How do H-bonding interactions control viscoelasticity and thixotropy of molecular gels? Insights from mono-, di- and tri-hydroxymethylated alkanamide gelators.

The structural and dynamic properties of molecular gels, made from 9 structurally-related mono-, di, and tri-hydroxymethylated alkanamide gelators, have been examined at different distance scales. The subtle changes in the gelator structures, in terms of the number of hydroxymethyl groups and the length of the alkanamide chain, have been correlated with the type of the self-assembled fibrillar networks and the viscoelasticity of the gels as well as the characteristics of the liquid as indicated by Hansen solubility parameters. Some of the gels exhibit high degrees of thixotropy and very rapid recovery after the cessation of destructive strain. Gelation efficiencies-based upon the range of liquids gelated, the critical gelator concentrations, and the gel-sol transition temperatures-depend upon both the length of the fatty acid chain and the number of hydroxymethyl groups: the best gelator of the series contains the longest alkyl chain examined (hexadecyl) and two hydroxymethyl groups (i.e., better than the gelators with one or three groups). FT-IR and powder X-ray diffraction data indicate that hydrogen-bonding and molecular packing modes in the gels and the neat gelator powders are very similar. Polarizing optical microscopy images of the gels show that the morphology of the gel networks can be tuned by changing the cooling processes used to transform the sols into gels. In total, the observations and conclusions derived provide useful insights into the relationship between gelator structure and gel properties. These data will be useful to those interested in the a priori design of new gelators or other molecules undergoing a variety of self-assembly processes that lead to robust thermal- and mechano-reversible materials.