Ionic residual conduction in the isotropic phase of a nematic liquid crystal

Volume 9, number4

iONIC

CHEMICAL PHYSICS LETTERS

RESIDUAL OF

CONDUCTION A NEMATIC

IN

THE

LIQUID

IS iGay I971

ISOTROPIC

PHASE

CRYSTAL

G. BRIERE, F. GASPARD and R. HERINO Wnioevsit& Scientifkpz et hGdkaZs de Gre-EnobZe, Laboratoire dc Physique Expkrimentule, Domaine Universtkzire, 38 - SainthYatiind’H&res, France

Received 14 December 1970

Electrical mcasuromunts were performed by tbc clcctrodialysis method at 124°C. an the isotropic pkhnseof a liquid crystal (E.P.P.H. : pfcthoxy phenyfazo) phenyl heptanoatn, nemetic raogc 6%119°C). A discussion of the results. based on the relaxation of ionic dissociation equilibrium dispraced by elec-

trolysis, shows that the electrical behaviour of the purest liquid is not specific hut arises from welldefined natural siow dissociation of impurities in minute concentrations, The dissociation and recombination kinetic cowlants of an overall equivalent elccrrolyte, and the mobilitics of resulting ions, zlrc

determined.

1. INTRODUCTION The extensive study of nematic liquid crystal properties under an applied electric field, has not yet allowed the authors to propose a tenable interpretation on measured iow conductivities. Some hypotheses, like field assisted dissociation of neutral. molecules [l], or charge carriers injection by the electrodes [2,3] have not been born out by experiment, owing to the complexity of the electrical con~ction behavfour. The observed conductivity of a sample under an applied electric stress arises from the superposition of various phenomena, which are difficult to control and separate, such as: 1. secondary electrochemical reactions of injection at the electrodes, 2. macroscopic space charges leading to large electric field disturbances, 3. convectitm currents from thermnl or electrohydrodynamic origin [2,3]. It has been

possible fn particular =xperimentaL conditions, to get rid of these erratic phenome~ and to obtain experimental evidence of the reproductive

feature of the resirtual bulk con&Ictivity in isotropic E.P.P.R., due to the ionic dissociation of residual impurities. These impurities are weak electrolytes and are supposed to give rise to the following equilibrium:

% (T’

=A+ A B V

i- B-

?z*=12-

=

?2*

%: ionic concentration, v : AB concentration, kD and k~ are the corresponding dissociation and recombinatllon kinetic constants. The kinetic rate of this natural equilibrium is characterize by the chemical relaxation time ~R=I/~(~R&RI# 4 3, introduced by Langevin. The study of the displacement by an electric field of the dissociation equilibrium of a weak electrofyte has been reported elsewhere 141. 0niy the main results xill be reported for discussion in the next section. 2. THEORY The discussion v&d in the case of a weak electrolyte exhtbit~g a slow chemical rate (in comparison Mth the electrolysis rate) in a Eondissociated solvent, enables the system to be characterized by the three main following points.

2.1 Tlie stabiowry current-uolfage curve The stationary current-voitagye curve shows two parts, before injection processes occur:

285

CHEMICAL PHYSICSLETTERS

Volume 9. number 4

15 May 1971

s : Ohm’s law is &eyed, in the a)O
2.2 Electrical ? elaxaiiovz of condzcctivity When the system is in a steady state during the application of a voltage greater than Vs., an electrical relaxation towards a new state of displaced equilibrium t ohserved, if a different voltage (also greater than V,) is suddenly applied. The intensity of the transient current obeys the equation: i(t) - is = (i. - is) m[-2f/7T]

;

(1)

fg is the current intensity for I = 0 [4]. 2.3. Chevnical relaxation of covlducfivily If the voltage is cancelled when the system is in a displaced equilibrium slate, then a relaxation towards the natural thermal equilibrium is observed. One can follow this behaviour by means of !ow-voltage impulse measurements in order to keep this natural relaxation undisturbed. We have : i(j) =ie “xp[f/Tl exp[//r]

- p +B ’

(2)

with P=

ie _ 2.0 i,;

“0 is the intensity when 1 = 0 and ie is the intensity when C = EQ[4]. Th ose different phenomena have been effectively observed in the isotropic phase of E.P.P.H., and the experimental results are in a rather good agreement with the crude theoretical model presented here. 3. EXPERIMENTAL The cell, shown in fig. 1, is provided with two stainless steel electrodes coated with 2b6

Fig. 1. Electrodialytic cell. 1. Observation window, 2. hollow elcctrodc with thermostated oil circulation, 3. liquid sample. 4. set screw for adjustment of the gap, 5. electrode support, 6. toric joins. perm-selec-

tive membranes.

ion exchange membranes (IONAC MC 3470 and MA 3475). By this means, one gets perfect nonblocking electrodes with fast electrochemicaI kinetics [5] and hindering secondary processes and injection phenomena in conditions of slow voltage increase. Ionic and molecular homogeneity in the bulk is maintained by a strong mechanical stirring inside the cell: the ionic concentration decrease by electrodialysis is the same in the whole volume of the cell, and the electric field remains uniform from the cathode to the anode. The cell

is put in a thermostatic

box,

temper-

ature is carefully controlled (* l/50’) by oil circulation inside the electrodes. Measurements were performed in the isotropic phase (T = 124OC), after chemical purification by successive fractional crystallisation and ionic purification by a forty eight hours electrodialysis, in order to stir out pre-existing ions arising from strong electrolytes or weak electrolytes with a fast kinetic of dissociation in comparison with the electrolysis rate [6]. 4. RESULTS 4.1. Stalionary curvent-voltage curve (fig. 2) After ionic purification by electrodialysis, the conduction level is reproductive, and the observed current corresponds to the displacement of the dissociation equilibrium by the field. For each voltage, a new displaced equi-

