Article pubs.acs.org/Langmuir
Influence of Anions and Alkyl Chain Lengths of N‑Alkyl‑n‑(R)‑12Hydroxyoctadecyl Ammonium Salts on Their Hydrogels and Organogels V. Ajay Mallia, Hyae-In Seo, and Richard G. Weiss* Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, D.C. 20057-1227, United States S Supporting Information *
ABSTRACT: The self-assembly and gelating characteristics of a set of N-alkyl-(R)-12-hydroxyoctadecylammonium salts (nHOA-X, where n = 0−6, 18 is the length of the alkyl chain on nitrogen, X = Cl, n = 3, and X = Br, NO3, and BF4) are described. Solid−solid phase transitions were observed for powders of n-HOA-Cl, and orthorhombic-type crystal packing arrangements and lattice spacings were calculated from X-ray diffractograms at 22 °C. The diffractogram of 3-HOA-Br indicates the presence of more than one morph at room temperature, and that of 3-HOA-I corresponds to a lamellar packing arrangement. Differences in the molecular packing arrangements of 3-HOA-X are reflected in their gelation abilities. The melting temperatures (Tgel) of the hydrogels of 3HOA-Br are higher than those of 3-HOA-Cl at the same concentrations, and 3-HOA-I failed to gelate any of the investigated liquids. 3-HOA-NO3 gelated only water and CCl4 and 3-HOA-BF4 formed only hydrogels. Plots of changes in conductivities of the 3-HOA-X salts (where X = Cl, Br, NO3 and BF4) as a function of temperature were used to calculate the critical aggregation concentrations (CGCs). Because the CGCs from the ‘falling drop’ method are nearly the same as those from the conductivity measurements, aggregation, nucleation, and gelation must occur within a very narrow 3-HOA-X concentration range. Tgel values of 2 wt % 3-HOA-Cl hydrogels (prepared by fast cooling of the sol phase) increased upon adding KCl up to 0.1 M. The effects can be attributed principally to the chloride anion rather than its cation partners. The properties of the hydrogels of 3-HOA-X do not follow the Hofmeister ranking rule. The variations in the counterions afford detailed insight into the behavior of 3-HOA-X in their neat solids and assemblies in gels as well as the processes accompanying gel formation in water and organic liquids.
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INTRODUCTION Low-molecular-mass organogelators (LMOGs) are molecules that can entrap a large volume of liquid, resulting in semisolid viscoelastic materials.1−6 Noncovalent interactions, such as H bonding, electrostatic forces, π−π stacking, and London dispersion forces, allow the LMOG molecules to interact physically to form 3D, metastable, self-assembled fibrillar networks (SAFINs).1 The 3D networks immobilize the liquid component on a macroscopic scale via surface tension and capillary forces.7 Interest in molecular gels has increased dramatically during the last few decades as the number of their potential and realized applications has grown8 and because they offer fundamental insights into how molecules are able to selfassemble into 1D objects.9−13 In addition, gels with ionic LMOGs are potentially important as electrolytes for lithium batteries, dye-sensitized solar cells, and capacitors.14 Although the range of structures of known LMOGs is extremely diverse, no comprehensive understanding of the relationship between mechanisms of gelation and the molecular structure of uncharged or ionic LMOGs exists currently. Controlling the noncovalent gelator−gelator, gelator−liquid, and liquid−liquid © 2013 American Chemical Society
