Tuning Organogel Properties by Controlling the Organic-Phase

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Tuning Organogel Properties by Controlling the Organic-Phase Composition Anh Bui and Nick Virgilio* CREPEC, Department of Chemical Engineering, Polytechnique Montréal, Montréal, Québec H3C 3A7, Canada ABSTRACT: The gel-to-sol and sol-to-gel transition temperatures of organogels comprising mixtures at various ratios of canola and mineral oils, gelified with 12-hydroxystearic acid, have been investigated by differential scanning calorimetry and rheological probing. The results reveal that controlling the organic liquid-phase composition in this model system allows the adjustment of the transition temperatures over a range of almost 20 °C and represents an easy and economical method for tuning organogel properties. Interestingly, the transition temperatures do not change linearly with the oil-phase composition, but deviate negatively toward the pure component having the lowest transition temperatures, in this case canola oil, suggesting perhaps a preferential interaction between 12-hydroxystearic acid and canola oil. hydrogen bonding with neighboring molecules, orient fiber growth. Recently, Wu et al. showed that HOA can also display two different polymorphic forms after crystallization and gel formation, depending on the polarity of the solvent used.18 Overall, the relative balance between the hydrophilic and hydrophobic local segments within this molecule allows fiber growth. Since then, much attention has focused on the synthesis and characterization of new organogelators derived from a wide variety of molecular families3,4 including monosaccharides,19−21 fatty acids, and steroids,5 with some displaying exceptional gelling properties.20 The choice of the solvent/gelator combination is an important factor that determines the properties of the resulting gel.22−24 If the affinity between the solvent and the gelator is too high, the gelator molecules completely dissolve in the solvent, and a solution is obtained; however, if the affinity is too low, precipitation of the gelator molecules occurs. A delicate balance must then be established between the gelator molecules and the solvent. This is reflected in the molecular structure of the gelator molecules, in which some segments favor contacts with the solvent whereas others avoid it, leading, in the case of organogels, to the development of long anisotropic fibers that are able to gel a given solvent. Recently, an approach based on the Hansen solubility parameters has proven to be quite useful and powerful for explaining and predicting whether a gel will be formed for a particular solvent/gelator pair .25−28 Intensive research activities have been directed toward the formulation of new organogelators to expand the available choice of molecules and toward the study of solvent/gelator interactions and effects during and after gelling. It appears, however, that relatively few reports have been published concerning the role of the organic-phase composition on the organogel properties. Yan et al. showed that ternary systems comprising glucono-appended 1-pyrenesulfonyl derivatives

1. INTRODUCTION Organogels and organogelators are a rapidly expanding and complementary family of materials to hydrogels.1−5 The intense research activities directed toward the synthesis of new organogelators stem in part from the various application fields in which these molecules could be employed, such as drug release systems, texture and rheology modifiers, oil spill recovery, and soft optical materials, to name a few.6−11 The formation of hydrogels relies, for example, on mechanisms such as thermoreversible helix formation (agar, etc.), ionic cross-linking (sodium alginate), and chemical crosslinking [poly(2-hydroxyethyl methacrylate) or pHEMA].12−15 The molecular causes related to the formation of organogels, however, differ significantly in origin and still pose some challenges to the scientific community. In the past two decades, many families of small organogelator molecules3−5 have been discovered and added to the rapidly expanding library of molecules allowing the gelation of organic liquids. One fundamental mechanism involves the aggregation of small organogelator molecules in the form of long, highly organized fibers. At the sol-to-gel transition temperature, sharp changes in the solvent flow and rheological properties are observed, as the liquid is immobilized in the fibrous network through capillary forces. The molecular architecture of organogelator molecules has been clearly identified as an important factor determining the ability to gel an organic phase. The presence of highly directional interactions (hydrogen bonding, π−π stacking, etc.) between the gelator molecules, combined with their affinity for the organic phase, allows for the formation of highly structured fibers. Terech et al. conducted a certain number of studies with 12-hydroxystearic acid (HOA), a well-known organogelator agent.16,17 They studied its gelling properties and molecular organization into fibers formed in various solvents. They showed that HOA molecules organize following a monoclinic structure, with the carboxylic acid groups in the lateral position and hydroxyl groups forming directional intermolecular hydrogen bonding oriented in the fiber’s longitudinal direction. Qualitatively, the hydroxyl groups on the C12 carbon of HOA hide within the fibers from the organic phase and, through © XXXX American Chemical Society

