Binary Diamondoid Building Blocks for Molecular Gels - Langmuir

Jun 5, 2014 - Adamantane is a type of diamondoid molecules that has a cage or globular shape with a diameter of 6.34 ± 0.04 Å.8 Anisotropic interact...
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Binary Diamondoid Building Blocks for Molecular Gels Mengwen Zhang and Charles F. Zukoski* Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: Adamantane is a type of diamondoid molecules that has a cage or globular shape with a diameter of 6.34 ± 0.04 Å.8 Anisotropic interactions between these truly nanoscopic particles can be induced through the derivatization of the diamondoid cage. Here we explore the gelation of paired systems of adamantane where attractions are introduced through van der Waals forces and hydrogen bonding. Gels are produced through the mixing of 1-adamantanecarboxylic acid (A1C) and 1adamantylamine (A1N). Upon mixing dimethyl sulfoxide solutions of these molecules at vanishing concentrations, these diamondoid molecules rapidly precipitate. A space-filling gel of the resulting aggregates is observed at approximately 3% by weight. These resulting gels have elastic moduli of 102−104 Pa in the 3−7 wt % concentration range. At a 1:1 mol ratio of 1adamantanecarboxylic acid (A1C) and 1-adamantylamine (A1N), the gel’s elastic modulus and yield stress increase as volume fractions ϕx and ϕy with x ≈ 4.2 and y ≈ 3.5. The dependencies of moduli and yield stress on the volume fraction display characteristics of colloidal gels. Transmission electron microscope (TEM) images indicate that the gels are formed from a network of interwoven and branched fibers which are composed of ∼30 nm crystallites that have undergone oriented aggregation to form fibers.

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INTRODUCTION Molecular gels have received much attention in the past decade as a type of molecular self-assembly.1,2 The gels are composed of fibrillar networks that form at very low concentrations and have potential applications in chemical sensing,3,4 oil recovery,5 regenerative medicine,6,7 and the construction of optical devices.5 These molecules interact via noncovalent anisotropic interactions such as H-bonding, π−π stacking, van der Waals, dipole−dipole, and coordination interactions to form a spacefilling network that entraps the solvent at concentrations of as low as ≤2 wt %.2 Here we introduce a class of molecular gels produced from derivatized adamantane (C10H16), a diamondoid molecule composed of a saturated carbon cage. The molecule is analogous to a typical colloidal particle in shape, except on a much smaller scale (diameter of 6.34 ± 0.04 Å).8 Previously, adamantane-based molecules have been used in several molecular gel systems.9−12 For instance, adamantane has been used in the formation of adamantane-peptide hydrogelators for cell culturing and recovery purposes.11 More recently, Harada et al. discovered a new class of adamantane-based gels formed through the host−guest interaction of β-cyclodextrins and an adamantane monomer. These βCD-Ad gels have exceptional shape recovery and stretching properties and have shown great promise in medical applications.12 In this case, we will be studying a binary gel that is formed from 1-adamantanecarboxylic acid (A1C) and 1-adamantylamine (A1N) dissolved in dimethyl sulfoxide (DMSO) (Figure 1). Anisotropic interactions are introduced with the addition of side groups (in this case, a −COOH and an −NH2). The adamantane core molecules experience van der Waals attractions while the amine and carboxylic acid groups experience hydrogen bonding. © 2014 American Chemical Society

Figure 1. (a) Three-dimensional representations of 1-adamantylamine (A1N) and 1-adamantane-carboxylic acid (A1C). (C, gray; H, white; N or O, black). (b) Colloidal analogues of A1N and A1C.

These are some of the classical interactions seen in single- and binary-component molecular and colloidal gel systems.13,14 As shown in Figure 1, the interaction between the two molecules closely resembles that of two colloidal particles with partial charges. The difference from conventional colloidal suspensions lies in the “particles” being subnanometer in size and being hollow, which will reduce the magnitude of the van der Waals attractions. Of particular interest to us is how closely the rheological behavior and structure of the binary molecular gel resemble those of colloidal systems. Here, we will first describe the gel, then analyze both its mechanical properties and structure, and finally explore the relationship between this molecular gel and typical colloidal gels. Received: January 22, 2014 Revised: May 29, 2014 Published: June 5, 2014 7540

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concentration. Additional scattering data were acquired at the George L. Clark X-ray Facility at UIUC to capture the q range (0.2 to 0.6 Å−1) that was not captured at the Argonne facility. The wavelength was adjusted to 1.54 Å, and a capillary tube was used as the sample holder.

