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> HAL Id: jpa-00210357 https://hal.archives-ouvertes.fr/jpa-00210357 Submitted on 1 Jan 1986 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. J. Physique 47 (1986) 1577-1585 SEPTEMBRE 1986, 1577 Classification Physics Abstracts 61.30E - 74.70M A nematic gap in mixtures of smectics K. A1 and Ad (*) 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) Des études de l’influence du rapport r des épaisseurs des couches smectiques des composants d’un mélange binaire sur 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 comme exemples. Quand r augmente la stabilité de la phase smectique en mélange diminue et pour r ~ 1,4 on observe un intervalle nématique séparant les régions des smectiques A1 et Ad. Pour le système binaire particulier 80CB-4DBT on a examiné les densités, les viscosités et les données de la diffusion des rayons X en fonction de la composition et température. Enfin les enthalpies des transitions de phase ont été mesurées. Le nematique 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 Résumé. en 2014 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 one 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 a nematic gap separating the smectics A1 and Ad is observed The density, viscosity and scattering of X-rays as a 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 a viscosity and structure characteristic for typical nematics. The binary system increases its molar 0.2. volume as a result of mixing and assumes a maximum in the concentration range x8OCB 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 components 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 was 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 smectic 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 is a monothe terminal position of the molecule layered smectic A (A, type), and the second compoan ester with cyano or nitro group in the ternent is partially a double-layered smecminal position tic A (Ad type). Next, we observed the nematic gap in binary mixtures 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 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 one a 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 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 homologous 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 consisting 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 compounds lie in the range of 70-80 OC and their melting points between 30 and 60 OC [15]. 4-octyloxy-4-cyanobiphenyl 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 molecule equal to 1.37. In the present work it is shown how the character of the phase diagram changes with the smectic layer - ture. exhaustive investi- gations of their physical properties. We have found compounds which are more convenient for this pur- Table I. of the components making up the mixthe variations of density, viscosity and Next, N - I and SA --+ N phase transitions the of enthalpy 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. spacing ratio The structural formulae, phase 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 compounds 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 temperature 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- transition temperatures, spacing (d) for compounds used for preparing the binary mixtures : lengths of molecules (1) and smectic layer 1579 phenyl)dioxane-1,3 molecule. The length of the alkyldioxylphenyl radical was taken according to the measurements of the authors quoted, while the NCS 3. Results. group 80CB-nDBT BINARY MIXTURES. - In figures la1 d, 2a-2d and 3a-3b phase diagrams of 80CB-nDBT binary mixtures are presented, where n varies successively from 12 to 2 in the order of the increasing smectic layer spacing ratio, r dSOCB/ dnDBT.The latter assumes 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 spacing 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, 80CB10DBT, 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 continuous. For the next pair, 80CB-7DBT (r 1.46) the is and of the smectic regions At Ad interruptcontinuity ed (Fig. 2a). The smectic phase decays in the middle of the concentration range and is replaced by the 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 purpose a heated stage VEB Analytic Dresden polarization 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 dilatometer 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 thermometer 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 specific density of the pure compounds (4DBT and 80CB) and of their mixtures allowed us to calculate the molar excess volumes of mixing defined as : was 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 viscosimeter 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 preliminary calibration by using standards (In of 99.999 % purity, Sn of 99.999 % purity). 3.1 EFFECT OF THE SMECTIC LAYER SPACING RATIO THE CHARACTER OF ON THE PHASE DIAGRAMS OF = = = = layer spacing ratio 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- Fig. on 1. the - The influence of the smectic nematic transition. 1580 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 temperature 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 0.10.25 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 concentrations 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 particularly 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 geometrical point of view it would seem that such an arrangement 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 transition 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. SAd phase Fig. on 2. the The influence of the smectic layer spacing ratio phase diagram. Bicomponent systems with a nematic - gap. 3. The influence of the smectic layer spacing ratio (r) the phase diagram. Bicomponent systems with nematic gap and high r. Fig. - on nematic phase. The observed nematic gap is immediately 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 OF THE DENSITY OF THE 80CB-3DBT MIXTURES WITH TEMPERATURE AND COMPOSITION. 