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

https://doi.org/10.1016/j.jcis.2016.10.008Get rights and content

Abstract

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.

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.

References (73)

  • P. Terech et al.

    Low molecular mass gelators of organic liquids and the properties of their gels

    Chem. Rev.

    (1997)
  • N. Amanokura et al.

    New sugar-based gelators bearing a p-nitrophenyl chromophore: remarkably large influence of a sugar structure on the gelation ability

    J. Chem. Soc. Perkin Trans.

    (1998)
  • N.M. Sangeetha et al.

    Supramolecular gels: functions and uses

    Chem. Soc. Rev.

    (2005)
  • P. Jonkheijm et al.

    Probing the solvent-assisted nucleation pathway in chemical self-assembly

    Science

    (2006)
  • M. Morales et al.

    Study and description of hydrogels and organogels as vehicles for cosmetic active ingredients

    J. Cosmet. Sci.

    (2009)
  • J.F. Toro-Vazquez et al.

    Development of organogels with candelilla wax and safflower oil with high triolein content

    J. Am. Oil Chem. Soc.

    (2007)
  • R.P. Sijbesma et al.

    Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding

    Science

    (1997)
  • P. Cordier et al.

    Self-healing and thermoreversible rubber from supramolecular assembly

    Nature

    (2008)
  • S. Burattini et al.

    A healable supramolecular polymer blend based on aromatic π-π stacking and hydrogen-bonding interactions

    J. Am. Chem. Soc.

    (2010)
  • W. Weng et al.

    Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels

    J. Am. Chem. Soc.

    (2006)
  • T.D. Hamilton et al.

    Thixotropic hydrogel derived from a product of an organic solid-state synthesis: properties and densities of metal-organic nanoparticles

    J. Am. Chem. Soc.

    (2011)
  • X. Huang et al.

    Kinetics of 5α-cholestan-3β-yl N-(2-naphthyl) carbamate/n-alkane organogel formation and its influence on the fibrillar networks

    J. Am. Chem. Soc.

    (2005)
  • X. Huang et al.

    Distinct kinetic pathways generate organogel networks with contrasting fractality and thixotropic properties

    J. Am. Chem. Soc.

    (2006)
  • H. Xiao et al.

    Distinct kinetic pathways generate organogel networks with contrasting fractality and thixotropic properties

    J. Am. Chem. Soc.

    (2006)
  • S. Michihiro et al.

    A stable single piece of unimolecularly π-stacked porphyrin aggregate in a thixotropic low molecular weight gel: a one-dimensional molecular template for polydiacetylene wiring up to several tens of micrometers in length

    J. Am. Chem. Soc.

    (2005)
  • M. Pritam et al.

    Regulation of a real-time self-healing process in organogel tissues by molecular adhesives

    Angew. Chem. Int. Ed.

    (2010)
  • M. Lescanne et al.

    Thixotropic organogels based on a simple N-hydroxyalkyl amide: Rheological and aging properties

    Langmuir

    (2004)
  • J. Brinksma et al.

    Rheology and thermotropic properties of bis-urea-based organogels in various primary alcohols

    Langmuir

    (2000)
  • J.F. Toro-Vazquez et al.

    Cooling rate effects on the microstructure, solid content, and rheological properties of organogels of amides derived from stearic and (R)-12-hydroxystearic acid in vegetable oil

    Langmuir

    (2013)
  • V.A. Mallia et al.

    Robust organogels from nitrogen-containing derivatives of (R)-12-hydroxystearic acid as gelators: comparisons with gels from stearic acid derivatives

    Langmuir

    (2009)
  • V.A. Mallia et al.

    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

    Soft Matter

    (2015)
  • X. Yu et al.

    Thixotropic and self-healing triggered reversible rheology switching in a peptide-based organogel with a cross-linked nano-ring pattern

    Soft Matter

    (2012)
  • C. Du et al.

    Supramolecular assembly of amelogenin nanospheres into birefringent microribbons

    Science

    (2005)
  • C.R. Strader et al.

    Fingolimod (FTY720): a recently approved multiple sclerosis drug based on a fungal secondary metabolite

    J. Nat. Prod.

    (2011)
  • S.R. Abulateefeh et al.

    Synthesis and characterization of new derivatives of alginic acid and evaluation of their iron (III) crosslinked beads as potential controlled release matrices

    Pharmaceut. Dev. Technol.

    (2014)
  • B.G. Bag et al.

    Self-assembly of esters of arjunolic acid into fibrous networks and the properties of their organogels

    Langmuir

    (2009)
  • Cited by (0)

    View full text