interactions to design new gelators is expected to remain a daunting challenge for the foreseeable future.7 As a result, most new LMOGs are discovered serendipitously or by small modifications of known LMOG structures rather than by a priori design.15 The challenges in the field are especially acute if new LMOGs capable of gelating both low- and high-polarity liquids are sought.15 In this regard, we have investigated LMOGs based on simple structures, such as derivatives of (R)-12-hydroxystearic acid (HSA), by converting them to amines and amides.16 Recently we reported that several ammonium salts derived from HSA, N-alkyl-(R)-12-hydroxyoctadecylammonium chlorides (n-HOA-Cl, where n = 0−6 or 18), gelate both water and organic liquids (i.e., they are ambidextrous).17 The structures contain clearly identifiable sites that are hydrophilic (the 12-hydroxy group, the cationic ammonium center, and the anion) and hydrophobic (the long octadecyl Received: February 27, 2013 Revised: April 17, 2013 Published: May 15, 2013 6476
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the point of maximum heat flow. Thermal gravimetric analyses (TGA) were performed on a TA 2910 differential scanning calorimeter interfaced to a TA Thermal Analyst 3100 controller while a slow stream of nitrogen flowed through the instrument cell. Powder X-ray diffraction (XRD) patterns of samples were obtained on a Rigaku R-AXIS image plate system with Cu Kα X-rays (λ = 1.54 Å) generated by a Rigaku generator operating at 46 kV and 40 mA with the collimator at 0.5 mm (to obtain 0.5-mm-diameter beams of Xrays20). Data processing and analyses were performed using Materials Data JADE (version 5.0.35) XRD pattern processing software. Samples were sealed in 0.5 mm glass capillaries (W. Müller, Schönwalde, Germany), and diffraction data were collected for 1 (neat powders) or 10 h (gels). IR spectra were recorded using a Varian 3100 FT-IR spectrometer with a PIKE MIRacle ATR sampling accessory. Conductivities were measured with a conductivity meter (Yellow Spring Instrument Co., model 31) using Au electrodes in a cell with a Teflon body designed by Prof. Robert de Levie. Rheological measurements were made on an Anton Paar Physica MCR 301 strain-controlled rheometer using a Peltier temperature controller and parallel plates (25 mm diameter) separated by 0.5 mm. Data were collected and analyzed using Rheoplus/32 Service V3.10 software (Anton Paar, Germany). Before data were recorded, each sample was placed between the shearing plates of the rheometer and heated to 80 °C to ensure that a solution/sol was present. It was cooled at ∼20 °C/min to 10 °C and incubated there for 15 min to reform the gel. The temperature was increased to 25 °C, and the sample was incubated there for 15 min before the experiment. To limit liquid evaporation, the rheometer sample compartment was fitted with a solvent trap, and thin layers of paper soaked in the sample liquid were kept inside the trap. AFM imaging was accomplished on a Bruker BioScope Catalyst with peak force tapping using a ScanAsyst-Air probe (k ≈ 0.4 N/m and tip radius < 10 nm). Hydrogel samples were prepared on microslides (Corning 75 × 25 × 1 mm3) using the fast-cooling protocol. Images were analyzed using Research Nanoscope 8.15 software.
chain of the parent amine and the N-alkyl chain). As such, the structure of the salts can be fine-tuned to modulate the noncovalent interactions that control molecular aggregation.
Here, we investigate how structural changes, principally of the anion, of neat N-alkyl-(R)-12-hydroxyoctadecylammonium salts, n-HOA-X (n-HOA-X, where n = 0−6, 18 and X = Cl; n = 3 and X = Br, NO3, and BF4), influence the phase behavior of neat solids and the ability of the salts to form gels. In addition, we report the conductance properties of the n-HOA-X (n = 3 and X = Cl, Br, NO3, and BF4) in water as a function of concentration as a means to determine their critical aggregation concentrations (CGCs),18 and we compare them with the macroscopically determined CGCs obtained by the falling drop method. Finally, from results with hydrogels of 3-HOA-Cl, it has been possible to discern that added chloride has a much larger influence on the gel properties than do their cations (Na+, K+, Ca2+, and Fe3+).
