W. E. ACREE, JR. AND J. S. CHICKOS

A  isStpce for 627 liquid crystals. .............. 1057
3. A histogram of the distribution of errors in
A0isoStpce(exp)-A0isoStpce(calc) for 2637
compounds used in deriving and validating
group parameters for estimating total phase
change entropies ......................... 1057
4. A histogram of the distribution of errors in
A0osoStpce(exp)-Ao Stpce(calc) for liquid crystals. 1058
5. A plot of the difference between experimental
and calculated A i'toS  as a function of the
number of carbon atoms. .................... 1058
6. A plot of the entropy of the benzene
hexa- n - alkanoates as a function of the number
of carbon  atoms.............. .............  1059
7. A comparison of the melting and clearing
temperatures of the odd thiocholesteryl
n-alkanoates .............................. 1060
8. A comparison of the melting and clearing
temperatures of the even thiocholesteryl
n-alkanoates ............................   1060
9. A comparison of the melting and clearing
temperatures of the alkyl
4 ' - methoxybiphenyl- 4 - carboxylates............ 1061
1. Introduction
Since their first discovery back in 1888, interest in the
properties and practical applications of liquid crystals has
increased dramatically.1'2 General acceptance of liquid crys-
tals as a distinct phase of matter was slow, occurring some 30
years since they were first reported. Liquid crystalline behav-
ior is found among numerous classes of compounds that in-
clude biphenyls, cholesterol esters, soaps, lipids, polymers,
and elastomers. More than 76000 compounds have been
identified as exhibiting liquid crystalline behavior.3(a)-(c) This
paper will review the total phase change enthalpies and en-
tropies of the more than 3000 compounds whose condensed
phase thermochemical properties have been studied.
Unlike most small molecules that behave isotropically
upon liquefaction, many molecules that are highly non-
spherical in shape, exhibit marked self-assembly in the liquid
phase that persists even upon continued heating. Cylindrical
rod, disk, and banana shaped molecular structures are among
those most frequently encountered exhibiting this behavior.
Loss of self-assembly can occur in stages and can be moni-
tored by changes in a variety of physical properties. In some
cases, liquid crystalline behavior is observed, only upon su-
percooling of the melt. In these instances, the melting of the
solid first produces an isotropic liquid; self-association is ob-
served upon supercooling below the melting temperature. Al-
though self-association may be prompted by the supercool-
ing of many nonspherically shaped molecular liquids, the
scattering of visible light by liquid crystals requires a higher
level of self-association not observed with most substances.

Though liquid crystals can be considered as a separate
phase of matter,2 they exhibit properties intermediate be-

tween anisotropic solids that are rigidly and uniquely ar-
ranged in a lattice with very little mobility, plastic crystals
that flow under stress and usually characterized by rotational
motion within a lattice, and isotropic liquids characterized by
free rotational and translational motion. If the forces of in-
teraction are sufficiently strong, a more limited form of self-
association can also be detected in the gas phase, as exem-
plified by the dimerization observed with some carboxylic
acids.
Of all the techniques used to study liquid crystals, thermal
analysis, while perhaps not the most sensitive, provides a
quantitative measure of the magnitude of the interactions re-
sponsible for self-assembly. A study of the thermal behavior
exhibited by liquid crystals may also provide insight into the
associative behavior of other molecules that behave noniso-
tropically but do not form liquid crystals. This study reports
the thermal behavior of some 3000+ liquid crystals and
compares the total molar phase change entropy (A0iSoStpce)
of a representative number of them to the total molar phase
change entropy of substances that are believed to melt to
isotropic liquids. In view of the large number of compounds
in the database, the calculated and experimental total molar
phase change entropy of some 667 entries on approximately
600 different compounds were compared. Compounds were
selected to include a variety of functional groups and struc-
tures. In order to simplify the calculations, members of ho-
mologous series were frequently chosen. If the compounds
selected included multiple independent determinations of
their thermal properties, all entries for the compound were
included in the analysis.
Throughout this article, Tfus, Tc1d, and Tiso are used to
distinguish between slightly different events and conditions.
The temperature, Tfus, refers to the temperature at which a
solid is converted to either an isotropic liquid or to a liquid
crystal. Tcl1d is used to refer to the clearing temperature if the
isotropic liquid is converted to the liquid crystal by super-
cooling the isotropic liquid below Tfus ; Tcl1d may be observed
experimentally at the temperature the supercooled liquid be-
comes cloudy. The term Tiso has been used to refer to tem-
peratures at which the liquid becomes isotropic above Tfus.
The relationship between these terms is defined as follows:
Tcld< Tfus Tiso
1.1. Phase Change Enthalpies
An examination of the phase change enthalpies of liquid
crystals reveals that these substances exhibit several thermal
transitions that can be detected. For most substances, the
largest enthalpic change occurs upon conversion of the solid
to a nematic or smectic phase. In a few cases, some highly
substituted anthraquinones (for example, see C124H21608,
C132H232Ol8 in Table 10), the largest enthalpy change ob-
served is associated with changes occurring at the mesomor-
phic stage.
It has been previously shown that the total molar phase

change enthalpy (A0TisoHtpce) associated in going from the
rigid solid to the isotropic liquid at the melting temperature

J. Phys. Chem. Ref. Data, Vol. 35, No. 3, 2006

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