Vohne

9,

nwllber

CIfEMICAL PHYSICS LETTERS

4

TIME

(s x10-*_I

.‘.,

a

t;

APPLIED

VOLTAGE

(Vxl@]

Stationary current density versus applied voltage. Characteristic curve of elcctrodialysed E. I?. P. ff. 1. LOW field Ohm’s law region (ue :- 3.2x10-11 Q-l cm-l. 2. saturation plateau (is ~5.0x10-8 Xcm-2). 3. injection region; - circles represent cxpcrimenlal Fig. 2.

values of i_; - full line corresponds to the theoretical [{l+(T /ZTR)311/2 - 11, equation [.$I:&, :: 8is(7njTT)2 7R and TT arc gpven in tab Te 1.

giving a stationary current (Ai/i < 1% during the last hour). The stationary current-voltage curve (fig. 2) is in good agreement with the theoretical curve and shows for lo-w voltages an ohmic part, followed by a saturation plateau, which leads to the numerical dcterminrxtion of the saturation current density i,=kDvel = 5.0 x 1W8 A cmm2, (1~~0.5cm). The third part, more complex, corresponds to accidental charge carriers injection by electrodes. This phenomenon, corresponding to oxidation or reductiun of neutral or dissociated impurities at the metal-membrane electrodes [7], appears generally at higher voltages in polar liquids. librium

stake

is reached,

4.2. Electrical relaxation (fig. 3) When the steady state is reached under 15OV, the voltage is suddenly changed to 75 V, The current shows initially an ohmic hehaviour, because the ionic concentraQon is unchanged and then decreases to the saturation current value, according to eq_ (11, with an ionic transit time, under 75V, ?T a 103sec. 4.3. ChemicaL rf?laxation (fig. 4) From the displaced equilibrium under SOV,

ELECTRICAL

RELAXATION

Fig. S. Electrical relaxation. 1. Intensity ratio versus time, according to eq. (I), 2. linearized graph of those variations.

the evolution of conductivity is followed, after the voltage is switched off, by the application of ten voIt impulses every ten minutes. The relaxation data check eq. (2) and Lead to the value of 7~ = 2.4x lo3 sec. The calculated values of the kinetic constants of the equilibrium and the mean mobilities of the resulting ions, deduced from these different measurements are reported in tabie 1. These values are not specific to the compound, and significant variations from one sample to another have been observed, depending on the purification leveL Nevertheless, whatever the purification processes may be, impurities are always present in mixWe concentration in the purest liquid samples. E.P.P.K. can be purified still further but the results, reported here, refer to the highest observed purity. The praposed phenomenologieat expbxnation agrees wiwith the results, but does not lead to the value of impurity concentration or a hypottiesis about the nature of the dissociation mechanism. These species may come either from chemical prcparation or from a slow degradation of the compound =Id the resulting ions, from an acid-base 287

Volume 9, number 4

CHEMICAL PHYSICS LETTERS

15 Mny 1971

Table 1

--

Measured values

is = R D UC

7R’

L-

5.0 x 10B8Acm’2

1

2(ki.tkD V)l/2

-= 21 7T = F(/.L++ p_)

_-

Calculated values -------.kB V = 3.8 x 1011 cm-3 set-1

= 2.4 x lo3 set

kR = 1.3 x IO-l9 cm3 se&

1.0 x 103 set

P+ +

TIME 6 hour+

2

cr- = 3.3 x 10-e cm2 v-1 set-1

that the apparent conductivity of E.P.PA arises from the displncement of ionic equilibrium of weak residual electrolytes. No intrinsic bulk conductivity of the compound can be proposed through residual conductivity measurements. The conductivity deduced from the current voltage curve, even with non-blocking and noninjecting electrodes, is largely dependent on the absolute value of the applied voltage, the cell geometry, and the time at which the measurement is made. Nevertheless, the complete kinetic study of the conduction, described here, can give the values of Lb, kR and $+ + /A_) Of the impurity-solvent system. The determination of their variations with temperature and orderdegree in the nematic phase will facilitate the access to the low field electrical properties of these ccmpounds.

REFERENCES L.A. Zanoni nnd L.A.Barton, Proc. I.E.E.E. 56 (1968) 1162. [ 21 G Durand, M. Veyssie. F. Rondelca and L. Lcger. [lJ G.H.Hcilmeier.

CHEMICAL

RELAXATION

Fig. 4. Chemical reliixation. 1. Intensity ratio versus time. according to cq. (2). 2. linearized graph of those variaticins. or salt dissociation

equilibrium.

5. CONCLUSTON

In the absence of charge injection,

28R

it appears

Compt. Rend. Acad. Sci. (Paris) 27OB (1970) 97. [3] K.Koelmans and A.M. van Bostel. Phys. Letters 32A (1970) 32. [4] G.Bricre and F.Gaspnrd. Chem. Phys. Letters 7 (1970) 537. [5] G.Briere and J.P.Gosse, J. Chim. Phys. 65 (1968) 1341. [6] S.Barret. G.Briere and F.Gaspard. J. Chim. Phys. 64 (1967) 1714. [7] G.Bricre. G.Cauqui~. B.Rose and D.Serve, J. Chim. Phys. 66 (1969) 44.