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Chapter 12
Structure-Property Comparison and Self-Assembly Studies of Molecular Gels Derived from (R)-12-Hydroxystearic Acid Derivatives as Low Molecular Mass Gelators V. Ajay Mallia1 and Richard G. Weiss*,2 1School
of Science and Technology, Georgia Gwinnett College, Lawrenceville, Georgia 30043, United States 2Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, United States *E-mail:
[email protected].
This chapter summarizes the self-assembly, structure and properties of molecular gels derived from amide and hydrazide derivatives of (R)-12-hydroxystearic acid (HSA) while comparing them to those of the parent acid. The nature of the aggregates is discussed at different length scales, starting from single molecule structures and progressing to packing within the self-assembled fibrillar networks (SAFINs) of these gels. The structural properties of the gelators are correlated with the bulk properties of their gels. Of note is the observation that many of these gels exhibit thixotropic properties (i.e., they are converted to sols upon application of destructive strain and then return to their gel state when the strain is released). Analyses of the factors within the SAFINs leading to their viscoelastic properties and to the ability of another derivative of HSA to form self-supporting and self-healing gels are presented as well. The results demonstrate how small structural modifications of derivatives of HSA can cause very large changes in the properties of their gels, some of which have important potential consequences to their potential applications.
© 2018 American Chemical Society Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Introduction Molecular gels are quasi-solid materials composed of small amount of a low molecular weight compound (a gelator) and a liquid (1–7). They are a subclass of a wide range of materials commonly called ‘soft matter’. In the appropriate temperature and concentration regimes, the gelator molecules self-assemble to form one-dimensional objects (e.g., fibers, strands, tapes or tubules), that further interact with each other and form 3-dimensional self-assembled fibrillar networks (SAFINs) (8, 9). The SAFINs are stabilized by weak intermolecular interactions such as H-bonding, π-π stacking, electrostatic interactions or London dispersion forces, and macroscopically entrap the liquid in which they reside (10). As such, they are sometimes referred to as ‘physical gels’ to differentiate them from ‘chemical gels’ in which the 3-dimensional networks are comprised of interacting polymeric chains (i.e., the ‘molecular gelators’ can be thought of as being covalently linked). Interest in molecular gels has increased significantly during the last two decades due to their fundamental importance to understand modes of molecular aggregation (11) and their potential applications (12). Many molecular gelators contain steroidal, aromatic, and other functional groups, or they may rely on ionic interactions between two molecules, neither of which is a gelator alone (1, 13, 14). Long n-alkanes are the simplest possible organic molecules that are gelators, and their SAFINs are stabilized by London dispersion interactions (15, 16).
Figure 1. Structures of(R)-12-hydroxystearic acid (1) and gelators based on it.
In this chapter, we summarize systematically some relationships between the properties of the gels and the structures of the molecular gelators and their aggregates at different distance scales that are derived from a molecule which is a di-substituted n-alkane, (R)-12-hydroxystearic acid (HSA; 1 Figure 1). It is obtained from a natural source—hydrolysis and hydrogenation of triglycerides found in the seeds of the castor plant. SAFIN structures of molecular gels of the parent molecule, HSA, have been studied extensively (17). X-ray and infrared studies, especially, have demonstrated that the carboxylic acid groups of HSA promote the formation of H-bonded sequences that stabilize SAFINs, and the 12-hydroxy groups participate in the stabilization as well—HSA is a much more efficient gelator than stearic acid (18–23). 228 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
As demonstrated by the large body of research with polypeptides, H-bonding interactions between primary or secondary amide groups can be even more effective than those between carboxylic moieties (24). Thus, the amide derivatives of HSA, 2 and 3 (R = alkyl), are efficient gelators. Also, H-bonding interactions of molecules with alkyl (25) and aryl hydrazide groups (26, 27) have been used to stabilize self-assembled structures. Thus, the gelating abilitiy of 4 and 5 (R = alkyl) have also been assessed as has the series of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides (6), in which an amide and a hydroxyl functional group are separated by an alkyl chain linker. SAFINs of the molecular gels of 6 are also stabilized by H-bonding involving the hydroxyl group at the C12 postion of the octadecanoyl chains, the terminal ω-hydroxyl groups on the N-alkyl chains, and the amide groups (28).