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EXPERIMENTAL SECTION
Details of material preparations, characterizations, and procedures are described in the Supporting Information. Gelator concentrations are expressed throughout as weight/liquid volume percentages (wt %). Samples to test gelation properties were prepared by cooling hot solutions/sols in flame-sealed glass tubes (∼5 mm i.d.) either by plunging them into an ice−water bath (∼5−10 °C) for 10 min (fastcooling method) or by leaving the solution/sol in a hot-water or oil bath while it cooled to ambient temperature (23−24 °C) over ∼3 h (slow-cooling method). Unless stated otherwise, all samples were prepared by fast cooling. In both cases, the tubes were kept at ambient temperatures for ∼1 h before their appearance was noted. If macroscopic phase separation was not visible and no flow was observed over periods of >10 s when the tubes were inverted, the samples were designated in a preliminary assessment as gels. Some of the samples that met this criterion were actually viscous dispersions rather than true gels; for purposes of simplicity, they will be referred to as gels also, except where their rheological properties are discussed (vide infra). Gelation temperatures (Tgel) were determined by the falling drop method:19 the sealed glass tubes were inverted and placed in a water bath that was heated at ca. 1.5 °C/min; the temperature ranges over which the gels fell under the influence of gravity are indicated as Tgel. Critical gelator concentrations (CGCs) were determined at 24 °C by the falling drop method using a series of fast-cooled gels in which the gelator concentrations were changed in 0.1 wt % increments. Gel samples for examining polarizing optical micrographs (POMs) were heated to their sol phases and placed rapidly in flattened glass capillary tubes (0.4 mm path length, VitroCom, NJ) that were flame-sealed. The samples were reheated to their sol phases and cooled prior to recording the POMs on a Leitz 585 SM-LUX-POL microscope equipped with crossed polarizers, a Leitz 350 heating stage, a Photometrics CCD camera interfaced to a computer, and an Omega HH503 microprocessor thermometer connected to a K (Chrome-Alome) thermocouple (Omega Engineering, Inc.). Differential scanning calorimetry (DSC) was carried out on a DSC Q200 calorimeter (TA Instruments, New Castle, DE) interfaced to a TA Thermal Analyst 3100 controller connected to an RCS90 cooling system. Heating and cooling (at 10 °C/min) were done in a 50 mL/ min stream of nitrogen. Samples were hermetically sealed in T-zero hermetic lids and pans that were weighed before and after DSC runs to determine whether any liquid had evaporated; unless stated otherwise, no liquid was lost. Transition temperatures from DSC are reported at
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RESULTS AND DISCUSSION Structures and Phase Transitions of Neat Salts: Chloride Salts. Molecular packing within crystalline materials is determined by thermodynamic factors (including tendencies to minimize free volume and the sum of the energies from attractive and repulsive van der Waals interactions) as well as dynamic considerations (including diffusion, epitaxial growth rates, and temperature-related disordering).21 Those factors are instrumental in determining the nature of 1D fibers of SAFINs as well.4−6,22 Although it is not possible to dissect these parameters here with the information at hand, some insights into the molecular packing modes can be gleaned. The melting temperatures of the salts increased with increasing N-alkyl chain length, maximizing at n-pentyl (5HOA-Cl) and decreasing thereafter for the n-hexyl (6-HOACl) and the n-octadecyl (18-HOA-Cl) homologues. No alternation of melting temperatures between odd- and evennumbered alkyl chain lengths (as is observed with n-alkanes23) was found; the association among the alkyl chains in these LMOGs cannot be as intimate as it is among n-alkane chains. When the N-alkyl chain was four or more carbon atoms in length, more than one solid phase could be detected, and the temperature range between phase transitions increased with increasing chain length (Supporting Information Table S1). Polymorphism is a well-known phenomenon in alkylammonium salts.24 From the POM optical textures, the transitions are solid−solid and solid−liquid; no evidence for a transition to a liquid-crystalline phase was found, although they are sometimes present in salts with long alkyl chains.25−27 6477
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Table 1. Appearances,a Tgel Values (°C),b Critical Gelator Concentrations (CGCs; wt % in Brackets), Periods of Stability (in Parentheses)c of Gels Containing ∼2 wt % 3-HOA-Cl,17 and Other 3-HOA-X in Various Liquids (in order of Decreasing Dielectric Constant of the Liquid Components36 at 20 or 25 °C) liquid (dielectric constant)
3-HOA-Cl
3-HOA-Br
3-HOA-I
3-HOA-NO3
3-HOA-BF4
water (80.1) acetonitrile (37.5) methanol (32.7) 1-butanol (17.5) benzyl alcohol (13.0) 1-octanol (10.3) toluene (2.4) CCl4 (2.2) n-dodecane (2.0) cyclohexane (2.0) n-hexane (1.9)
OG (76−77, >1 year) [0.3] OG (76c >1 year) [0.4] soln soln soln P CG (77−78, >1 year) [0.5] TG (76−77, >1 yeare [0.5] I P I
OG (83−84, >1 year) [0.3] OG (61−62, >1y) [0.6] soln P soln P CG (69, >1 year) [0.5] TG (74, >1 year) [0.5] I P I
P P soln soln soln P soln soln I I I
OG (80−81, >1 year) [0.5] P soln soln soln soln soln TG (59, >1 year) [0.8] I P I
OG (81, > 1y)d[0.6] soln soln P soln P soln soln I P I
a
OG, opaque gel; soln, solution; visc, viscous solution/sol; P, precipitated from sol; I, insoluble when heated in liquid; TG, translucent gel; CG, clear gel. bTemperature ranges from the falling drop method. cPhase separation observed. dTime between when a gel was prepared in a sealed container and when it underwent visually detectable phase separation after being kept at ∼24 °C. eSyneresis after 5 months.
