of the molar volume of 3.2 cm3 . mole - ’ accompanies the transition from the smectic to the isotropic phase in the mixture with a 0.175 molar fraction of 80CB. Here, however,

there is no direct SAt -+ ^I transition, but the transition

proceeds indirectly via the nematic phase : SAt

^N

and N - I. The changes of molar volume for the

latter transitions cannot be precisely determined in

view of the close vicinity of these transitions.

1582

Table III.

Values of ^{the thermal expansion} coefficients ^{a, the constant} po and molar volume change A V of phase

transition N -+ I or SA -+ ^I of pure 4DBT and 80CB and their mixtures.

The SA,, --+ ^{N phase} transition in _pure 80CB is not manifested by ^a change of the molar volume or of the coefficient _a, whereas the N - I phase transition is

accompanied by ^{a moderate} change ^{of volume}

AV

1.3 cm3.mole-t. Our result for _pure 80CB differs from the observation on 60CB-80CB mixture

by Cladis et al. [21]. They have found that l/p(1) changes ^its slope ^{at the} SA -+ ^N transition.

In the remaining binary mixtures tested (XSOCB

0.425, ^{0.75 and} 0.9) which reveal only ^one phase ^tran-

sition ^{in the} region ^{of the} mesophase flU - I) ^the

transition from the nematic to the isotropic phase ^is accompanied by ^an increase of molar volume equal ^to

or close to that observed ⁱⁿ _pure 80CB.

The density ^{of the} tested mixtures changes ^{with their} composition, ^the greatest changes being ^observed

at the limits of the binary system. ^These changes ^are

illustrated in figure 5 where the molar volumes of

mixing YS ^are presented ^{as a function of} temperature and composition. ^The binary system ^80CB-4DBT reveals a distinct increase of molar volume ^{as a} result of mixing except ^{for the} composition X SOCB

^0.95.

For the latter composition the molar volume decreases

(YS ^{assumes a} negative value) ^{as a result of} mixing.

The greatest changes ^{of the} mixing ^{volume YS or the} greatest changes ^of density ^{are observed at concen-}

trations of 80CB in the mixture below a molar frac- tion of 0.2. Thus introduction of 80CB into 4DBT

produces ^a strong increase of the molar volume. In the case of induced smectic phase ^the depression ^{of the}

Fig. ^5.

Molar volume of isothermal mixing ^{of the} system 4DBT-80CB

function of temperature.

molar volume achieves its maximum at approxima-

tely ^equimolar concentrations of the components [7].

ence the phenomenon ^of the nematic _gap in smectics is not directly ^{reverse to} that of smectic phase ^induc-

tion observed in nematics. Probably the mechanisms

of intermolecular reaction are in both phenomena

not only ^{reverse but also} completely different. This is also confirmed by ^the high ^{value of YE observed in the} isotropic phase. The maximum molar mixing ^volume

of 80CB and 4DBT in the isotropic phase ^is

3.2 cm3 . mole - 1 and is indeed higher ^{than the volume}

contraction of the isotropic phase ^{of the mixture in}

which the smectic phase is induced For instance, ^it reaches the value of about - 0.5 cm3 mole-1 for the

4-ethyl-4’ pentylazoxybenzene-PCB binary system [7].

If the isotropic liquid is formed from compounds ^{in the}

smectic phase, ^{the observed} change of volume is of

course still higher and for the tested system ^80CB- 4DBT, V’

^4.98 cm3 . .mole-1 (Fig. 5). ^{In the latter} binary system, the formation of the nematic from the smectic phase ^is accompanied by ^{an increase of the}

molar volume of V’

3 CM3 mole -1. This value is similar (considering ^{its absolute} value) ^to the observed decrease of molar volume accompanying the formation of the smectic from the nematic phase ^{in the} 4-ethyl-4’- pentyl-azoxybenzene-PCB system (VS = - 2.5cm3.

mole -1) [7].

3.3 VARIATION OF THE VISCOSITY OF 80CB-4DBT

MIXTURES WITH TEMPERATURE AND COMPOSITION.

