An Easy To Prepare Organic Salt as a Low Molecular Mass Organic Gelator Capable of Selective Gelation of Oil from Oil/Water Mixtures Darshak R. Trivedi, Amar Ballabh, and Parthasarathi Dastidar* Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364 002, Gujarat, India Received April 22, 2003 Revised Manuscript Received July 22, 2003 Quite a few low molecular mass organic gelators 1 (LMOGs) and their applications 2 have been reported so far. These physical gels obtained from LMOGs depend on relatively weak interactions, for example, hydrogen bonding, to form a 3-D network that immobilize the solvent molecules, leading to gel formation. During the course of our investigations in the area of crystal engineering of an organic acid-base adduct, 3 we re- cently found that a simple organic salt 4 can harden few organic solvents. We, therefore, launched an extensive search for new LMOGs based on organic salts. We decided to work on secondary ammonium (dicyclohexyl- ammonium) salt of chloro-substituted monocarboxylic acid (2-, 3-, and 4-chloro cinnamic acid) because of the following reasons: (i) The hydrogen bonding network in such salt is expected to be one-dimensional (through COO - ‚‚‚H-R 2 N + -H‚‚‚ - OOC hydrogen bonding), which is believed to be one of the prerequisites for a molecule to have potential to become a LMOG, 5 (ii) the presence of conjugated aromatic and alicyclic (cyclohexyl) moi- eties might help in interacting with nonpolar solvents, and (iii) Cl‚‚‚Cl nonbonded interactions 6 might play a role in the gelation process. To our delight, we observed that salt 1 (Scheme 1) is an excellent gelator for many organic solvents (nonpolar and polar) and oils (com- mercial fuels and edible oils). In an attempt to obtain insights into the gelation process, allied salts 2 and 3 were also prepared and investigated (Scheme 1). 1 is found to be an excellent gelator of many nonpolar and polar organic solvents whereas 2 displayed remark- able gelation ability only with nonpolar solvents. Both 1 and 2 belong to the super-gelator category as the minimum gel concentration (MGC) of these gelators is less than or equal to 1.0 wt %. 1 forms remarkably stable gel (T gel > 100 °C; see Supporting Information) with commercial fuels such as kerosene and diesel. However, salt 3 did not form gel with any solvent studied here (Table 1). SEM of gel of 1 in petrol (Figure 1a) and 2 in toluene (Figure 1b) displayed a typical fibrous network of varying thickness. In a petrol gel of 1, a population of fibers (ca. 5.5-16 μm) could easily be seen, while in a toluene gel of 2, an extremely compli- cated three-dimensional network of fibers (ca. 4-10 μm) is present. Understandably, the solvent molecules get immobilized in such network of fibers, resulting in gel formation. Selective gelation of oil from an oil/water mixture is considered important in containing an oil spill problem. 7 Interestingly, 1 displays a remarkable ability to gelate selectively an oil (either commercial fuel or edible oil) in a biphasic mixture of oil/water (1 mL/1 mL). 1 is either solubilized in such a mixture by heating with or without the presence of few drops of MeOH and the solution is left to equilibrate. Within a few hours, the oil layer is found to be completely gelled, leaving the water phase unaffected. The same observation is also seen when the experi- ment is carried out under vigorous shaking. By this way, 1 is found to gelate petrol, kerosene, cottonseed oil, sunflower oil, and coconut oil in such biphasic system of oil/water mixtures. 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