solid phases must be in a bent conformation, in an extended conformation but at an angle with respect to their layer planes (Supporting Information Figure S14), or both. A diffuse peak in the wide-angle region in the diffractogram recorded at 130 °C is indicative of significant disorganization of the alkyl portions of the molecules and less crystallinity in the higher-temperature phase. A similar comportment was found for 18-HOA-Cl (Supporting Information Figures S15 and S16). The IR spectrum of neat 3-HOA-Cl (Supporting Information Figure S17) includes broad peaks at 3316 and 3196 cm−1 that can be assigned to stretching modes of NH and OH bonds, respectively, that are involved in strong H-bonding interactions.30 From these and the other data, we suggest that the nHOA-Cl species self-assemble for the most part in layers with their alkyl parts packed to maximize London dispersion interactions and with the charged centers and hydroxyl groups positioned to achieve electrostatic pairing and extended hydrogen bonding. At higher temperatures, the dispersive interactions may be weakened sufficiently to allow more motions of the alkyl chains, possibly leading to different polymorphs (and eventual melting). Salts with Anions other than Chloride. The DSC heating thermogram for neat 3-HOA-Br includes solid−solid and solid−melt endotherms. (with a shoulder on the hightemperature side of the latter); the cooling thermogram shows the shoulder clearly separated (as an exotherm) as well as the other two transitions as exotherms (Supporting Information Figure S18). Consistent with these observations, the XRD diffractogram of the neat powder at room temperature indicates the presence of two coexisting morphs, one of which is consistent with an orthorhombic packing arrangement (Supporting Information Figure S19). The space group of the higher-temperature phase could not be assigned with the small number of diffraction peaks observed. All of the structural analyses performed on this salt, as well as repeated recrystallizations, indicate that the second morph is not a result of the presence of an impurity. An analysis of the reflections obtained from XRD studies of the compounds having polyatomic counterions, 3-HOA-NO3 (Supporting Information Figure S21) and 3-HOA-BF 4 (Supporting Information Figure S22), also exhibit orthorhombic packing arrangements. The d spacings calculated from the Bragg relationship for the peaks in the diffractogram of neat
Because the heats and temperatures of transition from DSC thermograms of n-HOA-Cl (n = 0 to 6 and 18) (Supporting Information Table S1) are reproducible in subsequent heating and cooling cycles, the salts are thermally stable within the temperature ranges explored. Before melting to its isotropic phase at 137 °C, 0-HOA-Cl underwent a crystal-to-crystal transition at 78.7 °C that was not detectable clearly by POM (Supporting Information Figures S2 and S3) and another at 132.8 °C that is detectable (Supporting Information Figure S4). The distances corresponding to the lowest-angle peaks in the diffractograms of 0-HOA-Cl are approximately twice its calculated extended length,28 suggesting a bilayer packing arrangement (Supporting Information Figure S5 and Table S2). However, indexing the crystal structure is difficult because of the small number of reflection peaks in the XRD diffractogram. Although more than one phase transition, including an exotherm on heating, was observed for 1-HOA-Cl as well (Supporting Information Table S1 and Figure S6), the thermogram of 2-HOA-Cl gave no evidence of polymorphism (Supporting Information Figure S7). Exothermic peaks, separated by small temperature increments, were seen in the cooling thermograms of 3-HOA-Cl (Supporting Information Figure S8); only one endotherm could be detected on heating, however. The lattice spacings (d, Å) of 3-HOA-Cl, calculated from the Bragg relationship and summarized in Supporting Information Figure S9 and Table S2, were indexed to an orthorhombic-type crystal.29 Three polymorphic forms are known for long-chain ammonium salts that are packed in layers: the α phase, in which the long molecular axes are orthogonal to the layer planes and the alkyl chains rotate about their long axes; the β form, in which the packing is like that of the α phase but the chains do not rotate about their long axes; and the γ form, in which the long molecular axes are at a nonorthogonal angle with respect to their layer planes and the chains do not rotate about their long axes.24 Consistent with the data from the DSC measurements for 6-HOA-Cl, its XRD diffractograms at 22 and 130 °C showed the presence of different morphs (Supporting Information Figure S14). The fully extended length calculated for 6-HOA-Cl, 34.7 Å, is longer than the Bragg distance for the lowest-angle diffraction peaks (assigned tentatively as the 001 reflection), 30 Å, in the diffractograms at both 22 and 130 °C (Supporting Information Figure S14). The molecules in the 6478
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Figure 1. Polarizing optical micrographs at 23 °C of gels of 5 wt % 3-HOA-Br in water (A, B) and 5 wt % 3-HOA-Br in toluene (C, D) prepared by the fast-cooling (A, C) or slow-cooling (B, D) protocols. The images were recorded with a full-wave plate.