The viscosities of mixtures in the isotropic and nematic phases vary with temperature according ^{to the} expo- nential equation :

The activation _{energy A} of mixtures in the nematic

phase is lower than that of _pure 80CB. The values of

A for mixtures containing 0.425, 0.75, 0.9 and 1.0 molar fractions of 80CB are 0.37 eV, ^0.37 eV, 0.43 eV and 0.48 eV, respectively.

The viscosities of nematic mixtures are much lower than that of _pure 80CB in the nematic phase ^{at the}

same temperature. ^{As far as the} viscosity ^{of 80CB-}

4DBT mixtures varies in the isotropic phase propor-

tionally ^to concentration, it reaches in the nematic

a minimum for the concentration _XSOCB

0.4. Hence

we can conclude that nematic mixtures with ^a _compo- sition corresponding ^to the central part of the nematic gap reveal the lowest viscosity. The mixture with X80CB

0.425 has a viscosity of 30.5 and 49.5 mPa. s at 30 and 20 OC, respectively. These values are only slightly higher ^than the viscosities of 4-alkyl-4’-cyano- biphenyls ^{and the same as those of} 5-alkyl-2-(4’- cyanophenyl)pyrimidines [22].

3.4 DSC MEASUREMENTS.

Figures ^{6a and 6b pre-}

sent the variation of clearing enthalpy (AHs_j ^or AHN-I) ^{and S- N} phase transition enthalpy (AHs-N)

with concentration of 80CB-4DBT mixtures. The

clearing enthalpy ^of pure 4DBT is 4 kJ. mole - 1 and that of _pure 80CB is smaller and amounts to 0.88 kJ . mole-1. The latter value is in fairly good

agreement ^{with that} given in the literature for this

compound (0.98 kJ.mole-’) [23]. The value of

AHS-N measured for our sample ^{is 22 J . mole - 1 and is}

smaller than that given by ^Cox (78 ^J. mole - 1) [23].

Fig. ^6.

Enthalpies of transitions SA - ^I

^{N -+} I (a) ^and SA - ^N (b) ^{of the} system ^4DBT-80CB.

The clearing enthalpy ^decreases rapidly ^to XSOCB

0.3 when 80CB is added to pure 4DBT, upon which it remains almost constant. In the concentration range XSOCB

0.3 to 0.95 the values. of enthalpy ^{lie on a} straight ^{line with a small} inclination (Fig. 6a). ^This

allows us to extrapolate ^{this line to} XSOCB

0 and to obtain the hypothetic enthalpy of the virtual transi- tion N - I in _pure 4DBT. The clearing enthalpy

of the nematic phase ^{in 4DBT} found in this _way is

AHN-I

^{0.3 kJ. .mole-1.} Extrapolation ^towards pure 80CB yields ^the clearing enthalpy ^{for this} compound equal ^{to 0.6 kJ . mole - 1. This} enthalpy ^could possibly

be the clearing enthalpy ^{of pure 80CB} composed ^of

monomers since in the mixture with 4DBT the state

consisting ^{of 80CB monomers is more} privileged ^than

in _pure 80CB. Particularly characteristic are the

changes ^of enthalpy ^{of the} SA -+ ^N phase ^transition

with concentration of the 80CB-4DBT mixture

(Fig. 6b). ^The enthalpy ^{of the} SA,, -+ ^N phase ^transi-

tion decreases rapidly ^{to zero} when 4DBT is added to pure 80CB (AHS-N

^{0 at} jCgocB

0.96). ^{On the} opposite side of the diagram, where 80CB is added to the _pure 4DBT, ^the enthalpy ^{of the} SAt -+ ^{N phase}

transition initially increases, ^and then decreases

rapidly ^{to zero} (AHS-N

^{0 at} XSOCB

0.22). ^{In the} vicinity of the nematic _gap the S -+ N phase ^transi-

tion becomes a transition of the second kind. This is

probably ^a general feature of systems ^{in which a} nematic _gap is observed between the smectic regions.

