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THE EFFECT OF THE MOLECULAR STRUCTURE OF THE CHOLESTERIC COMPONENT ON THE
COMPOSITION DEPENDENCE OF THE RECIPROCAL HELICAL PITCH IN BINARY
CHOLESTERIC MIXTURES
I. Gorina, M. Rubtsova, I. Chistjakov
To cite this version:
I. Gorina, M. Rubtsova, I. Chistjakov. THE EFFECT OF THE MOLECULAR STRUCTURE OF THE CHOLESTERIC COMPONENT ON THE COMPOSITION DEPENDENCE OF THE RECIP- ROCAL HELICAL PITCH IN BINARY CHOLESTERIC MIXTURES. Journal de Physique Collo- ques, 1979, 40 (C3), pp.C3-229-C3-233. �10.1051/jphyscol:1979345�. �jpa-00218741�
JOURNAL DE PHYSIQUE Colloque C3, supplément au no 4, Tome 40, Avril 1979, page C3-229
THE EFFECT OF THE MOLECULAR STRUCTURE
OF THE CHOLESTERIC COMPONENT ON THE COMPOSITION DEPENDENCE OF THE RECIPROCAL HELICAL PITCH
IN BINARY CHOLESTERIC MIXTURES
1. 1. GORINA, M. Ju. RUBTSOVA and 1. G. CHISTJAKOV The Crystallography Institute of the Academy of Sciences of the USSR,
Leninski prospect 59, Moscow, U.S.S.R.
Résumé. - Nous avons étudié la dépendance de concentration du pas inverse de la spirale des mélanges binaires du cholestéryle-chlorure (CC) avec des cholestéryle-benzoate (CB) substitués et B-haloïde-cholestéryle-alcanoates (HCA). La dépendance est linéaire pour les mélanges CC et HCA et non linéaire pour CC et CB. Les résultats expérimentaux sont interprétés en tenant compte de la construction électronique des molécules des composants des mélanges cholestériques.
Abstract. - Composition dependence of the reciprocal helical pitch has been investigated for binary mixtures of cholesteryl chloride (CC) with substituted cholesteryl benzoates (CB) and /3- halogeno cholesteryl alkanoates (HCA). The dependence is linear for CC
+
HCA and nonlinear for CC+
CB. The experimental results are considered, taking into account the electronic structures of the molecules of the components of the cholesteric mixture.1. Introduction. - The investigation of the compo- sition dedendence of the reciprocal helical pitch in binary cholesteric and nematic-cholesteric mixtures has been carried out in many laboratories in the world for the last decade [l-111. The experimental study shows that in cholesteric binary systems this depen- dence is close to linear [l, 3-5, 121 or is nonlinear [l, 10, 13, 141 and it is as a rule nonlinear in nematic- cholesteric mixtures [6-8, 10, 11, 151.
A series of theoretical models has been proposed [2, 6, 8, 10, 13, 161 to explain a nonlinear character of composition dependence of the pitch. The most interesting approach is, in Our opinion, the one of Pochan and Hinman [17] advanced for nematic- cholesteric mixtures. The authors proceed from an assurnption that nematic-cholesteric mixtures behave as liquid solutions of organic substances in the sense that a direct interaction exists between the molecules of nematic and cholesteric substances and causes the formation of associates sirnilar to molecular com- ,plexes. As is known it is the association that produces a nonlinear composition dependence of properties in solutions of organic substances [16]. Depending on the strength of the intermolecular interaction, the extent of association and the structure of the associates the composition dependence of, for example, the di- electric constant of the solution of organic compounds is expressed by curves similar to ones that express the composition dependence of the pitch in binary nema-
tic-cholesteric mixtures [7, 8, 10, 17, 181. Since, however, systematic experimental investigations of the nature of the nonlinearity in nematic-cholesteric mixtures are not available (because of the difficulties of experimenting perhaps), the idea of the associated state of the mixtures is not experimentally coniîrmed as yet. In this connection we believe that for under- standing the physical base of the intermolecular inter- actions in cholesteric and nematic-cholesteric mixtures the study of the influence of the molecular structure of the mixture components on the nature of 1/Â. vs concentration dependence has a large importance.
A similar study has been attempted recently for nematic-cholesteric mixtures [19].
In the present article we consider the effect of mole- cular structure and, in particular, of the electronic structure of the mixture components on the composi- tion dependence of reciprocal helical pitch in binary cholesteric mixtures. We consider also the possibility of the associated state in cholesteric mixtures.
