Comparison of Dipolar, H‑Bonding, and Dispersive
Interactions on Gelation Efficiency of Positional Isomers of Keto
and Hydroxy Substituted Octadecanoic Acids
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Abstract
A systematic study
of the importance of functional group position
and type on the gelator efficiencies of structurally simple, low molecular-mass
gelators is reported. Thus, the gelation abilities of a series of
positional isomers of ketooctadecanoic acid (<i>n</i>-KSA)
are compared in a wide range of liquids. The gelation abilities of
the <i>n</i>-KSA as a function of <i>n</i>, the
keto group position along the chain, are characterized by several
structural, thermal, and rheological techniques and are compared with
those of the corresponding hydroxyoctadecanoic acid isomers (<i>n</i>-HSA) and the parent molecule, octadecanoic acid (SA).
Analyses of the gels according to the strengths of functional group
interactions along the alkyl chain in terms of group position and
type are made. The conclusions derived from the study indicate that
gel stability is enhanced when the functional group is located relatively
far from the carboxylic headgroup and when group–group interactions
are stronger (i.e., hydrogen-bonding interactions are stronger in
the <i>n</i>-HSA than dipole interactions in the <i>n</i>-KSA, which are stronger than the London dispersion interactions
in SA). Co-crystals of the keto- and hydroxy-substituted octadecanoic
acids are found to be less efficient gelators than even the ketooctadecanoic
acids, due to molecular packing and limited group interactions within
the gelator networks