In a similar _way it was ascertained that the enthalpy

of mixtures of two smectics A1 changes [8].

3.5 X-RAY DIFFRACTION.

X-ray photographs ^of

all the tested samples of the 80CB-4DBT mixture

reveal the _presence of only ^one internal reflection in

the form of non-split spots distributed symmetrically

1584

with respect ^to the initial beam. The variation of the smectic (or quasi-smectic ^{in case} of nematic phase)

order spacing ^obtained from that reflexion is shown in figure ^{7. In the case of} low concentrations of 80CB in the mixture it is approximately proportional ^{to the} weighted ^mean lengths ^of the 80CB and 4DBT mole- cules, ^{and for} higher ^80CB concentrations

to the

weighted ^mean lengths of the 80CB dimer and 4DBT molecule. The internal reflexion which is observed ^{as a} sharp spot ^{in the cases} of the pure ^com-

pounds ^{becomes diffuse, in the} region of the nematic gap. The X-ray photographs ^{obtained for mixtures}

of the concentration XSOCB

^{0.425 and 0.75 are} typi-

cal for poorly ^ordered nematics which have an ordi- nary nematic phase with small short correlation length

of smectic-like ordering.

Our X-ray picture of the nematic phase existing

between the two smectic phases At ^and Ad ^{is thus diffe-}

rent from that obtained for the reentrant nematic

phase which also occurs between the SA1 ^and SAd phases. ^{In the case of the reentrant} phase ^{we observe}

the _presence of two internal reflexes, ^{due to} X-ray scattering, of different dimensions : one characteristic for the A, phase ^and the other for the Ad ^one [24].

4. Conclusion.

The main factor decisive for the appearance of dis- continuities of the smectic regions ^{in mixtures of}

smectic A, ^and Ad is the difference in the spacings ^of

the smectic layers. The nematic _gap is observed when the ratio of the smectic layer spacings ^{in the compo-}

nents of a mixture is >, 1.4. The nematic phase ^exist- ing ⁱⁿ the central region of the nematic _gap is a normal nematic phase ^{and does not} reveal the presence of

cybotactic ^smectic structures with molecular dimen- sions typical ^{of the} pure components; it also has a

viscosity ^{which is} typical of nematics. The difference in length ^{of the} alkyl ^{chains is not} essential for this behaviour either. The nematic gap is observed in the mixture of the pair ^of compounds ^{80CB-7DBT whose} alkyl chains differ by only ^one methylene group. It

seems that the chemical structure and direction of the

dipole ^{moment have some effect on the} ability ^{of the}

smectic system ^to transform into a nematic one. The

Fig. ^7.

^Smectic ordering spacings ^{in mixture 4DBT-}

80CB

a function of mole fraction from X-ray diffraction (inner reflection).

nematic gap ^{was so} far observed in mixtures of com-

pounds which have a polar ^{group in the terminal} posi-

tion with compounds in which the dipole ^{moment of}

other polar groups in the molecule is in accordance with the direction of the dipole ^moment of the termi- nal _{group. In} reference [14] ^{it was found} that, ^{if the} dipole ^{moments are} arranged in the molecule discor-

dantly, the nematic _{gap may} not be observed even if the condition regarding the smectic layer spacing ^ratio

is fulfilled

In our opinion ^the phenomenon which has been described in the present ^work may be significant ^from

both the practical and theoretical points of view. It allows us to obtain nematics of relatively ^low viscosity

from smectics and shows how ^{one can} _prevent ^the

formation of smectic phases in mixtures at low tem-

peratures. ^{In this} way the number of liquid crystalline

substances that _may be used for preparing ^nematic

mixtures or smectic mixtures with ^a definite nematic interval is increased

Besides the results obtained lead us to the conclu- sion that when identifying ^smectic phases by ^{the misci-} bility ^{method the} standard substance should be select- ed in such a way ^{as to ensure} that its smectic layer spacing ^be fairly ^{close to} that in the identified com-

pound

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