Received: June 21, 2013 Revised: August 23, 2013 Accepted: September 3, 2013

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dx.doi.org/10.1021/ie401965z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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with α,ω-diaminoalkane spacers and two miscible solvents, in this case water and tetrahydrofuran (THF), can form gels over a certain range of solvent compositions.25 Tuning of the gel transition temperature was possible over a range of approximately 10 °C. Interestingly, no gels were formed in the pure solvents because the gelators were insoluble in both. Cai et al. studied ternary systems consisting of a calix[4]arenebased dimeric-cholesteryl derivative in mixtures of n-decane and acetonitrile, two immiscible solvents (the gelator was soluble in n-decane but insoluble in acetonitrile).29 They showed that these systems can gel over a certain range of n-decane/ acetonitrile compositions. Furthermore, a certain number of articles have looked at the effect of adding two gelators to a single solvent.30,31 The possibility of controlling the sol-to-gel and gel-to-sol transition temperatures over considerable temperature ranges by tuning the organic-phase composition is indeed interesting for formulation considerations. Furthermore, such data could possibly yield some information about the local molecular composition near the organogelator molecules in the sol state, as well as in the vicinity of the fibers once they are formed. The objectives of this work were as follows: (1) to experimentally verify whether the gel-to-sol and sol-to-gel transition temperatures of organogels can be tuned by controlling the composition of the organic phase, (2) to quantify the results by calorimetric experiments, and (3) to determine whether the results deviate from a linear additive model. We used model systems consisting of miscible white mineral oil (MO) and canola oil (CO) with 12-hydroxystearic acid (HOA). These two oils are of significant importance and interest in many formulation applications in fields as diverse as the cosmetics and food industries, biomedical materials, and texture modifiers. HOA is a well-known organogelator molecule that has been thoroughly investigated in the scientific literature16,17,32,33 and that can form gels with both MO and CO.

and Δhsg), were determined using TA’s Universal Analysis software. 2.3. Rheological Measurements and Analysis. All rheological measurements were performed with an MCR502 stress-controlled rheometer from Anton Paar equipped with a Couette geometry (cup radius, 9.040 mm; bob radius, 8.330 mm). The samples were first heated over their gel-to-sol temperatures and poured into the geometry initially heated at 80 °C before the temperature was brought down to 10 °C at a rate of 2 °C/min. First, the gel’s linear viscoelastic (LVE) regime was determined by performing oscillatory stress sweep tests at an angular velocity of 0.5 rad/s. Then, temperature ramps were performed in the LVE regime at a deformation amplitude of γ0 = 0.1% and an angular velocity of ω = 0.5 rad/s. The samples were first heated and maintained at 80 °C for 5 min and then cooled at a rate of 2 °C/min to 10 °C. The sol-togel temperature corresponds to the sharp transition observed in the LVE properties. The samples were then heated again at the same rate back to 80 °C to obtain the gel-to-sol transition temperature.

3. RESULTS AND DISCUSSION 3.1. Visual Inspection of the Pure and Binary Organogels. Figure 1 shows gels comprising pure MO, pure

