Phase Change Enthalpies and Entropies of Liquid Crystals Page: 1,053
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PHASE CHANGE ENTHALPIES AND ENTROPIES OF LIQUID CRYSTALS
TABLE 1. Contributions to AisoStpce by the hydrocarbon portion of acyclic and aromatic molecules
Group value Group
Acyclic and aromatic carbon groups Group (J. K1- mol- 1) Ci
primary sp 3 CH3 - 17.6
secondary sp 3 > CHz 7.1 1.31 a
tertiary sp3 - CH< -16.4 0.60
quaternary sp 3 > C< - 34.8 0.66
secondary sp2 - CH2 17.3
tertiary sp2 -CH- 5.3 0.75
quaternary sp2 =C(R)- - 10.7
tertiary sp H- C 14.9
quaternary sp - C - 2.8
aromatic tertiary sp2 -CaH- 7.4
quaternary aromatic sp2 carbon adjacent to an sp3 atom -Ca(R)- -9.6
peripheral quaternary aromatic sp2 carbon adjacent to an sp2 atom -Ca(R)- -7.5
internal quaternary aromatic sp2 carbon adjacent to an sp2 atom -Ca(R)- -0.7
aThe group coefficient of 1.31 for CCH2 is applied only when the number of consecutive methylene groups equals or exceeds the sum of the remaining groups;
see the discussion in text; R: any alkyl or aryl group unless specified otherwise.
of most organic compounds is not as good of a group prop-
erty as the corresponding total molar phase change entropy
(A0 isStpce); this is particularly true for molecules that have
multiple phase changes occurring at various temperatures in-
termediate between T= 0 K and T= Tfus .4 A comparison of
A isoStpce calculated for molecules believed to be isotropic in
the liquid phase to the experimental total phase change en-
tropy of molecules forming liquid crystals would be informa-
tive with regards to the quantitative nature of the phase
changes occurring in liquid crystals.
2. Phase Change Entropies
2.1. Estimation of Total Phase Change Entropy
A method for estimation of the total phase change entropy
of a wide variety of organic molecules has appeared
recently.5 The method is based on group additivity. Only a
brief description of the details of the estimations are given
here; additional details can be found in the literature.5'6 In the
discussion that follows, it is important to bear in mind that
the terms primary, secondary, tertiary, and quaternary, are
based solely on the number of hydrogens attached to carbon,
3, 2, 1, 0, respectively. A listing of the group values used in
the estimations of the total phase change entropy of the test
compounds is provided in Tables 1-5. Group values for sev-
eral functional groups encountered in this study are not cur-
Table 1 provides the group values used to estimate the
acyclic hydrocarbon portion of the molecule. The contribu-
tions of the hydrocarbon portion of the molecule are obtained
by simply adding up the contribution of each group present.
The only exceptions to this rule occur whenever a functional
group listed in Tables 2 and 3 is attached directly to either a
tertiary or quaternary sp3 carbon or to a tertiary sp2 carbon;
in this case the contribution of each carbon is attenuated as
the product of the group coefficient and group value (CiGi).
Additionally, if the number of consecutive methylene groups
equals or exceeds the sum of the remaining groups, the con-
tribution of each CH2 is also evaluated as the product of
CiGi; other methylene groups in the molecule are not in-
cluded in this count and are treated normally.
The contribution of each functional group is treated in
much the same way. Functional groups are defined in Tables
2, 3, and 5. The contribution of each function group defined
in Table 2 depends on the total number of functional groups
present in the molecule. Each of these functional groups con-
tribute CkGk . The appropriate value of Ck is chosen from the
table on the basis of the total number of functional groups
TABLE 2. Contributions of acyclic functional groups used in estimating A0 isStpce of liquid crystals; functional groups dependent on the substitution patterns
Group value (Gk)a Group Coefficient (Ck)
(J. K- 1.mol- 1)
Functional groupsa Total number of functional groups; k= 2 3 4 5 6
chlorine R-Cl 10.8 1.5 1.5 1.5 1.5 1.5
two fluorines on an sp3 carbon R-CF2- R 13.2b 1.06 1.06 1.06 1.06 1.06
hydroxyl group R-OH 1.7 10.4 9.7 13.1 12.1 13.1
carboxylic acid R- C(=O) OH 13.4 1.21 2.25 2.25 2.25 2.25
aR: any alkyl or aryl group unless specified otherwise.
bThis group value has been modified from the value reported in an earlier version;6 use this value for each fluorine; see Table 3 for the fluorine value to use
in perfluorinated compounds and Ref. 3 for examples.
J. Phys. Chem. Ref. Data, Vol. 35, No. 3, 2006
Downloaded 25 Feb 2013 to 18.104.22.168. Redistribution subject to AIP license or copyright; see http://jpcrd.aip.orglabout/rights_and_permissions
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Acree, William E. (William Eugene) & Chickos, James S. Phase Change Enthalpies and Entropies of Liquid Crystals, article, July 17, 2006; [College Park, Maryland]. (digital.library.unt.edu/ark:/67531/metadc172344/m1/3/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.