Structural bases for mechano-responsive properties in molecular gels of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides. Rates of formation and responses to destructive strain

Abstract

The self-assembly and gelation behavior of a series of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides (HS-n-OH, where n = 2, 3, 4 and 5 is the length of the alkyl chain on nitrogen), as well as those of two ‘model’ compounds, N-(3-hydroxypropyl)octadecanamide (S-3-OH) and (R)-12-hydroxy-N-propyloctadecanamide (HS-3), have been investigated in a wide range of liquids. A unique aspect of some of the HS-n-OH gels is the degree and velocity of their recovery of viscoelasticity after the cessation of destructive shear. The recovery times vary from less than one second to hundreds of seconds, depending on the length of the ω-hydroxyalkyl group on nitrogen. The data indicate that the modes and dynamics of aggregation of the gelator molecules from incubation of a sol phase below the gel melting temperature, as analyzed by Avrami and fractal equations, cannot be used to explain the degree and dynamics of the thixotropy: sol-to-gel transformations involve assembly of 0-dimensional objects (i.e., individual gelator molecules) into 1-dimensional fibrils and then into 3-dimensional networks; recovery after mechano-destruction of gels requires only 1-dimensional to 3-dimensional re-assembly or re-association of 3-dimensional spherulitic objects. A model to understand the extreme sensitivity of the thixotropy on the length of the ω-hydroxyalkyl group in the HS-n-OH (which is based upon detailed comparisons among the dynamic properties of the gels, the morphologies of the neat gelators, and the fibrillar networks of the gels) invokes the importance of the cleavage and reformation of H-bonds between fibers at ‘junction zones’ or between spherulitic objects.

Mechano‐Responsive, Thermo‐Reversible, Luminescent Organogels Derived from a Long‐Chained, Naturally Occurring Fatty Acid

“…The morphologies of these thixotropic gels before and after the destruction–recovery process were investigated by POM. Based on the POM images and XRD diffractograms, the SAFiN s of these gels are crystalline; as mentioned, few gels with crystalline SAFiN s are thixotropic . Before the initial application of the destructive strain, the DODA gel SAFiN s consisted of bundles of fibrillar objects (Figure S11 in the Supporting Information).…”

“…They devised a model for correlating kinetics and network structure which provided information about the interplay between nucleation, fiber growth, branching, and interfiber interactions . A recent study by our group utilized both the Avrami and Dickinson theories to probe the aggregation and gelation processes of N ‐acyl ethanolamines in isostearyl alcohol . Here, these approaches are applied to gain insights into the formation of SAFiN s of DODA in 1‐octanol. …”

“…Also, we have exploited the emission to extract important details about the processes during the aggregation and deaggregation of DODA ; the information gleaned cannot be duplicated by the other analogues of the SA family produced to date. Surprisingly, some DODA ‐based gels also exhibit reversible mechano‐responsive properties (i.e., they are thixotropic), a characteristic rare in molecular gels with crystalline SAFiN s . Here, we suggest a model for the structural and molecular packing within the SAFiN s and use this model to explain the reversible changes in the emission and in the mechanical strength of the gels and their sols.…”

Gelating abilities of metal salts of ricinelaidic acid. Structural and rheological considerations

“…As discussed earlier, both D‐REA‐Ca and D‐REA‐Na can form thixotropic gels in nitrobenzene and/or water with rapid recovery of viscoelasticity after the cessation of destructive strain. We hypothesize that the 3D SAFiN s break into 1D fibers or bundles of them, but not revert to 0D objects (such as isolated molecules or small aggregates that exist in the sol phase) . Some of the disconnected 1D fibers then rejoin once the destructive strain ceases.…”

“…Some of the disconnected 1D fibers then rejoin once the destructive strain ceases. The longer recovery time and lower degree of recovery of D‐REA‐Ca gels than of D‐REA‐Na gels suggest a lower ability of D‐REA‐Ca to reform inter‐fiber interactions; [12, 21b, 22] the reorganization of the D‐REA‐Ca objects is more demanding entropically.…”