EXPERIMENTAL SECTION

Materials. Gelators 1-adamantanecarboxylic acid (C11H16O2) and 1-adamantylamine (C10H17N) were acquired from Sigma-Aldrich (Figure 1a). Both molecules have a diamondlike caged molecular structure. Reagent-grade DMSO (>99.9%) from Fisher Scientific was used as the solvent. Preparation of Gels. Stock solutions of 1-adamantanecarboxylic acid (A1C) and 1-adamantylamine (A1N) in DMSO were prepared individually. Each solution was held at 70 °C for 20 min until all of the gelating molecules had completely dissolved. The solutions were then cooled to room temperature and checked for clearness. To produce gels, solutions of A1C and A1N were rapidly mixed together for 30 s using a Thermolyne mixer. Rheology. Rheology measurements were performed using a Bohlin rheometer at room temperature. Cone-and-plate geometry (CP 4°/40 mm) with a gap space of 150 μm was employed. To investigate the effect of gelator concentration on rheology, the gels were prepared by mixing equimolar amounts of A1N and A1C at various concentrations. To investigate the effect of stoichiometry on gel rheology, the gels were prepared by mixing nonstoichiometric amounts of A1N and A1C at fixed concentrations. Amplitude sweep tests were performed at a frequency of 1 Hz at room temperature to measure the gel’s elastic modulus (G′) and viscous modulus (G″) as a function of shear stress (σ). The gels were presheared manually at first to remove heterogeneity from the sample. Amplitude sweeps were then performed at a frequency of 1 Hz three times for each sample. The linear elastic moduli (Go′), linear viscous moduli (Go″), and yield shear stress (σy) were determined in these experiments. At the yield stress σy, the elastic modulus equals the viscous modulus (G′ = G″) and the sample transforms from a solidlike (G′ > G″) state to a liquidlike (G′ ϕg, rloc/D ≈ 0.1(ϕg/ ϕ)1.6, then we predict that Go′ ≈ Go′g(ϕ/ϕg)4.2, where Go′g = 102ϕgkT/D3. For D = 30 nm, Go′g ≈ 380 Pa, which is on the order of what is observed in the data in Figure 5. While these estimates of the magnitude of the modulus are approximate at best, they show that if we were to assume that the adamantane gels studied here were composed of 30 nm particles that were aggregated due to a pair attraction with a width of ̈ mode approximately 10% of a particle diameter, then naive coupling theory could be used to predict the magnitude of the elasticity observed. The remarkable aspect of this theory is that it makes predictions of elasticity with no recognition of mesoscopic structure; that is, it shows a weak dependence on the nature of the aggregation, whether in a fractal, fibrous, or centrosymmetric manner.28−31 These rough calculations suggest that the elasticity and mechanics of the fibrous adamantane gels can be understood in terms of colloidal gel theories despite the fact that the mesoscopic structure of the gels is that of fibers composed of oriented aggregates of molecular crystals.

Figure 9. (a) TEM showing the fibrous network of the gel. (b) Slightly enlarged view of the fiber network. (c) TEM image showing a single fiber with a slightly rough surface. (d) Small aggregates with a size of 30−50 nm are observed in the enlarged view of the internal structure of a fiber. The electron diffraction pattern of the fiber is shown in the lower right corner.

Gelation Scheme. On the basis of both rheology and structural data, we propose that the gels are formed through the scheme shown in Figure 10. As supported by WAXS, TEM, and

Figure 10. Schematic of the gel’s aggregation scheme: A1C and A1N first aggregate into ∼30 nm crystallites, which further aggregate into fibers with varied diameters (200 nm to >1 μm).