Gelation Studies and SAFIN Structure As noted above, HSA gelates many liquids, including n-hexane, toluene, silicone oil and acetonitrile (29). The gel melting temperature (Tgel) of these gels depends on the concentration of the gelator and the nature of the liquid being gelated (Table 1). As an example, at 2 wt % of HSA, Tgel ofthe gel in silicone oil is higher than that in n-hexane. However, as a result of the stronger H-bonding interactions between primary or secondary amide functional groups than carboxylic acid groups (30), Tgel values of molecular gels of the primary amide (2) in hexane, silicone oil, toluene and acetonitrile are higher than those of the corresponding HSA gels. Consistent with the importance H-bonding interactions at the head groups of HSA derivatives in controlling the stability of their molecular gels, the Tgel values of the secondary amides (3) are generally lower than those of 2; H-bonding interactions among the molecules with secondary amide groups are weaker than those of primary amides. As noted, hydrazide groups exhibit strong intermolecular H-bonding interactions as well (30). For that reason, (R)-12-hydroxystearic acid hydrazide (4) is able to gelate alkanes, alcohols, and aromatic liquids among others. However, critical gelator concentrations (CGCs; i.e., the lowest concentrations of a gelator at which gels can form at room temperature) values for 4 are larger than those of of HSA, primary or secondary HSA-derived amides (29); in this regard, the hydrazide gelators are less efficient. Based on the very low CGC values of their gels in silicone oil, acetonitrile, and toluene (0.5 wt%) (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides are exceedingly effective gelators (28). CGC values of gels of 4 in n-hexane and in silicone oil are 3.9 and 1.5 wt %, respectively. Consistent with the observations with primary and secondary amide derivatives of the HSA gelators, the alkylated hydrazide, (R)-N-ethyl-12-hydroxyoctadecane hydrazide (5, R = ethyl), did not gelate hexane and decane or most of the alcohol liquids examined. However, 5wt % 5 (R = ethyl) was able to gelate aromatic liquids such as toluene and nitrobenzene (31). Also, (R)-12-hydroxy-N-(ω-hydroxyethyl)octadecanamide (6, n = 2) did not gelate n-hexane (28). 229 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 1. Appearancesa and Tgel Values (°C, in Parentheses) of Gels Containing 2 or 5 wt % of HSA Derivatives in Various Liquids. See Figure 1 for Structures. Liquid /gelator
1b,d
2b,d
3b d R = C2H5
4c
e
5c,e R = C2H5
6b, f n=2
n-Hexane
OG (59-60)
OG (91-92)
OG (81)
OG (107108)
P
I
Silicone oil
OG (73-74)
OG (98-100)
OG (86-87)
CG (108109)
OG (99-103)
OG (107-108)
Toluene
CG (44-45)
CG (65-67)
OG (57-58)
CG (75-78)
TG (72-75)
CG (82-83)
1-Octanol
Soln
OG (27-34)
P
OG (53-55)
Viscous solution
P
Methanol
Soln
Soln
Soln
TG
P
Soln
Acetonitrile
OG (45-48)
OG (53-54)
OG (56)
P
P
OG (67-70)
a Unless stated otherwise, all gels were prepared by a ‘fast-cooling’ protocol by placing hot sols/solutions of a gelator and liquid in the air or in a cold water bath until the sample reached room temperature: OG-opaque gel, TG-translucent gel, CG-clear gel, P-precipitate, I-insoluble, Soln-solution. b 2 wt % and c 5 wt %. d from reference (29), e from reference (31) and f from reference (28).