Figure 2. AFM images at 23 °C of hydrogels, prepared by the fast-cooling protocol, containing 2 wt % (A) 3-HOA-Cl and (B) 3-HOA-Br. The scale bar is 200 nm.
3-HOA-I correspond to a 1:1/2 :1/3 :1/4 :1/6 progression (Supporting Information Figure S20), which is consistent with a layered packing arrangement. In solid phases of a primary alkyl ammonium halide, the charged nitrogen atom resides next to its halide counterion and is hydrogen bonded to it. The energy of these H bonds usually follows the order Cl > Br > I. Thus, the ammonium center experiences a significant barrier to rotation.31 Decreasing the energy of the hydrogen-bonding interactions or increasing the temperature facilitates ammonium reorientations and may affect the preferred packing arrangements of 3-HOA-X. However, the solid phases of ethylammonium chloride, bromide, and iodide salts, perhaps as a consequence of their very short alkyl chain (and limited opportunities for stabilization by London dispersion forces), have the same molecular packing arrangement at room temperature.32
Structure and Properties of Molecular Gels. As noted, the ability to form SAFINs of hydrogels of 3-HOA-X and the packing within them depend on the delicate balance between the hydrophilic and hydrophobic interactions of the gelator molecules.13,33,34 In that regard, 3-HOA-I failed to gelate any of the liquids investigated (Table 1) whereas at equal gelator weights, Tgel values of hydrogels of 3-HOA-Br were higher than those of 3-HOA-Cl. Obviously, these differences must be attributed to differences in the aforementioned interactions and packing arrangements. The sizes of objects in SAFIN networks are known to be sensitive to the rate of cooling of the sol phases, and previous examples based on n-HOA-Cl derivatives reported17 (but not some other gelators with very different molecular structures) show that fast- and slow-cooling protocols do not alter gel melting temperatures (as is the case here with the n-HOA-X) or some other properties; the 6479
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neat gelator). Despite the clear differences between the fibrillar networks of 2.0 wt % 3-HOA-Br gels made by the fast- and slow-cooling protocols, their Tgel values, at least for the hydrogel and toluene gel, were the same within experimental error; similar results were found previously with 3-HOA-Cl as the LMOG.17 By contrast, the XRD (and POM, Figure 1) of the fast-cooled toluene gel of 3-HOA-Br gave no evidence for the presence of crystalline objects within its SAFIN. Although the molecular packing in the neat solid and hydrogel of 3HOA-Cl are known to be the same, they differ from the packing arrangements within the fibrillar networks of the organogels examined.17 Either the fibrillar objects in the fastcooled toluene gel of 3-HOA-Br are too small to be detected by our XRD instrument (N. B., Scherrer’s law)50 and the microscope or the packing arrangement is different from that of its hydrogel here as well. Upon the basis of the very large heats of melting found in the DSC studies for these gels (Supporting Information Figures S29−S32), the fibrillar networks must be crystalline. Additionally, rheological studies at 25 °C on fast-cooled samples of 3-HOA-Br toluene gels showed that they are not true gels; they are gel-like, weakly viscoelastic dispersions. (Supporting Information Figure S27). Thus, G′ and G″ values in the frequency sweep were not completely independent of the applied frequency over a range of at least 0.01−100 rad/s at 25 °C (Supporting Information Figure S28). Normalized enthalpies for the gel−sol and sol−gel phase transitions have been calculated for 3-HOA-Br hydrogels and toluene gels from their DSC heating and cooling thermograms. The value of Tgel increased as the concentrations of 3-HOA-Br were increased (i.e., as the fibrillar networks became fully formed; Supporting Information Figures S27−S30 and Table S3). The heating DSC thermogram of the hydrogel showed two endotherms, the second of which was less pronounced at higher 3-HOA-Br concentrations, and only one exotherm during the cooling of the sols (Supporting Information Figures S29 and S30). The lower-temperature transition is attributed to a gel− gel phase transition within the fibrillar networks.51 By contrast, two exotherms were observed during the cooling of the 3HOA-Br sols in toluene, but only one very broad endotherm