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Using Fourier Transform Infrared Spectroscopy to Examine Structure in Bisurea Organogels

Volume 61, Number 4 (April 2007) Page 379-387

Pierce, Aaron M.; Maslanka, Paul J.; Carr, Andrew J.; McCain, Karla S.

The structure of two bisurea organogels was examined by Fourier-transform infrared (FT-IR) spectroscopy. Organogels were prepared in benzene at different concentrations of gelator in order to determine the effect of concentration on the assembly of organogelator molecules. This work examined two types of bisurea organogelators, both with dodecyl alkyl tail groups. The two molecules differ only in the length of an alkyl chain separating their two urea groups: 6 carbons in the C6C12 organogelator (1,6-bis(3(3,5-didodecoxybinzyl)-urea-hexane) and 12 carbons in the C12C12 organogelator (1,12-bis(3(3,5-didodecoxybinzyl)-urea-dodecane). The degree of urea hydrogen bonding was determined from the position of the amide II band, and the conformational order of the alkyl chains in the organogelator was determined in the methylene bending region. Both gels showed a general trend of less hydrogen bonding and greater conformational disorder in the alkyl chains as the concentration of organogelator increased; however, the changes were smaller in the C12C12 gels. This decrease in structural order with increasing organogelator concentration is explained by the kinetics of gel formation; more concentrated gels solidify too quickly to assemble perfectly. The observed differences between the two organogelators are caused by the different structures into which these two similar molecules assemble. The C6C12 organogelator only assembles linearly, while the C12C12 organogelator can form sheets through brick-like packing, and these packing motifs were confirmed by scanning electron microscopy.