As shown in Figure 2A, the hydroxyl groups on HSA are connected by an unidirectional H-bonding network in their SAFIN networks. The periodicity of the molecular packing arrangement of the HSA was interpreted as the distance between double planes with a monoclinic crystal lattice in one unit cell, similar to those observed in many fatty acids (32). Although more than one XRD crystal structure of racemic HSA has been reported (33, 34), to the best of our knowledge, no single crystal structure of the (R) enantiomer of HSA has been determined. In benzene and acetonitrile gels, SAFINs of HSA exhibit monoclinic crystalline packing within fibers of rectangular ribbon-like aggegates (32). Comparison of XRD diffractograms of neat powders and 5 wt % 2 in silicone oil gels show that 2 in silicone oil gel show similar morphology as in the neat gelator. The Bragg distances of the low-angle peaks, indicated lamellar packing arrangement shown in Figure 2B (29). The positions of the diffraction peaks of the silicone oil gel of 12-hydroxy-N-propyloctadecanamide correspond to those of the neat gelator, and are consistent with a monolayer arrangement as shown in Figure 2C.
230 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 2. A) Structural model of the ribbon-like aggregates of HSA in organic solvents. The parallel vertical lines represent the direction of the H-bonding network. Adapted from reference . Two proposed packing arrangement of gelator molecules in gel aggregates: B) Calculated length of a conformationally extended dimeric unit of 2 = 52.8 A °; C) Calculated molecular length 12-hydroxy-N-propyloctadecanamide = 31.1 A °. Adapted from reference (29).
231 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. (A) Possible molecular packing models for (R)-12-hydroxystearic acid hydrazide (4), (B) (R)-N-decyl-12-hydroxyoctadecane hydrazide (5, R = decyl), and (C) (R)-12-hydroxy-N-(2-hydroxyethyl)octadecanamide (6, n = 2). A and B are reproduced with permission from reference (31). Copyright 2016 Wiley-VCH Verlag GmbH & Co. (C) reproduced with permission from reference (28). Copyright 2015 The Royal Society of Chemistry.
XRD diffractograms of the organogels and neat solids of (R)-12hydroxystearic acid hydrazide (4) and (R)-N-decyl-12-hydroxyoctadecane hydrazide (5, R = decyl) were also compared. The Bragg reflections of the XRD diffractograms indicated lamellar spacing. The lowest angle reflection for (R)-12-hydroxystearic acid hydrazide (4) was twice the calculated molecular length, indicating bilayer packing (see Figure 3A) (31). The lowest angle reflection for (R)-N-decyl-12-hydroxyoctadecane hydrazide corresponds to a distance close to the length of an extended molecule; the suggested packing arrangement for two of the molecules is shown in Figure 3B. Figure 3C shows a possible head-to-tail packing arrangement for (R)-12-hydroxy-N-(2-hydroxyethyl)octadecanamide (6, n =2) based on the structurally similar methyl ester of HAS (28). Radially-averaged, 2-dimensional small angle neutron scattering (SANS) curves of a gel comprised of 2 wt% 6 (n = 2) in toluene-d8 gel exhibited a Bragg reflection peak at Q = 1.2 Å, corresponding to a distance of 52.0 Å. The SANS data were also consistent with a bilayer packing arrangement of 6 (n = 2) in its toluene gel. The overall fit of the SANS curve indicates flexible cylinders with poly-radii and a radius of (Figure 4) (28). 232 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 4. Log–log plot of SANS intensity (I) versus Q profile of a 2 wt% 6 (n = 2) in toluene-d8 gel (●). The black line is the theoretical curve for cylinders with poly radii and a radius of Å, The arrow shows the second oscillation peak at d = 52.0 Å. Reproduced with permission from reference (28). Copyright 2015 The Royal Society of Chemistry.
Polarizing optical micrographs (POMs) in Figure 5 show images of the SAFINs of gels of 2 wt % 2 in silicone oil and in toluene. Both show spherulites, and they were larger when the sols were cooled more slowly Also, spherulitic objects were observed for 5 wt % 4 and 5 (R = C2H5) in silicone oil gels (Figure 6). POM images of a 2 wt % 6 (n = 2) in silicone gel exhibited fibrillar textures and the corresponding AFM image showed rope-like bundled fibers with ca. 100 nm diameters (Figure 7).