was evident in the heating endotherms of the toluene gels (Supporting Information Figures S31 and S32). These results were not totally unexpected because, as noted above, neat 3HOA-Br exhibited a crystal−crystal phase transition at 91.8 °C during heating and transitions at 106.5 and 85.6 °C during the cooling of its melt (Supporting Information Figure S18). The normalized enthalpies were calculated by multiplying the observed ΔH values by the inverse of the gelator concentrations minus the CGC; in this way, the normalized enthalpies for the gels can be compared directly with the heats found for the neat gelators (i.e., extrapolating to 100 wt % LMOG). The normalized enthalpies for the gel-to-sol transition of both the hydrogel and the toluene gel of 3-HOA-Br were significantly larger than the heat of melting for neat 3-HOA-Br. Similar observations were reported for other gel systems.52,53 They can be attributed either to the melting of the fibrillar networks into nonideal solutions (rather than the ideal ones that the data treatment assumes) or to the molecular packing arrangements of the neat solids and within the fibrillar networks being different. In these cases, the packing of these LMOGs within their gel fibers is a very important if not dominating factor: the molecular packing arrangements in neat 2-HOA-Cl and 3HOA-Cl and in the fibers of their lower-temperature hydrogel
clear manifestation of the rate of sol cooling on the SAFINs is the size of the fibrillar objects. For these reasons, we have performed experiments on gels prepared by the fast-cooling protocol except where POM images have been compared (Figure 1). Also, we reported earlier that varying the n-alkyl chain length (n) can drastically alter the self-aggregation properties of n-HOA-Cl in ways that are explicable on the bases of polarity and size.17 A similar dependence on the balance between hydrophilic and hydrophobic character has been reported for glutamate bolamphiphiles, whose sols in methanol/water mixtures lead to gels when the linker is butylene or pentylene and to a precipitate when it is hexylene.35 From rheological measurements, the upper limit of the linear viscoelastic regime of a hydrogel containing 2.1 wt % 3-HOABr was ca. 0.1% strain at 1 rad/s (at 25 °C); under similar conditions, the yield strain for the hydrogel of 3-HOA-NO3 was 0.5% (Supporting Information Figures S23 and Figure S24). The storage modulus (G′) and loss modulus (G″) values are independent of the applied frequency at 25 °C and 0.1% strain (Supporting Information Figures S25 and S26). Although G′ is higher than G″, both values are lowthe materials are weak gelsand they undergo phase separation at higher strain. The rheological properties of the hydrogels of the chloride salt are similar to those of the bromide and nitrate. The aggregates of 3-HOA-Br in its hydrogel and toluene gel, prepared using the fast-cooling protocol, cannot be seen clearly by POM at the magnifications available with our optical microscope (Figure 1A,C), but those made by the slow-cooling protocol are spherulitic (Figure 1B,D). Similar spherulite-like objects have been found in many other gel systems37,38 and nucleating solutions. 39 The branching patterns in the spherulites can be altered significantly by changing the thermodynamic driving force for their formation (i.e., by altering the incubation temperature or rate of cooling of the sol phases).40−47 Generally, more highly branched fibrillar networks and smaller spherulites result when Tgel − Tin (where Tin is the incubation temperature) for sols is larger (i.e., the driving force for phase separation in the supersaturated sol phase is larger).48 AFM images of the SAFINs of the 2 wt % hydrogels (fastcooled) of 3-HOA-Cl and 3-HOA-Br (Figure 2A,B) showed fibrillar bundles, similar to the results from cryo-SEM studies.17,49 Attempts to image the SAFINs of these gels in silicone oil and toluene by AFM were unsuccessful. XRD diffractograms (Figure 3) of the hydrogels (prepared by the fast-cooling protocol) of 3-HOA-Br were consistent with the presence of coexisting morphs in the SAFINs (as found for the
Figure 3. XRD diffractograms of 3-HOA-Br at 22 °C: neat solid (red); 5.0 wt % hydrogel (fast-cooled, blue); and 5.0 wt % toluene gel (fastcooled, green). The gel curves result from the empirical subtraction of solvent diffraction. 6480