3.2 VARIATION - 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 1581 Table II. g - mot’*)./br Interpolated molar volumes (V) of 4DBT (Mrool = chosen temperatures. 277.388 g mol-I) and 80CB (Mmol ° = 257.440 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 transition point the density varies with temperature according to the general linear relation : However, the thermal expansion coefficients a, summarized in table III, are different at different concentrations. 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 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 to the smectic Fig. 4. Densities of the system 4DBT-80CB of temperature. - as a function 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 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 or SA -+ I of pure 4DBT and 80CB and their mixtures. transition N -+ I 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 transition 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 concentrations of 80CB in the mixture below a molar fraction 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 A V of phase = = = Fig. 5. Molar volume of isothermal mixing of the system 4DBT-80CB as a function of temperature. - molar volume achieves its maximum at approximaequimolar concentrations of the components [7]. ence the phenomenon of the nematic gap in smectics is not directly reverse to that of smectic phase induction observed in nematics. Probably the mechanisms of intermolecular reaction are in both phenomena tely 1583 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 not 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 80CB4DBT, 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 = 3 CM3 mole -1. This value is molar volume of V’ 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. Fig. 6. Enthalpies of transitions SA - I or N mole -1) [7]. SA - N (b) of the system 4DBT-80CB. - 3.3 VARIATION OF THE VISCOSITY OF -+ I (a) and 80CB-4DBT MIXTURES WITH TEMPERATURE AND COMPOSITION. - The viscosities of mixtures in the isotropic and nematic phases vary with temperature according to the exponential 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 80CB4DBT mixtures varies in the isotropic phase proportionally to concentration, it reaches in the nematic a minimum for the concentration XSOCB 0.4. Hence we can conclude that nematic mixtures with a composition corresponding to the central part of the nematic gap reveal the lowest viscosity. The mixture with 0.425 has a viscosity of 30.5 and 49.5 mPa. s X80CB at 30 and 20 OC, respectively. These values are only slightly higher than the viscosities of 4-alkyl-4’-cyanobiphenyls and the same as those of 5-alkyl-2-(4’= = cyanophenyl)pyrimidines [22]. 3.4 DSC MEASUREMENTS. Figures 6a and 6b present 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]. 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 0.3 to 0.95 the values.of enthalpy lie on a XSOCB straight line with a small inclination (Fig. 6a). This 0 and to allows us to extrapolate this line to XSOCB obtain the hypothetic enthalpy of the virtual transition 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 transition decreases rapidly to zero when 4DBT is added to 0 at jCgocB pure 80CB (AHS-N 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 transition 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 molecules, 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 compounds becomes diffuse, in the region of the nematic gap. The X-ray photographs obtained for mixtures 0.425 and 0.75 are typiof the concentration XSOCB which have an ordinematics cal for poorly ordered correlation length small short with nary nematic phase of smectic-like ordering. Our X-ray picture of the nematic phase existing between the two smectic phases At and Ad is thus different 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 discontinuities 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 components of a mixture is &#x3E;, 1.4. The nematic phase existing 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 dimensions 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. 80CB - Smectic ordering spacings in mixture 4DBTfunction of mole fraction from X-ray diffraction as a (inner reflection). nematic gap was so far observed in mixtures of compounds which have a polar group in the terminal position 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 terminal group. In reference [14] it was found that, if the dipole moments are arranged in the molecule discordantly, 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 temperatures. 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 conclusion that when identifying smectic phases by the miscibility method the standard substance should be selected in such a way as to ensure that its smectic layer spacing be fairly close to that in the identified com- pound References [1] DOMON, M., BILLARD, J., J. Physique Colloq. [2] [3] [4] [5] 40 (1979) C3-413. SHARMA, N. K., PELZL, G., DEMUS, D., WEISFLOG, W., Z. Phys. Chemie (Leipzig) 281 (1980) 579. ENGELEN, B., SCHNEIDER, F., Z. Naturforsch. 33a (1978) 1077. FUKUI, M., MATSUNAGA, Y., Bull. Chem. Soc. Japan 54 (1981) 3137. CLADIS, P. E., Mol. Cryst. Liq. Cryst. 67 (1981) 177. [6] SZABON, J., JÁNOSSY, I., Advances in Liquid Crystal Research and Applications, ed. Bata, L. (Pergamon Press, Oxford-Akademiai Kiado, Budapest) 1980, p. 299. [7] SADOWSKA, K. 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