Regular ArticleHow do H-bonding interactions control viscoelasticity and thixotropy of molecular gels? Insights from mono-, di- and tri-hydroxymethylated alkanamide gelators
Graphical abstract
Introduction
Molecular gels are a type of soft matter comprised of a low concentration of a low-molecular-weight gelator (LMOG) and a liquid. The gel phases are formed through aggregation and nucleation of the LMOG molecules into self-assembled, three-dimensional networks (SAFINs) that immobilize the liquid component [1], [2], [3], [4]. The driving forces for the self-assembly can include non-covalent interactions like H-bonding, π-π stacking, donor–acceptor interactions and van der Waals interactions [5], [6], [7], [8].
Several properties of molecular gels make them attractive candidates for applications in the cosmetics [9], drug delivery [10], and food industries [11]. Those properties include thermal reversibility—they can be cycled between solutions/sols and gels phases by heating above and cooling below a characteristic temperature, Tgel—and sensitivity to external stimuli, such as mechanical stress that leads to their reversion to sol phases. However, some molecular gels recover part or nearly all of their viscoelasticity after cessation of destructive mechanical stress (i.e., they are partially or fully thixotropic). Thixotropic materials are useful in the food industry, to mitigate fracture and fatigue, and as bio- and self-healing materials [12], [13], [14]. Many of the known thixotropic molecular gels rely upon LMOGs that contain a metal ion [15], [16], derivatives of steroids (especially cholesterol [17], [18]) [19], [20], porphyrins [21], dianthracenes [22], simple amides [23], and ureas [24].
A structurally simple LMOG, N-3-hydroxypropyl dodecanamide, has been reported to produce thixotropic gels with toluene as the liquid [23]. In that case, the thixotropic behavior was induced by shear-induced alignment of rod-like objects and reformation of the connections among them after the cessation of shear. The degree of viscoelastic recovery of gels with cholesterol derivatives as LMOGs is influenced greatly by the morphology of the SAFINs: spherulitic aggregates, consisting of highly branched fibers, appear to favor recovery much more than networks consisting of rod-like objects [25]. More recently, we have studied the relationship between the molecular structures of amide and amine LMOGs based on (R)-12-hydroxystearic acid (12HSA) and their gelating abilities and thixotropic behaviors in organic liquids [26], [27]. The results indicate that the degree of viscoelastic recovery can be correlated qualitatively with the strength of hydrogen-bonding interactions among the LMOG molecules. Also, an interesting concentration dependence on thixotropic behavior has been reported for toluene gels prepared with some asymmetric peptide-based compounds [28]: within a concentration range of gelator, the gels exhibited thixotropic properties while those formed either below or above that range did not.
Here, the gelation efficiencies and thixotropic properties of molecular gels based upon a series of structurally simple gelators, m-HMMnA (Fig. 1, where m is the number of hydroxymethyl groups attached to the carbon atom adjacent to nitrogen and n is the length of an n-alkyl chain connected to an amide group), have been explored. The selection of hydroxymethyl groups is based upon their appearance in several natural products with physiological activity and drugs derived from them [29], [30], [31] and their use for the controlled release of drugs [32], as well as the ability to explore in detail the effect of increasing or decreasing H-bonding interactions in selective parts of the gelator structures [33], [34], [35], [36]. Previous studies with molecules of greater structural complexity have demonstrated that the placement and number of hydroxyl groups can have important consequences to how self-assembly occurs [37], [38], [39], [40], [41]. In addition, the ability to explore the balance among the influences to m-HMMnA self-assembly from H-bonding, van der Waals forces (through changing the length of the alkyl chains), and secondary amide group intermolecular interactions are built into the m-HMMnA structures. In that regard, the gelator efficiencies of the m-HMMnA have been correlated with structural analyses of the gel networks at different distance scales in order to discern the salient factors needed to produce gels with specific properties. Somewhat surprisingly, we find that the degree of viscoelastic recovery is not very sensitive to the amount of destructive strain applied to the gels; in essence, the SAFINs are being disrupted but not totally destroyed [27], [42]. However, the gel properties are very sensitive to the number of hydroxymethyl groups, the length of the alkanamide chain, the LMOG concentration, and the properties of the liquids as assessed by Hansen solubility parameters [43], [44], [45]. Contrary to our initial expectations, the 3-HMMnA series of gelators did not produce the strongest gels, and reasons for this observation are advanced based on the competition between intra- and inter-molecular H-bonding interactions within the SAFIN assemblies. We anticipate that even more robust thermal- and mechano-reversible molecular gels and other related self-assembling systems can be produced by judicious use of the information presented here.
Section snippets
Experimental section
Details of the synthetic procedures, purification methods, and characterizations of the m-NMMnA are included in the Supporting Information file. Figures and Tables in that file are prefixed with “S”.
Gelation studies
Unless stated otherwise, experiments were performed on gels made by the fast-cooling method. The gelation abilities of the m-HMMnA at 2 wt% concentrations were examined in 20 different liquids (Table 1). 1-HMM10A was able to gelate several liquids, such as chlorobenzene and nitrobenzene, but not n-alkanes. The m-HMM16A, with the longest alkyl chains examined, showed the best gelation abilities. Thus, 1-HMM16A was able to gelate 11 of the liquids tested, including more polar liquids (such as
Mechanical properties
The viscoelastic properties of the gels in nitrobenzene were studied quantitatively by rheology (Figs. 8 and S19). The storage modulus (G′) remained higher than the loss modulus (G″) [64] within the linear viscoelastic region (LVR), and both moduli were independent of the frequency over the range examined at 0.1% strain (i.e., within the LVR). These are the characteristics of true gels [3], [4], [65]. Both G′ and G″ values increased with increasing gelator concentration (as expected for
Conclusions
This study has correlated structural parameters at the molecular scale of gels made from a series of highly efficient molecular gelators, m-HMMnA, whose structures are based on the selective combination of hydroxymethyl and n-alkyl chains around an amide group. (i.e., at the sub-nanometer distances), with those obtained at the micron and larger length scales. Furthermore, the latter have been related to mechanical properties of the systems on the macro scale and interpreted, again, based on
Acknowledgments
YZ is grateful to the China Scholarship Council (CSC) for a fellowship to support her study at Georgetown University. We thank Drs. Mohan Zhang, Jingjing Li, V. Ajay Mallia and Xinran Zhang for their help with some data collection and useful discussions. RGW thanks the US National Science Foundation for its support of this research through Grant CHE-1502856.
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