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Conductance Studies of the Ammonium Salts in Aqueous Solutions and Hydrogels. Because the n-HOA-X species aggregate, their conductivities are expected to increase with increasing concentration, but in an attenuated fashion.54 Such dependencies are well documented for surfactant molecules that form micelles, and conductivity changes are a useful tool for calculating critical micellar concentrations.18 Surprisingly, we have been unable to find examples of the use of conductivity to determine the CGCs of LMOG-based gels. We demonstrate here the utility of such measurements to calculate CGC values of 3-HOA-X (X = Cl, Br, NO3, and BF4) in water at room temperature (Figure 5). Comparisons have been made with the values obtained by the falling drop method.19 The two sets of CGCs need not be the same because on heating a gel the falling drop method provides data related primarily to the loss of junction zones of a SAFIN whereas the conductivity measurements report on the degree to which the aggregates dissolve. At room temperature, both methods report on steady-state properties of the systems, and the concentration dependence of the LMOG on conductivity can be related to the degree of aggregation because the mobility of the ions and the number that are not ion-paired decrease as the concentration increases. In each case, the conductivity increases nearly linearly with increasing gelator concentration and then reaches a plateau value, which we interpret to be the gel region. Because the points of intersection between the two linear segments are very near the CGC values determined by the falling drop method (Supporting Information Table S4), the inception of aggregation and SAFIN formation must occur over very narrow concentration ranges of 3-HOA-X. Effect of Inorganic Salts on Hydrogels of 3-HOA-Cl. Simple electrolytes are known to affect the properties of water, which can lead to various changes in solute conformations and degrees of aggregation.55 Specific and selective salt ion effects have been investigated in protein folding and protein
phases are the same, and the heat of melting of each neat solid and the normalized value for its hydrogel was similar; the molecular packing within the fibers of the toluene gels does not match that of the neat solids, and the heats were quite different. On the basis of the heats of melting from the DSC studies (Supporting Information Figures S29, S30, S31, and S32 and Table S3), SAFIN networks are crystalline in nature. Fibrillar objects in the fast-cooled gels of 3-HOA-Br may be too small to characterize using POM or XRD.50 For the salts with a more complex anion than the halides, 3HOA-NO3 gelated water as well as CCl4, whereas 3-HOA-BF4 gelated only water (Table 1). Fiberlike structures were observed within each of these gels (Supporting Information Figures S33 and S34) and their XRD patterns are similar as well (Figure 4 and Supporting Information Figures S35 and S36).
Figure 4. XRD diffractograms at 22 °C: (A) neat solid 3-HOA-NO3 (green) and its 5.0 wt % hydrogel (red); (B) neat solid 3-HOA-BF4 (green) and its 5.0 wt % hydrogel (red). The hydrogel curves are obtained after the empirical subtraction of solvent diffractions.
An AFM image of the network of a 2 wt % hydrogel (fastcooled) of 3-HOA-NO3 (not shown) was different from those of the 3-HOA-Cl and 3-HOA-Br hydrogels, but it was not possible to ascribe it to a particular motif.
Figure 5. Concentration dependence of (A) 3-HOA-Cl, (B) 3-HOA-Br,(C) 3-HOA-NO3, and (D) 3-HOA-BF4 on conductance in water. Different symbols are for two separate experiments. 6481
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precipitation56 as well as in polymer folding and colloid stability. The structures and shapes of the assemblies formed by n-alkylammonium salts have been reported to be altered by the addition of NaCl.57 Most germane to this research is the observation that simple inorganic salts can influence the supramolecular structures and chiral organizations of some peptide-based hydrogelators.58 Here, KCl, NaCl, CaCl2, and FeCl3 were added to 2 wt % fast-cooled hydrogels of 3-HOA-Cl. The temperature at maximum heat flow (Tmax) from DSC thermograms and the gel melting temperatures by the falling drop method indicate that K+. Here, no significant difference in the gel melting temperatures or morphology was observed upon adding