2. MATERIALS AND METHODS 2.1. Materials and Solution Preparation. A commercial white mineral oil (MO) of low viscosity (about 10 cP), canola oil (CO), and 12-hydroxystearic (hydroxyoctadecanoic) acid (HOA, M = 300.48 g/mol) (Fisher Scientific) were used without further purification. HOA was mixed with 10 mL of oil at concentrations (cgel) ranging from 0.3 to 3 g of HOA/100 mL of oil. The solution was then heated at 80 °C for 30 min and stirred vigorously at 300 rpm until complete dissolution of the HOA. After dissolution, the samples were poured into 20 mL vials and stored at room temperature for 24 h. For binary CO/MO systems, the preparation protocol was similar to that just described, with the MO volume fraction (ϕMO) ranging from 0% to 100%. All subsequent tests were performed after at least 24 h of storage at room temperature. 2.2. Differential Scanning Calorimetry (DSC) Measurements and Analysis. The gel-to-sol and sol-to-gel transition temperatures (Tgs and Tsg, respectively) were determined by differential scanning calorimetry experiments and analysis using a Q-2000 DSC apparatus from TA Instruments. All samples were initially heated to 100 °C at a rate of 5 °C/min to erase their thermal history. The samples were subsequently cooled at a rate of 5 °C/min to 25 °C and then heated again to 100 °C at the same rate. The onset and peak temperatures of the gel-tosol (Tgs‑onset and Tgs‑peak) and sol-to-gel (Tsg‑onset and Tsg‑peak) transitions, as well as the associated specific enthalpies (Δhgs

Figure 1. Effect of the HOA gelator composition on the formation of organogels for (a) white mineral oil (MO), (b) canola oil (CO), and (c) 50/50 vol % binary solutions of MO and CO. For each photograph, from left to right: 0 (no organogelator), 0.3, 1, 1.5, 2, 2.5, and 3 g of HOA/100 mL of solution.

CO, and 50/50 (vol %) CO/MO, with cgel ranging from 0 (no gelator) to 3 g of HOA/100 mL of oil. At the lowest value of cgel (0.3 g/100 mL, second bottles from the left), different behaviors were observed. In mineral oil, an initial gelling was observed within the whole volume of the liquid phase. However, after a few days, significant syneresis is observed, as the mineral oil was gradually expelled from the gel. In canola B



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increasing ϕMO (Figure 3c) with a similar linear relation or slightly negative deviation. The gel-to-sol transition also became more clearly defined, with a narrowing of the peak in the thermograms (Figure 3a) as ϕMO increased. Inspection of the sol-to-gel transition (Figure 3d) reveals some differences with respect to the gel-to-sol transition. The onset temperature (Tsg‑onset) and peak temperature (minimum heat flow, Tsg‑peak) shifted toward higher values as ϕMO was increased : from 58.5 to 71 °C for Tsg‑onset and from 54 to 70.5 °C for Tsg‑peak. The sol-to-gel transition temperature could be adjusted over a range of almost 15 °C by tuning the organicphase composition while keeping the HOA concentration constant (as for the gel-to-sol transition). Analysis of Tsg as a function of ϕMO, however, reveals a significant negative deviation as compared to a linear trend (Figure 3e), whereas no such clear deviation can be observed for the specific enthalpy of the sol-to-gel transition (Δhsg) as a function of ϕMO (Figure 3f). Note, however, that it is more difficult to obtain specific enthalpies than transition temperatures from thermograms, because the limits of integration are not necessarily easy to determine. We verified these results by monitoring the linear viscoelastic (LVE) properties of the gels while performing temperature ramps. From 80 °C, the solutions were cooled to 10 °C at a rate of 2 °C/min and then heated back to 80 °C at the same rate. The transition temperatures appear as sharp transitions in the LVE properties (Figure 4). We identified the onset of the sol-to-gel transition (Tsg‑onset) as the last point associated with the solution state (Figure 4a) and the onset of the gel-to-sol transition (Tgs‑onset) as the last point of the gel state, although the latter is not as easy to identify (Figure 4b). Tsg‑onset was found to range from 61 to 74.7 °C as ϕMO increased, whereas Tgs‑onset ranged from approximately 55 to 70 °C, although it is more difficult to estimate Tgs at low MO compositions from the rheological data as compared to the DSC results. These results compare well with those from DSC experiments. For the gel-to-sol transition, the results might suggest an affinity between HOA and CO, which could explain the observed negative deviations. This is not unexpected considering the molecular structure of HOA.5 Using an analysis based on the Hansen solubility parameters,24−27 Gao et al. showed that it is crucial to carefully tune the hydrogen-bonding component if HOA is to form a gel in an organic solvent. If the hydrogen-bonding interactions are too strong, significant solvent−gelator interactions arise and disrupt gel formation. In our case, the hydrogen-bonding interactions should be stronger with canola oil, which could explain the observed lower transition temperatures for this system. However, the interactions were not strong enough to prevent gel formation. The next section considers this aspect more closely. The temperature range over which the gel-to-sol and sol-togel transitions can be tuned is quite significantnearly 20 °Cand should be a function of the components present in the organic phase. Adjusting the organic-phase composition should also have a significant effect on the level of undercooling during the nonisothermal cooling step. A calorimetric and microscopic study of Rogers and Marangoni on binary systems containing HOA and various solvents showed that the cooling rate has an impact on the HOA crystal nucleation behavior (induction time, number of nuclei, etc.) and gel microstructure (fiber length and branching, thickness, etc.).22 A future study on ternary systems should address these important aspects. Finally, a broad tuning range could then possibly be achieved if