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the literature data, A1N and A1C molecules associate via a three-dimensional hydrogen bonding network and van der Waals forces into ∼30 nm crystallites upon mixing in DMSO. SAXS, TEM, and DLS data suggest that these ∼30 nm crystallites further aggregate into large interwoven and branched crystalline fibers that have slightly rough surfaces and diameters varying from 200 nm to 1 μm. The resulting fibrous structure indicates that aggregation takes place between objects that themselves experience anisotropic interactions. Although we have not been able to pinpoint the exact mechanism for this anisotropic aggregation, we believe that it is the interplay of the van der Waals force, hydrogen bonding, and the insolubility of adamantane core molecules that contributes to this anisotropic aggregation. The fibers then

CONCLUSIONS Unlike typical molecular gelators that have expanded chemical structures with multiple aromatic rings for π−π stacking and are capable of forming many bonds with neighboring molecules, this binary gel is composed of small and compact, almost colloidlike, diamondoid molecules that are capable of forming a limited number of bonds. While ordered aggregates are formed when 1-adamantylamine and 1-adamantanecarboxylic acid dissolved in DMSO are mixed at very low concentrations, space-filling gels of these crystalline aggregates are formed for total adamantane concentrations of gel at 2.5−3 wt % and higher. The gels are composed of oriented aggregates of ∼30 nm crystallites that form branched fibers that are 0.2−1 μm 7544

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thick. We believe that it is the combination of van der Waals forces, hydrogen bonding, and the insolubility of adamantane core molecules that is responsible for this anisotropic aggregation. These structures are disrupted by shear but reform in a reversible manner upon cessation of shear. The gel stiffness depends on the limiting reactant (A1C or A1N) in the system, and the gel is stiffest at equimolar amounts of A1C and A1N. Gels formed from A1C and A1N exhibit rheological behavior similar to that seen in many colloidal gels. For instance, the moduli of the gels grow with volume fraction as Go′ ≈ ϕx (where x is 4.2) falls in a range commonly seen for colloidal gels. Furthermore, by treating the crystals as 30 nm colloidal particles, mode coupling theory captures the onset and magnitude of the elastic modulus of the gels. For future studies, there are several important issues that will be addressed: (i) the independence of crystal size on concentration, stoichiometry, or drying; (ii) the mechanism for the uniaxial anisotropic aggregation of the crystallites; and (iii) other similarities observed in the rheological behavior of this system and typical colloidal systems.

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ASSOCIATED CONTENT

S Supporting Information *

Crystal sizes plotted against volume fractions for gels with equimolar amounts of A1N to A1C and for various mole fractions of A1N (xA1N) for 5 wt % gels. Wide angle X-ray diffraction data for a 3 wt % gel phase-matched with the single crystal data for adamantane-1-ammonium 1-adamantanecarboxylate. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the U.S. Department of Energy, Division of Materials Science under award nos. DE-FG02-07ER46471 and DE-FG02-07ER46453, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. Research was carried out in part at the Frederick Seitz Materials Research Laboratory Central Facilities and George L. Clark X-ray Facility at the University of Illinois at Urbana-Champaign. We gratefully acknowledge Danielle Gray, director of the George L. Clark X-ray Facility at Noyes Laboratory, for help and discussions on X-ray data analysis.

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(27) Viehman, D.C.; Schweizer, K.S. Theory of gelation, vitrification, and activated barrier hopping in mixtures of hard and sticky spheres. J. Chem. Phys. 2008, 128, 084509. (28) Chen, Y.L.; Schweizer, K.S. Microscopic theory of gelation and elasticity in polymer-particle suspensions. J. Chem. Phys. 2004, 120, 7212−7222. (29) Ramakrishnan, S.; Zukoski, C.F. Microstructure and Rheology of Thermoreversible Nanoparticle Gels. Langmuir 2006, 22, 7833− 7842. (30) Gopalakrishnan, V.; Zukoski, C.F. Yielding Behavior of Thermoreversible Colloidal Gels. Langmuir 2007, 23, 8187−8193. (31) Shin, H.; Schweizer, K.S. Self-consistent phonon theory of the crystallization and elasticity of attractive hard spheres. J. Chem. Phys. 2013, 138, 084510.

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