Dilatometric Study and Pretransitional Effect at Nematic
to Isotropic Phase Transitions
in Calamitic Lyotropics
Madalina-R. PUICA a, Rodica MOLDOVAN b, Traian BEICA b, Stefan FRUNZA b, Wilfried SCHRANZ a
a - University of Vienna, Institute of Experimental Physics, Strudlhofgasse no.4, 1090 Vienna, Austria
b -National Institute of Materials Physics, Bucharest, R-76900, Romania
Abstract
Materials
A dilatometric study on the nematic to isotropic phase transition of lyotropic
liquid crystal system: SLS/water/decanol, at a nematic calamitic concentration, is
presented. The experimental results for the temperature dependence of the density at
normal pressure, show within the limits of experimental precision (± 1×10-5 g/cm3),
that the density has no jump at the nematic to isotropic phase transition. The obtained
result is brought in connection with a pretransitional effect, observed for the first time
at this lyotropic system, namely the appearance of isotropic elliptic domains in the
nematic matrix below the transition temperature.
The lyotropic liquid crystal was obtained from sodium lauryl sulphate in water, with
addition of decanol.
Components used:
Sodium
lauryl (dodecyl) sulphate (SLS) (Sigma 99% purity, molar mass M=288 g/mol)
 - the chemical formula: CH  CH   OSO  Na 
O
3
2 11
3
 - the structural formula:
Experimental techniques
CH3
The expansion of the liquid crystal as a function of temperature was investigated using
a dilatometer with capillary tube (of volume 1.45cm3). The length of liquid columns was
measured using an Abbé comparing microscope (Zeiss) with accuracy of 1m.
The errors in the density variation determinations: 110-5gcm-3 (Fig. 2).
CH2
CH2
CH2
CH2
CH2
CH2
O
S
O- Na+
O
CH2
CH2
CH2
Hydrophobic tail
CH2
CH2
Polar head
1-decanol (Merck, 99% purity , molar mass M=158 g/mol)

CH 3   CH 2 9  OH
 - the chemical formula:
tridistilled water (home-made).
The composition of the sample was chosen according to the phase diagram (Fig. 1) from literature1, in
order to obtain a calamitic lyonematic liquid crystal (Nc).
Fig. 3
Integrated system for polarized light
microscopy
Fig. 2
Experimental set-up for density
measurements.
Fig. 1: Partial phase diagram (wt %) for the system
SLS/ water/ decanol1 at 25°C.
The dot gives the composition of the investigated
nematic samples.
Sample composition:
Nc : 25.05% (SLS)+ 70.46 %(water) + 4.48 %(decanol)
The pretransitional effect for Nc mixtures, was checked using polarized light microscopy.
The temperature was controlled with accuracy of 0.01°C using an HS1 Microscope hot stage
(INSTEC). Photographs of the elliptic domains were taken with the aid of a digital image
capture system provided with Kodak MegaPlus (Edmund Scientific) camera (Fig. 3).
Experimental results
The dilatometric measurements were made at a constant variation of the temperature with a rate
of 1.5°C/hour. The temperature stability inside the micro chamber was 0.01ºC.
By observing the dilatometer reservoir between cross-polarizers, the temperature transition (T0)
between nematic and isotropic phase was determined at: T0= 29.6°C.
Usually when the temperature approaches the transition temperature, the sample,
observed between crossed polarizers, is gradually getting dark until, at a given temperature, it
becomes completely dark. For some samples, at a temperature of (0.2-0.1)°C below the
transition temperature, we observed small elliptic isotropic domains in the nematic matrix (a
pretransitional effect), as show in Fig. 5.
-3
0.00010
- / gcm
0.00005
b
L
0
-0.00005
-0.00010
26
T 30
0
28
32
 / gcm
-3
1.022
a
Fig. 5: Images of the isotropic domains appeared in the nematic matrix of a lyo-nematic
calamitic SLS/water/decanol mixture, within an 0.1°C interval under the nematic-isotropic
transition temperature. The arrow points the liquid crystal orientation direction.
1.021
1.020
1.019
26
Size evolution for an elliptic domain:
28
30
32
T /ºC
D (m)
Minor axis
180
160
Fig. 4: a) The temperature dependence of the density  for the Nc solution.
The straight line is the linear regression for the data, L=1.0351-5.13910-4T.
b) The deviation of the density  from the above linear variation L.
D= 36.7 + 566.7* T + 6589*( T)2
R = 0.99725
140
120
Major axis
100
D= 56.07 + 480.2* T + 8340.5*( T)2
R = 0.99831
80
60
Conclusions

- - Since our experimental results put in evidence no density jump at the nematic to
isotropic phase transition temperature (TN-I), we may conclude that, in case that this jump
exists, it must be smaller than the measurement resolution allowed by our method.
Similar results were obtained by Boden and Joley2 for another nematic system
(CsPFO/water), whose behavior also presents no jump of density at the TN-I temperature.

- - For the first time it was evidenced a pretransitional effect for the lyotropic system
SLS/water/decanol , at Nc composition. This could be related to the absence of density jump.
40
20
0.00
0.02
0.04
0.06
0.08
0.10
0.12
T-Tin (K)
Fig. 6: Size values of the major and minor axes of an elliptical isotropic domain,
successively photographed. Continuous curves represent the fit with a second order
equation.
Reference
1-Per-Ola
Quist, B.Halle, I.Furo, J.Chem.Phys. 96, 3875 (1992)
2-N. Boden, K.W. Jolley, Phys. Rev. A, 41, 8751-8758 (1992)