oil, gelified droplets and aggregates, but no homogeneous gel, were observed, and the system remained mostly liquid. Interestingly, a clear and complete gel, without significant syneresis (even after 1 month), was observed in the 50/50 vol % CO/MO solution (Figure 1c, second bottle from the left). A closer inspection clearly reveals these features (Figure 2).

Figure 2. Closer inspection of (a) the significant syneresis effect observed in MO gelified with 0.3 g of HOA/100 mL of oil, (b) canola oil with 0.3 g of HOA/100 mL (only some gelified droplets are observed, and the CO remains mostly liquid), (c) 50/50 (vol %) MO/ CO system gelified with 0.3 g of HOA/100 mL.

Homogeneous gels with no syneresis were observed for all three systems (100% CO, 100% MO and 50/50 vol % systems) at cgel values over 0.3 g of HOA/100 mL of oil (Figure 1, third bottles from the left and subsequent). As cgel increased, the gels became gradually more opaque, and at the highest concentration, the gel formed with MO became almost solid-like. The fact that no syneresis was observed in the mixed gel as compared to the pure MO gel could be due to the gelator network microstructure, as crystallographic differences probably exist between these two gels.22,34−38 The network formed by the gelator molecules is highly sensitive to the cooling rate and organic-phase properties.22 A network formed from few nuclei, with little branching and/or long fibers, should not retain the oil phase as well as the complementary case. However, adding too much CO will eventually lead to the formation of aggregates instead of a gel, as illustrated in Figure 1b. Carefully tuning the composition of the organic phase then represents an interesting potential method for obtaining stable gels at very low concentrations of gelator. 3.2. Thermal Properties of Organogels: Effect of the Organic-Phase Composition. The thermal properties of organogels comprising 2.5 g of HOA/100 mL of organic phase, with various CO/MO compositions, were characterized by DSC analysis. As the MO volume fraction was increased (ϕMO), the onset temperature (Tgs‑onset) and peak temperature (maximum heat flow, Tgs‑peak) of the gel-to-sol transition gradually shifted toward higher values (Figure 3a,b): from 46 to 71 °C for Tgs‑onset and from 62 to 76.5 °C for Tgs‑peak. These results indicate that it is possible to adjust the gel-to-sol transition temperature Tgs over a range of almost 20 °C by controlling the organic-phase composition while keeping the HOA concentration constant. The results also reveal a linear relation, or a slightly negative deviation, when plotting Tgs as a function of ϕMO (Figure 3b). The specific enthalpy of the gelto-sol transition (Δhgs) was also found to increase with C



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Figure 3. Effect of the organic-phase composition on organogel formation in gels containing 2.5 g of HOA/100 mL of oil. (a) DSC thermograms showing the gel-to-sol transition temperature (Tgs). From bottom to top: ϕMO = 0, 25, 50, 75, and 100 (vol % of MO). (b) Onset (○) and peak (●) temperatures associated with the gel-to-sol transition (Tgs‑onset and Tgs‑peak) as a function of ϕMO. (c) Specific enthalpy of the gel-to-sol transition (Δhgs) as a function of ϕMO. (d) DSC thermograms showing the sol-to-gel transition temperature. From bottom to top: ϕMO = 0, 25, 50, 75, and 100 (vol % of MO). (e) Onset (○) and peak (●) temperatures associated with the sol-to-gel transition (Tsg‑onset and Tsg‑peak) as a function of ϕMO. (f) Specific enthalpy of the sol-to-gel transition (Δhsg) as a function of ϕMO.