2. Experimental methods. - As an object for the investigation we chose two series of cholesteric substances dfierent in chemical structure and pro- perties : 1) para and ortho substituted cholesteryl benzoates (CB) and 2) P-haloid cholesteryl alkanoates (HCA). Some characteristics of these substances are given in table 1. We have studied the binary cholesteric mixtures of the compounds presented in table 1 with
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979345
1. 1. GORINA. M. Ju. RUBTSOVA AND 1. G. CHISTJAKOV
Some characteristics of cholesteryne derivatives
The derivatives of cholesteryl benzoate The derivatives of cholesteryl alcanoate
Substituent - H para CH, para C,H,
para C,H,O para NO, para Br ortho Br
TS-,, Tc,-,,, Rotation
OC OC sense
- - -
149 178 left
179 246 -
-
149 165 -
191,5 260 -
179 267,5 -
105 134 -
Ts-ch>
NO Substituent OC
-- - -
8 8-Cl propionate 85,8 (50,5) 9 8-Br propionate 92 (58) 10 8-Br butirate 124,5 11 8-Cl butirate 108 (82) 12 8-Br capronate 105,8 (75,8) 13 a-Br stearate 47,4 (**)
Tc,-,,, Rotation Pintr
OC sense A
- - - 132 left 3 300
120 -
129 - (*)
1!8
- (*)
Il9 2 700
50,5 - 2 O00
Note. - The temperatures of phase transitions for cholesteryl benzoates are taken from [20] ; P,,,, of p-haloid cholesteryl alcanoates are determined by the aiiihors ; (*) the value of P,,,, is not determined because nf fast crystallizatinn nf the mixture ; (**) given is the value of Tsm-ch.
the cholesteryl chloride (CC). To obtain the compo- sition dependence of the reciprocal helical pitch of the mixtures we measured the wavelength of the maximum selective reflection lo of the mixtures at the room temperature. According to the work [21]
A. = Z P , (1)
where Z is the average refractive index and P the helical pitch. Since, however, 7i is only slightly depen- dent on, for instance, the temperature [22] or compo-
cce ( ~ t ~ / ~ )
FIG. 1. - Composition dependence of the reciprocal helical pitch for binary cholesteric mixtures of cholesteryi chloride with haloid cholesteryl alcanoates : 1. 8-Cl cholesteryl propionate; 2. 8-Br
cholesteryl capronate ; 3. a-Br cholesteryl stearate.
sition of the mixture [23, 241 the dependence of 2, and P on these parameters is of the same type. For this reason in the experimental treatment
A,
is often used instead of the helical pitch P. We have obtained the curves for the dependence of l/AO in binary cholesteric mixtures on the weight concentration of CC ; these are presented in figure 1 and figure 2. From these curves values of 111 have been obtained by using the extrapolation methods described in 13, 141 and from the eq. (1) intrinsic helical pitchs Pint, have been$termined for the substances investigated; these are presented in table 1. The sense of rotation was determined by the contact method [26].
3. Experimental results and their discussion. -
It can be seen from figures 1 and 2 that the compo- sition dependences of l/Ao for the mixtures of CC with HCA and of CC with CB are sharply different : the plots for the first mixtures are close to linear, for the latter these shaw a nonlinear behaviour, the deviation from linearity (the curvature) being markedly chang- ing according with the chemical nature of the substi- tuent and its position in the phenyl nucleus of the cholesteryl benzoates in the following order :
(the curvature decreasing from left to right). In the mixtures CC
+
HCA the curvature is weak. In the mixtures of CC with a-Br-cholesteryl stearate (Fig. 1) the situation is analogous to that of binary cholesteric systems in which one or both components have the smectic mesophase, namely : when the percentage of the component that have both cholesteric and smectic mesophases (a-Br-cholesteryl stearate here) becomes large (80-85%)
the linear dependence changes into spikelike one 127,281. According to [27] the appearanceEFFECT OF MOLECULAR STRUCTURE OF CHOLESTERIC COMPONENT C3-231
- 2.4 1 I
b)
FIG. 2. - a, b Composition dependence of the reciprocal helical pitch for binary cholesteric mixtures of cholesteryl chloride with cholesteryl benzoates : 1. nonsubstituted cholesteryl benzoate (CB) ; 2. para N0,CB ; 3. para etoxy CB; 4. para n-butyl CB ;
5. ortho BrCB ; 6 . para BrCB ; 7. para metyl CB.
of the spike is explained by the heterogeneous struc- turization of the mixture because of the formation of smectic clusters in the pretransition range at the cholesteric-smectic interface. We don't consider this type of nonlinearity because it has been investigated earlier.