Figure 5. Polarizing optical micrographs of gels 2 wt % 2 (A) in silicone oil and (B) in toluene. Adapted from reference (29). 233 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 6. Polarizing optical micrographs of 5 wt % silicone oil gels of 4 (A) and 5 (R = C2H5) (B). Reproduced with permission from reference (31). Copyright 2016 Wiley-VCH Verlag GmbH & Co.
Figure 7. (A) Polarizing optical micrograph and (B) AFM image of a gel comprised of 2 wt % 6 (n =2) in silicone oil. Reproduced with permission from reference (28). Copyright 2015 The Royal Society of Chemistry.
Thixotropic Properties Thixotropy can be defined as the shear thinning of a viscoelastic material upon the application of a destructive strain and the subsequent recovery of the viscoelasticity after cessation of the strain (36). Thixotropic properties arise in molecular gels when the changes in applied strain result in a loss and gain of intermolecular interations among SAFINs (37–39). A recent review has highlighted and summarized the correlations between thixotropic and structural properties of molecular gels with crystalline networks (40). In this chapter, we summarize the efforts to understand why some structurally-simple molecular gelators which are derived from HSA (Figure 1) can form gels which are thixotropic. We extend our discussion of how the morphologies of the SAFINs are contolled by the different head groups of the HSA-related derivatives to the rheological properties of their gels. 234 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 8. A) Elastic (G’) (●) and viscous (G”) (▴) moduli as a function of time and application of different levels of strain and frequency to a 2.0 wt % 2 in silicone oil gel at 25 ºC. Adapted from reference (29). B) Thixotropic studies of 5 wt% 4 in silicone oil gel (G’ (●) and G” (●)). Reproduced with permission from reference (31). Copyright 2016 Wiley-VCH Verlag GmbH & Co.
For the thixotropic studies, the viscoelastic the elastic (G’) and viscous (G”) moduli were measured sequentially within the linear viscoelastic regime (LVR), at a destructive strain, and finally at theoriginal LVR condi tions. In all cases, the values of G’ were taken as the measure of recovery. For example, a 2 wt % silicone oil gel of HSA was found to recover 70% of its initial G’ value in 99%, their recovery times, ~16.6 and 2.5 min, respectively, were very different and much longer than the recovery times of the shorter homologues. The results show 236 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
that addition or subtraction of one methylene unit on the hydroxyalkyl chain attached to the amide nitrogen atom of the series of 6 molecules affects both the eventual recovery of the viscoelastic properties of the gels and the time required for that recovery. These observations indicate that the conformational changes within the head groups of the 6 homologues have an important role in determining the degree of intra- and inter-molecular H-bonding within their SAFINs and the ability of the subunits present immediately after the cessation of destructive strain to reassemble and lead again to gels. A possible mechanism for these processes in the isostearyl alcohol gels with the 6 homologues is shown in Figure 10.
Figure 10. Cartoon representation of a possible mechanism to explain the thixotropic behavior of SAFINs of 6 in isostearyl alcohol gels. Reproduced with permission from reference (28). Copyright 2015 The Royal Society of Chemistry.