Figure 4. Complex modulus G* of organogels containing 2.5 g of HOA/100 mL of oil as a function of temperature during the (a) cooling (from 80 to 10 °C) and (b) heating (from 10 to 80 °C) processes at 2 °C/min for various binary CO/MO blends (vol %): 100/0 (●), 75/25 (□), 50/50 (▲), 25/75 (◊), and 0/100 (★).

the transition temperatures of the pure components were significantly different. 3.3. Thermal Properties of Organogels: Effect of Organogelator Concentration. To further investigate a possible affinity between the HOA gelator and CO, we

prepared three distinct series of gels with increasing HOA concentrations (cgel = 0.3−3 g of HOA/100 mL of organic phase): (1) pure MO gels, (2) pure CO gels, and (3) CO/MO gels with a 50/50 vol % composition. As cgel was increased, the DSC thermograms for all three series showed increasing peak D



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Figure 5. Effects of HOA concentration (cgel = 0.3, 1, 1.5, 2, 2.5, and 3 g of HOA/100 mL of organic phase) on the thermal properties of pure MO gels (●), pure CO gels (■), and MO/CO gels (50/50 vol %) (⧫). (a) DSC thermograms showing the gel-to-sol transition of pure MO gels as cgel increases (top to bottom). (b) Gel-to-sol transition temperature (Tgs‑peak) as a function of cgel for all three organic-phase compositions. (c) Sol-to-gel transition temperature (Tsg‑peak) as a function of cgel for all three organic-phase compositions. (d) Plot of log(cgel) as a function of (1/Tgs‑peak) for all three compositions.

heights at both the gel-to-sol (Tgs‑peak) and sol-to-gel (Tsg‑peak) transition temperatures. Figure 5a shows the DSC thermograms corresponding to the gel-to-sol transitions in MO systems with cgel increasing from top to bottom. Figure 5b shows Tgs‑peak as a function of cgel. For all three series, Tgs‑peak increased as cgel was increased. The pure MO gels displayed the highest transition temperatures (from 71 to 76.5 °C), whereas the pure CO gels displayed the lowest ones (from 57 to 65 °C). All Tgs‑peak values for MO/CO gels fell between these two ranges (from 59 to 71 °C) but were systematically closer to the values for pure CO gel for identical HOA concentrations (especially at low cgel values), except at cgel = 3%. Similar trends were observed for Tsg‑peak (Figure 5c). Figure 5d displays the values of log(cgel) as a function of 1/ Tgs‑peak. This representation is based on a model (eq 1) developed by Eldridge and Ferry for the examination of the gelto-sol transition temperatures of gelatin gels39 and has subsequently been adapted by a number of authors to study the effects of the solvent phase and gelator concentration on organogel formation16,21,40,41 log(cgel) =

−Δhgs 2.303RTgs‐peak

Table 1. Gel-to-Sol Enthalpies Determined with Eq 1

a

system composition (CO/MO, vol %)

gel-to-sol specific enthalpya (Δhgs, kJ/mol)

100/0 0/100 50/50

163 437 161

For HOA, Δhf = 57.6 kJ/mol.

First, one can see that the slopes of the CO and MO/CO gels are nearly identical and yield similar Δhgs values of 163 and 161 kJ/mol of HOA, respectively, corresponding to approximately 3 times the enthalpy of fusion, Δhf, of pure HOA (57.6 kJ/mol). Similar trends for HOA-, cholesterol-, and monosaccharide-based organogels have been reported in the literature, with Δhgs values of various gelator/solvent pairs typically being equal to 2−3 times the value of Δhf of the gelator, suggesting an analogy between the two process.16,21,40,41 The MO gels, however, exhibit a much higher Δhgs value, which is about 2.5 times higher than the results for the CO and CO/MO gels and nearly 7.5 times the value of Δhf. Unusually high Δhgs values were also reported by Shinkai and co-workers for a few gelator/solvent pairs.40,41 Although the origin of such results is still unclear, the authors suggested that Δhgs combines at least two distinct energetic contributions: (1) the melting enthalpy of the fibers (which should be more or less constant and independent of the solvent used if the fibers comprise pure gelator molecules organized in a similar and regular structure) and (2) the enthalpy of dissolution of the gelator in the solvent phase, which is related to the molecular affinity between the two species and should depend on the solvent used. In our study, the polarity of the HOA molecules could explain why Δhgs is lower for the CO gels than the MO gels because