The behavior of the cholesteric systems (Figs. 1 and 2) indicates that the nature of interaction between the molecules in CC
+
CB systems is different fromthe one in CC
+
HCA systems and the cause is likely to consist in the difference of the electronic structure of the molecules of the mixture components. Pro- ceeding from the general assumptions of the theory of electronic structure of organic substances [29, 301 cholesteryl benzoates have to reveal resonance because we can assign to them two extreme indistinguishable electronic structures :Because of the resonance the molecules of CB are rigid and have high thermal stability (table 1). The presence of the substituents at the aromatic nucleus affects difFerently on the resonance of CB. The substi- tuents that are in para position, especially those that reveal resonance themselves (as for instance NO, group), strengthen resonance and accordingly increase the thermal stability of cholesteryl benzoates (table 1, no 4 and 5). The substituents at meta and especially at ortho positions inhibit the participation of the- COO-groups in the resonance of the phenyl nucleus.
Because of such inhibiting the resonance the molecules of CB become more flexible and their thermal stability decreases (table 1, no 7). The steric factor also inhibits the resonance. A clear interconnection exists between the changes of the resonance in the molecules of substituted cholesteryl benzoates (and therefore of their electronic structure) and the curvature of l/A, vs concentration curves (Fig. 2, a and b). The more detailed explanation will be given below.
As distinct from cholesteryl benzoates cholesteryl alcanoates (CA), haloid cholesteryl alcanoates and cholesteryl chloride are not capable to resonance ; their electronic structure is detennined by the induc- tive effect ( I J) which causes the polarization of the molecules because of shifting the electrons of o-bonds along the atomic chain :
CH, -+ CH2 -+ CH, -+ CH, -+ C-OCZ7H4, O
Il
For this reason we can consider the homological series of CA, HCA and CC as similar from the view of the electronic structure despite the daerence in their chemical structure and despite the fine distinctions associated with the direction and strength of the inductive effect depending on the nature of the substi- tuents at the saturated carbon atom and on the
C3-232 1. 1. GORINA, M. Ju. RUBTSOVA AND 1. G. CHISTJAKOV
hydrocarbon chain length [29, 301. Our qualitative consideration of the similarness and distinctions of the electronic structure of CA, HCA, CC and CB are confirmed by the spectra of intrinsic adsorption for the molecules of these substances [31].
Taking the above into account we can expect that when, for example, CC is mixed with HCA (or CC with CA, or HCA with HCA and etc.) additivity in the properties of these systems will be observed, since the molecules of the components will behave as inert diluents because of the similarity of their electronic structure. This is confirmed by composition depen- dence of 1/P we have obtained for the mixtures of CC with HCA : as it can be seen from figure 1 these plots are almost linear. This is confirmed also by the data avalaible in literature. According to [Il] the linear dependence is characteristic of the mixtures of CC with CA ; of CA (one with another) [2] ; of cholesteryl pelargonate with steryl chlorides [27, 281 ; of cho- lesteryl oleyl carbonate (COC) with CA [14] and with simple cholesterine esters 1121 and etc.
The slight deviations from linearity which occur in such systems seem to be the result of the fine distinc- tions (mentioned above) in electronic structure of component molecules, these distinctions affecting the intermolecular interactions. And, vice versa, we can expect that considerable deviations from additivity will be observed for the properties of the systems strongly different in electronic structure of their molecules, for instance, because of the specific interactions between the molecules different in nature, if these interactions make a considerable contribution into the (universal) van der Waals interactions [32, 331 and, perhaps, cause the formation of molecular associates in the mixture. Indeed, the concentration plots of 112, for mixtures of CC
+
CB are not linear (Fig. 2, a and b). The substituent groupes such as NO,, OC,H, strengthen the resonance in the mole- cules of CB in comparison to the nonsubstituted CB, and as a result the difference in electronic structure of the molecules in systems of CB+
CC increases what correlates with increase of the curvature of the l/Â, vs concentration plots (Fig. 2a, curves 1, 2, 3). The substituent groups Cl, CH,, C,H,, on the contrary, decrease the resonance in the CB and accordingly the curvature of the l/ÂO vs concentration plots becomes less markedly expressed in comparison to the non- substituted CB (Fig. 2a, curve 1 ; Fig. 2b) and especially in comparison to para-NO, and para-OC,H, substi- tuted CB (Fig. 2a, curves 2 and 3 ; Fig. 2b). TheRefer [l] ADAMS, J., HAAS, W., WYSOCKI, J., Bulf. Amer. Phys. Soc.
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The following fact should be noted : as we men- tioned above, the mixtures of CC with CA (or with COC and etc.) have a linear composition dependence of the reciprocal pitch. However, the mixtures of the first member of the homological series of CA (the cholesteryl formiate) with CC or with other members of the series reveal nonlinearity [ll], which can also be explained by the difference in electronic structure of the component molecules. Indeed, as it appears from the highly resolving NMR spectroscopic inves- tigation of the cholesteryl alcanoates [34] cholesteryl formiate differs from the other homologues : its C-O bond of the-C-O-group is similar in nature
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