Self-Standing Gels and Diffusion Studies Many molecular gels are not sufficiently strong to be free-standing solids. Some have been strengthened by incorporating polimerizable groups, such as methacrylate, within the gelator structureand then polymerizing the SAFINs (43). After such treatments, the gels are no longer classified as molecular (physical); they are polymer (chemical). However, there are few reports of self-standing sorbitol-based molecular gels (44, 45), and Dastidar and coworkers have developed a broad selection of two component salt gelators (14), some of which form self-standing and self-healing gels (46). Of the HSA-derived gelators and their gels, those of ethylene glycol and 2 or 5 wt% (R)-12-hydroxystearic acid hydrazide (4) exhibit self-standing and selfhealing properties. The stiffness and the load-bearing force (Fb) of cylindrical blocks of the gels were measured using compression tests by decreasing the gap between parallel rheometer plates. Fb and rate of increase of force increased from 10 to 47 N and 1.23 to 1.95 Nm/m on increasing the gelator concentration from 2 to 5 wt% (Figure 11). 237 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 11. Compression curves of normal force versus gap distance for (R)-12-hydroxystearic acid hydrazide gels. a) gels at 2 and 5 wt% gelator in ethylene glycol and in ethylene glycol/DMF mixtures. b) 5 wt% gelator in ethylene glycol (black, 1) and repeated cycles of compression (squares, 2a-6a) and extension (circles, 2b-6b) below the load bearing force, Fb; red (2), blue (3), green (4), cyan (5), and magenta (6) are from the first, second, third, fourth, and fifth cycles, respectively. Note that the color in these figures will only be available in the online version. Compression (and extension) speeds were 1 mm/s. Reproduced with permission from reference (31). Copyright 2016 Wiley-VCH Verlag GmbH & Co.
Figure 12. Gel blocks of R)-12-hydroxystearic acid hydrazide in ethylene glycol: a) 5 wt% and b) 2 wt% gelator. ( c) Two 5 wt% gelator blocks placed in contact with each other; the lower one contains methylene blue. d) The two blocks in (c) after 17 h, showing their self-healing and the diffusion of methylene blue between them. Reproduced with permission from reference (31). Copyright 2016 Wiley-VCH Verlag GmbH & Co. 238 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Self-standing gels based on (R)-12-hydroxystearic acid hydrazide (4) and mixtures of ethylene glycol and DMF exhibit similar stiffness, although they became weaker at higher DMF contents (Figure 11). However, none of the homologues of 5 examined yielded a self-standing gel in any of the liquids tested. Self healing of an (R)-12-hydroxystearic acid hydrazide in ethylene glycol gel was demonstrated as shown in Figure 12. First, a cylindrical gel block was divided into two and one part was submerged into a solution of methylene blue in ethylene glycol. After remaining in contact for 17 h, the two pieces had merged and some of the methylene blue had diffused into the undoped part. The diffusion of an anionic dye, methylene blue, and a cationic dye, erythrosine B, from a gel of 2 wt% (R)-12-hydroxystearic acid hydrazide in ethylene glycol into ethylene glycol liquid was determined quantitatively using Fick’s second law (eqn (2)) (47, 48).
Mt is the total amount of dye released during the measurement time t, M∞ is the total amount of dye that was in the gel phase at time = 0, λ is the gel thickness; and D is the diffusion coefficient. D was calculated from the slope of a plot of Mt2 as a function of time, t. The diffusion coefficients of methylene blue and erythrosine B at 25 °C were found to be 7.59x10-12 and 6.04x10-12 m2 s-1, respectively. These values are ca. 10% of the self-diffusion rate of neat ethylene glycol (49); The gel matrix slows diffusion. After ca. 2.5 days, 53% of the anionic and 48% of the cationic dye had been released into the ethylene glycol liquid. These results suggest that the gels may be useful in slowing the release of drugs.
Conclusions We have summarized the properties of molecules with simple structures derived from HSA as organogelators and the properties of their gels. The natures of the aggregates of the gelators, starting from the individual molecules and proceeding to SAFINs, have been described. Those results have been correlated with various properties and efficiencies of the gels, especially those related to thixotropy, self-standing, and self-healing. Specifically treated here are amide and hydrazide modifications of the head group of HSA and their effects on the ability of the molecules to attain H-bonding networks which promote SAFINs and gelation of liquids. An ultimate goal of this and related research is to devise predictive models for how and when a gelator molecule will be able to gelate a specific solvent. Although we are far from achieving that end, progress is being made, and results like those summarized here should aid in reaching it. 239 Horkay et al.; Gels and Other Soft Amorphous Solids ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Acknowledgments We thank the U.S. National Science Foundation for its support of the portion of the research in this chapter conducted at Georgetown through grants CHE-1147353 and -1502856. We are also grateful to the researchers at Georgetown who have contributed to the results presented here.
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