+b (1)

where cgel is the gelator concentration, Tgs‑peak is the gel-to-sol transition temperature, Δhgs is the specific enthalpy of the process, R is the gas constant, and b is the intercept at the origin. Δhgs was obtained by determining the slope of a data set on the graph. For the three studied series, linear trends can be observed (except at very low concentrations of gelator, as observed in previous publications39−41). The obtained Δhgs values are reported in Table 1 and compared to the specific enthalpy of fusion (Δhf) of pure HOA. E



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(3) Abdallah, D. J.; Weiss, R. G. Organogels and Low Molecular Mass Organic Gelators. Adv. Mater. 2000, 12, 1237−1247. (4) van Esch, J.; Feringa, B. L. New Functional Materials Based on Self-Assembling Organogels: From Serendipity towards Design. Angew. Chem., Int. Ed. 2009, 39, 2263−2266. (5) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133− 3159. (6) John, G.; Jadhav, S. R.; Menon, V. M.; John, V. T. Flexible Optics: Recent Developments in Molecular Gels. Angew. Chem., Int. Ed. 2012, 51, 1760−1762. (7) Vidyasagar, A.; Handore, K.; Sureshan, K. M. Soft Optical Devices from Self-Healing Gels Formed by Oil and Sugar-Based Organogelators. Angew. Chem., Int. Ed. 2011, 50, 8021−8024. (8) Mukherjee, S.; Mukhopadhyay, B. Phase-Elective Carbohydrate Gelator. RSC Adv. 2012, 2, 2270−2273. (9) Jadhav, S. R.; Vemula, P. K.; Kumar, R.; Raghavan, S. R.; John, G. Sugar-Derived Phase-Selective Molecular Gelators as Model Solidifiers for Oil Spills. Angew. Chem., Int. Ed. 2010, 49, 7695−7698. (10) Vintiloiu, A.; Leroux, J.-C. Organogels and Their Use in Drug DeliveryA Review. J. Controlled Release 2008, 125, 179−192. (11) Murdan, S.; Gregoriadis, G.; Florence, A. T. Novel Sorbitan Monostearate Organogels. J. Pharm. Sci. 1999, 88, 608−614. (12) Vlierberghe, S. V.; Dubruel, P.; Schacht, E. Biopolymer-Based Hydrogels As Scaffolds for Tissue Engineering Applications: A Review. Biomacromolecules 2011, 12, 1387−1408. (13) Slaughter, B. V.; Khurshid, S. S.; Fisher, O. Z.; Khademhosseini, A.; Peppas, N. A. Hydrogels in Regenerative Medicine. Adv. Mater. 2009, 21, 3307−3329. (14) Hoare, T. R.; Kohane, D. S. Hydrogels in Drug Delivery: Progress and Challenges. Polymer 2008, 49, 1993−2007. (15) Peppas, N. A.; Hilt, J. Z.; Khademhosseini, A.; Langer, R. Hydrogels in Biology and Medecine: From Molecular Principles to Bionanotechnology. Adv. Mater. 2006, 18, 1345−1360. (16) Terech, P.; Pasquier, D.; Bordas, V.; Rossat, C. Rheological Properties and Structural Correlations in Molecular Organogels. Langmuir 2000, 16, 4485−4494. (17) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Organogels and Aerogels of Racemic and Chiral 12-Hydroxyoctadecanoic Acid. Langmuir 1994, 10, 3406−3418. (18) Wu, S.; Gao, J.; Emge, T. J.; Rogers, M. A. Solvent-Induced Polymorphic Nanoscale Transitions for 12-Hydroxyoctadecanoic Acid Molecular Gels. Cryst. Growth Des. 2013, 13, 1360−1366. (19) Gronwald, O.; Shinkai, S. Sugar-Integrated Gelators of Organic Solvents. Chem.Eur. J. 2001, 7, 4329−4334. (20) Luboradzki, R.; Gronwald, O.; Ikeda, A.; Shinkai, S. SugarIntegrated “Supergelators” Which Can Form Organogels with 0.03− 0.05% [g mL−1]. Chem. Lett. 2000, 10, 1148−1149. (21) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Sugar-Integrated Gelators of Organic SolventsTheir Remarkable Diversity in Gelation Ability and Aggregate Structure. Chem.Eur. J. 1999, 5, 2722−2729. (22) Rogers, M. A.; Marangoni, A. G. Solvent-Modulated Nucleation and Crystallization Kinetics of 12-Hydroxystearic Acid: A Nonisothermal Approach. Langmuir 2009, 25, 8556−8566. (23) Hirst, A. R.; Coates, I. A.; Boucheteau, T. R.; Miravet, J. F.; Escuder, B.; Castelletto, V.; Hamley, I. W.; Smith, D. K. LowMolecular-Weight Gelators: Elucidating the Principles of Gelation Based on Gelator Solubility and a Cooperative Self-Assembly Model. J. Am. Chem. Soc. 2008, 130, 9113−9121. (24) Zhu, G.; Dordick, J. S. Solvent Effect on Organogel Formation by Low Molecular Weight Molecules. Chem. Mater. 2006, 18, 5988− 5995. (25) Yan, N.; Xu, Z.; Diehn, K. K.; Raghavan, S. R.; Fang, Y.; Weiss, R. G. How Do Liquid Mixtures Solubilize Insoluble Gelators? SelfAssembly Properties of Pyrenyl-Linker-Glucono Gelators in Tetrahydrofuran-Water Mixtures. J. Am. Chem. Soc. 2013, 135, 8989−8999.

dissolution should be more favorable in CO. Furthermore, a local molecular neighborhood rich in CO near HOA molecules could explain why the behavior of the CO/MO gels is much closer to that of pure CO gels at the gel-to-sol and sol-to-gel transitions. More investigations will be conducted on this aspect.

4. CONCLUSIONS The gel-to-sol and sol-to-gel transition temperatures of organogels can be controlled by using mixtures of organic liquids. In this article, we have demonstrated that the transition temperatures of canola and mineral oil gels prepared with smalls amount of 12-hydroxystearic acid (HOA) can be adjusted over a range of almost 20 °C. This is an interesting result that can lead to simple strategies for formulation applications in fields such as texture modifiers (e.g., for food, paints, and inks), oil spill recovery, and controlled drug delivery. Furthermore, the syneresis effects observed in mineral oil gels at low HOA contents can be significantly reduced by the addition of canola oil. DSC and rheological analyses revealed that the transition temperatures are systematically lower in canola oil, as compared to mineral oil, at all HOA concentrations. At a given HOA concentration, the mixing of canola and mineral oils allows one to control the transition temperatures continuously between those of the two pure oils, but the transitions are not linear functions of the composition. Instead, systematic negative deviations toward the canola oil transition temperatures are observed at all oil mixture ratios, suggesting an affinity between canola oil and the HOA gelator. Similar results were obtained when we analyzed the effect of the HOA concentration on the transition temperatures of pure canola and mineral oils, as compared to the results obtained with a 50/50 vol % blend of the two oils. Further investigations are required to explain the exact causes of these results. Combining organic liquids appears to be an interesting and easy approach to controlling the gel-to-sol and sol-to-gel transition temperatures of organogels. Based on the reported results, to maximize the range of accessible transition temperatures, one should use organic liquids with very different transition temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 1-514-340-4711 ext. 4524. Fax: 1-514-340-4159. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Polytechnique Montréal and the Department of Chemical Engineering with the PIED program (start-up funds). The authors thank Mrs. Claire Cerclé and Mrs. Melina Hamdine for their support and assistance.

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REFERENCES

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